Paper No 162

1

Abstractmdash Trace gas sensing and analysis by Tunable Diode

Laser Absorption Spectroscopy (TDLAS) has become a robust and reliable technology accepted for industrial process monitoring and control quality assurance environmental sensing plant safety and infrastructure security Sensors incorporating well-packaged wavelength-stabilized near-infrared (12 - 20 microm) laser sources sense over a dozen toxic or industrially-important gases Recently developed mid-IR lasers particularly quantum cascade devices spanning wavelengths of 3-12 microm can sense in real-time sub-ppm concentrations of many hydrocarbons

A large emerging application for TDLAS is standoff sensing of chemical vapors eg leaks from natural gas pipelines Employing a 10 mW DFB laser the eye-safe battery-powered 6-pound handheld Remote Methane Leak Detector (RMLD) illuminates a non-cooperative topographic surface and analyzes returned scattered light to deduce the presence of excess methane For aerial surveying replacing the handheld transceiver with a large-aperture telescope and adding an EDFA to the laser transmitter extends the standoff distance to 3000 m By selecting a laser source having an appropriate wavelength the standoff TDLAS tool detects trace concentrations of non-methane hazardous gases including several high-priority TICs and emissions from illicit chemical production laboratories

This paper also describes concepts for miniature integrated optic TDLAS sensors that combine a laser source sampling section and detector on a monolithic semiconductor materials system substrate Such chip-scale low-power integrated optic gas-phase chemical sensors may enable low-cost mass production so that many hundreds or thousands of such sensors can be distributed cost-effectively over a wide area of interest and communicate via wireless networks

Manuscript received August 28 2008 This work was supported in part by

the US Environmental Protection Agency the US Department of Energy National Energy Technology Laboratory the US Air Force the Northeast Gas Association Physical Sciences Inc and Heath Consultants Inc

M B Frish is with Physical Sciences Inc Andover MA 01810 USA (phone 978-689-0003 fax 978-689-3232 e-mail frishpsicorpcom)

R T Wainner is with Physical Sciences Inc Andover MA 01810 USA (phone 978-689-0003 fax 978-689-3232 e-mail wainnerpsicorpcom)

M C Laderer was with Physical Sciences Inc Andover MA 01810 USA He is currently at the Georgia Institute of Technology Department of Mechanical Engineering Atlanta GA 30332 USA (phone 404-894-3200 fax 404-894-8736 e-mail matthew_laderergatechedu)

B D Green is with Physical Sciences Inc Andover MA 01810 USA (phone 978-689-0003 fax 978-689-3232 e-mail greenpsircorpcom)

M G Allen is with Physical Sciences Inc Andover MA 01810 USA (phone 978-689-0003 fax 978-689-3232 e-mail allenpsicorpcom)

Index Termsmdashchemical sensors spectroscopy standoff detection TDLAS

I TDLAS OVERVIEW DLAS is a configurable gas sensing technology that is commercially successful in markets demanding sensitive

detection with minimal false positive occurrences and minimal down time [1]-[7] Fig 1 shows a generalized TDLAS architecture utilized by practitioners to monitor chemical combustion or manufacturing processes

Fig 1 TDLAS gas detector system

The measurement principle is absorption spectroscopy Many gaseous molecules absorb light at specific wavelengths (called absorption lines) For example Fig 2 presents the absorbance spectra for CO2 and H2O across a 1 m path at standard temperature and pressure calculated using the HITRAN database According to the Beer-Lambert law

( )[ ] ( )αminus=sdotsdotminussdotsdot= exp-exp 000 vvv IlNvvgSII (1)

where Iν is the transmitted intensity of laser radiation of wavenumber ν through an absorbing medium and Iν0 is the initial laser intensity The exponential term α conventionally called the absorbance represents the attenuation of Iν0 by absorption where S is the strength of an absorption line g(ν-ν0) expresses the absorption lineshape N is the molecular

Standoff and Miniature Chemical Vapor Detectors Based on Tunable Diode Laser

Absorption Spectroscopy Mickey B Frish Richard T Wainner Matthew C Laderer B David Green and Mark G Allen

T

Paper No 162

2

Fig 2 Absorption spectra of CO2 and H2O density of absorbing molecules in the gas phase and l is the path length of the laser across the region of absorbing molecules N is deduced using explicit knowledge or measurement of each of the other terms in (1) N is commonly expressed as a concentration in ppm or when path length is variable the measurement is expressed as a combination of Nl or ppm-m

In TDLAS a diode laser emitting light at a well-defined but adjustable or tunable wavelength usually samples a single gas absorption line Typically each TDLAS system is built using a laser having a specific design wavelength chosen to correspond to a specific absorption line of the target analyte gas that is free of interfering absorption from other molecules Accurate control of the laser injection current and temperature achieves rapid and precise tuning over a range of plusmn 2 nm around the specified wavelength The laser emission linewidth is narrower than gas absorption linewidths This may be contrasted with NIR absorption techniques that sample with broadband sources and measure absorption from multiple lines across a fairly broad range of frequencies TDLAS thus offers the advantage of selectivity for a target trace gas absent with spectral interferences from other background gases Compact

TDLAS systems operate continuously at ambient temperatures from -20C to 50C using about 500 mW of electrical power without maintenance cryogenics or operator attendance

TDLAS sensors commonly exploit the laserrsquos fast tuning capability to rapidly (gt 10 kHz typically) modulate the wavelength causing it to sweep back and forth across an absorption feature at a precise modulation frequency ωm [8] [9] The response to the wavelength modulation is an amplitude modulation at the detector illustrated in Fig 3 The amplitude modulation arises from two sources 1) the transmitted laser power follows the wavelength modulation and thus modulates sinusoidally at frequency ωm and 2) the absorption due to target gas Since the wavelength crosses the target gas absorption line twice for each modulation cycle the amplitude modulation due to absorption occurs at precisely twice the modulation frequency 2ωm Via lock-in amplification the signal processor measures the average values of the amplitude modulations at ωm and 2ωm These are called the F1 and F2 signals respectively F1 is proportional to the received laser power while F2 is proportional to the received laser power and the target gas absorbance Thus the ratio F2F1 is proportional to the absorbance only and is independent of the received laser power as long as it exceeds sensor noise

The modulation and lock-in detection technique is called as wavelength modulation spectroscopy (WMS) WMS measures absorbances of 10-5 or less with one second or faster response and can provide ppm to ppb chemical detection limits depending on the spectroscopic properties of the target gas and the sampling pathlength WMS is thus a highly sensitive and gas-specific form of spectroscopic gas analysis Table I is a partial list of target gases that TDLAS has successfully measured [2] [5] [6] [10]-[15] Dividing the values in Table I by the optical path length the laser beam transits through the gas sample determines the measurable concentration in ppm For example the 1 ppm-m detection limit for CO2 combined with a 100 m round-trip optical path yields a sensitivity to an average of 001 ppm = 10 ppb CO2 Table II lists high-priority Toxic Industrial Compounds (TICs) and compares with demonstrated TDLAS sensing capabilities

Fig 3 WMS spectroscopy Laser wavenumber ν modulates sinusoidally across a spectral absorption line with modulation depth plusmn δ and modulation frequency ωm creating an amplitude modulated detector signal

Paper No 162

3

TABLE I SOME GASES MEASURED BY NEAR-INFRARED TDLAS

Gas Detection Limit

(ppm-m) Gas Detection Limit

(ppm-m) HF 02 HCN 02 H2S 200 CO 400 NH3 50 CO2 10 H2O 10 NO 300 CH4 10 NO2 02 HCl 02 O2 500

H2CO 50 C2H2 02

TABLE II ABILITY OF TDLAS TO DETECT HIGH PRIORITY TICS

Ammonia Arsine Boron trichloride Boron trifluoride Carbon disulfide Chlorine Diborane Ethylene oxide Fluorine Formaldehyde Hydrogen bromide Hydrogen chloride

Hydrogen cyanide Hydrogen fluoride Hydrogen sulfide Nitric acid fuming Phosgene Phosphorus trichloride Sulfur dioxide Sulfuric acid Tungsten hexafluoride

Proven

Likely

Possible

Unknown

II STANDOFF TDLAS Each TDLAS sensor incorporates a Measurement Path or

Gas Sampling Head Confirable options for these sections include Extractive Open Path and Standoff [16]-[19] Extractive sampling draws the gas sample through an optical measurement chamber The optical path length within the chamber is designed to provide the required sensitivity to the target gas A multi-pass optical configuration such as a Herriot cell may be utilized to provide a long optical path length within a small volume Open Path and Standoff configurations transmit the laser beam along a line of sight that can be up to few hundreds of meters in length

Fig 4 illustrates Standoff TDLAS [8] A transceiver unit transmits the laser beam which like a flashlight illuminates a distant diffuse surface The transceiver collects laser light backscattered from the illuminated surface and concentrates the received laser power onto the photodetector The photo-detector converts the received laser power into an electrical signal transmitted to a signal processor Fig 5 shows a commercial handheld TDLAS transceiver for gas sensing applications [20] Its nominal 10 mW transmitted laser power combined with its 10 cm diameter receiver provides a standoff range of up to 30 m

The transceiver shown in Fig 5 is part of the product known as the Remote Methane Leak Detector or RMLDtrade [21]Its specific handheld battery-powered configuration was

developed for walking surveys for leaks from municipal natural gas pipelines The RMLD is replacing flame ionization detectors and simplifying the work of surveyors who periodically walk the length of each buried gas pipe RMLD detects methane with 5 ppm-m sensitivity at 10 Hz It communicates gas concentration via an audio output an LCD alphanumeric display and a serial output that may be attached to a separate computer

Fig 6 shows a new configuration of the RMLD platform developed for leak surveying while driving city streets [22]-[24] This unit integrates an RMLD transceiver with a video camera and a rangefinder in a platform mounted on a joystick-controlled tip-tilt platform A laptop computer mounted in the vehicle cabin synthesizes data from all three sensors to automatically identify leaks and capture a video image of the leak location The rangefinder corrects for ambient methane as optical path length changes

Fig 7 presents data acquired while driving past leaks during field trials Additional data (not shown) demonstrates high probability detection of methane plumes having path- integrated concentrations of 20 ppm-m from leaks smaller than 1 scfh while driving past at speeds up to 40 kmh

By replacing the 10 cm diameter receiver of the handheld transceiver with a 40 cm diameter receiver (using a modified astronomical telescope) the standoff range increases to approximately 1200 ft for detecting methane plumes having path-integrated concentrations exceeding about 100 ppm-m [25] This capability is suitable for aerial pipeline leak surveying from rotary or fixed wing aircraft Fig 8 illustrates the configuration of an aerial system installed in a Cessna 206 a single-engine 4-seat fixed wing aircraft The transceiver aims through a 50 cm diameter hole in the aircraft floor Fig 9 shows data acquired during preliminary airborne testing [24]

Fig 4 Standoff TDLAS

III INTEGRATED OPTICS TDLAS SENSOR VISION AND CHALLENGES

Many gas sensing applications particularly for homeland security and defense demand low-cost distributed networks comprising hundreds or thousands of individual mass-produced sensor elements These networked sensors serve as alarms they sense and report abnormal events To preclude

Paper No 162

4

Fig 5 Handheld TDLAS Transceiver 10 mW transmitted power combined with 10 cm diameter receiver provides 5 ppm-m methane detection limit at standoff ranges up to 100 m

Fig 6 Mobile RMLD platform with photo of unit mounted on a box truck cabin roof

Fig 7 Methane leak detection with Mobile RMLD 10 scfh leak rate 10 m standoff range 10 kmh driving speed

Paper No 162

5

Fig 8 Long range transceiver system Photograph shows transceiver component details drawing shows aircraft installation configuration schematic shows system interconnections and optional EDFA for amplifying laser power and extending standoff range to 2000 m with a detection sensitivity of about 10000 ppm-m [25] [26]

Fig 9 Data acquired with aerial RMLD flying in a Cessna 206 making several passes over a plume from a natural gas leak flowing at about 1000 scfh

Paper No 162

6

false alarms the sensors must be sensitive and specific to the targeted threat key attributes of TDLAS However measurement accuracy and precision requirements are less stringent gas concentrations errors of 20 more can be tolerated without compromising the basic sensor alarm function

TDLAS has the potential to meet this need However although TDLAS technology is not much more complex than that of compact disk players and recorders the smallest and lowest cost currently-available TDLAS sensors fit in a smoke-alarm type package weighing about three pounds and costing ~$10000 Fig 10 shows an example This cost limits TDLAS applications to low-volume markets where the expensive sensor provides an economic payback Thus TDLAS remains too expensive bulky and power-hungry to deploy practically in the envisioned autonomous distributed sensor networks The high cost of TDLAS results from using 1) laser packages designed in the early 1990rsquos by and for the telecommunications industry that are produced in relatively low volumes and cost about $1000 each even though the laser chips alone could cost only tens of dollars each if produced in quantities of tens of thousands 2) bulk optical components often hand-polished with sophisticated coatings 3) control and data processing electronics built from commercial discrete components

Fig10 Battery-powered TDLAS sensor on 6rdquo x 6rdquo circuit board

Integrating micro-machined optical waveguide Sampling Elements with laser sources and detectors to create monolithic TDLAS sensors conceptualized in Fig 11 offer the potential for low-cost mass-production Within the Waveguide Sampling Element the laser power is constrained to follow a transparent mechanical structure but the waveguide structure can be designed to force a fraction of the guided laser power into an evanescent field in the surrounding air where the targeted chemical is sensed [27] Evanescent waveguides can be fabricated from silicon or indium phosphide semiconductor wafers Assuming production of tens of thousands of such devices their cost will be comparable to that of the laser chips a few tens of dollars each To provide relatively long (~ 1m) effective optical pathlengths the waveguides can include wavelength-scale resonant structures such as ring

resonators of Fig 11 that that function similarly to multi-pass or resonant bulk optical cells [28] [29] As described above TDLAS sensors routinely measure changes in optical power due to absorption that are 1 part in 100000 of the average power received by the detector yielding the detection limits listed in Table I When used with an evanescent waveguide sampling element only the evanescent portion of the power is subject to absorption therefore the minimum detection limit must be multiplied by the ratio of the total power to the evanescent power Assuming 10 of the power is in the evanescent portion then the minimum measurable absorbance (or noise-equivalent absorbance NEA) would be 10-4 Because measurement accuracy and precision are not key requirements for these sensors the waveguide sensing elements need not perfectly replicate each other facilitating high production yields (albeit difficult to quantify at this early conceptual stage of development)

Fig 11 Examples of ring resonator waveguide sensing element providing long optical pathlengths integrated on a monolithic platform with a laser source and detector to form an integrated optic TDLAS sensor Not to scale

A key challenge to achieving this capability is reducing laser power consumption Near-IR TDLAS power consumption results primarily from laser thermal control wherein laser temperature is maintained near 300 K (room temperature) with plusmn10mK precision A typical laser package illustrated by Fig 12 includes a thermo-electric cooler (TEC) platform a thermistor a microlens to project laser light onto an optical fiber facet an optical isolator to prevent optical fiber backreflections from disrupting laser performance and a monitor photodiode for measuring laser output Because this laser package style enabled industrial quality TDLAS sensors it has become a de facto TDLAS standard However for the high-volume gas sensing applications the sophisticated laser packaging is both unnecessary and detrimental to performance TDLAS laser packaging requirements are not technically complex but are unique to the needs of TDLAS and have not been explored by the industry

The microlenses and optical isolator in the laser package couple laser light into an optical fiber The optical fiber transmits the laser light to a remote sensor head which samples the target gas Integrating on a monolithic platform an optically-resonant cavity or waveguide that samples the target gas eliminates the need for and complexity of optical elements associated with the optical fiber and the remote sensor head

The TEC function is to stabilize laser temperature by transferring the laserrsquos waste heat to a heat sink However the TEC is very inefficient the tiny laserrsquos waste heat is much

Paper No 162

7

Fig 12 Traditional telecommunications laser package 14-pin butterfly style less than the power demanded by the TEC itself to perform its function Much of the current TDLAS electronics is dedicated to laser thermal control specifically to regulating the current powering the TEC Since the TEC draws and wastes considerable power its control electronics must withstand currents ~1 A forcing use of discrete and inefficient electronic components Techniques for eliminating this wasted power are needed Successful elimination of waste heat and associated controls combined with limiting laser optical power to about 1 mW should reduce the total system power consumption to less than 100 mW for continuous operation Since intermittent sampling at perhaps 1 ndash 10 duty cycle is acceptable to achieve the alarm function average power consumption of 1 ndash 10 mW is potentially achievable

IV CONCLUSION Commercial TDLAS sensors are accepted as rugged

reliable industrial gas analyzers Worldwide thousands are installed permanently or utilized as portable sensors at industrial plants for safety process measurement and control and environmental and emissions monitoring The relatively recent introduction of Standoff TDLAS is enabling use of this technology for battery-operated hand-portable mobile and aerial surveying for natural gas pipeline leaks We anticipate that applications for similar surveying to detect gases indicative of hostile or illicit drug and chemical laboratory activities will emerge in the near future

We envision that over the next few years TDLAS will continue its evolution into smaller integrated deveices that will eventually serve mass market applications Technical developments needed to achieve this vision include laser sources designed specifically for TDLAS These lasers should minimize power consumption and harness waste heat

while providing excellent thermal and wavelength stability Such sources would enable miniature or monolithic integrated waveguide TDLAS sensors These miniature sensors can be combined with ASIC electronics and wireless transmitters to form networks serving defense and security applications

ACKNOWLEDGMENTS The authors gratefully acknowledge the support of the US

Environmental Protection Agency the US Department of Energy National Energy Technology Laboratory the US Air Force the Northeast Gas Association Physical Sciences Inc and Heath Consultants Inc

REFERENCES [1] M B Frish and F Klein ldquoTrace gas monitors based on tunable diode

laser technology an introduction and description of applicationsrdquo 5th International Symposium on Gas Analysis by Tunable Diode Lasers Freiburg Germany VDI Berichte 1366 1998

[2] M Druy M B Frish and W J Kessler ldquoFrom laboratory technique to process gas sensor - the maturation of tunable diode laser absorption spectroscopyrdquo Spectroscopy vol 21 no 3 pp 14-18 March 2006

[3] M G Allen ldquoDiode laser absorption sensing of gas dynamic and combustion flowsrdquo an invited review for Measurement Science and Technology vol 9 no 4 pp 545-562 (1998)

[4] D S Bomse ldquoDiode lasers finding trace gases in the lab and the plantrdquo Photonics Spectra vol 29 no 6 1995

[5] D E Cooper and R U Martinelli ldquoNear-infrared diode lasers monitor molecular speciesrdquo Laser Focus World November 1992

[6] M G Allen et al ldquoIn-situ and stand-off sensing using QCIC laser technology from 3 - 100 micronsrdquo Paper 5732-28 SPIE Integrated Optoelectronic Devices Photonics West Conference January 2005

[7] D M Sonnenfroh et al ldquoMid-IR gas sensors based on quasi-cw room-temperature quantum cascade lasersrdquo AIAA 38th Aerospace Sciences Meeting January 2000 Paper No 2000-0641

[8] G C Bjorklund ldquoFrequency modulation spectroscopy a new method for measuring weak absorptions and dispersionsrdquo Opt Lett vol 5 1980

[9] D S Bomse A C Stanton and J A Silver ldquoFrequency modulation and wavelength modulation spectroscopies comparison of experimental methods using a lead-salt diode laserrdquo Applied Optics vol 31 1992

[10] C Gmachl etal Engineering Research Center on Mid-Infrared Technologies for Health and the Environment (MIRTHE) First Annual Report Princeton University 2007

[11] ME Webber MB Pushkarsky and C Kumar N Patel ldquoOptical detection of chemical warfare agents and toxic industrial chemicals Simulationrdquo J Appl Phys vol 97 113101 2005

[12] M Pushkarsky A Tsekoun IG Dunayevskiy R Go and C Kumar N Patel ldquoSub-parts-per-billion level detection of NO2 using room temperature quantum-cascade lasersrdquo Proceedings of the National Academy of Sciences vol 103 pp 10846-10849 2006

[13] C M Gittins et al ldquoRemote sensing of chemical contamination using quantum cascade lasersrdquo presented at the Solid State Lasers Technology Conference June 2002

[14] D M Sonnenfroh M B Frish R T Wainner and M G Allen ldquoMid-IR quantum cascade laser sensor for tropospherically important trace gasesrdquo Final Report prepared for US Environmental Protection Agency under Order No 4C-R348-NASA PSI-2857TR-1971 November 2004

[15] J M Hensley W T Rawlins D B Oakes D M Sonnenfroh and M G Allen A quantum cascade laser sensor for SO2 and SO3 2005 CLEOIQELS May 2005 Paper No CTuY4

[16] M B Frishet al ldquoThe evolution and application of trace gas analyzers based on tunable diode laser absorption spectroscopyrdquo Invited Presentation 19th International Forum on Process Analytical Chemistry (IFPAC) Washington DC January 2005

[17] M B Frish MA White and M G Allen ldquoHandheld laser-based sensor for remote detection of toxic and hazardous gasesrdquo Presentation at Water Ground and Air Pollution Monitoring and Remediation Conference Boston MA November 2000 SPIE Paper No 4199-05

Paper No 162

8

[18] M B Frish R T Wainner B D Green M C Laderer and M G Allen ldquoStandoff gas leak detectors based on tunable diode laser absorption spectroscopyrdquo SPIE Paper No 6010-13 Optics East Boston MA 23-26 October 2005

[19] R T Wainner B D Green M G Allen M A White J Stafford-Evans and R Naper ldquoHandheld battery-powered near-IR TDL sensor for stand-off detection of gas and vapor plumesrdquo Applied Physics B vol 75 pp 249-254 2002

[20] M B Frish et al ldquoStandoff sensing of natural gas leaks evolution of the remote methane leak detector (RMLD)rdquo Invited Paper in Conference on Lasers and Electro-opticsQuantum Electronics and Laser Science and Photonic Applications Systems and Technologies 2005 Optical Society of America Washington DC 2005 (Presentation JThF3 at Photonic Applications Systems Technologies (PhAST) Conference Baltimore MD May 2005)

[21] A G Fabiano J Rutherford S Chancey and M B Frish ldquoRemote methane leak detector advanced prototype to beta developmentrdquo in Proceedings of Natural Gas Technologies 2005 Gas Technology Institute Des Plaines IL (Presented at NGT III Conference Orlando FL January 2005)

[22] MB Frish et al ldquoExtended performance handheld and mobile sensors for remote detection of natural gas leaksrdquo Phase II Final Report PSI-1402TR-1979 Physical Sciences Inc Andover MA March 2005 A portion of this report is included in ldquoField testing of remote sensor gas leak detection systemsrdquo Final Report for Project No 1810485 Southwest Research Institute San Antonio TX (December 2004)

[23] M B Frish et al ldquoDevelopment of a prototype mobile RMLDrdquo Final Report prepared for NYSEARCH PSI 1488-TR-2274 Physical Sciences Inc Andover MA December 2007

[24] M B Frish et al ldquoThe next generation of TDLAS analyzersrdquo SPIE Paper 6765-5 Optics East Boston MA 17 September 2007

[25] R T Wainner M B Frish M C Laderer M G Allen and B D Green ldquoTunable diode laser wavelength modulation spectroscopy (TDL-WMS) using a fiber-amplified sourcerdquo CLEO QELS lsquo07ndashConference on Lasers and Electro-Optics Quantum Electronics and Laser Science Conference Baltimore MD 6-11 May 2007

[26] R T Wainner et al ldquoHigh altitude aerial natural gas leak detection systemrdquo Final Report Prepared for US Department of Energy under Grant No DE-FC26-04NT42268 PSI-1454TR-2211 April 2007

[27] A B Buckman Guided Wave Photonics Saunders College Publishing 1992

[28] C Manolatou et al ldquoHigh-density integrated opticsrdquo J Lightwave Technol vol17 no9 pp1682ndash1692 Sept 1999

[29] K K Lee ldquoTransmission and routing of optical signals in on-chip waveguides for silicon microphotonicsrdquo PhD thesis MIT 2001

Paper No 162

2

Fig 2 Absorption spectra of CO2 and H2O density of absorbing molecules in the gas phase and l is the path length of the laser across the region of absorbing molecules N is deduced using explicit knowledge or measurement of each of the other terms in (1) N is commonly expressed as a concentration in ppm or when path length is variable the measurement is expressed as a combination of Nl or ppm-m

In TDLAS a diode laser emitting light at a well-defined but adjustable or tunable wavelength usually samples a single gas absorption line Typically each TDLAS system is built using a laser having a specific design wavelength chosen to correspond to a specific absorption line of the target analyte gas that is free of interfering absorption from other molecules Accurate control of the laser injection current and temperature achieves rapid and precise tuning over a range of plusmn 2 nm around the specified wavelength The laser emission linewidth is narrower than gas absorption linewidths This may be contrasted with NIR absorption techniques that sample with broadband sources and measure absorption from multiple lines across a fairly broad range of frequencies TDLAS thus offers the advantage of selectivity for a target trace gas absent with spectral interferences from other background gases Compact

TDLAS systems operate continuously at ambient temperatures from -20C to 50C using about 500 mW of electrical power without maintenance cryogenics or operator attendance

TDLAS sensors commonly exploit the laserrsquos fast tuning capability to rapidly (gt 10 kHz typically) modulate the wavelength causing it to sweep back and forth across an absorption feature at a precise modulation frequency ωm [8] [9] The response to the wavelength modulation is an amplitude modulation at the detector illustrated in Fig 3 The amplitude modulation arises from two sources 1) the transmitted laser power follows the wavelength modulation and thus modulates sinusoidally at frequency ωm and 2) the absorption due to target gas Since the wavelength crosses the target gas absorption line twice for each modulation cycle the amplitude modulation due to absorption occurs at precisely twice the modulation frequency 2ωm Via lock-in amplification the signal processor measures the average values of the amplitude modulations at ωm and 2ωm These are called the F1 and F2 signals respectively F1 is proportional to the received laser power while F2 is proportional to the received laser power and the target gas absorbance Thus the ratio F2F1 is proportional to the absorbance only and is independent of the received laser power as long as it exceeds sensor noise

The modulation and lock-in detection technique is called as wavelength modulation spectroscopy (WMS) WMS measures absorbances of 10-5 or less with one second or faster response and can provide ppm to ppb chemical detection limits depending on the spectroscopic properties of the target gas and the sampling pathlength WMS is thus a highly sensitive and gas-specific form of spectroscopic gas analysis Table I is a partial list of target gases that TDLAS has successfully measured [2] [5] [6] [10]-[15] Dividing the values in Table I by the optical path length the laser beam transits through the gas sample determines the measurable concentration in ppm For example the 1 ppm-m detection limit for CO2 combined with a 100 m round-trip optical path yields a sensitivity to an average of 001 ppm = 10 ppb CO2 Table II lists high-priority Toxic Industrial Compounds (TICs) and compares with demonstrated TDLAS sensing capabilities

Fig 3 WMS spectroscopy Laser wavenumber ν modulates sinusoidally across a spectral absorption line with modulation depth plusmn δ and modulation frequency ωm creating an amplitude modulated detector signal

Paper No 162

3

TABLE I SOME GASES MEASURED BY NEAR-INFRARED TDLAS

Gas Detection Limit

(ppm-m) Gas Detection Limit

(ppm-m) HF 02 HCN 02 H2S 200 CO 400 NH3 50 CO2 10 H2O 10 NO 300 CH4 10 NO2 02 HCl 02 O2 500

H2CO 50 C2H2 02

TABLE II ABILITY OF TDLAS TO DETECT HIGH PRIORITY TICS

Ammonia Arsine Boron trichloride Boron trifluoride Carbon disulfide Chlorine Diborane Ethylene oxide Fluorine Formaldehyde Hydrogen bromide Hydrogen chloride

Hydrogen cyanide Hydrogen fluoride Hydrogen sulfide Nitric acid fuming Phosgene Phosphorus trichloride Sulfur dioxide Sulfuric acid Tungsten hexafluoride

Proven

Likely

Possible

Unknown

II STANDOFF TDLAS Each TDLAS sensor incorporates a Measurement Path or

Gas Sampling Head Confirable options for these sections include Extractive Open Path and Standoff [16]-[19] Extractive sampling draws the gas sample through an optical measurement chamber The optical path length within the chamber is designed to provide the required sensitivity to the target gas A multi-pass optical configuration such as a Herriot cell may be utilized to provide a long optical path length within a small volume Open Path and Standoff configurations transmit the laser beam along a line of sight that can be up to few hundreds of meters in length

Fig 4 illustrates Standoff TDLAS [8] A transceiver unit transmits the laser beam which like a flashlight illuminates a distant diffuse surface The transceiver collects laser light backscattered from the illuminated surface and concentrates the received laser power onto the photodetector The photo-detector converts the received laser power into an electrical signal transmitted to a signal processor Fig 5 shows a commercial handheld TDLAS transceiver for gas sensing applications [20] Its nominal 10 mW transmitted laser power combined with its 10 cm diameter receiver provides a standoff range of up to 30 m

The transceiver shown in Fig 5 is part of the product known as the Remote Methane Leak Detector or RMLDtrade [21]Its specific handheld battery-powered configuration was

developed for walking surveys for leaks from municipal natural gas pipelines The RMLD is replacing flame ionization detectors and simplifying the work of surveyors who periodically walk the length of each buried gas pipe RMLD detects methane with 5 ppm-m sensitivity at 10 Hz It communicates gas concentration via an audio output an LCD alphanumeric display and a serial output that may be attached to a separate computer

Fig 6 shows a new configuration of the RMLD platform developed for leak surveying while driving city streets [22]-[24] This unit integrates an RMLD transceiver with a video camera and a rangefinder in a platform mounted on a joystick-controlled tip-tilt platform A laptop computer mounted in the vehicle cabin synthesizes data from all three sensors to automatically identify leaks and capture a video image of the leak location The rangefinder corrects for ambient methane as optical path length changes

Fig 7 presents data acquired while driving past leaks during field trials Additional data (not shown) demonstrates high probability detection of methane plumes having path- integrated concentrations of 20 ppm-m from leaks smaller than 1 scfh while driving past at speeds up to 40 kmh

By replacing the 10 cm diameter receiver of the handheld transceiver with a 40 cm diameter receiver (using a modified astronomical telescope) the standoff range increases to approximately 1200 ft for detecting methane plumes having path-integrated concentrations exceeding about 100 ppm-m [25] This capability is suitable for aerial pipeline leak surveying from rotary or fixed wing aircraft Fig 8 illustrates the configuration of an aerial system installed in a Cessna 206 a single-engine 4-seat fixed wing aircraft The transceiver aims through a 50 cm diameter hole in the aircraft floor Fig 9 shows data acquired during preliminary airborne testing [24]

Fig 4 Standoff TDLAS

III INTEGRATED OPTICS TDLAS SENSOR VISION AND CHALLENGES

Many gas sensing applications particularly for homeland security and defense demand low-cost distributed networks comprising hundreds or thousands of individual mass-produced sensor elements These networked sensors serve as alarms they sense and report abnormal events To preclude

Paper No 162

4

Fig 5 Handheld TDLAS Transceiver 10 mW transmitted power combined with 10 cm diameter receiver provides 5 ppm-m methane detection limit at standoff ranges up to 100 m

Fig 6 Mobile RMLD platform with photo of unit mounted on a box truck cabin roof

Fig 7 Methane leak detection with Mobile RMLD 10 scfh leak rate 10 m standoff range 10 kmh driving speed

Paper No 162

5

Fig 8 Long range transceiver system Photograph shows transceiver component details drawing shows aircraft installation configuration schematic shows system interconnections and optional EDFA for amplifying laser power and extending standoff range to 2000 m with a detection sensitivity of about 10000 ppm-m [25] [26]

Fig 9 Data acquired with aerial RMLD flying in a Cessna 206 making several passes over a plume from a natural gas leak flowing at about 1000 scfh

Paper No 162

6

false alarms the sensors must be sensitive and specific to the targeted threat key attributes of TDLAS However measurement accuracy and precision requirements are less stringent gas concentrations errors of 20 more can be tolerated without compromising the basic sensor alarm function

TDLAS has the potential to meet this need However although TDLAS technology is not much more complex than that of compact disk players and recorders the smallest and lowest cost currently-available TDLAS sensors fit in a smoke-alarm type package weighing about three pounds and costing ~$10000 Fig 10 shows an example This cost limits TDLAS applications to low-volume markets where the expensive sensor provides an economic payback Thus TDLAS remains too expensive bulky and power-hungry to deploy practically in the envisioned autonomous distributed sensor networks The high cost of TDLAS results from using 1) laser packages designed in the early 1990rsquos by and for the telecommunications industry that are produced in relatively low volumes and cost about $1000 each even though the laser chips alone could cost only tens of dollars each if produced in quantities of tens of thousands 2) bulk optical components often hand-polished with sophisticated coatings 3) control and data processing electronics built from commercial discrete components

Fig10 Battery-powered TDLAS sensor on 6rdquo x 6rdquo circuit board

Integrating micro-machined optical waveguide Sampling Elements with laser sources and detectors to create monolithic TDLAS sensors conceptualized in Fig 11 offer the potential for low-cost mass-production Within the Waveguide Sampling Element the laser power is constrained to follow a transparent mechanical structure but the waveguide structure can be designed to force a fraction of the guided laser power into an evanescent field in the surrounding air where the targeted chemical is sensed [27] Evanescent waveguides can be fabricated from silicon or indium phosphide semiconductor wafers Assuming production of tens of thousands of such devices their cost will be comparable to that of the laser chips a few tens of dollars each To provide relatively long (~ 1m) effective optical pathlengths the waveguides can include wavelength-scale resonant structures such as ring

resonators of Fig 11 that that function similarly to multi-pass or resonant bulk optical cells [28] [29] As described above TDLAS sensors routinely measure changes in optical power due to absorption that are 1 part in 100000 of the average power received by the detector yielding the detection limits listed in Table I When used with an evanescent waveguide sampling element only the evanescent portion of the power is subject to absorption therefore the minimum detection limit must be multiplied by the ratio of the total power to the evanescent power Assuming 10 of the power is in the evanescent portion then the minimum measurable absorbance (or noise-equivalent absorbance NEA) would be 10-4 Because measurement accuracy and precision are not key requirements for these sensors the waveguide sensing elements need not perfectly replicate each other facilitating high production yields (albeit difficult to quantify at this early conceptual stage of development)

Fig 11 Examples of ring resonator waveguide sensing element providing long optical pathlengths integrated on a monolithic platform with a laser source and detector to form an integrated optic TDLAS sensor Not to scale

A key challenge to achieving this capability is reducing laser power consumption Near-IR TDLAS power consumption results primarily from laser thermal control wherein laser temperature is maintained near 300 K (room temperature) with plusmn10mK precision A typical laser package illustrated by Fig 12 includes a thermo-electric cooler (TEC) platform a thermistor a microlens to project laser light onto an optical fiber facet an optical isolator to prevent optical fiber backreflections from disrupting laser performance and a monitor photodiode for measuring laser output Because this laser package style enabled industrial quality TDLAS sensors it has become a de facto TDLAS standard However for the high-volume gas sensing applications the sophisticated laser packaging is both unnecessary and detrimental to performance TDLAS laser packaging requirements are not technically complex but are unique to the needs of TDLAS and have not been explored by the industry

The microlenses and optical isolator in the laser package couple laser light into an optical fiber The optical fiber transmits the laser light to a remote sensor head which samples the target gas Integrating on a monolithic platform an optically-resonant cavity or waveguide that samples the target gas eliminates the need for and complexity of optical elements associated with the optical fiber and the remote sensor head

The TEC function is to stabilize laser temperature by transferring the laserrsquos waste heat to a heat sink However the TEC is very inefficient the tiny laserrsquos waste heat is much

Paper No 162

7

Fig 12 Traditional telecommunications laser package 14-pin butterfly style less than the power demanded by the TEC itself to perform its function Much of the current TDLAS electronics is dedicated to laser thermal control specifically to regulating the current powering the TEC Since the TEC draws and wastes considerable power its control electronics must withstand currents ~1 A forcing use of discrete and inefficient electronic components Techniques for eliminating this wasted power are needed Successful elimination of waste heat and associated controls combined with limiting laser optical power to about 1 mW should reduce the total system power consumption to less than 100 mW for continuous operation Since intermittent sampling at perhaps 1 ndash 10 duty cycle is acceptable to achieve the alarm function average power consumption of 1 ndash 10 mW is potentially achievable

IV CONCLUSION Commercial TDLAS sensors are accepted as rugged

reliable industrial gas analyzers Worldwide thousands are installed permanently or utilized as portable sensors at industrial plants for safety process measurement and control and environmental and emissions monitoring The relatively recent introduction of Standoff TDLAS is enabling use of this technology for battery-operated hand-portable mobile and aerial surveying for natural gas pipeline leaks We anticipate that applications for similar surveying to detect gases indicative of hostile or illicit drug and chemical laboratory activities will emerge in the near future

We envision that over the next few years TDLAS will continue its evolution into smaller integrated deveices that will eventually serve mass market applications Technical developments needed to achieve this vision include laser sources designed specifically for TDLAS These lasers should minimize power consumption and harness waste heat

while providing excellent thermal and wavelength stability Such sources would enable miniature or monolithic integrated waveguide TDLAS sensors These miniature sensors can be combined with ASIC electronics and wireless transmitters to form networks serving defense and security applications

ACKNOWLEDGMENTS The authors gratefully acknowledge the support of the US

Environmental Protection Agency the US Department of Energy National Energy Technology Laboratory the US Air Force the Northeast Gas Association Physical Sciences Inc and Heath Consultants Inc

REFERENCES [1] M B Frish and F Klein ldquoTrace gas monitors based on tunable diode

laser technology an introduction and description of applicationsrdquo 5th International Symposium on Gas Analysis by Tunable Diode Lasers Freiburg Germany VDI Berichte 1366 1998

[2] M Druy M B Frish and W J Kessler ldquoFrom laboratory technique to process gas sensor - the maturation of tunable diode laser absorption spectroscopyrdquo Spectroscopy vol 21 no 3 pp 14-18 March 2006

[3] M G Allen ldquoDiode laser absorption sensing of gas dynamic and combustion flowsrdquo an invited review for Measurement Science and Technology vol 9 no 4 pp 545-562 (1998)

[4] D S Bomse ldquoDiode lasers finding trace gases in the lab and the plantrdquo Photonics Spectra vol 29 no 6 1995

[5] D E Cooper and R U Martinelli ldquoNear-infrared diode lasers monitor molecular speciesrdquo Laser Focus World November 1992

[6] M G Allen et al ldquoIn-situ and stand-off sensing using QCIC laser technology from 3 - 100 micronsrdquo Paper 5732-28 SPIE Integrated Optoelectronic Devices Photonics West Conference January 2005

[7] D M Sonnenfroh et al ldquoMid-IR gas sensors based on quasi-cw room-temperature quantum cascade lasersrdquo AIAA 38th Aerospace Sciences Meeting January 2000 Paper No 2000-0641

[8] G C Bjorklund ldquoFrequency modulation spectroscopy a new method for measuring weak absorptions and dispersionsrdquo Opt Lett vol 5 1980

[9] D S Bomse A C Stanton and J A Silver ldquoFrequency modulation and wavelength modulation spectroscopies comparison of experimental methods using a lead-salt diode laserrdquo Applied Optics vol 31 1992

[10] C Gmachl etal Engineering Research Center on Mid-Infrared Technologies for Health and the Environment (MIRTHE) First Annual Report Princeton University 2007

[11] ME Webber MB Pushkarsky and C Kumar N Patel ldquoOptical detection of chemical warfare agents and toxic industrial chemicals Simulationrdquo J Appl Phys vol 97 113101 2005

[12] M Pushkarsky A Tsekoun IG Dunayevskiy R Go and C Kumar N Patel ldquoSub-parts-per-billion level detection of NO2 using room temperature quantum-cascade lasersrdquo Proceedings of the National Academy of Sciences vol 103 pp 10846-10849 2006

[13] C M Gittins et al ldquoRemote sensing of chemical contamination using quantum cascade lasersrdquo presented at the Solid State Lasers Technology Conference June 2002

[14] D M Sonnenfroh M B Frish R T Wainner and M G Allen ldquoMid-IR quantum cascade laser sensor for tropospherically important trace gasesrdquo Final Report prepared for US Environmental Protection Agency under Order No 4C-R348-NASA PSI-2857TR-1971 November 2004

[15] J M Hensley W T Rawlins D B Oakes D M Sonnenfroh and M G Allen A quantum cascade laser sensor for SO2 and SO3 2005 CLEOIQELS May 2005 Paper No CTuY4

[16] M B Frishet al ldquoThe evolution and application of trace gas analyzers based on tunable diode laser absorption spectroscopyrdquo Invited Presentation 19th International Forum on Process Analytical Chemistry (IFPAC) Washington DC January 2005

[17] M B Frish MA White and M G Allen ldquoHandheld laser-based sensor for remote detection of toxic and hazardous gasesrdquo Presentation at Water Ground and Air Pollution Monitoring and Remediation Conference Boston MA November 2000 SPIE Paper No 4199-05

Paper No 162

8

[18] M B Frish R T Wainner B D Green M C Laderer and M G Allen ldquoStandoff gas leak detectors based on tunable diode laser absorption spectroscopyrdquo SPIE Paper No 6010-13 Optics East Boston MA 23-26 October 2005

[19] R T Wainner B D Green M G Allen M A White J Stafford-Evans and R Naper ldquoHandheld battery-powered near-IR TDL sensor for stand-off detection of gas and vapor plumesrdquo Applied Physics B vol 75 pp 249-254 2002

[20] M B Frish et al ldquoStandoff sensing of natural gas leaks evolution of the remote methane leak detector (RMLD)rdquo Invited Paper in Conference on Lasers and Electro-opticsQuantum Electronics and Laser Science and Photonic Applications Systems and Technologies 2005 Optical Society of America Washington DC 2005 (Presentation JThF3 at Photonic Applications Systems Technologies (PhAST) Conference Baltimore MD May 2005)

[21] A G Fabiano J Rutherford S Chancey and M B Frish ldquoRemote methane leak detector advanced prototype to beta developmentrdquo in Proceedings of Natural Gas Technologies 2005 Gas Technology Institute Des Plaines IL (Presented at NGT III Conference Orlando FL January 2005)

[22] MB Frish et al ldquoExtended performance handheld and mobile sensors for remote detection of natural gas leaksrdquo Phase II Final Report PSI-1402TR-1979 Physical Sciences Inc Andover MA March 2005 A portion of this report is included in ldquoField testing of remote sensor gas leak detection systemsrdquo Final Report for Project No 1810485 Southwest Research Institute San Antonio TX (December 2004)

[23] M B Frish et al ldquoDevelopment of a prototype mobile RMLDrdquo Final Report prepared for NYSEARCH PSI 1488-TR-2274 Physical Sciences Inc Andover MA December 2007

[24] M B Frish et al ldquoThe next generation of TDLAS analyzersrdquo SPIE Paper 6765-5 Optics East Boston MA 17 September 2007

[25] R T Wainner M B Frish M C Laderer M G Allen and B D Green ldquoTunable diode laser wavelength modulation spectroscopy (TDL-WMS) using a fiber-amplified sourcerdquo CLEO QELS lsquo07ndashConference on Lasers and Electro-Optics Quantum Electronics and Laser Science Conference Baltimore MD 6-11 May 2007

[26] R T Wainner et al ldquoHigh altitude aerial natural gas leak detection systemrdquo Final Report Prepared for US Department of Energy under Grant No DE-FC26-04NT42268 PSI-1454TR-2211 April 2007

[27] A B Buckman Guided Wave Photonics Saunders College Publishing 1992

[28] C Manolatou et al ldquoHigh-density integrated opticsrdquo J Lightwave Technol vol17 no9 pp1682ndash1692 Sept 1999

[29] K K Lee ldquoTransmission and routing of optical signals in on-chip waveguides for silicon microphotonicsrdquo PhD thesis MIT 2001

Paper No 162

3

TABLE I SOME GASES MEASURED BY NEAR-INFRARED TDLAS

Gas Detection Limit

(ppm-m) Gas Detection Limit

(ppm-m) HF 02 HCN 02 H2S 200 CO 400 NH3 50 CO2 10 H2O 10 NO 300 CH4 10 NO2 02 HCl 02 O2 500

H2CO 50 C2H2 02

TABLE II ABILITY OF TDLAS TO DETECT HIGH PRIORITY TICS

Ammonia Arsine Boron trichloride Boron trifluoride Carbon disulfide Chlorine Diborane Ethylene oxide Fluorine Formaldehyde Hydrogen bromide Hydrogen chloride

Hydrogen cyanide Hydrogen fluoride Hydrogen sulfide Nitric acid fuming Phosgene Phosphorus trichloride Sulfur dioxide Sulfuric acid Tungsten hexafluoride

Proven

Likely

Possible

Unknown

II STANDOFF TDLAS Each TDLAS sensor incorporates a Measurement Path or

Gas Sampling Head Confirable options for these sections include Extractive Open Path and Standoff [16]-[19] Extractive sampling draws the gas sample through an optical measurement chamber The optical path length within the chamber is designed to provide the required sensitivity to the target gas A multi-pass optical configuration such as a Herriot cell may be utilized to provide a long optical path length within a small volume Open Path and Standoff configurations transmit the laser beam along a line of sight that can be up to few hundreds of meters in length

Fig 4 illustrates Standoff TDLAS [8] A transceiver unit transmits the laser beam which like a flashlight illuminates a distant diffuse surface The transceiver collects laser light backscattered from the illuminated surface and concentrates the received laser power onto the photodetector The photo-detector converts the received laser power into an electrical signal transmitted to a signal processor Fig 5 shows a commercial handheld TDLAS transceiver for gas sensing applications [20] Its nominal 10 mW transmitted laser power combined with its 10 cm diameter receiver provides a standoff range of up to 30 m

The transceiver shown in Fig 5 is part of the product known as the Remote Methane Leak Detector or RMLDtrade [21]Its specific handheld battery-powered configuration was

developed for walking surveys for leaks from municipal natural gas pipelines The RMLD is replacing flame ionization detectors and simplifying the work of surveyors who periodically walk the length of each buried gas pipe RMLD detects methane with 5 ppm-m sensitivity at 10 Hz It communicates gas concentration via an audio output an LCD alphanumeric display and a serial output that may be attached to a separate computer

Fig 6 shows a new configuration of the RMLD platform developed for leak surveying while driving city streets [22]-[24] This unit integrates an RMLD transceiver with a video camera and a rangefinder in a platform mounted on a joystick-controlled tip-tilt platform A laptop computer mounted in the vehicle cabin synthesizes data from all three sensors to automatically identify leaks and capture a video image of the leak location The rangefinder corrects for ambient methane as optical path length changes

Fig 7 presents data acquired while driving past leaks during field trials Additional data (not shown) demonstrates high probability detection of methane plumes having path- integrated concentrations of 20 ppm-m from leaks smaller than 1 scfh while driving past at speeds up to 40 kmh

By replacing the 10 cm diameter receiver of the handheld transceiver with a 40 cm diameter receiver (using a modified astronomical telescope) the standoff range increases to approximately 1200 ft for detecting methane plumes having path-integrated concentrations exceeding about 100 ppm-m [25] This capability is suitable for aerial pipeline leak surveying from rotary or fixed wing aircraft Fig 8 illustrates the configuration of an aerial system installed in a Cessna 206 a single-engine 4-seat fixed wing aircraft The transceiver aims through a 50 cm diameter hole in the aircraft floor Fig 9 shows data acquired during preliminary airborne testing [24]

Fig 4 Standoff TDLAS

III INTEGRATED OPTICS TDLAS SENSOR VISION AND CHALLENGES

Many gas sensing applications particularly for homeland security and defense demand low-cost distributed networks comprising hundreds or thousands of individual mass-produced sensor elements These networked sensors serve as alarms they sense and report abnormal events To preclude

Paper No 162

4

Fig 5 Handheld TDLAS Transceiver 10 mW transmitted power combined with 10 cm diameter receiver provides 5 ppm-m methane detection limit at standoff ranges up to 100 m

Fig 6 Mobile RMLD platform with photo of unit mounted on a box truck cabin roof

Fig 7 Methane leak detection with Mobile RMLD 10 scfh leak rate 10 m standoff range 10 kmh driving speed

Paper No 162

5

Fig 8 Long range transceiver system Photograph shows transceiver component details drawing shows aircraft installation configuration schematic shows system interconnections and optional EDFA for amplifying laser power and extending standoff range to 2000 m with a detection sensitivity of about 10000 ppm-m [25] [26]

Fig 9 Data acquired with aerial RMLD flying in a Cessna 206 making several passes over a plume from a natural gas leak flowing at about 1000 scfh

Paper No 162

6

false alarms the sensors must be sensitive and specific to the targeted threat key attributes of TDLAS However measurement accuracy and precision requirements are less stringent gas concentrations errors of 20 more can be tolerated without compromising the basic sensor alarm function

TDLAS has the potential to meet this need However although TDLAS technology is not much more complex than that of compact disk players and recorders the smallest and lowest cost currently-available TDLAS sensors fit in a smoke-alarm type package weighing about three pounds and costing ~$10000 Fig 10 shows an example This cost limits TDLAS applications to low-volume markets where the expensive sensor provides an economic payback Thus TDLAS remains too expensive bulky and power-hungry to deploy practically in the envisioned autonomous distributed sensor networks The high cost of TDLAS results from using 1) laser packages designed in the early 1990rsquos by and for the telecommunications industry that are produced in relatively low volumes and cost about $1000 each even though the laser chips alone could cost only tens of dollars each if produced in quantities of tens of thousands 2) bulk optical components often hand-polished with sophisticated coatings 3) control and data processing electronics built from commercial discrete components

Fig10 Battery-powered TDLAS sensor on 6rdquo x 6rdquo circuit board

Integrating micro-machined optical waveguide Sampling Elements with laser sources and detectors to create monolithic TDLAS sensors conceptualized in Fig 11 offer the potential for low-cost mass-production Within the Waveguide Sampling Element the laser power is constrained to follow a transparent mechanical structure but the waveguide structure can be designed to force a fraction of the guided laser power into an evanescent field in the surrounding air where the targeted chemical is sensed [27] Evanescent waveguides can be fabricated from silicon or indium phosphide semiconductor wafers Assuming production of tens of thousands of such devices their cost will be comparable to that of the laser chips a few tens of dollars each To provide relatively long (~ 1m) effective optical pathlengths the waveguides can include wavelength-scale resonant structures such as ring

resonators of Fig 11 that that function similarly to multi-pass or resonant bulk optical cells [28] [29] As described above TDLAS sensors routinely measure changes in optical power due to absorption that are 1 part in 100000 of the average power received by the detector yielding the detection limits listed in Table I When used with an evanescent waveguide sampling element only the evanescent portion of the power is subject to absorption therefore the minimum detection limit must be multiplied by the ratio of the total power to the evanescent power Assuming 10 of the power is in the evanescent portion then the minimum measurable absorbance (or noise-equivalent absorbance NEA) would be 10-4 Because measurement accuracy and precision are not key requirements for these sensors the waveguide sensing elements need not perfectly replicate each other facilitating high production yields (albeit difficult to quantify at this early conceptual stage of development)

Fig 11 Examples of ring resonator waveguide sensing element providing long optical pathlengths integrated on a monolithic platform with a laser source and detector to form an integrated optic TDLAS sensor Not to scale

A key challenge to achieving this capability is reducing laser power consumption Near-IR TDLAS power consumption results primarily from laser thermal control wherein laser temperature is maintained near 300 K (room temperature) with plusmn10mK precision A typical laser package illustrated by Fig 12 includes a thermo-electric cooler (TEC) platform a thermistor a microlens to project laser light onto an optical fiber facet an optical isolator to prevent optical fiber backreflections from disrupting laser performance and a monitor photodiode for measuring laser output Because this laser package style enabled industrial quality TDLAS sensors it has become a de facto TDLAS standard However for the high-volume gas sensing applications the sophisticated laser packaging is both unnecessary and detrimental to performance TDLAS laser packaging requirements are not technically complex but are unique to the needs of TDLAS and have not been explored by the industry

The microlenses and optical isolator in the laser package couple laser light into an optical fiber The optical fiber transmits the laser light to a remote sensor head which samples the target gas Integrating on a monolithic platform an optically-resonant cavity or waveguide that samples the target gas eliminates the need for and complexity of optical elements associated with the optical fiber and the remote sensor head

The TEC function is to stabilize laser temperature by transferring the laserrsquos waste heat to a heat sink However the TEC is very inefficient the tiny laserrsquos waste heat is much

Paper No 162

7

Fig 12 Traditional telecommunications laser package 14-pin butterfly style less than the power demanded by the TEC itself to perform its function Much of the current TDLAS electronics is dedicated to laser thermal control specifically to regulating the current powering the TEC Since the TEC draws and wastes considerable power its control electronics must withstand currents ~1 A forcing use of discrete and inefficient electronic components Techniques for eliminating this wasted power are needed Successful elimination of waste heat and associated controls combined with limiting laser optical power to about 1 mW should reduce the total system power consumption to less than 100 mW for continuous operation Since intermittent sampling at perhaps 1 ndash 10 duty cycle is acceptable to achieve the alarm function average power consumption of 1 ndash 10 mW is potentially achievable

IV CONCLUSION Commercial TDLAS sensors are accepted as rugged

reliable industrial gas analyzers Worldwide thousands are installed permanently or utilized as portable sensors at industrial plants for safety process measurement and control and environmental and emissions monitoring The relatively recent introduction of Standoff TDLAS is enabling use of this technology for battery-operated hand-portable mobile and aerial surveying for natural gas pipeline leaks We anticipate that applications for similar surveying to detect gases indicative of hostile or illicit drug and chemical laboratory activities will emerge in the near future

We envision that over the next few years TDLAS will continue its evolution into smaller integrated deveices that will eventually serve mass market applications Technical developments needed to achieve this vision include laser sources designed specifically for TDLAS These lasers should minimize power consumption and harness waste heat

while providing excellent thermal and wavelength stability Such sources would enable miniature or monolithic integrated waveguide TDLAS sensors These miniature sensors can be combined with ASIC electronics and wireless transmitters to form networks serving defense and security applications

ACKNOWLEDGMENTS The authors gratefully acknowledge the support of the US

Environmental Protection Agency the US Department of Energy National Energy Technology Laboratory the US Air Force the Northeast Gas Association Physical Sciences Inc and Heath Consultants Inc

REFERENCES [1] M B Frish and F Klein ldquoTrace gas monitors based on tunable diode

laser technology an introduction and description of applicationsrdquo 5th International Symposium on Gas Analysis by Tunable Diode Lasers Freiburg Germany VDI Berichte 1366 1998

[2] M Druy M B Frish and W J Kessler ldquoFrom laboratory technique to process gas sensor - the maturation of tunable diode laser absorption spectroscopyrdquo Spectroscopy vol 21 no 3 pp 14-18 March 2006

[3] M G Allen ldquoDiode laser absorption sensing of gas dynamic and combustion flowsrdquo an invited review for Measurement Science and Technology vol 9 no 4 pp 545-562 (1998)

[4] D S Bomse ldquoDiode lasers finding trace gases in the lab and the plantrdquo Photonics Spectra vol 29 no 6 1995

[5] D E Cooper and R U Martinelli ldquoNear-infrared diode lasers monitor molecular speciesrdquo Laser Focus World November 1992

[6] M G Allen et al ldquoIn-situ and stand-off sensing using QCIC laser technology from 3 - 100 micronsrdquo Paper 5732-28 SPIE Integrated Optoelectronic Devices Photonics West Conference January 2005

[7] D M Sonnenfroh et al ldquoMid-IR gas sensors based on quasi-cw room-temperature quantum cascade lasersrdquo AIAA 38th Aerospace Sciences Meeting January 2000 Paper No 2000-0641

[8] G C Bjorklund ldquoFrequency modulation spectroscopy a new method for measuring weak absorptions and dispersionsrdquo Opt Lett vol 5 1980

[9] D S Bomse A C Stanton and J A Silver ldquoFrequency modulation and wavelength modulation spectroscopies comparison of experimental methods using a lead-salt diode laserrdquo Applied Optics vol 31 1992

[10] C Gmachl etal Engineering Research Center on Mid-Infrared Technologies for Health and the Environment (MIRTHE) First Annual Report Princeton University 2007

[11] ME Webber MB Pushkarsky and C Kumar N Patel ldquoOptical detection of chemical warfare agents and toxic industrial chemicals Simulationrdquo J Appl Phys vol 97 113101 2005

[12] M Pushkarsky A Tsekoun IG Dunayevskiy R Go and C Kumar N Patel ldquoSub-parts-per-billion level detection of NO2 using room temperature quantum-cascade lasersrdquo Proceedings of the National Academy of Sciences vol 103 pp 10846-10849 2006

[13] C M Gittins et al ldquoRemote sensing of chemical contamination using quantum cascade lasersrdquo presented at the Solid State Lasers Technology Conference June 2002

[14] D M Sonnenfroh M B Frish R T Wainner and M G Allen ldquoMid-IR quantum cascade laser sensor for tropospherically important trace gasesrdquo Final Report prepared for US Environmental Protection Agency under Order No 4C-R348-NASA PSI-2857TR-1971 November 2004

[15] J M Hensley W T Rawlins D B Oakes D M Sonnenfroh and M G Allen A quantum cascade laser sensor for SO2 and SO3 2005 CLEOIQELS May 2005 Paper No CTuY4

[16] M B Frishet al ldquoThe evolution and application of trace gas analyzers based on tunable diode laser absorption spectroscopyrdquo Invited Presentation 19th International Forum on Process Analytical Chemistry (IFPAC) Washington DC January 2005

[17] M B Frish MA White and M G Allen ldquoHandheld laser-based sensor for remote detection of toxic and hazardous gasesrdquo Presentation at Water Ground and Air Pollution Monitoring and Remediation Conference Boston MA November 2000 SPIE Paper No 4199-05

Paper No 162

8

[18] M B Frish R T Wainner B D Green M C Laderer and M G Allen ldquoStandoff gas leak detectors based on tunable diode laser absorption spectroscopyrdquo SPIE Paper No 6010-13 Optics East Boston MA 23-26 October 2005

[19] R T Wainner B D Green M G Allen M A White J Stafford-Evans and R Naper ldquoHandheld battery-powered near-IR TDL sensor for stand-off detection of gas and vapor plumesrdquo Applied Physics B vol 75 pp 249-254 2002

[20] M B Frish et al ldquoStandoff sensing of natural gas leaks evolution of the remote methane leak detector (RMLD)rdquo Invited Paper in Conference on Lasers and Electro-opticsQuantum Electronics and Laser Science and Photonic Applications Systems and Technologies 2005 Optical Society of America Washington DC 2005 (Presentation JThF3 at Photonic Applications Systems Technologies (PhAST) Conference Baltimore MD May 2005)

[21] A G Fabiano J Rutherford S Chancey and M B Frish ldquoRemote methane leak detector advanced prototype to beta developmentrdquo in Proceedings of Natural Gas Technologies 2005 Gas Technology Institute Des Plaines IL (Presented at NGT III Conference Orlando FL January 2005)

[22] MB Frish et al ldquoExtended performance handheld and mobile sensors for remote detection of natural gas leaksrdquo Phase II Final Report PSI-1402TR-1979 Physical Sciences Inc Andover MA March 2005 A portion of this report is included in ldquoField testing of remote sensor gas leak detection systemsrdquo Final Report for Project No 1810485 Southwest Research Institute San Antonio TX (December 2004)

[23] M B Frish et al ldquoDevelopment of a prototype mobile RMLDrdquo Final Report prepared for NYSEARCH PSI 1488-TR-2274 Physical Sciences Inc Andover MA December 2007

[24] M B Frish et al ldquoThe next generation of TDLAS analyzersrdquo SPIE Paper 6765-5 Optics East Boston MA 17 September 2007

[25] R T Wainner M B Frish M C Laderer M G Allen and B D Green ldquoTunable diode laser wavelength modulation spectroscopy (TDL-WMS) using a fiber-amplified sourcerdquo CLEO QELS lsquo07ndashConference on Lasers and Electro-Optics Quantum Electronics and Laser Science Conference Baltimore MD 6-11 May 2007

[26] R T Wainner et al ldquoHigh altitude aerial natural gas leak detection systemrdquo Final Report Prepared for US Department of Energy under Grant No DE-FC26-04NT42268 PSI-1454TR-2211 April 2007

[27] A B Buckman Guided Wave Photonics Saunders College Publishing 1992

[28] C Manolatou et al ldquoHigh-density integrated opticsrdquo J Lightwave Technol vol17 no9 pp1682ndash1692 Sept 1999

[29] K K Lee ldquoTransmission and routing of optical signals in on-chip waveguides for silicon microphotonicsrdquo PhD thesis MIT 2001

Paper No 162

4

Fig 5 Handheld TDLAS Transceiver 10 mW transmitted power combined with 10 cm diameter receiver provides 5 ppm-m methane detection limit at standoff ranges up to 100 m

Fig 6 Mobile RMLD platform with photo of unit mounted on a box truck cabin roof

Fig 7 Methane leak detection with Mobile RMLD 10 scfh leak rate 10 m standoff range 10 kmh driving speed

Paper No 162

5

Fig 8 Long range transceiver system Photograph shows transceiver component details drawing shows aircraft installation configuration schematic shows system interconnections and optional EDFA for amplifying laser power and extending standoff range to 2000 m with a detection sensitivity of about 10000 ppm-m [25] [26]

Fig 9 Data acquired with aerial RMLD flying in a Cessna 206 making several passes over a plume from a natural gas leak flowing at about 1000 scfh

Paper No 162

6

false alarms the sensors must be sensitive and specific to the targeted threat key attributes of TDLAS However measurement accuracy and precision requirements are less stringent gas concentrations errors of 20 more can be tolerated without compromising the basic sensor alarm function

TDLAS has the potential to meet this need However although TDLAS technology is not much more complex than that of compact disk players and recorders the smallest and lowest cost currently-available TDLAS sensors fit in a smoke-alarm type package weighing about three pounds and costing ~$10000 Fig 10 shows an example This cost limits TDLAS applications to low-volume markets where the expensive sensor provides an economic payback Thus TDLAS remains too expensive bulky and power-hungry to deploy practically in the envisioned autonomous distributed sensor networks The high cost of TDLAS results from using 1) laser packages designed in the early 1990rsquos by and for the telecommunications industry that are produced in relatively low volumes and cost about $1000 each even though the laser chips alone could cost only tens of dollars each if produced in quantities of tens of thousands 2) bulk optical components often hand-polished with sophisticated coatings 3) control and data processing electronics built from commercial discrete components

Fig10 Battery-powered TDLAS sensor on 6rdquo x 6rdquo circuit board

Integrating micro-machined optical waveguide Sampling Elements with laser sources and detectors to create monolithic TDLAS sensors conceptualized in Fig 11 offer the potential for low-cost mass-production Within the Waveguide Sampling Element the laser power is constrained to follow a transparent mechanical structure but the waveguide structure can be designed to force a fraction of the guided laser power into an evanescent field in the surrounding air where the targeted chemical is sensed [27] Evanescent waveguides can be fabricated from silicon or indium phosphide semiconductor wafers Assuming production of tens of thousands of such devices their cost will be comparable to that of the laser chips a few tens of dollars each To provide relatively long (~ 1m) effective optical pathlengths the waveguides can include wavelength-scale resonant structures such as ring

resonators of Fig 11 that that function similarly to multi-pass or resonant bulk optical cells [28] [29] As described above TDLAS sensors routinely measure changes in optical power due to absorption that are 1 part in 100000 of the average power received by the detector yielding the detection limits listed in Table I When used with an evanescent waveguide sampling element only the evanescent portion of the power is subject to absorption therefore the minimum detection limit must be multiplied by the ratio of the total power to the evanescent power Assuming 10 of the power is in the evanescent portion then the minimum measurable absorbance (or noise-equivalent absorbance NEA) would be 10-4 Because measurement accuracy and precision are not key requirements for these sensors the waveguide sensing elements need not perfectly replicate each other facilitating high production yields (albeit difficult to quantify at this early conceptual stage of development)

Fig 11 Examples of ring resonator waveguide sensing element providing long optical pathlengths integrated on a monolithic platform with a laser source and detector to form an integrated optic TDLAS sensor Not to scale

A key challenge to achieving this capability is reducing laser power consumption Near-IR TDLAS power consumption results primarily from laser thermal control wherein laser temperature is maintained near 300 K (room temperature) with plusmn10mK precision A typical laser package illustrated by Fig 12 includes a thermo-electric cooler (TEC) platform a thermistor a microlens to project laser light onto an optical fiber facet an optical isolator to prevent optical fiber backreflections from disrupting laser performance and a monitor photodiode for measuring laser output Because this laser package style enabled industrial quality TDLAS sensors it has become a de facto TDLAS standard However for the high-volume gas sensing applications the sophisticated laser packaging is both unnecessary and detrimental to performance TDLAS laser packaging requirements are not technically complex but are unique to the needs of TDLAS and have not been explored by the industry

The microlenses and optical isolator in the laser package couple laser light into an optical fiber The optical fiber transmits the laser light to a remote sensor head which samples the target gas Integrating on a monolithic platform an optically-resonant cavity or waveguide that samples the target gas eliminates the need for and complexity of optical elements associated with the optical fiber and the remote sensor head

The TEC function is to stabilize laser temperature by transferring the laserrsquos waste heat to a heat sink However the TEC is very inefficient the tiny laserrsquos waste heat is much

Paper No 162

7

Fig 12 Traditional telecommunications laser package 14-pin butterfly style less than the power demanded by the TEC itself to perform its function Much of the current TDLAS electronics is dedicated to laser thermal control specifically to regulating the current powering the TEC Since the TEC draws and wastes considerable power its control electronics must withstand currents ~1 A forcing use of discrete and inefficient electronic components Techniques for eliminating this wasted power are needed Successful elimination of waste heat and associated controls combined with limiting laser optical power to about 1 mW should reduce the total system power consumption to less than 100 mW for continuous operation Since intermittent sampling at perhaps 1 ndash 10 duty cycle is acceptable to achieve the alarm function average power consumption of 1 ndash 10 mW is potentially achievable

IV CONCLUSION Commercial TDLAS sensors are accepted as rugged

reliable industrial gas analyzers Worldwide thousands are installed permanently or utilized as portable sensors at industrial plants for safety process measurement and control and environmental and emissions monitoring The relatively recent introduction of Standoff TDLAS is enabling use of this technology for battery-operated hand-portable mobile and aerial surveying for natural gas pipeline leaks We anticipate that applications for similar surveying to detect gases indicative of hostile or illicit drug and chemical laboratory activities will emerge in the near future

We envision that over the next few years TDLAS will continue its evolution into smaller integrated deveices that will eventually serve mass market applications Technical developments needed to achieve this vision include laser sources designed specifically for TDLAS These lasers should minimize power consumption and harness waste heat

while providing excellent thermal and wavelength stability Such sources would enable miniature or monolithic integrated waveguide TDLAS sensors These miniature sensors can be combined with ASIC electronics and wireless transmitters to form networks serving defense and security applications

ACKNOWLEDGMENTS The authors gratefully acknowledge the support of the US

Environmental Protection Agency the US Department of Energy National Energy Technology Laboratory the US Air Force the Northeast Gas Association Physical Sciences Inc and Heath Consultants Inc

REFERENCES [1] M B Frish and F Klein ldquoTrace gas monitors based on tunable diode

laser technology an introduction and description of applicationsrdquo 5th International Symposium on Gas Analysis by Tunable Diode Lasers Freiburg Germany VDI Berichte 1366 1998

[2] M Druy M B Frish and W J Kessler ldquoFrom laboratory technique to process gas sensor - the maturation of tunable diode laser absorption spectroscopyrdquo Spectroscopy vol 21 no 3 pp 14-18 March 2006

[3] M G Allen ldquoDiode laser absorption sensing of gas dynamic and combustion flowsrdquo an invited review for Measurement Science and Technology vol 9 no 4 pp 545-562 (1998)

[4] D S Bomse ldquoDiode lasers finding trace gases in the lab and the plantrdquo Photonics Spectra vol 29 no 6 1995

[5] D E Cooper and R U Martinelli ldquoNear-infrared diode lasers monitor molecular speciesrdquo Laser Focus World November 1992

[6] M G Allen et al ldquoIn-situ and stand-off sensing using QCIC laser technology from 3 - 100 micronsrdquo Paper 5732-28 SPIE Integrated Optoelectronic Devices Photonics West Conference January 2005

[7] D M Sonnenfroh et al ldquoMid-IR gas sensors based on quasi-cw room-temperature quantum cascade lasersrdquo AIAA 38th Aerospace Sciences Meeting January 2000 Paper No 2000-0641

[8] G C Bjorklund ldquoFrequency modulation spectroscopy a new method for measuring weak absorptions and dispersionsrdquo Opt Lett vol 5 1980

[9] D S Bomse A C Stanton and J A Silver ldquoFrequency modulation and wavelength modulation spectroscopies comparison of experimental methods using a lead-salt diode laserrdquo Applied Optics vol 31 1992

[10] C Gmachl etal Engineering Research Center on Mid-Infrared Technologies for Health and the Environment (MIRTHE) First Annual Report Princeton University 2007

[11] ME Webber MB Pushkarsky and C Kumar N Patel ldquoOptical detection of chemical warfare agents and toxic industrial chemicals Simulationrdquo J Appl Phys vol 97 113101 2005

[12] M Pushkarsky A Tsekoun IG Dunayevskiy R Go and C Kumar N Patel ldquoSub-parts-per-billion level detection of NO2 using room temperature quantum-cascade lasersrdquo Proceedings of the National Academy of Sciences vol 103 pp 10846-10849 2006

[13] C M Gittins et al ldquoRemote sensing of chemical contamination using quantum cascade lasersrdquo presented at the Solid State Lasers Technology Conference June 2002

[14] D M Sonnenfroh M B Frish R T Wainner and M G Allen ldquoMid-IR quantum cascade laser sensor for tropospherically important trace gasesrdquo Final Report prepared for US Environmental Protection Agency under Order No 4C-R348-NASA PSI-2857TR-1971 November 2004

[15] J M Hensley W T Rawlins D B Oakes D M Sonnenfroh and M G Allen A quantum cascade laser sensor for SO2 and SO3 2005 CLEOIQELS May 2005 Paper No CTuY4

[16] M B Frishet al ldquoThe evolution and application of trace gas analyzers based on tunable diode laser absorption spectroscopyrdquo Invited Presentation 19th International Forum on Process Analytical Chemistry (IFPAC) Washington DC January 2005

[17] M B Frish MA White and M G Allen ldquoHandheld laser-based sensor for remote detection of toxic and hazardous gasesrdquo Presentation at Water Ground and Air Pollution Monitoring and Remediation Conference Boston MA November 2000 SPIE Paper No 4199-05

Paper No 162

8

[18] M B Frish R T Wainner B D Green M C Laderer and M G Allen ldquoStandoff gas leak detectors based on tunable diode laser absorption spectroscopyrdquo SPIE Paper No 6010-13 Optics East Boston MA 23-26 October 2005

[19] R T Wainner B D Green M G Allen M A White J Stafford-Evans and R Naper ldquoHandheld battery-powered near-IR TDL sensor for stand-off detection of gas and vapor plumesrdquo Applied Physics B vol 75 pp 249-254 2002

[20] M B Frish et al ldquoStandoff sensing of natural gas leaks evolution of the remote methane leak detector (RMLD)rdquo Invited Paper in Conference on Lasers and Electro-opticsQuantum Electronics and Laser Science and Photonic Applications Systems and Technologies 2005 Optical Society of America Washington DC 2005 (Presentation JThF3 at Photonic Applications Systems Technologies (PhAST) Conference Baltimore MD May 2005)

[21] A G Fabiano J Rutherford S Chancey and M B Frish ldquoRemote methane leak detector advanced prototype to beta developmentrdquo in Proceedings of Natural Gas Technologies 2005 Gas Technology Institute Des Plaines IL (Presented at NGT III Conference Orlando FL January 2005)

[22] MB Frish et al ldquoExtended performance handheld and mobile sensors for remote detection of natural gas leaksrdquo Phase II Final Report PSI-1402TR-1979 Physical Sciences Inc Andover MA March 2005 A portion of this report is included in ldquoField testing of remote sensor gas leak detection systemsrdquo Final Report for Project No 1810485 Southwest Research Institute San Antonio TX (December 2004)

[23] M B Frish et al ldquoDevelopment of a prototype mobile RMLDrdquo Final Report prepared for NYSEARCH PSI 1488-TR-2274 Physical Sciences Inc Andover MA December 2007

[24] M B Frish et al ldquoThe next generation of TDLAS analyzersrdquo SPIE Paper 6765-5 Optics East Boston MA 17 September 2007

[25] R T Wainner M B Frish M C Laderer M G Allen and B D Green ldquoTunable diode laser wavelength modulation spectroscopy (TDL-WMS) using a fiber-amplified sourcerdquo CLEO QELS lsquo07ndashConference on Lasers and Electro-Optics Quantum Electronics and Laser Science Conference Baltimore MD 6-11 May 2007

[26] R T Wainner et al ldquoHigh altitude aerial natural gas leak detection systemrdquo Final Report Prepared for US Department of Energy under Grant No DE-FC26-04NT42268 PSI-1454TR-2211 April 2007

[27] A B Buckman Guided Wave Photonics Saunders College Publishing 1992

[28] C Manolatou et al ldquoHigh-density integrated opticsrdquo J Lightwave Technol vol17 no9 pp1682ndash1692 Sept 1999

[29] K K Lee ldquoTransmission and routing of optical signals in on-chip waveguides for silicon microphotonicsrdquo PhD thesis MIT 2001

Paper No 162

5

Fig 8 Long range transceiver system Photograph shows transceiver component details drawing shows aircraft installation configuration schematic shows system interconnections and optional EDFA for amplifying laser power and extending standoff range to 2000 m with a detection sensitivity of about 10000 ppm-m [25] [26]

Fig 9 Data acquired with aerial RMLD flying in a Cessna 206 making several passes over a plume from a natural gas leak flowing at about 1000 scfh

Paper No 162

6

false alarms the sensors must be sensitive and specific to the targeted threat key attributes of TDLAS However measurement accuracy and precision requirements are less stringent gas concentrations errors of 20 more can be tolerated without compromising the basic sensor alarm function

TDLAS has the potential to meet this need However although TDLAS technology is not much more complex than that of compact disk players and recorders the smallest and lowest cost currently-available TDLAS sensors fit in a smoke-alarm type package weighing about three pounds and costing ~$10000 Fig 10 shows an example This cost limits TDLAS applications to low-volume markets where the expensive sensor provides an economic payback Thus TDLAS remains too expensive bulky and power-hungry to deploy practically in the envisioned autonomous distributed sensor networks The high cost of TDLAS results from using 1) laser packages designed in the early 1990rsquos by and for the telecommunications industry that are produced in relatively low volumes and cost about $1000 each even though the laser chips alone could cost only tens of dollars each if produced in quantities of tens of thousands 2) bulk optical components often hand-polished with sophisticated coatings 3) control and data processing electronics built from commercial discrete components

Fig10 Battery-powered TDLAS sensor on 6rdquo x 6rdquo circuit board

Integrating micro-machined optical waveguide Sampling Elements with laser sources and detectors to create monolithic TDLAS sensors conceptualized in Fig 11 offer the potential for low-cost mass-production Within the Waveguide Sampling Element the laser power is constrained to follow a transparent mechanical structure but the waveguide structure can be designed to force a fraction of the guided laser power into an evanescent field in the surrounding air where the targeted chemical is sensed [27] Evanescent waveguides can be fabricated from silicon or indium phosphide semiconductor wafers Assuming production of tens of thousands of such devices their cost will be comparable to that of the laser chips a few tens of dollars each To provide relatively long (~ 1m) effective optical pathlengths the waveguides can include wavelength-scale resonant structures such as ring

resonators of Fig 11 that that function similarly to multi-pass or resonant bulk optical cells [28] [29] As described above TDLAS sensors routinely measure changes in optical power due to absorption that are 1 part in 100000 of the average power received by the detector yielding the detection limits listed in Table I When used with an evanescent waveguide sampling element only the evanescent portion of the power is subject to absorption therefore the minimum detection limit must be multiplied by the ratio of the total power to the evanescent power Assuming 10 of the power is in the evanescent portion then the minimum measurable absorbance (or noise-equivalent absorbance NEA) would be 10-4 Because measurement accuracy and precision are not key requirements for these sensors the waveguide sensing elements need not perfectly replicate each other facilitating high production yields (albeit difficult to quantify at this early conceptual stage of development)

Fig 11 Examples of ring resonator waveguide sensing element providing long optical pathlengths integrated on a monolithic platform with a laser source and detector to form an integrated optic TDLAS sensor Not to scale

A key challenge to achieving this capability is reducing laser power consumption Near-IR TDLAS power consumption results primarily from laser thermal control wherein laser temperature is maintained near 300 K (room temperature) with plusmn10mK precision A typical laser package illustrated by Fig 12 includes a thermo-electric cooler (TEC) platform a thermistor a microlens to project laser light onto an optical fiber facet an optical isolator to prevent optical fiber backreflections from disrupting laser performance and a monitor photodiode for measuring laser output Because this laser package style enabled industrial quality TDLAS sensors it has become a de facto TDLAS standard However for the high-volume gas sensing applications the sophisticated laser packaging is both unnecessary and detrimental to performance TDLAS laser packaging requirements are not technically complex but are unique to the needs of TDLAS and have not been explored by the industry

The microlenses and optical isolator in the laser package couple laser light into an optical fiber The optical fiber transmits the laser light to a remote sensor head which samples the target gas Integrating on a monolithic platform an optically-resonant cavity or waveguide that samples the target gas eliminates the need for and complexity of optical elements associated with the optical fiber and the remote sensor head

The TEC function is to stabilize laser temperature by transferring the laserrsquos waste heat to a heat sink However the TEC is very inefficient the tiny laserrsquos waste heat is much

Paper No 162

7

Fig 12 Traditional telecommunications laser package 14-pin butterfly style less than the power demanded by the TEC itself to perform its function Much of the current TDLAS electronics is dedicated to laser thermal control specifically to regulating the current powering the TEC Since the TEC draws and wastes considerable power its control electronics must withstand currents ~1 A forcing use of discrete and inefficient electronic components Techniques for eliminating this wasted power are needed Successful elimination of waste heat and associated controls combined with limiting laser optical power to about 1 mW should reduce the total system power consumption to less than 100 mW for continuous operation Since intermittent sampling at perhaps 1 ndash 10 duty cycle is acceptable to achieve the alarm function average power consumption of 1 ndash 10 mW is potentially achievable

IV CONCLUSION Commercial TDLAS sensors are accepted as rugged

reliable industrial gas analyzers Worldwide thousands are installed permanently or utilized as portable sensors at industrial plants for safety process measurement and control and environmental and emissions monitoring The relatively recent introduction of Standoff TDLAS is enabling use of this technology for battery-operated hand-portable mobile and aerial surveying for natural gas pipeline leaks We anticipate that applications for similar surveying to detect gases indicative of hostile or illicit drug and chemical laboratory activities will emerge in the near future

We envision that over the next few years TDLAS will continue its evolution into smaller integrated deveices that will eventually serve mass market applications Technical developments needed to achieve this vision include laser sources designed specifically for TDLAS These lasers should minimize power consumption and harness waste heat

while providing excellent thermal and wavelength stability Such sources would enable miniature or monolithic integrated waveguide TDLAS sensors These miniature sensors can be combined with ASIC electronics and wireless transmitters to form networks serving defense and security applications

ACKNOWLEDGMENTS The authors gratefully acknowledge the support of the US

Environmental Protection Agency the US Department of Energy National Energy Technology Laboratory the US Air Force the Northeast Gas Association Physical Sciences Inc and Heath Consultants Inc

REFERENCES [1] M B Frish and F Klein ldquoTrace gas monitors based on tunable diode

laser technology an introduction and description of applicationsrdquo 5th International Symposium on Gas Analysis by Tunable Diode Lasers Freiburg Germany VDI Berichte 1366 1998

[2] M Druy M B Frish and W J Kessler ldquoFrom laboratory technique to process gas sensor - the maturation of tunable diode laser absorption spectroscopyrdquo Spectroscopy vol 21 no 3 pp 14-18 March 2006

[3] M G Allen ldquoDiode laser absorption sensing of gas dynamic and combustion flowsrdquo an invited review for Measurement Science and Technology vol 9 no 4 pp 545-562 (1998)

[4] D S Bomse ldquoDiode lasers finding trace gases in the lab and the plantrdquo Photonics Spectra vol 29 no 6 1995

[5] D E Cooper and R U Martinelli ldquoNear-infrared diode lasers monitor molecular speciesrdquo Laser Focus World November 1992

[6] M G Allen et al ldquoIn-situ and stand-off sensing using QCIC laser technology from 3 - 100 micronsrdquo Paper 5732-28 SPIE Integrated Optoelectronic Devices Photonics West Conference January 2005

[7] D M Sonnenfroh et al ldquoMid-IR gas sensors based on quasi-cw room-temperature quantum cascade lasersrdquo AIAA 38th Aerospace Sciences Meeting January 2000 Paper No 2000-0641

[8] G C Bjorklund ldquoFrequency modulation spectroscopy a new method for measuring weak absorptions and dispersionsrdquo Opt Lett vol 5 1980

[9] D S Bomse A C Stanton and J A Silver ldquoFrequency modulation and wavelength modulation spectroscopies comparison of experimental methods using a lead-salt diode laserrdquo Applied Optics vol 31 1992

[10] C Gmachl etal Engineering Research Center on Mid-Infrared Technologies for Health and the Environment (MIRTHE) First Annual Report Princeton University 2007

[11] ME Webber MB Pushkarsky and C Kumar N Patel ldquoOptical detection of chemical warfare agents and toxic industrial chemicals Simulationrdquo J Appl Phys vol 97 113101 2005

[12] M Pushkarsky A Tsekoun IG Dunayevskiy R Go and C Kumar N Patel ldquoSub-parts-per-billion level detection of NO2 using room temperature quantum-cascade lasersrdquo Proceedings of the National Academy of Sciences vol 103 pp 10846-10849 2006

[13] C M Gittins et al ldquoRemote sensing of chemical contamination using quantum cascade lasersrdquo presented at the Solid State Lasers Technology Conference June 2002

[14] D M Sonnenfroh M B Frish R T Wainner and M G Allen ldquoMid-IR quantum cascade laser sensor for tropospherically important trace gasesrdquo Final Report prepared for US Environmental Protection Agency under Order No 4C-R348-NASA PSI-2857TR-1971 November 2004

[15] J M Hensley W T Rawlins D B Oakes D M Sonnenfroh and M G Allen A quantum cascade laser sensor for SO2 and SO3 2005 CLEOIQELS May 2005 Paper No CTuY4

[16] M B Frishet al ldquoThe evolution and application of trace gas analyzers based on tunable diode laser absorption spectroscopyrdquo Invited Presentation 19th International Forum on Process Analytical Chemistry (IFPAC) Washington DC January 2005

[17] M B Frish MA White and M G Allen ldquoHandheld laser-based sensor for remote detection of toxic and hazardous gasesrdquo Presentation at Water Ground and Air Pollution Monitoring and Remediation Conference Boston MA November 2000 SPIE Paper No 4199-05

Paper No 162

8

[18] M B Frish R T Wainner B D Green M C Laderer and M G Allen ldquoStandoff gas leak detectors based on tunable diode laser absorption spectroscopyrdquo SPIE Paper No 6010-13 Optics East Boston MA 23-26 October 2005

[19] R T Wainner B D Green M G Allen M A White J Stafford-Evans and R Naper ldquoHandheld battery-powered near-IR TDL sensor for stand-off detection of gas and vapor plumesrdquo Applied Physics B vol 75 pp 249-254 2002

[20] M B Frish et al ldquoStandoff sensing of natural gas leaks evolution of the remote methane leak detector (RMLD)rdquo Invited Paper in Conference on Lasers and Electro-opticsQuantum Electronics and Laser Science and Photonic Applications Systems and Technologies 2005 Optical Society of America Washington DC 2005 (Presentation JThF3 at Photonic Applications Systems Technologies (PhAST) Conference Baltimore MD May 2005)

[21] A G Fabiano J Rutherford S Chancey and M B Frish ldquoRemote methane leak detector advanced prototype to beta developmentrdquo in Proceedings of Natural Gas Technologies 2005 Gas Technology Institute Des Plaines IL (Presented at NGT III Conference Orlando FL January 2005)

[22] MB Frish et al ldquoExtended performance handheld and mobile sensors for remote detection of natural gas leaksrdquo Phase II Final Report PSI-1402TR-1979 Physical Sciences Inc Andover MA March 2005 A portion of this report is included in ldquoField testing of remote sensor gas leak detection systemsrdquo Final Report for Project No 1810485 Southwest Research Institute San Antonio TX (December 2004)

[23] M B Frish et al ldquoDevelopment of a prototype mobile RMLDrdquo Final Report prepared for NYSEARCH PSI 1488-TR-2274 Physical Sciences Inc Andover MA December 2007

[24] M B Frish et al ldquoThe next generation of TDLAS analyzersrdquo SPIE Paper 6765-5 Optics East Boston MA 17 September 2007

[25] R T Wainner M B Frish M C Laderer M G Allen and B D Green ldquoTunable diode laser wavelength modulation spectroscopy (TDL-WMS) using a fiber-amplified sourcerdquo CLEO QELS lsquo07ndashConference on Lasers and Electro-Optics Quantum Electronics and Laser Science Conference Baltimore MD 6-11 May 2007

[26] R T Wainner et al ldquoHigh altitude aerial natural gas leak detection systemrdquo Final Report Prepared for US Department of Energy under Grant No DE-FC26-04NT42268 PSI-1454TR-2211 April 2007

[27] A B Buckman Guided Wave Photonics Saunders College Publishing 1992

[28] C Manolatou et al ldquoHigh-density integrated opticsrdquo J Lightwave Technol vol17 no9 pp1682ndash1692 Sept 1999

[29] K K Lee ldquoTransmission and routing of optical signals in on-chip waveguides for silicon microphotonicsrdquo PhD thesis MIT 2001

Paper No 162

6

false alarms the sensors must be sensitive and specific to the targeted threat key attributes of TDLAS However measurement accuracy and precision requirements are less stringent gas concentrations errors of 20 more can be tolerated without compromising the basic sensor alarm function

TDLAS has the potential to meet this need However although TDLAS technology is not much more complex than that of compact disk players and recorders the smallest and lowest cost currently-available TDLAS sensors fit in a smoke-alarm type package weighing about three pounds and costing ~$10000 Fig 10 shows an example This cost limits TDLAS applications to low-volume markets where the expensive sensor provides an economic payback Thus TDLAS remains too expensive bulky and power-hungry to deploy practically in the envisioned autonomous distributed sensor networks The high cost of TDLAS results from using 1) laser packages designed in the early 1990rsquos by and for the telecommunications industry that are produced in relatively low volumes and cost about $1000 each even though the laser chips alone could cost only tens of dollars each if produced in quantities of tens of thousands 2) bulk optical components often hand-polished with sophisticated coatings 3) control and data processing electronics built from commercial discrete components

Fig10 Battery-powered TDLAS sensor on 6rdquo x 6rdquo circuit board

Integrating micro-machined optical waveguide Sampling Elements with laser sources and detectors to create monolithic TDLAS sensors conceptualized in Fig 11 offer the potential for low-cost mass-production Within the Waveguide Sampling Element the laser power is constrained to follow a transparent mechanical structure but the waveguide structure can be designed to force a fraction of the guided laser power into an evanescent field in the surrounding air where the targeted chemical is sensed [27] Evanescent waveguides can be fabricated from silicon or indium phosphide semiconductor wafers Assuming production of tens of thousands of such devices their cost will be comparable to that of the laser chips a few tens of dollars each To provide relatively long (~ 1m) effective optical pathlengths the waveguides can include wavelength-scale resonant structures such as ring

resonators of Fig 11 that that function similarly to multi-pass or resonant bulk optical cells [28] [29] As described above TDLAS sensors routinely measure changes in optical power due to absorption that are 1 part in 100000 of the average power received by the detector yielding the detection limits listed in Table I When used with an evanescent waveguide sampling element only the evanescent portion of the power is subject to absorption therefore the minimum detection limit must be multiplied by the ratio of the total power to the evanescent power Assuming 10 of the power is in the evanescent portion then the minimum measurable absorbance (or noise-equivalent absorbance NEA) would be 10-4 Because measurement accuracy and precision are not key requirements for these sensors the waveguide sensing elements need not perfectly replicate each other facilitating high production yields (albeit difficult to quantify at this early conceptual stage of development)

Fig 11 Examples of ring resonator waveguide sensing element providing long optical pathlengths integrated on a monolithic platform with a laser source and detector to form an integrated optic TDLAS sensor Not to scale

A key challenge to achieving this capability is reducing laser power consumption Near-IR TDLAS power consumption results primarily from laser thermal control wherein laser temperature is maintained near 300 K (room temperature) with plusmn10mK precision A typical laser package illustrated by Fig 12 includes a thermo-electric cooler (TEC) platform a thermistor a microlens to project laser light onto an optical fiber facet an optical isolator to prevent optical fiber backreflections from disrupting laser performance and a monitor photodiode for measuring laser output Because this laser package style enabled industrial quality TDLAS sensors it has become a de facto TDLAS standard However for the high-volume gas sensing applications the sophisticated laser packaging is both unnecessary and detrimental to performance TDLAS laser packaging requirements are not technically complex but are unique to the needs of TDLAS and have not been explored by the industry

The microlenses and optical isolator in the laser package couple laser light into an optical fiber The optical fiber transmits the laser light to a remote sensor head which samples the target gas Integrating on a monolithic platform an optically-resonant cavity or waveguide that samples the target gas eliminates the need for and complexity of optical elements associated with the optical fiber and the remote sensor head

The TEC function is to stabilize laser temperature by transferring the laserrsquos waste heat to a heat sink However the TEC is very inefficient the tiny laserrsquos waste heat is much

Paper No 162

7

Fig 12 Traditional telecommunications laser package 14-pin butterfly style less than the power demanded by the TEC itself to perform its function Much of the current TDLAS electronics is dedicated to laser thermal control specifically to regulating the current powering the TEC Since the TEC draws and wastes considerable power its control electronics must withstand currents ~1 A forcing use of discrete and inefficient electronic components Techniques for eliminating this wasted power are needed Successful elimination of waste heat and associated controls combined with limiting laser optical power to about 1 mW should reduce the total system power consumption to less than 100 mW for continuous operation Since intermittent sampling at perhaps 1 ndash 10 duty cycle is acceptable to achieve the alarm function average power consumption of 1 ndash 10 mW is potentially achievable

IV CONCLUSION Commercial TDLAS sensors are accepted as rugged

reliable industrial gas analyzers Worldwide thousands are installed permanently or utilized as portable sensors at industrial plants for safety process measurement and control and environmental and emissions monitoring The relatively recent introduction of Standoff TDLAS is enabling use of this technology for battery-operated hand-portable mobile and aerial surveying for natural gas pipeline leaks We anticipate that applications for similar surveying to detect gases indicative of hostile or illicit drug and chemical laboratory activities will emerge in the near future

We envision that over the next few years TDLAS will continue its evolution into smaller integrated deveices that will eventually serve mass market applications Technical developments needed to achieve this vision include laser sources designed specifically for TDLAS These lasers should minimize power consumption and harness waste heat

while providing excellent thermal and wavelength stability Such sources would enable miniature or monolithic integrated waveguide TDLAS sensors These miniature sensors can be combined with ASIC electronics and wireless transmitters to form networks serving defense and security applications

ACKNOWLEDGMENTS The authors gratefully acknowledge the support of the US

Environmental Protection Agency the US Department of Energy National Energy Technology Laboratory the US Air Force the Northeast Gas Association Physical Sciences Inc and Heath Consultants Inc

REFERENCES [1] M B Frish and F Klein ldquoTrace gas monitors based on tunable diode

laser technology an introduction and description of applicationsrdquo 5th International Symposium on Gas Analysis by Tunable Diode Lasers Freiburg Germany VDI Berichte 1366 1998

[2] M Druy M B Frish and W J Kessler ldquoFrom laboratory technique to process gas sensor - the maturation of tunable diode laser absorption spectroscopyrdquo Spectroscopy vol 21 no 3 pp 14-18 March 2006

[3] M G Allen ldquoDiode laser absorption sensing of gas dynamic and combustion flowsrdquo an invited review for Measurement Science and Technology vol 9 no 4 pp 545-562 (1998)

[4] D S Bomse ldquoDiode lasers finding trace gases in the lab and the plantrdquo Photonics Spectra vol 29 no 6 1995

[5] D E Cooper and R U Martinelli ldquoNear-infrared diode lasers monitor molecular speciesrdquo Laser Focus World November 1992

[6] M G Allen et al ldquoIn-situ and stand-off sensing using QCIC laser technology from 3 - 100 micronsrdquo Paper 5732-28 SPIE Integrated Optoelectronic Devices Photonics West Conference January 2005

[7] D M Sonnenfroh et al ldquoMid-IR gas sensors based on quasi-cw room-temperature quantum cascade lasersrdquo AIAA 38th Aerospace Sciences Meeting January 2000 Paper No 2000-0641

[8] G C Bjorklund ldquoFrequency modulation spectroscopy a new method for measuring weak absorptions and dispersionsrdquo Opt Lett vol 5 1980

[9] D S Bomse A C Stanton and J A Silver ldquoFrequency modulation and wavelength modulation spectroscopies comparison of experimental methods using a lead-salt diode laserrdquo Applied Optics vol 31 1992

[10] C Gmachl etal Engineering Research Center on Mid-Infrared Technologies for Health and the Environment (MIRTHE) First Annual Report Princeton University 2007

[11] ME Webber MB Pushkarsky and C Kumar N Patel ldquoOptical detection of chemical warfare agents and toxic industrial chemicals Simulationrdquo J Appl Phys vol 97 113101 2005

[12] M Pushkarsky A Tsekoun IG Dunayevskiy R Go and C Kumar N Patel ldquoSub-parts-per-billion level detection of NO2 using room temperature quantum-cascade lasersrdquo Proceedings of the National Academy of Sciences vol 103 pp 10846-10849 2006

[13] C M Gittins et al ldquoRemote sensing of chemical contamination using quantum cascade lasersrdquo presented at the Solid State Lasers Technology Conference June 2002

[14] D M Sonnenfroh M B Frish R T Wainner and M G Allen ldquoMid-IR quantum cascade laser sensor for tropospherically important trace gasesrdquo Final Report prepared for US Environmental Protection Agency under Order No 4C-R348-NASA PSI-2857TR-1971 November 2004

[15] J M Hensley W T Rawlins D B Oakes D M Sonnenfroh and M G Allen A quantum cascade laser sensor for SO2 and SO3 2005 CLEOIQELS May 2005 Paper No CTuY4

[16] M B Frishet al ldquoThe evolution and application of trace gas analyzers based on tunable diode laser absorption spectroscopyrdquo Invited Presentation 19th International Forum on Process Analytical Chemistry (IFPAC) Washington DC January 2005

[17] M B Frish MA White and M G Allen ldquoHandheld laser-based sensor for remote detection of toxic and hazardous gasesrdquo Presentation at Water Ground and Air Pollution Monitoring and Remediation Conference Boston MA November 2000 SPIE Paper No 4199-05

Paper No 162

8

[18] M B Frish R T Wainner B D Green M C Laderer and M G Allen ldquoStandoff gas leak detectors based on tunable diode laser absorption spectroscopyrdquo SPIE Paper No 6010-13 Optics East Boston MA 23-26 October 2005

[19] R T Wainner B D Green M G Allen M A White J Stafford-Evans and R Naper ldquoHandheld battery-powered near-IR TDL sensor for stand-off detection of gas and vapor plumesrdquo Applied Physics B vol 75 pp 249-254 2002

[20] M B Frish et al ldquoStandoff sensing of natural gas leaks evolution of the remote methane leak detector (RMLD)rdquo Invited Paper in Conference on Lasers and Electro-opticsQuantum Electronics and Laser Science and Photonic Applications Systems and Technologies 2005 Optical Society of America Washington DC 2005 (Presentation JThF3 at Photonic Applications Systems Technologies (PhAST) Conference Baltimore MD May 2005)

[21] A G Fabiano J Rutherford S Chancey and M B Frish ldquoRemote methane leak detector advanced prototype to beta developmentrdquo in Proceedings of Natural Gas Technologies 2005 Gas Technology Institute Des Plaines IL (Presented at NGT III Conference Orlando FL January 2005)

[22] MB Frish et al ldquoExtended performance handheld and mobile sensors for remote detection of natural gas leaksrdquo Phase II Final Report PSI-1402TR-1979 Physical Sciences Inc Andover MA March 2005 A portion of this report is included in ldquoField testing of remote sensor gas leak detection systemsrdquo Final Report for Project No 1810485 Southwest Research Institute San Antonio TX (December 2004)

[23] M B Frish et al ldquoDevelopment of a prototype mobile RMLDrdquo Final Report prepared for NYSEARCH PSI 1488-TR-2274 Physical Sciences Inc Andover MA December 2007

[24] M B Frish et al ldquoThe next generation of TDLAS analyzersrdquo SPIE Paper 6765-5 Optics East Boston MA 17 September 2007

[25] R T Wainner M B Frish M C Laderer M G Allen and B D Green ldquoTunable diode laser wavelength modulation spectroscopy (TDL-WMS) using a fiber-amplified sourcerdquo CLEO QELS lsquo07ndashConference on Lasers and Electro-Optics Quantum Electronics and Laser Science Conference Baltimore MD 6-11 May 2007

[26] R T Wainner et al ldquoHigh altitude aerial natural gas leak detection systemrdquo Final Report Prepared for US Department of Energy under Grant No DE-FC26-04NT42268 PSI-1454TR-2211 April 2007

[27] A B Buckman Guided Wave Photonics Saunders College Publishing 1992

[28] C Manolatou et al ldquoHigh-density integrated opticsrdquo J Lightwave Technol vol17 no9 pp1682ndash1692 Sept 1999

[29] K K Lee ldquoTransmission and routing of optical signals in on-chip waveguides for silicon microphotonicsrdquo PhD thesis MIT 2001

Paper No 162

7

Fig 12 Traditional telecommunications laser package 14-pin butterfly style less than the power demanded by the TEC itself to perform its function Much of the current TDLAS electronics is dedicated to laser thermal control specifically to regulating the current powering the TEC Since the TEC draws and wastes considerable power its control electronics must withstand currents ~1 A forcing use of discrete and inefficient electronic components Techniques for eliminating this wasted power are needed Successful elimination of waste heat and associated controls combined with limiting laser optical power to about 1 mW should reduce the total system power consumption to less than 100 mW for continuous operation Since intermittent sampling at perhaps 1 ndash 10 duty cycle is acceptable to achieve the alarm function average power consumption of 1 ndash 10 mW is potentially achievable

IV CONCLUSION Commercial TDLAS sensors are accepted as rugged

reliable industrial gas analyzers Worldwide thousands are installed permanently or utilized as portable sensors at industrial plants for safety process measurement and control and environmental and emissions monitoring The relatively recent introduction of Standoff TDLAS is enabling use of this technology for battery-operated hand-portable mobile and aerial surveying for natural gas pipeline leaks We anticipate that applications for similar surveying to detect gases indicative of hostile or illicit drug and chemical laboratory activities will emerge in the near future

We envision that over the next few years TDLAS will continue its evolution into smaller integrated deveices that will eventually serve mass market applications Technical developments needed to achieve this vision include laser sources designed specifically for TDLAS These lasers should minimize power consumption and harness waste heat

while providing excellent thermal and wavelength stability Such sources would enable miniature or monolithic integrated waveguide TDLAS sensors These miniature sensors can be combined with ASIC electronics and wireless transmitters to form networks serving defense and security applications

ACKNOWLEDGMENTS The authors gratefully acknowledge the support of the US

Environmental Protection Agency the US Department of Energy National Energy Technology Laboratory the US Air Force the Northeast Gas Association Physical Sciences Inc and Heath Consultants Inc

REFERENCES [1] M B Frish and F Klein ldquoTrace gas monitors based on tunable diode

laser technology an introduction and description of applicationsrdquo 5th International Symposium on Gas Analysis by Tunable Diode Lasers Freiburg Germany VDI Berichte 1366 1998

[2] M Druy M B Frish and W J Kessler ldquoFrom laboratory technique to process gas sensor - the maturation of tunable diode laser absorption spectroscopyrdquo Spectroscopy vol 21 no 3 pp 14-18 March 2006

[3] M G Allen ldquoDiode laser absorption sensing of gas dynamic and combustion flowsrdquo an invited review for Measurement Science and Technology vol 9 no 4 pp 545-562 (1998)

[4] D S Bomse ldquoDiode lasers finding trace gases in the lab and the plantrdquo Photonics Spectra vol 29 no 6 1995

[5] D E Cooper and R U Martinelli ldquoNear-infrared diode lasers monitor molecular speciesrdquo Laser Focus World November 1992

[6] M G Allen et al ldquoIn-situ and stand-off sensing using QCIC laser technology from 3 - 100 micronsrdquo Paper 5732-28 SPIE Integrated Optoelectronic Devices Photonics West Conference January 2005

[7] D M Sonnenfroh et al ldquoMid-IR gas sensors based on quasi-cw room-temperature quantum cascade lasersrdquo AIAA 38th Aerospace Sciences Meeting January 2000 Paper No 2000-0641

[8] G C Bjorklund ldquoFrequency modulation spectroscopy a new method for measuring weak absorptions and dispersionsrdquo Opt Lett vol 5 1980

[9] D S Bomse A C Stanton and J A Silver ldquoFrequency modulation and wavelength modulation spectroscopies comparison of experimental methods using a lead-salt diode laserrdquo Applied Optics vol 31 1992

[10] C Gmachl etal Engineering Research Center on Mid-Infrared Technologies for Health and the Environment (MIRTHE) First Annual Report Princeton University 2007

[11] ME Webber MB Pushkarsky and C Kumar N Patel ldquoOptical detection of chemical warfare agents and toxic industrial chemicals Simulationrdquo J Appl Phys vol 97 113101 2005

[12] M Pushkarsky A Tsekoun IG Dunayevskiy R Go and C Kumar N Patel ldquoSub-parts-per-billion level detection of NO2 using room temperature quantum-cascade lasersrdquo Proceedings of the National Academy of Sciences vol 103 pp 10846-10849 2006

[13] C M Gittins et al ldquoRemote sensing of chemical contamination using quantum cascade lasersrdquo presented at the Solid State Lasers Technology Conference June 2002

[14] D M Sonnenfroh M B Frish R T Wainner and M G Allen ldquoMid-IR quantum cascade laser sensor for tropospherically important trace gasesrdquo Final Report prepared for US Environmental Protection Agency under Order No 4C-R348-NASA PSI-2857TR-1971 November 2004

[15] J M Hensley W T Rawlins D B Oakes D M Sonnenfroh and M G Allen A quantum cascade laser sensor for SO2 and SO3 2005 CLEOIQELS May 2005 Paper No CTuY4

[16] M B Frishet al ldquoThe evolution and application of trace gas analyzers based on tunable diode laser absorption spectroscopyrdquo Invited Presentation 19th International Forum on Process Analytical Chemistry (IFPAC) Washington DC January 2005

[17] M B Frish MA White and M G Allen ldquoHandheld laser-based sensor for remote detection of toxic and hazardous gasesrdquo Presentation at Water Ground and Air Pollution Monitoring and Remediation Conference Boston MA November 2000 SPIE Paper No 4199-05

Paper No 162

8

[18] M B Frish R T Wainner B D Green M C Laderer and M G Allen ldquoStandoff gas leak detectors based on tunable diode laser absorption spectroscopyrdquo SPIE Paper No 6010-13 Optics East Boston MA 23-26 October 2005

[19] R T Wainner B D Green M G Allen M A White J Stafford-Evans and R Naper ldquoHandheld battery-powered near-IR TDL sensor for stand-off detection of gas and vapor plumesrdquo Applied Physics B vol 75 pp 249-254 2002

[20] M B Frish et al ldquoStandoff sensing of natural gas leaks evolution of the remote methane leak detector (RMLD)rdquo Invited Paper in Conference on Lasers and Electro-opticsQuantum Electronics and Laser Science and Photonic Applications Systems and Technologies 2005 Optical Society of America Washington DC 2005 (Presentation JThF3 at Photonic Applications Systems Technologies (PhAST) Conference Baltimore MD May 2005)

[21] A G Fabiano J Rutherford S Chancey and M B Frish ldquoRemote methane leak detector advanced prototype to beta developmentrdquo in Proceedings of Natural Gas Technologies 2005 Gas Technology Institute Des Plaines IL (Presented at NGT III Conference Orlando FL January 2005)

[22] MB Frish et al ldquoExtended performance handheld and mobile sensors for remote detection of natural gas leaksrdquo Phase II Final Report PSI-1402TR-1979 Physical Sciences Inc Andover MA March 2005 A portion of this report is included in ldquoField testing of remote sensor gas leak detection systemsrdquo Final Report for Project No 1810485 Southwest Research Institute San Antonio TX (December 2004)

[23] M B Frish et al ldquoDevelopment of a prototype mobile RMLDrdquo Final Report prepared for NYSEARCH PSI 1488-TR-2274 Physical Sciences Inc Andover MA December 2007

[24] M B Frish et al ldquoThe next generation of TDLAS analyzersrdquo SPIE Paper 6765-5 Optics East Boston MA 17 September 2007

[25] R T Wainner M B Frish M C Laderer M G Allen and B D Green ldquoTunable diode laser wavelength modulation spectroscopy (TDL-WMS) using a fiber-amplified sourcerdquo CLEO QELS lsquo07ndashConference on Lasers and Electro-Optics Quantum Electronics and Laser Science Conference Baltimore MD 6-11 May 2007

[26] R T Wainner et al ldquoHigh altitude aerial natural gas leak detection systemrdquo Final Report Prepared for US Department of Energy under Grant No DE-FC26-04NT42268 PSI-1454TR-2211 April 2007

[27] A B Buckman Guided Wave Photonics Saunders College Publishing 1992

[28] C Manolatou et al ldquoHigh-density integrated opticsrdquo J Lightwave Technol vol17 no9 pp1682ndash1692 Sept 1999

[29] K K Lee ldquoTransmission and routing of optical signals in on-chip waveguides for silicon microphotonicsrdquo PhD thesis MIT 2001

Paper No 162

8

[18] M B Frish R T Wainner B D Green M C Laderer and M G Allen ldquoStandoff gas leak detectors based on tunable diode laser absorption spectroscopyrdquo SPIE Paper No 6010-13 Optics East Boston MA 23-26 October 2005

[19] R T Wainner B D Green M G Allen M A White J Stafford-Evans and R Naper ldquoHandheld battery-powered near-IR TDL sensor for stand-off detection of gas and vapor plumesrdquo Applied Physics B vol 75 pp 249-254 2002

[20] M B Frish et al ldquoStandoff sensing of natural gas leaks evolution of the remote methane leak detector (RMLD)rdquo Invited Paper in Conference on Lasers and Electro-opticsQuantum Electronics and Laser Science and Photonic Applications Systems and Technologies 2005 Optical Society of America Washington DC 2005 (Presentation JThF3 at Photonic Applications Systems Technologies (PhAST) Conference Baltimore MD May 2005)

[21] A G Fabiano J Rutherford S Chancey and M B Frish ldquoRemote methane leak detector advanced prototype to beta developmentrdquo in Proceedings of Natural Gas Technologies 2005 Gas Technology Institute Des Plaines IL (Presented at NGT III Conference Orlando FL January 2005)

[22] MB Frish et al ldquoExtended performance handheld and mobile sensors for remote detection of natural gas leaksrdquo Phase II Final Report PSI-1402TR-1979 Physical Sciences Inc Andover MA March 2005 A portion of this report is included in ldquoField testing of remote sensor gas leak detection systemsrdquo Final Report for Project No 1810485 Southwest Research Institute San Antonio TX (December 2004)

[23] M B Frish et al ldquoDevelopment of a prototype mobile RMLDrdquo Final Report prepared for NYSEARCH PSI 1488-TR-2274 Physical Sciences Inc Andover MA December 2007

[24] M B Frish et al ldquoThe next generation of TDLAS analyzersrdquo SPIE Paper 6765-5 Optics East Boston MA 17 September 2007

[25] R T Wainner M B Frish M C Laderer M G Allen and B D Green ldquoTunable diode laser wavelength modulation spectroscopy (TDL-WMS) using a fiber-amplified sourcerdquo CLEO QELS lsquo07ndashConference on Lasers and Electro-Optics Quantum Electronics and Laser Science Conference Baltimore MD 6-11 May 2007

[26] R T Wainner et al ldquoHigh altitude aerial natural gas leak detection systemrdquo Final Report Prepared for US Department of Energy under Grant No DE-FC26-04NT42268 PSI-1454TR-2211 April 2007

[27] A B Buckman Guided Wave Photonics Saunders College Publishing 1992

[28] C Manolatou et al ldquoHigh-density integrated opticsrdquo J Lightwave Technol vol17 no9 pp1682ndash1692 Sept 1999

[29] K K Lee ldquoTransmission and routing of optical signals in on-chip waveguides for silicon microphotonicsrdquo PhD thesis MIT 2001