toptica thz brochure
TRANSCRIPT
FemtoFiber®
LasersLasers for TerahertzPulsed Femtosecond and CW Tunable Diode Lasers
A Passion for Precision.
Terahertz waves represent the last “final frontier” on the spectral map. Situated between infrared light and electrically generated radio waves, terahertz radiation exhibits a very individual interaction with matter. Meanwhile “terahertz” has become the preferred technology for the next generation of security screening equipment because of the ability to detect hazardous substances hidden in materials like clothes, plastics and paper. Furthermore, the possibility to record high-resolution “fingerprints” — chemically sensitive terahertz spectra — has produced new analytical tools for medical imaging, packaged goods inspection and the analysis of pharmaceuticals.
In practical use, the terahertz regime extends from frequencies of 0.3 THz (a wavelength of 1 mm) to 10 THz (30 µm) — offering unique imaging properties. Compared to radio waves, the short wavelengths of terahertz rays provide the necessary spatial resolution for 3D micro imaging. Another
Terahertz Radiation:Innovative Solutions closingthe Spectral Gap
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Image left: complete range of lasers for
terahertz generation: pulsed femtosecond
fi ber lasers and CW DFB diode lasers.
Image right: tunable diode lasers
for CW terahertz generation in
OEM package.
advantage, terahertz waves are non-ionizing and hence do not cause physiological damage associated with X-rays.
Among the many approaches capable to generate terahertz waves, lasers are the preferred choice. The preference is due to the high level of spectral and temporal control that can be exercised with laser light. Furthermore, the higher powers now available with pulsed or CW lasers permit the efficient use of nonlinear frequency conversion schemes.
Early research into terahertz generation predominantly employed ultrashort laser pulses. More recent work also exploits the fact that the optical beat frequency of two slightly detuned CW lasers lies in the terahertz regime. Accurate frequency control of the two seed lasers, either in the visible or the near-infrared, offers a very elegant way of producing narrow-band — and tunable — terahertz beat frequencies.TOPTICA, as one of the world's most
innovative laser suppliers, has been cooperating with researchers in the terahertz arena from the beginning. As a result TOPTICA is able to offer the most suitable lasers for both approaches, pulsed and CW. Our FemtoFiber® lasers provide the perfect parameters for optical rectification and photoconductive switches — at the necessary wavelengths. In addition, our tunable DFB diode laser systems can be amplified to attractive power levels using all-diode technology. Finally, our proprietary techniques of laser frequency control allow for highest resolution in difference frequency mixing applications.
The latest development of emitters and antennas has provided the necessary tool kit for terahertz technology. TOPTICA has established partnerships with suppliers of terahertz equipment for spectroscopy, imaging and consulting. Customers who want to discuss their terahertz applications with TOPTICA will find a competent and knowledgeable partner.
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Continuous-wave (CW) terahertz radiation is obtained by means of difference frequency mixing. Two laser beams of slightly different wavelengths are superimposed on a dedicated antenna structure which serves as terahertz emitter. The irradiated antenna, usually a biased GaAs photomixer, emits an electromagnetic wave at the difference frequency of the two lasers.
For effi cient terahertz generation, some key requirements must be met. In order to employ GaAs photomixers, the laser wavelength has to be shorter than that corresponding to the semiconductor bandgap (approx. 870 nm). The laser must also be powerful and, as many terahertz absorption lines are tens or hundreds of GHz broad, have a large mode-hop-free tuning range.
TOPTICA's distributed feedback (DFB) diode lasers combine superb spectral properties with a rugged design well-suited for demanding “real-world” applications. With 150 mW output power in the 850 – 860 nm range and a mode-hop-free tuning range of several nanometers, they are the premium
Diode Lasers for High-ResolutionCW Terahertz Generation
choice for effi cient difference frequency mixing. By sweeping the difference between the two optical frequencies, the terahertz radiation is tuned continuously across several hundred GHz. The range of accessible difference frequencies can be tailored to the experimental requirements. Combining DFB diodes at customized center wavelengths yields terahertz tuning ranges of 0 to 2 THz or 1 to 3 THz — frequencies that cover the absorption signatures of a multitude of relevant substances, including explosives.
A commonly used experimental set-up of a two-color laser source is shown above. Each laser beam passes an optical isolator and is coupled into a polarization-maintaining (PM) fi ber combiner. The combiner provides two outputs — one for terahertz generation, one for detection — with each output comprising the two laser frequencies. When the experiment requires power levels in the 500 – 1000 mW range, the two DFB lasers can be coupled together into a BoosTA® semiconductor tapered amplifi er. Tuning range and spectral properties are
maintained and the necessary overlap of the two beams on the antenna is automatically secured.
For ultimate frequency control, the DFB lasers are combined with TOPTICA's patented iScan® interferometer. This combination yields a laser source with the following features:
· terahertz frequency stability and linewidth in the 1 MHz range· locking to any difference frequency desired· full computer control of the laser frequency via RS 232 or USB interface· precisely linear frequency scans even across several hundred GHz (fi g. below)
DFB tunable diode lasers
· For CW terahertz generation via difference frequency mixing
· 0 to 2 THz or 1 to 3 THz continuous tuning,
e.g. the frequency difference of 852 nm and 855 nm corresponds to 1.2 THz
· Optional high-power, 2-color amplifi er (up to 750 mW)
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Images left to right:
Dual-color laser source for CW terahertz
generation, with fi ber-optic beam
combination.
Visible image and terahertz image of a
ceramic blade hidden in cloth (courtesy
of TeraView Ltd).
Terahertz spectra of Lactose and RDX
(plastic explosive), recorded with two
TOPTICA DL-DFB lasers (courtesy of
TeraView Ltd).
Recommended Laser
Basic product packageSYST DL DFB THz(2 DFB laser heads, control electronics, analog interface)
Laser power 2 x 150 mW free beam, 2 x 75 mW fi ber output
Laser wavelengths 853 nm, 855 nm, 860 nm (others on request)
Difference frequency tuning0 to 2.2 THz (853 nm + 855 nm)0.6 to 2.9 THz (855 nm + 860 nm)
Difference frequency resolution < 0.005 THz
Tuning speed Up to 0.1 THz/s
OptionsFiber coupling, fi ber-optic beam combination,beat signal detector for frequency synchronization
High power package SYST DL DFB THz with BoosTA amplifi er
Laser powerTyp. 750 mW free beam, 2 x 175 mW fi ber output(2-color output)
Precision frequency control packageiScan® interferometer for high resolutionterahertz spectroscopy
Difference frequency resolutionTyp 1.5 x 10-6 THz (beat linewidth @ 80 ms)< 1 x 10-6 THz on request
TOPTICA follows a policy of continuous product improvements.Specifi cations are subject to changes without notice.
Security screening with terahertz radiationResearchers at TeraView Ltd. (Cambridge, UK) used TOPTICA's DL-DFB lasers for CW terahertz imaging and spectroscopy. In a feasibility study, engineers imaged weapons hidden beneath clothing.*
In the trial, they concealed a ceramic scalpel blade behind a layer of black denim cloth to simulate a knife hidden in a pocket of jeans. Although the cloth was impenetrable to visible light, at 0.5 THz the hidden blade
was clearly discernible. The experiment demonstrates the ability of terahertz imaging to reveal non-metallic objects, which present a particular challenge for traditional detection techniques. The researchers went on to show that plastic explosives could be located and identifi ed due to their unambiguous spectral fi ngerprint.
* see I.S. Gregory et al., Appl. Phys. Lett. 86,
204104 (2005).
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Femtosecond laser pulses provide a powerful means for generating broadband terahertz pulses. When a femtosecond pulse excites a semiconductor material with photon energies above the bandgap, free charge carriers are generated which may subsequently be accelerated by internal or external bias fi elds. According to Maxwell's equations, the corresponding current onset and switch-off will lead to the emission of an electromagnetic fi eld transient. Due to the sub-picosecond timescale on which these processes occur, the spectrum of the emitted transient is located in the terahertz regime. The terahertz spectra generated by femtosecond laser pulses exhibit a bandwidth of several THz. Currently, the most established emitter technologies are based on GaAs antennas, for which a laser excitation wavelength around 800 nm is required. Other common generation mechanisms for terahertz radiation, also at 1550 nm, include intra-pulse difference frequency generation or optical rectifi cation in nonlinear crystals or at surfaces.
In time-domain terahertz spectroscopy, the electric fi eld of the pulses is interrogated by
Femtosecond Fiber Lasers for Time-domain Terahertz Spectroscopy
means of optical sampling. To this end, the pulses of a second ultrafast laser beam and the terahertz pulses are superimposed onto a receiver, such as an electro-optical switch made of low-temperature grown GaAs (LT-GaAs). An alternative detection technique named “electro-optic sampling” relies on the birefringence which the terahertz fi eld induces in an optically nonlinear crystal. In both cases, the femtosecond sampling pulses are scanned in a stepwise manner through the terahertz pulses in order to reconstruct the electric waveform. Spectral information is extracted from Fourier transforms of the recorded time traces. Comparison of spectra, with and without samples inserted into the terahertz beam path, reveals the “absorption fi ngerprint” characteristic of the materials under test.
FemtoFiber® lasers are perfectly suited for time-domain terahertz spectroscopy. The FemtoFiber® laser cabinet of the FFS series offers models for both wavelength regimes currently in use, 775 and 1550 nm, and in each case provides the optimum specifi cations concerning pulse duration and output power. Furthermore, these
lasers are based on low-cost telecom components with a very long lifetime and they are compact and economical. This means that time-domain terahertz research no longer requires bulky and expensiveTi:Sapphire lasers, but can now use reliable and turnkey FemtoFiber® technology.
FemtoFiber® versus Ti:Sapphire: a test In collaboration with researchers at EKSPLA UAB (Vilnius, Lithuania), TOPTICA recently verifi ed the suitability of the FFS laser for replacing Ti:Sapphire lasers in terahertz set-ups. A FFS.SYS-SHG laser system was used to generate 775 nm radiation with 150 fs pulse duration and 100 mW of average output power, thus remaining just below the damage threshold of the photoconductive switches. The signals were highly reproducible and the signal-to-noise ratio was a factor of 2 higher than with the Ti:Sapphire laser. These results indicate that FemtoFiber® provides the most suitable laser technology for time-domain terahertz spectroscopy, better even than more expensive and powerful Ti:Sapphire lasers.
Femtosecond fi ber lasers
· For pulsed terahertz generation via optical rectifi cation or photoconductive switches
· 775 nm or 1550 nm
· Up to 2 x 250 mW power
· Optional: ASOPS technique to replace mechanical delay line
Images left to right:
FFS.SYS-SHG – ideal laser parameters
for terahertz generation at 775 nm.
One of the terahertz transients recorded
in the collaboration with EKSPLA UAB.
The corresponding terahertz spectrum,
ranging from 0.1 to 3 THz, is deduced
by Fourier transform of the transients.
ASOPS system consisting of two
synchronised FemtoFiber® lasers with
control hard- and software.
Recommended Laser
Applications at 1550 nm
Laser system FemtoFiber® laser (FFS.SYS)
Pulse duration < 100 fs
Repetition frequency 100 MHz
Average power > 250 mW (for enhanced power please contact TOPTICA)
Applications at 775 nm
Laser system FemtoFiber® laser with second harmonic generation (FFS.SYS-SHG)
Pulse duration < 150 fs
Repetition frequency 100 MHz
Average power > 90 mW (for enhanced power please contact TOPTICA)
Asynchronous optical sampling (ASOPS)
Laser system 2 FemtoFiber® lasers (FFS.SYS or FFS.SYS-SHG)
Controller FFS-SYNC-PLL
Sampling range 800 ps or customer specifi ed
RMS jitter < 300 fs (10 Hz – 100 kHz)
TOPTICA follows a policy of continuous product improvements.Specifi cations are subject to changes without notice.
Asynchronous optical sampling (ASOPS)The ASOPS technique permits optical sampling of the terahertz pulses without a moving delay line. Traditionally one uses a mechanical delay line in order to scan the femtosecond sampling pulses with respect to the terahertz pulses. The centimeter-long translations easily introduce diffi culties concerning mechanical vibrations, varying spot sizes and pointing instabilities. Besides avoiding these diffi culties, ASOPS also allows for much faster scanning.
TOPTICA's ASOPS system incorporates two FemtoFiber® lasers with the same repetition rate. One of the lasers is used for generating the terahertz pulses, the other for sampling them. Controlled modulations
of the cavity length of one of the lasers make it run alternately faster and slower than the other laser. For one roundtrip of the laser pulse inside the fi ber oscillator, the modulation corresponds to only 0.1 µm of motion of the mechanical delay line. However, each roundtrip the delay accumulates. After 1 million roundtrips, for example, the modulation already corresponds to a delay of 10 cm. The sweep of the sampling pulses with respect to the terahertz pulses is controlled by phase-locked loop (PLL) electronics with highest stability (jitter < 300 fs), enabling the highest bandwidths and speed for the optical sampling process.
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TOPTICA Photonics AGLochhamer Schlag 19D-82166 Graefelfi ng/MunichGermanyPhone: +49 89 85837-0Fax: +49 89 [email protected]
Distributors
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Australia & New ZealandLastek Pty. Ltd.Mr. Alex StancoThebarton CampusUniversity of Adelaide10 Reid Street5031 Thebarton, SAAustralia Phone: +61 8 8443 8668Fax: +61 8 8443 [email protected]
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ItalyLaser 2000 ItaliaMrs. Beate SauterVia Vetulonia, 22I-00183 Roma, ItalyPhone: +39 06 9727 0695Fax: +39 06 9725 [email protected]
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TaiwanSLEO Photonics Co. Ltd. Mr. Jimmy Chao6F, No. 2, Lane 74An-der StreetHsin Tien City, Taipei CountyTaiwan 231, R.O.C.Phone: +886 2 2211 5408Fax: +886 2 2211 [email protected]
United Kingdom & IrelandMr. Howard Potter Unit 4H Lansbury Estate Woking, Surrey, UK, GU21 2EP Great BritainPhone: +44 1483 799 030 Fax: +44 1483 799 [email protected] www.toptica.com
USA & CanadaTOPTICA Photonics, Inc. (TPI)Scientifi c: Mr. Alain BourdonIndustry: Mr. Mark Tolbert61, Union StreetWestfi eld, MA 01085, U.S.A.Phone: +1 413 562 5406Fax: +1 413 562 [email protected]
Distributorsonly for Optical Disc Testing:
TaiwanOmega Scientifi c Taiwan Ltd. Mr. James Ting3F-3, No.415, Sec. 4Sinyi Road110 Taipei CityTaiwan R.O.C.Phone: +886 2 8780 5228Fax: +886 2 8780 [email protected]
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Every other country not listed above:
TOPTICA Photonics AGLochhamer Schlag 19D-82166 Graefelfi ng/MunichGermanyPhone: +49 89 85837-0Fax: +49 89 [email protected]