PSEUDO-RANDOM AMPLITUDE OR TONE BURST MODULATION

COMBINED WITHCAVITY-ENHANCED DETECTION

CHRIS HOVDE, STEVE M. MASSICK and DAVID S. BOMSE

Support from DOE and USDA

Pulsed CRDS is sort of like differential lidar…

…except the beam is never far away, mirrors are more stable than the atmosphere, the sample is homogeneous

laser

detector

laser detector

Pseudo-random modulation uses a special digital code with interesting properties

• Almost a 50% duty cycle• Autocorrelation function is almost a delta

function• Fast inverse transforms exist– Computationally reasonable– Analogous to FFT

Pseudo-random experiment links modulation and detection

laser detector

1023 bit code64 cycles

1023 length buffer

ADC

Inverse Transform

Cylindrical Mirror Cavity

Cylindrical mirror cavity suppresses mode noise similar to off-axis ICOS

Beam traces out Lissajous pattern on each mirror

Large number of passes results in reduced mode coupling noise

Results from AM ICOS

10 us

5 MHz clock, 1023 bit code, average of three sets of 64 cycles = 13 ms data acquisition time

Tone burst is a variation of wavelength or frequency modulation

EOM

laser

RF Switch

RF Oscillator

LASER SPECTRUMModulation OFF

LASER SPECTRUMModulation ON

CO2 Absorption Feature

Pseudo-Random Tone Burst directly measures the wavelength-dependent

signal

Pseudo-random tone burst apparatus with cylindrical mirror

EOM

laser

detector

Open Cylindrical

Mirror Cavity

ADCRF Switch

RF Oscillator

Optical Filter

Pseudo-Random Tone Burst of CO2

Air + CO2

Off-wavelength

5 MHz clock, 1023 bit code, average of ten sets of 64 cycles = 40 ms data acquisition time

Advantages and disadvantages

• Sort of like Off-Axis ICOS, but get range –resolved signal– But need a fast, large area detector and much more

complicated electronics• Sort of like CRDS with cw lasers, but don’t need

to lock laser to cavity or switch laser off• Laser is on all the time– High duty factor– All the laser noise– Shot noise of total power corrupts weakest signals

Conclusions

• First demonstration of pseudo-random modulation with cavity-enhanced detection

• Technique combines some advantages of off-axis ICOS with some advantages of ring down

• Technique combines some disadvantages too.• Probably best when detector noise dominates