CHRIS HOVDE, STEVE M. MASSICK and DAVID S. BOMSE PSEUDO-RANDOM AMPLITUDE OR TONE BURST MODULATION COMBINED WITH CAVITY-ENHANCED DETECTION CHRIS HOVDE, STEVE M. MASSICK and DAVID S. BOMSE Support from DOE and USDA

Pulsed CRDS is sort of like differential lidar… laser detector …except the beam is never far away, mirrors are more stable than the atmosphere, the sample is homogeneous 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 ADC Cylindrical Mirror Cavity 1023 bit code 64 cycles 1023 length buffer Inverse Transform

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 LASER SPECTRUM Modulation OFF laser LASER SPECTRUM Modulation ON EOM RF Oscillator RF Switch CO2 Absorption Feature

Pseudo-Random Tone Burst directly measures the wavelength-dependent signal

Pseudo-random tone burst apparatus with cylindrical mirror laser Optical Filter EOM detector Open Cylindrical Mirror Cavity ADC RF Oscillator RF Switch

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