1. 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

2. 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

3. 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

4. Pseudo-random experiment links modulation and detection
laser
detector
ADC
Cylindrical Mirror Cavity
1023 bit code
64 cycles
1023 length buffer
Inverse Transform

5. 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

6. Results from AM ICOS
10 us
5 MHz clock, 1023 bit code, average of three sets of 64 cycles = 13 ms data acquisition time

7. 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

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

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

10. 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

11. 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

12. 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