Post on 17-Dec-2015
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 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