Transcript HovdePseudoRandom

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
Cylindrical
Mirror
Cavity
1023 bit code
64 cycles
ADC
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
100000
10000
Noise
voltage
1000
noise floor
100
noise
10
1
0
20
40
Round trips
60
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
EOM
RF
Oscillator
LASER SPECTRUM
Modulation ON
RF
Switch
CO2 Absorption
Feature
photocurrent
Pseudo-Random Tone Burst directly
measures the wavelength-dependent
signal
time
Pseudo-random tone burst apparatus
with cylindrical mirror
laser
Optical
Filter
EOM
RF
Oscillator
RF
Switch
detector
Open
Cylindrical
Mirror
Cavity
ADC
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 offaxis ICOS with some advantages of ring down
• Technique combines some disadvantages too.
• Probably best when detector noise dominates