broadband mid-infrared comb-resolved fourier transform spectroscopy kevin f. lee a. mills, c. mohr,...
TRANSCRIPT
Broadband Mid-infrared Comb-Resolved Fourier Transform Spectroscopy
Kevin F. LeeA. Mills, C. Mohr, Jie Jiang,
Martin E. FermannP. Masłowski
Spectroscopy
absorption spectra of gases
midinfrared• strong fundamental
vibrations
New system
midinfrared comb & FTIR
• bright• 30 mW• coherent
• broadband• parallel
– fast
• comb resolution• 200 kHz linewidth
– optically referenced• referenced to GPS
Comparison
other methods
• single-frequency laser• bright• precise• narrowband• slow scan
• LED, glowbar• broadband• incoherent• low resolution
• VIPA spectrometer• comb-resolved• narrowband ~10 nm
midinfrared comb & FTIR
• bright• 30 mW• coherent
• broadband• parallel
– fast
• comb resolution• 200 kHz linewidth
– optically referenced• referenced to GPS
Absorption Lines
measure line parameters
need resolution
• linewidths• Doppler broadened to ~ 100 MHz
• want resolution ~ 10 MHz~ 0.0003 cm-1
need speed
• multiple lines• temperatures• pressures
11 Torr
150 Torr
Frequency combs
stable train of laser pulses
• stable frequency comb
active coherent lock
• first tooth • carrier envelope offset
(CEO)
• optical tooth to a reference
• whole comb stable
Midinfrared comb
OPO comb
• inherits repetition rate
• same optical reference
scan comb lines by tuning reference wavelength
• access any frequency
Sample
100 m path length multipass cell
• uses coherence & power
• increases absorption
parts-per-billion sensitivity
• minimum detectable absorption
• 10-6 cm-1Hz-1/2
• low pressures and concentrations
Spectrometer
Fourier transform spectrometer (FTIR)
• record intensity vs. path delay
• time domain data• FT to get spectrum
• broadband
for 10 MHz resolution
• 30 m optical path delay
• too long, use comb
Comb-resolved FTIR
match FTIR to comb
• alternating pixels with and without comb lines
• spectrum with 200 kHz linewidth elements
our FTIR• 16 cm stage• 8 pass• 234 MHz• 2'x2' breadboard
Absolute frequency
comb frequencies from measured repetition rate
• GPS referenced
FTIR self-calibrated
150 MHz mismatch with HITRAN
1 / frep
Measure CO in Ar
50+ CO absorption lines
scan comb by ~30 MHz steps
• 13 comb-resolved spectra
• 1 000 000+ comb teeth
CO fundamental
Comb Scan of CO
11 TorrCO in Ar2 ppm
line P62119.7 cm-1
Comb Scan of CO
11 TorrCO in Ar2 ppm
line P62119.7 cm-1
Comb Scan of CO
11 TorrCO in Ar2 ppm
line P62119.7 cm-1
Comb Scan of CO
11 TorrCO in Ar2 ppm
line P62119.7 cm-1
Comb Scan of CO
11 TorrCO in Ar2 ppm
line P62119.7 cm-1
Comb Scan of CO
11 TorrCO in Ar2 ppm
line P62119.7 cm-1
Comb Scan of CO
11 TorrCO in Ar2 ppm
line P62119.7 cm-1
Comb Scan of CO
11 TorrCO in Ar2 ppm
line P62119.7 cm-1
Comb Scan of CO
11 TorrCO in Ar2 ppm
line P62119.7 cm-1
Comb Scan of CO
11 TorrCO in Ar2 ppm
line P62119.7 cm-1
Comb Scan of CO
11 TorrCO in Ar2 ppm
line P62119.7 cm-1
Comb Scan of CO
11 TorrCO in Ar2 ppm
line P62119.7 cm-1
Comb Scan of CO
11 TorrCO in Ar2 ppm
line P62119.7 cm-1
Comb Scan of CO
170 MHz
30 MHz spacing(chosen)
234 MHz FTIR resolution
11 TorrCO in Ar2 ppm
line P62119.7 cm-1
150 kHz linewidth
Lineshape analysis
simple Voigt profile at high pressure
see speed dependence at low pressure
Voigt profile
speed dependentVoigt profile
Residuals
404 Torr 27 Torr
Pressure dependence
fit with 4 pressures
• compare to Wehr et al. 2006
• shifting– 17% match
• broadening – 4% match
50 lines * 4 pressures
• 1 hour by comb
• 1 month by cw-CRDSCO fundamental P7
Summary
OPO comb & FTIR
broadband midIR
comb-resolved