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