Broadband Mid-infrared Comb-Resolved Fourier Transform Spectroscopy Kevin F. Lee A. Mills, C. Mohr, Jie Jiang, Martin E. Fermann P. 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 ~ 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 cm -1 Hz -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 / f rep

Measure CO in Ar 50+ CO absorption lines scan comb by ~30 MHz steps 13 comb-resolved spectra comb teeth CO fundamental

Comb Scan of CO 11 Torr CO in Ar 2 ppm line P cm -1

Comb Scan of CO 11 Torr CO in Ar 2 ppm line P cm -1

Comb Scan of CO 11 Torr CO in Ar 2 ppm line P cm -1

Comb Scan of CO 11 Torr CO in Ar 2 ppm line P cm -1

Comb Scan of CO 11 Torr CO in Ar 2 ppm line P cm -1

Comb Scan of CO 11 Torr CO in Ar 2 ppm line P cm -1

Comb Scan of CO 11 Torr CO in Ar 2 ppm line P cm -1

Comb Scan of CO 11 Torr CO in Ar 2 ppm line P cm -1

Comb Scan of CO 11 Torr CO in Ar 2 ppm line P cm -1

Comb Scan of CO 11 Torr CO in Ar 2 ppm line P cm -1

Comb Scan of CO 11 Torr CO in Ar 2 ppm line P cm -1

Comb Scan of CO 11 Torr CO in Ar 2 ppm line P cm -1

Comb Scan of CO 11 Torr CO in Ar 2 ppm line P cm -1

Comb Scan of CO 170 MHz 30 MHz spacing (chosen) 234 MHz FTIR resolution 11 Torr CO in Ar 2 ppm line P cm kHz linewidth

Lineshape analysis simple Voigt profile at high pressure see speed dependence at low pressure Voigt profile speed dependent Voigt profile Residuals 404 Torr27 Torr

Pressure dependence fit with 4 pressures compare to Wehr et al shifting –17% match broadening –4% match 50 lines * 4 pressures 1 hour by comb 1 month by cw-CRDS CO fundamental P7

Summary OPO comb & FTIR broadband midIR comb-resolved