Dual Comb Spectroscopy

Dual-comb MIR Fourier transform spectroscopy

Mid-infrared spectroscopy offers unparalleled sensitivity for the detection of trace gases, solids and liquids, based on the existence of strongest tell-tale vibrational bands in the 3-12 µm band. The technique of frequency-comb Fourier transform spectroscopy [1-2], and especially dual-comb spectroscopy [3-6] is capable of extremely fast data acquisition combined with superior spectral resolution and broadband spectral coverage. The development of the dual-comb spectroscopy in the mid-IR was not as dramatic as in the near-IR, because of lack of sufficiently broadband and mutually coherent sources.

In our group, we pioneered a new platform for mid-infrared dual-comb Fourier-transform spectroscopy. It is based on a pair of ultra-broadband subharmonic optical parametric oscillators (OPOs) pumped by two phase-locked thulium-fibre combs. Such OPOs operating at degeneracy are ideal coherent frequency dividers whose output is inherently frequency- and phase-locked to the pump [7-12]. The ‘instantaneous’ spectrum of frequency combs produced via subharmonic generation can be well above an octave [12]. Our dual-comb spectroscopy system provides fast (7 ms for a single interferogram), moving-parts-free, simultaneous acquisition of 350,000 spectral data points, spaced by 115-MHz intermodal interval over the whole 3.1–5.5 µm spectral range. Parallel detection of 22 trace molecular species in a gas mixture, including isotopologues containing such isotopes as 13C, 18O, 17O, 15N, 34S, 33S and deuterium, with part-per-billion sensitivity and sub-Doppler resolution has been demonstrated [13]. The technique also features:

  • absolute optical frequency referencing to atomic clock,
  • high degree of mutual coherence between the two mid-infrared combs  with a relative comb-tooth linewidth of 25 mHz
  • possibility of obtaining mode-resolved spectra with a finesse of 4,000
  • capability of coherent averaging of > 400,000 interferograms
  • feasibility for kHz-scale spectral resolution in a comb-tooth scanning mode
Fig. 1. Dual comb setup
Fig. 2. Dual-comb spectrometer based on two subharmonic OPOs. A pair of phase-locked Tm-doped fiber lasers (λ≈1.93 µm, frep =115 MHz, ∆frep =138.5 Hz) was used to pump a twin OPO system. The OPO output beams were combined, passed through a multipass gas cell, detected with an InSb (77K) detector, digitized, and Fourier transformed to retrieve an optical spectrum.
Fig. 3. Dual comb spectra of a mixture of gases. (a) Optical spectrum (log scale) retrieved from a single coherently averaged interferogram (Nave =100,000), when the gas cell was evacuated. Absorption dips originate from atmospheric gases outside the cell. (b) Optical spectrum with the gas cell filled with a mixture of ten gases (OCS, N2O, NO, CO, CH4, C2H6, C2H4, C2H2, CO2 and H2O) in N2 buffer gas at 3 mbar total pressure. The two curves are vertically offset for clarity.
Fig. 4. Spectra of trace molecules detected in ambient air at 10 mbar pressure. (a) CO, (b) N2O, (c) CH4, (d-f) three isotopologues of CO2, and (g-i) three isotopologues of H2O. Theoretical (HITRAN) absorbance spectra are shown as inverted peaks.

Molecules … singing

Molecules get back to phase every 2π/ΔΩ sec, where ΔΩ is the interval of angular velocity quantization

Hear a mixture of seven molecules singing. (Molecular vibrations were slowed down by 25 billion times  and converted to sound). The total event lasts ~ 3 ns in real time.

Solo: CO molecule

Solo: CH4 molecule

Solo: COS molecule

Solo: N2O molecule

[1] D. Mazzotti, et al., Frequency-comb-based absolute frequency measurements in the mid-infrared with a difference-frequency spectrometer, Opt. Lett. 30, 997 (2005)

[2] F. Adler, et al., Mid-infrared Fourier transform spectroscopy with a broadband frequency comb, Opt. Express 18, 21861 (2010)

[3] F. Keilmann, C. Gohle, and R. Holzwarth, Time-domain mid-infrared frequency-comb spectrometer, Opt. Lett. 29, 1542 (2004)

[4] B. Bernhardt, et al., Mid-infrared dual-comb spectroscopy with 2.4 μm Cr2+:ZnSe femtosecond lasers, Applied Physics B 100, 3-8 (2010)

[5] Z. Zhang, T. Gardiner, and D. T. Reid, Mid-infrared dual-comb spectroscopy with an optical parametric oscillator, Opt. Lett., 38, 3148 (2013)

[6] E. Baumann, et al., Spectroscopy of the methane ν3 band with an accurate midinfrared coherent dual-comb spectrometer, Phys. Rev. A 84, 062513 (2011)

[7] K.L. Vodopyanov, S.T. Wong, and R.L. Byer, Infrared frequency comb methods, arrangements and applications. US patent 8,384,990 (2013).

[8] N. Leindecker, A. Marandi, R.L. Byer, and K.L. Vodopyanov, Broadband degenerate OPO for mid-infrared frequency comb generation. Opt. Express 19, 6304 (2011).

[9] A. Marandi, N. Leindecker, V. Pervak, R.L. Byer, and K.L. Vodopyanov, Coherence properties of a broadband femtosecond mid-IR optical parametric oscillator operating at degeneracy. Opt. Express 20, 7255 (2012).

[10] K.F. Lee, C. Mohr, J. Jiang, P.G. Schunemann, K.L. Vodopyanov, and M. E. Fermann, Midinfrared frequency comb from self-stable degenerate GaAs optical parametric oscillator. Opt. Express 23, 26596 (2015).

[11] C. Wan,P. Li, A. Ruehl, and I. Hartl, Coherent frequency division with a degenerate synchronously pumped optical parametric, oscillator, Opt. Lett. 43, 1059 (2018).

[12] V. O. Smolski, H. Yang, S. D. Gorelov, P. G. Schunemann, and K. L. Vodopyanov, Coherence properties of a 2.6 -7.5-µm frequency comb produced as subharmonic of a Tm-fiber laser, Opt. Lett. 41, 1388-1391 (2016).

[13] A. Muraviev, V. O. Smolski, Z. E. Loparo, and K. L. Vodopyanov, Massively parallel sensing of trace molecules and their isotopologues with broadband subharmonic mid-infrared frequency combs, Nature Photon. 12, 209-214 (2018).

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