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 telltale vibrational bands in the 3-12 µm band. The technique of frequency-comb based 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. Recently, we demonstrated a 1.5-octave -wide frequency comb operation of a subharmonic GaAs optical parametric oscillator (OPO) centered at  4 µm and demonstrated its high degree of coherence with respect to the pump source [7-9]. Our present dual-comb system is based on a pair of such GaAs OPOs that are pumped by two fully stabilized phase-coherent thulium-fiber laser combs at λ≈2 µm. The sub-Hertz relative lenewidth between the two OPO combs allows coherent averaging of the interferogram over seconds. We demonstrate sensitive and massively parallel detection of a mixture of molecular species (CO2, 13CO2, N2O, CH4 and H2O) in the whole range of 3.2 to 5.3 µm (frequency span 1240 cm-1) with mode-resolved (115 MHz spacing)  accuracy. We were able to acquire 300 000 spectral points with intermodal resolution in few seconds.

Fig. 1. Dual-comb spectrometer based on two subharmonic OPOs. A pair of phase-locked Tm-doped fiber lasers (λ≈2 µm, frep =115 MHz, ∆frep =69 Hz) was used to pump a twin OPO system. The OPO output beams were combined, passed through a gas cell, detected with an InSb detector, digitized, and Fourier transformed to retrieve an optical spectrum. A lon-gpass (> 2.5 µm) filter was used to cut the pump radiation.

Fig. 2. Dual-comb interferogram and retrieved RF and optical spectra. (a) The original interferogram obtained from the InSb detector and (b) RF spectrum obtained via Fast Fourier Transform. The optical spectrum was obtained via up-scaling the frequency axis by frep/Δfrep. Absorption features due to different molecules (N2O, CH4, CO2, isotopic CO2, and H2O) are indicated by arrows.

Fig. 3.  Dual-comb mid-IR spectra. Normalized transmission of a gas cell for (a) nitrous oxide (N2O) (186 ppm in 1-atm N2 buffer gas) and (b) methane (CH4) (1250 ppm in 1-atm N2 buffer gas). Also shown are spectra for:  (c) isotopic carbon dioxide (13CO2) and (d) water (H2O) and methane (CH4) – all present in the atmosphere at trace amounts. Theoretical spectra are inverted for clarity.

Singing molecules: Magnificent seven (NO, OCS, CO, N2O, CO2, H2O and CH4).

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[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] N. Leindecker, et al., Octave-spanning ultrafast OPO with 2.6-6.1 µm instantaneous bandwidth pumped by femtosecond Tm-fiber laser, Opt. Express 20, 7047 (2013)

[8] K. F. Lee, et al., Carrier envelope offset frequency of a doubly resonant, nondegenerate, mid-infrared GaAs optical parametric oscillator, Opt. Lett. 38, 1191 (2013)

[9] 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).