Mid-infrared dual-comb spectroscopy with absolute frequency calibration using a passive optical reference

Abstract

We demonstrate an absolute-frequency-calibrated mid-infrared dual-comb spectrometer by using a reference absorption cell. The source is based on a singly-resonant OPO containing two MgO:PPLN crystals in a common ring cavity, synchronously pumped by two mode-locked Yb-fiber lasers. The repetition-rate of the two pumps are stabilized while their offset frequencies and the OPO cavity length are not actively controlled. The reference spectrum is used to correct the frequency fluctuations in the sample spectrum providing a high-quality averaged spectrum with spectral resolution of 6 GHz and calibration precision of 120 MHz, without adding any complexity to the experimental setup or signal processing.

Time-resolved mid-infrared dual-comb spectroscopy

“…The time-domain interferograms in the output of the two photodetectors are digitized, a Blackman apodization function is applied to the both (sample and reference) interferograms, and followed by a Fourier transformation to yield the corresponding absorption spectra. The apodization function is used to minimize the ringing effect around the (narrow) absorption lines 40 , which also limits the effective measurement time-window of each interferogram to ~120 μs around the central burst. Therefore, each individual interferogram is measured in 120 μs with a repetition period of 1/∆f rep (≈4 ms).…”

Broadband Time-Resolved Absorption and Dispersion Spectroscopy of Methane and Ethane in A Plasma Using a Mid-Infrared Dual-Comb Spectrometer

“…For DCS, one pump comb had a slightly different repetition frequency (frep) with respect to the second comb, so that the difference in the repetition frequency (Δfrep) could be set between 200 to 300 Hz. A 10 MHz reference clock was used for referencing all of the frequency sources used in frep control electronics, triggering of the data acquisition, and modulating the discharge [30,50].…”

“…We use the optical frequency position of this reference absorption line for removing the fceo fluctuations and frequency calibration of the dual-comb spectrum. More details on absolute frequency calibration can be found in [50].…”

“…The recorded interferograms are time stamped and post-processed using a Matlab-based program. In order to improve the signal-to-noise ratio and remove the ringing effect around the narrow absorption lines in the sample and reference spectra, a Blackman apodization function is applied to the interferograms in the post processing routine [50]. Afterward, a Fast Fourier Transform (FFT) is applied to each individual reference and sample interferogram yielding the reference and sample spectra in the RF domain.…”

“…To remove these shifts, we use the known position of an absorption line in the reference spectrum (~3038.5 cm -1 ) and correct for the shifts in both dispersion and absorption spectra of the sample. The absorption and dispersion spectra are averaged after the shift correction, yielding high SNR and absolute-frequency-calibrated spectra [50].…”