Dual-comb mid-infrared spectroscopy with free-running oscillators and absolute optical calibration from a radio-frequency reference

Abstract

By using free-running independent femtosecond OPOs with a repetition-rate difference of 500 Hz we demonstrate methane absorption spectroscopy with spectral coverage simultaneously spanning the methane P, Q and R branches and with a resolution of 0.5 cm. Absolute optical frequency calibration with an accuracy of 0.25 cm (0.27 nm) is achieved from simultaneous repetition-rate and carrier-envelope-offset frequency measurements, without the need for any optical reference. The calibration technique allows registration and averaging of consecutively acquired dual-comb spectra, leading to a high quality and low-noise absorbance measurement in good agreement with the HITRAN database.

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

“…Compared to fully stabilized dual-comb spectrometers based on femtosecond OPOs [13], our developed system is less complicated and demanding; but it provides coarser spectral resolution and frequency precision. On the other hand, compared to similar OPObased DCS systems using cross-correlation of the measured spectrum for offset correction [18,19], our method provides an absolute-frequency-calibrated spectrum, with a consistent and good frequency precision, long term stability and spectral resolution; fully independent from the SNR of the sample spectrum. Finally, compared to conventional Fourier transform spectrometers, our method provides similar spectral resolution and precision, but the measurement time is a few orders of magnitude shorter than a typical mechanical FTS.…”

“…One can also use two singly-resonance OPOs (SROPOs), which eliminates the need for active stabilization of the length of the OPO cavities. DCS is demonstrated by using two freerunning mode-locked lasers pumping two singly-resonance OPOs [18,19]. In the first demonstration, the repetition rate difference (Δf rep ) as well as the f ceo of the two pump lasers were recorded along with the time domain interferogram.…”

“…The Δf rep and f ceo values were used to correct for distortions in the spectrum, due to their instantaneous fluctuations. The residual linear frequency mismatch between the individual spectra was revealed by a cross-correlation algorithm and compensated before averaging, yielding a spectral resolution of 0.5 cm −1 for the averaged spectra [18]. In the second demonstration the f ceo of the pumps were not monitored, which simplified the setup due to elimination of two f-2f interferometers.…”

Singly and doubly resonant femtosecond optical parametric oscillators for precision spectroscopy from the near-to mid-infrared

“…After Fourier transforming, these frequencies were used to perform the calibration procedure independently on each spectrum. Full details of this procedure appear in [28]. The result is shown in Fig.…”

“…Simultaneously the 10 th harmonic of ΔfREP was recorded directly in the time domain to provide an average ΔfREP value for each individual interferogram, after which the corresponding RF spectrum was scaled to the optical domain by multiplying its frequency sampling interval by a factor of fREP / DfREP. By using a full-spectrum cross-correlation technique, each spectrum was accurately co-aligned with every other spectrum [7,28]. In this way, the systematic frequency shifts between all the members of a dataset containing more than one hundred consecutively acquired spectra were corrected to provide a single high-quality spectrum.…”