A quantitative mode-resolved frequency comb spectrometer

Abstract

We have developed a frequency-comb spectrometer that records 35-nm (4 THz) spectra with 2-pm (250 MHz) spectral sampling and an absolute frequency accuracy of 2 kHz. We achieve a signal-to-noise ratio of ~400 in a measurement time of 8.2 s. The spectrometer is based on a commercial frequency comb decimated by a variable-length, low-finesse Fabry Pérot filter cavity to fully resolve the comb modes as imaged by a virtually imaged phased array (VIPA), diffraction grating and near-IR camera. By tuning the cavity length, spectra derived from all unique decimated combs are acquired and then interleaved to achieve frequency sampling at the comb repetition rate of 250 MHz. We have validated the performance of the spectrometer by comparison with a previous high-precision absorption measurement of H¹³C¹⁴N near 1543 nm. We find excellent agreement, with deviations from the expected line centers and widths of, at most, 1 pm (125 MHz) and 3 pm (360 MHz), respectively.

Comb-locked frequency-swept synthesizer for high precision broadband spectroscopy

“…A multi-spectrum fitting with a Voigt profile was found sufficient to obtain featureless residuals within the noise level, which increases around the line peak because of the shorter ring-down time. The extrapolated zero-pressure line center of 189955768025 (25) kHz is in agreement with previous highly accurate determinations 7,38 well within our statistical uncertainty of 25 kHz. This indirectly provides an upper limit also for our systematic errors.…”

“…Alternatively, direct comb spectroscopy (DCS) employs OFCs to directly probe broad absorption spectra and eventually enable detection of multiple gases at high temporal resolution. Several techniques can be adopted to extract the spectroscopic content impressed by the gas on the OFC modes, via dual-comb spectroscopy [9][10][11][12][13][14][15][16][17] , Fourier Transform spectroscopy (FTS) [18][19][20][21][22] , high-dispersion spectroscopy through Virtually-Imaged-Phased Arrays (VIPAs) [23][24][25][26] , or scanning microresonators 27 . In all cases individual comb modes may be addressed, combining the large spectral band with a frequency accuracy ultimately determined by the comb itself.…”

High-precision gas refractometer by comb-mode-resolved spectral interferometry

“…The FFP cavity is directly manufactured by optical fibers so that no alignment or mode-matching is required. The FSR of the cavity is set to 5 GHz (transmitting every 20th comb mode) because it is over three times the spectrometer resolution and can result in an optimal 2D spectrum without cross-talk 37 . The length of the FFP cavity is electronically stabilized to the highest intensity peak of the transmission via a dither-locking technique (refer to “methods” section for details).…”

“…Specifically speaking, cross-correlations of the images are carried out with a 5 × 5 pixel kernel and the kernel is one imaged spot that resembles the profile of all resolved comb modes best 38 . Because the matched filter is the optimum linear technique to detect the existence of a signal mixed with white noise, the imaged spots that are only partially transmitted by the FFP cavity are easily removed from the camera image 37 . To eliminate the influence of common-mode brightness variations in the optical system, the reconstructed 1D spectral interferograms have been normalized to the envelope of the comb spectrum (Fig.…”

Self-corrected chip-based dual-comb spectrometer

“…The Lorentzian width (pressure broadening), centre and depth of each line are left as free parameters. We use the same approach as the one described in [45] to fit the data and suppress the slowly varying background. The dominant structure left in the fit residuals is due to weak hot-band transitions.…”