Sensitive and broadband measurement of dispersion in a cavity using a Fourier transform spectrometer with kHz resolution

Abstract

Optical cavities provide high sensitivity to dispersion since their resonance frequencies depend on the index of refraction. We present a direct, broadband, and accurate measurement of the modes of a high finesse cavity using an optical frequency comb and a mechanical Fourier transform spectrometer with a kHz-level resolution. We characterize 16000 cavity modes spanning 16 THz of bandwidth in terms of center frequency, linewidth, and amplitude. We retrieve the group delay dispersion of the cavity mirror coatings and pure N2 with 0.1 fs 2 precision and 1 fs 2 accuracy, as well as the refractivity of the 3ν1+ν3 absorption band of CO2 with 5 × 10 -12 precision. This opens up for broadband refractive index metrology and calibration-free spectroscopy of entire molecular bands.OCIS codes : (140.4780) Optical resonators; (300.6300) Spectroscopy, Fourier transform; (260.2030) Physical optics, dispersion.Fabry-Perot cavities in combination with narrow linewidth continuous wave (cw) lasers are versatile tools for ultra-sensitive measurements of displacement, absorption, and dispersion. For example, high precision measurements of minute length variation of Fabry-Perot cavities enable detection of gravitational waves [1]. A pressure sensor based on the measurement of gas refractivity inside a cavity can outperform a manometer [2]. Cavity-enhanced molecular absorption [3,4] and dispersion [5,6] spectroscopies, which rely on the measurement of intracavity absorption losses and dispersion induced shifts of the cavity modes, respectively, provide complementary information about the molecular transitions and high sensitivity to absorption/dispersion. However, cw lasers allow such measurements only over narrow bandwidths, typically in the sub-THz range. Optical frequency combs, whose spectra consist of thousands of equidistant narrow lines, can probe cavity modes over a much broader bandwidth. In cavity-enhanced optical frequency comb absorption spectroscopy, spectra of entire molecular bands can be acquired with high resolution in short acquisition times [7][8][9][10]. Combs are also an ideal tool for measurements of broadband cavity dispersion induced either by the cavity mirror coatings or intracavity samples. However, previous demonstrations [11-13] did not fully benefit from the high frequency accuracy provided by the comb and suffered from poor spectral resolution (at the THz level).Recent advances in comb-based Fourier transform spectroscopy have provided means to measure spectra over the entire comb bandwidth with resolution directly given by the comb linewidth, using either dual-comb spectrometers [14][15][16][17] or mechanical Fourier transform spectrometers (FTS) [18]. Here we use a frequency comb and a mechanical FTS with sub-nominal resolution [18] to directly measure broadband transmission spectra of a high finesse cavity with high signal-to-noise ratio and frequency precision and accuracy. We fully characterize the cavity modes in terms of amplitude, width, and center frequency. From the shift of the cavity mod...

Optical Frequency Comb Photoacoustic Spectroscopy

“…If needed, spectral features narrower than f rep can be measured with no ILS distortion using the method of comb-based FTS with sub-nominal resolution, in which interferograms with length matched precisely to c/f rep are measured. 23,28,29 The detection limit of OFC-PAS can be improved by several measures. First of all, a factor of 40 improvement can be expected using available high-power mid-IR OFC sources 30 with an output power of up to 1.5 W, compared to 36 mW of the signal comb used in this work.…”

“…Using an optical frequency comb (OFC) as a light source for Fourier transform spectrometers (FTS), in the so-called combbased FTS, allows much faster acquisition (of the order of seconds) of spectra with high signal-to-noise ratio (S/N), 22 and resolution down to kHz range with no ILS contribution. 23 Here, we report the first demonstration of photoacoustic spectroscopy using an optical frequency comb. We call the technique: optical frequency comb photoacoustic spectroscopy (OFC-PAS).…”

Quantitative modeling of complex molecular response in coherent cavity-enhanced dual-comb spectroscopy

“…With high acquisition rates (up to 2 × 1/Δ rep ) and a highthroughput data stream amenable to actively controlled coherent averaging [60], postprocessing [57], and/or adaptive sampling [58], deep averaging of fitted spectroscopic parameters is possible, and therefore so is high precision. Although intensity measurements of cavity transmission modes can also measure complex molecular response (via mode position and line width) [92][93][94][95], those techniques each require laser scanning, and therefore are inherently…”

Broadband calibration-free cavity-enhanced complex refractive index spectroscopy using a frequency comb

“…1, follows in large the one used in ref. [19]. The resonances of a Fabry-Perot cavity are probed by an Er:fiber frequency comb, and the light transmitted through the cavity is analyzed with a fast-scanning Fourier transform spectrometer (FTS) with sub-nominal resolution [18].…”

“…A Lorentzian function is fitted to each cavity mode to retrieve its center frequency, width, and amplitude. We have previously demonstrated that cavity mode frequencies can be used for highprecision retrieval of broadband intracavity group delay dispersion [19]. Here we utilize the full potential of cavity mode characterization and retrieve dispersion and absorption spectra of three combination bands of CO 2 from the cavity mode shifts and broadenings, as well as the corresponding cavity-enhanced transmission spectra from the cavity mode amplitudes.…”