Complex Gas Spectroscopy Through Hz-Level Cavity ModeMeasurements With a Comb-Based VIPA Spectrometer

G. Kowzan∗1, D. Charczun1, A. Cygan1, D. Lisak1, R. S. Trawinski1, P. Masłowski11. Institute of Physics, Faculty of Physics, Astronomy and Informatics, Nicolaus Copernicus University in Torun, Ul. Grudziadzka 5, 87-100

Torun, Poland

Cavity mode-width spectroscopy [1] and cavity mode-dispersion spectroscopy [2] techniques provide a wayto simultaneously determine absorption and dispersion of a sample by measuring widths and positions of en-hancement cavity resonances. It was shown recently that the latter can also be efficiently combined with opticalfrequency combs (OFCs) to perform dispersion measurements in a broad spectral range [3]. Here, we presentbroadband measurements of 7-kHz-wide cavity resonances at Hz-level precision, resulting in complex spectra ofCO second overtone transitions [4]. To the best of our knowledge, the 7-kHz-wide cavity resonances shown inthis work are the narrowest spectral features measured directly with a frequency comb. Moreover, the presentedtechnique is capable of retrieving shapes of such cavity modes in several seconds, enabling future ultra-precise,broadband and time-resolved spectroscopic measurements.

Fig. 1: Left part: (a) Measured cavity mode shape with a Lorentzian fit and fit residuals. The uncertainties formode width γ (HWHM) and ν0 are 1σ fit uncertainties. Right part: absorption (b) and dispersion (c) spectrum ofthe 0-3 band of CO-N2 at 714 Torr with fit residuals based on complex Voigt profiles..

We couple a near-infrared OFC into a high-finesse optical cavity (F = 18500) and tightly lock the comb teethto the cavity modes, while absolute frequencies of the cavity modes are stabilized to an Rb frequency standard.The OFC is Vernier-filtered by the cavity to obtain 4-GHz spacing between comb modes and transferred to a VIPAspectrometer [5]. The VIPA spectrometer with resolution of 600 MHz and spectral bandwidth of 20 nm clearlyresolves individual comb lines. The shapes of the cavity modes are retrieved by tuning their positions with respectto the OFC and recording the transmitted spectra. Each of the modes is subsequently fitted to a Lorentzian function(shown on Fig. 1a) to obtain its width and position. The mode widths are converted to absorption coefficient and themode position shifts to refractive index and both are fitted to the complex Voigt profiles of probed transitions. Weobtain signal-to-noise ratio of 190 for absorption spectrum and of 380 for dispersion spectrum with expected betterperformance for longer averaging times. We also report on broadband determination of group-delay dispersion ofthe cavity mirrors based on observed cavity mode positions.

The research was supported by the National Science Center, Poland, project nos. 2015/18/E/ST2/00585,2016/21/N/ST2/00334. G. K. was supported by the National Science Centre, Poland scholarship 2017/24/T/ST2/00242.

References[1] A. Cygan, D. Lisak, P. Morzynski, M. Bober, M. Zawada, E. Pazderski, and R. Ciuryło, Opt. Express 21, 29744 (2013).[2] A. Cygan, S. Wójtewicz, M. Zaborowski, P. Wcisło, R. Guo, R. Ciuryło, and D. Lisak, Meas. Sci. Technol. 27, 045501 (2016).[3] L. Rutkowski, A. C. Johansson, G. Zhao, T. Hausmaninger, A. Khodabakhsh, O. Axner, and A. Foltynowicz, Opt. Express 25, 21711(2017).[4] G. Kowzan, D. Charczun, A. Cygan, R. S. Trawinski, D. Lisak, and P. Masłowski, Conference on Lasers and Electro-Optics (2018), PaperSTu3N.8 (Optical Society of America, 2018)[5] S. A. Diddams, L. Hollberg, and V. Mbele, Nature 445, 627 (2007).

∗Corresponding author: gkowzan@fizyka.umk.pl