Mode-resolved Cavity-enhanced Vernier Spectroscopy Using an Interband Cascade Laser Frequency Comb Lukasz A. Sterczewski 1 , Tzu-Ling Chen 2 , Douglas C. Ober 2 , Charles R. Markus 2 , Chadwick L. Canedy 3 , Igor Vurgaftman 3 , Clifford Frez 1 , Jerry R. Meyer 3 , Mitchio Okumura 2 , and Mahmood Bagheri 1 1 Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California 91109, USA 2 Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, CA, 91125, USA 3 Code, 5613, Naval Research Laboratory, Washington, DC 20375, USA Author e-mail address: lukasz.a.sterczewski@jpl.nasa.gov Abstract: We demonstrate a mid-IR mode-resolved Vernier optical frequency comb spectrometer with an interband cascade laser source. The free-running system provides 35 meters of effective path length for monitoring 1-THz-broad spectra at 3.63 µm. 1. Introduction Chip-scale optical frequency combs (OFCs) [1] are revolutionizing the detection of trace chemicals in gas form, by providing high-resolution, broadband spectroscopy at a rapid sampling speed. Currently, dual-comb spectroscopy (DCS) [2] is arguably the most widely-used approach for accessing mode-resolved spectral information. Despite its many advantages, however, DCS can be challenging experimentally because high mutual coherence between the two comb lasers must be maintained via analog [3] or digital [4] feedback control loops, and precise broadband microwave digitizing is needed. Moreover, high sensitivity requires enhancement of the interaction length via a multi-pass Herriott cell or optical cavity that adds an extra layer of complexity to the OFC spectrometer. In this work, we demonstrate a simplified Vernier spectroscopy approach [5] that provides robust, real-time sensing. As illustrated in Fig. 1, we employ a high-finesse (F=1800) optical cavity in combination with a single-pixel photodetector to simultaneously realize both optical path enhancement and the full spectral resolution of a single free-running interband cascade laser (ICL) OFC. 2. Vernier spectrometer Vernier cavity PZT L 2 L 1 ISO Piezo voltage Ramp up a b c d=3.09 cm F = 1800 2dF/ effective path (35 meters) Cavity mirrors Photodetector signal ICL Comb PD DAQ Time (ms) 1.2 ms Ramp down Vernier noise floor FTIR (~2 s) d Vernier spectrometer (1.4 ms) e Frequency (kHz) + 9.6956 GHz 100 kHz span 10 dB/div. Fig. 1. Vernier OFC spectrometer. (a) Schematic of the experimental setup. Collimated light from a free- running ICL comb is mode-matched to the cavity using a lens (L1) preceded by an optical isolator (ISO). Voltage tuning of the piezoelectric transducer (PZT) enables the cavity to act as a mode-selecting optical filter. A single-element photodetector (PD) converts light into an electrical signal that is captured by a digitizer (DAQ). (b) Example of a time-domain trace acquired with a 7 MS/s 12-bit digitizer. (c) Photograph of the open-path Vernier cavity with effective path length 35 m. (d) Optical spectrum measured with a Fourier Transform spectrometer (FTIR) in ~2 seconds. (e) Optical spectrum measured in 1.4 ms by scanning the mirror spacing of the Vernier cavity. Because the optical linewidth of ~300 kHz at 1 ms increases slightly with frequency due to the accumulation of timing jitter, a lower coupling efficiency around 2765 cm -1 causes the comb lines to have amplitudes ~1 dB lower than those at 2737 cm -1 . Nevertheless, the full comb bandwidth is transmitted through the cavity. SM4N.3 CLEO 2021 © OSA 2021 © 2021 The Author(s) Authorized licensed use limited to: CALIFORNIA INSTITUTE OF TECHNOLOGY. Downloaded on December 11,2021 at 00:54:53 UTC from IEEE Xplore. Restrictions apply.