Broadband 1-GHz mid-infrared frequency comb

Hoghooghi et al. Light: Science & Applications (2022)11:264 https://doi.org/10.1038/s41377-022-00947-w ARTICLE Official journal of the CIOMP 2047-7538 www.nature.com/lsa Open Access Broadband 1-GHz mid-infrared frequency comb Nazanin Hoghooghi1 ✉, Sida Xing2,3, Peter Chang2,3, Daniel Lesko2,4, Alexander Lind2,3, Greg Rieker1 and Scott Diddams 2,3,5 ✉ 1234567890():,; 1234567890():,; 1234567890():,; 1234567890():,; Abstract Mid-infrared (MIR) spectrometers are invaluable tools for molecular fingerprinting and hyper-spectral imaging. Among the available spectroscopic approaches, GHz MIR dual-comb absorption spectrometers have the potential to simultaneously combine the high-speed, high spectral resolution, and broad optical bandwidth needed to accurately study complex, transient events in chemistry, combustion, and microscopy. However, such a spectrometer has not yet been demonstrated due to the lack of GHz MIR frequency combs with broad and full spectral coverage. Here, we introduce the first broadband MIR frequency comb laser platform at 1 GHz repetition rate that achieves spectral coverage from 3 to 13 µm. This frequency comb is based on a commercially available 1.56 µm mode-locked laser, robust all-fiber Er amplifiers and intra-pulse difference frequency generation (IP-DFG) of few-cycle pulses in χ(2) nonlinear crystals. When used in a dual comb spectroscopy (DCS) configuration, this source will simultaneously enable measurements with μs time resolution, 1 GHz (0.03 cm−1) spectral point spacing and a full bandwidth of >5 THz (>166 cm−1) anywhere within the MIR atmospheric windows. This represents a unique spectroscopic resource for characterizing fast and non-repetitive events that are currently inaccessible with other sources. Introduction Coherent MIR (3–25 µm) light sources are critical to the advancement of various scientific fields. This is particularly true for spectroscopic sensing and imaging, where such sources access the molecular “fingerprint” region (6.7–20 µm), enabling chemical specificity while also improving the minimum detection sensitivity limit. Spectroscopy and imaging systems using CW MIR lasers have shown unprecedented sensitivity1,2. Broadband MIR optical frequency combs3 can further enhance the performance of spectroscopic and imaging systems by offering three important characteristics: high brightness, full instantaneous spectral coverage, and high spectral resolution. When combined with a fast, broadband, and high-resolution detection scheme such as dual-comb spectroscopy (DCS), MIR frequency comb spectrometers Correspondence: Nazanin Hoghooghi () or Scott Diddams () 1 Precision Laser Diagnostics Laboratory, University of Colorado, Boulder, CO 80309, USA 2 Time and Frequency Division, National Institute of Standards and Technology, Boulder, CO 80305, USA Full list of author information is available at the end of the article have the potential for recovering full spectral fingerprint information at MHz rates. A significant body of frequency comb spectroscopy employs dispersive and Fourier transform spectrometers4–6, but here we restrict our attention to DCS with its simplicity that stems from a single-element detector and freedom from mechanical delay stages. The majority of existing broadband MIR frequency combs are generated through nonlinear down-conversion of near-infrared (NIR) frequency combs, either through parametric oscillation or difference frequency generation (DFG) techniques. These sources typically have 50–200 MHz comb tooth spacing, which is defined by the repetition rate of the fundamental NIR frequency combs driving the nonlinear process. Such ~100 MHz repetition rate MIR frequency combs have been used in a DCS configuration for atmospheric sensing7, trace gas detection7–11, and studies related to wildfire spread12, where the time scale of the events under study is on the order of seconds. However, a wide range of scientific studies, such as those in combustion13–15 and biological reactions16, would benefit from MIR DCS systems with increased © The Author(s) 2022 Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons license, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons license and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/. Hoghooghi et al. Light: Science & Applications (2022)11:264 Page 2 of 7 Addressing these challenges, we demonstrate the first 1-GHz MIR frequency comb with spectral coverage from 3 to 13 µm. A key aspect of this advance is the use of soliton self-compression in highly nonlinear fiber (HNLF) to generate NIR pulses centered at 1.56 µm with average power of 2.3 W and duration as short as 8.1 fs (1.5 optical cycles). In a simple single-pass geometry, these ultrashort pulses drive intra-pulse difference frequency generation (IP-DFG) in χ(2) nonlinear crystals, yielding MIR powers as high as 6.2 mW. Our approach is built off a commercial 1.56 µm source29 and established Er-fiber amplifiers and fiber components, all of which combine to provide a robust, reproducible, and broad bandwidth MIR frequency comb platform for high-speed molecular spectroscopy in settings beyond the research lab. speed (μs time resolution), while maintaining broad spectral coverage and high spectral resolution. Since the measurement speed of DCS scales as the square of the repetition rate17, significant gains are achieved by scaling broadband MIR frequency combs from the MHz range to the GHz. In particular, frequency combs with ~1 GHz repetition rate strike an attractive balance between speed and spectral resolution in scenarios of expanding interest. For example, they enable DCS measurements over ~5 THz of spectral bandwidth with 10 µs time resolution, while still providing the necessary spectral resolution for gas phase measurements from atmospheric to combustion and exoplanet relevant temperatures and pressures. These benefits were highlighted previously in the near infrared18, but have not been fully extended to the MIR, where only a handful of 1 GHz MIR frequency combs exist19–25. It should be noted that the multi-GHz chip-based MIR frequency combs generated from electrooptic combs26 quantum cascaded lasers27 or microcombs28 enable sub-microsecond spectral acquisition. However, most of these sources have relatively narrow spectral coverage and comb tooth spacing of >10 (...truncated)