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

Riccardo Gotti,Yuriy Mayzlin,Davide Gatti,Bidoor Alsaif,Julian Robinson-Tait,Thomas Puppe,Felix Rohde,Marco Lamperti,Szymon Wójtewicz,Patrick Leisching,Rafal Wilk,Wilhelm G Kaenders,Paolo Laporta,Marco Marangoni

doi:10.1038/s41598-020-59398-1

Abstract

Frequency combs have made optical metrology accessible to hundreds of laboratories worldwide and they have set new benchmarks in multi-species trace gas sensing for environmental, industrial and medical applications. However, current comb spectrometers privilege either frequency precision and sensitivity through interposition of a cw probe laser with limited tuning range, or spectral coverage and measurement time using the comb itself as an ultra-broadband probe. We overcome this restriction by introducing a comb-locked frequency-swept optical synthesizer that allows a continuous-wave laser to be swept in seconds over spectral ranges of several terahertz while remaining phase locked to an underlying frequency comb. This offers a unique degree of versatility, as the synthesizer can be either repeatedly scanned over a single absorption line to achieve ultimate precision and sensitivity, or swept in seconds over an entire rovibrational band to capture multiple species. The spectrometer enables us to determine line center frequencies with an absolute uncertainty of 30 kHz and at the same time to collect absorption spectra over more than 3 THz with state-of-the-art sensitivity of a few 10−10 cm−1. Beyond precision broadband spectroscopy, the proposed synthesizer is an extremely promising tool to force a breakthrough in terahertz metrology and coherent laser ranging.

Highlights

Frequency combs have made optical metrology accessible to hundreds of laboratories worldwide and they have set new benchmarks in multi-species trace gas sensing for environmental, industrial and medical applications
Several techniques can be adopted to extract the spectroscopic content impressed by the gas on the optical frequency combs (OFCs) 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]
An equivalent synthesizer for the optical domain should adopt an OFC as a reference and a widely tunable cw laser to generate the single-frequency signal; a frequency sweep over a broad spectral range in a coherent fashion is not trivial with an OFC because of the ambiguity that arises whenever the cw laser either superimposes a comb mode or lies halfway between two comb modes

Summary

Introduction

Frequency combs have made optical metrology accessible to hundreds of laboratories worldwide and they have set new benchmarks in multi-species trace gas sensing for environmental, industrial and medical applications. In this paper we are proposing a third versatile way to perform spectroscopy with OFCs, which combines the multi-THz spectral coverage of DCS with the 10-kHz level accuracy on the line centers of CAS while keeping the measurement time of seconds over the full band, both with and without an enhancement cavity Such an approach, hereafter referred to as comb-locked frequency-swept synthesis (CLFSS), is based on the phase locking of a mode-hop-free tunable cw laser to an OFC whose frequency axis is smoothly and seamlessly shifted within the comb spectral range, at speeds up to 1 THz/s and over spectral bands as large as 10 THz. CLFSS ensures precise optical frequency generation and imprints the OFC coherence to the scanning laser, so that both absolute accuracy and narrow linewidth are available for high precision spectroscopy. A similar agreement is found on the line centers of the ν1 + ν3 band of C2H2 investigated in a cavity-free setup

Methods

Results

Conclusion