Coherent cavity-enhanced dual-comb spectroscopy.

Adam J Fleisher,Zachary D Reed,Joseph T Hodges,David A Long,David F Plusquellic

doi:10.1364/oe.24.010424

Abstract

Dual-comb spectroscopy allows for the rapid, multiplexed acquisition of high-resolution spectra without the need for moving parts or low-resolution dispersive optics. This method of broadband spectroscopy is most often accomplished via tight phase locking of two mode-locked lasers or via sophisticated signal processing algorithms, and therefore, long integration times of phase coherent signals are difficult to achieve. Here we demonstrate an alternative approach to dual-comb spectroscopy using two phase modulator combs originating from a single continuous-wave laser capable of > 2 hours of coherent real-time averaging. The dual combs were generated by driving the phase modulators with step-recovery diodes where each comb consisted of > 250 teeth with 203 MHz spacing and spanned > 50 GHz region in the near-infrared. The step-recovery diodes are passive devices that provide low-phase-noise harmonics for efficient coupling into an enhancement cavity at picowatt optical powers. With this approach, we demonstrate the sensitivity to simultaneously monitor ambient levels of CO2, CO, HDO, and H2O in a single spectral region at a maximum acquisition rate of 150 kHz. Robust, compact, low-cost and widely tunable dual-comb systems could enable a network of distributed multiplexed optical sensors.

Highlights

Dual-comb spectroscopy has shown great potential as a fast, accurate, and high-resolution alternative to existing interferometric methods that require lengthy integration times and precision moving parts (e.g., Fourier transform spectroscopy) [1,2]
While comb generation based on electro-optic phase modulators (EOMs) was first demonstrated in the 1990s for metrology [15,16,17] and interferometry [18], only recently have the advantages of dual-EOM comb systems been realized for spectroscopy [19,20,21,22]
In order to address current challenges in dual-comb spectroscopy pertaining to relative comb coherence and cost-effective, robust instrument development, we demonstrate here dual-EOM comb spectroscopy with a combination of three novel enhancements: 1) the inclusion of step-recovery diodes as passive, in-line, hands-free devices for generating coherent RF pulses, 2) the design of a low-bandwidth phase-locked loop which allowed for > 2 hours of mutual comb coherence without software corrections, and 3) a real time data streaming and time-domain coherent averaging procedure which improved data throughput to nearly 100 %

Summary

Introduction

Dual-comb spectroscopy has shown great potential as a fast, accurate, and high-resolution alternative to existing interferometric methods that require lengthy integration times and precision moving parts (e.g., Fourier transform spectroscopy) [1,2]. EOM combs offer 1) inherently high relative phase coherence between the two arms of the interferometer owing to its self-heterodyne nature [24,25], and 2) the frequency-agility and precision of the driving low-phase-noise microwave electronics for exquisite control of spectral coverage, resolution, scan speed and down-conversion bandwidth. These features make for an efficient and facile way to couple all comb teeth into narrow resonance modes of an enhancement cavity for high resolution and ultra-sensitive spectroscopic detection.

Methodology of comb generation

Long-term coherence without complexity

Findings

Conclusion