11-μs time-resolved, continuous dual-comb spectroscopy with spectrally filtered mode-locked frequency combs

Abstract

Broadband dual-comb spectroscopy (DCS) based on portable mode-locked fiber frequency combs is a powerful tool for in situ, calibration free, multi-species spectroscopy. While the acquisition of a single spectrum with mode-locked DCS typically takes microseconds to milliseconds, the applications of these spectrometers have generally been limited to systems and processes with time changes on the order of seconds or minutes due to the need to average many spectra to reach a high signal-to-noise ratio (SNR). Here, we demonstrate high-speed, continuous, fiber mode-locked laser DCS with down to 11 μs time resolution. We achieve this by filtering the comb spectra using portable Fabry-Perot cavities to generate filtered combs with 1 GHz tooth spacing. The 1 GHz spacing increases the DCS acquisition speed and SNR for a given optical bandwidth while retaining a sufficient spacing to resolve absorption features over a wide range of conditions. We measure spectra of methane inside a rapid compression machine throughout the 16 ms compression cycle with 133 cm -1 bandwidth (4000 comb teeth) and 1.4 ms time resolution by spectrally filtering one of the combs. By filtering both combs, we measured a single-shot, 25 cm -1 (750 comb teeth) spectrum of CO around 6330 cm -1 in 11μs. The technique enables simultaneously high-speed and high-resolution DCS measurements, and can be applied anywhere within the octavespanning spectrum of robust and portable fiber mode-locked frequency combs I.

Broadband 1-GHz mid-infrared frequency comb

“…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 infrared 18 , but have not been fully extended to the MIR, where only a handful of 1 GHz MIR frequency combs exist [19][20][21][22][23][24][25] . It should be noted that the multi-GHz chip-based MIR frequency combs generated from electrooptic combs 26 quantum cascaded lasers 27 or microcombs 28 enable sub-microsecond spectral acquisition.…”

A hybrid integrated dual-microcomb source

“…There exist a number of rapidly developing approaches for implementing dual-comb techniques. Various dualcomb systems are based on conventional fibre modelocked lasers (MLLs) [24][25][26][27].…”

Architecture for microcomb-based GHz-mid-infrared dual-comb spectroscopy

“…And while offering extremely short acquisition times, these rates are too high for spectroscopy of many species, leading to spectral under-sampling of gas samples. For balance between acquisition rate and spectral resolution, DCS at GHz rates is considered to be relatively optimal for sensing of ambient gases (with linewidths narrower than 10s of GHz) 24 – 26 . Special efforts have been directed to reduce near-IR microcomb rates to the single-digit GHz range 27 , 28 , but these require very high Q resonators to reduce increased threshold pumping power associated with larger mode volumes.…”