High Coherence Mid-Infrared Dual Comb Spectroscopy Spanning 2.6 to 5.2 microns (1709.07105v1)

Abstract: Mid-infrared dual-comb spectroscopy has the potential to supplant conventional high-resolution Fourier transform spectroscopy in applications that require high resolution, accuracy, signal-to-noise ratio, and speed. Until now, dual-comb spectroscopy in the mid-infrared has been limited to narrow optical bandwidths or to low signal-to-noise ratios. Using a combination of digital signal processing and broadband frequency conversion in waveguides, we demonstrate a mid-infrared dual-comb spectrometer that can measure comb-tooth resolved spectra across an octave of bandwidth in the mid-infrared from 2.6-5.2 $\mu$m with sub-MHz frequency precision and accuracy and with a spectral signal-to-noise ratio as high as 6500. As a demonstration, we measure the highly structured, broadband cross-section of propane (C3H8) in the 2860-3020 cm-1 region, the complex phase/amplitude spectrum of carbonyl sulfide (COS) in the 2000 to 2100 cm-1 region, and the complex spectra of methane, acetylene, and ethane in the 2860-3400 cm-1 region.

• The paper demonstrates a high-coherence mid-infrared dual-comb spectrometer that achieves comb-tooth resolved spectra over an octave with SNRs up to 6500 and sub-MHz precision.
• The paper employs advanced digital signal processing and difference frequency generation to convert near-infrared outputs into mid-infrared light, reducing measurement time to one-seventh of traditional FTS.
• The paper showcases the spectrometer’s capability for real-time detection of complex molecular spectra, paving the way for applications in atmospheric monitoring and industrial control.

High Coherence Mid-Infrared Dual Comb Spectroscopy Spanning 2.6 to 5.2 Microns

Dual-comb spectroscopy (DCS) has emerged as a technologically advanced alternative to Fourier transform spectroscopy (FTS), primarily due to its high resolution, absolute frequency accuracy, and rapid acquisition rates. Despite the impressive advances seen in near-infrared DCS applications, where a wider availability of laser sources exists, mid-infrared DCS technology has lagged, hindered by limitations in signal-to-noise (SNR) ratio and coherence over broad optical bandwidths. This paper introduces a high coherence mid-infrared dual-comb spectrometer achieving comb-tooth resolved spectra across an octave from 2.6 to 5.2 microns, promising SNRs up to 6500, effectively challenging conventional high-resolution FTS.

Key Contributions and Findings

• Spectrometer Design: Employing a combination of digital signal processing techniques and broadband frequency conversion via difference frequency generation (DFG), this spectrometer delivers high coherence DCS within the mid-infrared domain. Notably, the use of difference frequency generation converts near-infrared laser outputs into mid-infrared light with precise frequency management, offering enhanced power capability and simple operation.
• Resolution and Accuracy: The system demonstrates sub-MHz frequency precision and accuracy. This capability is showcased in measuring the structured, broadband cross-section of propane, consisting of features as narrow as 0.01 cm$^{-1}$ (300 MHz). This precision parallels high-resolution FTS yet is achieved with only 1/7th the time, significantly optimizing throughput.
• Application to Complex Molecules: Through the high coherence achieved, the paper details the dual-comb spectrometer's competency in measuring complex broadband spectra of large molecules, achieving comparable fidelity to traditional methods while circumventing typical calibration corrections, hence streamlining data acquisition processes.
• Data Processing Techniques: The spectrometer's performance is further augmented by real-time digital processing that corrects phase and timing drifts, facilitating coherent averaging that sustains high SNR. The digital signal processing implementation is versatile across spectral regions, highlighting a strategic enhancement to maintaining coherence.
• Implications for Broader Applications: The methodology benefits from compact and streamlined optical setup, suggesting utility in portable applications beyond laboratory settings. Potential domains include outdoor atmospheric monitoring and industrial process control, where broad spectral coverage can facilitate the detection of large molecules amidst various gas backgrounds.

Numerical and Comparative Analysis

• Spectral Signal-to-Noise Ratio: The spectrometer achieves peak SNR on the order of 94/√s, validating its competitive efficacy even relative to solution-specific narrow-band DCS reports.
• Figure-of-Merit: An average figure-of-merit of 9 x 10$^9$/√s underscores its capability, positioned favorably against previous narrow-band counterparts.

Future Directions and Considerations

This work lays foundational advancements enabling DCS to advance into non-laboratory settings, with promising applicability to real-world challenges. Future innovations may include further reduction in system size and enhancement in functionality, possibly integrating quantum cascade lasers and other novel mid-infrared sources to push the boundaries of spectral range and measurement accuracy further. Additionally, improvements in noise reduction, such as employing balanced detectors similar to near-infrared usage, could alleviate interference issues, further augmenting this technology's viability for expansive, non-intrusive monitoring tasks.

In conclusion, this research exemplifies significant strides in mid-infrared spectroscopy, bridging existing performance gaps with established spectroscopic paradigms, and setting a precedent for adaptive, rapid, and accurate spectral measurement across broad optical domains.