Open-path dual-comb spectroscopy is emerging as a new method for atmospheric gas sensing. For example, this technique can detect methane leaks in oil and gas fields that represent lost revenue and contribute significantly to increased ambient methane concentrations, and therefore to global warming. In a related example, open-path dual-comb spectroscopy can provide greenhouse gas monitoring in urban environments. However, adoption of dual-comb spectroscopy for any application will depend critically on cost. Current laboratory systems are quite expensive. Ideally, the cost of future portable systems must be lowered—but without sacrificing performance and with improved robustness. Philippe Guay and coauthors describe an elegant approach that does just that, enabling up to an order of magnitude cost reduction without sacrificing performance.

They exploit advanced digital phase correction algorithms to replace the self-referenced frequency combs with two free-running combs, thereby removing the standard, but expensive, f-2f interferometer, with significant cost savings. In addition, they use their digital phase correction to remove the requirement for a “fast PZT,” which improves comb robustness. Critically, they then demonstrate that this system can still acquire precise, high signal-to-noise ratio spectra of methane, whose absorption lies in a spectral band that is far from the output of the underlying femtosecond fiber lasers or reference CW lasers.

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