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Direct comb multi-heterodyne interference spectroscopy

We present a comb-based spectroscopic method that enables simultaneous detections of multiple gases by adopting an erbium-doped fiber femtosecond laser as a single broadband probing beam. The method takes multiple continuous-wave diode lasers as the frequency references, each being assigned to its distinct gas absorption line. The interference of the probing femtosecond laser with the diode lasers produces multi-heterodyne beats in the radio frequency domain, which are captured using a high-speed photodetector and electronically processed to identify the absorption lines of interest with a comb-mode spectral resolution. The experimental result of this study demonstrates that two gas absorption lines of H13CN and 12CO2, separated by a 23 nm spectral offset, can be detected concurrently at a 10 μs update rate with a 100 kHz spectral resolution. The proposed method finds applications in not only fundamental spectral line measurements for atomic and molecular physics but also diverse practical uses for remote sensing of trace and toxic gas molecules.