High-Precision Dual-Comb Sensing Method Keeps Compact Footprint

Dual-comb photothermal spectroscopy (DC-PTS), demonstrated for the first time by a joint team from the Chinese University of Hong Kong and the Changchun Institute of Optics, Fine Mechanics and Physics (CIOMP), could provide a highly sensitive solution to broadband gas sensing and measurement. Where conventional frequency comb spectroscopy requires a sophisticated spectrometer or a high-bandwidth photodetector to resolve the comb line, and a bulky gas cell to obtain sufficient sensitivity, thereby limiting its use outside laboratory environments, the DC-PTS approach provides superresolution, sensitive broadband gas sensing with just a small sample volume and in a compact configuration.

The technique blends dual-comb spectroscopy, a broadband spectral measurement approach that can be used for gas sensing, with photothermal spectroscopy, a gas sensing method usually done using a single-wavelength pump laser. By combining photothermal spectroscopy with a dual-comb source, the Changchun researchers realized a compact gas sensor with background-free, ultrasensitive, broadband detection capabilities.

Instead of directly measuring the comb light, DC-PTS sensitively detects comb-absorption-inducted photothermal waves with an optical interferometer. The interferometer confines the light and gas in a hollow-core fiber with an inner diameter just tens of microns in size, to achieve significant light-gas interaction with a submicroliter sample volume.

The newly demonstrated sensing method is based on the researchers’ discovery that photothermal-induced modulations of the refractive index at various frequencies can be measured precisely using a dual-comb multiheterodyne method. The researchers found that when two phase-locked combs are transmitted through a gas sample simultaneously, the beating process causes an amplitude modulation of an individual comb line at a frequency that can be easily read by an interferometer. This allows the detection of frequency combs in the optical frequency domain to be down-shifted to the detection of refractive index modulations in the radio-frequency domain.

The researchers used an electro-optic, dual-comb source to generate a photothermal refractive index modulation, which was detected by an in-line Fabry-Pérot interferometer. The dual-comb source generated a multiheterodyne modulation of the refractive index of a gas sample in a hollow-core, antiresonant fiber. A moderate optical span of hundreds of gigahertz was mapped to the frequency response range of the interferometer.

The method of dual-comb photothermal spectroscopy offers precise broadband gas sensing without bulky equipment. Courtesy of professor Qiang Wang/Changchun Institute of Optics, Fine Mechanics and Physics.

Using the electro-optic comb source, the researchers measured the photothermal spectra of acetylene over a broad spectral range of more than 1 THz. With an average optical power of 15 mW for the dual-comb source and a fiber length of 7 cm, the researchers demonstrated a minimum detection limit of 8.7 ppm of acetylene over the coherent averaging time of 1000 s, in a hollow-core fiber with a total sample volume of only 0.17 μL.

The photothermal spectra of acetylene pumped by the dual-comb source could be obtained by the Fourier transform of the interferograms generated from the interferometer within a measurement time as short as 10 ms, the researchers said.

To the best of the researchers’ knowledge, this is the first demonstration of broadband photothermal detection using frequency combs.

As a compact, versatile gas sensor with high sensitivity, high resolution, fast response, and broadband detection capabilities, DC-PTS could be useful for many industry sectors, including the energy and environmental sectors. Although the researchers demonstrated DC-PTS for electro-optic combs, they believe their technique also could be used for mid-infrared combs generated from microresonators, quantum cascade lasers, and interband cascade lasers, with stronger gas absorption and reduced size for integration as the result.

The research was published in Nature Communications (www.doi.org/10.1038/s41467-022-29865-6).