Optical frequency comb, a Nobel Prize-winning device, can emit numerous evenly spaced and phase-coherent laser lines simultaneously. Besides metrology, attosecond science, and optical communication, frequency comb has recently become an extremely powerful tool for high-precision spectroscopy. By resolving the absorption spectrum from the transmitted comb light, people can obtain fast multi-gas detection with unprecedented spectral bandwidth and resolution.
However, 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, limiting its wider applications outside the laboratory.
In a new study published in Nature Communications, a joint team led by Prof. WANG Qiang from the Changchun Institute of Optics, Fine Mechanics and Physics (CIOMP) of the Chinese Academy of Sciences and Prof. REN Wei from the Chinese University of Hong Kong, demonstrated dual-comb photothermal spectroscopy (DC-PTS). Rather than directly measuring the comb light, this new method sensitively detects comb-absorption-inducted photothermal waves by an optical interferometer, which confined the light and gas in a hollow-core fiber with an inner diameter of tens of microns to obtain a significantly enhanced light-gas interaction and a sub-microliter sample volume.
The principle of this sensing method was based on the discovery that the photothermal-induced modulations of the refractive index at various frequencies can be precisely measured using the dual-comb multi-heterodyne method. When two phase-locked combs are transmitted through the gas sample simultaneously, the beating process causes the amplitude modulation of individual comb line at a unique frequency that can be easily read out by an interferometer. Hence, the detection of frequency combs in the optical frequency domain is down-shifted to the detection of refractive index modulations in the radio-frequency domain.
With a homemade electro-optic comb source, the researchers measured the photothermal spectra of acetylene over a broad spectral range of more than 1 THz, and demonstrated a minimum detection limit of 8.7 parts-per-million (ppm) of acetylene in a hollow-core fiber with a total sample volume of only 0.17 μL.
"Recent developments in spectroscopy have witnessed the establishment of dual-comb techniques. In this work the authors demonstrate dual-comb photothermal spectroscopy providing gas sensing with superfine resolution and high sensitivity,” summarized by Nature Communications.
The proposed DC-PTS showed the promising potential for achieving a versatile gas sensor with high sensitivity, high resolution, fast response, broadband detection, and quite compact size. These combined advantages pave the way for many multidisciplinary applications in energy, environment, etc.