image: Figure | Working principle and application scenarios of photon-level dual-comb spectroscopy. a, schematic illustration of photon-counting statistical process. The dual-comb single-photon interference signal is reconstructed through statistical accumulation of photon arrival-time patterns. b, schematic configuration of the compact all-fiber system, enabling eye-safe and low-power consumption long-path turbulent remote sensing. Photon events are recorded as a time-series of “01” code sequences via the common-mode trigger protocol, which are subsequently used to reconstruct spectral information as depicted in (a). c, this field-deployable system is suitable for challenging applications, such as large-scale open-path environmental monitoring, and non-cooperative open-path DCS without the need for retroreflectors.
Credit: Wei Zhong et al.
Laser spectroscopy is a powerful tool for atmospheric gas analysis, yet traditional methods face challenges due to atmospheric turbulence and significant energy loss. While dual-comb spectroscopy offers rapid wide-spectrum analysis, it often struggles with sensitivity, particularly in turbulent or harsh weather conditions, and its reliability for long-path monitoring remains limited.
In a recent study published in Light: Science & Applications, a research team led by Professor Xianghui Xue from the University of Science and Technology of China, in collaboration with Professor Wei Ren from The Chinese University of Hong Kong, introduced an innovative photon-level dual-comb spectroscopy system. This system enables the reconstruction of spectral information from discrete single photons by using the single-photon detector, a significant advancement for atmospheric remote sensing.
The researchers conducted a thorough investigation into single-photon interference mechanisms between two combs, and photon arrival-time behaviors in a photon-counting setup. They developed a common-mode triggering protocol that mitigates optical path fluctuations caused by turbulence or optical fiber length wandering, allowing for more reliable measurements. In laboratory experiments, they successfully acquired 20-nm bandwidth HCN absorption spectra under simulated turbulent conditions, even with severe optical path jitter. Remarkably, the system maintained kHz spectral resolution and long-term stability at ultra-low energy levels (4 attowatts per comb line), achieving new records in dual-comb detection sensitivity and spectral bandwidth.
To demonstrate practical applications, the team developed a portable fiber-based system and conducted the first single-photon open-path dual-comb spectroscopy experiment. Over a 3.3-km open path through complex environments—including areas with high turbulence and dense urban traffic—the system successfully monitored variations in CO₂, H₂O, and HDO concentrations with high spectral resolution. The advancement addresses long-standing limitations in detection sensitivity for open-path dual-comb spectroscopy and robustness for photon-counting detection, offering scalable solutions for real-time trace gas analysis in challenging environments.
The photon-level dual-comb spectroscopy system utilizes a compact, room-temperature InGaAs-based SPAD, along with a common-mode signal sensing protocol. This design allows for the precise detection of ultra-weak signals through individual photon arrival times, while compensating for optical path fluctuations caused by atmospheric turbulence. By utilizing the arrival information of these photon events, terahertz-level broadband dual-comb interference with attowatt-level sensitivity per comb line can be successfully reconstructed —10 orders of magnitude better than conventional dual-comb spectroscopy. The operational principle lies in converting probabilistic photon arrival-time patterns into coherent spectral information via statistical analysis. This technology facilitates hundred-kilometer open-air broadband spectroscopy using a compact, eye-safe, low-power, and field-deployable all-fiber optical system.
“During the month-long trial of the system, we coincided with three earthquakes (magnitudes 4.7, 4.9, and 4.8). Despite the shaking on the 16th floor, the system remained operational, with only minor adjustments needed following the 4.9-magnitude quake.” recalled by the first author Wei Zhong.
Looking ahead, the team expressed their ambitions:
“While we’ve achieved 15-minute time-resolution open-path spectral detection for multiple greenhouse gases and their isotopes, we believe there is big room for improvement. By integrating increasingly advanced single-photon detector arrays and segmented detection configurations, we aim to accelerate the detection speed, meeting the real-time demands of applications ranging from instant industrial leak monitoring to tracking chemical changs in extreme weather.”
“This breakthrough not only enhances our capability to monitor atmospheric gases with unprecedented sensitivity but also paves the way for next-generation optical sensing networks that are robust, low-power, and scalable. We envision its widespread adoption in global environmental monitoring grids, smart industrial inspection, and even space-based remote sensing, contributing to sustainable, data-driven solutions,” the scientists concluded.
Journal
Light Science & Applications
Article Title
roadband photon-counting dual-comb spectroscopy with attowatt sensitivity over turbulent optical paths