The quantification of greenhouse gases, airborne pollutants, and hazardous gases is typically performed in an open-path scenario, a versatile method for real-life applications in ambient atmosphere. In particular, laser absorption spectroscopy has emerged as a rapid, non-intrusive technique for this purpose. Optical frequency combs, characterized by their narrow linewidth and expansive spectrum, offer both high resolution and swift response for spectroscopic measurements. Dual-comb spectroscopy (DCS) stands out, boasting even faster update rates than its single-comb counterpart.

In a recent study, researchers from the teams of Professors Haoyun Wei and Yan Li at Tsinghua University, China, introduced a novel dual-purpose compensation technique. This method effectively counters both the temporal jitter inherent in dual free-running optical frequency comb sources, and the contributing noise arising from turbulent disturbances encountered in open paths.

Figure 1: Experimental configuration for phase-sensitive open-path DCS employing two free-running optical frequency combs (Menlo Systems FC1500).

The team utilized two Menlo Systems FC1500 combs with repetition frequency of 250 MHz and a slight offset in the repetition frequency of 290 Hz, in order to achieve enhanced phase-sensitive detection in an open-path DCS setup (Fig. 1). The signal (S) and local oscillator (LO) combs both cover a broad spectrum of 6250 – 6660 cm⁻¹ (1.5 – 1.6 μm), overlapping with rovibrational bands in gas-phase CO₂ and H₂O. They demonstrated the strength of the compensation technique in a measurement, where spectra of a 900 m long turbulent air path in atmosphere were extracted, ranging from 6250 to 6660 cm⁻¹.

Figure 2: Broadband atmospheric amplitude and phase measurement spectra

The results of the experiment are presented in Figure 2, where the measurement data is compared to theoretical calculations. The residual error of the measured amplitude spectrum is considerably less than 0.01 for the strong overlapping features of CO₂ and H₂O. At the same time, the average residual of the phase spectrum is 0.2 mrad, corresponding to a refraction-index variation of about 6×10⁻¹⁴ over the target path. This result is comparable to measurements using a coherently phase-locked comb scheme.

In a second experiment, the team demonstrate the capability of the system for dynamic emission monitoring in a field-test scenario simulating pipeline leakage. The retrieved concentrations from the amplitude and phase spectra show deviation of less than 0.82 ppm.

Concluding from their findings, with further improvement of the method and by employing a more compact and portable commercial dual-comb spectrometer, the team from Tsinghua University believes that the dual-purpose compensation DCS can offer real-time and accurate gas-absorption information for environmental protection, atmospheric science and industrial operation.

Author: Zhaoyang Tai, Menlo Systems

Original publication:

X. Chen, Ch. Huang, J. Li , M. Lu, Y. Li, and H. Wei:
Phase-Sensitive Open-Path Dual-Comb Spectroscopy with Free-Running Combs
Phys. Rev. Appl. Vol. 19, 044016 (2023)
doi: 10.1103/PhysRevApplied.19.044016