Recent advances and applications of off‐axis integrated cavity output spectroscopy

Abstract

The off-axis integrated cavity output spectroscopy (OA-ICOS) represents a suitable solution in many applications with a simple structure and high sensitivity. The off-axis configuration allows the continuous spectrum output without cavity mode structure when a continuous wave laser is used while preserving the optical resonator's ability to enhance the optical path by tens of thousands of times. This review summarizes the recent advances and applications of OA-ICOS in recent decades. The evolutionary process and basic principle of OA-ICOS are introduced. Various approaches for improving the technical performance and enabling simultaneous multigas measurement are reported. Finally, the various field applications of OA-ICOS technology and the potential development directions are discussed.

Spectroscopic techniques to analyze stable carbon isotopic compositions of carbon dioxide, methane and volatile organic compounds

“…Similar to other cavity-based methods, a stable optical cavity is used in OA-ICOS to enhance effective path length, which usually consists of two highly reflective spherical mirrors under stable conditions. 36 As indicated in Fig. 2, OA-ICOS is injecting light off-axis to alter the resonances of the cavity, allowing multiple transverse modes to be simultaneously pumped in order to produce a dense cavity mode spectrum.…”

“…37 Once the so-called “re-entrant condition” is satisfied, the light precisely returns to the entry point after multiple reflections and repeats the same path. 34,36–38 This condition is determined by both specific curvature and spacing of the mirrors in the cavity. 34 When target molecules enter the optical cavity, the molecules interact with light in the cavity, and absorption signals are obtained by temporal integration of laser intensities transmitted through the cavity.…”

“…OA-ICOS techniques have drawn broad interest due to their simple structures and high sensitivities. 1,7,36,38–41 Jost et al designed the first OA-ICOS instrument for determining 13 C/ 12 C and 18 O/ 16 O ratios from CO 2 rotation/vibration fine structures at three different combined bands. Their preliminary results showed that the accuracy of δ 13 C and δ 18 O at 0.8 Hz is 3.3‰ and 2.8‰, respectively, and the measurements were conducted over a period of 10 hours for pure CO 2 gas samples at 26 h Pa. By averaging 100 spectra over a subset of the data, they achieved an accuracy of 1.6‰ for δ 13 C and 0.8‰ for δ 18 O.…”

Re-injection off-axis integrated cavity output spectroscopy for the simultaneous detection of N2O, H2O and CO with a mid-infrared QCL laser

“…The cavity mode noise is suppressed by smoothing the mode structure, and the detection sensitivity and interference immunity have also been improved. 15–18 Due to the much higher gas absorption intensity in the mid-infrared region compared to that in the near-infrared region, the combination of a mid-infrared quantum cascade laser (QCL) and OA-ICOS can achieve higher measurement sensitivity. 19,20 However, off-axis integrating cavity is non-mode-matched optical cavity, where the high energy in the fundamental mode is spread out into dense higher-order modes, the output energy is greatly reduced.…”

Spectroscopic detection of radiocarbon dioxide based on optical feedback linear cavity enhanced absorption spectroscopy

“…Off‐axis integrated cavity output spectroscopy (OA‐ICOS) is an alternative method that is immune to the variation of the coupling coefficient. In OA‐ICOS, 28 the incident light is misaligned with respect to the cavity axis, resulting in the excitement of many high transverse modes. This significantly shortens the free spectral range (FSR) and consecutive cavity transmission can be observed.…”