Near-infrared optical-feedback linear cavity-enhanced absorption spectroscopy

Cavity-enhanced absorption spectroscopy (CEAS) utilizes multiple reflections of light between two cavity mirrors to increase the interactions between a laser beam and gas, thus improving detection sensitivity. In CEAS, noise predominantly arises from a low laser-to-cavity coupling efficiency and fluctuations of the cavity mode amplitude. To address such noise, researchers have developed optical-feedback cavity-enhanced absorption spectroscopy technology (OF-CEAS), which locks the laser frequency to the frequency of the cavity longitudinal mode. As a result, it narrows the laser linewidth and improves the laser-to-cavity coupling efficiency. To avoid direct reflections from the optical cavity for optical feedback, traditional OF-CEAS employs a three-mirror V-shaped resonator. However, based on experimental verification, we have found that when the feedback phase is properly controlled, light directly reflected by the optical cavity does not cause optical feedback, and the laser can be locked to the resonant cavity mode from the optical cavity. Therefore, we propose OF-CEAS based on a linear F-P cavity. The symmetry of the transmission cavity mode is used to generate the error signal for feedback phase control. Then, 101 consecutive cavity transmission modes with a stable amplitude and broad width were observed. Finally, we detected methane standard gas with a concentration of 32×10-6 and obtained the OF-CEAS absorption signal. Based on the signal to noise ratio, the detection sensitivity down to 0.54×10-6 (1σ) was estimated.