Design of a Laser Gas Sensor With Phase Compensation in Free Space Using Half-Mirror and Mixer

A laser gas sensor with phase compensation in free space is designed based on a half-mirror and mixer in this study. The half-mirror is employed to split the returned beam into transmitted and reflected beams in free space, thereby generating two distinct path signals that carry highly correlated phase fluctuation information. A phase correction method in free space is then proposed to extract the phase information from the two signals, which is subsequently combined with a mixer to effectively mitigate phase fluctuations caused by atmospheric turbulence. To verity the performance of the designed laser gas sensor, NH3 is selected as the target gas for the experiment. Test results demonstrate that the low limit of detection for 5 ppm NH3 is 8 ppb ( $1\sigma $ ) in static experiments. Additionally, in free space, the fluctuation power spectral density (PSD) is suppressed below the 20-Hz offset frequency. These results indicate that the sensor effectively mitigates phase fluctuations in free space and holds significant potential for applications in airborne observation, ground-based remote sensing, and satellite remote sensing, offering a pathway for high-resolution measurements.