The LIDAR principle is used in a variety of fields, including large-scale pipeline facility management, industrial disaster safety control, and atmospheric environmental monitoring, to employ the remote gas detection technique. In this study, we designed and implemented a remote detection method for N2O gas leaks using absorption spectroscopy based on frequency modulation of a Mid-IR quantum cascade laser (QCL) with a wavelength of 4.5 μm. We direct the frequency-modulated beam, locked to a single absorption line of N2O, to a leak hole on a target surface within a range of approximately 50 m. For area scanning around the leak point, we use a galvano scanner to deflect the probe beam. The back-scattered beam from the diffuse target surface is then collected by a Cassegrain telescope with a diameter of 300 mm and detected by an InSb photo-detector with high photon sensitivity. To process the detected signal, we utilize fundamental and second harmonic detection with a lock-in amplifier, resulting in a relative gas concentration expressed as the second harmonic signal normalized by the fundamental signal. Our test results demonstrate that this proposed method can detect gas leaks as small as 0.005 sccm at a distance of 50 m.

Light detection and ranging (LiDAR) is one of the most efficient technologies to obtain the topographic and bathymetric map of coastal zones, superior to other technologies, such as sound navigation and ranging (SONAR) and synthetic aperture radar (SAR). However, the measurement results using LiDAR are vulnerable to environmental factors. To achieve a correspondence between the acquired LiDAR data and reality, error sources must be considered, such as the water surface slope, water turbidity, and seafloor slope. Based on the knowledge of those factors’ effects, error corrections can be applied. We concentrated on the effect of the seafloor slope on LiDAR waveforms while restricting other error sources. A simulation regarding in-water beam scattering was conducted, followed by an investigation of the correlation between the seafloor slope and peak timing of return waveforms. As a result, an equation was derived to correct the depth error caused by the seafloor slope.