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.

Optical dimensional metrology has playing a long-term key role from high-precision engineering to large-scale industrial manufacturing. Various methods of optical dimensional metrology have been proposed and demonstrated to respond to the ever-growing industrial demands as well as fundamental science demands for the measurement precision and range. However, most of them demonstrated under laboratory conditions have a long way to go outside the laboratory. Here, we present a progress review on optical modulation technique-based dimensional metrology, which has already been used in real applications and has been commercialized. Amplitude modulation (AM) and frequency modulation (FM) based dimensional measurement techniques are described with their operating principles, and recent progresses and applications in 3D imaging are presented in this review.