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.

Material extrusion (ME) type 3D printing has been widely utilized through various types of systems depending on the fabrication methods, materials, and precision to fabricate complex workpieces that cannot be made with conventional methods. This study provides basic considerations in response to the current demands on performance evaluation of ME type 3D printing related to dimensions as well as the realization of the guidelines to be established in the near future. As a simple specimen for these purposes, 2D and 3D hole-plates were designed and fabricated by using a ME type entry-level 3D printer. For evaluation of dimensions on the specimen, both specimens were measured by a calibrated tool-maker’s microscope which is to length standard. The measurement parameters were the center position of the holes, the diameter of the holes, and the circularity error of the holes.

A spectral-domain interferometer with dual reference paths and orthogonal polarization states to avoid measurement errors when interference signals overlap is proposed and realized. In our previous study, by using dual reference mirrors, two inherent problems of the spectral-domain interferometer, the non-measurable range and the directional ambiguity problem, were successfully solved. However, because of the overlap of interference signals, the absolute distance values were distorted. In this study, the polarization states of beams from two reference paths were made orthogonal to eliminate the interference signal between them, so that the overlap can be essentially avoided. First, we performed a numerical simulation on the measurement error with respect to the degree of superposition of the interference signals. Simulation results show that with the previous method the measurement error can be up to approximately 1 μm within the overlap region, but the proposed method drastically reduced this error to below 100 nm. Then, the proposed method was experimentally realized and verified. In conclusion, the proposed method can measure the absolute distances without the inherent problems as well as the measure errors caused by the overlap of the interference signals.

To measure the depth of the through silicon vias on 300 mm silicon wafers, a measuring machine was developed. Based on the preceding research in a laboratory environment, the machine was designed and built by modifying the optical probe for reducing the mass, combining a visible optical microscope to monitor the location of the measuring points, and constructing the metrology frame for large silicon wafers. The depths of the three different-sized through silicon vias were measured repeatedly to estimate the repeatability. Moreover, comparative measurement was carried out to verify the measured depth values. The total measurement time was also estimated by measuring 110 through silicon vias at different locations. According to the measurement results, the measurement performance satisfied the technical requirements of the industry in terms of repeatability, accuracy, and measurement time.