This paper presents an underwater high-precision line laser three-dimensional (3D) scanning (LLS) system with rotary scanning mode, which is composed of a low illumination underwater camera and a green line laser projector. The underwater 3D data acquisition can be realized in the range of field of view of 50° (vertical) × 360° (horizontal). We compensate the refraction of the 3D reconstruction system to reduce the angle error caused by the refraction of light on different media surfaces and reduce the impact of refraction on the image quality. In order to verify the reconstruction effect of the 3D reconstruction system and the effectiveness of the refraction compensation algorithm, we conducted error experiments on a standard sphere. The results show that the system’s underwater reconstruction error is less than 0.6 mm within the working distance of 140 mm~2500 mm, which meets the design requirements. It can provide reference for the development of low-cost underwater 3D laser scanning system.

A novel TT-type resonator was proposed for the first time, to our knowledge, to realize differential photoacoustic (PA) detection for trace gas measurement. The special design of the TT-type resonator allows us to install the microphone at the resonant center of the acoustic field to maximize the use of the absorption-induced PA signal. To meet the requirement of low gas consumption and easy integration, the TT-type resonator-based PA cell was fabricated as a fiber-coupled module with an inner volume of only 1.1 ml. For validation, the TT-type PA cell was integrated to a photoacoustic spectroscopy (PAS) system for acetylene detection. As a result, a linearity of 0.99999 was achieved in a concentration range from 0 to 5000 ppm with a noise equivalent sensitivity of 101 ppb. The proposed TT-type resonator contributes a new style of PA cell structure to the field of PAS gas detection, combining the advantages of easy integration, low gas consumption, differential detection, and photoacoustic enhancement together.

Fixed wavelength modulation spectroscopy (FWMS) featured as that wavelength is modulated around a constant value instead of a dynamic scanning ramp in the standard wavelength modulation spectroscopy (WMS). To eliminate the influence of center wavelength drift on absorption measurement in the FWMS system, a software-based proportional integral derivative (PID) controller is developed to lock the laser wavelength at methane line 1653.72 nm for CH ₄ detection in this study. Moreover, two crucial parameters of PID, gain and integral time are optimized to improve the laser wavelength locking performance. As a result, the laser wavelength is locked to the target CH ₄ line with a 1σ fluctuation less than 406 kHz (better than 3.7×10 ^-3 pm) with a robust 100% locking success rate, which is the best result reported so far to our knowledge. Based on the wavelength locked FWMS CH ₄ sensor, linearity is tested to be R ² = 0.998 in low concentration range of 0-1000 ppm. Detection sensitivity is 52 ppb at 0.1 s and could be improved to 36 ppb if the integration time is extended to 1.4 s. Compared with standard WMS, CH ₄ detection stability is improved by 5 times.

In this paper, we propose a new method to improve the position measurement accuracy for Laguerre-Gaussian beams on a quadrant detector (QD). First, the error effects of the detector diameter and the gap size are taken into account, and the position error compensation factor is introduced into the conventional formula. Then, in order to reduce the number of parameters, the concept of effective radius is proposed. Thus, a new analytical expression is obtained with a best fit using the least square method. It is verified by simulation that this approach can reduce the maximum error by 97.4% when the beam radius is 0.95 mm; meanwhile, the root mean square errors under different radii are all less than 0.004 mm. The results of simulation show that the new method could effectively improve the accuracy of the QD measurement for different radii. Therefore, the new method would have a good prospect in the engineering practice of beam position measurements.