In addition to the atmospheric measurement, detection of dissolved carbon oxides and hydrocarbons in a water region is also an important aspect of greenhouse gas monitoring, such as CH₄ and CO₂. The first step of measuring dissolved gases is the separation process of water and gases. However, slow degassing efficiency is a big challenge which requires the gas detection technology itself with low gas consumption. Photoacoustic spectroscopy (PAS) is a good choice with advantages of high sensitivity, low gas consumption, and zero background, which has been rapidly developed in recent years and is expected to be applied in the field of dissolved gas detection. In this study, a miniaturized differential photoacoustic cell with a volume of 7.9 mL is designed for CH₄ and CO₂ detection, and a dual differential method with four microphones is proposed to enhance the photoacoustic signal. What we believe to be a new method increases photoacoustic signal by 4 times and improves the signal to noise ratio (SNR) over 10 times compared with the conventional single-microphone mode. Two distributed feedback (DFB) lasers at 1651 nm and 2004nm are employed to construct the PAS system for CH₄ and CO₂ detection respectively. Wavelength modulation spectroscopy (WMS) and 2^nd harmonic demodulation techniques are applied to further improve the SNR. As a result, sensitivity of 0.44 ppm and 7.39 ppm for CH₄ and CO₂ are achieved respectively with an integration time of 10 s. Allan deviation analysis indicates that the sensitivity can be further improved to 42 ppb (NNEA=4.7×10⁻¹⁰cm⁻¹WHz^−1/2) for CH₄ and 0.86 ppm (NNEA=5.3×10⁻¹⁰cm⁻¹WHz^−1/2) for CO₂ when the integration time is extended to 1000 s.

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.

An underwater hyperspectral imager (UHI) detection system is designed and developed, and a push-broom scan imaging mode was used. The telescope system adopts an image-side telecentric design with 22 mm focal length and 11 ∘ × 11 ∘ field of view. A prism–grating–prism structure was used as the spectral unit of spectrometer system to reduce spectral line bending. The UHI system reaches 5 nm spectral resolution in spectral range from 400 nm to 800 nm, and its modulation transfer function is greater than 0.62 at characteristic frequency in spectral range. An underwater hyperspectral detection experiment was carried out with an electric turntable, and the UHI system is of good imaging quality and spectral resolution, being able to distinguish various underwater targets.

Photoacoustic cell is an essential core component in a photoacoustic spectroscopy (PAS) gas sensing system, which is used to enhance and measure the absorption-induced acoustic signal. For certain applications, miniaturized photoacoustic cells with tiny inner volume are preferred to reduce the gas consumption. In this paper, a photoacoustic cell with inner volume of only 0.5 mL is designed based on differential Helmholtz resonators. After an initial simulation stage by the finite element analysis based on COMSOL software, the photoacoustic cell is fabricated by 3D printing. To facilitate laser power coupling, a micro fiber collimator is customized and integrated with photoacoustic cell to guide the laser beam. New concept of photoacoustic detector is proposed and integrated to a PAS system for trace acetylene detection. As a result, an excellent linearity of 0.99999 is achieved in the concentration range of 0 - 5000 ppm. A 1σ sensitivity of 0.13 ppm is measured within a response time of 10 s, the minimum detection limit is evaluated to be 0.3 ppm. Allan deviation analysis indicate that the detection sensitivity can be improved to 16.6 ppb at an integration time of 910 s.