To measure natural gas hydrate, a mid-infrared dissolved CO2 measurement system is designed using the tunable diode laser spectroscopy with ultra-compact multipass cell. The QCL laser of 4319 nm has a scanning range from 2315.05 to 2315.40 cm(-1), which contains (CO2)-C-12 and (CO2)-C-13 for isotope measurement and wide range of concentration detection. The optimized 3f + 1f wavelength modulation method reduces the limit of detection to 7.1 ppbv. An ultra-compact multi-pass cell with effective optical path length of 6.3 m and small volume of 15 ml is designed to achieve the system response time of 40 s. The dissolved CO2 sensing system has been used in deep sea natural gas hydrate detection.

A compact and high-sensitivity sensing system is required for the exploration of natural gas hydrates (NGHs) to measure dissolved CO2 in seawater. In this study, a mid-infrared (MIR) CO2-sensing system was investigated by wavelength modulation spectroscopy (WMS). The system contained an interband cascade laser (ICL) operating in MIR wavelength, an optical multipass gas cell (MPGC), and an MIR mercury cadmium telluride detector. A combination of three absorption lines was utilized to achieve measurement with a wide CO2-concentration range. Additionally, we proposed and introduced in detail a method of optimizing the operating temperature, pressure, and modulation depth. The optimum minimum detection limit (MDL) of 11 ppbv was achieved when the integration time was 145 s. In the northern South China Sea, a deep-sea test was performed, and the test results validated the satisfactory performance of the sensing system for deep-sea NGH exploration.

To solve the contradiction between high performance and portability of tunable diode laser absorption spectroscopy (TDLAS) systems, we propose a multipass cell (MPC) design method based on differential evolution (DE) algorithm, and realize intelligent optimization design of MPC with small volume and long optical path. MPC is the core component of the TDLAS system. The optical path of MPC determines the detection accuracy of the TDLAS system. The optical path is affected by the position and angle of incident laser, mirror spacing, and other parameters. The DE algorithm is used to optimize the parameters that affect the performance of MPC, and a compact MPC with a spherical mirror diameter of 25.4 mm, a laser reflection number of 183, an optical path of 5.98 m, and a mirror spacing of 31.7 mm is designed. Under the same spherical mirror condition, the detection limit of MPC designed based on the DE algorithm is increased from 0.91 to 0.76 ppm compared with the MPC commonly used seven-ring spot pattern. This has a guiding and exemplary role in the design and development of high space utilization MPC.

Multipass cell (MPC), with a long effective optical path length (OPL), is an effective method to improve measurement sensitivity in laser absorption spectroscopy for trace gas sensing. We present a space equation (SE) method to design MPC, which can solve inaccuracy caused by using paraxial-matrix (PM) method. The location and dimension of light spot pattern can be calculated accurately and intuitively through SE method. Meanwhile, actual effective OPL can be accurately calculated, SE method can reduce the interference effect, the direction of each ray can be obtained accurately, and, hence, designing the angle of outgoing ray is simple. By adjusting the mirror curvature, mirror space, incident angle, and incident position, we realized f o ur dense spot patterns with different multiple reflections in MATLAB simulation. We calculated the shape changes of light spot after multiple reflection and its influence on the design of outgoing ray hole size accurately. We also considered the evolution of dense pattern and found a method easy to obtain multiple ring pattern. We verified the validity of the proposed model and built a compact MPC with seven rings dense spot pattern. Such MPC offers an effective OPL of 36.48 m with 198 times of reflection. We verified the tunable diode laser absorption spectroscopy (TDLAS) technology to measure 610-ppmv standard methane. In contrast to PM method, the improvement of the signal-to-noise ratio (SNR) of 1.36 times is achieved when the volume of MPC is reduced by 3 times. In addition, the SNR can be improved to 6.6 times, while the volume of MPC is the same. Results validated the ability of the SE method to design MPC.

The study proposes a 30 X continuous zoom lens for 610 X 512 cooled mid-wave infrared focal plane detector array. Furthermore, we propose an optical system design that separates the lens from the front fixed group as a two-speed moving group, and combines the two-speed moving group, variable magnification group, compensation group, and rear fixed group into a composite continuous zoom system. To realize the above model, we design a cooling-type medium-wave infrared continuous zoom optical system based on the theory of diffractive optics. It has a working band of 3.7-4.8 mu m, F-number of 1, continuously variable focal length of 12-360 mm, and total optical length of only 160 mm. We optimize the image quality and cam curves of the system at 6 focal lengths. The system offers advantages of a large variable ratio, miniaturized structure, excellent image quality, and smooth cam curve, which meets the requirements of infrared thermal imagers.