Recently, a high sensitivity and compact laser absorption spectroscopy gas sensor based on a novel compact dense-pattern multipass cell (DP-MPC) in conjunction with a fiber-coupled distributed feedback diode laser was developed by the research group of Prof. Xiaoming Gao from the Anhui Institute of Optics and Fine Mechanics (AIOFM), which has been published in Sensors & Actuators: B. Chemical (220, 1000-1005, 2015).
The sensors based on laser absorption spectroscopy offer the unique advantages for fast, self-calibration, highly sensitive, nonintrusive and in situ quantification without any sample preparation. Therefore, Laser absorption spectroscopy sensors were found widely applications in environmental monitoring, medial diagnostics or industrial process control. According to the Beer–Lambert absorption law, the detection sensitivity by absorption spectroscopy is proportional to the molecule absorption path length. In general, sensitive detection scheme is achieved with a traditional multipass cell. However, for the compact Herriott configuration using conventional design, the number of the light passes inside the cell is limited by the overlapping of light spots on the cell mirrors, which limits the achievable optical path length within a compact cell size. Therefore, the development of compact, handheld and highly sensitivity sensors based on absorption spectroscopy was limited.
Recently, with an aim to create gas cells that deliver long absorption path lengths in a smaller package, Prof. Xiaoming GAO and Dr. Kun LIU et al. developed a novel DP-MPC. The developed DP-MPC consisted of two 2" silver coated concave spherical mirrors separated by a distance of 12 cm. With a careful optical design and alignment, a novel dense pattern of 7 circles was configured on the mirror after laser beam passing 215 times between two spherical mirrors without spot overlapping and an effective optical path length of 26 m was achieved with a sample volume of 280 cm3. Highly sensitive detection of atmospheric methane (CH4) was performed by the DP-MPC in conjunction with a fiber-coupled distributed feedback diode laser operating at 1.653 μm. High performance with a minimum detection limit of 100 ppb in 1 ms and a measurement precision of < 80 ppb have been achieved using wavelength modulation spectroscopy approach.
This low cost, compact and wavelength-independent widely usable DP-MPC is a powerful new tool for laser-based sensing field applications, in particular for gas sensing when small sensor footprint, long optical path length, and simple construction are high priorities, such as in harsh environment and weight-limited unmanned aerial vehicle (UAV) or balloon-embedded observations.
The work was supported by the grants from the National Natural Science Foundation of China and Youth Innovation Promotion Association of CAS.
Photograph of the beam pattern visualized using a red diode laser beam (left) and software simulated pattern (right).
Ambient CH4 concentration variation during two days continuous measurements