Highly Sensitive Carbon Dioxide Gas Sensor Based on 2 m Laser Diode and Herriott Multi-pass Absorption Cell (Invited)

The measurement of carbon dioxide plays a very important role in breath diagnostics,monitoring industrial safety, monitoring greenhouse gases, observing industrial chemical reactions, measuring combustion completeness,and avoiding fires. Tunable Diode Laser Absorption Spectroscopy (TDLAS) technology has the advantages of high selectivity,high sensitivity,and strong anti-interference ability. The TDLAS systems with a multi-pass cell increase the minimum detection limit by increasing the length of the absorption path. Increasing the absorption path has strict requirements on the divergence characteristics of the laser,and requires enough space. By designing the multi-pass absorption cell,the requirements on optical beam quality can be reduced. The volume of the system can be compressed,and the spatial resolution of the measurement can be improved. In this paper,a highly-sensitive carbon dioxide gas sensor based on a 2 mu m laser diode and a Herriott multi-pass absorption cell is designed. The basic parameters of the Herriott multi-pass absorption cell are calculated by paraxial approximation. The solution includes the incident position and the incident angle of the incident light,and the distance between the two spherical concave mirrors. According to the simulation results, a Herriott type multi-path absorption cell is fabricated. The multi-pass cell can fold the optical path with an optical path length of 2.6 m within a length of 54 mm. The minimum detection limit of the system is related to the absorption line intensity of the selected absorption peak. In order to reduce the cost of the system,a distributed feedback laser diode with a center wavelength of 2.004 mu m is used to cover the strong absorption line of carbon dioxide molecules at 4 989.9 cm(-1). The second harmonic signal with zero background noise at the detection frequency can be detected by 2f-wavelength modulation spectroscopy technology. This technique effectively reduces the noise of the system. The signal measured by the system has white noise,and the method of signal averaging requires a long measurement time. A self-programming real-time Kalman filtering technology on the software level helps to improve the anti-interference ability of the system. Interference by background white noise and abrupt changes to the signal can be avoided. The experimental results show that with the developed TDLAS sensor,the minimum detection limit for carbon dioxide can reach 0.18x10(-6) with an integration time of 1 s. After the self-programmed real-time Kalman filter,the minimum detection limit can be further improved to 0.13x10(-6), resulting in an increase of similar to 30%. The TDLAS sensor was used to continuously monitor the indoor carbon dioxide concentration for 8 h. An outdoor 24 h continuous monitoring for carbon dioxide concentration on the building roof of Jinan University. This work demonstrates the stability and robustness of the developed TDLAS sensor. The sensor can respond in real time to changes of carbon dioxide concentration in the ambient air. The real-time Kalman filter carried by the system has better stability,which can effectively improve the minimum detection limit of the system. The optical multi-pass can be accurately designed by the method of paraxial and non-paraxial approximation. In the next work,a dense multi-pass absorption cell can be designed to fold longer optical paths between the same two mirror. However,the light spot of the dense multi-pass absorption cell will become irregular with the increase of the number of reflections. In this case,the incident hole and exit hole of the cell need to be manufactured with higher precision. If the multi-passed light cannot pass through the exit hole completely,the interference of the absorption cell will increase resulting in a decrease in signal to noise ratio. The Herriott multi-path absorption cell can avoid this problem very well.