Fiber-optic Method Based On Spectral Absorption Of Hydrogen Sulfide Gas Sensor

As in recent years hydrogen sulfide (H₂S) gas has leaked out frequently in production and daily life, the safety issues on H₂S gas has gradually become the focus of attention, which made the requirement to develop a on-line real-time, fast, efficient H₂S gas sensing with high sensitivity,and low detection limit an ever more urgent one. Although a traditional non-optical H₂S gas sensor can achieve a low detection limit, it is vulnerable to be influenced by other gas components via cross-sensitivity, besides it has other disadvantages, including relatively slow response, low reusable rate, short life span and difficulty to achieve on-line continuous monitoring. For large-scale monitoring, traditional non-optical sensors for distributed detection of H₂S gas will be very expensive. An optical fiber H₂S gas sensor based on NIR absorption spectrum has been proposed in this paper. If combined with the weak optical signal detection technology and the optical technology, the sensor can meet all kinds of H₂S gas detection needs in production and daily life. Besides, the sensor can realize distributed detections of various gases if a tunable laser light source is applied.Based on the theory of selective absorption of NIR spectrum and Lambert-Beer law, a differential absorption detection method combined with a narrow-band light source has been used to detect the concentration of H₂S gas. The single isolated rotational absorption lineν1+v2+v3 around 1578 nm is chosen as the center absorption line of H₂S gas, which has an absorption intensity of approximately 1.3×10^-22cm^-1/(molecule·cm^-2)^-1. A narrow-band tunable laser with a 3dB width of 0.0157nm is used as the light source. The 400-nm gas absorption cell consists of 4 serial 100-mm short absorption cells, and each short absorption cell is formed by two micro-GRIN lens. To eliminate the adverse influence originating from the fluctuation of the source power, a differential system which compares the signal with the reference light is adopted.Through the analysis on the absorption spectra of H₂S gas at normal temperature and pressure, it is shown that the absorption coefficient of the center frequency is about 1.5×10^-2cm^-1·atm^-1, and the spectral broadening is mainly affected by the molecules' collision, and either of the Lorentzian or Voigt line-type is suitable for H₂S gas absorption spectroscopy. As commercial DFB lasers for gas detection are expensive, a narrow-band tunable fiber laser has been designed and constructed in this paper. The wavelength of the fiber laser is tuned by changing the temperature and the length of the Bragg grating. The temperature sensitivity and the voltage sensitivity are approximately 0.04066nm/℃and 0.00126nm/V respectively by tests. Finally, gas absorption experiments for proving the feasibility of H₂S gas detection are carried out, and the calibration of the system has been obtained by data processing and fitting. The gas sensitivity of this system is about 0.00035μw/ppm, and the minimum resolution of the system is about 3271ppm. The absorption coefficient of the center frequency of H₂S gas acquired in laboratory testing is about 2.6×10^-2cm^-1·atm^-1.