| Laser intra-cavity absorption spectroscopy has been of concern to scholars from various countries since it was proposed. Because of its high sensitivity, it is widely applied to the detection of weak absorption in gas, liquid and plasma. With the development of optical fiber technology, it is possible to measure intra-cavity absorption spectroscopic signal using a distributed structure, but there are no mature commercial instruments available yet. In this paper, an all-fiber, quasi-distributed intra-cavity absorption spectroscopic gas sensing system based on the fiber ring laser is presented, tested and applied to trace gas analysis.Based on the analysis of the components and simplification of the transfer functions, the generation conditions of intra-cavity absorption spectroscopic signal are discussed, and the work models of sensing system including steady-state model and transient model are derived. According to the models, the amplified spontaneous emission spectrum of fiber ring laser and intra-cavity absorption spectrum of gas are calculated and simulated. It is confirmed that using wavelength sweep to acquire intra-cavity absorption spectroscopic signal with the sensing system is feasible.To decrease the interference caused by amplified spontaneous emission and etalon noise, low-frequency wavelength sweep combined with wavelength modulation technique is applied to the system. The modulation parameters are determined through theoretical analysis and systematic investigations. Seventeen absorption lines for 1% acetylene gas are observed in the wavelengths region from 1526.5 nm to 1537 nm. To enhance the sensor performance, the selection of the tunable F-P filter and the mode of wavelength scanning are optimized. Twenty-seven absorption lines are observed between the wavelengths of 1525 nm to 1545 nm in wavelength sweep sensing experiments with the new system. The obvious improvement of sensitivity indicates that the optimization is efficient. The optimal optical path length of gas absorption for this system is studied using gas cells with different lengths.Spectra extraction is studied with four methods including wavelet transform zero-crossing, wavelet transform iteration and wavelet transform combined with morphology to extract absorption spectra from experimental spectral data. The correlation analysis of low-frequency modulation spectrum indicates that the harmonic spectrum is proportional to the Fourier component of modulation spectrum, which can be calculated by the discrete Fourier transform. For intra-cavity absorption modulation spectroscopy, the calibration method is proposed that the calculation of gas concentration and wavelength of absorption line are performed on the basis of the second harmonic and third harmonic, respectively. The quantitative measurement ability of the system is analyzed by using single-line method and multi-line average method to calculate concentration calibration error respectively.To avoid the uncertainty of the wavelength values of the absorption lines, the method to calibrate the wavelength value by introducing the Bragg gratings as reference for intra-cavity absorption spectroscopy is investigated. Firstly, introduce the Bragg grating array as wavelength reference. The peaks of gas absorption spectrum and grating reflection spectrum are found through Gaussian fitting. Secondly, use known grating wavelengths to create the calibration curve between driving voltages of F-P optic filter and the wavelengths by polynomial fitting. Finally, calculate the center wavelengths of absorption lines according to the driving voltages corresponding to the peaks. The experimental results show that the error in wavelength calibration is reduced to 0.1 nm, which meets the requirement for multi-component gas analysis. |
PDF Full Text Request