Study On Intra-cavity Gas Analytical Technology Based On Erbium Doped Fiber Ring Laser

The detection of pollutant, flammable or explosive gases is an important contribution to the environmental protection and safety of manufacturing. Unfortunately the conventional gas monitoring methods are not able to detect and measure multi-gas real-time and simultaneously for some are sensitive to just one certain gas, needing chemical reagent, or simply are too expensive. Therefore the goal of this work was to develop a multi-gas monitoring system, which could real-time, simultaneously and sensitively detect and measure multi-gas qualitatively and quantitatively.The main idea and scheme to achieve the goal are as the following: with intra-cavity gas sensors being placed inside the erbium-doped fiber ring laser cavity, sensitivity may be greatly improved. Also, the wide spectrum range of the Erbium-doped fiber laser covers the overtone absorption lines of a number of important gases, thus may allow for multi-gas detection within the system. In addition, wavelength modulation technology could be applied to this system to further improve its measuring sensitivity and precision. Acetylene can be used as an example to explore the feasibility of this intra-cavity absorption system and analyze its precision of qualitative and quantitative measurement.To fulfill the goal mentioned above, the main work completed is as follows:First, a prototype of the intra-cavity gas detecting system based on the Erbium-doped fiber ring laser has been built and some primary factors affect the sensitivity of the system have been discussed. High accuracy qualitative detection is achieved by using fiber Bragg gratings to determine the wavelength of the absorption line, which ensures the feasibility of multi-gas measurement. The performances of quantitative measurements, including repeat accuracy, measurement precision and detection limit and so on, have also been evaluated by calculating the absorbance of standard gas. Factors inducing errors as well as the methods to eliminate them are discussed in this paper.Second, data processing approaches, which are designed in accordance with the features of the scanning spectrum of intra-cavity gas detecting system based on the Erbium-doped fiber ring laser, have been studied to eliminate the influence of background noise and the continuous spectrum envelope. Due to the multi-resolution feature of the wavelet transform, the signal that is reconstructed, after using the Birge-Massart strategy to deal with the wavelet coefficient, could be de-noised without losing the power of the absorption line. Wavelet transform zero-crossing method is shown to have the ability to fit the continuous spectrum envelope with high similarity, thus reducing the errors of spectrum extraction and improving the retrieval accuracy.Third, the application of the wavelength modulation technology in this intra-cavity system has been investigated. First, the feasibility of wavelength modulation technology is validated by numerical simulation. Then, experiments have been completed which confirm the advantages of the second harmonic spectrum. Experiments show that significant improvements can be achieved in repeat accuracy, measurement precision and detection limit by means of the wavelength modulation as opposed to use of the static scanning spectrum.