Photoacoustic Spectrometer Based On Combination Of Fiber Laser And Fiber Amplifier

Trace gas detection plays a very important role in the fields of atmospheric pollution monitoring, power transformer diagnosis, human health diagnosis. Technologies such as optical sensors and laser spectrum are widely applied in trace gas detection. Near infrared photoacoustic spectroscopy is widely studied and applied for its advantages such as high sensitivity, large dynamic detection range, fast response, and online detection in trace gas detection.The theory of photoacoustic spectroscopy including photonic absorption and the generation of photoacoustic signal is discussed in detail under the assumption of double energy level system. And also, technologies of wavelength modulation and second harmonic detection are introduced, which are the theoretical basis for trace gas detection based on photoacoustic spectroscopy.As we know that it is the first time the combination of tunable erbium doped fiber laser (TEDFL) and erbium doped fiber amplifier (EDFA) is applied to photoacoustic spectroscopy detection. The principle of light amplification of erbium ion, the principle and structure of ring optical fiber laser and EDFA with double-direction-pump are introduced. Advantages of the TEDFL based photoacoustic spectroscopy system are also given out after the comparison between TEDFL and tunable diode lasers (TDL).A photoacoustic spectrometer based on TEDFL and EDFA is built up and trace acetylene and ammonia are detected by choosing absorption lines in 1.5μm band with the system. Signal to noise ratio (SNR) is improved by using wavelength modulation and second harmonic detection. The experiment results show that under the conditions of room temperature and atmospheric pressure, the detection limit of acetylene and ammonia reaches 1.3ppb and 6ppb respectively and there is an excellent linear relationship between second harmonic peak values and concentrations. Significances of this paper are as follows: Firstly, the system detection sensitivity limits for acetylene and ammonia gases could meet most demands of acetylene and ammonia detection in the fields of environment monitoring, industry, agriculture, and medicine. And this work would be helpful for pushing the wide application of photoacoustic spectroscopy technique and accelerating the steps that photoacoustic spectroscopy walks out from lab to industry.