Research On Active Laser Heterodyne Spectroscopy Gas Telemetry

With the development of economy,there is an increasing demand for gas detection in various scenarios such as atmospheric environment monitoring,industrial process control,and pollution emission monitoring.Laser absorption spectroscopy(LAS)can perform long-distance gas remote sensing,which has obvious advantages over other gas detection technologies including electrochemical method,flame photometry,and gas chromatography.The existing LAS remote sensing technology is usually based on a direct detection method,so that its detection distance and signal-to-noise ratio(SNR)are particularly dependent on the echo optical power,which results in greatly limited detection performance in non-cooperative target remote sensing applications.For the same absorption spectrum,the achievable SNR of the heterodyne detection system is usually 3-4 orders of magnitude higher than that of the direct detection system.Therefore,heterodyne detection can be used instead of the traditional direct detection,namely active laser heterodyne spectroscopy(ALHS).In ALHS,the emitted laser is divided into signal light and local oscillation light.After the signal light performs the remote sensing task,the echo signal light and the local oscillation light are combined for heterodyne detection.It is expected to improve the detection performance of non-cooperative target remote sensing.This dissertation has carried out research on ALHS to solve the problems of the lack of gas remote sensing theory,lack of research using near-infrared and all-fiber structure system,and lack of wavelength modulation technology,achieved the following results:(1)The measurement model of ALHS gas remote sensing technology was established.Both direct absorption based ALHS(DA-ALHS)and wavelength modulation based ALHS(WM-ALHS)were theoretically analyzed.The transfer function of the system was deduced,and the forward models for these two spectral detection techniques were built by Simulink to simulate the ALHS signals according to the set remote sensing conditions.Furthermore,the inversion models were also established to invert the gas concentration by processing the heterodyne signal received by the photodetector.(2)The wavelength modulation spectroscopic measurement model was improved for large absorption gas detection.In the process of simulation,it was found that the existing normalized second harmonic method by the first harmonic(WMS-2f/1f)has a large calculation error under high absorption conditions.An improved WMS-2f/1f model under large absorption was proposed.Combining the gas absorption and the background signal into the calculation of the first harmonic.By simulation,this method reduced gas concentration calculation error from 5.42%to 0.39%at an absorption rate of 21%,when compared with the existing calculation method.(3)A set of near-infrared ALHS gas remote sensing system based on all-fiber structure was designed and verified by experiments.The DA-ALHS and WM-ALHS measurement systems were built using a butterfly packaged distributed feedback laser with a pigtail of 1.65?m and a fiber splitter and coupler,with CH₄ as the target gas and an aluminum plate as a reflector at a distance of 1m.The detection limit of DA-ALHS is 120 ppmv in the measurement time of 120 seconds,while the detection limit of WM-ALHS using 4f/1f method is 95.13 ppmv,and the detection limit using 2f/1f method is15 ppmv.This dissertation innovatively combines heterodyne detection technology with tunable laser absorption spectroscopy technology.From measurement model to experimental research,it can provide experience and reference for follow-up research,and shows the application prospect of this technology in the field of remote sensing.