Study On High-Speed Middle-Infrared Frequency Modulation Spectroscopy Based On All-Optical Modulation Of Quantum Cascade Laser

As a high-performance mid-infrared coherent light source,quantum cascade laser has the advantages of high power,narrow line width,tunable output wavelength and room temperature operation.It allows the laser subbands to be designed to match any wavelength in range from 3 ?m to 100 ?m.At present,the quantum cascade laser is widely used in wireless optical communication and spectroscopy for gas detection.Quantum cascade lasers are unipolar devices in which electrons are the only carriers.It is an advantage which achieves all optical modulation by near-infrared illuminating active range of quantum cascade laser facet,the amplitude modulation and frequency modulation can be achieved at same time,and all optical modulation avoid the effect of parasitic capacitance,its modulation bandwidth is predicted to be more than 100 GHz in theory.With the development of society and economy,the problem of environmental pollution is becoming increasingly conspicuous,which seriously threatens people's health and further development of the society and economy,so it attracts much attention of the country all over the world.For atmospheric environmental monitoring,the gas detection requires high sensitivity,low detection limits,real-time,wide range,long distance,and so on.Compared to other gas detecting technologies,laser spectroscopy based gas detecting technology is one of the main development trends because it has the advantages of high detection speed and sensitivity,low detection limits,contactless and without chemical reaction.To take full advantage of the infrared absorption gas fingerprint spectrum,the laser spectroscopy based gas detecting technologycan realize qualitative and quantitative detections.Compared to regular laser spectral technologies,frequency modulation spectrum technology has the advantages of high signal to noise ratio,high sensitivity and low detection limits.But the middle infrared frequency modulation gas detection spectrum technology is not yet a mature technology,especially because of its slow detecting speed,so it is difficult to realize real-time gas detection.In view of the existing all-optical modulation techniques,this paper carried out the research of amplitude modulation and frequency modulation.Researching the effects of each parameter on the amplitude modulation and frequency modulation,and selecting the optimal parameters.This project supported by the national natural science fund(The infrared quantum cascade laser's basic theory and technology research of high speed all optical modulation.61077057).The existing gas detection technologies could not take into account the detection speed and sensitivity,and detection limits,simultaneously.To solve the problem,combining the advantages of infrared spectrum gas detection with the outstanding characteristics of quantum cascade laser,we present a high-speed middle-infrared frequency modulation spectroscopy based on all-optical modulation of quantum cascade laser.To use the near-infrared light beam,it can realize the high-speed all-optical modulation of quantum cascade laser.A high-speed middle-infrared frequency modulation spectroscopy-based gas detection system was built andrealized the highspeed and high sensitivity,and low limits detection of Carbon monoxide.Compared todirect absorption spectroscopy detection technology,the high-speed middle-infrared frequency modulation spectroscopy-based gas detection technology of this dissertation reduced the detecting gas concentration by 7 times and increased the detection sensitivity by 2 times.The work of this dissertationeffectively solved the conflict between the detection speed and sensitivity,and detection limits,and realized real-time,high sensitivity,and low limits gas detection.The main contributions of this dissertation are listed as follows:(1)The optimal physical model of quantum cascade laser is proposed,to address the restrictions such as unable to conduct the research on the optical properties of laser and electron temperature at the same time,which brought by the traditional model of quantum cascade laser.Numerical algorithm was established,and C++ programs was written based on the model.Numerical simulation of the output characteristics of quantum cascade laser was accomplished,including optical radiation and electron temperature.What's more,the effect of electron temperature on laser output characteristics was further studied,which laid a foundation for further study of all-optical modulation mechanism of quantum cascade lasers theoretically.(2)Based on the theory of all-optical modulation,all-optical modulation experiment system of mid-infrared(4.6 ?m)distributed feedback quantum cascade lasers was established.High-speed modulated of output light intensity and wavelength was accomplished by stimulating the active area of the output end face of quantum cascade lasers by using near infrared coherent light.All-optical modulation experiment of quantum cascade lasers was conducted,in which the effect of light power of near infrared,as well as wavelength and frequency modulation on the quantum cascade laser I-V characteristics,I-L characteristics,the intensity modulation depth and frequency modulation coefficient.The best scan pulse width(200 ns)and the best modulation frequency(200 MHZ)were optimized.The optimal near-infrared light modulation wavelength(1550 nm)was determined,combining with the photo-luminescence experiments.(3)The high-speed mid-infrared frequency modulation spectroscopy gas detection system based on quantum cascade laser was built.Its wavelength scan time is 200 ns,repetition frequency is 10 KHz,all-optical modulation wavelength is 1550 nm,and modulation frequency is 200 MHz.The system's wavelength calibration wascompleted.And the main performance parameters,such as: center wavelength,spectral scanning range,spectral resolution,stability and so on have been experimentally tested.The toxic Carbon monoxide(CO)was selectedas detection object.And its FM spectra were obtained within 200 ns.The FM spectra of CO gas which has different concentration were measured.Thereby the FM spectrum CO gas concentration detection work curvewas obtained.CO gas concentrationswere quantitative tested.In the same optical system,the comparative experiment between the high-speed mid-infrared laser frequency modulation spectroscopy gas detection method and the traditional direct absorption spectra gas detectionmethod was completed.Experimental results show that the gas detection sensitivity of high-speed mid-infrared laser frequency modulation spectroscopy gas detection methodimprove 2 times,while the gas detection limitreduce 7 times,so the method greatly improves the performance of gas detection.Further experimental studieshave shown that increasing the near-infrared modulated optical power,is expected to make thedetection sensitivity and detection limit of high-speed mid-infrared laser frequency modulation spectroscopy detection methodbeen further improved significantly.