Driving Technology And Detection Performance Of Quantum Cascade Laser For Differential Absorption Lidar In VOCs Concentration Detection

Volatile organic compounds(VOCs)are important foregoing of air pollution,they have serious hazard to the environment and human health.Nowadays,as China's VOCs emissions are increasing rapidly,and large-scale unmanaged emissions is continuing to increase,well-organized VOCs emissions,especially in heavy polluted areas,is particularly necessary,and detection of VOCs is its first step.Differential absorption lidar(DIAL)is an effective method to obtain the spatial concentration profile of gas,as the absorption lines of VOCs are mainly located in the mid-infrared band,a DIAL system based on quantum cascade lasers(QCL)is proposed in this thesis.The QCL,as the detection light source,can generation the mid-infrared band laser,united with the DIAL of concentration profile detection,the proposed system can achieve the VOCs concentration effectively.In this thesis,our research focus on the driving technology of QCL,and the detection performance of DIAL for VOCs is also carried out preliminarily,the main work includes:(1)The scattering and absorption characteristics of the laser in the atmosphere are analyzed firstly,so as the gas concentration equation is deduced,then the factors that affect the detection performance are analyzed based on the DIAL theory.A DIAL system based on the QCL is proposed,and the detection performance with the absorption line broadening is studied in the open light-path.(2)A circuit model is constructed based on the electron rate equation and photon rate equation of QCL.Thus,the DC,transient and AC characteristics of the circuit model are simulated based on PSpice,then,the relationship between laser wavelength,power,and temperature,current are constructed,which will give the foundation for the selection and driving design of QCL.(3)A driving scheme,which is used to adjust the wavelength of QCL,with the current and temperature control model,is well designed.In order to restrain overshoot and noise,a high-precisio n DC.power with soft-start and filter is presented firstly.Then,a current control circuit,composed of DDS,amplification,voltage-current conversion and overcurrent protection circuit,generating the triangular superimposed sinusoidal signal for the wavelength adjustment is carried out.A temperature control circuit main by the LTC1923,is also proposed,which is expected to achieve the accuracy of 0.3?.Finally,a core controller with MCU C8051F020 is set up,and the program implement the drive control is provided.(4)The detection performance of formaldehyde and benzene based on DIAL using QCL is carried out,the optimization of detection wavelength for formaldehyde and benzene is performance based on selection principle of absorption spectrum and the effects of interfering gases in the atmosphere.Our results show that the DIAL system can detect Ippm formaldehyde within 580 meters and lppm benzene within 600 meters in the open length atmosphere,which can apply in the concentration detection of formaldehyde and benzene in the heavy chemical pollution area.The driving technology and detection performance of QCL for DIAL in VOCs concentration detection,is a preliminary exploration for the spatial detection of VOCs,which will give a reference for the effective detection and control of VOCs emissions in China.