Equivalent Circuit Model Of Four-Level Quantum Cascade Laser And Its Application

With the development of science and technology, a fast and high precious gas-detection system is in dire need. The traditional ways have some disadvantages, like low precision, being affected and so on. So they can’t meet the demand. Using a principle that the gas can absorb middle Infrared light, we can sovle this problem. And this method is fast, high precious, unaffected and convenient. Lack of proper light source has been limiting the use of this technology.Under this background, we do some studying work about problems of gas-detection system. Firstly, it is about the light. Quantum cascade laser (QCL) is a new kind of laser and can laser light of high quality in room temperature. So it is fit for being the source of high precious gas-detection system. Secondly, it is that the singal of gas-detection system is very weak. Only through amplifying the singal can the system reach a high sensitive and precious level.In this papre, we adopt an approach based on rate eqution. Through analyzing the level scheme and operational principle of QCL, we get the rate equations of QCL. Based on this, an equivlent circuit model of four-level QCL is build. Using the PSpice software, we do some simulation of the built equivalent circuit model of QCL inculuding DC, transient and AC simulation. We obtain the relationship between output optical power and current. Besides, the threshold current, turn-on time, turn-off time and 3-dB bandwidth of QCL is also gotton. The equivalent circuit-model is very useful for the design of driving circuit of QCL and it is also critical for designing the Optoelectronic Integrated Circuit (OEIC) of QCL. Then in the process of using, we can take full advantage of the characters of QCL in order to satisfy the need of gas-detection system.In order to improve the sensitivity, a gas-detection system based on opto-acoustic method is designed. When the frequence of laser beam is in accordance with the eigenfrequency of the cavity, the cavity will occur in resonance. Thus the opto-acoustic signal will be lagerly amplified. Using the finite element method, we calculate the eigenfrequencies of cylindrical cavity. The calculated result is consistent with the eigenfrequencies in the theory. A T-shape cavity is proposed, which can better suppress the interference of noise. Using COMSOL software, we calculate the eigenfrequencies of T-shape cell and the distribution of pressure inside. Besides, we use Labview software to deal with the photoacoustic signal and we develop related program.