The Design And Realization On Digital Driver Of Laser Diode

With the rapid development of science and technology, the techniques of semiconductor laser diode have been applied widely in many fields including industry, high tech and science research. Semiconductor laser diode has the excellent character of high efficiency, small size, low weight, high reliability and direct modulation. But semiconductor laser diode is a kind of device with high power density and sky-high quantum efficiency. Even very little change of drive current or temperature will cause its great change of output light power and device parameters. These problems put forward higher request to the laser diode driver. However, with low control precision, discommodious operation and low safety, the traditional laser diode drivers in home market can not fit the request. So the research on laser diode driver has practical significance. Based on the original analog laser diode driver, this paper designed a digital laser diode driver, which has overcome most of these shortcomings and made up for the deficiencies of the existing products.In this design, we use both analog circuit and digital circuit. The digital part includes STN89C51 and its control program, whose function is to control the whole system to work in phase. The microcontroller uses the D/A converter to change the digital code to the analog voltage, which provides the input voltage for the constant current source. It also uses the A/D converter to collect and to send back the current parameters of the system to ensure the system work correctly. At the same time, it sends the system status and the output parameters to the LCD screen to display. And the function of limiting current is also realized by the SCM program. The analog part is mainly composed of the constant current source circuit and two protection circuits. The constant current circuit converts the voltage signal to current to drive the laser diode. In the design of the constant current circuit we use two feedbacks: the current feedback and the power feedback. We used a sampling resistance to convert the current to voltage and magnified it to realize the current feedback. The purpose of this feedback is to raise the stability of the output current. We use a photoelectric diode (PD) to realize the power feedback. The PD samples the light power of the laser diode and converts it to proper voltage signal, which is sent back to the feedback port. The purpose of the power feedback is to raise the stability of the light power of the laser diode. In the analog part, we designed two protection circuits: preventing surge circuit and delayed soft-start circuit. The preventing surge circuit uses a relay and a RC charging circuit. The relay is opened when the possible surge impacting time is over, which prevents the possible damage to laser diode efficiently. The soft-start circuit uses the discharge character of the RC circuit to release the input voltage of the constant current source softly from zero to the normal value, which makes the output current raise from zero to the normal value softly. This protection method can avoid the possible damage to the laser diode due to the sudden change of the current.The whole paper is divided to five chapters. In chapter one, we explain the significant of the research and the development status of the laser diode driver interiorly and overseas. Chapter two is the theory foundation of this paper. In this chapter, we introduce some basic theories of the laser diode and its driver. Chapter three is the holistic explain of the design project. In this chapter, we firstly bring out the system design project and then expound according to the system blueprint. Afterwards we introduce the character of some component including the microcontroller, D/A converter, amplifier and MOS FET. Chapter four is the core part of the paper. In this chapter, we introduce the design and realization of the system in detail. In the hardware part, we introduce the design project and the working principle of the coder circuit, D/A convert circuit and protection circuit etc. In the software part, we introduce the design idea and the flow of main program and sub programs. In the last part of chapter four, we show the character test of the system. We explain in detail the test measure, the using instruments and the test result. In chapter five we conclude the paper and bring out the next research idea.