Design Of High-power Driver For Acousto-optic Q Switch

Acoustic Optic Q-Switch Driver(AOQSD) is a kind of RF pulse-power signal source. By driving Acoustic Optic Q-Switch placed in Laser’s resonant cavity, the Q value can be adjusted, consequently, the high peak power, high pulse repetition frequency(HPRF) and narrow pulse width laser can be achieved. AOQSD is a very important part of high-power solid-state laser system. It has been widely used in the civilian laser processing field such as laser marking, laser cutting, laser engraving, and the defense equipment including laser radar, laser ranging, photoelectric confrontation and so on.In this paper, based on the laser application system, the performance of AOQSD including power, efficiency, pulse fall-time and EMC has been analyzed. Moreover, it has discussed the basic theory of power amplifying circuit of the driver in detail. The main designing contents include frequency source, modulation circuit, amplifier circuit, protection circuit and other functional circuit; also include EMC and thermal designing of the driver. To simplifies the design of oscillating circuit, it select oscillator as the frequency source. Use the variable gain amplifier(VGA) as the modulator of Q switch driver has been proposed firstly, which has fast modulation speed, broken ratio, good linearity, and the overall performance is better than the traditional modulation method such as gate modulator, drain modulator, RF switch modulator etc. The LDMOS is used as power amplifying device. It designed bias circuit and matching circuit of power amplifier based on ADS simulation software to improve the design efficiency, designed temperature compensation circuit for gate voltage bias on the analysis of thermal characteristics of LDMOS devices. By analyzing the cause of failure factors in the driver, it presents the structure of closed-loop protection which core technology includes over-voltage protection, over-current protection, temperature protection, SWR protection, and designed current protection circuit based on high side current detection techniques; designed the standing-wave protection circuit that used directional coupler and detector as the core. The design principles and methods of electromagnetic compatibility are introduced from several aspects, such as grounding, PCB design, filtering, shielding and so on. Thermal analysis was carried out on the LDMOS device which is the main heating device in the driver. The heat dissipation area and mode are determined to the maximum dissipated power in the whole temperature range.According to the circuit designing, the AOQSD prototype can be achieved, moreover, the performance of the prototype has been tested. Output power 105 W, efficiency 65%, pulse fall-time 92 ns, harmonic suppression-35 dB, on-off ratio 68 dB, full temperature working range of power fluctuation is less than 1dB. The test results show that the all design goal of this paper can be reached.