Research On Miniaturization Of Frequency Control System For Radio Frequency Excited CO₂Waveguide Laser

Radio-frequency-excited waveguide CO2laser can be used in fields of THz laserpump, laser radar, heterodyne detection, which require the high standards of the CO2laser frequency stabilization. So, it is very significant to study frequency stabilization ofRF waveguide CO2laser.To meet the requirements of CO2coherent laser radar system, in this paper, wedesigns a frequency stability control system to control the frequency of dual-channelZ-fold radio-frequency-excited waveguide CO2laser. The control system includes twoparts: active frequency stabilization system and offset frequency locking system. Theactive frequency stabilization system stabilizes the main vibration frequency of pulsedlasers with the method of finding the minimum pulse time. Meanwhile, the offsetfrequency locking system stabilizes the heterodyne frequency between local oscillatorand main oscillator near a given value by controlling the cavity length of local laser.The control system uses one detector unit to provide the input signals of frequencystabilization system and offset frequency locking system at the same time. By applyingfor FPGA and PIC microcontroller, the frequency control system consists of signalshaping circuit, FPGA, PIC microcontroller, DAC, high voltage amplifier and so on, asdesigned system caters to the requirement of miniaturization,The main research contents of this paper are as follows:(1)A time counter and offset frequency discriminator are designed on FPGA. Thephase-locked loop doubles the clock frequency to improve the accuracy of build-up time.High probability value and average filter is conducted to reduce the error time andfrequency. We complete the conversion of build-up time and frequency, display the dataon the digital, and also transmit them to the microcontroller.(2)PIC microcontroller is selected as the control system, which controls thefrequency of two lasers, and the frequency data is processed by using the interruptprogram. Internal DAC of PIC is used to provide the reference voltage of the filtershaping circuit. This reference voltage will be changed as the amplitude of offsetfrequency is changed. Internal timer1of PIC is used as time-sharing system to outputthe voltage loaded on master and local lasers, and internal SPI of PIC is used to realizethe serial communication with TLC5615.(3)In this paper, we build a simulation platform to verify the correctness of thecontrol system. According to the experimental results, the frequency stabilizationsystem works correctly and the results of the simulation experiment show that the offsetfrequency can be controlled in less than±0.5MHz.(4)In further experiment of the control capability of the system on the frequencystabilization and offset frequency locking of dual-channel RF active waveguide CO2 laser, the experimental results show that the frequency stability reaches10-8, and theprecision of offset frequency locking reaches±5MHz.In this paper, we design a frequency stability control system. FPGA is used toachieve the accurate measurement of pulse build-up time and offset frequency. PICmicrocontroller is selected as the control system, it processes the data of pulse build-uptime and frequency by Time sharing. It also controls the local oscillator and mainoscillator respectively and realizes the functions of active frequency stabilization systemand offset frequency locking system. The frequency stability reaches10-8, and theprecision of offset frequency locking reaches±5MHz.The experimental results showthat the system has the advantages of smaller size, compact structure, lower cost, highprecision of frequency stability and offset frequency locking.