Objective Laser diode has been widely used in laser scanners, optical storage, laser printers, optical fiber communications, laser pointers, laser spectroscopy and other fields because of their light weight, high efficiency, small size, low power drive, high conversion efficiency, and direct modulation. As a high-efficiency photoelectric converter, temperature has a great impact on its performance and life; If it is serious, it will cause the increase of threshold current, the shift of emission wavelength, the reduction of service life and other adverse effects. In order to prolong the service life of laser diode and stabilize the functional parameters, a temperature control system with high precision, fast response, high stability and good reliability must be designed to control the temperature of semiconductor lasers, so as to meet the needs of different environments of semiconductor lasers. Therefore, a digital analog hybrid temperature control system based on FPGA is designed.
Methods The temperature conditioning method of variable temperature control zero point is adopted. This method is based on iteration and multi-objective optimization algorithm to determine the optimal number of zero points, the optimal position of zero points, and the minimum number of ADC bits (Fig.3). A three-wire Wheatstone bridge is adopted to reduce the influence of wiring resistance and connection point resistance (Fig.2). The control method of anti-windup PID (AWPID) is adopted to reduce overshoot and speed up response (Fig.6). The Full bridge Synchronous Buck drive circuit (H-BUCK) adopts T-type capacitance network to reduce the ripple (Fig.7).
Results and Discussions After six iterations of the multi-objective optimization algorithm, the constraint conditions for the temperature conditioning method of variable temperature control zero point were met. The optimal number of zero points within the current temperature control range was determined to be six, with the optimal zero point positions at −33 ℃, −12 ℃, 8 ℃, 29 ℃, 49 ℃, and 70 ℃. Additionally, the minimum number of ADC bits required was found to be 10 (Fig.5). The temperature conditioning method of variable temperature control zero points reduces the requirements for amplifier and ADCs. The test results show that the accuracy in the whole temperature range is 0.02 ℃ (Tab.4), which is an improvement of 89.7% compared to the accuracy of a single fixed zero point (with the zero point set at 25 ℃) (Tab.4). The control strategy of AWPID reduces the overshoot from 9.13% to 1.5%, and shortens the stabilization time from 41 s to 30 s.
Conclusions In order to improve the accuracy, integration, response speed and reduce the cost of laser diode temperature control system, a digital analog hybrid temperature control system based on FPGA is designed. A three-wire Wheatstone bridge is used. Aiming at the nonlinear error of thermistor and bridge, a temperature conditioning method of variable temperature control zero point is adopted. The method is based on iteration and multi-objective optimization method to improve the temperature control accuracy. The measured results indicate that the temperature control system employing variable temperature control zero points achieves a temperature control accuracy of ± 0.02 ℃ within the temperature range of −45 ℃ to 75 ℃. This accuracy is an 89.7% improvement compared to the maximum temperature control accuracy of 0.1951 ℃ achieved by the temperature control system with a single temperature control zero point. Another advantage of the variable temperature control zero point is that it reduces the system cost. By reducing the voltage range of the Wheatstone bridge, it reduces the requirements for the common mode range of the instrumentation amplifier. By increasing the number of zero points, it reduces the ADC bits. By reducing the temperature range at each zero point, it reduces the signal-to-noise ratio (SNR) requirements of the ADC. The H-BUCK uses a T-type capacitor network to further reduce the ripple and increase the stability, Extend the service life and accuracy of thermoelectric cooler (TEC). According to the characteristics of large temperature lag and high delay, the temperature control strategy adopts the AWPID automatic control method to reduce overshoot and accelerate speed. The test results show that, compared with the PID control strategy, the AWPID control strategy reduces the overshoot from 9.13% to 1.5%, and improves the stability time from 41 s to 30 s. The stability test shows that the temperature control system can maintain the temperature control accuracy of ± 0.02 ℃ for a long time, meeting the requirements of high accuracy. The system has the characteristics of high precision, high integration and low cost, and can meet the multi field and requirements of high precision laser diode temperature control system.
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