Objective As an important optoelectronic device for optical fiber communication, the 980 nm semiconductor laser has the advantages of a wide wavelength range, easy modulation, and high efficiency. It can excite ground-state erbium ions to a metastable state, thereby paving the way for stimulated radiation. It is thus an optimal pumping source for the erbiumdoped fiber amplifier (EDFA). However, as an important factor influencing the power stability and spectral shift of the 980 nm semiconductor laser, temperature affects the stability of the output wavelength of the EDFA. The traditional control methods, mainly using the microcontroller unit ( MCU) to implement the proportional- integral-derivative ( PID) algorithm, have the weaknesses of cumbersome hardware circuits and control processes. In this paper, the field programmable gate array (FPGA) with high speed and flexibility is used as the main controller to switch the internal finite state machine (FSM), thereby achieving the automatic temperature control of the 980 nm semiconductor laser and stabilizing the output power of the laser. Then, the above method is applied to an EDFA system, and the results show that it reduces the shift and improves the stability of the output spectrum. The proposed temperature control method is expected to promote the development and application of the temperature control of semiconductor lasers. Methods The structure and properties of the 980 nm semiconductor laser are investigated, and the analysis shows that the thermoelectric cooler (TEC) and the thermistor can respectively be used as the actuating element and the temperature sensor. Besides, the FPGA, as a core control component, is utilized to control the analog- to-digital chip ( ADC) collecting the voltage of the thermistor and further to obtain the internal temperature of the semiconductor laser. In addition, the FSM is adjusted and switched to different states in real time to control the magnitude and direction of the current flowing into the TEC. The above measures are taken to achieve automatic temperature control. An FPGA-based experimental device with an EDFA system is built to verify the feasibility of the proposed method. For this purpose, an experiment is conducted under different temperatures by analyzing the variations of output power-current ( PI)curves of the 980 nm semiconductor laser and the output spectra of the EDFA with time and temperature. The results prove the feasibility of the proposed method. Results and Discussions This paper proposes a real-time temperature control method for the 980 nm semiconductor laser that uses the FPGA to automatically switch the internal FSM (Fig. 7). An FPGA- based experimental device with an EDFA system is built ( Fig. 9) to verify the feasibility of the proposed method. The experimental results show that when the threshold temperature of the FSM is set to +/- 0. 2., the temperature of the laser within 60 min largely remains stable, with a maximum temperature difference of smaller than 0. 4. (Fig. 11). When the temperature of the 980 nm semiconductor laser stabilizes at 25., the proposed temperature control method increases the goodness of linear fit of the PI curve by 23. 07% and reduces the wavelength shift of the EDFA by 62. 5% within 60 min ( Fig. 13). The application of the proposed temperature control method effectively ensures the stability of the output power of the semiconductor laser and that of the output wavelength of the amplifier. Conclusions As the current temperature control methods for semiconductor lasers are faced with the slow transmission speed and complex structure of the controller, an FPGA-based real-time temperature control method for the 980 nm semiconductor laser is presented in this paper. The method uses the FPGA as the main controller to obtain the internal temperature of the semiconductor laser in real time by measuring the voltage of the thermistor. The FSM state could be switched in real time according to the collected temperature, thereby controlling the direction and magnitude of the current flowing into the TEC in the semiconductor laser. In this way, the internal temperature control of the semiconductor laser is achieved. An FPGA- based experimental device with an EDFA system is built to analyze the temperature control effectiveness of automatic switching of the internal FSM by the FPGA when the temperature is 25. and the working time is 60 min. When the threshold temperature of the FSM is set to 0. 2., the temperature of the semiconductor laser is generally 0. 4. under control. The goodness of linear fit of the PI curve of the 980 nm semiconductor laser is improved by 23. 07% from 0. 9584 to 0. 9784. The maximum shift and variance of the output wavelength of the EDFA are respectively reduced from 40 pm to 14 pm and from 14. 4 pm to 5. 4 pm. The wavelength shift and variance are thus respectively reduced by 65% and 62. 5%. The simple constant current source is combined with devices inside the semiconductor for temperature acquisition and cooling, and the internal FSM program is controlled by the FPGA to achieve temperature control. The proposed method, with a simple structure and fast speed, is of great significance for promoting the development and application of the temperature control of semiconductor lasers.
