Semiconductor Laser Stabilization System Based On FPGA

As a type of semi-custom digital circuit device,FPGA has advantages such as anti-interference,reproducibility,and stability.It can also design circuits according to user needs,with high flexibility and integration.It has a wide range of applications in industrial automation,control,and computer fields.In recent years,digital circuits based on FPGA have shown broad application prospects in the field of quantum optics research.Semiconductor lasers,as an important basic light source in the field of quantum optics,have the characteristics of simple structure,long working life,wide wavelength coverage range,and low threshold current.In order to further obtain narrow output frequency linewidth and high tunable range,an external cavity is introduced to form an external cavity diode laser.Through passive frequency stabilization and active frequency stabilization methods,the output frequency of the laser is further stabilized and the output linewidth is narrowed.However,in traditional laser active frequency stabilization systems,the components are discrete and expensive.To improve system integration and save space and funds we propose using FPGA as the core circuit element to stabilize the laser’s output frequency.The FPGA can simultaneously perform a series of functions such as laser frequency scanning,modulation/demodulation,feedback control and signal monitoring while achieving semiconductor laser frequency locking relative to cesium atomic saturated absorption spectrum.This research greatly improves the integration of the semiconductor laser frequency stabilization system while its frequency fluctuations at the same level as commercial controllers.The main work of this article is as follows:（1）The factors causing frequency fluctuations in external cavity diode lasers was analyzed,corresponding formulas and magnitudes for the corresponding fluctuations was provided.（2）A semiconductor laser frequency stabilization system based on cesium atomic saturated absorption spectrum was constructed,with an FPGA as the core feedback circuit device.The system performs frequency scanning,modulation,demodulation,feedback control,and signal monitoring on the laser to lock it to the corresponding frequency of cesium atomic saturated absorption spectrum.（3）The frequency fluctuations of the laser after applying current feedback was analyzed and the laser was locked to the saturation absorption peaks of|6²S_1/2,（?）=4（?）（?）|6²P_3/2,（?）=4,5（?）and|6²S_1/2,（?）=4（?）（?）|6²P_3/2,（?）=5（?）.iithin a short time（30 ms）,frequency fluctuations of 2.2（0.4）MHz and 1.8（0.3）MHz were respectively obtained.The laser’s frequency fluctuation within 80 s was 3.0（0.3）MHz.（4）The stability of the laser output frequency before and after locked was analyzed.Based on the Allan deviation and spectrum before and after locking,it was confirmed that the output frequency stability of the semiconductor laser was significantly improved by applying current feedback control to the external cavity semiconductor laser through FPGA.The scheme used in this article can also be applied to applications such as optical cavity locking and laser phase locking.In the future,automatic search and lock of reference frequencies such as atomic saturated absorption spectrum and cavity resonant peaks can be achieved through software and hardware programming.In addition to its application in the field of optics,Py RPL can also be used for data acquisition,storage,and display of experimental data as it has modules such as oscilloscope,spectrum analyzer,network analyzer,etc.The PI controller can also be used for various feedback control applications.