Design of quadrature-demodulated digital Pound–Drever–Hall frequency stabilization system with automatic relocking

The problem that the traditional Pound–Drever–Hall (PDH) frequency stabilization method cannot compensate for the phase fluctuations of the heterodyne interference signal in real time has been theoretically analyzed. To solve the problem of the traditional PDH frequency stabilization method, and to improve the robustness of the PDH system, a quadrature-demodulated digital PDH (QDD-PDH) frequency stabilization system with automatic relocking has been designed. This system can separate the amplitude and phase of the heterodyne interference signal using the digital quadrature phase-sensitive detection technology, and it calculates the PDH error signal in real time, which can be fed back to adjust the laser cavity length so that the laser frequency can be stabilized to the resonant frequency of the referenced Fabry–Pérot cavity (FPC). In addition, the FPC transmission signal is used to continuously monitor the operating status of the QDD-PDH system; that is, when the voltage of the transmission signal is lower than the preset threshold voltage, the system will automatically operate in the frequency relocking mode. The characteristics of the frequency discrimination, frequency stabilization, and automatic relocking of the QDD-PDH system have been investigated experimentally, and the results have indicated that the frequency discriminating sensitivity is 159.44 mV/MHz, the frequency drift is less than 0.40 MHz within an hour, with a frequency stability of 7.19×10−11 at a 10-s averaging time. The successful percentage of the frequency relocking is 97.6%, and the average relocking time is 26.2 s. Such a QDD-PDH frequency stabilization system with automatic frequency relocking will find wide applications in the fields of laser interferometry, precision spectrometry, etc.