To solve the problems of narrow linear dynamic range and weak anti-interference ability of the Pound-Drever-Hall (PDH) technique, a PDH frequency stabilization method based on two modulation depths and two error signals is proposed herein. The PDH technique is widely used in the fields of laser frequency or optical resonant cavity locking. The traditional PDH technique usually utilizes a modulation depth of 1.08 rad to obtain the most sensitive error signal. However, the traditional PDH technique, used for frequency stabilization, is susceptible to environmental disturbances and loss of lock owing to the narrow linear dynamic range of error signals. In addition, only when the phase of a local demodulation signal matches the phase of an interference signal reflected by the cavity, the error signal with the highest sensitivity can be obtained. Currently, most methods manually adjust the initial phase of the local demodulation signal to achieve phase matching; these methods exhibit low accuracy and cannot realize automatic locking easily. Therefore, an adaptive locking mechanism having large modulation depth with large linear dynamic range error signal and small modulation depth with high-sensitivity error signal is developed to achieve frequency stabilization with strong anti-interference ability and high precision.

First, a digital quadrature demodulation technique was used to accurately extract the phase of the interference signal to achieve automatic matching between the phases of the local demodulation and interference signals. Second, a new error signal (S_pre) was realized using the transmitted power signal P_tran and traditional error signal S_PDH to enlarge the linear dynamic range of the PDH frequency stabilization system. Then, S_pre corresponding to the large modulation depth was used to realize fast capture and prelocking. Finally, S_PDH corresponding to the small modulation depth was used to realize precise locking. After locking, the modulation depths and error signals could be automatically switched according to the amplitude change in P_tran, realizing frequency stabilization with a large linear dynamic range and high sensitivity in the PDH technique. A frequency stabilization control system based on a field-programmable logic gate array (FPGA) was developed, and a locking test was conducted on a Fabry-Perot cavity. The experimental results show that the adaptive locking mechanism with double modulation depths and double error signals can greatly improve the anti-interference ability of the locking system with precision locking.

Considering the influence of phase mismatch and narrow linear dynamic range on the frequency stabilization accuracy of the PDH technique, an adaptive frequency stabilization method with a large linear dynamic range based on two modulation depths and two error signals is proposed herein. The phase of the interference signal is obtained using the digital quadrature demodulation technique to realize phase matching between the interference and local demodulation signals to improve the sensitivity of the error signal S_PDH obtained using the PDH technique (Fig. 3). To improve the anti-interference ability of the locking system, S_pre with a large linear dynamic range is constructed and combined with S_PDH and P_tran (Fig. 4). The adaptive locking mechanism using large modulation depth to obtain S_pre and small modulation depth to obtain S_PDH is designed herein (Figs. 5 and 6). Thus, the proposed locking mechanism has the highest sensitivity and linear dynamic range, affording high precision and strong anti-interference locking. A locking control system based on FPGA was designed herein (Fig. 7), and a locking test was conducted on the Fabry-Perot cavity. The test results show that the linear dynamic range of S_pre corresponding to β= 1.80 rad can reach 6.04 nm (Fig. 8), which is ~3.4 times that of S_PDH corresponding to β= 1.08 rad. The automatic switching and locking mechanism based on two modulation depths and two error signals can realize relocking of the Fabry-Perot cavity after instantaneous detuning (Figs. 10 and 11). The long-term relative stability of the Fabry-Perot cavity is 5.72×10^-9 (Fig. 12). Therefore, the proposed adaptive PDH frequency stabilization method can achieve long-term precise locking of the optical cavity/laser frequency.

This study proposes an adaptive frequency stabilization mechanism using two modulation depths and two error signals to modify the traditional PDH technique to achieve large linear dynamic range, high locking accuracy, and strong anti-interference ability. The test results show that the linear dynamic range of S_pre corresponding to a large modulation depth of 1.80 rad can reach 6.04 nm, which is ~3.4 times that of S_PDH (1.78 nm) corresponding to a small modulation depth of 1.08 rad. The adaptive switching and locking mechanism using two modulation depths and two error signals can substantially improve the anti-interference ability of the locking system, with precision locking. The relative stability of the locked cavity reaches 5.72×10^-9 within 3 h. Thus, the proposed method can be widely used in fields such as laser frequency locking and resonant cavity locking.