Objective  For a long time, continuous-wave single-longitudinal-mode tunable lasers have served as superior light sources for applications such as atomic lithography, ultra-precision metrology, quantum optics, and cold atom physics, owing to their wide tuning range, high output power, and ultra-narrow linewidth. However, their frequency stability is susceptible to environmental temperature fluctuations, mechanical vibrations, and acoustic noise. Furthermore, the inherent free-running frequency stability is often insufficient to meet the stringent requirements for frequency stability and accuracy in high-precision laser frequency measurements. To address this issue, this article demonstrates an offset frequency locking system for a tunable Ti:Sapphire laser based on an optical phase-locked loop (OPLL). By utilizing an optical frequency comb as the frequency reference and leveraging the high sensitivity of the OPLL technique, the system successfully achieves long-term, high-stability frequency locking of the Ti:Sapphire laser.

Methods  This article demonstrates an offset frequency locking system for a tunable Ti:Sapphire laser based on an optical phase-locked loop. In this system, an optical frequency comb serves as the master laser and the Ti:Sapphire laser as the slave laser. A beat note detection module was built to obtain the beat signal between the two lasers, as shown in Fig.4. This beat signal, along with a reference frequency from a signal generator, was processed by a self-developed OPLL circuit, which generated a feedback control signal to tune the frequency of the Ti:Sapphire laser. The system successfully achieved offset frequency locking of the Ti:Sapphire laser to the optical frequency comb, with the locking performance illustrated in Fig.7.

Results and Discussions  The offset frequency locking system based on the optical phase-locked loop (OPLL) developed in this article employs an optical frequency comb as the master laser. As shown in Fig.5, the frequency comb exhibits high stability, with fluctuations in its repetition rate and offset frequency remaining within ±0.00004 Hz and ±0.004 Hz, respectively, over 3 hours. This high stability makes the optical frequency comb an ideal reference for achieving high-precision laser frequency stabilization. The frequency fluctuation of the beat signal after system locking is presented in Fig.7, which compares the laser's performance under free-running and locked conditions. In the experiment, the beat signal was first recorded using a frequency counter for 20 minutes without active locking. The system was then locked with a reference frequency set at 50 MHz, and frequency counting continued for 3 hours. The results show that the beat signal was successfully locked to the 50 MHz reference, with frequency fluctuations within 3 kHz, demonstrating high-stability frequency locking of the Ti:Sapphire laser. To evaluate the stability and continuous tuning capability of the OPLL system, the initial beat frequency was set to 30 MHz while maintaining lock. By varying the reference frequency, the beat signal was tuned from 30 MHz to 80 MHz in 5 MHz steps, both upward and downward. As shown in Fig.8, the system maintained lock throughout the entire tuning range. The relative Allan deviation of the OPLL system is shown in Fig.9. The values obtained are 4.6 × 10⁻⁶ at 1 s and 3.2 × 10⁻⁷ at 1000 s integration time.

Conclusions  This article introduces an offset frequency locking system for a tunable Ti:Sapphire laser based on an optical phase-locked loop. The system utilizes the PZT modulation port of the Ti:Sapphire laser for frequency modulation. A beat note detection setup between the laser and an optical frequency comb was established to obtain a high signal-to-noise ratio beat signal. This beat signal, together with an external reference signal, was fed into a custom-designed OPLL circuit to implement closed-loop negative feedback control, thereby achieving offset frequency locking of the laser to the optical frequency comb. Experimental results demonstrate that the developed OPLL system can successfully lock the frequency of the Ti:Sapphire laser to the frequency comb with a defined offset. Compared to conventional free-running Ti:Sapphire lasers, he frequency stability is significantly improved, addressing the issue of poor frequency stability in free-running operation. However, since the frequency modulation of the Ti:Sapphire laser relies on a PZT-based control port with limited frequency response, the achieved frequency stability after offset locking remains on the order of kHz. Future work will focus on integrating the OPLL with an acousto-optic modulator (AOM) or electro-optic modulator (EOM) to further enhance the frequency stability of the Ti:Sapphire laser.