High-precision laser spectroscopic techniques, crucial for applications such as trace gas sensing, impose stringent and often conflicting demands on both ultra-narrow laser linewidth and high-accuracy frequency calibration. Conventional solutions struggle to simultaneously meet these requirements: optical frequency combs offer superior frequency markers but entail complex and costly parallel detection systems, while traditional tunable lasers are simple but lack precise frequency calibration. To bridge this gap, this paper proposes and experimentally validates a novel scheme for generating a frequency-discrete, narrow-linewidth semiconductor laser based on optical feedback locking to a high-finesse ultrastable optical cavity.
The scheme employs a high-finesse linear Fabry-Pérot cavity—with a free spectral range (FSR) of 680 MHz and a mirror reflectivity of 99.994%—as the fundamental frequency reference. Its high-performance stabilization is realized through a synergistic dual-loop control mechanism. First, the resonantly filtered optical field from the high-finesse ultrastable cavity is fed back into a distributed feedback (DFB) semiconductor laser. This optical feedback acts upon the laser's active region, effectively suppressing its phase noise. Second, a low-bandwidth electronic servo loop dynamically stabilizes the optical feedback phase via a piezoelectric transducer, enabling robust and sustained frequency locking. Notably, the optical feedback locking range spans nearly the entire cavity FSR (~680 MHz), ensuring exceptional robustness against perturbations.
Under this dual-feedback locked state, the laser exhibits a unique operating characteristic. Continuous scanning of the laser injection current produces an output that is continuous in the time domain but discrete in the frequency domain. The output frequency undergoes precise, deterministic stepwise tuning, where each frequency jump is exactly equal to the cavity FSR. This generates a series of discrete, narrow-linewidth frequency points, achieving a wide tuning range exceeding tens of gigahertz (e.g., 26.5 GHz containing 39 distinct frequency steps in a single scan cycle).
The laser's performance was rigorously characterized through heterodyne beating with an optical frequency comb. Experimental results demonstrate a linewidth reduction from 1.31 MHz in free-running operation to 3.93 kHz under locked conditions, corresponding to a suppression by three orders of magnitude. The frequency calibration capability was evaluated by analyzing the repeatability of each discrete frequency step during periodic scanning, yielding an average standard deviation better than 1.69 MHz, surpassing the calibration precision of typical wavemeters (>10 MHz) by an order of magnitude. Furthermore, the system exhibits excellent long-term stability, with a measured frequency drift of only 17.27 MHz over 24 hours, underpinned by stringent environmental controls including temperature stability of ±0.001°C.
The proposed laser architecture successfully synthesizes the key advantages of disparate technologies. It provides high frequency-calibration accuracy, using the stable cavity FSR as a precise "optical ruler." Concurrently, it retains the practical benefits of traditional scanning lasers: its serial output enables complete spectral acquisition using a single photodetector, drastically simplifying the detection scheme. This fusion of precision, wide tunability, robustness, and practical simplicity offers a highly competitive light-source solution, paving the way for low-cost, compact spectroscopic sensing systems with high sensitivity and accuracy.