With the rapid and diversified development of the national economy, the current domestic atmospheric environment remains complex. So lidar is in high demand as a critical tool for monitoring air pollution and greenhouse gases. In atmospheric remote sensing lidar system, high-spectral-resolution lidar (HSRL) and differential absorption lidar (DIAL) play significant roles in fields such as atmospheric composition detection and environmental monitoring due to their high precision and high sensitivity. A high-power, narrow-linewidth, and frequency tunable 1064 nm continuous-wave (CW) single-frequency laser can be used as the light source for HSRL via pulse modulation and power amplification. It can also generate short-wave and mid-infrared lasers through nonlinear optical frequency conversion techniques such as optical parametric oscillators (OPOs), providing DIAL with rich wavelength options. This paper aims to develop a broadband, mode-hop-free, tunable CW single-frequency Nd∶YVO₄ laser based on a unidirectional figure-eight ring cavity structure. In the future, this laser will be integrated as a core component into mobile-platform lidar systems, providing robust support for air pollution and greenhouse gases monitoring.

In this work, we develope a mode-hop-free, continuously tunable single-frequency laser at 1064 nm, which features a compact structure, high output power, and a broad frequency tuning range. The figure-eight ring cavity consists of four mirrors (M1?M4). M1 and M4 also serve as the pump light input and laser output couplers, respectively. The laser is pumped by a fiber-coupled laser diode. The gain medium is an Nd∶YVO₄ crystal with natural birefringence. Compared to the Nd∶YAG crystal, Nd∶YVO₄ has a higher absorption coefficient, a broader absorption bandwidth, and can directly generate linearly polarized lasers. To ensure single-longitudinal-mode operation, an optical unidirectional device composed of a terbium gallium garnet (TGG) crystal and a half-wave plate (HWP) is added into the cavity to eliminate the spatial hole burning. To achieve high power output, an etalon with a free spectral range (FSR) much larger than the cavity’s FSR is incorporated to narrow the effective gain bandwidth and suppress competing longitudinal modes. Intracavity insertion of a noncritically phase-matched LiB₃O₅ (LBO) crystal enables 532 nm laser output, which coincides with the absorption line of iodine molecules, facilitating long-term frequency stabilization. Finally, through coordinated control of the laser crystal temperature and piezoelectric ceramics (PZT)-driven cavity mirror, continuous mode-hop-free frequency tuning over a range of 14 GHz is achieved.

Experimental results demonstrate that the laser designed in this work exhibits excellent performance in all aspects. Under a pump power of 25 W, the maximum output powers of the fundamental-wave and second-harmonic-wave laser reach 6.5 W and 2.1 W, with slope efficiencies of 26.12% and 8.36%, respectively (Fig. 4). The power stabilities over 4 h are 0.23% and 0.31% (Fig. 5). Within 1.35 h, the fundamental-wave power fluctuation is approximately ±0.238 pm (±63.1 MHz) (Fig. 7). The PZT tuning coefficient of the fundamental-wave is about -0.58 pm/V (-153.53 MHz/V) (Fig. 8), while the temperature tuning coefficient is about -26.89 pm/℃ (-7.12 GHz/℃) (Fig. 9). By employing the coordinated control of crystal temperature and cavity length, mode-hop-free wavelength tuning over a range of 53.16 pm (14.08 GHz) is achieved (Fig. 10). Compared to the method of coordinated control of the etalon angle and PZT-driven mirror, the wavelength tuning approach proposed in this paper eliminates the need for complex mechanical and electronic control systems. Although the experimental results align well with theoretical predictions, the current frequency tuning method requires further refinement. Due to the inherent thermal inertia of crystal temperature adjustment, the tuning speed must remain moderate, and nonlinearity is observed during the initial and final stages of frequency tuning (Fig. 10). Additionally, hysteresis effects in the PZT actuator introduce deviations in wavelength tuning linearity. These issues will be systematically addressed in future work.

This paper presents a continuous-wave single-frequency Nd∶YVO₄ laser based on a figure-eight ring cavity, achieving 6.5 W of fundamental-wave and 2.1 W of second-harmonic wave laser output, with power stabilities of 0.23% and 0.31% over 4 h, respectively. Continuous wavelength tuning across a 14 GHz range is achieved by driving a PZT mounted cavity mirror to adjust the resonator length while synchronizing with laser mode variations induced by crystal temperature tuning. This laser overcomes the limitations of conventional cavity length control methods, which only allow continuous tuning within a single longitudinal mode. And, in contrast to approaches that require coordinated control of the etalon angle and cavity length, our design eliminates the need for complex control systems involving galvanometer actuators and phase-locked electronics. However, its tuning range and speed are still inferior to what can be achieved with etalon angle control, and further improvements are needed for the linearity of the control algorithm. Finally, the wavelength tuning performance is experimentally verified by scanning the absorption spectrum of iodine molecules. Potential applications of this laser include lidar systems or serving as a pump source for mid-infrared OPOs.