高功率高效率低噪声单频 Er∶YAG 非平面环形腔激光器（特邀）

Objective 1.6 μm“eye-safe”single-frequency lasers are important laser sources for applications such as coherent Doppler wind lidar, high-precision ranging, and differential absorption lidar (DIAL). In these applications, achieving high power, high efficiency, narrow linewidth, and low noise output simultaneously is a core requirement for laser source design. Since being proposed in the 1980s, monolithic non-planar ring oscillator (NPRO) lasers have been one of the mainstream solutions for obtaining high-quality single-frequency lasers due to their unique monolithic resonant cavity design, excellent mechanical stability, and low noise characteristics. However, limited by the reabsorption loss of the quasi-three-level system and thermal effects at high power, the power and efficiency of room-temperature Er∶YAG NPRO lasers still have capacity for improvement compared to the theoretical Stokes limit. Cryogenic cooling technology can effectively reduce the population of the lower laser level, converting the quasi-three-level system into a four-level system, and significantly improving the thermal physical properties of the crystal. This work aims to develop a high-power, high-efficiency, and low-noise single-frequency laser system by introducing cryogenic cooling technology to the Er∶YAG NPRO design. Methods The experimental setup uses a monolithic Er∶YAG NPRO crystal. The crystal dimensions are 12 mm×14 mm×4 mm, and the doping concentration is 0.8%. Pump absorption bleaching is a common issue in Er∶YAG at cryogenic temperatures. This occurs because the absorption cross-section at the main peak (1532.3 nm) increases drastically, leading to a low saturation intensity. To address this, we employed a resonant pumping strategy targeting a“satellite”absorption peak at 1532.5 nm. The pump source is a narrow-linewidth fiber laser with a full width at half maximum (FWHM) of 0.2 nm. This spectral width ensures efficient absorption along the gain medium. It also avoids the deep saturation associated with the main peak. The NPRO was mounted in a cryostat and cooled to 80 K using liquid nitrogen. A magnetic field of approximately 0.4 T was applied to enforce unidirectional traveling-wave operation. This setup ensured stable single-frequency output. We studied the laser performance at both room temperature (300 K) and cryogenic temperature (80 K). Output power, slope efficiency, and spectral characteristics were recorded. The laser linewidth was measured using the delayed self-heterodyne method with a 30 km delay fiber. Noise properties were analyzed using a Mach‒Zehnder interferometer. Results and Discussions Cryogenic cooling significantly enhanced the laser performance. At 80 K, we observed a mode competition phenomenon. The laser initially oscillated at a novel wavelength of 1660.5 nm (max power 329 mW) at low pump powers, which is reported for the first time in an Er∶YAG NPRO. As pump power increased, the oscillation locked to 1645.7 nm. At this wavelength, a maximum continuous-wave single-frequency output power of 22.5 W was achieved. The system demonstrated an optical-to-optical conversion efficiency of 77.9% and a slope efficiency of 82.6%. To the best of our knowledge, these represent the highest efficiencies reported for Er∶YAG lasers and the highest single-frequency power for NPRO structures to date. At the maximum power, the laser integrated linewidth was measured to be 2.6 kHz, and the phase noise was measured to be 10^-5 rad/ Hz (1 kHz) and 10^-8 rad/ Hz (1 MHz). The beam quality M^2-factors were measured to be 1.31 and 1.34 in x and y directions, respectively. Additionally, a continuous wavelength tuning range of 0.58 nm was achieved by varying the temperature, covering the methane absorption line. Conclusions We have demonstrated a high-efficiency, cryogenically cooled Er∶YAG NPRO pumped by a 1532.5 nm fiber laser. The system achieved record-breaking efficiency and power output for this architecture. The results indicate that cryogenic Er∶YAG NPROs are reliable solutions for generating high-power, low-noise 1.6 µm single-frequency radiation, with the new 1660.5 nm wavelength expanding the potential spectral coverage for remote sensing applications.