Since the advent of lasers, a single-frequency laser has always been a key research direction of lasers. A single-frequency all-solid-state laser has the advantages of narrow linewidth and stable frequency, which is widely used in lidar, gravity wave detection, coherent communication, and so on. Many research institutions and enterprises have studied the tunable performance, frequency performance, and noise characteristics of nonplanar ring oscillator (NPRO) single-frequency lasers. However, most researches are based on the cavity structure to develop a desktop test system. And commercial products are difficult to meet the requirements of narrow linewidth, small volume, and high reliability, and thus difficult to meet the requirements of space field. The greatest particularity of space laser is the particularities of test environment and operation environment. The laser has to experience not only vibration and shock, but also the rapid changes of atmospheric pressure and temperature in the process of laser emission. These raise high requirements for the performance and reliability of the laser. Therefore, it is necessary to develop a laser with characteristics of miniaturization, high performance, and high reliability, which can adapt to the extreme space environment and ensure the normal operation in orbit.

In this study, we design a new structure of an NPRO crystal with a dimension of 12 mm (length)×8 mm (width)×3 mm (height). First, to achieve a single frequency output and high slope efficiency, the transmission of the coupling coating is optimized to 97% for s-polarization and 85% for p-polarization. Then we develop a small all fiber coupled nonplanar ring cavity solid-state laser. The device is mainly composed of pump fiber assembly, Nd∶YAG NPRO crystal, and coupled fiber assembly. The beam from the pump fiber is first focused by lens and then injected into the NPRO crystal. The beam from the NPRO crystal is focused into the coupled fiber. The core diameter of the pump fiber is 105 μm with a numerical aperture of NA=0.22. The coupling fiber is a panda polarization maintaining fiber and its core diameter is 6 μm with a numerical aperture of NA=0.12. Semiconductor cooler, thermistor, and laser crystal are integrated in the laser housing by metal welding. The pump optical fiber assembly and the coupling optical fiber assembly are fixed outside the laser housing by laser welding. The permanent magnet is fixed on the cover plate by adhesive. Finally, the cover plate is welded on the laser housing by the parallel seam welding technology.

To satisfy the needs of high reliability, miniaturization, and air tightness in the space field, a compact all fiber coupled NPRO solid-state laser is developed. The output power of the single-mode polarization maintaining fiber is nearly 600 mW under a 1.5 W pump power (Fig. 4). The power stability and frequency stability of the laser are tested. The power stability is 0.16%@2 h(Fig. 5) and the frequency drift is less than 20 MHz@3 h(Fig. 7). The linewidth is about 182.87 Hz(Fig. 6), and the polarization contrast is better than 20 dB. The temperature tuning coefficient is about -2.8 GHz/℃, and the 9 GHz mode hopping free tuning is obtained(Fig. 8). The PZT mechanical tuning coefficient is 2.7 MHz/V(Fig. 9). The output power tuning coefficient is -4.96 kHz/mW(Fig. 10). The tuning coefficient is between PZT mechanical tuning coefficient and temperature tuning coefficient, which can be used in the actual frequency stabilization process. The laser passes the mechanical test (the root mean square accelerated speed of random vibration is 19.8g, where g is the accelerated speed of gravity) and the temperature test (-20-65 ℃), and the output power change is less than 5% before and after the test(Fig. 11).

A compact all fiber coupled NPRO solid-state laser is developed. The laser has a linewidth of around 100 Hz. The temperature tuning coefficient is about -2.8 GHz/℃, and 9 GHz mode hopping free tuning is obtained. The PZT mechanical tuning coefficient is 2.7 MHz/V. The output power tuning coefficient is -4.96 kHz/mW. The laser passes various environmental routine tests, and the output power change is less than 5% before and after the tests. The solid-state laser is very suitable for high working temperature and mechanical environment. It has the advantages of narrow linewidth, low noise, small volume, low cost, and mature manufacturing. It has a great application potential in high-precision gravitational wave detection, lidar, frequency conversion and others.