Seed lasers have the advantages of narrow linewidths, low noise, high-frequency stability, and tuning capabilities. They are widely used in optical frequency standards, gravitational wave detection, light-controlled phased-array radars, quantum optics, and space-coherent communication. Most recent studies, in addition to remaining in the desktop stage, have primarily focused on reducing the structure of the split crystal, increasing the output power of the seed laser, and reducing the relative intensity noise. The adaptability and long life of seed lasers in space environments have not been verified, thus limiting their application. Due to the complexity of space environments, extreme temperature changes are accompanied by vibrations and shocks. In orbit, seed lasers must maintain low-noise and stable-frequency operations, and their lifetime characteristics are crucial. Therefore, developing an integrated, low-noise, high-frequency, tunable, and high-reliability laser is necessary to satisfy the normal operational requirements of lasers in extreme space environments.

A compact solid-state laser with a structure size of 62 mm×42 mm×16 mm was designed by integrating a pump source and a nonplanar ring oscillator (NPRO) crystal. Optical components such as pump chips, crystals, lenses, thermoelectric cooler (TEC), hot-face resistors, and covers were welded and encapsulated using metal welding, laser welding, and hermetic packaging technologies. The pump source adopted a main and backup dual-chip structure, and the output power of a single chip was approximately 1 W. When the main and backup chips were tasked with working independently, the output light was pumped into the crystal after focusing, which coincided with the intrinsic laser mode, and the light from the crystal was focused onto the polarization-maintaining fiber. The coupling-fiber assembly was fixed to the housing output by laser welding, and permanent magnets were glued onto the cover. Metal welding technology, laser welding technology, and hermetic packaging technology were used to weld and package the pump chips, crystals, lenses, TEC, hot-surface resistors and other optical components, as well as the cover plates.

The performance of the compact double-chip sealed seed laser was analyzed and the results are presented as follows. When the chip is pumped at 576 mW, the output power of the seed laser and conversion efficiency reach 133.7 mW and 37.2%, respectively [Fig. 3(a)]. The linewidth frequency noise and relative intensity noise (RIN) of the seed laser were next analyzed. Results show that the seed laser linewidth is 201 Hz, the frequency noise is 5.2 Hz/Hz @10 kHz (Fig. 4), and the RIN is -170 dBc/Hz @100 MHz (Fig. 5). A narrow linewidth and low noise indicate that the seed laser has good frequency jitter and power stability. The tuning characteristics of the seed laser were next tested. Results show that the temperature tuning coefficient is -2.99 GHz/℃, and the continuous tuning range is 44.97 GHz (Fig. 6). The piezoelectric transducer (PZT) tuning coefficient is 3.34 MHz/V, and the continuous tuning range is 315 MHz (Fig. 7). The corresponding tuning method can be selected based on the specific requirements of the laser. The seed laser was then subjected to temperature, mechanical (random, sinusoidal, and shock), irradiation, and thermal vacuum tests in the space environment. Results reveal that the power change is 0.16%, and the wavelength change is 1.7 pm (Fig. 8). Following an accelerated-life test of 2400 h, the power of the laser decreases by 4.5% (Fig. 10), and the laser can be expected to work in orbit for 8 a. The linewidth, frequency noise, and RIN of the NPRO laser change only minimally and still meet the requirements for laser use (Fig. 11). Finally, the primary backup can be switched to further improve the reliability of the seed laser.

A primary/backup compact seed laser with an output power of 260 mW, linewidth of less than 201 Hz, and RIN of -170 dBc/Hz @100 MHz was developed. The temperature tuning coefficient of the laser is -2.99 GHz/°C, and the PZT tuning coefficient is 3.34 MHz/V, which can achieve continuous tuning of 315 MHz. After completing temperature, mechanics, irradiation, and thermal vacuum tests in the space environment, the laser has a power change of 0.16% and a wavelength change of 1.7 pm. Accelerated lifetime tests were simultaneously conducted on the seed lasers to verify their performance. In addition to traditional power, a lifetime characterization method for the linewidth and noise was established. Within the acceleration lifetime of 2400 h, the laser power, linewidth, and noise changes are minimal, meeting the requirements for narrow linewidth and low-noise laser use. The expected orbital lifetime is greater than 8 a. In addition, the main/backup pump can be switched without affecting the overall seed laser performance, further improving the reliability of the laser. The results reveal that the problems of large mechanical size, large linewidth, high noise, insufficient life test data, and unclear risk of on-orbit operation of seed lasers are effectively solved, providing a test basis for the application of these lasers in gravitational wave detection, laser communication, and lidar.