The primary goal of this study is to develop a high-power, low-noise, single-frequency fiber laser system for third-generation ground-based gravitational wave detection. These detectors require continuous, high-power lasers with extremely low-intensity noise, as gravitational wave signals are exceedingly faint and susceptible to interference from laser noise. This study introduces a 1550 nm single-frequency fiber laser system capable of outputting 20 W power with low relative intensity noise (RIN) over the critical frequency range of 10 Hz to 10 kHz. The objective is to establish a reliable light source for gravitational wave detection systems and provide a solid foundation for future advancements in gravitational wave astronomy.

A master oscillator power amplifier (MOPA) configuration is used to amplify an ultralow noise fiber laser seed source. The laser system consists of a three-stage, all-fiber amplification setup that sequentially amplifies a 1550 nm seed laser. In the first stage, a polarization-maintaining erbium-ytterbium-doped fiber (EYDF) boosts initial power by several watts. The second stage, utilizing a larger fiber core, further increases power while managing nonlinear effects. The final stage employs a large mode area (LMA) fiber to achieve a high output power of up to 20 W, minimizing nonlinear effects such as stimulated Brillouin scattering (SBS) and stimulated Raman scattering (SRS). Several noise-reduction techniques are incorporated to achieve stable, low-noise operation. Thermal management, essential due to the heat generated during amplification, is achieved by mounting gain fibers in thermally conductive grooves within water-cooled aluminum plates. A high-precision temperature control system maintains fiber temperature stability within ±0.01 ℃, ensuring uniform heat dissipation, protecting the system, and stabilizing power output. To further reduce noise from sources like mechanical vibrations, air currents, and electromagnetic interference, the laser system is housed in an acrylic wind shield with electromagnetic shielding. A self-developed low-noise photodetector and high-precision voltage reference source are integrated into an optical-electrical feedback control loop. The photodetector monitors laser intensity noise, allowing the feedback system to dynamically adjust the pump current based on real-time signals, thereby minimizing laser intensity noise in the critical frequency range for gravitational wave detection.

The results show that the designed single frequency fiber laser system can meet the requirements for third-generation ground-based gravitational wave detection. The system achieves an output power of 20 W with a highly stable intensity profile. When the output power of fiber laser system is 20 W, the RIN at 10 Hz and 2 kHz are approximately 10^-5/Hz^1/2 and below 4×10^-8/Hz^1/2, respectively. Laser intensity noise is significantly reduced, particularly in the critical 10 Hz to 10 kHz frequency range. The system noise performance is enhanced by the combination of environmental noise isolation and the implementation of a self-developed low-noise photodetector and voltage reference source. The photodetector operates with a noise level as low as 2×10^-9/Hz^1/2 over the 10 Hz to 10 kHz range, ensuring that the system feedback loop can effectively suppress intensity noise without being influenced by the detector electronic noise. The voltage reference source is designed with multi-stage filtering and electromagnetic shielding, which provides a stable baseline for the feedback system and further improves the laser noise suppression capability. The LMA gain fiber in the main amplifier plays a critical role in suppressing nonlinear effects that typically arise at high power levels. The research also highlights the importance of careful thermal management in high-power fiber lasers. The temperature control system ensures that the heat generated during amplification is efficiently dissipated, preventing thermal accumulation that destabilizes the system or damages the gain fiber. This approach ensures stable laser operation over extended periods, which is a key requirement for gravitational wave detection.

This study successfully develops a high-power, low-noise, single-frequency fiber laser system optimized for ground-based gravitational wave detection. The system achieves 20 W output power with exceptionally low-intensity noise across the frequency range of 10 Hz to 10 kHz, making it well-suited for gravitational wave interferometers. The integration of a low-noise photodetector, high-precision voltage reference source, and optical-electrical feedback control effectively suppresses noise, meeting the stringent requirements of third-generation gravitational wave detectors. The findings of this study lay a crucial foundation for future advancements in gravitational wave detection technology. The high power and low-noise performance of the laser system ensures its capability to measure tiny space time distortions caused by gravitational waves, significantly enhancing gravitational wave detector sensitivity. Future work will focus on optimizing the system further by improving the feedback loop gain and bandwidth, and enhancing resistance to environmental noise. With these improvements, the laser system can provide robust support for high-precision gravitational wave detection.