## 1 Introduction  
Single-frequency lasers are used for a wide variety of commercial and scientific applications including material processing, manufacturing, lidar, and gravitational-wave (GW) detectors [1; 2; 3]. The GW detector has extremely strict requirements in terms of intensity noise performance and primary laser stability. Intensity noise would induce fluctuating forces on the test mass of GW detectors, which can severely reduce GW detector sensitivity [4]. A high-power, low-noise, highly stable, long-lifetime laser system is a key component of the GW detector. The most promising candidate for such a laser system is the master oscillator power amplifier (MOPA) architecture [5]. In the MOPA architecture, a low-power, low-noise, narrow-linewidth laser is amplified through a fiber amplifier to achieve high-power output. The advantages of the MOPA laser system are the high efficiency of the fiber amplifier and the power scalability of the overall laser system. On the other hand, the fiber amplifier might induce more power fluctuations and broaden the linewidth of the seed laser [6; 7]. Accordingly, active power stabilization is required for the MOPA laser system used in GW detectors. Limited by ground vibration, gradient noise, and limited interferometer size (\(\sim\)kilometers long), ground-based GW detectors cannot receive GW signals below 10 Hz [8]. Space-borne GW detectors are expected to provide more information about the universe through GWs in the frequency range from 0.1 mHz to 1 Hz [9]. Space-borne GW detectors have been proposed, such as the Laser Interferometer Space Antenna (LISA) [10], Tian-Qin [11], and Taiji [12]. All the MOPA lasers in these schemes require single-frequencyradiation with 2-5 W of output power in a diffraction-limited beam and a relative power stability of better than \(2\times 10^{-4}/\sqrt{\text{Hz}}\) at a Fourier frequency of 1 mHz [13]. To date, many studies have been devoted to low-noise lasers for LISA [14; 15; 16; 17; 18]. The baseline architecture for the laser consists of a low-power, low-noise master oscillator followed by a power amplifier stage with \(\sim\)2 W output. As early as 2005, [PERSON] reported a MOPA laser with an output power of 1 W [16]. In this work, a quartz glass wedge in free space was used for stable laser sampling. In the electronic feedback loop, a voltage reference (AD587 KN) was employed, and the temperature coefficient of AD587 KN was up to 10 ppm/K. The best RIN result of \(-90\) dBc/Hz @ 1 mHz was obtained through electronic feedback to the pump diode current, temperature stabilization of the photodiode, quartz wedge, and voltage reference. In 2017, [PERSON] compared the intensity noise between a stand-alone nonplanar ring oscillator (NPRO) and a planar-waveguide external-cavity diode laser (PW-ECL) built by Redfern Integrated Optics Incorporated (RIO) [17]. The results show that the PW-ECL intensity noise is significantly lower than that of NPRO at frequencies from 0.1 mHz to 1 mHz. The amplifier contained a semiconductor optical amplifier (SOA) as a preamplifier and a fiber amplifier as a power amplifier. The laser system used a robust mechanical design and temperature stabilization to suppress fiber-length variations. By stabilizing the amplifier pump-diode current, an amplitude noise attenuation of \(\sim\)30 was achieved at a frequency of 0.1 mHz. In 2018, [PERSON] reported a MOPA design including an extended cavity diode laser (ECDL) as the master oscillator, and two amplification stages as the power amplifier with a 2.1 W output power [18]. However, the RIN below the frequency of 3 mHz still fell behind the requirement of LISA. In 2019, [PERSON] reported a highly stable MOPA laser system with a 2.5 W output [9]. The adopted micro NPRO (m-NPRO) has the lowest noise level. By stabilizing the pump-diode current in the amplifier, the intensity noise at approximately 1 mHz can be greatly improved. Recently, Chinese space GW detection projects have been developing quickly. TianQin-1 and Taiji-1 have successfully completed in-orbit tests.