Objective Narrow-linewidth fiber lasers have a wide range of applications in fields such as long-distance communication, nonlinear frequency conversion, and gravitational wave detection. To achieve higher output power, beam synthesis techniques are used to combine multiple fiber lasers into a single beam. However, nonlinear effects limit the scalability in high-power narrow-linewidth fiber lasers. Among these effects, stimulated Brillouin scattering (SBS) has the lowest threshold and can induce self-pulsing, characterized by high peak power, narrow pulse width, and randomness. The master oscillator power amplifier (MOPA) structure, utilizing a phase-modulated single-frequency laser seed source, effectively suppresses the SBS effect without significantly broadening the linewidth during multistage amplification. Therefore, it is crucial to investigate the influence of different phase modulation techniques on SBS suppression. Methods To study the effects of different phase modulation methods on spectral linewidth and the SBS threshold, we construct a linearly polarized, narrow-linewidth, all-fiber laser system. This system includes a phase-modulated single-frequency laser, a two-stage linearly polarized preamplifier, and a one-stage linearly polarized main amplifier. The phase modulation module includes a white noise signal source, a pseudo-random binary sequence (PRBS) signal source, a low-pass filter, an RF amplifier, and an electro-optical modulator. By varying the modulation frequency and other parameters of these two signals, we observe changes in spectral linewidth and SBS threshold. In addition, the power and spectra of the output laser and backscattered light are measured and converted into time-domain signals for a detailed analysis of their characteristics. Results and Discussions In the linearly polarized narrow-linewidth fiber laser system, no self-pulsing effect occurs before or after the SBS threshold is reached when the unmodulated single-frequency narrow-linewidth fiber laser is amplified (Fig. 2). However, with white noise signal (WNS) phase modulation, the self-pulsing effect can occur before the output power reaches the SBS threshold (Fig. 3). Increasing the modulation frequency broadens the linewidth and raises the SBS threshold (Fig. 4). However, as modulation depth increases, the linewidth continues to widen, and the SBS threshold, which initially rises, eventually decreases. Cascaded WNS phase modulation performs better by achieving a higher SBS threshold at a similar linewidth compared to single-stage WNS phase modulation (Fig. 5). PRBS phase modulation is less prone to inducing the self-pulsing (Fig. 7). Shorter PRBS code lengths result in narrower linewidths and higher SBS thresholds (Fig. 8). As the modulation frequency increases, the SBS threshold initially rises and then decreases (Fig. 9). The modulation depth significantly affects the linewidth, and both linewidth and SBS threshold are positively correlated with modulation depth (Fig. 10). Finally, by applying phase modulation to the seed source and using a three-stage MOPA structure, a linearly polarized all-fiber laser system is built. This system achieves a linewidth of 0.081 nm (21.8 GHz) at a central wavelength of 1055 nm, with a slope efficiency as high as 87.0% and a polarization extinction ratio (PER) exceeding 13.5 dB. At a laser output power of 1.25 kW, no SBS effect or self-pulsing is observed, and the SBS threshold enhancement factor reaches 57.8 (Figs. 11-13). Conclusions In this paper, we experimentally investigate the effects of different modulation modes and the variation of modulation parameters on the linewidth and SBS threshold of high-power narrow-linewidth fiber lasers. Before modulation, single-frequency lasers are less prone to generating a self-pulsing effect. WNS phase modulation can produce a much wider linewidth than the modulation frequency, is easily adjustable, and offers a high SBS threshold. Cascaded WNS phase modulation can enhance the SBS threshold by around 25.1% while maintaining a similar linewidth, thereby supporting higher laser output power. PRBS phase modulation is less likely to induce self-pulsing, resulting in a slightly wider linewidth than the modulation frequency, making it more suitable for high-power narrow-linewidth fiber lasers. A shorter PRBS code length provides better SBS suppression. Appropriately increasing the modulation frequency and depth can enhance the SBS threshold for both phase modulation methods. Finally, a linearly polarized, narrow-linewidth, all-fiber laser system with a center wavelength of 1055 nm is built using cascaded WNS phase modulation in a three-stage amplified MOPA structure. This system achieves an SBS threshold enhancement factor of 57.8, with no self-pulsing or SBS effects at a laser output power of 1.25 kW, a linewidth of 0.081 nm (21.8 GHz), a slope efficiency of 87.0%, a signal-to-noise ratio of 32 dB, and a PER of 13.5 dB. These findings provide a crucial experimental foundation for the development of phase modulation systems and beam synthesis techniques for high-power narrow-linewidth fiber lasers.
