High-power narrow-linewidth fiber lasers are widely used in coherent synthesis and spectral synthesis; however, their power expansion is limited owing to stimulated Brillouin scattering. Common methods for inhibiting stimulated Brillouin scattering include the design and fabrication of stimulated Brillouin scattering suppression fibers and changing the temperature field and stress field distributions of the fibers. However, these methods entail complicated processing and can easily produce noise. In recent years, phase modulation of the light field has become the main method for suppressing stimulated Brillouin scattering. In the linewidth range of 50 GHz, single-stage phase modulation has a limited threshold boost for stimulated Brillouin scattering. In this study, we report a high-power narrow-linewidth fiber laser based on a cascaded pseudo-random binary sequence and sinusoidal phase modulation. The proposed method is expected to contribute to the power scaling amplification of narrow-linewidth fiber lasers in the 50-GHz linewidth range.

In this study, the appropriate pseudo-random binary sequence phase modulation parameters and low-pass-filter cutoff frequency are selected so that the unit spectral linewidth has the greatest suppression of stimulated Brillouin scattering. The effects of the modulation frequency and depth of sinusoidal phase modulation on the laser spectrum are studied. By changing the modulation frequency and depth, a spectral form with a fundamental frequency as high as the sideband of the ±1 level is obtained. After cascading the pseudo-random binary sequence and sinusoidal phase modulation, the spectrum shows a near-flat-top morphology, which exhibits good stimulated Brillouin scattering suppression. According to the theoretical research results, a high-power narrow-linewidth fiber laser based on cascaded phase modulation is constructed, and the output powers and stimulated Brillouin scattering thresholds are compared at root mean square (RMS) linewidths of 20 GHz and 46 GHz.

Based on previous research, when the pseudo-random binary sequence phase modulation depth is 0.55π and the ratio of filter cutoff frequency to modulation frequency is 0.53, the modulation spectrum exhibits a near-flat-top morphology (Fig. 2). According to theoretical research, the depth of sinusoidal modulation influences the number of spectral lines and the relative intensity of spectral lines, and the results are shown in Fig. 3. As the modulation depth increases, the number of spectral lines increases and the relative intensity of the spectral lines also changes. When the modulation frequency is reduced, the spectral line spacing and spectral linewidth are also significantly reduced. When the sinusoidal modulation frequency is 9.7 GHz and the modulation amplitude is 0.458π, the modulation spectrum has three single frequencies (Fig. 4). After cascading the pseudo-random binary sequence and sinusoidal phase modulation, the flatter spectral morphology than that from single-stage pseudo-random binary phase modulation is obtained, and the stimulated Brillouin scattering suppression is better. Based on the above research, a narrow-linewidth laser is built, and the output power and backward transmission power at RMS linewidths of 20 GHz and 46 GHz are monitored in the experiment (Fig. 7). When the RMS linewidth is 20 GHz and the output power is 2.2 kW, the backward power increases exponentially. When the RMS linewidth is 46 GHz,the output power is close to the SBS threshold. Because the stimulated Brillouin scattering phenomenon is not observed in the experiment, the power is increased to 4.93 kW, yielding a stimulated Brillouin scattering threshold enhancement factor of ~328, a system slope efficiency of 78%, and a beam quality factor (M²) below 1.2.

In this study, the physical mechanism by which the cascaded pseudo-random binary sequence and sinusoidal phase modulation is used to broaden the laser spectrum to suppress stimulated Brillouin scattering is investigated. The effects of pseudo-random binary sequence modulation frequency and mode length on the spectrum are theoretically studied. Under the optimal ratio of the filter cutoff frequency to the pseudo-random binary sequence modulation frequency, the unit linewidth well suppresses stimulated Brillouin scattering. Through theoretical simulations, the influence of the modulation depth and modulation frequency of the sinusoidal signal on the laser spectrum is obtained. Based on theoretical guidance, a narrow-linewidth single-fiber laser based on cascaded phase modulation is built for the experiment. The cascaded pseudo-random binary sequence and sinusoidal phase modulation is used to widen the seed source spectrum. Compared with the stimulated Brillouin scattering thresholds and output powers under different RMS linewidths, the output power finally reaches 4.93 kW after amplification by the four-stage optical fiber when the RMS linewidth of the seed source is 46 GHz. The system slope efficiency is 78% and the beam quality factor M² is below 1.2.