Effect of Laser Linewidth of Kilowatt Narrow-Linewidth Thulium-Doped Fiber Amplifier on SBS Threshold (Invited)

Objective 2 mu m high-power narrow-linewidth fiber lasers have a wide range of application demands in frontier applications and scientific research fields, such as coherent optical communications, Lidar, remote sensing, and next-generation gravitational wave detection, owing to their favorable spatiotemporal coherence. Generally, the power scaling of narrow-linewidth fiber lasers is limited by the stimulated Brillouin scattering effect. Using phase modulation to broaden the linewidth of a single-frequency seed source to reduce the SBS gain peak, researchers achieved >1 kW narrow-linewidth laser output for high-power fiber amplifiers in the 1.0-1.5 mu m band, thus proving that phase modulation is the most promising technical path currently to achieve kilowatt-level narrow-linewidth fiber-laser operations. Compared with 1.0-1.5 mu m Yb- or Er-doped fiber lasers, 2 mu m-band Tm-doped fibers with a larger mode field area can effectively increase the SBS threshold, thus demonstrating significant potential in achieving high-power single-frequency laser operations. Thus far, 2 mu m Tm-doped fiber lasers have achieved 316 W single-frequency outputs and 1.1 kW laser powers with a narrow linewidth of similar to 5 GHz. However, the effect of the laser linewidth on the SBS suppression capability of kilowatt-level narrow-linewidth thulium-doped fiber amplifiers has not been investigated comprehensively. In this study, we constructed a thulium-doped fiber laser system with >1 kW power and a linewidth of <500 MHZ based on pseudo-random binary sequence phase modulation and a multihotspot pumping scheme. Additionally, we investigated the effect of the seed-source linewidth on the SBS threshold in thulium doped fiber amplifier (TDFA), both theoretically and experimentally. Methods First, based on the theoretical model of the SBS threshold enhancement factor after filtered pseudo-random binary sequence (PRBS) signal modulation, the effects of the PRBS7 signal modulation frequency and seed linewidth on the SBS threshold enhancement factor of a thulium-doped fiber amplifier were analyzed. In the experiment, a kilowatt-class all-fiber narrow-linewidth thulium-doped fiber amplifier system was constructed based on pseudo-random binary phase-modulation technology and a multihotspot pumping scheme design, and the laser performance was characterized. Finally, the experimental and theoretical results of the effect of laser linewidth on the SBS threshold were compared and discussed. Results and Discussions The theoretical simulation results show that the SBS threshold enhancement factor first increases and then saturates as the PRBS7 modulation frequency increases under different filter bandwidths. Additionally, the saturated SBS threshold enhancement factor is positively correlated with the filter bandwidth, as shown in Fig. 4 (a). In the experiment, the SBS threshold is defined as the forward laser power when the reverse optical power reaches 1000 times the forward output power. The measured SBS threshold of the thulium-doped fiber amplifier is approximately 257 W under an unmodulated single-frequency seed-source injection with a linewidth of 1 MHz. When the seed linewidth is modulated using a PRBS signal with a modulation frequency of 1 GHz and broadened to 116, 232, and 480 MHz, the SBS threshold increases approximately linearly to 547, 683, and 1020 W, respectively, with a slope efficiency of 1.36 W/MHz. In other words, the thulium doped fiber amplifier can achieve SBS threshold enhancement factors of 2.13, 2.66, and 3.97, respectively, while the linewidth of the seed source is broadened 116, 232, and 480 times, respectively. The experimental results agree well with the theoretical SBS threshold enhancement factor curve at a modulation frequency of 1 GHz, as shown in Fig. 4 (b). The output power of the thulium-doped fiber amplifier increases linearly to 1025 W at a pump power of 1980 W while the seed linewidth is fixed at 480 MHz, thus resulting in a slope efficiency of 52.6%. The central wavelength is 1998 nm, with a signal-to-noise ratio of > 46 dB. This shows that the linewidth broadening of the seed source through phase modulation can linearly enhance the SBS threshold, thereby effectively suppressing the SBS effect in thulium-doped fiber amplifiers and realizing a narrow linewidth kilowatt-level laser operation. Conclusions By performing PRBS phase modulation to broaden the linewidth of the single-frequency seed source, the relationship between the SBS threshold and laser linewidth in a kilowatt-level narrow-linewidth thulium-doped fiber amplifier was investigated theoretically and experimentally. The thermal effect of the thulium-doped gain fiber under high-power laser operation was alleviated by adopting a multihotspot pumping design. An output power of 1025 W with a slope efficiency of 52.6% is achieved under a seed-source linewidth of 480 MHz. The central wavelength of the laser output is 1998 nm and the signal-to-noise ratio is >46 dB. The SBS threshold of the thulium-doped fiber amplifier increases linearly with the seed linewidth broadening, thus resulting in an SBS enhancement factor of 4 at a seed linewidth of 480 MHz. The results of this study can provide a technical reference for the optimization of 2 mu m kilowatt-level single-frequency thulium-doped fiber laser systems.