Objective A pulsed fiber laser with high power, high energy, and good beam quality has significant application value in laser cleaning, deep penetration welding, and other fields. The development of double-cladding fiber, laser diodes, and passive devices has allowed the unprecedented improvement of the output power of nanosecond pulsed fiber lasers. A master oscillation power amplification (MOPA) structure is commonly used to achieve a nanosecond pulsed laser output with high average power, high single-pulse energy, and excellent beam quality, which expands the range of applications. However, this laser output improvement is limited by the parasitic oscillation caused by amplified spontaneous emission (ASE) and the stimulated Raman scattering (SRS) caused by a high peak power. Large-mode-area gain fiber is commonly used to suppress the SRS and achieve a laser output with high average power and high pulse energy. However, this also leads to serious deterioration of the beam quality. Commercial 30 μm/250 μm fibers can achieve nanosecond pulsed laser output near the diffraction limitation, but cannot simultaneously achieve high average power and high pulse energy. With the longitudinal variation of the core cladding structure, tapered ytterbium-doped fiber (T-YDF) can suppress SRS and beam-quality deterioration, realizing a laser output with high average power, high single-pulse energy, and excellent beam quality. This provides a suitable method for improving the output performance of pulsed lasers. Herein, a T-YDF with a total length of 4 m and tapered region length of 1.5 m is proposed, which experimentally provides obvious suppression effects of the SRS and beam quality deterioration. Methods The T-YDF shown in Fig. 1 is fabricated by modified chemical vapor deposition (MCVD) process with solution doping technology (SDT). This T-YDF can be divided into two regions along the longitudinal direction. The first is a tapered region with a gradual variation in the diameters of the core and cladding at a fixed ratio of 0.124. The diameter of the core varies from 31 μm to 62 μm, and the diameter of the inner cladding varies from 250 μm to 500 μm. The length is 1.5 m. The second is a large uniform region with core and inner-cladding diameters of 62 μm and 500 μm, respectively, with a length of 2.5 m. The numerical aperture (NA) of the T-YDF is approximately 0.06, with the 31 μm/250 μm end serving as the signal input end and the 62 μm/500 μm end serving as the signal output end. The laser performance of the T-YDF is investigated using an all-fiber nanosecond pulsed MOPA system with a forward pumping configuration, which is depicted in Fig. 3. As the contrast fibers, a 31 μm/250 μm uniform ytterbium-doped fiber (YDF) and 50 μm/400 μm YDF are prepared using the same preform rod to experimentally verify the suppression effects of the T-YDF on the SRS and beam-quality deterioration. Results and Discussions A nanosecond laser output with an average power of 832 W, a peak power of 24.8 kW, and a pulse energy of 8.32 mJ is achieved based on the T-YDF after bending optimization experiments (Fig. 4). The threshold power of the SRS effect occurring in the spectrum is 551 W, and the SRS suppression ratio at maximum output power is approximately 48.5 dB. Beam quality factors Mx2 and My2 are 3.506 and 3.465, respectively. The 50 μm/400 μm YDF prepared from the same preform rod is used in bending optimization experiments under the same system conditions, achieving a laser output with a maximum average power of 773 W, peak power of 23.3 kW, and pulse energy of 7.73 mJ (Fig. 5). The threshold power of the SRS effect occurring in the spectrum is 420 W, and the SRS suppression ratio at the maximum output power is approximately 44.3 dB. Beam quality factors Mx2 and My2 are 4.897 and 4.744, respectively. Compared with that in the 50 μm/400 μm YDF, the threshold power of the SRS effect increases from 420 W to 551 W in the T-YDF, an improvement of approximately 31%, and the beam quality factor decreases from ~4.8 to ~3.5. The experimental results show that the T-YDF has preferable suppression effects on the SRS effect and beam-quality deterioration. Conclusions Based on MCVD process and SDT, T-YDF with core diameter of 31‒62 μm and inner cladding diameter of 250‒500 μm is fabricated, which includes a 1.5 m long tapered region and 2.5 m long large uniform region. Based on the all-fiber nanosecond MOPA system, a laser output with an average power of 832 W, a peak power of 24.8 kW, a single pulse energy of 8.32 mJ, and a pulse width of 336 ns is achieved by a forward pumping configuration at a repetition rate of 100 kHz. The output of the single-fiber nanosecond pulsed laser with the best beam quality at the average power and single pulse energy level is realized based on the homemade T-YDF. Compared with that in the 50 μm/400 μm YDF produced using the same preform rod, the threshold of the SRS effect increases by approximately 31%, and the beam quality factor decreases from approximately 4.8 to approximately 3.5. The experimental results show that the tapered fiber has great potential for SRS suppression and beam-quality improvement. A new single-module realization method for high-power nanosecond pulsed laser combination is provided. Further optimization of the fiber structure parameters is expected to achieve a pulse laser output with higher power and better beam quality. © 2024 Science Press. All rights reserved.
