Narrow-linewidth picosecond fiber lasers concurrently exhibit advantages such as high peak power and high spectral purity, and have been widely used in the fields of nonlinear frequency conversion, cold processing of materials, precise ranging, and ultrafast spectroscopy. Taking nonlinear frequency conversion as an example, obtaining blue-green laser output based on frequency doubling of the 1 μm band laser is a highly mature technical approach. Owing to the matching characteristics of the frequency doubling crystal, to achieve a relatively high frequency doubling conversion efficiency, the 1 μm band laser as the fundamental frequency light often demands high peak power, narrow linewidth, and high brightness. At present, the master oscillator power amplifier (MOPA) configuration is typically employed to realize narrow-linewidth, high-brightness, and high-peak-power picosecond pulsed fiber lasers. Nevertheless, the laser output generated by this MOPA structure is restricted by amplified spontaneous emission (ASE) and stimulated Raman scattering (SRS). Super-large mode area gain fiber is commonly utilized to suppress SRS for achieving laser output with higher peak power. However, this often results in deterioration of beam quality. Conventional few-mode fiber laser can achieve laser output near the diffraction limit, but cannot simultaneously achieve high average power and high pulse energy. Incorporating the merits of the above-mentioned two types of fiber laser, this paper proposes a picosecond pulsed fiber laser simultaneously achieving high peak power and high brightness picosecond pulsed laser output.

The experimental system is shown in Fig. 1. The picosecond pulsed fiber laser is composed of an oscillator seed and a three-stage amplification structure. The narrow-linewidth picosecond pulsed seed source is a linear cavity semiconductor saturable absorber mirror (SESAM) mode-locked fiber oscillator operating at 1064 nm, whose structure is shown in Fig. 2. A 976 nm semiconductor laser diode (LD) pumps a single-clad polarization-maintaining ytterbium-doped active fiber with a core absorption of 250 dB/m through a 976/1064 nm wavelength division multiplexer (WDM). A SESAM with a modulation depth of 32% and a relaxation time of 3 ps is used as the high-reflection mirror to form the resonant cavity. When the saturable absorber is in the saturated state, the signal light passes through the saturable absorber completely and is reflected by the end mirror with nearly 100% reflection to ensure the stability of the oscillation system. To reduce the mode-locking threshold, a fiber Bragg grating (FBG) with a 3 dB bandwidth of 0.1 nm, a center wavelength of 1064.2 nm, and a reflectivity of 70% is used as the coupling grating at the output end of the oscillator system. The seed light is amplified in the main amplification system after passing through two pre-amplification stages. The gain fiber in the main amplification stage has a core diameter of 30 μm, a cladding absorption of 7.2 dB/m at 976 nm, a core numerical aperture of 0.06, an inner cladding diameter of 250 μm, and a cladding numerical aperture of 0.46. This experiment explores the influence of the properties of the signal light injected into the main amplification system on the final output laser power and Raman suppression ratio.

The output power of the secondary preamplifier is controlled at 1.5 W. In the main amplification system, the length of the gain fiber is 2.3 m, and the fiber length of the passive devices such as cladding power stripper (CPS) and quartz block head (QBH) at the back end is optimized to 1.5 m. When the pump power is increased to 140 W, the maximum output power is 103 W, with a slope efficiency of 72.8%. The output results are shown in Fig. 5. Figure 5(a) shows the variation of the output laser power with the pump power. Figure 5(b) shows the output spectrum at the maximum output power of 103 W. At this time, due to the influence of the self-phase modulation (SPM) effect, the output spectrum is significantly broadened, as shown in the inset in Fig. 5(b). The SRS suppression ratio is 35 dB, and the polarization extinction ratio is 15 dB. Further power amplification is limited by the SRS effect. Figure 5(c) shows a schematic diagram of the final output pulse duration, which is broadened to 32.4 ps, with a repetition rate of 29 MHz and a 3 dB spectral width of 0.42 nm. The beam quality and beam waist profile at the maximum output power measured by the beam quality analyzer are shown in Fig. 5(d), with a beam quality factor M² of 1.11 (transverse beam quality factor Mx2=1.13 and longitudinal beam quality factor My2=1.09).

This paper discusses a narrow-linewidth picosecond pulsed fiber laser with a MOPA structure. The mode-locked seed source built independently by SESAM is amplified through three stages to achieve high peak-power laser output. The experiment investigates the influence of the injection power of the main amplification stage on the Raman threshold of the final output signal during the picosecond pulse amplification process. The main amplification system adopts high-absorption polarization-maintaining gain fibers. By optimizing the output power of the secondary pre-amplification and the system structure, a laser output with a central wavelength of 1064.2 nm, an average power of 103 W, a repetition rate of 29 MHz, a pulse duration of 32.4 ps, and a peak power of 110.9 kW is achieved. At this time, the Raman suppression ratio is 35 dB, the polarization extinction ratio is 15 dB, and the M² is 1.11. Further power enhancement is limited by the SRS effect.