Fig. 1. Schematic diagram of the structure of PS-LPFG. When the output laser transmits through the PS-LPFG, the broadened laser will be coupled from the core to the cladding and the signal laser continues to transmit.

Fig. 2. Simulated transmission spectrum of (a) LPFG and PS-LPFG with resonance wavelength 1080 nm at a period of 578 μm and a period number of 140; (b) PS-LPFGs with different periods at a period number of 140 and an index modulation amplitude of 8 × 10^–5; (c) PS-LPFGs with different period numbers at a period of 578 μm and an index modulation amplitude of 8 × 10^–5; and (d) PS-LPFGs with different index modulation amplitudes at a period of 578 μm and a period number of 140.

Fig. 3. Inscribing system of PS-LPFG based on a point-by-point scanning technique.

Fig. 4. (a) Temperature curve of dynamic-high-temperature annealing and (b) transmission spectrum of the fabricated PS-LPFG after annealing.

Fig. 5. High-power MOPA system for evaluating performance of suppressing spectral broadening.

Fig. 6. Output spectra of the evaluation system (a) without and (b) with PS-LPFG and comparison of the output spectra with and without the PS-LPFG at the highest power (c) in the range of signal laser and (d) in the range of SRS, and (e) output power versus pump power without or with PS-LPFG with the beam quality and profile of the output.

Table 1. Linewidth of output laser before and after splicing the PS-LPFG (at maximum output power).