Nanosecond Fiber Laser Step-Tunable From 3.87 to 4.5 μm in HBr-Filled Hollow-Core Silica Fibers

We report here a wavelength widely step-tunable nanosecond 4 μm fiber laser based on HBr-filled hollow-core fibers (HCFs), which is pumped by an acousto-optic modulated narrow linewidth 2 μm thulium-doped fiber amplifier (TDFA) seeded by several thermally tunable diode lasers. The modulated pump pulses excite the first overtone transition of the HBr molecules in HCFs with single-pass configuration, resulting in a number of mid-infrared laser emissions containing both P and R branch fundamental transition lines from 3.87 μm to 4.5 μm. To the best of our knowledge, this is the first tunable pulsed mid-infrared fiber laser beyond 3.8 μm, and it is also the largest tuning range achieved to date for any pulsed fiber laser not by nonlinear effects. The maximum average power of 550 mW associated with a single pulse energy of 0.37 μJ (pulse duration 20 ns, pulse repetition rate 1.5 MHz) is achieved at 4.17 μm, and the laser slope efficiency is about 15.6%. The mid-infrared laser shows a good fundamental mode with a measured beam quality factor M2 of about 1.15. The time and frequency domain characteristics are also experimentally investigated in detail. The versatility of such gas-filled HCF laser offers much flexibility in selected molecule gain media and HCF species for power scaling and wavelength extending of pulsed mid-infrared fiber lasers.