High-Peak-Power Picosecond Fiber Laser at 2050 nm and Its Application to Cascaded Raman Light $>2~\mu \text{m}$

In this letter, an all-fiber master oscillator power amplifier system at 2050 nm was built, which incorporated a picosecond mode-locked fiber laser as the master oscillator followed by a two-stage thulium-doped fiber amplifier (TDFA). The master oscillator could provide picosecond laser pulses with temporal width of 36 ps at a pulse repetition rate of 13.6 MHz. Using the two-stage double-cladding TDFA, the output average power of the seed oscillator was boosted from 8 mW to 23.5 W, corresponding to a total gain of 34.7 dB. The maximum output peak power reached 54.2 kW. Moreover, we demonstrated that the picosecond laser pulses at 2050 nm could be used as the pump source to generate cascaded Raman (CR) shifted stokes light toward the mid-infrared region using a piece of fiber. Output spectrum incorporated three orders of stokes lights (centered at 2250, 2500, and 2800 nm, respectively) was measured in the experiment, while the wavelength of the third-order stokes light was limited, because it located at 2800 nm in the huge infrared loss region of the silica fiber. Numerical simulation results matched with the experimental results very well, indicating that the infrared fiber loss is the key limiting factor and optimization must be took fully to obtain CR fiber light sources above 2 μm.