Watt-level ultrafast 1.75 μm laser system based on thulium-doped core and terbium-doped cladding fluoride fibers

We report watt-level femtosecond pulses in the 1.75 $μ$m region using a thulium-doped core, terbium-doped cladding fluoride (Tm:Tb:ZBLAN) fiber laser system. The seed pulse is generated through stimulated Raman scattering in a silica fiber pumped by an erbium-doped fiber laser. The soliton is subsequently amplified through a multi-stage Tm:Tb:ZBLAN amplifier. The tunability of our chirped fiber Bragg grating stretcher, matched with a Treacy compressor, compresses the pulse to 217 fs. Our system generates ~250 nJ of single-pulse energy, with a corresponding average power of ~1 W at a 4 MHz repetition rate. The laser system is suitable for multiphoton microscopy.