Supercontinuum generation from ∼1.9 to 4.5 μm in ZBLAN fiber with high average power generation beyond 3,8 μm using a thulium-doped fiber amplifier

A mid-IR supercontinuum (SC) fiber laser based on a thulium-doped fiber amplifier (TDFA) is demonstrated. A continuous spectrum extending from similar to 1.9 to 4.5 mu m is generated with similar to 0.7W time-average power in wavelengths beyond 3.8 mu m. The laser outputs a total average power of up to similar to 2.6W from similar to 8.5m length of ZrF4-BaF2-LaF3-AlF3-NaF (ZBLAN) fiber, with an optical conversion efficiency of similar to 9% from the TDFA pump to the mid-IR SC. Optimal efficiency in generating wavelengths beyond 3.8 mu m is achieved by reducing the losses in the TDFA stage and optimizing the ZBLAN fiber length. We demonstrate a novel (to our knowledge) approach of generating modulation instability-initiated SC starting from 1.55 mu m by splitting the spectral shifting process into two steps. In the first step, amplified approximately nanosecond-long 1.55 mu m laser diode pulses with similar to 2.5kW peak power generate a SC extending beyond 2.1 mu m in similar to 25m length of standard single-mode fiber (SMF). The similar to 2 mu m wavelength components at the standard SMF output are amplified in a TDFA and coupled into ZBLAN fiber leading to mid-IR SC generation. Up to similar to 270nm SC long wavelength edge extension and similar to 2.5x higher optical conversion efficiency to wavelengths beyond 3.8 mu m are achieved by switching an Er:Yb-based power amplifier stage with a TDFA. The laser also demonstrates scalability in the average output power with respect to the pulse repetition rate and the amplifier pump power. Numerical simulations are performed by solving the generalized nonlinear Schrodinger equation, which show the long wavelength edge of the SC to be limited by the loss in ZBLAN. (C) 2011 Optical Society of America