Ultra-low-noise supercontinuum in normal-dispersion ZBLAN fibers pumped at 1.85 µm

We demonstrate, for the first time to the best of our knowledge, ultra-low-noise supercontinuum (SC) generation in normal-dispersion fluoride fibers pumped by femtosecond (fs) pulses. We have investigated two elliptical-core polarization-maintaining (PM) ZBLAN fibers with core dimensions of 6.7 × 2.7 μm and 8.9 × 4.1 μm, experimentally measured to have normal dispersion up to 3.77 μm and 3.25 μm, respectively; the smaller-core fiber yields ultra-low-noise SC spanning 1.537-2.196 μm with a minimum relative-intensity noise (RIN) of 0.22% at 1.7 μm, and the larger-core fiber yields 1.507-2.250 μm with 0.36% at 2.0 μm. To aid the generation of low-noise SC, we developed an all-PM thulium chirped-pulse amplifier delivering 58 fs pulses at 1.85 μm, 210 mW average power at 40 MHz, with 0.41% RIN, seeded by a part of an ultra-low-noise SC using a 1.55 μm fs laser and an all-normal-dispersion (ANDi) silica fiber for precise seed control. These results establish a robust, alignment-free pathway to extend ultra-low-noise ANDi-fiber SC towards the mid-infrared using PM fluoride fibers.