Ultra-low-noise supercontinuum in normal-dispersion ZBLAN fibres pumped at 1.85 $\mu$m

Abstract

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