Supercontinuum Generation with Ultralong Ring Fibre Lasers in Highly Nonlinear Fibres

Recently, a new family of passively mode-locked femtosecond pulse fibre oscillators, relying on dispersion and nonlinearity management and using polarization-insensitive InN-based semiconductor saturable absorber mirrors (SESAMs) in a multi-kilometer standard fibre ring with Erbium Doped Fibre Amplifier (EDFA)-based amplification, was reported [1]. This family of ultralong ultrafast ring fibre lasers (UURFLs) displays pulsed lasing with low repetition rates, low-chirp and high-power pulses centered at 1560 nm in the hundreds of fs range, without requiring chirped-pulse amplification or external stages. Moreover, cavity lengths of up to 25 km were later demonstrated with the assistance of distributed Raman amplification, still supporting ultrashort pulse generation with pulse energies above μJ [2]. Real-time monitoring via time-stretch dispersive Fourier transform (TS-DFT) reveals the rich dynamics of this laser family, capable of operating in different harmonic regimes and power ranges [3], [4], ideal for applications such as supercontinuum generation (SCG) for environmental monitoring [5] in standard telecommunications fibre. This work investigates SCG and pulse compression in different kinds of highly-nonlinear fibre, from the output of UURFLs of up to 2.4 km. Our results demonstrate pulsed supercontinuum, with spectral broadening exceeding 800 nm FWHM from average laser output powers of just about 30 mW. Pulse compression to below 20 femtoseconds is achieved, with most of the supercontinuum energy contained with the ultrashort pulses.