Ultralong passively mode-locked ring fibre lasers in the femtosecond range assisted by Raman amplification

•New family of Raman-assisted passively mode-locked ultrafast fiber ring lasers.•Record length 10 km and 25 km ring resonators with ultrashort pulses achieved for the first time.•High energy pulses produced with native repetition frequencies as low as 20 kHz.•Suitable for power-demanding applications like materials processing or SC generation. A novel fibre ring laser architecture reliant on the use of Raman-assisted nonlinearity management over ultralong cavities is used for the first time to generate ultrashort pulses in ring resonators of up to 25.2 km. Using this approach, stable generation of pulses under 200 fs with estimated maximum energies of the order of μJ and an ultra-low repetition rate of 20 kHz is reported in passively mode-locked ultra-long ring fibre lasers of 10 km, overcoming previous limitations to pulse duration imposed by dispersive effects. The proposed system operates in the telecommunications infrared C-band, and relies on an Erbium-doped fibre amplifier, a polarization-insensitive InN-based semiconductor saturable absorber mirror and standard telecommunication fibres, and does not require external amplification or compression stages. Real-time monitoring of the output signal through the time-stretch dispersive Fourier transform technique shows that the average laser optical spectrum corresponds to that of the individual pulses. The presented devices constitute a new family of ultrafast fibre oscillators with unique characteristics suited for a broad range of applications.