Femtosecond 78-nm Tunable Er:Fibre Laser Based on Drop-Shaped Resonator Topology

We report for the first time on study of a passively mode-locked Er-doped fibre laser with ultra-wide tuning (from 1524 to 1602 nm) of the central wavelength of its femtosecond output (503-940 fs). The recently proposed drop-shaped cavity topology for fibre lasers enabled a compact widely tuneable femtosecond laser with a single discrete element, a reflective diffraction grating used for wavelength tuning of the output radiation. The key features of the proposed laser are: spectral tuning does not require adjustment of any other cavity elements; stable generation with a single pulse per round trip is provided over the entire wavelength tuning range; and the output pulses are close to transform limited and feature relatively high (>58 dB) signal-to-noise ratio of laser inter-mode beats indicating high quality of mode locking across the entire wavelength tuning range. The proposed laser configuration relying on the new drop-shaped cavity topology simultaneously delivers stability and reliability of mode locking, possibility of broad-range wavelength tunability, and adjustability of the output pulse repetition rate of the laser (which is important in various metrological and other applications).