Multi-wavelength, nano-second actively mode-locked Yb-fiber oscillator with 100 nm wide Raman broadened spectrum

Abstract

An all-normal dispersion, actively mode-locked Yb-fiber oscillator has been demonstrated which delivers multi-pulsing output with Raman broadened spectrum spanning more than 100 nm. Pulses of nano-second duration at 953.81 kHz repetition rate were obtained due to an elongated cavity consisting of 200 m long single mode fiber introducing high nonlinearity. The individual pulses of the multi-pulsing output corresponded to different wavelengths in the output spectrum and concurrently generated multiple first order Raman Stokes sidebands at close wavelength intervals. This resulted in a smoother broadband spectrum compared to usually obtained Raman broadened spectrum in normal dispersion regime which contains multiple higher order Stokes lines at large wavelength intervals when pumped by a single nano-second pulse. Analysis of the spectral power content revealed that nearly 50% of the output power contained in the Raman band around the 1120 nm region. By amplification of the generated pulses and further non-linear broadening through an ultra-high numerical aperture (UHNA) fiber, a broad spectrum having a wavelength span of nearly 260 nm with 1.5 Watt average power has been demonstrated showing application capability of the designed source.