Passive spectral tailoring via random fibre gratings for linewidth management in a kilowatt cascaded Raman fibre oscillator

Spectral broadening in guided-wave optics, an effect intrinsic to the interplay of dispersion and nonlinearity, presents a major obstacle to achieving the high spectral purity required for applications in nonlinear frequency conversion and precision spectroscopy. This effect is especially detrimental in high-power cascaded Raman fibre lasers (CRFLs). Leveraging a concept analogous to wavefront shaping but implemented passively, we introduce and validate a novel architecture for CRFLs incorporating random fibre grating-assisted spectral tailoring. A theoretical model is developed to show that the randomized reflectance profile of the grating induces wavelength-selective feedback, promoting intracavity mode competition and noise suppression through enhanced coherent enhancement of selected spectral components. Experimentally, we demonstrate a maximum output power exceeding 1 kW at 1185 nm from the cascaded Raman fibre oscillator. The random grating provides enhanced spectral control, achieving a 3-dB linewidth of approximately 1.0 nm. Notably, it also significantly suppresses the spectral wings, reducing the 10- and 30-dB linewidths to 2.9 and 9.5 nm, respectively. This work represents the first implementation of built-in passive spectral tailoring in an all-fibre nonlinear dynamical system, offering insights into the control of complex light fields in both temporal and frequency domains throughout cascaded nonlinear processes.