Fourier spectral shaper assisted feedback for wavelength and linewidth control of cascaded Raman fiber lasers

Cascaded Raman fiber lasers (CRFLs) with wavelength-independent feedback can provide power at any wavelength in near-IR regions. However, broad feedback leads to a broad output spectrum, decreasing spectral power density at a desired wavelength. The output characteristics of CRFLs can be controlled by controlling the feedback. We propose using a reflective Fourier spectral shaper with a spatial mask for arbitrary spectral control of the feedback. We achieved coarse wavelength tuning of ∼140 nm using three Raman shifts with a fine wavelength tuning of >13 nm within each Raman band while using a fixed wavelength pump. We achieved ∼3× linewidth reduction with a multi-watt output power and high spectral purity of >94%.