An optical source configured for providing output light for providing input signal light or pump light can comprise pump source for pumping a four wave mixing (FWM) process with light pulses (“FWM pump light”); a FWM element in optical communication with the pump source, the FWM element configured for hosting the FWM process to generate, responsive to the FWM pump light, pulses of FWM signal light and FWM idler light having different wavelengths. The optical source can be configured such that the output light comprises pump light having a pumping wavelength or as input signal light having a gain wavelength for pumping or seeding an amplifying optical device comprising a gain material for providing optical gain. The gain material can have absorption and emission spectra defining gain and pumping wavelengths at which, respectively, the gain material is arranged in the device to provide optical gain via a process of stimulated emission responsive to being pumped.