An optical fiber is optically coupled to an optical multiplexer. First and second wavelength-selective reflectors are formed onto the optical fiber. The first wavelength selective reflector is configured to reflect radiation of a first wavelength and the second wavelength reflective selector is configured to reflect radiation of a second wavelength that is longer than the first wavelength. A resonant laser cavity is formed between transmission fiber acting as distributed Rayleigh mirror and first and second wavelength selective reflectors. The first and second wavelength-selective reflectors and the optical fiber are configured such that Raman scattering and gain in the transmission fiber converts pump radiation at a pump wavelength less than the first wavelength to radiation of the first wavelength and also convert radiation of the first wavelength to radiation of the second wavelength.