Abstract: A multi-wavelength Raman laser comprising a Raman fiber forming part of an optical ring, preferably most parts of which are formed by optical fiber. The ring includes a periodic filter selectively transmitting optical power at a number of equally spaced wavelengths corresponding to the wavelengths of the laser output. Radiation at the signal wavelengths propagating in one direction on the ring is tapped onto an output fiber. An optical isolator is formed either in the ring or fiber tapped into the ring to suppress radiation propagating in the reverse direction on the ring. A frequency shifter may be inserted into the loop to prevent power from concentrating in a single mode, thereby allowing wide band lasing. A band pass filter may be inserted into the loop to restrict the lasing band.