A source of optical radiation for generation of narrow line width, high power optical pulses, the source including: a main fibre amplifier including an optical fibre and a pump laser coupleable into the optical fibre, wherein the optical fibre is a rare-earth doped silica glass optical fibre and wherein the optical fibre is a large mode area fibre configured to support optical peak power of at least twenty kiloWatts; a first laser source coupleable into the large mode area optical fibre, wherein the first laser source is a pulsed laser source; a second laser source coupleable into the large mode area optical fibre; wherein the first laser source emits radiation centred around a first wavelength and the second laser source emits radiation centred around a second wavelength other than the first wavelength, wherein the optical radiation of the first laser source is amplified by stimulated emission of rare earth ions within the large mode area optical fibre , wherein the first and second wavelengths are selected so that the second wavelength is shifted from the first wavelength by a frequency that corresponds to the first Stokes mode within a Raman frequency band of the large mode area optical fibre, wherein input power of the first laser source and optical gain of the main fibre amplifier are selected so that during amplification, peak optical power within the large mode area optical fibre is substantially at or above a threshold for stimulated Raman scattering, wherein the narrow line width, high power optical pulses output from the optical fibre are substantially around the second wavelength, wherein the high power optical pulses have less than 100 picometer line width and multi-kiloWatt peak power. a main fibre amplifier including an optical fibre and a pump laser coupleable into the optical fibre, wherein the optical fibre is a rare-earth doped silica glass optical fibre and wherein the optical fibre is a large mode area fibre configured to support optical peak power of at least twenty kiloWatts; a first laser source coupleable into the large mode area optical fibre, wherein the first laser source is a pulsed laser source; a second laser source coupleable into the large mode area optical fibre; wherein the first laser source emits radiation centred around a first wavelength and the second laser source emits radiation centred around a second wavelength other than the first wavelength, wherein the optical radiation of the first laser source is amplified by stimulated emission of rare earth ions within the large mode area optical fibre , wherein the first and second wavelengths are selected so that the second wavelength is shifted from the first wavelength by a frequency that corresponds to the first Stokes mode within a Raman frequency band of the large mode area optical fibre, wherein input power of the first laser source and optical gain of the main fibre amplifier are selected so that during amplification, peak optical power within the large mode area optical fibre is substantially at or above a threshold for stimulated Raman scattering, wherein the narrow line width, high power optical pulses output from the optical fibre are substantially around the second wavelength, wherein the high power optical pulses have less than 100 picometer line width and multi-kiloWatt peak power. <EOS>