Abstract: Apparatus comprising an optical fiber laser having at least two laser cores and means arranged to combine laser output from the at least two laser cores at a first end of the optical fiber laser to produce a combined laser output having one or more beat signals.

Abstract: Apparatus comprising an optical fibre laser having at least two laser cores and means arranged to combine laser output from the at least two laser cores at a first end of the optical fibre laser to produce a combined laser output having one or more beat signals.

Abstract: This invention relates to methods and apparatus for covert illumination. The method involves illuminating the scene with light which is tuned to an absorption resonances of atmospheric oxygen gas. Light tuned to have a wavelength corresponding to an oxygen absorption wavelength travelling through the atmosphere is therefore attenuated by absorption. The method therefore teaches that a low power illumination source can be used to illuminate nearby objects, thus allowing the detail thereof to be resolved, and the illuminating light reflected from the object, or even the light source itself, will be undetectable beyond a certain range due to atmospheric absorption. The illumination may be used in conjunction with the use of low light level imagers to image a scene. Alternatively the illumination could be used for short range optical communication which will be undetectable at much longer range.