Patent ID: 6978057

Claim:
An optical waveguide for outputting light of a substantially single predetermined wavelength, the optical waveguide comprising: a light conducting medium ( 2 ) defining a longitudinally extending optical path ( 15 ) for guiding the light, the optical path ( 15 ) extending longitudinally between respective spaced apart first and second ends ( 8 , 9 ), and a means ( 20 , 21 ) for causing partial longitudinal reflections of the light along the optical path ( 15 ) at at least two spaced apart partial reflecting locations ( 20 ) along the optical path ( 15 ) for deriving light of the predetermined wavelength, wherein the means ( 20 , 21 ) for causing the partial reflections comprises a refractive index altering means ( 21 ) for altering the effective refractive index of the light conducting medium ( 2 ) presented to light passing along the optical path ( 15 ) at each of the at least two reflecting locations ( 20 ) for causing the partial reflections, and the means for causing the partial reflections locates the reflecting locations ( 20 ) along the optical path ( 15 ) at distances from the first end ( 8 ) along the optical path ( 15 ) which are functions of the product of the actual length of the optical path ( 15 ) and the actual refractive index of the light conducting medium ( 2 ) defining the optical path, less the sum of the products of the lengths of the reflecting locations ( 20 ) and the differences between respective effective refractive indices of the reflecting locations ( 20 ) and the actual refractive index of the light conducting medium defining the optical path ( 15 ), so that account is taken of alteration to the actual length of the optical path ( 15 ) resulting from the effect of the alteration of the effective refractive index at the respective reflecting locations on the actual length of the optical path ( 15 ), so that the distances of the reflecting locations along the optical path from the first end are such that the standing waves set up between the first end and each of the reflecting locations, and the standing wave or waves set up between any two of the reflecting locations, and the standing wave set up between the first and second ends, are all in harmonic relationship with each other.