| 摘要 |
An optical device comprises an optical waveguide, such as a single mode optical fibre (11), underlying a first layer (20) of material, such as a thin film, which has a refractive index higher than the refractive index of the waveguide (11) and which forms a planar waveguide capable of supporting and a guiding at least one propagation mode of a higher order than, but matching the phase velocity of, the propagation mode or modes in the underlying waveguide. A reflection diffraction grating (4) is provided on or adjacent to the surface of the first layer (2) remote from the waveguide (11). The arrangement is such that an optical signal which is coupled from the waveguide (11) into the first layer (2) is reflected by the reflection diffraction grating (4) and is coupled back into the waveguide. |
