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Abstract

A optical medium, such as an angled distributed reflector, e.g., an angular grating, or .alpha.-DFB laser diode or a waveguide wavelength selective filter, has a mean optical axis defining an optical cavity for substantially confined light propagation within the device. An angular grating is provided in at least a portion of the optical cavity forming a grating region permitting light to propagate along the optical cavity in two coupled waves or modes incident along the angular grating, a first incident propagating wave substantially parallel with respect to the mean optical axis and a second incident propagating wave at an angle with respect to the mean optical axis. At least one preferential coupling region in the optical cavity at an interface or boundary with the grating region to receive both propagating waves and provide for preferential treatment to the first incident propagating wave by coupling propagating light in the second incident propagating wave into the first incident propagating wave prior to light output from the medium via the non-grating region, forming a boundary with the grating region. The angle of the boundary at the interface of these respective regions is chosen to be substantially collinear with a propagation direction of the second incident propagating wave so that propagating light in the second incident propagating wave will substantially enter the preferential coupling region from the grating region via the boundary in a propagation direction substantially parallel with the mean optical axis.

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