OPTICAL DEVICES

摘要

Passive, self-adjusting and tracking optical wavelength filters (20) are described. The filters are absorptive and can be of either transmissive or reflective type. The filters comprise an unpumped doped optical waveguide configured so that signals of different wavelength are spatially decoupled to some extent. The self-adjustment of the filter centre wavelength is achieved by the combined effects of the power-dependent saturable absorption, provided by an appropriate dopant, and partial longitudinal hole burning povided by the spatial decoupling of the different wavelengths. External cavity lasers using this type of filter in the external cavity are also described. This external cavity configuration can provide stable single frequency operation of, for example, a semiconducteur laser. By using a saturable absorber for the external cavity (e.g. an erbium doped fibre), longitudinal mode-hopping can be suppressed, ensuring single frequency operation.