1988-12-19
1989-11-21
Sikes, William L.
350 9612, 350 9613, 350 9615, G02B 634
Patent
active
048817910
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
The invention relates to an optical device.
There is currently considerable interest in the use of high-reflectivity grating filters for use as feedback and filtering elements in, for example, fibre lasers. At present, the generation of high reflectivity grating filter involves etching gratings formed in photoresist on top of polished directional couplers and providing an overlying layer of oil with a refractive index matching that of the underlying waveguide. An example of this is described in "High-Reflectivity Monomode-Fibre Grating Filters" Electronics Letters, 13th March 1986, Vol. 22, No. 6, pages 341-343.
The generation of these filters involves complex fabrication procedures which are difficult to circumvent and expensive.
SUMMARY OF THE INVENTION
In accordance with the present invention, an optical device comprises an optical waveguide underlying a first layer of material which has a refractive index higher than the effective refractive index of the waveguide and which forms a planar waveguide capable of supporting and 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; and a diffraction grating provided on or adjacent to at least one surface of the first layer, the arrangement being such that an optical signal with a selected wavelength which is coupled from the waveguide into the first layer impinges on the diffraction grating and is coupled back into the waveguide.
This invention provides an alternative approach to the etching of gratings directly onto the waveguide by applying the grating in a high index overlay structure. The wavelength which is selected can be predetermined but in some cases the refractive index of the first layer of material could be tuned. This might be possible if the material of the first layer was electro-optic (eg. a liquid crystal).
In most cases several optical modes will be coupled into the first layer but only one will be reflected. However, in other cases just one optical mode might be coupled in the first layer.
The diffraction grating is preferably provided on or adjacent to the surface of the first layer remote from the waveguide. This enables the diffraction grating to be made independently of the remainder of the device. However it could be provided on the surface adjacent the waveguide or two diffraction gratings could be provided, one on each surface.
Preferably, the device further comprises a second layer (or superstrate) overlying the first layer with the surface of the second layer facing the first layer being provided with the diffraction grating.
Typically, the second layer will form a non-guiding superstrate having a substantially planar surface and a refractive index which is lower than the effective refractive index of the first layer for a given mode of propagation.
The first layer and the superstrate are preferably in intimate contact. This obviates the need for an index matching liquid between the first layer and the superstrate. The waveguide and the first layer are also preferably in intimate contact but may be spaced a small distance apart. For effective coupling, close proximity of the waveguide and first layer are required for strong field coupling, providing a degree of lateral confinement of the field in the first layer.
The waveguide conveniently comprises an optical fibre, and is preferably an optical single mode fibre.
In another form of the invention, the waveguide may comprise a waveguide associated with, or forming part of, an integrated optics device.
The optical device according to the invention may be used in a wide variety of applications but is particularly suited for use as feedback or filtering elements in fibre lasers.
The diffraction grating preferably comprises a reflection diffraction grating although a phase grating without a reflective layer could also be used.
Reflection at the diffraction grating will occur when the Bragg condition is satisfied, that is: index of the guided mode in th
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Mallinson Stephen R.
Millar Colin A.
British Telecommunications public limited company
Healy Brian M.
Sikes William L.
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