1987-08-21
1989-09-26
Lee, John D.
350 9617, G02B 626
Patent
active
048695670
DESCRIPTION:
BRIEF SUMMARY
This invention relates to coupler devices, and in particular to optical coupler devices.
According to the present invention, a frequency selective optical coupler device comprises a 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.
The first layer may be overlayed by a further layer forming a non-guiding substrate 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/or the substrate may be uniform or non-uniform in comparison, and preferably of uniform refractive index.
The first layer and substrate are preferably in intimate contact, but may instead be spaced a small distance apart, for example byy one or more intervening layers, or an air gap. The waveguide and the first layer are also preferably spaced a small distance apart, but may be in intimate contact. For effective coupling, close proximity of the waveguide and first layer is 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 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 first layer and/or the substrate may be provided with means, such as electrodes, for example, to modify the optical properties of the associated layer, thereby to permit control of the operation of the coupler device.
Coupler devices according to the invention may be used to form optical modulators, filters, and other devices operating on an optical signal at distinct positions along the length of an optical fibre, preferably a single mode fibre.
An important aspect of the present invention is that the index of the planar waveguide may be appreciably greater than the effective index of the waveguide, that is to say, in the case where the waveguide is a fibre, and the planar waveguide a film for example, that
The refractive index (n) of a material is understood herein to be the bulk refractive index of the material as determined bymeans of an Abbe refractometer, for example. The effective refractive index (ne) is the ratio of the speed of light in vacuo (c) to the phase velocity (vp) of the guided mode concerned.
The refractive index of the first layer is conveniently at least 1.degree./o to 10.degree./o, and preferably at least 15.degree./o to 20.degree./o, greater than the effective refractive index of the waveguide. However, it has been calculated that efficient coupling will be obtained even in cases where the refractive index of the first layer is 60.degree./o higher than the effective refractive index of the waveguide.
The present invention will now be described further by way of examples and with reference to the accompanying drawings, of which:
FIG. 1 is a schematic sectional view through a channel dropping optical filter device according to the present invention;
FIG. 2 is a mode dispersion diagram;
FIG. 3 is a graph showing transmission vs wavelength for the half-coupler of the device of FIG. 1 with a thin overlay film;
FIG. 4 shows the output of a device with a thicker overlay;
FIGS. 5A and 5B illustrate the effects of a solid wave guide overlay.
Referring first to FIG. 1 of the drawings, an optical channel dropping filter or coupler in accordance with the present invention comprises a polished optical fibre half-coupler 1 incorporating an optical single mode fibre wave guide, 11 and underlying a first layer formed by a thin film 2. A substrate in the form of a fused silica slide 3 is mounted on top of the film 2. This serves to form a boundary for the film, as it has lower refractive index a
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Brierley Michael C.
Mallinson Stephen R.
Millar Colin A.
British Telecommunications public limited company
Heartney Phan T.
Lee John D.
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