Optical waveguides – With optical coupler – Input/output coupler
Patent
1993-03-12
1994-05-24
Gonzalez, Frank
Optical waveguides
With optical coupler
Input/output coupler
385 48, G02B 626
Patent
active
053156757
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to an optical tap for coupling light into, or out of an optical fibre.
2. Related Art
In optical communications, the radiation used is not necessarily in the visible region of the electromagnetic spectrum, and so the words `optical` and `light` when used in this specification are not to be interpreted as implying any limitation to the visible spectrum. For example, the wavelengths preferred for transmission through silica optical fibres are in the infra red region, because the loss minima of silica fibres occur at 1.3 and 1.5 microns.
In many `in field` situations, it is necessary for an engineer to sample the signals that are being transmitted along an optical fibre, or to transmit signals to, for example, a telephone exchange. This may be for maintenance or testing purposes, or so that the engineer can receive and transmit communications.
The specification of our published patent application W088/07689 describes an optical tap that can be temporarily attached to an optical fibre, the fibre being bent, and the tap collecting light that couples out due to the local perturbation caused by the bend. With the arrangement described in that specification, the level of light that is coupled out exhibits a non-linear wavelength dependence. Moreover, a large proportion (98-99% of total core power), escapes from the core along optical paths nearly tangential to the fibre bend so this type of tap has a low coupling efficiency, and can only be used as an intrusive device. The reason for the high optical loss is that leakage occurs over the entire length of the bend, and continues for some distance afterwards, as fibre cladding modes formed by the presence of the bend eventually radiate out of the fibre.
SUMMARY OF THE INVENTION
The present invention provides optical tap apparatus for coupling light into, or out of, an optical fibre, the apparatus comprising an optical head having a V-shaped recess, and a member adapted to deform the optical fibre into the recess, the recess being shaped and sized to subject the optical fibre to a tight curve of short arcuate length when the fibre is deformed into the recess whereby light can be coupled into, or out of, the optical fibre at a punctiform region of its curved portion, wherein the angle at the apex of the V-shaped recess is within the range of from 152.degree. to 179.degree..
The advantage of this type of optical tap is that light can enter or leave the optical fibre at a region which is substantially a point. Consequently, the tap has a much higher coupling efficiency than known optical taps which rely on coupling occurring over substantially the entire length of a fibre bend. In practice, the recess and the abutment member are sized and shaped to subject the optical fibre to a curve whose radius of curvature lies in the range of from 125 .mu.m to 4 mm.
Advantageously, the optical head is a block made of transparent acrylic material, and the block is shaped so that a side surface thereof subjects light travelling towards, or away from, said punctiform region of the optical fibre to total internal reflection, whereby the light enters or leaves the block via a surface thereof which is generally parallel to the recessed surface.
Conveniently, the abutment member is constituted by a resilient protrusion provided on a mandrel. The resilient protrusion may be a generally hemispherical member whose radius is 3 mm and which is made of soft rubber.
In a preferred embodiment, the apparatus is such that the abutment member applies, in use, a pressure lying within the range of from 60 g to 500 g to the optical fibre. Conveniently, the recess has a width of 20 mm, and a depth within the range of from 0.1 to 2.5 mm. Preferably, the recess has a rounded cross-section, the radius of curvature of which lies within the range of from 2 mm to 3 mm.
In a preferred embodiment, the apparatus further comprises a detector for detecting light which is coupled from the optical fibre into the optical head. The detecto
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Dennis Simon M.
Downing Roland W.
Drouet Dominik
Ferguson David A.
James Simon M.
British Telecommunications public limited company
Gonzalez Frank
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