Optical waveguides – Directional optical modulation within an optical waveguide
Patent
1992-03-27
1994-06-14
Lobo, Ian J.
Optical waveguides
Directional optical modulation within an optical waveguide
385 43, 385 28, 385 1, G02B 2602
Patent
active
053217772
DESCRIPTION:
BRIEF SUMMARY
This invention relates to modulation techniques for optical communication systems, and in particular to modulation of light guided in an optical fibre.
In many instances, it is desirable to be able to modulate light within an optical fibre, without conducting light from the fibre through a separate modulation element. One such technique, for providing a clip-on supervisory channel, is disclosed in the specification of our copending patent application 8909944.4, in which the preferred embodiment comprises a small radius mandrel mounted on a loudspeaker coil, the mandrel impinging on an optical fibre to impart transient deformation to the fibre in accordance with the coil movement. This provides amplitude modulation of the light in the fibre, by virtue of differential losses from the fibre dependent upon the deformation.
A significant problem within an optical communication network is the provision of flexibility points at which expansion, or rearrangement, of the network can subsequently be made. For flexibility, it is desirable to maximise the number of potential coupling or tapping points. This is not possible if there is significant loss at the tapping points not utilised.
The aim of the present invention is to provide in-fibre modulation, and also to provide low-loss potential tapping points.
The present invention provides an optical fibre assembly comprising an optical fibre having a suspended tapered portion, and means for applying a perturbation to the tapered portion to vary the percentage transmission through the fibre, wherein the assembly is such that, in use, the perturbation applying means distorts the waveguide geometry of the tapered portion causing coupling of an optical beam carried by the tapered portion into higher order bound modes.
Preferably, the suspended tapered portion of the optical fibre is supported, under tension, between two support members.
Advantageously, the tapered portion comprises a fibre tapered by heating and drawing, the minimum radius of the tapered portion being 20 microns. Alternatively, the tapered portion comprises a fused coupler.
Preferably, the perturbation applying means comprises a vibratile member, such as a piezo-electric device. The vibratile member may impart a lateral deformation, or changes in tension, to the tapered portion.
The invention also provides a method of regulating the transmission power through an optical fibre, the method comprising the steps of applying a perturbation to a suspended tapered portion of the fibre, whereby the waveguide geometry of the tapered portion is distorted causing, in use, coupling of the modes of an optical beam carried by the tapered portion into higher order bound modes.
The invention will now be described in greater detail, by way of example, with reference to the accompanying drawings, in which:
FIG. 1 illustrates an embodiment of the invention in which a tapered fibre is subjected to lateral deformation;
FIG. 2 shows an embodiment in which a fused coupler is subjected to varying longitudinal tension; and
FIG. 3 shows a modified form of the embodiment shown in FIG. 2 in which a fused coupler is connected to a reflector loop.
Referring to the drawings, FIG. 1 shows an optical fibre 1 which carries an optical beam such as a CW laser beam. A portion 2 of the fibre 1 that has previously been tapered, for example to a minimum radius of about 20 microns (although larger or smaller minimum radii are possible), is supported by a substrate 3 having arms 3a attached to the fibre, for example by glue or cement, at each side of the tapered portion, so that the tapered portion is suspended between the arms. It has been found that a suitable tapered portion 2, having an overall reduced diameter length of about 27 mm, can be produced by heating and drawing a standard optical fibre at 1700.degree. C. The insertion loss of such a tapered portion 2 is typically 0.05 dB. The supporting substrate 3 may be of any suitable material, but is preferably made of silica or some other material having a thermal expansion matched to that of t
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Achurch Nicholas E.
Hicks Andrew M.
Hornung Stephen
Wilkinson Iain J.
British Telecommunications public limited company
Lobo Ian J.
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