GB1596405A - Optical fibres - Google Patents

Description

(54) IMPROVEMENTS IN OR RELATING TO OPTICAL FIBRES (71) We, THE PLESSEY COMPANY LIMITED, a British Company, of Vicarage Lane, Ilford, Essex, do hereby declare the invention for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: The present invention relates to optical apparatus for use in association with optical fibres.

In particular the invention provides a coupler for use with single multimode optical fibres. A known coupler is described in our co-pending application No. 48337/74 (Serial No. 1538195) in which mode coupling is obtained by constraining the optical fibre into a sinusoidal form. One problem with such a form is that since the bends in the fibre are equal in two directions substantially equal output beams are produced in these directions. This is inconvenient since it is preferable to collect the output on only one side of the fibre.

It is an object of the present invention, therefore, to provide apparatus which causes the output to be predominantly on one side of a fibre.

Accordingly the present invention provides a method of extracting light laterally from a single multimode optical fibre including releasably constraining an optical fibre in a shape in the form of a partially sinusoidal curve modified by superimposing a relatively gentle curve thereon.

In a particular form the apparatus comprises first and second soft transparent pads for applying pressure to an optical fibre, a rigid backing plate for each of the soft transparent pads, means for applying a predetermined pressure to at least one of the rigid backing pads, the rigid backing pads being so shaped that on application of said predetermined pressure the fibre is constrained to take . up said partially sinusoidal shape.

Embodiments of the present invention will now be described, by way of example with reference to the drawings accompanying the provisional specification in which: Figure 1 illustrates the desired partial sinusoidal shape of an optical fibre, and Figure 2 shows apparatus according to the present invention for imparting such a partial sinusoidal shape to an optical fibre.

Referring now to Figure 1 a fibre 10 is shown constrained to lie in a partial sinusoidal shape. This shape is achieved by imparting a gentle curve of radius-R to a sinusoidal shape. This results in the radii R, and R2 becoming different, R, now being much greater than R2. This results in mode loss MLl being very much greater than ML2.

We thus have Rl > R2 and ML, ML2 The distance D is the critical coupling wavelength for the chosen mode.

In a practical embodiment where Rl=2R2 where R23 cms for a fibre of 85 ,um core diameter (N. A. of 0.17), and D=l.9 mms the mode loss power will be predominantly upwards. It is, therefore, possible to arrange the detectors on the upper side and for these detectors to receive substantially more power than in the known apparatus as described in our above referred to copending application.

Figure 2 shows apparatus for imparting the partial sinusoidal shape to the fibre.

First and second soft transparent rubber pads 20, 21 (e.g. silicone rubber) are attached to respective backing plate 22, 23.

The fibre 24 to be shaped is placed between the soft rubber pads and pressure P is applied to the backing plates. Due to the curved serrated front face of plate 23 the distortion will be partially transferred to the fibre which will assume the required partial sinusoidal shape.

Other forms of apparatus may be easily envisaged for example, specially shaped silicon rubber pads or transparent plastic pads of a partial sinusoidal shape.

WHAT WE CLAIM IS:- 1. A method of extracting light laterally from a single multimode optical fibre, including releasably constraining an optical fibre in a shape in the form of a partially sinusoidal curve modified by superimposing a relatively gentle curve thereon.

2. Apparatus for carrying out the method of claim 1 in which means for constraining the optical fibre comprises first and second soft transparent pads for applying pressure to the optical fibre a rigid backing plate for each of said soft transparent pads, means for applying a predetermined pressure to at least one of said backing pads, the rigid backing pads being so shaped that on application of said predetermined pressure the optical fibre is constrained to take up said modified sinusoidal shape.

3. Apparatus substantially as described with reference to the drawings accompanying the provisional specification.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (3)

**WARNING** start of CLMS field may overlap end of DESC **. WHAT WE CLAIM IS:-

1. A method of extracting light laterally from a single multimode optical fibre, including releasably constraining an optical fibre in a shape in the form of a partially sinusoidal curve modified by superimposing a relatively gentle curve thereon.

2. Apparatus for carrying out the method of claim 1 in which means for constraining the optical fibre comprises first and second soft transparent pads for applying pressure to the optical fibre a rigid backing plate for each of said soft transparent pads, means for applying a predetermined pressure to at least one of said backing pads, the rigid backing pads being so shaped that on application of said predetermined pressure the optical fibre is constrained to take up said modified sinusoidal shape.

3. Apparatus substantially as described with reference to the drawings accompanying the provisional specification.