GB2388670A

GB2388670A - Coupling multicore optic fibre cores to one single core optic fibre

Info

Publication number: GB2388670A
Application number: GB0211446A
Authority: GB
Inventors: James Gordon Burnett
Original assignee: Qinetiq Ltd
Current assignee: Qinetiq Ltd
Priority date: 2002-05-18
Filing date: 2002-05-18
Publication date: 2003-11-19
Also published as: WO2003098294A1; GB0211446D0; AU2003240998A1

Abstract

A multicore optical fibre 5 is connected to a single core of a single core fibre 10 using a first GRIN lens 2 of 0.25 pitch for connecting to the multicore fibre, a second GRIN lens 3 of 0.25 pitch for connecting to the single core fibre and a diffractive optical element 4 connecting optically to both GRIN lenses for splitting incident wavefronts from N different cores into N Ò 1-order wavefront pairs, whereby output from a plurality of cores within the multicore fibre 5 is received by the core of the single core fibre 10.

Description

1 2388670

Fibre Optic Connector The invention concerns a method of connecting each core of a multi-core optical fibre 5 to the core of a single core optical fibre, and to a connector for connecting such two fibre types. Such a connector allows for the simultaneous interrogation of N cores in the multicore fibre through a common single-mode fibre download.

Optical fibres are known components having a core along which light can pass either 10 In single mode or multi mode transmission, surrounded by an optically insulating sheath. The fibre may be single or multicore. Typically the fibre cores are of a glass material having a diameter of several microns upwards, e.g. 50 to 500,um. In a multicore fibre the cores may be several tens of Am apart, e.g. 30 to 1 00pm apart.

15 One use of multicore fibres is in the monitoring of structures where the fibre is incorporated into the structure to measure bend. It is necessary to read the outputs from different cores to obtain a bend measurement. One problem concerns coupling light from the cores to other fibres leading to detectors and other components for calculating bend etc. One arrangement for connecting a multiple core optical fibre to 20 single core fibres is described in US patent 6,078,708.

According to this invention the above problem is solved by use of two similar GRIN lenses optically connected together through a diffractive optical element and connecting the multicore fibre to one GRIN lens and a single core fibre to the other 25 GRIN lens.

( - 2 According to this invention a method of coupling cores in a multicore optical fibre to a single core of a single core fibre includes the steps of: 5 providing a first GRIN lens of 0.25 pitch for connecting to the multicore fibre; providing a second GRIN lens of 0.25 pitch for connecting to the single core fibre; and providing a diffractive optical element connecting optically to both GRIN lenses for 10 splitting incident wavefronts from N different cores into N + 1-order wavefront pairs, whereby output from a plurality of cores within the muiticore fibre is received by the single core of the single core fibre.

15 According to this invention a connector for coupling cores in a multicore optical fibre to a single core of a single core fibre includes: a first GRIN lens of 0.25 pitch for connecting to the multicore fibre; 20 a second GRIN lens of 0.25 pitch for connecting to the single core fibre; and a diffractive optical element connecting optically to both GRIN lenses for splitting incident wavefronts from N different cores into N 1- order wavefront pairs, 25 whereby output from a plurality of cores within the multicore fibre is received by the core of the single core fibre.

- 3 The optical fibres transmit visible light and/or other wavelengths such as IR and UV.

The diffractive optical element may be formed by holographic techniques. For 5 example it may be formed from a photoresist material on one end of a GRIN lens illuminated by light from a multicore fibre fixed to the other end of the lens.

The invention may also be used to connect a plurality of single core optical fibres to the muiticore optical fibre. In this case the DOE needs to be different to that used for 10 a single fibre.

GRIN lenses are known commercially obtainable optical components; for example obtainable from Nippon Sheet Glass Company. The GRIN lenses are described as being of 0.25 pitch. This is the theoretical ideal, true at one wavelength; in practice 15 deviation from the ideal is satisfactory.

The term pitch relates to the number of cycles that are associated with the sinusoidal trajectory of an optical ray propagating from the input face of the GINR lens to its output face. The sinusoidal trajectory of an optical ray propagating along a GRIN lens 20 Is a consequence of the quadratic refractive index profile of the GRIN lens. An optical ray that propagates along a ray path trajectory equal to one cycles of a sinusoid has a pitch of 1.0.

A 0.25 pitch lens will propagate rays through a quarter of a sinusoid cycle and 25 therefore all rays emanating from a point on the input face of a 0.25 GRIN lens (provided these ray propagate within the numerical aperture of the GRIN lens) will exit the GRIN lens at its output face colinearly (i.e. they will describe a collimated beam).

f The invention will now be described, by way of example only, with reference to the accompanying drawings in which: Figure 1 is a diagrammatic view of a connector between a multicore optical fibre and 5 a single core optical fibre; Figure 2 is an enlarged end view of the multicore fibre having four cores on a square grid; 10 Figure 3 is an enlarged end view of the single core fibre; and Figure 4 is a schematic view of Figure 1 showing wave traces.

The connector 1 of Figures 1, 2 comprises a first graded index (GRIN) lens 2 15 connected optically to a second GRIN lens 3 through a diffractive optical element (DOE) 4. A multicore optical fibre 5 having N cores is connected optically to the first GRIN lens. Both GRIN lens 3, 4 are 0.25 pitch length. In the specific example N=4, and the four cores 6, 7, 8, 9 are arranged on a square grid with each core offset from fibre axis by as seen in Figure 2. An optical fibre 10 having a single core 11 is 20 connected optically to the second GRIN lens 3.

The whole assembly of GRIN lenses 2, 3 and fibres 5, 10 may be held in position by optical cement and all contained in a potting material for robustness. Alternatively the components may be held in position by a holder 12, which may in two parts 13, 14 25 separable for connecting to different elements.

- 5 The DOE 4 is arranged to split an incident wavefront light into N +1order wavefront pairs, Figure 4. Each wavefront pair has a predetermined tilt offset leading to 2N images on the GRIN lens 2 surface butted to the multicore fibre 5. In the specific 5 example, light 15 from the single core fibre 6 is diffracted into two paths 16, 16' into two opposite cores 6, 8 of the multicore fibre 5. Similarly, light 17 from core 6 is diffracted into two paths 18 one into the single core 11 and the other path 18' onto the end face of the second GRIN lens 3. Likewise, light 19 from core 8 is diffracted into two paths, one 20 onto the single core 11 and the other path 20' onto the end face of 10 the second GRIN lens 3.

The DOE 4 may be formed by holographic techniques. For example a layer of a photoresist material may be spun onto the end of a GRIN lens. On the other end is fixed the multicore fibre 5. Light is directed down the multicore fibre 5, through the 15 GRIN lens onto the photoresist to form interference patterns in the photoresist. The photoresist is then photographically developed by selective removal of exposed/non exposed (?) material and hardened by a fixture material. The second GRIN lens 3 is then attached to the DOE 4.

20 One use of the connector shown is in a bend sensor as described in WO 985921 9A2.

The two cores 8, 9 may be used as two arms of a Michelson interferometer in which case the end of the multicore fibre 5 is cleaved square and coated to maximise reflectivity back along the fibre. Light is directed from the single core fibre 10 into both cores 8, 9. Light travels to the end of the cores 8, 9 arranged within a 25 component whose bending is to be detected. Bending of these cores 8, 9 will produce changes in their relative length" On reflection from the core ends, light is coupled into the single core fibre and transported back to a detector where interference gives rise to a signal which is a function of the optical path difference between the two cores 8, 9 and hence the amount of bend.

Claims

- 6 Claims.

1. A method of coupling cores in a multicore optical fibre to a single core of a single core fibre including the steps of: providing a first G RIN lens of 0.25 pitch for connecting to the multicore fibre; providing a second G RIN lens of 0.25 pitch for connecting to the single core fibre; and: 10 providing a diffractive optical element connecting optically to both G RIN lenses for splitting incident wavefronts from N different cores into N + 1-order wavefront pairs, whereby output from a plurality of cores within the multicore fibre is received by the single core of the single core fibre.

2. The method of claim 1 wherein the diffractive optical element is formed by exposure of light from the cores of the multicore optical fibre and through the G RIN lens onto a layer of photoresist material followed by development of the photoresist material.

3. A connector for coupling cores in a multicore optical fibre (5) to a single core of a single core fibre (10) including: a first G RIN lens (2) of 0.25 pitch for connecting to the multicore fibre (5); a second GRIN lens (3) of 0.25 pitch for connecting to the single core fibre (10); and a diffractive optical element (4) connecting optically to both G RIN lenses (2, 3) for splitting incident wavefronts from N different cores into N + 1-order wavefront pairs, whereby output from a plurality of cores (6, 7, 8, 9) within the multicore fibre (5)is received by the core (1 1) of the single core fibre (10).

4. The connector of claim 3 wherein the optical fibres (5, 10) and GRIN lenses (2, 3) 35 are supported within a holder (12).