GB2228799A - Optical star coupler - Google Patents

Abstract

An NxN optical star coupler comprises two GRIN lenses 52, 53 spaced apart from one another and having a partial reflector 54 placed therebetween. Light incident on a lens via an input I is focussed onto the partial reflector 54 where part of it is reflected and returns through that lens 52 to an output O, and the remainder passes through the other lens 53 where it emerges via another output O. Optical fibres 55, 56, 57, 58 couple N outputs to N inputs. <IMAGE>

Description

OPTICAL COUPLER The present invention relates to optical couplers, and particularly, but not exclusively, to star couplers for distributing light from each of several inputs to each of the same number of outputs.

Optical fibres are extensively used for telecommunication purposes to carry optical data through teleccmunications networks. Optical couplers are important elements of such networks, as they allow light in one fibre to be coupled to one or more other fibres.

A know 2x2 coupler is described in US patent No. 4557566 by Kikuchi et al. The coupler comprises two spherical graded refractive index lenses each having a core of uneven refractive index of the shape of a sphere or hemisphere and surrounded by cladding forming a spherical shell and a rod disposed on the periphery of the core. A beam splitter is disposed between the lenses, and the abutting surfaces may be joined by means of a suitable adhesive.

It is known to form star couplers having a plurality of inputs and a plurality of outputs (NxN coupler) by interconnecting an appropriate number of 2x2 couplers. A disadvantage of this known way of forming an NxN coupler is that it is expensive, as the number of basic 2x2 couplers required increases exponentially with the number of inputs and outputs the coupler is required to have. A further disadvantage is that the manufacture of such couplers is time consuming as it is necessary to splice many fibres. The present invention seeks to provide an optical coupler which overcomes these disadvantages.

Accordingly, a first aspect of the present invention provides an NxN optical coupler having N(log2N) inputs and N(log2N) outputs and comprising (N(log2N))/2, 2x2 couplers, each 2x2 coupler comprising a first and a second input and a first and a second output; and a first optical coupling means common to all the 2x2 couplers formed from a partial reflector located between a first and a second collimating means and arranged such that light from each input is collimated by one of the collimating means, a portion of it is reflected by the partial reflector to be focussed by that one of the collimating means to one of the outputs, and the remaining portion is transmitted through the partial reflector to the other collimating means which focusses it on the other output; and a second optical means coupling N(log2N)-N of the outputs to N(log2N)-N of the inputs so as to form an NxN star coupler with the remaining N outputs and N inputs.

By spatially multiplexing the N, 2x2 star couplers according to the present invention it is possible to form a NxN star coupler using only one first coupling means.

This obviates the need to use a large number of basic 2x2 couplers to form the NxN coupler and when N is large, results in smaller, more compact and robust NxN couplers.

In addition the manufacture of NxN couplers made in accordance with the invention is substantially less time-consuming than that of the known NxN coupler described above.

Preferably-, the second optical means is formed from optical fibres which may be single optical fibres, each fibre connecting an output to an input, or, when the signal being transmitted by the coupler is two dimensional, a coherent bundle of a plurality of optical fibres.

According to a second aspect of the present invention there is provided an optical coupler comprising N, NxN optical couplers each according to the first aspect of the present invention all having the first optical coupling means in common.

The first optical means may comprise a y-cut lithium niobate substrate having a titanium indiffused planar waveguide.The first and second collimating means may then comprise first and second Luneburg lenses respectively, and the partial reflector may be a surface grating beam combiner. Such a coupler maybe simply fabricated by using integrated optic techniques such as photolithography.

The first optical means could also comprise a crystal silica substrate in which a planar waveguide is formed by doping a region in a top layer of the silica.

Alternatively the first optical means comprises a silicon substrate having a silica indiffused planar waveguide.

The first and second collimating means may alternatively comprise first and second Fresnel lenses respectively.

By forming the first optical means from either a silica substrate, or a silicon substrate, well known microprocessing techniques may be used to align the coupler with other optical devices.

The partial reflector may alternatively comprise a grafting, or a combination of two or more gratings formed using holographic techniques. The partial reflector may thus be designed to split light incident on it in a particular way, eg. into more than two portions.

In a preferred embodiment, the first and second collimating means are first and second graded refractive index (GRIN) lenses respectively, and the partial reflector is a partially reflecting coating on an end face of the first GRIN lens which is joined to an end face of the second GRIN lens. The first and second lenses and the partially reflecting coating are thus each held in a fixed position relative to each other. This results in the first optical means being robust and compact.

Additionally, a first optical coupling means comprising-a GRIN lens is particularly suitable for use in spatially multiplexing several NxN star couplers, as the inputs and outputs of each coupler will lie in a separate line at the end faces of each lens remote from the coating.

Embodiments of the invention will now be described by way of example only with reference to the accompanying drawings in which: Figure 1 is a schematic diagram of a known 2x2 optical coupler; Figure 2 is a schematic diagram of a second known 2x2 coupler; Figure 3 is a diagramatic cross-sectional view of a 2x2 coupler as shown in Figure 2, formed from graded refractive index (GRIN) lenses; Figure 4 is a schematic diagram of a 4x4 star coupler composed of 2x2 couplers of the type shown in figure 1; Figure 5 is a schematic diagram 4x4 optical star coupler in accordance with the first aspect of the present invention, composed of 2x2 couplers of the type shown in Figure 2; Figure 6 is a perspective view of a second embodiment of a 4x4 optical star coupler made in accordance with the first aspect of the present invention; and Figure 7 is a perspective view of a star coupler made in accordance with the second aspect of the invention comprising M, NxN star couplers according to the first aspect of the invention.

Referring to figure 1, a known 2x2 optical coupler is shown generally by the numeral 1 and has input ports I and 12 and output ports Ol and 02. Light entering the coupler at input I1 is guided by fibre 2. At point 3 of the fibre 2, the fibre 2 is spliced to a second fibre 4. The splice joining the fibre 3 to fibre 4 may be formed by stripping the primary coating from a short length of each of the fibres 2, 4. The stripped portions are then heated whilst being stretched, and are thus each formed into biconical tapers. The fibres 2, 4 may-then be fused together at their respective biconical tapers to form the splice 3. At the splice 3, light entering input I1 will be split such that a portion is coupled to the fibre 4, and a portion remains in fibres 2.The portion of light coupled to fibre 4 will emerge at output 02 whilst the portion of light remaining in the fibre 2 will emerge at output 01. Similarly light entering I2 is coupled-to Ol and 02.

Referring to Figure 2, a second known 2x2 star optical coupler comprises two lenses 21 and 22 having the same focal length f. Lens 21 has two focal planes P1 and P2, and lens 22 has two focal planes P2 and P3. Thus P2 is common to both lenses. The distance between Pl and P3 is equivalent to four times the focal length f. Lying in the plane P2 is a 1:1 transflector 25 which operates as an equal intensity, polarisation insensitive beam splitter which transmits half of the power incident on it and reflects the remainder. The coupler has two inputs 11 and I2, and two outputs Ol and 02 corresponding to the functionally equivalent similarly formed ports of Figure 1. Light from input I1 will be incident on lens 21 which collimates it and directs it towards transflector 25. The transflector 25 causes a portion of the light to be-reflected back onto the lens 21 which focusses it onto output 01. The transflector 25 allows the remainder of the light to pass through it. This portion is then incident on the lens 22 which focusses it onto output 02. Similarly, light entering the coupler at input I2 will emerge via outputs Ol and Referring to figure 3, a particular form of the 2x2 optical star coupler shown schematically at Figure 2 is shown. Lenses-31 and 32 are GRIN lenses. The transflector 33 is a coating formed on one end of lens 31. The coating is attached to lens 32 by means of epoxy resin. This arrangement results in a compact and robust coupler, which may be easily moved without the necessity of realignment of the lenses 31, 32 and the transflector 33.

Referring to figure 4, a known 4x4 star coupler 41 is shown having four inputs Ill 12, I3 and 141 and four outputs 011 02, O3 and 04. The coupler 41 comprises four 2x2 couplers of the type shown in figure 1, and here marked C1, C2, C3 and C4. A coupler may be denoted generally by the term Cj. Each coupler C.

has two inputs 11 (Cj) and I2 (Cj), and two outputs Ol (C.) and 02 (C > ). Light entering any one of the inputs Ill I2, I3 and 14 will emerge via all the outputs 011 021 03 and 04 in a known manner.

Referring to figure 5, a 4x4 optical star coupler 51 in accordance with invention is shown. Here, as N=4, N(log2N) = 4 and N(log2N))/2 = 2, the coupler 51 comprises 4 inputs and 4 outputs and 2, (2x2) couplers.

The coupler 51 comprises lenses 52, 53 and partial reflector 54. The coupler 51 comprises four spatially multiplexed 2x2 optical star couplers of the type shown in figure 2. The couplers C1, C2, C3 and C4 are equivalent to the similarly designated couplers of figure 4. The lenses 52, 53, and the partial reflector 54 are common to all the couplers C1, C2, C3, C4 to form the equivalent coupling as between inputs and outputs of the couplers Cj of Figure 4.The coupler 51 has four inputs I1, 121 13 and I4, and four outputs 01' 2' 3 and -04. Light entering any one of the inputs Il to 14 will emerge simultaneously at all of the outputs Ol to 04 in the equivalent way to Figure 4.

Optical fibres 55, 56, 57 and 58 each link an output of a coupler to an input of another coupler. This is so that light from an input will emerge from all four outputs 011 021 03' 04.

The number of spatially multiplexed 2x2 couplers may be varied to vary the size of the NxN coupler as desired.

Referring to figure 6, a further embodiment of a coupler according to the invention is shown. The coupler 61 comprises a single crystal silicon substrate 62 having a planar single mode silica waveguide 63. A fixed surface grating beam combiner 64 acts as a partial reflector. It is fabricated by the holographical exposure of a suitable photosensitive material to form a grating in a known manner. The presence of the periodic overlay produces a periodic modulation of the mode index which has the effect of a thick phase grating on the guided waves. Two Fresnel lenses 65, 66, are formed on the either side of the grating 64. Light entering any one of the inputs I1 to 14 will emerge from all of the outputs Ol to 04 in an equivalent way to figure 4.Optical fibres 81, 82, 83, 84 each link an output of a coupler to an input of another coupler. This is so that light from an input will emerge from all four outputs 011 021 031 04.

Referring to Figure 7, an optical star coupler 70 in accordance with the second aspect of the invention comprises three spatially multiplexed 4x4 star couplers of the type shown in figure 5. The coupler 70 comprises two GRIN lenses 71 and 72 and a partial reflector 73. The coupler 70 has three sets of 4 inputs and 4 outputs.

Optical fibres 74-85 each link an output of a coupler to the input of another coupler. This results in light from an input emerging from all 4 outputs associated with that input.

Each of the optical couplers described in figures 1 to 7, are reversible. That is light may enter the 'outputs' and emerge via the 1inputs', as well as, as described, enter the inputs and emerge via the outputs.

In this specification, the term "optical" is intended to refer to that part of the electromagnetic spectrum which is generally known as the visible region together with those parts of the infrared and ultraviolet regions at each end of the visible region which are capable of being transmitted by dielectric optical waveguides such as optical fibres.

Claims (9)

1. An NxN optical coupler having N(log2N) inputs and N(log2N) outputs and comprising (N(log2N))/2, 2x2 star couplers, each 2x2 star coupler comprising a first and a second input and a first and a second output; and a first optical coupling means common to all the 2x2 couplers formed from a partial reflector located between a first and a second collimating means and arranged such that light from each input is collimated by one of the collimating means, a portion of it is reflected by the partial reflector to be focussed by that one of the collimating means to one of the outputs, and the remaining portion is transmitted by the partial reflector to the other collimating means which focusses it on the other output; and a second optical coupling means coupling N(log2N)-N of the outputs to N(log2N)-N.of the inputs so as to form an NxN star coupler with the remaining N outputs and N inputs.

2. A coupler as claimed in claim 1, wherein the second optical means is formed by optical fibres.

3. An optical coupler comprising X, NxN optical couplers each as claimed in claim 1 or claim 2, all having the first optical coupling means in common.

4. A coupler as claimed in any one of the preceding claims, wherein the first and second collimating means are first and -second GRIN lenses respectively.

5. A coupler as claimed in claim 4 wherein the partial reflector is a partially reflecting coating on an end face of the first GRIN lens, and is joined to an end face of the second GRIN lens.

6. A coupler as claimed in any one of claims 1, 2 or 3, wherein the first optical means is formed im a silicon substrate.

7. A coupler as claimed in claim 6, wherein thefirst and second collimating means are first and second Fresnel lenses respectively.

8. A coupler as claimed in claim 6 or claim 7, wherein the partial reflector is a surface grating beam combiner.

9. A coupler substantially as herein described with reference to any one of figures 5, 6 and 7.