Patent Description:
Optical waveguide crosspoints are known. Such crosspoints are invariably of an '+' shape. Each of the four arms of the crosspoint is a 1X1 multimode interference section designed to have the property whereby an optical signal at one end is re-imaged at the other end. An optical signal which enters (for example) the left arm of the crosspoint from a waveguide is re-imaged at the center of the crosspoint and then re-imaged again at the end of the right arm of the crosspoint where it is received by a further waveguide. An optical signal which enters the top arm of the crosspoint is re-imaged at the center of the crosspoint and then re-imaged again at the end of the bottom arm where it is received by a further waveguide.

A typical example of such an optical waveguide crosspoint is disclosed in <NPL>. Each arm of the crosspoint has a width direction parallel to its end face and a length direction normal to the width direction. The width of each arm (ie each MMI section) is small, typically of the order <NUM>. The correct length of an MMI section is proportional to the square of its width and accordingly the width the length ratio for each MMI section is large, typically of the order <NUM>.

Such a crosspoint can work well. There is negligible loss in signals moving between the left and right arms and also the top and bottom arms in the crosspoint. The crosspoint only works well is if is a '+' shape. If one were to move the relative positions of the arms to form a 'T' or an 'L' the crosspoint would show a high loss. The fact that any crosspoints in an optical circuit have to be of a '+' shape is a significant constraint in the design of optical circuits.

<NPL> discloses a design technique for low loss waveguide crossing using the self-imaging properties of multimode interference (MMI) structures. The technique explots the observation that optical fields with a small I/e width relative to that of the MMI structure in the image reformed region experience negligible propagation loss even in the absence of lateral guiding mechanism around the region. This allows the introduction of a slab in the MMI region.

The present invention seeks to overcome the problems of the prior art.

Accordingly, the present invention provides an optical waveguide crosspoint comprising.

Due to the relative dimensions of the multimode interference sections the compound multimode interference section shows minimal loss in signals passed between the primary input and output waveguides and also the secondary input and output waveguides whilst being a T shape rather than the known '+' shape. There is no need to gang MMI sections together which improves tolerance to variations in wavelength and relaxes manufacturing tolerances.

Preferably the width of the first multimode section is less than <NUM>%, more preferably less than <NUM>% of the length of the second multimode section.

Preferably the width of the second multimode interference section is less than <NUM>%, more preferably less than <NUM>%, more preferably less than <NUM>% of the length of the first multimode interference section.

Preferably the first multimode interference section is dimensioned to have a primary re-imaging length of L1 at wavelength λ1 and a secondary re-imaging length at λ1 of less than L1, the second multimode interference section comprising a symmetry axis extending from the centre of the input face to the centre of the output face the symmetry axis of the second multimode interference section being arranged a distance from the input face of the first multimode interference section equal to the secondary reimaging length.

Preferably the secondary re-imaging length is <NUM>. 5L1, such that the two arms of the T are of equal length.

Preferably the first multimode interference section is a single multimode interference section.

Alternatively the first multimode interference section is a ganged multimode interference section at λ1, re-imaging a single image at the secondary reimaging length.

Preferably the optical waveguide crosspoint further comprises an optical source connected to the at least one primary input optical waveguide, the optical source being adapted to provide a signal at wavelength λ1.

Preferably the second multimode interference section is dimensioned to have a primary re-imaging length of L2 at λ2.

Preferably the optical waveguide crosspoint further comprises an optical source connected to at least one secondary input optical waveguide, the optical source being adapted to provide a signal at wavelength λ2.

Preferably the first multimode interference section comprises a plurality of primary output optical waveguides connected to the output face.

Preferably the first multimode interference section comprises a plurality of primary input optical waveguides connected to the input face.

Preferably the second multimode interference section comprises a plurality of secondary output optical waveguides connected to the output face.

Preferably the second multimode interference section comprises a plurality of secondary input optical waveguides connected to the input face.

Preferably the width of each multimode interference section is at least <NUM>, more preferably at least <NUM>, more preferably at least <NUM>.

The present invention will now be described by way of example only and not in any limitative sense with reference to the accompanying drawings in which.

<FIG> shows a typical optical waveguide <NUM> in vertical cross section. The optical waveguide <NUM> comprises an AlGaAs substrate <NUM>. Arranged on the substrate <NUM> is a GaAs core <NUM>. Arranged on the core <NUM> is an AlGaAs cap <NUM>. The vertical arrangement of layers achieves vertical confinement of the light within the core layer <NUM>; lateral confinement of the light is secured by restricting the said layers <NUM>,<NUM> to a raised ridge structure by means of etching or other procedure. The optical waveguide <NUM> can support one or more optical modes along the waveguide <NUM>. The theory of such waveguides <NUM> is well known and will not be described in detail. The invention is not limited to AlGaAs waveguides <NUM> or to the etched laminar waveguide type. Waveguides <NUM> made in silicon, silica or InP/InGaAsP for example have equivalent properties.

<FIG> shows two such optical waveguides <NUM> from above crossing each other at a crosspoint <NUM>. From the point of view of each individual waveguide <NUM> the crosspoint <NUM> represents a short section lacking any lateral confinement. As an optical signal consisting of a light beam crosses the crosspoint <NUM>, diffraction causes the light beam to spread out as shown. As the signal re-enters the waveguide <NUM> on the other side of the crosspoint <NUM> the light beam is clipped. This results in power loss and may also excite undesirable higher order even modes of the waveguide <NUM>.

Shown in <FIG> is a known multimode interference (MMI) section <NUM>. The multimode interference section <NUM> comprises an input face <NUM>, an output face <NUM> and side walls <NUM> extending therebetween. The multimode interference section <NUM> has a length L between the input and output faces <NUM>,<NUM>. An input optical waveguide <NUM> is connected to the center of the input face <NUM>. An output optical waveguide <NUM> is connected to the center of the output face <NUM>. In use an optical signal passes from the input optical waveguide <NUM> into the MMI section <NUM>. The abrupt widening as the signal passes from the narrow input waveguide <NUM> into the wider MMI section <NUM> excites higher even order guided modes which create well controlled interference patterns over the length of the MMI section <NUM>. Because the modal propagation constants are in a well-known geometric relationship the launch profile is recreated at a distance equal to the beat length of the two lowest order modes from the input face. The MMI section <NUM> can essentially be considered to have a lens like property, focussing the launch profile at this distance from the input face <NUM>. This distance is commonly referred to as the re-imaging length. The re-imaging length is wavelength dependent and also dependent on the geometry of the MMI section <NUM>. Typically the input optical waveguide <NUM> is connected to an optical signal source <NUM> which provides a signal at a wavelength for which the re-imaging length of the MMI section <NUM> is equal to L. In that way a fundamental mode (or any even mode) signal provided to the MMI section <NUM> by the input optical waveguide <NUM> is imaged at the center of the output face <NUM> of the MMI section <NUM> at the join with the output optical waveguide <NUM>. Even order modes have a symmetric field profile while odd modes have an anti-symmetric profile. Even mode signals have a field maximum at the center, an even number of zero crossings and an odd number of lobes. Odd orders have a zero crossing at the center, an odd number of zero crossings and an even number of lobes.

The multimode interference section <NUM> shown in <FIG> is a 1x1 multimode interference section, having one input optical waveguide <NUM> and one output optical waveguide <NUM>. Any odd order modes of the signal passing along the input optical waveguide <NUM> are re-imaged towards the outer edges of the output face <NUM> and so do not pass from the MMI section <NUM> into the output waveguide <NUM>. Such multimode interference sections <NUM> are therefore used as mode filters to reject the odd-order modes in the optical circuit which are typically generated by asymmetric disturbances such as waveguide bends.

Shown in <FIG> is an alternative embodiment of a known MMI section <NUM>. This multimode interference section <NUM> is a 1x2 multimode interference section, being connected to one input optical waveguide <NUM> and two output optical waveguides <NUM>. In this embodiment the multimode interference section <NUM> creates two images of the input signal provided by the input optical waveguide <NUM>, one at each of the output optical waveguides <NUM>. Such an MMI section <NUM> can be considered to be an MMI section <NUM> similar to that of <FIG> but of half the length and which makes use of the dual image which naturally occurs half way along the length of the 1x1 MMI section <NUM>. Such MMI sections <NUM> find common application as optical power splitters or in reverse as combiners.

Shown in <FIG> is a further embodiment of a known multimode interference section <NUM>. This relatively large multimode interference section <NUM> supports many waveguided modes and so re-images at a plurality of points along the length of the multimode interference section <NUM> as shown. As before, the wavelength used with the multimode interference section <NUM> is such that the multimode interference section <NUM> re-images at the length L so imaging the signal received from the input optical waveguide <NUM> to the output optical waveguide <NUM>. Re-imaging at the full length of the MMI section <NUM> is referred to as re-imaging at the primary re-imaging length. Re-imaging at a length less than the length of the MMI section <NUM> is referred to as re-imaging at a secondary re-imaging length.

Shown in <FIG> is a known optical waveguide crosspoint <NUM> similar to that disclosed in Chen and Poon. The optical waveguide crosspoint <NUM> is shaped as an '+'having four arms <NUM> (North, East, South and West respectively). Attached to each arm <NUM> is an optical waveguide <NUM>,<NUM>,<NUM>,<NUM>. Each arm <NUM> is a 1X1 multimode interference section <NUM>. Each multimode interference section <NUM> has a re-imaging length of L at λ1. Attached to two of the optical waveguides <NUM>,<NUM> are optical sources <NUM>,<NUM>. Each source <NUM>,<NUM> provides an optical signal at wavelength λ1. The width of each MMI section <NUM> is small, typically of the order <NUM> and accordingly the width to length ratio of each MMI section <NUM> is large, typically of the order <NUM>.

In use an optical signal passes from an optical waveguide <NUM>,<NUM> into the associated multimode interference section <NUM>. The signal is re-imaged at the center of the crosspoint <NUM> and then again at the end of the opposite multimode interference section <NUM> where it is received by a further waveguide <NUM>,<NUM> as shown.

Considering the East-West arms <NUM> of the optical waveguide crosspoint <NUM> the electromagnetic field along the side walls of the multimode interference sections <NUM> varies with distance from the input and output faces <NUM>,<NUM>. Close to the input and output faces <NUM>,<NUM> the electromagnetic field is concentrated at the center of the input and output faces <NUM>,<NUM> and is negligible at the side walls. Moving away from the input and output faces <NUM>,<NUM> the electromagnetic field spreads out to fill each MMI section <NUM> reaching a peak at the side walls. Moving further away from the input and output faces <NUM>,<NUM> the electromagnetic fields are concentrated into an image of the input/output waveguide profile at the center of the optical waveguide crosspoint <NUM>. To each side of this image in the North South direction the electromagnetic field is relatively small. As the optical signal passes from the East arm <NUM> to the West arm <NUM> it is briefly unconfined as it crosses the North-South arms <NUM>. At this crossing point however the electromagnetic field is concentrated way from the side walls and hence the optical signal is unaffected by this lack of confinement.

The optical waveguide crosspoint <NUM> works well when it is a '+' shape. If (for example) the East and South arms <NUM> are removed to form an 'L' shaped optical waveguide crosspoint <NUM> then the crosspoint <NUM> works less well. In this case the output optical waveguides <NUM>,<NUM> are moved to new positions (shown dotted). Now the unguided crossing width is too large compared to the MMI length and is all at one side of the focus of the L. The optical waveguide crosspoint <NUM> therefore shows a high loss.

In <FIG> each arm <NUM> is considered to be a single MMI section <NUM>. The East and West arms <NUM> (for example) are two single MMI sections <NUM> ganged back to back. The East and West arms <NUM> together can alternatively be considered to be a single ganged MMI section <NUM>. The difference between a single MMI section <NUM> and a ganged MMI section <NUM> is that a ganged MMI section <NUM> will re-image the image at the input face <NUM> part way along its length whereas a single MMI section <NUM> will not. A ganged MMI section <NUM> can be notionally divided into a plurality of single MMI sections <NUM>.

Shown in <FIG> is an embodiment of an optical waveguide crosspoint <NUM> not according to the invention. The optical waveguide crosspoint <NUM> comprises first and second single multimode interference sections <NUM>,<NUM>. Each single multimode interference section <NUM>,<NUM> comprises input and output faces <NUM>,<NUM> and sidewalls <NUM> extending therebetween. The length of the first single multimode interference section <NUM> from the input face <NUM> to the output face <NUM> is L1. The first single multimode interference section <NUM> has a primary re-imaging length of L1 at wavelength A1. The length of the second single multimode interference section <NUM> from the input face <NUM> to the output face <NUM> is L2 and has a primary re-imaging length of L2 at λ2. Each single MMI section <NUM>,<NUM> has a symmetry axis <NUM> parallel to and equally spaced apart from its sidewalls <NUM>. The width of the first single multimode interference section <NUM> normal to its symmetry axis <NUM> is W1. The width of the second single multimode interference section <NUM> normal to its symmetry axis <NUM> is W2.

Connected to the input face <NUM> of the first single MMI section <NUM> is a primary input optical waveguide <NUM>. Connected to the output face <NUM> of the first single MMI section <NUM> is a primary output optical waveguide <NUM>. An optical source <NUM> providing an optical signal having an even mode component at λ1 is connected to the primary input optical waveguide <NUM>.

Connected to the input face <NUM> of the second single MMI section <NUM> is a secondary input optical waveguide <NUM>. Connected to the output face <NUM> of the second single MMI section <NUM> is a secondary output optical waveguide <NUM>. An optical source <NUM> providing an optical signal having an even mode component at λ2 is connected to the secondary input optical waveguide <NUM>.

Each of the waveguides <NUM>,<NUM>,<NUM>,<NUM> is connected to its associated single MMI section <NUM>,<NUM> along the symmetry axis <NUM> of the single MMI section <NUM>,<NUM> as shown.

The first and second single multimode interference sections <NUM>,<NUM> intersect to form a compound multimode interference structure <NUM>. In this embodiment the compound multimode interference structure <NUM> is of an 'L' shape. The output face <NUM> of the first single MMI section <NUM> forms part of a side wall <NUM> of the second single MMI section <NUM>. The output face <NUM> of the second single MMI section <NUM> forms part of the side wall <NUM> of the first single MMI section <NUM>.

The multimode interference sections <NUM>,<NUM> are dimensioned such that the width of each multimode interference section <NUM>,<NUM> is less than <NUM>% of the length of the other multimode interference section <NUM>,<NUM>. More preferably the width of each multimode section <NUM>,<NUM> is less than <NUM>% of the length of the other multimode interference section <NUM>,<NUM>. More preferably the width of each multimode interference section <NUM>,<NUM> is less than <NUM>% of the length of the other multimode interference section <NUM>,<NUM>.

In use a signal at wavelength λ1 passes from the input face <NUM> of the first single MMI section <NUM> to the output face <NUM> of the first single MMI section <NUM>. Close to the output face <NUM> of the first single MMI section <NUM> the electromagnetic field produced by this signal is concentrated away from the side walls <NUM> of the first single MMI section <NUM>. As the width of the second single MMI section <NUM> is small compared to the length of the first single MMI section <NUM> the behaviour of the first single MMI section <NUM> is insensitive to the presence of the second single MMI section <NUM>. The signal therefore passes along the primary path between the primary waveguides <NUM>,<NUM> without significant loss. The same is true for the second single MMI section <NUM> and the signal at A2 passes along the secondary path from one secondary waveguide <NUM> to the other <NUM> without significant loss. This enables signals to pass through the L shaped composite multimode interference structure <NUM> without loss of power or generation of higher order modes in both primary and secondary paths.

In an alternative embodiment not according to the invention the lengths of the two single multimode interference sections <NUM>,<NUM> are equal. In a further alternative embodiment of the invention the widths of the two single multimode interference sections <NUM>,<NUM> are equal. In a further alternative embodiment of the invention both the widths and lengths of the first and second single multimode interference sections <NUM>,<NUM> are equal. In this embodiment both optical sources <NUM>,<NUM> provide optical signals at the same wavelength.

In the above embodiments the first and second single multimode interference sections <NUM>,<NUM> have primary re-imaging lengths of L1 and L2. At least one of these single multimode interference sections may have one or more secondary re-imaging lengths less than L1 or L2 respectively. In this case images are formed part way along the single multimode interference section <NUM>,<NUM> in addition to at its output face <NUM>.

Shown in <FIG> is a further embodiment of an optical waveguide crosspoint <NUM> not according to the invention. This embodiment is similar to that of <FIG> except the first single MMI section <NUM> has a plurality (in this case two) primary optical output waveguides <NUM> connected to its output face <NUM>. The first single MMI section <NUM> is therefore a power splitter, based on a length equal to half the 1x1 re-imaging length. The two single MMI sections <NUM>,<NUM> will generally differ in width as well as length. Several variants of this are possible. In a first alternative embodiment the second single MMI section <NUM> is proximate to the input face <NUM> of the first single MMI section <NUM>, rather than the output face <NUM>. In a further alternative embodiment the first single MMI section <NUM> comprises a plurality of primary optical input waveguides <NUM> in addition to the plurality of primary optical output waveguides <NUM>. In a further alternative embodiment the first single MMI section <NUM> comprises a single primary optical input waveguide <NUM> spaced apart from the symmetry axis <NUM> and opposite one of the primary optical output waveguides <NUM>. In a further embodiment the second single MMI section <NUM> comprises a plurality of secondary input optical waveguides <NUM> or a plurality of secondary output optical waveguides <NUM> or both.

Shown in <FIG> is an embodiment of an optical waveguide crosspoint <NUM> according to the invention. In this embodiment the first and second MMI sections <NUM>,<NUM> intersect to form a compound multimode interference structure <NUM> in the shape of a 'T' as shown. The second MMI section <NUM> forms the stem of the T with the output face <NUM> of the second MMI section <NUM> forming part of a side wall <NUM> of the first MMI section <NUM>. The first MMI section <NUM> forms the arms of the T. The first multimode interference section <NUM> has a length L1 from the input face <NUM> to the output face <NUM> and a width W1 normal to the symmetry axis <NUM>. The symmetry axis <NUM> extends from the center of the input face <NUM> to the center of the output face <NUM> as before. The second multimode interference section <NUM> has a length L2 from its input face <NUM> to its output face <NUM> and a width W2 normal to its symmetry axis <NUM>. Its symmetry axis <NUM> extends from the center of its input face <NUM> to the center of its output face <NUM>. In this embodiment the two arms of the T are the same length, <NUM>.

The width of the first multimode interference section <NUM> is less than <NUM>% of the length of the second multimode interference section <NUM>. More preferably the width of the first multimode interference section <NUM> is less than <NUM>%, more preferably less than <NUM>% of the length of the second multimode interference section <NUM>.

Whilst the width of the second multimode interference section <NUM> is not as tightly constrained as the width of the first multimode interference section <NUM> it is preferred that the width of the second multimode interference section <NUM> is less than <NUM>% of the length of the first multimode interference section <NUM>. More preferably the width of the second multimode interference section <NUM> is less than <NUM>%, more preferably less than <NUM>% of the length of the first multimode interference section <NUM>.

Connected to the primary input optical waveguide <NUM> is an optical source <NUM> which provides an optical signal having an even mode component at wavelength λ1. The first MMI section <NUM> is dimensioned to have a primary re-imaging length of L1 and a secondary re-imaging length of <NUM>. In this embodiment the first MMI section <NUM> is a single MMI section. The re-image formed at <NUM>. 5L1 could for example be a dual image. In an alternative embodiment the first MMI section <NUM> is a ganged MMI section having a single image at the input face <NUM>, the output face <NUM> and <NUM>. As the arms of the T are of the same length the central symmetry axis <NUM> of the second MMI section <NUM> is a distance <NUM>. 5L1 from the input face <NUM> of the first MMI section <NUM>. More generally, the symmetry axis <NUM> of the second multimode interference section <NUM> is arranged a distance from the input face <NUM> of the first multimode interference section <NUM> equal to the secondary re-imaging length. At this distance from the input face <NUM> of the first MMI section <NUM> the electromagnetic field created by the passage of the first signal along the first MMI section <NUM> is concentrated away from the side walls <NUM> of the first MMI section <NUM>. The first signal is therefore unaffected by the presence of the second MMI section <NUM>.

An optical source <NUM> adapted to provide a signal at λ2 is connected to the secondary input optical waveguide <NUM>. The second MMI section <NUM> is a single MMI section and has a primary re-imaging length of L2 at A2. Close to the output face <NUM> of the second MMI section <NUM> the electromagnetic field of the signal at λ2 is concentrated away from the side walls <NUM> of the second MMI section <NUM> and is therefore largely unaffected by the presence of the first MMI section <NUM> due to the relatively small width of the first multimode interference section <NUM>. The composite geometry is taken into account when calculating the re-imaging lengths L1 and L2. For example, the second MMI section <NUM> experiences an effective widening where is it merged into the first MMI section <NUM>. This has a small effect on the optimum value of L2.

In an alternative embodiment of the invention one of the arms of the T is wider than the other In this case the image formerly formed at <NUM>. 5L1 is formed closer to one of the input or output faces <NUM>,<NUM> of the first MMI section <NUM> than the other. Accordingly, in such an embodiment the arms of the T are of different lengths so ensuring the second MMI section <NUM> crosses the first MMI section <NUM> at the correct distance from the input face <NUM> of the first MMI section <NUM> ie at the position where the image is formed in the first MMI section <NUM>.

Shown in <FIG> is a further embodiment of an optical waveguide crosspoint <NUM> according to the invention. This embodiment is similar to that of <FIG> except the second multimode interference section <NUM> includes a plurality (in this case two) secondary output optical waveguides <NUM> connected to its output face <NUM>. The second multimode interference section <NUM> is again a single MMI section and again has a primary re-imaging length of L2 at λ2. In this case the second multimode interference section <NUM> is a power splitter and produces two images at L2 each of which is received by a corresponding secondary output optical waveguide <NUM>. The first MMI section <NUM> is dimensioned to have a primary re-imaging length of L1 at λ1 and a secondary re-imaging length of <NUM>. The first MMI section <NUM> could be a single MMI section and accordingly this could be a dual image at <NUM>. Alternatively the first MMI section <NUM> could be a ganged MMI section with single images at the input face <NUM>, the output face <NUM> and <NUM>.

In a further embodiment of the invention the first multimode interference section <NUM> is either a single or ganged MMI section and is dimensioned to have multiple secondary re-imaging lengths at λ1, at least one of which is not at <NUM>. The second multimode interference section <NUM> is a single MMI section and is arranged with its symmetry axis <NUM> at a distance from the input face <NUM> of the first multimode interference section <NUM> equal to one of these re-imaging lengths and not equal to <NUM>. 5L1 such that the two arms of the T are of unequal length.

In a further alternative embodiment of the invention the first multimode interference section <NUM> of the T shaped compound multimode interference structure <NUM> comprises a plurality (typically two) primary output optical waveguides <NUM> connected to its output face <NUM>. The first multimode interference section <NUM> is dimensioned to re-image a plurality of images at the output face <NUM> of the first multimode interference section <NUM> each of which is received by a corresponding primary output optical waveguide <NUM>. In a further embodiment of the invention the first multimode interference section <NUM> comprises a plurality (typically two) of primary input optical waveguides <NUM> connected to its input face <NUM>. In a further alternative embodiment of the invention the second multimode interference section <NUM> comprises a plurality of secondary input optical waveguides <NUM> connected to its input face <NUM>.

An important feature of the current invention is that it employs known optical structures. 1x1 MMI sections are commonly employed as odd-mode filters in optical circuits while 1x2 MMI sections are commonly employed as power splitters. These may be present in the circuit close to where the cross point is needed. By passing a secondary waveguide at right angles through such a mode filter or power splitter at a suitable point along its length the mode filter or power splitter can in addition to its primary function provide a low loss optical waveguide crossing point for the primary path (that includes the MMI). By additionally adding a suitably dimensioned MMI to the secondary path intersection both primary and secondary paths may be optimised for low loss.

In all of the above embodiments the symmetry axis <NUM> of the second MMI section <NUM> is substantially normal to the symmetry axis <NUM> of the first MMI section <NUM>. This minimises loss and unwanted mode conversion. Substantially normal preferably means within five degrees, more preferably within two degrees, more preferably with one degree.

The width of the input/output ('interconnect') waveguides <NUM>,<NUM>,<NUM>,<NUM> is substantially less than the width of the MMI sections <NUM>,<NUM>. As an optical guided wave passes from an interconnect waveguide <NUM>,<NUM>,<NUM>,<NUM> into an MMI section <NUM>,<NUM> the step change in cross section causes controlled and intentional excitation of higher order modes which is the origin of the re-imaging effect in the MMI sections <NUM>,<NUM>.

Claim 1:
An optical waveguide crosspoint (<NUM>) comprising
first and second multimode interference sections (<NUM>, <NUM>), each comprising an input face (<NUM>), an output face (<NUM>) and sidewalls (<NUM>) extending therebetween, the distance between the input face and output face for each multimode interference section being the length of the multimode interference section, the lengths of the first and second multimode interference sections (<NUM>, <NUM>) being L1 and L2 respectively;
the first multimode interference section (<NUM>) being a single or ganged multimode interference section and the second multimode interference section (<NUM>) being a single multimode interference section;
at least one primary input optical waveguide (<NUM>) connected to the input face (<NUM>) of the first multimode interference section (<NUM>);
at least one primary output optical waveguide (<NUM>) connected to the output face (<NUM>) of the first multimode interference section (<NUM>);
the first multimode interference section (<NUM>) comprising a symmetry axis (<NUM>) extending from the center of the input face (<NUM>) to the center of the output face (<NUM>);
at least one secondary input optical waveguide (<NUM>) connected to the input face (<NUM>) of the second multimode interference section (<NUM>);
at least one secondary output optical waveguide (<NUM>) connected to the output face (<NUM>) of the second multimode interference section (<NUM>);
characterised in that
the first and second multimode interference sections (<NUM>, <NUM>) intersect to form a T shaped compound multimode interference structure with the first multimode interference section (<NUM>) forming the arms of the T and the second multimode interference section (<NUM>) forming the stem of the T; and,
the width of the first multimode interference section (<NUM>) in a direction normal to its symmetry axis (<NUM>) being less than <NUM>% of the length of the second multimode interference section (<NUM>).