WO2009030916A1

WO2009030916A1 - Method of modulating a beam of light and optical external modulator

Info

Publication number: WO2009030916A1
Application number: PCT/GB2008/003002
Authority: WO
Inventors: Haxha Shyqyri; Terry Gorman
Original assignee: The University Of Kent
Priority date: 2007-09-05
Filing date: 2008-09-05
Publication date: 2009-03-12
Also published as: GB2452505A; GB0717240D0; WO2009030916A8

Abstract

An optical external modulator, includes a substrate (1), provided with an optical channel waveguide circuit. The optical channel waveguide circuit has an optical input (2) and an optical output (3), and includes at least a first junction (5), at which a waveguide channel (4) splits into multiple outgoing waveguide channels (6,7) forming arms of an interferometer, at least sections (13, 16) of two of the multiple outgoing waveguide channels (6,7) adjacent the first junction (5) defining a plane in which they lie, and at least a second junction (8), at which multiple incoming waveguide channels (6,7) forming arms of an interferometer combine to allow optical beams directed towards the optical output (3) to recombine. The second junction (8) is positioned at a distance from the plane in a transverse direction thereto.

Description

METHOD OF MODULATING A BEAM OF LIGHT AND OPTICAL EXTERNAL MODULATOR

The invention relates to an optical external modulator, including a substrate, provided with an optical channel waveguide circuit, the optical channel waveguide circuit having an optical input and an optical output, and including at least a first junction, at which a waveguide channel splits into multiple outgoing waveguide channels forming arms of an interferometer, two of the multiple outgoing waveguide channels defining a plane in which they lie, and at least a second junction, at which multiple incoming waveguide channels forming arms of an interferometer combine to allow optical beams directed towards the optical output to recombine.

The invention also relates to a method of modulating a beam of light, including: splitting the beam of light into at least two outgoing beams; guiding the outgoing beams through respective waveguide channels in a substrate and changing an optical permeability of the substrate material at at least one of the waveguide channels, two of the outgoing beams defining a plane in which they are propagated; and re-combining incoming beams carrying energy from the beam of light and propagated through respective waveguide channels in a substrate, wherein an optical permeability of the substrate at at least one of the waveguide channels is changed.

Respective examples of such an external modulator and method are known. US 2006/0056002 discloses an optical digital external modulator that includes an optical waveguide formed in an x-cut lithium niobate substrate. The optical waveguide includes a first Y-branch, a first interferometer arm, a second interferometer arm and a second Y-branch, which collectively form a Mach-Zehnder interferometer. At the end of the substrate opposing the input/output end, the waveguide and a mirror form first and second directional couplers. A travelling- wave electrode structure is provided near the optical waveguide such that the first part of the first interferometer arm is disposed between a ground electrode and a hot electrode, while the first part of the second interferometer arm is disposed between another ground electrode and the hot electrode. Similarly, the second part of the first interferometer arm is disposed between another ground electrode and a further hot electrode, while the second part of the second interferometer arm is disposed between yet another ground electrode and the further hot electrode. In operation, light is input into the modulator from the input port and is output through the output port. More specifically, the light input through the input port propagates through the optical waveguide until it is split at the first Y- branch, where it then propagates equally along the two isolated paths corresponding to the two interferometer arms. When the light propagating through each arm of the interferometer reaches the corresponding directional couplers, it is reflected back along the corresponding second parts of the interferometer arms. When a time- varying drive voltage corresponding to an RF data modulation signal is applied to the travelling-wave electrode structure, the electro-optical effect causes the relative velocity of the light propagating through the two interferometer arms to change, thus creating a phase shift and producing constructive or destructive interference at the second Y-branch. The constructive and/or destructive interference produces an output amplitude modulated optical signal, wherein the modulation corresponds to the modulation of the RF data signal. The reflective design makes the optical modulator more compact and allows light to enter and exit the optical modulator from the same side. A problem of the known modulator is that it is not very compact in the lateral direction (relative to the direction of propagation through the interferometer arms) . It thus requires a relatively large substrate surface.

It is an object of the invention to provide an optical external modulator and method of the types defined in the opening paragraphs that provide for relatively long channels connecting the input of the modulator to the output but allow for a substrate with a small footprint.

This object is achieved by the optical external modulator according to the invention, which is characterised in that the second junction is positioned at a distance from the plane in a transverse direction thereto.

The optical waveguide circuit has an optical input and an optical output as well as the first and second junctions, which are consequently positioned between the optical input and output. Because the incoming waveguide channels at the second junction lie outside the plane of the outgoing waveguide channels, it follows that the optical waveguide circuit is "folded" . The total optical path length between the optical input and output is relatively large for a given set of dimensions of the substrate in the plane defined by the two outgoing waveguide channels. In effect, use is made of a third dimension to allow the optical path length to be increased.

In an embodiment, the first and second junctions are arranged such that beams of light travelling from the first junction through the outgoing waveguide channels adjacent the first junction are propagated in a direction having at least a component in parallel and oppositely directed to beams propagated through the incoming waveguide channels towards the second junction adjacent the second junction. An effect is that, where the optical input connects directly to the first junction and the second junction connects directly to the optical output, the optical input and output can be positioned on the same side of the substrate without the need for a substrate with a large footprint. A relatively large optical path length through the interferometer or interferometers is provided using a relatively compact substrate. This is achieved by propagating the light in parallel directions at different levels, e.g. with respect to the thickness of the substrate.

In an embodiment, outgoing waveguide channels at the first junction correspond to respective incoming waveguide channels at the second junction, and each waveguide channel includes at least two sections, each section being in a plane with a corresponding section of another of the waveguide channels, such that at least two of the planes are at an angle to each other.

An effect is to maximise the length of the optical waveguide channels forming arms of the interferometer. These optical waveguide channels "fold" back to increase their length without extending the lateral dimensions of the substrate in directions parallel to the plane defined by the two outgoing waveguide. By increasing the optical path lengths of the waveguide channels forming arms of the interferometer, the maximum phase shift to be introduced per unit length can be smaller to achieve full modulation of the light provided at the optical input. For a given electro- optically active material, this implies lower electric fields can be used with advantages in terms of driver voltage, dissipation, etc. Similar advantages are achieved where use is made of an acousto-optically active material. In a variant, the planes in which consecutive sections of a waveguide channel lie intersect at substantially a right angle to directions of propagation of a light beam along the channel sections.

An effect is that corresponding parallel sections of the waveguide channels will be of generally equal length. The equality of length is therefore achievable by means of a relatively simple manufacturing method. It is not necessary to introduce loops or other intricate shapes to match the lengths of optical waveguide channels forming the arms of a particular interferometer. The effect of matching the optical path lengths of the waveguide channels forming the arms of an interferometer is to improve the stability to environmental fluctuations, e.g. temperature stability.

Another embodiment of the optical external modulator includes at least two interferometers in series, each including a junction at which a waveguide channel splits into multiple waveguide channels and a junction at which the multiple waveguide channels re-combine, wherein waveguide channels forming the arms of an interferometer lie generally entirely in a single plane.

An effect is to make it easier to match the lengths of waveguide channels forming arms of an interferometer whilst still making use of a third dimension to keep the modulator compact.

An embodiment of the optical external modulator includes a waveguide channel comprising at least two sections, a first section being positioned at a first surface of the substrate, and a second section being positioned at an edge surface of the substrate. Positioning the sections of a waveguide channel at a surface of the substrate makes the optical external modulator relatively simple to manufacture. By positioning a second section at an edge surface of the substrate, the optical path length of the waveguide channel is made relatively long for given dimensions of the first surface of the substrate.

In a variant, the waveguide channel includes a third section, connecting the first section and the second section and provided at a chamfered edge of the substrate.

The chamfered edge makes it possible to avoid sharp turns in the waveguide channel, which might lead to heavy losses of optical beam power.

In an embodiment, the optical external modulator includes at least one reflecting structure for re-directing light from a first of two consecutive sections of a waveguide channel to a second of the two consecutive sections of the waveguide channel.

An effect is to be able to extend the optical path length of waveguide channels by using a third dimension of the substrate, and yet reduce leakage of beam power where the beam is "folded" .

An embodiment of the optical external modulator includes an electrode assembly for applying an electric field to at least part of the optical channel waveguide circuit, each electrode having at least two contiguous sections at an angle to each other and generally parallel to planes in which respective sections of waveguide channels forming interferometer arms lie. Thus, a single electrode is usable to subject waveguide channel sections at an angle to each other to a varying electrical field. The drive voltage is relatively small for a given substrate footprint, due to the increase in optical path length of the waveguide channels forming the arms of an interferometer. By also "folding" the electrodes, this is achieved with minimal or no increase in the number of electrodes, allowing common driving circuits to be used.

An embodiment of the optical external modulator includes an electrode assembly for applying an electric field to at least part of the optical channel waveguide circuit, which electrode assembly is provided in a travelling wave configuration.

An effect is to increase the upper bound of the bandwidth within which light can be modulated. Radio-frequency (RF) waves can travel at the same velocity along the electrodes as that of the optical waves travelling along the optical waveguide circuit. This eliminates the transit time problem, which is that the electro-optic retardation changes if the modulator medium does not experience the same electric field along the optical path along which the light is propagated.

An embodiment of the optical external modulator includes an electrode assembly for applying an electric field to at least part of the optical channel waveguide circuit and further includes a buffer layer of electrically insulating material, positioned between the electrode assembly and the optical channel waveguide circuit.

The buffer layer makes it possible to manufacture the modulator in the so- called Z-cut configuration. Even in the alternative X-cut configuration (in which optical waveguide channels are located below gaps between electrodes as opposed to being placed directly below, but insulated by a buffer layer from, the electrodes) , the buffer layer provides a mechanism for matching the velocity of the RF waves along the electrodes to the velocity of the optical waves propagated through the waveguide circuit. By varying the buffer layer thickness, one can also match the characteristic impedance of the device to the RF generator, typically

50 Ω. When this is achieved simultaneously with RF and optical velocity matching, the bandwidth of the modulator is optimised. In the Z-cut topology, a buffer layer is always required, to limit optical losses due to lossy metal electrodes. The buffer layer could be left out in an X-cut topology if velocity and impedance matching could be achieved by other means, but the buffer layer is relatively easy to implement and provides additional freedom of design.

In an embodiment, the substrate includes a cavity, bounded by inner surfaces, and the optical channel waveguide circuit is provided at outer surfaces of the substrate facing away from the inner surfaces.

An effect is to provide an alternative mechanism for velocity matching between the RF waves travelling along the electrodes and the optical waves propagated through the waveguide circuit. Additionally, impedance matching between the electrode assembly and the driver circuit can be achieved by varying the distance between the inner and outer surfaces - the effective substrate thickness can be increased or decreased. The cavity may be at least partially filed with a material having relative permittivity different from those of air and the material of the substrate, in order to achieve velocity /impedance matching.

In an embodiment of the optical external modulator, the substrate is at least partly comprised of an anisotropic crystalline material, and the optical external modulator has an X-cut configuration, in which an optical waveguide channel forming an arm of an interferometer is placed to one side of a gap separating at least parallel sections of a first and second electrode arranged to apply an electric field.

The X-cut configuration is easier to manufacture where optical waveguide channels forming interferometer arms also have sections extending in a transverse direction to the plane defined by the two of the multiple outgoing waveguide channels. That is because, in the X-cut configuration, the electric field will remain directed in the same direction relative to the dielectric axes of the waveguide material (generally parallel to the planes in which the waveguide channel sections lie) . In the Z-cut configuration, the electric field is perpendicular to the plane in which the waveguide channel sections lie. It is therefore different with respect to the dielectric axes of the waveguide material for the different sections, unless the substrate is composed of different sections.

In a variant, an optical waveguide channel forming a second arm of the same interferometer is placed to one side of a gap separating at least parallel sections of the first and a third electrode in a generally symmetrical configuration with respect to a plane of symmetry containing at least the section of the first electrode.

This variant tends to minimise "chirp" . The waveguide channels forming the first and second arms of the same interferometer experience a symmetrical change in refractive index due to the positioning of the electrodes. Moreover, the enhanced effect due to the symmetrical change in refractive index generally leads to an enhancement of the modulation, with lower drive voltages as a consequence.

According to another aspect, the method of optically modulating a beam of light according to the invention is characterised in that the incoming beams are re-combined at a point located at a distance from the plane in a transverse direction thereto.

The method allows the use of relatively long optical path lengths through the waveguide channels of which the refractive index is modulated in a set-up having a relatively small extent in directions parallel to the plane defined by the outgoing beams at the first junction.

An embodiment of the method includes the use of an electro-optical external modulator according to the invention.

The invention will be explained in further detail with reference to the accompanying drawings, in which:

Fig. 1 is a first schematic cross-sectional view of an electro-optical external modulator;

Fig. 2 is a second schematic cross-sectional view of the electro-optical external modulator;

Fig. 3 is a top plan view of the electro-optical external modulator;

Fig. 4 is an end plan view of the electro-optical external modulator; and Fig. 5 is a second schematic cross-sectional view of the electro-optical external modulator; and

Fig. 6 is a schematic cross-sectional view of a second electro-optical external modulator.

An electro-optical external modulator in one possible configuration will be described herein in detail by way of example. Alternative configurations will be indicated where appropriate. The modulator includes a generally rectangular substrate 1. The substrate 1 is made of a crystalline material exhibiting the linear electro-optic effect (also known as the Pockels effect) . The material is anisotropic. A commonly used suitable material is lithium niobate. An electro-optical external modulator made of lithium niobate has low drive voltage, modulation across a broad band of wavelengths, low optical insertion losses and good linearity. Alternative materials are listed, for example, in Table 9.2 of Yariv, A and Yeh, P. , "Photonics" , 6^lh edition, 2007. For example, the material may also be a polymer-based substance that exhibits an enhanced electro- optic effect. It is noted that at least some of the effects described herein are also achieved in an embodiment in which the substrate 1 is made of a material exhibiting an acousto-optical effect (also known as photo-elastic effect) .

The substrate 1 is provided with an optical channel waveguide circuit. In the embodiment illustrated in the drawings, the optical channel waveguide circuit comprises an optical input 2 and an optical output 3. A first waveguide channel 4 leads to a first junction 5, at which the first waveguide channel 4 splits into a second waveguide channel 6 and a third waveguide channel 7, forming arms of an interferometer. The second and third waveguide channels 6,7 recombine at a second junction 8 into a fourth waveguide channel 9, which leads to the optical output 3.

The electro-optical external modulator includes an electrode assembly comprising a hot electrode 10 and first and second ground electrodes 11,12. In use, the second waveguide channel 6 and the third waveguide channel 7 are subjected to electric fields of approximately equal magnitude and opposite sign. This field tends to changes the refractive index of the waveguide material, introducing a phase difference between the light travelling through the second and third waveguide channels 6,7. When the light recombines at the second junction 8, interference causes a change in intensity relative to the intensity of the light entering at the optical input 2. Thus, the second and third waveguide channels 6,7 form the arms of a Mach-Zehnder interferometer. The voltage difference that yields a phase difference of π between the light waves travelling through the arms of the interferometer is known as the half-wave voltage V_n. It is inversely proportional to the lengths of the second and third waveguide channels 6,7, over which they are subjected to an electric field.

To reduce the half- wave voltage for given wavelengths of light, electro- optic coefficient of waveguide material and waveguide thickness, the second and third waveguide channels 6,7 are made as long as possible given the dimensions of the substrate 1 in directions orthogonal to the direction of the smallest dimension of the substrate 1. The second waveguide channel 6 includes a first section 13 adjacent the first junction 5, a second section 14 and a third section 15. Similarly, the third waveguide channel 7 includes a first section 16, a second section 17 and a third section 18. The first sections 13, 16 define a first plane generally parallel to a first, top surface of the substrate 1 , in which first plane they lie. This plane extends in the X- and Z-directions indicated in the drawings. Light is propagated generally in the Z-direction through the first sections 13, 16. The third sections 15,18 are positioned adjacent the second junction 8 in a second plane generally parallel to the first plane and at a distance in the Y-direction to the first plane, i.e. in a direction transverse to the first plane.

In the illustrated embodiments, the second plane is located at a second surface of the substrate 1, opposite to the first surface. In another embodiment, the third sections 15,18 extend through the substrate 1 , rather than being positioned at a surface. The illustrated embodiment is relatively easy to manufacture, however.

The second sections 14,17 are positioned at an edge surface of the substrate 1 , in a plane at substantially a right angle to the directions of propagation of light beams through the first and second sections 13, 16, 15,18. This plane is thus also at an angle to the first plane. In this way, the second and third waveguide channels 6,7 extend along three surfaces of the substrate 1. Since they are, in use, subjected to an electric field along their entire lengths, the half-wave voltage V_π is made as small as possible.

To a slightly lesser extent, the same effect is achieved in an alternative embodiment (not shown) by providing a series of interferometers, one on each of three surfaces of the substrate 1 , with the optical outputs of the first two being connected to the optical inputs of the respective succeeding interferometers. This makes it slightly easier to match the lengths of waveguide channels forming the arms of any one particular interferometer. In this type of optical modulator, the interferometers each include a junction at which a waveguide channel splits into multiple waveguide channels and a junction at which the multiple waveguide channels re-combine, and the waveguide channels forming the arms of an interferometer lie generally entirely in a single plane. Matching the lengths of the arms of the interferometer is thus reduced to a two- dimensional problem.

In the illustrated electro-optical external modulator, the substrate 1 is provided with chamfered edges. Also, the first sections 13, 16 of the second and third waveguide channels 6,7 are connected to the second sections 14, 17 by first and second edge sections 19,20. Likewise, the second sections 14, 17 are connected to the third sections 15,18 by third and fourth edge sections 21 ,22. The edge sections 19-22 are provided at the chamfered edges of the substrate 1. Surfaces 23-26 at the sides facing away from the substrate 1 have been made reflective to reduce optical losses at the chamfered edges of the substrate 1 even further. In the illustrated embodiment, the reflective surfaces 23-26 are provided with a coating of a reflective material. Similarly low optical losses are obtained in alternative embodiments of the electro-optical external modulator by means of an extraneous mirror - which may be a micro-electromechanical mirror - a prism or a photonic crystal structure at the interface between waveguide channels or channel sections in the first plane and waveguide channels or channel sections in a plane at an angle thereto. In those alternative embodiments, a reflective surface is formed by a surface of the prism or formed by periodic structures, e.g. cavities in the photonic crystal.

With a view to minimising the loss of optical power, the chamfered edges are generally at an angle to a normal to the plane in which the first sections 13, 16 of the second and third waveguide channels 6,7 w ith a value in the range of 30° to 60° . It is observed that the optical loss is also minimised in this way in the absence of the reflective surfaces 23-26, simply by decreasing the sharpness of the bend in the second and third waveguide channels 6,7.

Alternatives to the illustrated electro-optical external modulator are possible in which electrodes are provided on either side of the second and third waveguide channels 6,7 and connected to a driver circuit in such a way as to establish a generally constant potential along the electrodes 10, 11,12. However, in order to maximise the modulation bandwidth, the electrode assembly is provided in a travelling wave configuration in the embodiments used herein as examples. That is to say that an RF modulation signal is provided in the form of a travelling wave.

A signal source applies the modulation signal at one end of the hot electrode 10 and each of the first and second ground electrodes 11,12.

An impedance-matching termination is provided at the other end of each pair of electrodes. The RF wave velocity is generally matched to the wave velocity of the light propagated through the optical waveguide channel structure to avoid reductions due to the finite transit time of light through the interferometer arms in the phase shift introduced in the arms of the interferometer.

The hot electrode 10 and the first and second ground electrodes 11, 12 each comprise first sections 28,29,30, second sections 31 ,32,33 and third sections 34,35,36, respectively. The first sections 28,29,30 run substantially parallel to the first sections 13, 16 of the second and third waveguide channels 6,7. The second sections 31,32,33 run substantially parallel to the second sections 14,17 of the second and third waveguide channels 6,7. The third sections 34,35,36 run substantially parallel to the third sections 15, 18 of the second and third waveguide channels 6,7.

In designing the electro-optical external modulator, it is desirable also to match the characteristic impedance of the electrodes to the internal impedance of the signal generator used as a driver. Parameters that are variable include the thickness of the electrodes 10,11, 12 and the thickness of a buffer layer 27 provided between the electrodes 10, 11, 12 and the optical channel waveguide circuit. The buffer layer 27 is made of an electrically insulating material, e.g. silicon dioxide.

Referring specifically to Fig. 2, the electro-optical external modulator described herein by way of example is provided in the X-cut configuration. The X and Y axes correspond in direction to the ordinary axes of the (anisotropic) crystalline substrate 1. The first section 13 of the second waveguide channel 6 is placed to one side of a gap separating the first section 28 of the hot electrode 10 and the first section 29 of the first ground electrode 11. The first section 16 of the third waveguide channel 7 is placed to one side of a gap separating the first section 28 of the hot electrode 10 and the first section 30 of the second ground electrode 12. The electric field applied to the first sections 13,16 of the second and third waveguide channels 6,7 is thus parallel to the X- direction.

An alternative embodiment is provided in the Z-cut configuration, in which the crystal axes are oriented differently. In the Z-cut configuration, the crystallographic Z-axis (or extraordinary axis) is parallel to the direction indicated as the y-axis in Fig. 2 -this is the vertical direction in that drawing. In the Z-cut configuration, the first section 13 of the second waveguide channel 6 would be placed to one side of (that is to say, underneath when seen as in Fig. 2) the hot electrode 10, and the first section 16 of the third waveguide channel 7 is placed to one side of (again, underneath when seen as in Fig. 2) the first section 30 of the second ground electrode 12, with only the buffer layer 27 between the electrodes and the waveguide channels. The first and second ground electrodes 11, 12 would be generally equidistant from the hot electrode 10 in such a configuration, so that the optical waveguide channels 6,7 would experience opposite but unequal modulating fields. If sections corresponding to the second sections 14, 17 of the second and third waveguide channels 6,7 are present in a modulator in Z-cut configuration, then the substrate 1 is provided as a composite substrate. A component of the composite substrate at the edge has a different crystal orientation relative to components provided with the first and third sections 13,16, 15, 18 of the second and third waveguide channels 6,7, in order to maintain the same orientation of the applied electric field relative to the crystal axes. A graded transition between components would make it possible to avoid reflections along the second and third waveguide channels 6,7.

To minimise chirp, the illustrated configuration is generally symmetrical with respect to a plane of symmetry containing the hot electrode 10.

Since the crystalline material of which the substrate 1 is made is generally linear, the second and third waveguide channels 6,7 are thus subjected to electric fields of opposite sign but generally equal magnitude, and a change in refractive index of opposite sign but generally equal magnitude results. In fact, the electric field strengths are enhanced by the X-cut configuration.

The optical channel waveguide circuit can be manufactured by LN:Ti indif fusion or annealed proton exchange. The former method is more common. The optical channel waveguide circuit can be etched to produce a ridge waveguide device. An effect is to reduce the half- wave voltage V_n and to broaden the modulation bandwidth, as well as to make possible the use of thinner electrodes to achieve velocity matching.

To facilitate velocity matching, an alternative electro-optical external modulator, illustrated in Fig. 6, includes a cavity 37, obtained by slotting a central region of the substrate 1. The cavity 37 is bounded by inner surfaces 38,39,40 facing away from outer surfaces of the substrate 1 at which the optical channel waveguide circuit is. provided. The cavity comprises air in this example, but could be filled with another material having a different dielectric constant from that of the material of the substrate 1. In this way, the impedance of the microwave circuit, as well as the microwave effective index can be adjusted by adjusting the distance between the inner surfaces 38-40 and the outer surfaces at which the optical channel waveguide circuit is provided. In an alternative embodiment, the buffer layer 27 could be dispensed with in the X-cut configuration.

In all the above embodiments, compactness of the substrate 1 is achieved in combination with a relatively low half-wave voltage V_π, due to the provision of optical waveguide channels at more than one level. Use is made of a third dimension to provide relatively long interferometer arms. The invention is not limited to the embodiments described above, which may be varied within the scope of the accompanying claims. For example, the optical input 2 and optical output 3 are located at the same edge surface of the substrate 1 in the illustrated embodiment. Light may enter and exit in different along different directions in other embodiments. The electro-optical external modulators are generally biased at half the half-wave voltage V_π when applying the method of modulating a beam of light as described herein. In some embodiments, a separate electrode assembly may be provided to achieve the biasing.

Claims

1. Optical external modulator, including a substrate (1) , provided with an optical channel waveguide circuit, the optical channel waveguide circuit having an optical input (2) and an optical output (3) , and including at least a first junction (5), at which a waveguide channel (4) splits into multiple outgoing waveguide channels (6,7) forming arms of an interferometer, at least sections (13,16) of two of the multiple outgoing waveguide channels (6,7) adjacent the first junction (5) defining a plane in which they lie, and at least a second junction (8) , at which multiple incoming waveguide channels (6,7) forming arms of an interferometer combine to allow optical beams directed towards the optical output (3) to recombine, characterised in that the second junction (8) is positioned at a distance from the plane in a transverse direction thereto.

2. Optical external modulator according to claim 1, wherein the first and second junctions are arranged such that beams of light travelling from the first junction (5) through the outgoing waveguide channels (6,7) adjacent the first junction (5) are propagated in a direction having at least a component in parallel and oppositely directed to beams propagated through the incoming waveguide channels (6,7) towards the second junction (8) adjacent the second junction (8) .

3. Optical external modulator according to claim 1 or 2, wherein outgoing waveguide channels at the first junction (5) correspond to respective incoming waveguide channels at the second junction (8), and wherein each waveguide channel includes at least two sections (13-22) , each section being in a plane with a corresponding section of another of the waveguide channels (6,7) , such that at least two of the planes are at an angle to each other.

4. Optical external modulator according to claim 3, wherein the planes in which consecutive sections (13-22) of a waveguide channel (6,7) lie intersect at substantially a right angle to directions of propagation of a light beam along the channel sections.

5. Optical external modulator according to claim 1 or 2, including at least two interferometers in series, each including a junction at which a waveguide channel splits into multiple waveguide channels and a junction at which the multiple waveguide channels re-combine, wherein waveguide channels forming the arms of an interferometer lie generally entirely in a single plane.

6. Optical external modulator according to any one of claims 1-5, including a waveguide channel (6,7) comprising at least two sections, a first section (13,16) being positioned at a first surface of the substrate (1), and a second section (14,17) being positioned at an edge surface of the substrate (1) .

7. Optical external modulator according to claim 6, wherein the waveguide channel includes a third section (19,20) , connecting the first section (13, 16) and the second section (14, 17) and provided at a chamfered edge of the substrate (1) .

8. Optical external modulator according to any one of claims 1-7, including at least one reflecting structure (23-26) for re-directing light from a first of two consecutive sections (13, 16,14, 17) of a waveguide channel to a second of the two consecutive sections (14, 17,15, 18) of the waveguide channel (6,7) .

9. Optical external modulator according to any one of claim 1-8, including an electrode assembly for applying an electric field to at least part of the optical channel waveguide circuit, each electrode (10-12) having at least two contiguous sections (28-36) at an angle to each other and generally parallel to planes in which respective sections (13-18) of waveguide channels (6,7) forming interferometer arms lie.

10. Optical external modulator according to any one of claims 1-9, including an electrode assembly for applying an electric field to at least part of the optical channel waveguide circuit, which electrode assembly is provided in a travelling wave configuration.

11. Optical external modulator according to any one of claims 1-10, including an electrode assembly for applying an electric field to at least part of the optical channel waveguide circuit and further including a buffer layer (27) of electrically insulating material, positioned between ^■ the electrode assembly and the optical channel waveguide circuit.

12. Optical external modulator according to any one of claims 1-11 , wherein the substrate (1) includes a cavity, bounded by inner surfaces (38-40) and wherein the optical channel waveguide circuit is provided at outer surfaces of the substrate (1) facing away from the inner surfaces.

13. Optical external modulator according to any one of claims 1-12, wherein the substrate (1) is at least partly comprised of an anisotropic crystalline material and wherein the optical external modulator has an X- cut configuration, in which an optical waveguide channel (6,7) forming an arm of an interferometer is placed to one side of a gap separating at least parallel sections of a first (10) and second electrode (11, 12) arranged to apply an electric field.

14. Optical external modulator according to claim 13, wherein an optical waveguide channel forming a second arm of the same interferometer is placed to one side of a gap separating at least parallel sections of the first and a third electrode in a generally symmetrical configuration with respect to a plane of symmetry containing at least the section of the first electrode (10) .

15. Method of modulating a beam of light, including: splitting the beam of light into at least two outgoing beams; guiding the outgoing beams through respective waveguide channels (6,7) in a substrate (1) and changing an optical permeability of the substrate material at at least one of the waveguide channels, two of the outgoing beams defining a plane in which they are propagated; and recombining incoming beams carrying energy from the beam of light and propagated through respective waveguide channels (6,7) in a substrate (1) , wherein an optical permeability of the substrate (1) at at least one of the waveguide channels is changed, characterised in that the incoming beams are re-combined at a point located at a distance from the plane in a transverse direction thereto.

16. Method according to claim 15, including the use of an optical external modulator according to any one of claims 1-14.