GB2383425A

GB2383425A - Integrated optical device

Info

Publication number: GB2383425A
Application number: GB0130274A
Authority: GB
Inventors: Barros Sara Otero
Original assignee: Bookham Technology PLC
Current assignee: Lumentum Technology UK Ltd
Priority date: 2001-12-19
Filing date: 2001-12-19
Publication date: 2003-06-25
Also published as: GB0130274D0; GB0207215D0; GB2385145A

Abstract

An integrated optical device for actively splitting and/or switching the optical power of an optical signal. The device comprises at least one input 1 and output 3, a waveguide 5 and a refractive index control portion (fig. 2). The control portion varies the refractive index of a part of the waveguide relative to an adjacent part and creates, or contributes to an interface between the two parts. The optical power is then reflected or transmitted to the desired output. The control portion may be an electrode which heats or which injects a current into the controlled part. Alternative charge carrier injection is performed by a pn or pin diode which may be at the upper surface of the waveguide. The waveguide may of the slab or rib type and may be made of silicon.

Description

<Desc/Clms Page number 1> integrated Optical Device The present invention relates to integrated optical devices, and in particular relates to actively controlled integrated optical splitters and integrated optical switches.

Various types of integrated optical splitters and switches are known.

One type of switch is a so-called beam steering switch, in which the optical power of an optical signal is selectively directed to each of two or more outputs by refraction of the optical beam. An example of this type of switch is disclosed in International Patent Application No. PCT/GB01/00880 published as WO 01/67172 (Bookham Technology PLC). This discloses an integrated optical device for selectively directing light from one or more input waveguides to one or more output waveguides, the device comprising: a slab waveguide; one or more input waveguides for directing light into the slab waveguide; one or more output waveguides for receiving light from the input waveguide (s) after it has travelled through the slab waveguide; and adjustment means for adjusting the refractive index of one or more refracting portions of the slab waveguide through which the light travels so as to refract the light as it passes therethrough, whereby transmission of light between the input and output waveguide (s) can be selectively controlled. The adjustment means may be a heater, or other means may be used, for example charge carrier injection, or depletion.

The present invention provides an integrated optical device which is constructed, and functions differently to that disclosed in WO 01/67172. At least in its broadest aspects, the invention may function either as an active optical splitter (in which the optical power of one or more optical signals is split between two or more outputs) or as an optical switch (in which the optical power of an optical switch is selectively directed exclusively to one output at a time).

Accordingly, the present invention provides an integrated optical device for actively splitting and/or switching the optical power of an optical signal, the device comprising: at least one input and a plurality of outputs; an optical waveguide arranged to propagate optical signals at least part of the distance between the input (s) and the outputs; and refractive index control means arranged to vary the refractive index of at least one reflecting/refracting portion of the waveguide relative to the refractive index of an immediately adjacent portion of the waveguide, thereby providing, or at least contributing to the provision of, an interface between the portions; said variation determining, in use, the proportions of the optical power of an optical signal directed to respective said outputs from the input (s), any of the optical power directed to at least one of the outputs being so directed by means of reflection at the interface.

The embodiments of the invention which function as optical switches may be regarded as a sub-set of the larger set of embodiments which function as optical splitters, since (at least in the context of the present invention) a switch may be regarded as a splitter in which the proportion of the optical power directed to all but one of the outputs at any one time, is zero.

In preferred embodiments of the invention, at least one of the outputs of the device receives optical power directed to it substantially without any of the optical power it receives having been reflected by the reflecting/refracting portion of the waveguide.

The integrated optical device according to the invention preferably comprises a semiconductor device; particularly preferred is a silicon device, for example formed on a silicon chip. Most preferred is a so-called silicon-oninsulator device comprising an upper layer of silicon in which the optical components are formed, and a lower optical confinement layer, preferably of silica (which, in the field of integrated electronics acts as an insulator, hence the name).

The (or each) input, and the outputs of the device preferably comprise optical waveguides. Preferably the input and output waveguides are rib waveguides, in which an elongate rib portion of the semiconductor material projects above a slab region of the semiconductor material situated on each elongate side of the rib portion.

At least in the broadest aspects of the invention, the optical waveguide of the device which includes the (or each) reflecting/refracting portion may comprise substantially any type of integrated waveguide. For example, this waveguide may, at least in some embodiments of the invention, be a rib waveguide. In preferred embodiments, however, the waveguide which includes the (or each) reflecting/refracting portion comprises a slab waveguide. The slab waveguide (i. e. a generally planar waveguide having two dimensions parallel to its plane significantly larger than the third dimension perpendicular to the plane) preferably confines optical signals in a vertical direction (i. e. a direction perpendicular to the plane of the slab) but substantially does not confine the optical signals in a horizontal direction (i. e. parallel to the plane of the slab) so that optical signals propagated through the slab waveguide spread out in the plane of the slab as they propagate.

It is to be understood that the terms "upper", "lower", "vertical", "horizontal", and the like are used in this specification merely to denote the relative positions of features of the integrated device based upon the arbitrary designation of the optical confinement layer (e. g. the buried silica layer) being below the waveguides and the plane of the confinement layer being a horizontal plane. These designations do not refer to any particular orientation of the device in space.

Preferably the (or each) input and the outputs, at least for embodiments in which they comprise waveguides, abut directly against the waveguide which includes the (or each) reflecting/refracting portion (e. g. the input and

output waveguides may be integral with the waveguide which includes the (or each) reflecting/refracting portion). For example, where the input and output waveguides comprise rib waveguides they preferably abut directly against a slab waveguide which includes the reflecting/refracting portion (s).

The (or each) reflecting/refraction portion is a portion of a waveguide (preferably a slab waveguide, as already mentioned) whose refractive index may be varied relative to the refractive index of the immediately adjacent portion of the waveguide by means of the refractive index control means. The refractive index control means may be generally any means which is capable of altering the refractive index of a portion of semiconductor material. For example, the control means may comprise thermal means, i. e. heating and/or cooling means. Additionally or alternatively the control means may comprise electrical charge carrier injection means. For example, the control means may comprise one or more diodes, e. g. one or more p-n diodes, preferably one or more p-i-n diodes.

For embodiments of the invention in which the refractive index control means comprises one or more thermal means, the thermal means preferably comprise electrically resistive heating elements. For example, the control means may comprise one or more heating elements positioned on upper and/or lower surfaces of the slab waveguide. Electrically conductive tracks in or on the device preferably interconnect the heating elements with a source of electrical current, which is in turn preferably controlled by a computer which forms part of the refractive index control means, and which consequently controls the variation in the refractive index of the reflecting/refracting portion (s) as required for switching and/or splitting of the optical signals.

For embodiments of the invention in which the refractive index control means additionally or alternatively comprises one or more electrical charge carrier injection means, as already mentioned, the (or each) charge carrier injection means preferably comprises a diode, for example a p-n diode,

especially a p-i-n diode. For example, the (or each) charge carrier injection means may comprise a plurality of doped regions of the semiconductor waveguide (e. g. the semiconductor slab waveguide). The doped regions may be configured in any suitable way that enables the required refractive index change in the waveguide. For example, the doped regions may be located in or on upper and/or lower surfaces of the waveguide. The (or each) diode may be a vertical diode (e. g. in which the doped regions are arranged above and below each other in a direction generally perpendicular to the plane of a slab waveguide). Additionally or alternatively, the (or each) diode may be a lateral diode (e. g. in which the doped regions are arranged generally in the same plane parallel to the plane of a slab waveguide). If lateral diodes are used, the doped regions are preferably provided in, or on, an upper surface of a slab waveguide. For example, the doped regions may extend downwardly from an upper surface of the waveguide and/or they may be provided in recesses produced (e. g. etched) in an upper surface of the waveguide.

For embodiments in which the slab waveguide is formed from silicon, any p-doped regions preferably comprise boron or aluminium, and any ndoped regions preferably comprise phosphorous or arsenic.

As for embodiments in which the refractive index control means comprises one or more thermal means, electrically conductive tracks in or on the device preferably interconnect the doped regions with a source of electrical current, which is itself preferably controlled by a computer which forms part of the control means.

In some embodiments of the invention, the reflecting/refracting portion (s) may, for example, be formed from a different composition to that of an immediately adjacent portion of the waveguide. In such embodiments, therefore, an interface between the reflecting/refracting portion and the immediately adjacent portion is always present. Preferably, however, the material of the reflecting/refracting portion (s) is substantially identical to that of

the immediately adjacent portion of the waveguide. Preferably, the (or each) reflecting/refracting portion is defined substantially entirely by the extent, shape and configuration of the refractive index variation which the refractive index control means is able to generate in the waveguide. That is, the (or each) reflecting/refracting portion is preferably a"virtual"portion which becomes physically distinguished from the immediately adjacent portion of the waveguide only when the refractive index control means changes its refractive index relative to the immediately adjacent portion of the waveguide.

The (or each) reflecting/refracting portion preferably functions substantially as a prism (e. g. a"virtual prism"). Any refractive index mismatch between the reflecting/refracting portion and an immediately adjacent portion of the waveguide produced by the refractive index control means preferably creates the interface between the reflecting/refracting portion and the immediately adjacent portion of the waveguide on the opposite side of the interface. The presence of the interface between two regions of differing refractive index creates the possibility of reflection of light at the interface. In this way, the optical power of an optical signal may be split into two portions, or the entirety of the optical power of the signal may be re-directed from its original path. That is, if there is partial reflection of an optical signal at the interface (i. e. only a proportion of the total power of the signal is reflected) the signal is split into two portions, and the device is acting as a splitter. However, if there is total reflection of the signal at the interface (i. e. the entirety of the power of the signal is reflected) the signal remains intact and is merely re-directed (for example to one of a plurality of outputs) by being reflected, and the device is, for example, acting as a switch. Consequently, as indicated earlier, when the device according to the invention is functioning as a switch, this may be regarded as a splitter in which the proportion of the optical power directed to one output is 100%, and the proportion of the optical power directed to the (or each) other output is 0%. This is arranged, according to the invention by either 100% reflectance at the interface or by the absence of the interface due to the absence of a refractive index mismatch

between the reflecting/refracting portion and the portion of the waveguide immediately adjacent thereto, such an absence causing 100% transmission between the two portions.

The proportion of the power of an optical signal which is reflected at the interface depends upon the degree of mismatch between the refractive indices on opposite sides of the interface, and also upon the angle of incidence of the optical signal with respect to the interface. If the device is intended to operate as a switch, the refractive index mismatch and the angle of incidence are arranged so that there is total reflection at the interface. If the device is intended to be operated as a splitter, the refractive index mismatch and the angle of incidence are arranged so that there is only partial reflection at the interface. If the refractive index mismatch is arranged by the refractive index control means such that the refractive index of the reflecting/refracting portion is made lower than that of the immediately adjacent portion of the waveguide on the opposite side of the interface, then reflection at the interface will be external to the reflecting/refracting portion. However, if (as is preferred) the refractive index of the reflecting/refracting portion is made higher than that of the immediately adjacent portion of the waveguide on the opposite side of the interface, then reflection at the interface will be internal to the reflecting/refracting portion (i. e. internal reflection).

Heating the (or each) reflecting/refracting portion increases its refractive index. Injecting electrical charge carriers into the (or each) reflecting/refracting portion decreases its refractive index. For embodiments of the invention in which the refractive index is varied by heating, a temperature increase of several tens of degrees Centigrade may, for example, produce a refractive index change of the order of approximately 0.01. For embodiments of the invention in which the refractive index is varied by electrical charge carrier injection, this is preferably arranged to produce smaller changes in refractive index, for example of the order of 0.001 (greater charge carrier injection densities could produce greater changes in refractive

index, but could also lead to unacceptably high optical absorption). However, by arranging the angle of incidence suitably, such small changes in refractive index may still produce the required degree of reflectivity at the interface.

Advantageously, in some preferred embodiments of the invention, there is a plurality of reflecting/refracting portions. Preferably each

reflecting/refracting portion is a portion of one and the same waveguide as each other reflecting/refracting portion. Advantageously, the reflecting/refracting portions may be arranged in series to provide a cascading branching splitter or switch. Preferably at least one (and more preferably only one) of the outputs of the device receives optical power directed to it substantially without any of the optical power it receives having been reflected by a reflecting/refracting portion of the waveguide. Additionally or alternatively, any of the optical power directed to at least one of the outputs preferably is so directed by means of internal reflection in at least two reflecting/refracting portions.

For example, for a device according to the invention which includes n reflecting/refracting portions, the device preferably includes a total of n+1 outputs. Preferably, only one of the outputs receives optical power directed to it substantially without any of the optical power it receives having been reflected by any of the reflecting/refracting portions, and each other output preferably receives optical power directed to it by means of reflection at a respective unique integer number of the reflecting/refracting portions, the number being between one and n.

In some preferred embodiments of the invention, particularly those in which there is more than one reflecting/refracting portion, the device preferably includes one or more reflectors (in addition to the reflecting/refracting portions). The (or each) reflector preferably directs optical power of an optical signal between a respective reflecting/refracting portion and an input or output of the device (such directing of the optical power by a

reflector may, for example, be via one or more other reflectors of the device). The, or each, reflector may, for example comprise an edge of the slab waveguide, reflection from the edge occurring by total internal reflection within the slab waveguide. Preferably, each reflector causes convergence of the reflected optical signal, e. g. in the manner of a parabolic reflector. This convergence may, for example, be used to counteract the effect of divergence of the optical signal as it propagates through the slab waveguide and/or it may be used to focus an optical signal on a respective output.

Advantageously, in some particularly preferred embodiments of the invention, the, or each reflecting/refracting portion of the device may have a variable shape (e. g. between two or more shapes) and/or variable orientation (e. g. between two or more orientations) with respect to the or each input and/or output. For example, the refractive index control means may not only determine the refractive index of the reflecting/refractive portion (relative to an immediately adjacent portion) it may also determine the shape and/or orientation of the reflecting/refracting portion. This has the advantage of introducing an additional method of determining the degree of reflectance of an optical signal at an interface between the reflecting/refracting portion and an immediately adjacent portion which is controllable by the refractive index control means. This is because, by controlling the shape and/or orientation of the reflecting/refracting portion, the refractive index control means may control the angle of incidence between an optical signal and the interface. As explained earlier, the angle of incidence is one of the factors affecting the reflectivity at the interface.

The invention will now be described, by way of example, with reference to the accompanying drawings, of which: Figure 1 (views (a) and (b) ) is a schematic illustration of an embodiment of an integrated optical device according to the invention;

Figure 2 is a schematic illustration of an embodiment of a variable shape reflecting/refracting portion of an integrated optical device according to the invention; and Figure 3 is a schematic illustration of a refractive index control means in the form of an array of heaters.

Figure 1 is a schematic illustration of an embodiment of an integrated optical device according to the invention. The device comprises an input 1 and three outputs 3, the input and output being in the form of rib waveguides.

A slab optical waveguide 5 is indicated schematically by the dashed outline, and this waveguide is arranged to propagate optical signals from the input 1 to the outputs 3. The slab waveguide 5 includes refractive index control means (not shown) arranged to vary the refractive index of two reflecting/refracting portions 7a and 7b of the waveguide relative to the refractive index of the remainder of the waveguide. The refractive index control means may, for example, comprise heaters, e. g. electrical resistance heaters, located above and/or below the slab waveguide. Additionally or alternatively the refractive index control means may comprise electrical charge carrier injection means, for example p-n diodes (e. g. p-i-n diodes) located in or adjacent to the slab waveguide. For example, the refractive index control means may comprise a plurality of spaced apart p-doped and n-doped regions provided on the upper surface of the slab waveguide 5 (preferably provided in recesses in the upper surface). Whether they are heaters, diodes, or both, however, the refractive index control means are arranged and configured such that they may vary the refractive index of the slab waveguide in portions of the slab waveguide having the shapes and positions as indicated by reference numerals 7a and 7b, thereby defining the reflecting/refracting portions of the slab waveguide.

The device also includes a plurality (five as illustrated) of reflectors 9. The reflectors 9 (a to e) comprise parabollically curved etched edges at the periphery of the slab waveguide or within a recess provided in the slab

waveguide, which reflect the optical signals by total internal reflection. (Figure 1 (a) shows the reflectors schematically as being flat.) The device illustrated in Figure 1 functions as follows. An optical signal (indicated by arrows 11) enters the slab waveguide 5 of the device via the input rib waveguide 1. The signal is then reflected by parabolic reflector 9a, the reflector 9a also collimating the signal which had spread laterally (in the plane of the slab waveguide) as it propagated from the input 1 to the reflector 9a. The collimated optical signal is directed at an angle of 900 to its original input direction by the reflector 9a so that it enters the first reflecting/refracting portion 7a.

The fate of the optical signal as it enters the first reflecting/refracting portion 7a is determined by the refractive index control means for that portion.

If the device is functioning as an optical splitter, the refractive index of the portion 7a is arranged such that a proportion of the signal is propagated through the portion 7a and directed towards the second reflector 9b, and another proportion of the signal is reflected by internal reflection at an interface 13a between the portion 7a and an immediately adjacent portion of the slab waveguide 5. The relative proportions of the optical power of the signal which are reflected at the interface 13a and not reflected at the interface 13a are determined by the difference between the refractive indices of the portion 7a and the immediately adjacent portion of the slab waveguide 5, and also by the angle of incidence o-see Figure 1 (b) which shows a detail of portion 7a from Figure 1 (a). Any optical power which is not reflected at the interface 13a is refracted by the portion 7a and impinges on reflector 9b, which re-directs and focuses that proportion of the signal on output waveguide 3a. Any optical power which is reflected at the interface 13a is re-directed through 900 and enters the second reflecting/refracting portion 7b.

Instead of acting as a splitter, the device may act as a switch, in which case the refractive index control means will vary the refractive index of the portion 7a such that either none of the optical power of the optical signal is reflected at interface 13a (in which case all of the optical power of the signal will exit the device via output 3a), or all of the optical power of the optical signal is directed to the second reflecting/refracting portion 7b.

The fate of any optical power of an optical signal entering the second reflecting/refracting portion 7b is similarly determined by the refractive index control means (not shown) for that portion. That is, none of the optical power may be reflected at interface 13b, or all of it may be reflected at the interface, or a proportion of the optical power may be reflected and a proportion of the power may not be reflected.

Any optical power reflected at interface 13b of portion 7b is directed to reflector 9c which re-directs and focuses the optical power on output 3b, via which it exits the device. Any optical power not reflected at interface 13b is refracted by portion 7b and directed to reflector 9d which is an etched surface formed in the slab waveguide. All of the optical power so directed to reflector 9d is reflected and re-directed to reflector 9e which in turn re-directs, reflects and focuses the optical power on output 3c, via which it exits the device.

The device illustrated in Figure 1 consequently may function as a 3 way optical splitter or as a 1x3 optical switch. The device may be described as a "cascading and branching" splitter or switch.

It should be understood that some embodiments of the invention will be arranged to function only as splitters, and other embodiments may be arranged to function only as switches. Other embodiments, however, may be arranged to function either as splitters or as switches, in which case the switch function is, in essence, a limiting case of the splitter function in which the proportion of the optical power directed to all but one of the outputs is zero.

Figure 2 is a schematic illustration of an embodiment of a variable shape reflecting/refracting portion 15 of a device according to the invention. In this embodiment, the reflecting/refracting portion 15 of the device comprises two generally superimposed reflecting/refracting sub-portions 15a and 15b (also labelled, respectively as"Active Area Pattern 1"and"Active Area Pattern 2"). The refractive index control means of this portion 15 not only varies the refractive index of the portion relative to that of the immediately adjacent portions of the waveguide, but also varies the shape of the portion between each of the two sub-portions 15a and 15b. This consequently provides another way of determining the proportion (if any) of the optical power of an optical signal reflected at the interface, because by varying the shape of the portion 15 the orientation of the interface is varied.

Consequently, the angle of incidence of an optical signal upon the interface thereby varies, and this is a factor which determines the proportion of the optical power which is reflected.

For example, when sub-portion 15a is actuated (i. e. provided by the refractive index control means) the angle of incidence os is relatively large, and therefore a relatively small refractive index difference between sub-portion 15a and the remainder of the waveguide is needed to cause reflection at interface 17a. However, when sub-portion 15b is actuated, the angle of incidence 02 is relatively small, and therefore a relatively large refractive index difference between sub-portion 15b and the remainder of the waveguide is needed to cause reflection at interface 17b. This feature may be used, for example, to lessen the degree of refractive index change needed to provide the desired optical signal splitting or switching. For example, by using only sub-portion 15a when mainly or only reflection is required (e. g. Beam 1 in Figure 2) and

by using only sub-portion 15b when mainly or only transmittance is required (e. g. Beam 2 in Figure 2).

Figure 3 shows an embodiment of a refractive index control means in the form of a triangular array of heaters 19 provided on the top of the silicon slab waveguide. The array preferably comprises a series of resistance heaters, e. g. comprising a narrow strip of aluminium or tungsten, e. g. a titanium, tungsten, gold alloy, and may be between 5 and 20 microns wide and 0.5 to 2.0 microns thick with a spacing between adjacent strips (centre to centre) of about 30 to 50 microns.

An example comprises strips of tungsten alloy around 1 micron thick, 10 microns wide and at 40 micron intervals.

The array of heaters may be arranged in a variety of patterns and the lines can be individually controlled to provide the required temperature change and/or temperature gradients across the device.

Claims

CLAIMS 1. An integrated optical device for actively splitting and/or switching the optical power of an optical signal, the device comprising: at least one input and a plurality of outputs; an optical waveguide arranged to propagate optical signals at least part of the distance between the input (s) and the outputs; and refractive index control means arranged to vary the refractive index of at least one reflecting/refracting portion of the waveguide relative to the refractive index of an immediately adjacent portion of the waveguide, thereby providing, or at least contributing to the provision of, an interface between the portions; said variation determining, in use, the proportions of the optical power of an optical signal directed to respective said outputs from the input (s), any of the optical power directed to at least one of the outputs being so directed by means of reflection at the interface.
2. A device according to Claim 1, in which optical signals received by the outputs from the input (s) are propagated by said at least one reflecting/refracting portion of the waveguide.
3. A device according to Claim 1 or Claim 2, in which at least one of the outputs receives optical power directed to it substantially without any of the optical power it receives having been reflected by a said reflecting/refracting portion of the waveguide.
4. A device according to any preceding claim, comprising at least two reflecting/refracting portions.
5. A device according to Claim 4, in which each additional reflecting/refracting portion is a portion of one and the same optical waveguide.

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6. A device according to Claim 4, in which at least one reflecting/refracting portion is a portion of an additional optical waveguide.
7. A device according to any one of claims 4 to 6, in which any of the optical power directed to at least one of the outputs is so directed by means of reflection by at least two reflecting/refracting portions.
8. A device according to any one of claims 4 to 7, which includes n reflecting/refracting portions and a total of n+1 outputs.
9. A device according to Claim 8, in which only one of the outputs receives optical power directed to it substantially without any of the optical power it receives having been reflected by any of the reflecting/refracting portions, each other output receiving optical power directed to it by means of reflection at a respective unique integer number of the reflecting/refracting portions, the number being between one and n.
10. A device according to any preceding claim, in which the, or each, interface is provided substantially only by the variation in the refractive index of the, or each, respective reflecting/refracting portion which is provided by the refractive index control means.
11. A device according to any preceding claim, in which the refractive index control means is also arranged to vary the shape and/or orientation of the, or each, reflecting/refracting portion.
12. A device according to Claim 11, in which the, or each, reflecting/refracting portion has at least two shapes and/or orientations, which are arranged to provide differing predetermined ranges of the proportion of the optical power of an optical signal which is reflected.

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13. A device according to Claim 11 or Claim 12, in which the, or each, reflecting/refracting portion has two shapes and/or orientations, one of which is arranged to provide only or mainly reflection of the optical power of an optical signal, and the other of which is arranged to provide only or mainly transmission of the optical power of an optical signal.
14. A device according to any preceding claim, in which the, or each, optical waveguide arranged to propagate optical signals at least part of the distance between the input and the outputs comprises a slab waveguide.
15. A device according to any preceding claim, in which the input (s) and/or the outputs comprise rib waveguides.
16. A device according to any preceding claim, which comprises a silicon integrated optical device.
17. A device according to any preceding claim, in which the refractive index control means comprises heating and/or cooling means.
18. A device according to any preceding claim, in which the refractive index control means comprises electrical charge carrier injection means.
19. A device according to claim 18, in which the electrical charge carrier injection means comprises one or more p-n diodes.
20. A device according to claim 19, in which the, or each, p-n diode comprises a p-i-n diode.
21. A device according to claim 19 or claim 20, in which the, or each, p-n diode comprises at least one p-doped region and at least one n-doped region, each region provided in or on an upper surface of the waveguide

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which is arranged to propagate optical signals at least part of the distance between the input (s) and the outputs.
22. A device according to claim 21, in which at least one of the doped regions is provided in a recess in the upper surface.