GB2438222A

GB2438222A - Multimode interference waveguide coupler

Info

Publication number: GB2438222A
Application number: GB0609724A
Authority: GB
Inventors: Robert Walker
Original assignee: Bookham Technology PLC
Current assignee: Lumentum Technology UK Ltd
Priority date: 2006-05-17
Filing date: 2006-05-17
Publication date: 2007-11-21
Also published as: GB0609724D0

Abstract

A multimode interference (MMI) coupler comprises an MMI waveguide 15, at least one first waveguide 17,18 optically connected to a first end of the MMI waveguide, and two or more second waveguides 19,20 optically connected to an opposite second end of the MMI waveguide. The MMI waveguide comprises a first region 23 having a first structure and a second region 25 having a second structure that is different to the first structure, thereby causing a fixed difference between the effective refractive indices of the first and second regions. The different structures of the MMI waveguide may constitute different heights H1,H2 or different compositions of the waveguide (31, Fig 5). Tuning electrodes (41, Fig.7) may be present.

Description

Multimode Interference Coupler

Technical Field of the Invention

The present invention relates to integrated optical devices, for example semiconductor optical devices, and particularly relates to multimode interference couplers.

Background of the Invention

Multimode interference (MMI) couplers for splitting and/or combining light between waveguides are well known in the field of integrated optical devices. The term "coupler" is used herein to mean a splitter and/or a combiner, since the functions of light splitting and light combining are generally determined merely by the direction in which the light propagates through the device. Various different versions of MMI couplers exist, and they may have single input waveguide and a plurality output waveguides, or vice versa, or they may have a plurality of input waveguides and a plurality of output waveguides.

Figure 1 is a schematic illustration of a known type of MMI coupler fabricated as a semiconductor integrated optical device. The MMI coupler comprises an MMI waveguide 1, two input waveguides 3 optically connected to a first end of the MMI waveguide, and two output waveguides 5 optically connected to an opposite second end of the MMI waveguide. The MMI coupler may have only a single input waveguide or only a single output waveguide, or it may have a greater number of input and/or output waveguide. The input and output waveguides 3 and 5, and the MMI waveguide 1, are fabricated as ridge waveguides, such that each waveguide includes a respective ridge 7 extending across an etched plateau region 9. As is well known, for both strongly and weakly guiding waveguides much (or most) of the light normally propagates through the semiconductor material below the etched plateau 9, but the light is guided in the lateral dimension parallel to the plateau by the effect of the ridges 7 on the light. Normally (at least for strongly guiding waveguides), there is a guiding semiconductor core layer of the device situated below the etched plateau 9, in which much (or most) of the light normally propagates.

The input and output waveguides 3 and 5 respectively are substantially single mode waveguides (i.e. single mode or nearly single mode), and the MMI waveguide 1 is a multimode waveguide in which a plurality of modes (excited by asymmetric, non-matched input from input waveguide 3) create an interference pattern that has the well-known property of "self-imaging", or "re-imaging". The locations in the MMI waveguide at which images of the input light occur depend upon several criteria, such as the length and width of the MMI waveguide, the lateral positions of the input waveguide(s), the wavelength of the light, and the effective refractive index of the MMI waveguide. Consequently, the optical power received by the output waveguide, or received by each output waveguide (if there is more than one output waveguide), generally depends on these criteria and also on the lateral position of the, or each, output waveguide. Thus, for example, the optical power splitting ratio of a splitter that utilises an MMI waveguide can be determined by such criteria. The MMI coupler illustrated in Figure 1, when used as a splitter, would generally split the optical power substantially equally between the two output waveguides.

The optical power splitting ratio of an MMI splitter can also be dictated, in part, by the shape of the MMI waveguide. Figure 2 is a schematic illustration of a known type of MMI coupler that is commonly referred to as a "butterfly" coupler, because of its shape in which the MMI waveguide 11 narrows to a waist 13 in the middle of its length. (Other features of the coupler that are substantially the same as those shown in Figure 1, have the same reference numerals as Figure 1.) Such a device is typically used where a somewhat unequal split between the two output waveguides 5 is required without excessive loss. Other types of MM! coupler are also known, including the "inverse butterfly" (or "barrel") coupler, which increases in width from each end to a widest part in the middle of its length.

In addition to such "passive" MMI couplers, actively controlled MMI couplers are also known. Such active couplers include one or more electrodes located on the MM! waveguide, for applying an electric current or an electric field to localised regions of the MMI waveguide in order to vary the effective refractive index of the MM! waveguide in those regions. In this way, the self-imaging of the light in the MMI waveguide can be modified, thereby actively tuning the optical power splitting ratio of an MM! coupler. US Patent No. 6,571,038 Bi discloses such tuneable MMI couplers.

Summary of the Invention

The present invention seeks to provide an MMI coupler in which a fixed (non-tuneable) optical power distribution may be tailored in a new and inventive way.

Accordingly, a first aspect of the invention provides a multimode interference (MM!) coupler comprising an MMI waveguide, at least one first waveguide optically connected to a first end of the MMI waveguide, and two or more second waveguides optically connected to an opposite second end of the MM! waveguide, wherein the MM! waveguide comprises a first region having a first structure and a second region having a second structure that is different to the first structure, thereby causing a fixed difference between the effective refractive indices of the first and second regions.

By providing the MM! waveguide with first and second regions having different structures, which structures cause a fixed difference between the effective refractive indices of the first and second regions, the optical power distribution within the MM! waveguide during use, and thus for example the fixed (permanent, passive) optical power splitting ratio between two or more output waveguides, may be tailored in a new and inventive way. Depending upon the particular use to which the MMI coupler is put (e.g. as a splitter, a combiner, or a combiner/splitter), the (or each) first waveguide may be an input waveguide, and thus the second waveguides will be output waveguides.

Alternatively, the second waveguides may be input waveguides, and thus the (or each) first waveguide will be an output waveguide.

It will be understood by skilled persons that effective refractive indices are normally dependent on wavelength. Thus, while the MMI coupler according to the invention may be operated at substantially any convenient wavelength, it will be understood that the fixed difference between the effective refractive indices of the first and second regions is a fixed difference at any particular wavelength with which the coupler is operated.

The first and second regions may, for example, be designated such that the effective refractive index of the second region is lower than that of the first region.

At least in the broadest aspects of the invention, the waveguides of the MMI coupler may be strongly guiding waveguides or weakly guiding waveguides (or the MMI coupler may include one or more strongly guiding waveguides and one or more weakly guiding waveguides).

In preferred embodiments of the MMI coupler, the waveguides are ridge waveguides, and thus the MMI waveguide comprises an MMI section of the ridge waveguides. Preferably the first structure comprises a first height of the MMI waveguide and the second structure comprises a second height of the MMI waveguide, which is different to the first height. Thus, for example, the heights may be heights of the ridge of the MMI section. The heights of the MM! waveguide (or MMI section) will normally be determined by etching processes, and thus they may alternatively be described in terms of relative etching depths; however, for conceptual simplicity the term "height" is used herein.

The second height of the MMI waveguide may be lower than the first height, in which case (as mentioned above) the effective refractive index of the second region will be lower than that of the first region. For example, the second height might be no more than two thirds of the height of the first height, e.g. the second height might be no more than half of the height of the first height. Alternatively, the second height of the MMI waveguide may be higher than the first height, in which case the effective refractive index of the second region will be higher than that of the first region. For example, the first height might be no more than two thirds of the height of the second height, e.g. the first height might be no more than half of the height of the second height. However, the relative heights of the first and second structures that are required for any particular situation will normally depend upon the overall layer structure of the device, and in practice they may be determined (following a knowledge of the present specification) by computer modelling or by trial and error, for example. Generally speaking, the greater the height differential, the greater is the effective refractive index differential.

Additionally or alternatively, the first structure may comprise a first composition of the MMI waveguide and the second structure may comprise a second composition of the MMI waveguide. For example, the first and/or the second composition may include one or more quantum wells (e.g. a quantum well intermixed (QWI) composition). The first and/or the second composition may also, or instead, include implanted ions.

In especially preferred embodiments of the invention, the MMI coupler includes a plurality of second regions (e.g. two second regions, but there may be more than two such second regions). Additionally or alternatively there may be two or more first regions.

A said first region of the MMI coupler may extend along at least part of the length of the MM! waveguide, e.g. in a laterally central position of the MM! waveguide. In some preferred embodiments of the invention, the (or each) first region has a width no greater than half of the width of the MMI waveguide; for example, the (or each) first region may have a width in the range from a quarter to a half of the width of the MM! waveguide, e.g. approximately a third of the width of the MMI waveguide (especially, but not exclusively, for embodiments having a single first region and two second regions). The (or each) first region preferably extends along at least a longitudinally central position of the MM! waveguide, but it may extend along substantially the entire length of the MM! waveguide, for example.

The, or each, second region preferably extends along at least part of the length of the MM! waveguide, e.g. in a laterally peripheral (i.e outer, or edge) location of the MM! waveguide. In some preferred embodiments of the invention, the (or each) second region has a width no greater than half of the width of the MM! waveguide; for example, the (or each) second region may have a width in the range from a quarter to a half of the width of the MM! waveguide, e.g. approximately, a third of the width of the MM! waveguide (especially, but not exclusively, for embodiments having a single first region and two second regions) The (or each) second region preferably extends along at least a longitudinally central position of the MMI waveguide, but it may extend along substantially the entire length of the MMI waveguide, for

example.

In some embodiments of the invention, the first and/or second region(s) have widths that vary along at least part of their length. (Generally, if the width of the first region varies then the width(s) of the second regions will consequently vary, but in some versions of the invention there may be another region between the first and second regions, so this is not always necessarily the case.) The MM! coupler may include one or more further regions, each having a respective structure that is different to the first and second structures.

In some embodiments of the invention, the MM! coupler may include one or more electrodes located on one or more parts of the MM! waveguide and arranged to vary the refractive index of the part(s) of the MMI waveguide. It will be understood that the first and second regions of the MM! waveguide still provide the fixed difference between the effective refractive indices of the first and second regions, at least when there is no actuation of the electrode(s). However, the use of one or more electrodes provides the ability to vary the refractive index of one or more parts of the MM! waveguide (which may be one or more parts of the first and/or second regions), thereby providing the ability to tune the splitting ratio of the MM! coupler, for

example.

The MM! coupler according to the invention preferably is fabricated in one or more semiconductor materials. Generally, any semiconductor materials suitable for use as waveguides may be used, including "silicon-on-oxide" materials. In practice, the semiconductor materials may, for example, include 111-V semiconductors (i.e. semiconductors formed from elements belonging to groups III and V of the periodic table of the elements), but other semiconductor materials may be used. Particular examples include indium phosphide (InP) and/or gallium arsenide (GaAs) based systems, for example comprising indium gallium arsenide phosphide (InGaAsP) and/or indium aluminium gallium arsenide (InAlGaAs).

A second aspect of the invention provides an integrated optical device comprising at least one MMI coupler according to the first aspect of the invention, and at least one other optical component optically coupled therewith.

For example, the integrated optical device may comprise a Mach-Zehnder interferometer including two couplers, at least one of which is a said MMI coupler.

Brief Description of the Drawings

Embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings, of which: Figure 1 is a schematic illustration of a known type of MMI coupler fabricated as a semiconductor integrated optical device; Figure 2 is a schematic illustration of a known type of "butterfly" MMI coupler; Figure 3 is a schematic illustration of a first embodiment of an MMI coupler according to the invention; Figure 4 views (a) and (b) are schematic illustrations of the embodiment of the invention illustrated in Figure 3, showing the effect of the structure of the MMI waveguide on light propagating through the coupler; Figure 5 is a schematic illustration of a second embodiment of an MMI coupler according to the invention; Figure 6 is a schematic illustration of a further embodiment of an MMI coupler according to the invention; and Figure 7 illustrates an embodiment of the present invention that is similar to that illustrated in Figure 3, with the addition of tuning electrodes.

Detailed Description of the Invention

Figures 1 and 2 have been described above.

Figure 3 is a schematic illustration of a first embodiment of an MMI coupler according to the invention, fabricated as an integrated semiconductor device. The MMI coupler of Figure 3 differs from the known type of MMI coupler illustrated in Figure 1 by virtue of the structure of the MMI waveguide 15. The illustrated MM! coupler according to the invention comprises an MM! waveguide 15, two first waveguides 17 and 18 optically connected to a first end of the MM! waveguide, and two second waveguides 19 and 20 optically connected to an opposite second end of the MM! waveguide. The waveguides are ridge waveguides, each of which includes a respective ridge 7 extending across an etched plateau region 9 of the semiconductor device.

The MMI waveguide 15 comprises an MM! section 21 of the ridge 7 of the waveguides, the MM! waveguide comprising a first region 23 having a first structure and two second regions 25 having a second structure that is different to the first structure. The first structure (i.e. of the first region 23) comprises a first height H1 of the MM! section 21, and the second structure (i.e. of the second regions 25) comprises a second height H2 of the MM! section, which in the embodiment illustrated in Figure 3 is lower than the first height. (As explained above, in alternative embodiments of the invention the second height is higher than the first height. The skilled person will readily understand that such alternative embodiments of the invention may have the appearance of the embodiment illustrated in Figure 3 except that the first region 23 has the lower height H2 and the second regions 25 have the greater height H1.) As illustrated, height H2 is approximately half that of height H1, but other height differentials are possible. As explained above, the height differential may be chosen according to particular requirements. The first region 23 extends along substantially the entire length of the MM! waveguide in a laterally central position of the MMI waveguide, and has a width W1 of -10 -approximately one third of the width of the MMI waveguide. The second regions 25 extend along substantially the entire length of the MMI waveguide in a laterally peripheral position of the MMI waveguide, and each of them has a width W2 of approximately one third of the width of the MMI waveguide (i.e. approximately the same width as that of the first region 23). The second regions 25 preferably are formed by selectively etching those regions to a greater depth than the first region 23. The effect is to produce an MMI waveguide 15 having a central ridge 23 extending along its length.

The height differential between the first region 23 and the second regions 25 causes a fixed difference between the effective refractive indices of the first and second regions. The optical effect of this is illustrated in Figure 4, which shows two schematic cross- sectional views of the MMI coupler of Figure 3 (which utilises strongly guiding waveguides), including contours representing the optical power distribution as modelled for the coupler when used as a splitter with the first waveguide 17 used as an input waveguide, and the two second waveguides 19 and 20 used as output waveguides. View (a) of Figure 4 is a schematic cross-sectional diagram of the MMI coupler through a plane perpendicular to the longitudinal axes of the waveguides (i.e. perpendicular to the overall direction of propagation of light along the waveguides). In this view, a buried guiding core layer 27 of the MMI waveguide is shown. As indicated by the contours, the light is approximately centred on this guiding core layer 27. For a conventional MMI coupler of the type illustrated in Figure 1, the interference of the light modes within the MMI waveguicie produces re-imaging such that the optical power is split substantially equally between the two output waveguides 19 and 20.

However, because the MMI waveguide according to the invention has the first and second structures provided by the first and second regions 23 and 25 which have a height differential (H1 -H2), the interference pattern within the MMI waveguide is modified such that the optical power distribution between the two output waveguides 19 and 20 is as illustrated in Figure 4 (b). In this case, approximately 2.5 times as much of the optical power is coupled into -11 -the output waveguide 20 as is coupled into the output waveguide 19. The shapes and sizes (especially the difference in heights) of the first and second regions 23 and 25 of the MMI waveguide 15 of other MMI couplers may be modified to produce different fixed optical power splitting ratios.

The ability to tailor the optical power distribution (e.g. splitting) in the manner illustrated in Figure 4 by means of structural variations in the MMI waveguide can be extremely useful, because it provides great versatility in design and fabrication. One example of a device where this is useful is a Mach-Zehnder interferometer that utilises an MMI coupler for the splitter and/or the combiner such that two of the waveguides (e.g. two first waveguides or two second waveguides) connected to the MMI waveguide constitute the arms of the interferometer. It can be useful to provide unequal optical splitting (and corresponding asymmetric recombining) between the arms of the interferometer, in order to provide a built-in optical chirp, for

example.

Figure 5 illustrates an alternative way in which to provide the first and second regions of the MMI waveguide of a coupler according to the invention.

In this embodiment, a laterally central first region 29 of the MMI waveguide is formed from the same semiconductor material(s) as the input and output waveguides 3 and 5, whereas two laterally peripheral second regions 31 of the MMI waveguide have a different composition to that of the first region 29 and the waveguides 3 and 5. The different composition of the second regions 31 may be formed by quantum well intermixing and/or ion implantation, for

example.

The illustrations of the present invention in Figures 3 to 7 have shown MMI couplers that have two input waveguides and two output waveguides.

However, it will be appreciated that MMI couplers according to the present invention may also have a different number of input and/or output waveguides than those that have been illustrated.

-12 -It will be appreciated that an MMI coupler comprising a greater number of regions and corresponding structures also falls within the scope of the present invention. The different structures may be provided by regions with different heights and/or different compositions, e.g. similarly to Figures 3 and 5.

Figure 6 is a schematic illustration of a further embodiment of an MMI coupler according to the invention. The MMI waveguide 33 of the MMI coupler comprises first, second and third regions 35, 37 and 39 having corresponding structures comprising first, second and third heights H1, H2 and H3 respectively. In the illustrated embodiment of Figure 6 the first height H1 is greater than the second height H2, which is in turn greater than the third height H3. In an alternative embodiment the first height H1 may be less than the second height H2, which may in turn be less than the third height H3, for

example.

Figure 7 illustrates an embodiment of the present invention that is similar to that illustrated in Figure 3, with the addition of tuning electrodes 41 on the second region 25. Such tuning electrodes 41 may be used in combination with a further electrode (not shown) on the opposite surface 43 of the MMI coupler to produce localised refractive index changes in the portions of the MMI waveguide 21 beneath the tuning electrodes. Such refractive index changes may be used to provide active control of the split ratio of the MMI coupler.

It will be understood that many other embodiments also fall within the scope of the invention, as defined in the appended claims.

Claims

-13 -Claims 1. A multimode interference (MMI) coupler comprising an MMI

waveguide, at least one first waveguide optically connected to a first end of the MMI waveguide, and two or more second waveguides optically connected to an opposite second end of the MMI waveguide, wherein the MMI waveguide comprises a first region having a first structure and a second region having a second structure that is different to the first structure, thereby causing a fixed difference between the effective refractive indices of the first and second regions.

2. An MMI coupler according to claim 1, in which the effective refractive index of the second region is lower than that of the first region.

3. An MMI coupler according to claim 1 or claim 2, in which the waveguides are ridge waveguides, and in which the MMI waveguide comprises an MMI section of the ridge waveguides.

4. An MMI coupler according to any preceding claim, in which the first structure comprises a first height of the MMI waveguide and the second structure comprises a second height of the MMI waveguide, which is different to the first height.

5. An MMI coupler according to claim 4 when dependent upon claim 3, in which the heights are heights of the ridge of the MMI section.

6. An MMI coupler according to claim 4 or claim 5, in which the second height is lower than the first height.

7. An MMI coupler according to claim 4 or claim 5, in which the second height is greater than the first height.

-14 - 8. An MMI coupler according to claim 6 or claim 7, in which the lower height is no more than two thirds of the height of the greater height.

9. An MMI coupler according to claim 6 or claim 7, in which the lower height is no more than half of the height of the greater height.

10. An MMI coupler according to any preceding claim, in which the first structure comprises a first composition of the MMI waveguide and the second structure comprises a second composition of the MMI waveguide.

11. An MMI coupler according to claim 10, in which the second composition includes one or more quantum wells.

12. An MMI coupler according to claim 11, in which the second composition comprises a quantum well intermixed (QWI) composition.

13. An MMI coupler according to any one of claims 10 to 12, in which the second composition includes implanted ions.

14. An MMI coupler according to any preceding claim, in which the MMI waveguide includes a plurality of second regions.

15. An MMI coupler according to any preceding claim, in which the first region extends along at least part of the length of the MMI waveguide in a laterally central location thereof.

16. An MMI coupler according to any preceding claim, in which the first region has a width no greater than half of the width of the MMI waveguide.

-15 - 17. An MMI coupler according to any preceding claim, in which the first region has a width in the range from a quarter to a half of the width of the MM! waveguide.

18. An MMI coupler according to any preceding claim, in which the first region has a width that varies along at least part of its length.

19. An MMI coupler according to any preceding claim, in which the first region extends along substantially the entire length of the MM! waveguide.

20. An MMI coupler according to any preceding claim, in which the, or each, second region extends along at least part of the length of the MMI waveguide in a laterally peripheral location thereof.

21. An MMI coupler according to any preceding claim, in which the, or each, second region has a width no greater than half of the width of the MMI waveguide.

22. An MM! coupler according to any preceding claim, in which the, or each, second region has a width in the range from a quarter to a half of the width of the MM! waveguide.

23. An MMI coupler according to any preceding claim, in which the, or each, second region extends along substantially the entire length of the MM! waveguide.

24. An MM! coupler according to any preceding claim, in which the, or each, second region has a width that varies along at least part of its length.

-16 - 25. An MM! coupler according to any preceding claim, including one or more further regions, each having a respective structure that is different to the first and second structures.

26. An MM! coupler according to any preceding claim, including one or more electrodes located on one or more parts of the MM! waveguide and arranged to vary the refractive index of the part(s) of the MM! waveguide.

27. An integrated optical device comprising at least one MM! coupler according to any preceding claim, and at least one other optical component optically coupled therewith.

28. An integrated optical device according to claim 22, comprising a Mach-Zehnder interferometer including two couplers, at least one of which is a said MM! coupler.