Method for modifying the combining or splitting ratio of a multimode interference coupler

Method for modifying the splitting or combining ratio of a first multimode interference (MMI) coupler (100), which first coupler is arranged to convey light from one or several input waveguides to one or several output waveguides, wherein a film (103a) of a material is arranged over the first coupler, wherein the film is strained so that a force is applied by the film to the surface of the first coupler, and so that the refractive index profile in the material of the first coupler changes as a consequence of the force, and wherein the splitting or combining ratio is modified as a consequence of the changed refractive index profile.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method for modifying the combining or splitting ratio of a multimode interference coupler, as well as a corresponding method for modifying the combining or splitting ratio of a circuit comprising several such couplers. It also relates to a coupler comprising a strained film.

2. Description of the Related Art

Multimode interference (MMI) couplers are well-known in the art, and are for instance used as combiners and splitters in integrated optical circuits. Light launched into an access port of an MMI coupler traverses the length of the MMI region in several modes towards one or several output ports at an opposite side of the coupler. Because of the self-imaging effect of the coupler, its geometry can be determined so as to give rise to a certain light intensity distribution at the output port or ports for certain wavelengths. In its function as a combiner, the coupler comprises at least two input ports. As a splitter, the coupler comprises at least two output ports. A frequently used configuration is a 2×2 MMI coupler, comprising two input ports and two output ports.

In a 2×2 MMI coupler, it is customary to designate the transmission from an input port to an output port that is situated on the same side of the coupler centerline as the bar state transmission, and to designate transmission to an output port situated on the other side of the coupler centerline as the cross state transmission.

In many applications, it is desirable for a 2×2 MMI coupler to have a combining or splitting ratio of 1. The splitting ratio of a 2×2 MMI coupler for light that is launched from one of the input ports can be defined as the power transmitted in the cross state divided by power transmitted in the bar state. For example, the splitting ratio is the cross output power divided by the bar output power. In the case of a splitter with two output ports, a splitting ratio of 1 thus implies a symmetric division of light power between the output ports. A ratio of 1 is desirable for example where the coupler is used in a Mach-Zehnder interferometer based modulator for laser communication, such as the one described in WO 2011/043718 A1.

However, once an MMI coupler has been manufactured, for instance as a part of an integrated optical circuit, it is difficult to correct any deficiencies of the coupler negatively affecting the resulting splitting or combining ratio.

Several methods have been proposed to control the splitting and/or combining ratio of MMI couplers:

U.S. Pat. No. 5,689,597 discloses a so-called butterfly-shaped or inverse butterfly shaped 2×2 port optical MMI coupler, which achieves asymmetric splitting or combining. The transmission properties of a butterfly MMI are a sensitive function of the MMI dimensions, where for example only a 0.2 um change in the maximum or minimum width of the MMI can have a very large influence on the combining or splitting ratio. Accordingly, it is difficult to alter the combining or splitting ratio of a butterfly MMI coupler after it has been fabricated by subsequent modifications of the MMI shape.

U.S. Pat. No. 6,571,038 describes different methods for achieving a tunable splitting ratio in a 2×2 port MMI coupler, such as by placing electrodes at the location of optical images in the MMI and injecting current there. This requires the application of electrodes on the coupler, along with the additional cost and energy consumption of such injection, as well as control circuitry and space requirements for the accommodation of such circuitry. Another variant according to this document is to illuminate locations of optical images in the MMI with a light beam. This requires the presence of an apparatus to generate and focus a light beam onto the said locations.

The article Trung-Thanh Le and Laurence W. Cahill, “The Design of SOI-MMI Couplers with Arbitrary Power Splitting Ratios Using Slotted Waveguide Structures”, LEOS Annual Meeting Conference Proceedings, 2009 proposes an asymmetric 2×2 splitter or combiner fabricated with a deeply etched slot along the length of the MMI, at or near the center of the MMI. The slot must have optically smooth sidewalls and a precisely controlled depth, and it should also have a flat bottom surface, in order to achieve the desired splitting ratio while keeping scattering loss and reflections to a minimum. This not only results in more complicated manufacturing, but also limits the possible material systems.

SUMMARY OF THE INVENTION

The present invention provides for a way to modify the splitting and/or combining ratio of an MMI coupler in a simple and inexpensive way, and especially provides a way to correct the splitting and/or combining ratio of an existing such coupler, in order to achieve a combining or splitting ratio of 1 for the coupler.

Hence, the invention relates to a method for modifying the splitting or combining ratio of a first multimode interference (MMI) coupler, which first coupler is arranged to convey light from one or several input waveguides to one or several output waveguides, wherein a film of a material is arranged over the first coupler, which film is strained so that a force is applied by the said film to the surface of the first coupler, and so that the refractive index profile in the material of the first coupler changes as a consequence of the said force, and wherein the said splitting or combining ratio is modified as a consequence of the said changed refractive index profile.

Furthermore, the invention relates to a multimode interference (MMI) coupler, which coupler is arranged to convey light from one or several input waveguides to one or several output waveguides, where the circuit comprises a film of a strained material arranged over the coupler and arranged to apply a force to the surface of the coupler, and so that the refractive index profile in the material of the coupler, and as a consequence also the splitting or combining ratio of the coupler, is different as a consequence of the said force as compared to the case without said force.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1ais a simplified lateral cross-section, i.e. a section which is perpendicular to the direction of light propagation, of a prior art 2×2 MMI coupler10a. The cross-section is taken through the multimode region12of the ridge waveguide13of the coupler10a, between the input and output ports. The coupler10ais arranged on a substrate11, from which the ridge13protrudes a distance14. As is conventional as such for such MMI couplers, the light modes propagate along the ridge waveguide13, with vertical confinement of the light provided by one or more vertical waveguiding layers12a, and lateral confinement provided by the sidewalls of the MMI. The waveguiding layers are characterized by a higher index of refraction that the layers situated above and below, which are typically referred to as the cladding layers. In general, the propagating light is partly contained within the ridge itself, and may also extend partly into the optical material beneath the ridge. The direction of light propagation D is into the plane ofFIG. 1a. The letter D is consistently used throughout the drawings to denote the principal direction of light propagation through a component.

An MMI coupler10bwith an alternative MMI cross section is shown inFIG. 1b, where the layer or layers12bthat provide the vertical waveguiding are situated below the base of the ridge, rather than within the ridge. This MMI geometry results in weaker lateral waveguiding than inFIG. 1a. In this geometry, a portion of the vertically guided light extends up into the ridge material.

Still an alternative MMI coupler10cwith an MMI cross section is shown inFIG. 1c, where the layer or layers12cthat provide the vertical waveguiding are vertically arranged so that the base of the ridge that defines the MMI width is located within the vertical waveguiding layer or layers12c.

Apart from the position of waveguiding layers12a,12band12c, couplers10a,10band10cofFIGS. 1a,1band1care identical and share reference numerals. It is realized that the present invention is applicable to each of the types of couplers illustrated inFIGS. 1a,1band1c.

The couplers according to the invention are preferably, as is the case of the coupler10a, designed as ridge waveguides13, and are preferably also formed on the same semiconductor substrate and share the same or substantially the same material structure as other optical components on the same optical semiconductor circuit as the coupler. The substrate11and the ridge13may incorporate stacks of different material layers, in order to achieve desirable optical properties in terms of light propagation, optical gain, losses and so forth, which considerations are conventional as such. It is preferred that the couplers on the same integrated optical circuit or wafer are formed on the same semiconductor substrate and share the same or substantially the same material structure. This is preferably the case both for the substrate11and the ridge13.

The operation of a 2×2 port interference MMI coupler that operates as a splitter or combiner involves the propagation of many modes within the multimode region, but the operation of a 2×2 coupler according to the invention can be understood by considering the amplitudes of the two lowest order modes that are excited by the light injected at one launch port for the specific case of a 2×2 coupler designed to operate in the restricted paired interference regime.

In this design regime, for a 2×2 MMI having an effective optical width Weff, the light is launched substantially along an axis that is laterally displaced from the centerline of the MMI by

-Weff6
for one input port, and

+Weff6
for the other input port. Weffis approximately equal to the physical width of the MMI in the case of an MMI having strongly guiding sidewalls. The preferred length of the MMI is approximately equal to

2⁢⁢Nguide⁢Weff23⁢λ0,
where λ0is the free space radiation wavelength, and where Nguideis the effective refractive index of light that is propagating within the vertical 1 dimensional slab mode that corresponds to the given combination of vertical waveguide and cladding layers.

In this type of 2×2 coupler, light launched by fundamental mode light from one input port typically excites the fundamental and first higher order mode within the MMI region to a greater extent than the excitation of all other propagating modes.

FIG. 2aillustrates, in a simplified top view, the optical power splitting in the prior art 2×2 restricted paired interference MMI coupler10a, through which light propagates in the direction D, through the MMI region12when the light is launched from the input waveguide16to a pair of output waveguides18,19.17is another input waveguide. We now consider the two lowest order modes20,30propagating through the MMI region12.

With the launch of light into one the input ports of one16of the waveguides, the initial excitation of modes in the MMI region12is a superposition of these two modes20,30. This modal superposition within the MMI region12approximately reproduces the light that is launched at the illuminated input port, and results in approximately zero amplitude at the other input port, via which the waveguide17is connected to the MMI region12. Of course, only a coarse reconstruction of the launched mode at the start of the MMI region12is achieved at the illuminated input port, along with an imperfect zero at the other input port, because in this simplified illustration of the coupler operation we have considered only the two lowest order modes within the MMI region12, rather than all available propagating and radiation modes that are available within the MMI region12.

When these modes20,30have propagated to the far end of the MMI region12, the phase relationship between the modes20and30results in a symmetric division of light power between the respective output ports of waveguides18,19. The complex number coordinate systems in connection to each waveguide16,17,18,19illustrate the situation at the port of each respective waveguide16,17,18,19.

FIGS. 1dand1eare similar toFIG. 1a, but show a cross-section of two different exemplary MMI couplers100according to the invention.FIGS. 1dand1eshare reference numerals for corresponding parts. Hence, the coupler100is arranged on a substrate101on which a ridge structure103is arranged.FIGS. 1dand1e, similarly toFIG. 1a, show a respective lateral cross-section through the MMI region102of the coupler. The ridge103protrudes a certain distance104from the substrate surface. The distance104is preferably less than 10 μm, most preferably between 1 and 5 μm.

Furthermore,FIG. 2bcorresponds toFIG. 2a, and shows, in a similar manner and with reference numerals shared withFIGS. 1dand1e, a simplified top view of the MMI coupler100. The coupler100is arranged to convey light incident from input waveguides106,107, via corresponding input ports, into the MMI region102, and again out through output waveguides108,109, via corresponding output ports. Again, the two most fundamental modes110,120are considered for ease of understanding. Complex number coordinate systems show the light at each of the respective ports.

According to the invention, and as shown in the simplified and exemplary embodiments ofFIGS. 1dand1e, a film103a,103bof a certain material is arranged over the coupler100. InFIG. 1d, the film103ais arranged in a relatively thick layer, of thickness105afrom the substrate101surface, so that the complete ridge structure103is covered by the material103a. InFIG. 1e, the material103bis arranged in a shallower layer, of thickness105b, so that the ridge structure103is only partly covered by the film103b, effectively protruding above the film103bas viewed from the side as inFIG. 1e. The thickness105ais preferably between 20% and 200% of the distance104. In bothFIGS. 1dand1e,102adenotes the waveguiding layers103aas shown inFIG. 1a.

According to the invention, the material of the film103a,103bis mechanically strained, so that a force is applied by the film103a,130bto the surface of the coupler100. The force can be a tension or a compression force, or a combination of the two across different parts of the coupler100, resulting in a non-uniform mechanical stress field within the material of the MMI region102.

Such strain can be accomplished in different ways, including depositing the film103a,103bmaterial in a relaxed, unstrained state, on top of the coupler100at a first temperature, after which the temperature is changed to a second temperature so that the deposited film material shrinks or expands as a consequence of the change in temperatures. For example, a polymer can be used which when deposited at a first, higher temperature will shrink when the temperature drops to the second, lower temperature, thereby exerting a force onto the surface of the geometric structure of the substrate101and the ridge structure103as a consequence of said shrinking. In similar ways, it is possible to use a material which expands under a temperature change.

However, for some types of film materials, the chemical bonds between the film and the semiconductor surfaces, and/or the bonds formed within one or more of the films that have been deposited onto or attached to the semiconductor surfaces, can cause mechanical strain within the semiconductor without being subjected to a subsequent temperature or material property change, even if the film material is deposited in solid state, such as for instance by evaporation. If such materials and deposition techniques are used, no subsequent method step after the deposition itself will thus be necessary.

As a third option, it is possible to use a material which shrinks or expands as a consequence of curing or any other physical process, which process permanently modifies at least one material property of the film material and which process is applied in a processing step after an initial deposition of the unstrained or strained film material. In this case, the permanent change of the said at least one material property has the direct or indirect consequence of shrinking or expanding the film material, leading to similar results as a changed temperature with respect to MMI strain. Hence, as an alternative or addition to a film material with built-in strain or a subsequent temperature changing step, subsequent processing steps aiming at permanently modifying at least one material property of the deposited or attached film materials comprise optical or chemical curing, heat treatment, ion bombardment, intermixing, reflow, diffusion, and ion implantation.

A variety of methods can be used to deposit or attach one or more films of various material types onto semiconductor surfaces, for example by spinning, sputtering, soldering, crystal growth, flame hydrolysis, wafer bonding, electro-plating, ion beam deposition, evaporation (comprising ion assisted and plasma assisted), chemical vapour deposition (CVD), or plasma enhanced CVD.

Suitable film materials comprise, but are not limited to dielectrics, polymers, metals, crystals and glasses. In particular, preferred materials comprise benzocyclobutene (BCB), silicon dioxide (SiO2), silicon Nitride (SiN), silicon oxide (SiOx), silicon oxy nitride (SiOxNy), photopolyemide, or combinations thereof.

The skilled person realizes that the resulting net strain pattern onto the MMI from the film or stack of films will depend on the film thicknesses, deposition conditions and routine.

As a consequence of said force imparted onto the coupler100, the refractive index profile in the material of the coupler100will change, especially in the MMI region102and especially within and adjacent to the waveguiding layers12a,12b,12c,102a. According to the invention, the splitting or combining ratio of the coupler100is modified as a consequence of this changed refractive index profile, and will thus be different than the splitting or combining ratio of the corresponding coupler10without a strained film. In other words, the splitting or combining ratio of a multimode interference (MMI) coupler is modified by the arrangement of the strained film103a,103b.

With the strained film in place, the refractive index profile of the optical MMI material is modified more strongly along the outer edges of the MMI region102than in the center. As a result, while all modes are affected by the index change, the higher order mode120is more strongly affected than the fundamental mode110, because a greater proportion of its power distribution is located close to the MMI region102sidewalls.

Accordingly, the phase relationship of the two modes110,120when these arrive at the output end of the MMI region102will be modified as compared to the case illustrated inFIG. 2a, where there is no strained film. Specifically, for a film103a,130bthat applies tension force to the MMI region102, the constructive interference of the two modes110,120generally results in greater power for the cross state transmission than for the bar state transmission, as illustrated by the complex number coordinate systems inFIG. 2b.

Thus, the application of a strained film according to the invention provides an inexpensive method for modifying the splitting and/or combining ratio of an MMI coupler, not requiring any of the difficult or expensive methods and peripheral equipment used in the prior art described initially. Specifically, neither fabrication nor operation of control electrodes is required, and no illumination by external light beams is required. In the preferred embodiment that the strained film103a,130bthat covers the MMI region102also extends laterally away from the coupler100, no critical alignment of the film pattern with respect to the MMI region102is required during fabrication.

The present invention can advantageously be applied to an MMI coupler100having two input waveguides106,107and/or two output waveguides108,109, preferably a 2×2 MMI coupler.

Especially preferred is to apply the invention to such an MMI coupler, in such a way so as to retroactively correct the splitting or combining ratio in case a previous production step resulted in an MMI coupler with a splitting or combining ratio which was outside of a desired target range. Preferably, the above described modification of the splitting or combining ratio is carried out so that the splitting or combining ratio changes towards a value of 1.

It is furthermore preferred that the method step described herein of arranging a strained film over an existing MMI coupler is used as a one-time method step with the aim of permanently modifying or trimming the splitting and/or combining ratio of the said MMI coupler.

Hence, the present invention can be used within the production of MMI couplers, as an inexpensive post-processing method step for trimming or modifying the splitting and/or combining ratio of MMI couplers, such as after on-wafer testing or as a response to a request for a customization of certain MMI couplers, before shipping the manufactured products, as well as to retroactively correcting the performance of produced MMI couplers that do not satisfy certain set requirements. Such trimming will be permanent and will not require any other arrangements in order to maintain the ratio modification over time.

It is preferred that the strained film103a,103bis applied over the entire length of the MMI region102in the direction D. Thus, the film103awould preferably cover the whole width, preferably wider than the width, and preferably also the whole length, of the MMI region102as shown inFIG. 2b. In the partially covered case, the film103bwould surround the whole MMI region102inFIG. 2b, with the top surface of the MMI region102and possibly also the waveguides106-109protruding from above the surface of the film103b. That the film103a,103bis applied over the entire length of the MMI region102results in the effect of the film strain also being distributed over the entire length of the MMI region102, and as a consequence the required change in refractive index of material in the MMI region102to obtain a certain magnitude of modification in the coupler splitting or combining ratio can be substantially smaller than the change in index that must be achieved for the same ratio modification when that index change must be achieved by actions focused at a few optical image spots within the MMI region102.

Whereas the film103a,103bcan have a net tensile or compressive effect on the MMI region102, it is possible that it applies tensile forces onto certain parts of the MMI region102and compressive forces onto certain other parts thereof, depending on the exact geometry of the application and on the material properties, in terms of elasticity etc., of the film103a,103bmaterial. Specifically, in some applications it is advantageous to apply multiple layers of the same or different film materials, one on top of the other. Possibly, an etching step such as the one described below may be applied between the applications of consecutive layers of film material, so that different layers of material have different coverage in relation to the coupler100. This way, the exact effect of the strained film can be tailor made by varying film coverage, depth and/or composition across the surface of the MMI coupler100, possibly in combination with one or several intermediate or finalizing etching steps across selected parts of the coupler100or across its entire surface.

Hence, according to a preferred embodiment, the film103a,103bis partly etched away after being arranged over the coupler100, so that the force exerted onto the MMI region102as a consequence of the film material strain is different after said etching away as compared to the same situation without etching. Such etching may for instance comprise etching away the film103acompletely over one or several areas of the coupler100, resulting in a film103blike the one illustrated inFIG. 1e, where the MMI region102and possibly other components, such as waveguides106-109, protrude a certain distance above the film103bsurface. Alternatively, the etching may result in the film merely being somewhat thinner, so that the thickness105a,105bdecreases.

FIGS. 3a,3band3care graphs showing experimental measurement data of the combining ratio (Y axis) of three different 2×2 MMI optical combiners as a function of optical frequency (X axis, unspecified scale but approximately covering the conventional fiber communications C-band, in other words frequencies corresponding to wavelengths of 1.53-1.57 μm).FIG. 3ashows the combining ratio versus optical frequency for an MMI coupler which is fully encapsulated by a strained film according to the invention.FIG. 3bis a corresponding graph when the film is partly etched away, andFIG. 3cshows the combining ratio when the film is entirely etched away, all three graphs covering the same range of optical frequencies. It should be noted thatFIG. 3crepresents the conventional case, with no strained film present.

It is clear fromFIGS. 3a-3cthat the choice of film thickness affects the combining ratio of the combiner, and that a reasonably flat combining ratio is maintained over the entire C-band for all three cases. In the experiment illustrated inFIGS. 3a-3c, the combining ratio decreased in a direction away from 1 as a result of increased film thickness. However, it is clear that such a decrease may be useful for instance if the combining ratio is higher than desired in the absence of a strained film. Alternatively, a film material with opposite strain properties could be used in the illustrated case to instead increase the combining ratio from a value below 1.

The present invention can particularly advantageously be applied in one and the same method step to several MMI couplers, arranged as parts of the same integrated optical circuit. This situation is illustrated inFIG. 4, wherein a simplified Mach-Zehnder modulator400circuit exemplifies such a circuit. It is realized that, in practical applications, considerably more complicated and larger circuits can be treated in an analogue manner. The circuit400comprises input waveguides401a,401b, a first 2×2 MMI coupler in the form of a splitter402, two intermediary waveguides403,404, a second 2×2 MMI coupler in the form of a combiner405, and two output waveguides406a,406b. A film407of a polymer material is deposited, in a first step, across the complete surface of the circuit400, including all of its components. Thereafter, in a second optional step, the film407is etched down to a certain depth over a portion of or the whole of the MMI region of one or both 2×2 MMI regions, so that, where the etch has been applied, the waveguides and couplers401-406, all of which protrude from the substrate surface, also protrude from the surface of the film407, as described above.

Many times, the MMI couplers comprised in a circuit such as the one shown inFIG. 4, which has been manufactured as a unit, will suffer from similar or identical performance issues in terms of splitting and/or combining ratios. This is in particular true in the case of components formed on the same semiconductor substrate and sharing the same or substantially the same material structure (above). By subjecting several MMI components, or preferably the whole circuit, to the same strained film407, all such performance issues can be corrected in a very efficient and simple, single method step, across the complete circuit400. It is, of course, also possible to subject only some parts of the circuit400to the strained film, or to selectively apply certain layers of film, or certain etching steps, to only one or several parts or components on the same circuit400.

It is preferred that at least two of the thus subjected couplers402,405are formed on the same semiconductor substrate and share the same or substantially the same material structure, hence sharing the same or substantially the same substrate and material profile. This will result in predictable effects from the application of the strained film407.

FIG. 5shows a semiconductor wafer500comprising a plurality of integrated circuits501spread across its surface, each comprising at least one, preferably a plurality, of MMI couplers. It is realized that it is also possible that individual MMI components may be spread across the wafer500surface without being arranged in complete circuits. In a manner similar to the one described above with reference toFIG. 4, a film502of a strained material is applied to the wafer500surface, preferably partly or completely covering each of a plurality, more preferably partly or completely covering all, the circuits501or individual components, with similar advantages as described above.

The present invention can be applied without any modification of the standard fabrication sequence. For example, it is preferred that the film, apart from subjecting the MMI component surface to tension and/or compression forces, also serves as an insulating film that is deposited over the wafer to achieve other purposes, such as to provide electrical insulation between contact electrodes and the semiconductor surface in other parts of the optical device. Furthermore, the partial or complete removal of the film, by etching as described above, in the vicinity of the MMI component could be done at the same time as the film is cleared away from the top of some other semiconductor surfaces, to allow the formation of electrical contacts on those surfaces, while not making contacts on the surface of the MMI. In both of these examples, the utility of the film can be achieved without any changes in a standard optical device fabrication process sequence.

In the following, a detailed exemplary embodiment of the present invention will be described.

Hence, in a preferred embodiment, we consider a planar InP/InGaAsP-based semiconductor 2×2 MMI type optical coupler that is disposed on a planar InP substrate, where the 2×2 rectangular MMI region has a length of 221 μm along the direction of light propagation, and a width of 12.4 μm in the lateral direction, i.e. perpendicularly to the direction of light propagation. The two input and two output access port waveguides are symmetrically disposed about the MMI centerline, and have centerlines that are separated by a distance of 4.2 μm. The rectangular MMI shape and all of the input and output port waveguides are strongly laterally guided, and have lateral dimensions that have been defined by a 3 μm deep dry etch process. A stack of nominally unstrained layers of InGaAsP alloys and InP forms a vertical waveguide within the circuit that confines light to propagate in a plane that is parallel to the substrate surface, approximately 2 μm from the top surface of the circuit, and characterized by a slab or one-dimensional vertical waveguide effective refractive index of 3.27 at the illumination wavelength of 1.55 μm.

For an ideal 2×2 MMI coupler having the dimensions described above, light power that is launched into the fundamental mode in one of the input ports will be divided substantially equally between the two output ports. In other words, the power transmitted in the bar and cross state operation of the 2×2 MMI coupler is expected to be equal, or equivalently the ratio power transmission to the two output ports is expected to be 1.

However, if a wafer of optical circuits have been fabricated, and on-wafer testing reveals that the bar to cross state power splitting ratio of the 2×2 MMI couplers in a majority of the circuits on the wafer substantially exceeds 1, or if the circuits are characterized by a higher optical power loss in an optical path that precedes or follows one of the 2×2 MMI couplers in each circuit, such that a reduced bar to cross state power transmission ratio is required to advantageously alter the flow of optical power within the circuit, then a reduction in the bar to cross state power transmission would be desired.

To reduce the bar to cross state transmission, a planarizing polymer film of benzocyclobutene (BCB) was deposited on the wafer at 280° C., to cover the 2×2 MMI couplers. When the BCB film contracted, as the wafer was cooled to room temperature, the semiconductor surface was placed in tension, which resulted in a lateral profile of the refractive index within the MMI that provided a reduction of the bar to cross state transmission ratio of the 2×2 MMI's. The degree of reduction could have been modulated by the thickness of the BCB film, the film deposition conditions, or by restricting the extent of the film to cover only a portion of the MMI. In this manner, wafer-wide trimming of the circuit performance, and even trimming of the performance of individual circuits, was possible. The rectangular MMI having deeply etched sidewalls approximately 3 μm deep was covered by a planarizing BCB film with a total thickness of 5 μm above the surface where the deep dry etch terminated, in other words a BCB film that terminated 2 μm above the top surface of the MMI coupler. This way, a reduction in the bar to cross state power transmission ratio of approximately 20% was achieved.

Alternatively, a film material with the opposite sign of strain could have been applied, achieving an increase rather than a decrease in the bar to cross state power transmission ratio.