Optical assembly

Abstract

The invention relates to an optical assembly comprising a first optical circuit and a second optical circuit. The invention further relates to an optical device, wherein the first optical circuit and the second optical circuit are fixedly connected to each other. In addition, the invention relates to a method for manufacturing the optical device. According to the invention, a flexible waveguide end of a waveguide on the second optical circuit is used, which flexible waveguide end extends upwards from the second optical circuit to optically couple to a waveguide on the first optical circuit.

Description

[0001] The present invention relates to an optical assembly comprising a first optical circuit and a second optical circuit. The present invention further relates to an optical device, wherein the first optical circuit and the second optical circuit are fixedly connected to each other. In addition, the present invention relates to a method for manufacturing an optical device.

[0002] An optical assembly according to the preamble of claim 1 is known from WO 2016 / 131906. In this known optical assembly, the first optical circuit is an optically active circuit and comprises an optically active substrate, at least one first optical waveguide formed in the optically active substrate, and a first tapered recess in the optically active substrate arranged near a first end of the at least one first optical waveguide, the first tapered recess having a bottom surface, an end surface, and a pair of side surfaces extending from the bottom surface.

[0003] In the known optical assembly, the second optical circuit is an optically passive circuit formed by an interposer comprising at least one second optical waveguide. The interposer enables coupling of the at least one first waveguide on the optically active circuit to at least one optical fiber. More specifically, at one end of the interposer, the at least one second waveguide is coupled to the optical fiber, while at the other end, the at least one second waveguide is coupled to the at least one first optical waveguide of the optically active circuit.

[0004] By completely removing the substrate underneath the end of the at least one second optical waveguide of the optically passive circuit, a finger is formed. This finger is visible from both the top and the bottom of the interposer structure, which can help the finger to align into the first tapered recess of the optically active circuit. In this way, the end of the at least one second waveguide on the interposer becomes flexible, allowing it to be arranged in the first tapered recess. Typically, a plurality of fingers is formed to connect a plurality of second optical waveguides on the optically passive circuit to a corresponding plurality of first optical waveguides on the optically active circuit.

[0005] The permanent fiber-to-chip coupling via the interposer can be provided by permanently fixing the finger in place. This can be achieved using an index matching adhesive when the adhesive is in the optical path between the interposer and the at least one first waveguide of the optically active circuit. Alternatively, an adhesive such as solder can be used outside the first tapered recess for fixedly connecting the interposer to the optically active circuit.

[0006] The electrical connection between the interposer and the optically active circuit can be achieved using one or more metal layers that cover the finger and are in contact with metal pads on the optically active circuit.

[0007] In known optical assemblies, the alignment of the fingers is performed using the fingers themselves as visual indicators. This is made possible by completely removing the substrate underneath them. Furthermore, the alignment requires the interposers to be positioned at an angle with respect to the optically active circuit. This complicates the alignment, reduces the reliability of the coupling between the interposers and the optically active circuit, and only allows one interposer to be connected to the optically active circuit at a time.

[0008] It is an object of the present invention to provide an optical assembly in which the above-mentioned problems associated with the coupling of the first optical circuit and the second optical circuit do not occur or at least to a lesser extent.

[0009] This object is achieved using an optical assembly according to claim 1, characterized in that the first optical circuit further comprises a second tapered recess arranged near the second end of the at least one first optical waveguide, the second tapered recess having a bottom surface, an end surface, and a pair of side surfaces extending from the bottom surface, and the second optical circuit further comprises a first recess arranged below the first end of the at least one second optical waveguide, and a second recess arranged below the second end of the at least one second optical waveguide.

[0010] The first end of the second optical waveguide and the second end of the second optical waveguide form a first flexible end and a second flexible end, respectively, which, in the absence of an external force acting thereon, extend away from the second optical substrate in at least partially opposite directions.

[0011] According to the invention, the first flexible end and the second flexible end are configured to be received in the first tapered recess and the second tapered recess, respectively, with the aim of allowing the first flexible end and the second flexible end to exert a spring force onto the end surface of the first tapered recess and the second tapered recess, respectively, when the first optical circuit and the second optical circuit are fixedly coupled. When the first optical circuit and the second optical circuit are fixedly connected, the first optical waveguide having the first tapered recess arranged near its first end is optically coupled to the second optical waveguide having the first recess arranged near its first end via the end surface of the first tapered recess, and the first optical waveguide having the second tapered recess arranged near its second end is optically coupled to the second optical waveguide having the second recess arranged near its second end via the end surface of the second tapered recess.

[0012] Furthermore, the first optical circuit is one of an optically active circuit and an optically passive circuit, and the second optical circuit is the other of the optically active circuit and the optically passive circuit.

[0013] Due to the spring force applied to the end surfaces, the first optical circuit can be at least partially clamped when connecting the first optical circuit and the second optical circuit. Furthermore, the spring force ensures a low-loss optical coupling between the first flexible end and the first end of the first optical waveguide and between the second flexible end and the second end of the first optical waveguide.

[0014] The first optical circuit can be an optically active circuit and the second optical circuit can be an optically passive circuit. In the context of the present invention, an optically active circuit is an optical circuit which at least mostly increases optical energy by converting electrical energy supplied to the optically active circuit into optical energy. In contrast, an optically passive circuit is a circuit which at least mostly only dissipates optical energy.

[0015] The first end of the first optical waveguide and the second end of the first optical waveguide can be end portions at opposite sides of the same first optical waveguide. Alternatively, the first end of the first optical waveguide and the second end of the first optical waveguide can be end portions of different first optical waveguides, which are preferably arranged in parallel.

[0016] The first optical circuit can further comprise one or more first contact pads and a first alignment mark. In addition, the second optical circuit can further comprise one or more second contact pads and a second alignment mark. In this case, the one or more first contact pads are configured to be fixedly connected to the one or more second contact pads, e.g. using a solder, a solder paste or a conductive adhesive. Furthermore, the first optical circuit and the second optical circuit can be configured such that, when the first optical circuit and the second optical circuit have been moved relative to each other to bring the first alignment mark and the second alignment mark into a predefined lateral registration and the first optical circuit and the second optical circuit are subsequently brought closer together without substantially changing the lateral registration, the first flexible end and the second flexible end are guided by at least one of the bottom surface and the side surfaces of the first taper groove and the second taper groove, respectively, towards the first end of the first optical waveguide and the second end of the first optical waveguide, thereby enabling the optical coupling between the first flexible end and the first end of the first optical waveguide and between the second flexible end and the second end of the first optical waveguide.

[0017] The first and second alignment marks allow for an accurate lateral alignment. For example, both the first and second optical substrates are substantially planar structures extending in the x- and y-directions. Prior to connecting the first and second optical circuits, the circuits are moved relative to each other in the x- and / or y-directions until the distance between the first and second alignment marks in these directions satisfies a predetermined value. For example, the alignment marks can be considered aligned when the distance in the y-direction is equal to 500 + / - 5 microns and the distance in the x-direction is equal to 300 + / - 5 microns. In particular embodiments, the alignment marks are considered aligned when the distance in both directions is less than 5 microns. Thus, the mutual alignment of the first and second optical circuits can be relatively coarse, e.g., with tolerances much larger than the feature size (e.g., width) of at least one of the first or second optical waveguides. Due to the self-alignment of the flexible end in the tapered groove, the final alignment between the flexible end and the at least one first optical waveguide is facilitated, which final alignment typically has to be more accurate than the above-mentioned 5 microns.

[0018] During the alignment of the first and second alignment marks, the first and second flexible ends do not engage any of the surfaces of the first and second tapered grooves, respectively. However, once the alignment marks are aligned, the first and second optical circuits are moved relative to each other in the z-direction. During this movement, the first and second flexible ends will engage the first and second tapered grooves, respectively. More specifically, the first and second flexible ends will engage the bottom surface and at least the side surface of the first and second tapered grooves, respectively. Due to the shape of the first and second tapered grooves, which tapers outwardly away from the first or second end of the first optical waveguide, respectively, the first and second flexible ends will be guided towards the first or second end of the first optical waveguide, respectively. Due to the upward curvature of the first and second flexible ends, it is ensured that the first and second flexible ends engage the first or second tapered groove, respectively, even though the first and second optical circuits are still spaced apart in the z-direction. Furthermore, the upward curvature ensures that sufficient force is exerted by the end surface of the first and second tapered grooves onto the first and second flexible ends, respectively, and vice versa, to help the correct positioning of the first and second flexible ends relative to the first optical waveguide and to ensure a low-loss optical coupling. More specifically, the end surface provides a controllable end position, allowing the flexible end to stop at the same position. Furthermore, since the fixed connection between the first and second optical circuits is achieved using the one or more first and second contact pads, this positioning does not need to be disturbed by the presence of any adhesive on the second optical waveguide or in the first or second tapered groove.

[0019] The distance between the first flexible end and the second flexible end can be larger than the distance between the end surface of the first taper groove and the end surface of the second taper groove before the first optical circuit and the second optical circuit are joined to each other, and the distance between the first flexible end and the second flexible end is smaller than the distance between the end surface of the first taper groove and the end surface of the second taper groove if the first flexible end and the second flexible end are bent into alignment with the corresponding second optical waveguide. In this way, it can be ensured that the first flexible end and the second flexible end have to be deformed, preferably elastically deformed, thereby exerting a spring force onto the end surfaces of the first taper groove and the second taper groove, respectively.

[0020] The first flexible end and the second flexible end can extend away from the second optical substrate along a first direction and a second direction, respectively, each of the first direction and the second direction having a component parallel to the second optical substrate and a component perpendicular to the second optical substrate, wherein the parallel components corresponding to the first flexible end and the second flexible end are directed in at least partially opposite directions. By exerting the spring forces from at least partially opposite directions, at least partial clamping of the first optical circuit by the second optical circuit can be achieved. Furthermore, each parallel component can be split into a second component along a third direction and a third component along a fourth direction perpendicular to the third direction, wherein the second component corresponding to the first direction is opposite to the second component corresponding to the second direction, and / or wherein the third component corresponding to the first direction is opposite to the third component corresponding to the second direction. In this way, the spring forces can be applied symmetrically. Here, a component is considered opposite to another component if it has an opposite sign and, further preferably, an equal magnitude.

[0021] The optical assembly can comprise a plurality of the first taper grooves of the corresponding plurality of first optical waveguides, a plurality of the second taper grooves of the corresponding plurality of first optical waveguides, a plurality of the first flexible ends of the corresponding plurality of second optical waveguides, and a plurality of the second flexible ends of the corresponding plurality of second optical waveguides, each first flexible end and second flexible end being configured to be received in a respective first taper groove and second taper groove, respectively, the first flexible end and the second flexible end each extending away from the second optical substrate along a respective first direction and a respective second direction, respectively, each of the respective first direction and the respective second direction having a component parallel to the second optical substrate and a component perpendicular to the second optical substrate. Each respective parallel component can be split into a second component along a third direction and a third component along a fourth direction perpendicular to the third direction, and a sum of the second components and / or a sum of the third components can be substantially zero. Preferably, it can be achieved that a combined spring force exerted onto the first optical circuit can be substantially zero in at least one of the third direction and the fourth direction.

[0022] The at least one second optical waveguide can comprise a first layer arranged on the second layer. Further, when the first optical circuit and the second optical circuit are fixedly connected, the first layer is arranged between the second layer and the first optical substrate. The first layer can be in tensile strain with respect to the second layer, or the second layer can be in compressive strain with respect to the first layer. Due to the imbalance of the strain in the second optical waveguide, when material under the first end of the second optical waveguide is etched away in order to create the first groove, the first end of the second optical waveguide will bend upwards, thereby creating the first flexible end. The flexibility of the first flexible end of the second optical waveguide is related to the width and thickness of the second optical waveguide. Similar considerations apply when the second flexible end is formed.

[0023] The first layer can be provided only on the second layer in areas corresponding to the first flexible end and the second flexible end. In other areas of the at least one second optical waveguide, the first layer can be omitted, since no strain difference is required.

[0024] The maximum angle of inclination of the first flexible end and the second flexible end with respect to a plane parallel to the second optical substrate can be in the range between 1 degree and 45 degrees, and more preferably between 3 degrees and 30 degrees. Outside this range, during the mutual movement of the first optical circuit and the second optical circuit in the z-direction, the first flexible end and the second flexible end are at risk of breaking, or they will not extend far enough into the first tapered groove or the second tapered groove, respectively. In addition, the length of the first flexible end and the second flexible end can be in the range between 50 micrometers and 1000 micrometers. Additionally or alternatively, the width of the first flexible end and the second flexible end can be in the range between 0.5 micrometers and 20 micrometers, and the thickness of the first flexible end and the second flexible end can be in the range between 50% and 150% of the width of the first flexible end and the second flexible end, respectively.

[0025] The width of the at least one first optical waveguide can be in the range between 0.1 micrometers and 10 micrometers, and the thickness of the at least one first optical waveguide can be in the range between 0.1 micrometers and 5 micrometers.

[0026] Various functions can be implemented in the optically active circuit and the optically passive circuit. For example, at least one passive element of a group of passive elements consisting of filters, combiners, polarization converters, and splitters can be implemented in the optically passive circuit. Additionally or alternatively, at least one active element of a group of active elements consisting of lasers, semiconductor optical amplifiers, phase modulators, amplitude modulators, and photodetectors can be implemented in the optically active circuit. In the latter case, the optically active circuit can comprise one or more electrical terminals connected to the at least one active element for receiving or outputting electrical signals and / or for receiving electrical power, wherein at least one of the one or more electrical terminals is connected to and / or formed by a respective first contact pad of the one or more first contact pads. Thus, the same structure that is used for making electrical connections required for operating the at least one active element can be used for fixedly connecting the optically active circuit and the optically passive circuit.

[0027] A first optical waveguide of the at least one first optical waveguide can terminate in the corresponding first tapered recess and / or second tapered recess. For example, the corresponding first tapered recess and / or second tapered recess can comprise a first section in which the first optical waveguide of the at least one first optical waveguide terminates. This first section can be wider than a last portion of the first optical waveguide but smaller than the first flexible end or the second flexible end of the corresponding second optical waveguide to be received in the corresponding first tapered recess or second tapered recess. The corresponding first tapered recess and / or second tapered recess can comprise a second section connected to the first section. The second section can be wider than the first section and the first flexible end or the second flexible end of the corresponding second optical waveguide to be received in the corresponding first tapered recess or second tapered recess. Furthermore, a border between the first section and the second section can be configured to form an end surface of the corresponding first tapered recess or second tapered recess against which the first flexible end or the second flexible end of the corresponding second optical waveguide to be received in the corresponding first tapered recess or second tapered recess abuts after the one or more first contact pads and the second contact pad have been fixedly connected.

[0028] Alternatively, an end facet of the first optical waveguide arranged in the vicinity of the corresponding first tapered recess or second tapered recess can also form the end surface of the tapered recess.

[0029] The optically active circuit can further comprise a first contact ring arranged around the at least one first optical waveguide, and the optically passive circuit can further comprise a second contact ring arranged around the at least one second optical waveguide, wherein the first contact ring and the second contact ring are configured to be fixedly connected to each other so as to provide a seal for the at least one first optical waveguide and the at least one second optical waveguide. Such a seal can be beneficial to protect the coupling between the at least one first optical waveguide and the at least one second optical waveguide from environmental conditions such as humidity and contaminants and / or to protect the exposed facet of the flexible end and / or to protect the exposed facet of the at least one first optical waveguide.

[0030] The one or more first contact pads and the one or more second contact pads can each comprise one or more metal layers to allow them to be connected using soldering, preferably eutectic soldering, or bonding, preferably eutectic bonding, using simultaneous application of heat and pressure. Prior to connecting the first contact pads and the second contact pads, a solder or another adhesive can be applied to at least one of the first contact pads and the second contact pads. Similar considerations apply to the first contact ring and the second contact ring.

[0031] The first optical substrate or the second optical substrate corresponding to the optically active circuit can comprise a first cladding layer having a first doping type, a second cladding layer having a second doping type different from the first doping type, and an active layer arranged between the first cladding layer and the second cladding layer. The at least one first optical waveguide or the second optical waveguide as part of the optically active circuit can be formed by etching through at least a portion of the first cladding layer. Additionally or alternatively, the optically active circuit can comprise a first metal contact to the first cladding layer connected to a respective one of the one or more first contact pads or second contact pads and a second metal contact to the second cladding layer connected to a respective one of the one or more first contact pads or second contact pads.

[0032] The first cladding layer can comprise a first sub-layer, a second sub-layer, and a first etch stop layer arranged between the first sub-layer and the second sub-layer. The first etch stop layer can be a stop layer for stopping a particular wet chemical etching step. The at least one first optical waveguide or the second optical waveguide as part of the optically active circuit can be formed by etching up to the first etch stop layer. Similarly, the second cladding layer can comprise a first sub-layer, a second sub-layer, and a second etch stop layer arranged between the first sub-layer and the two sub-layers, wherein the second metal contact has been formed by etching up to the second etch stop layer and a metal layer arranged in a cavity formed by said etching. The second etch stop layer can be a stop layer for stopping a particular wet chemical etching step.

[0033] The first and second cladding layers can be made of indium phosphide, gallium arsenide or gallium antimonide and / or wherein the second cladding layer is arranged on an indium phosphide, gallium arsenide or gallium antimonide substrate having the same doping type as the second cladding layer. The active layer can be made of arsenide gallium indium phosphide, aluminum indium gallium arsenide or antimonide based materials. However, the present invention is not limited to these materials.

[0034] The optically passive substrate can comprise a guiding layer, a carrier substrate and a cladding layer arranged between the guiding layer and the carrier substrate. In this case, the at least one first optical waveguide or the second optical waveguide as part of the optically passive circuit has been formed by etching through the cladding layer and the guiding layer, preferably up to the carrier substrate. Additionally or alternatively, the first and second flexible ends of the at least one second optical waveguide as part of the second optical circuit have been formed by partially etching the carrier substrate below the cladding layer. Furthermore, a stress layer can be arranged on the guiding layer, the stress layer being under tensile strain and configured to at least partially cause the upward bending of the first and second flexible waveguides. The stress layer can be a silicon nitride layer Si x N y However, the present invention is not limited thereto.

[0035] The guiding layer can be a silicon layer, preferably a single crystalline layer. Furthermore, the cladding layer can be a silicon oxide layer Si x O y and the carrier substrate can comprise a silicon substrate.

[0036] Alternatively, the guiding layer can be embedded into the cladding layer, wherein the guiding layer is a silicon nitride layer Si x N y and the cladding layer is a silicon oxide layer Si x O y In this case, the at least one first optical waveguide or the second optical waveguide as part of the optically passive circuit can be formed by etching through the cladding layer, preferably up to the carrier substrate.

[0037] The optical assembly can further comprise a further optical active circuit configured as the aforementioned optical active circuit. In such a case, the optically passive circuit can be configured to be coupled to the further optical active circuit in the same manner as to the aforementioned optical active circuit.

[0038] According to a second aspect, the present invention relates to an optical device comprising an optical assembly as described above, wherein the optical active circuit, and for the moment the further optical active circuit, is fixedly connected to the optically passive circuit.

[0039] According to a third aspect, the invention relates to a method for manufacturing the optical device as described above, in that the first optical circuit comprises one or more first contact pads and first alignment marks, and the second optical circuit further comprises one or more second contact pads and second alignment marks. The method comprises moving the first optical circuit and the second optical circuit relative to each other to bring the first alignment marks and the second alignment marks into a predefined lateral registration. The method further comprises the steps of, after having obtained the predefined lateral registration, bringing the first optical circuit and the second optical circuit closer together without substantially changing the lateral registration, during which movement the first flexible end and the second flexible end are guided towards the first end of the first optical waveguide and the second end of the first optical waveguide, respectively, by at least one of the bottom surface and the side surfaces of the first tapered groove and the second tapered groove, thereby enabling optical coupling between the first flexible end of the first optical waveguide and the first end, and between the second flexible end of the first optical waveguide and the second end. As a final step, the first optical circuit and the second optical circuit are fixedly connected by fixedly connecting the one or more first contact pads to the one or more second contact pads.

[0040] Next, the invention will be described in more detail with reference to the accompanying drawings, in which:

[0041] Figure 1A and Figure 1B schematically cross-sections of coupling an optically active circuit to an optically passive circuit according to the invention are shown;

[0042] Figure 2A and Figure 2B top views of coupling an optically active circuit to an optically passive circuit according to the invention are shown, and Figure 2C and Figure 2D perspective views of a tapered groove and the arrangement of a flexible end in such a groove, respectively, are shown;

[0043] Figure 3A and Figure 3B optical devices and a wafer holding a plurality of said devices, respectively, are shown;

[0044] Figures 4A to 4D different processing steps for manufacturing an optically active circuit according to the invention are shown;

[0045] Figures 5A to 5D different processing steps for manufacturing an optically passive circuit based on a silicon-on-insulator technology are shown;

[0046] Figures 6A to 6D different processing steps for manufacturing an optically passive circuit are shown, in which a silicon nitride layer embedded in a silicon oxide layer is used to form an optical waveguide; and

[0047] Figure 7AAn exploded view is shown with the flexible end distanced from the second optical substrate along the direction in which it extends, and Figure 7B An example is shown in which the forces applied by the flexible end in different directions substantially compensate for each other.

[0048] In Figure 1A The assembly 100 is shown in a non-coupled state, comprising an optically active circuit 10 and an optically passive circuit 20. The optically active circuit 10 is schematically shown as having an optically active substrate 11 comprising an optically active layer 12. Here, a first optical waveguide is formed using the optically active layer 12. At opposite ends of this waveguide, tapered grooves 13A, 13B are provided. Further, a first contact pad 14 is provided, typically implemented using one or more metal layers. As shown, the distance between the end walls of the tapered grooves 13A, 13B is equal to the distance D2.

[0049] The optically passive circuit 20 comprises an optically passive substrate 21 in which a second optical waveguide 22 is formed. Grooves 23 are arranged near the ends of the waveguide 22. Due to the removal of parts of the substrate 21 underneath these ends, flexible ends 22A, 22B are formed, which are bent upwards at an angle Θ with respect to a plane parallel to the substrate 21. In addition, a second contact pad 24 is provided, typically made using one or more metal layers. Further, as shown, the distance between the top ends of the ends 22A, 22B is equal to the distance D3. Moreover, the distance between the top ends of the ends 22A, 22B is equal to the distance D1 when the ends 22A, 22B are parallel to the substrate 21. As shown, D3 > D2 > D1.

[0050] Figure 1B The assembly 100 is shown in a coupled state. As shown, the end 22A has been guided towards the end walls of the tapered grooves 13A, 13B, such that an optical signal can be efficiently transferred from the optical layer 12 to the waveguide 22. The flexible ends 22A, 22B are bent upwards at an angle Θ' with respect to a plane parallel to the substrate 21, where Θ' < Θ. In the uncoupled state, if the flexible ends 22A, 22B are bent upwards at an angle Θ', the distance between the top ends of the ends 22A, 22B would be smaller than D2. In this way, it is ensured that when coupling the optically active circuit 10 and the optically passive circuit 20, the flexible ends 22A, 22B will deform, preferably elastically deform, thereby exerting a force on the end surfaces or walls of the tapered grooves 13A, 13B.

[0051] Further, the first contact pad 14 is fixedly connected to the second contact pad 24, for example through an intermediate solder layer or through a eutectic bond. These contact pads can be used to transfer electrical signals and / or electrical power from the optically passive circuit 20 to the optically active circuit 10.

[0052] Figure 2A and Figure 2BA top view is shown illustrating the process of coupling the optically active circuit 10 to the optically passive circuit 20. Figure 2C and Figure 2D Perspective views are shown illustrating the arrangement of the tapered recesses 13A, 13B and the flexible ends 22A, 22B in the recesses 13A, 13B, respectively.

[0053] As shown, a first alignment mark 15 is arranged on the optically active substrate 11. Similarly, a second alignment mark 25 is arranged on the optically passive substrate 21. To couple the circuits 10, 20, the alignment marks 15, 25 are used to position the optically active circuit 10 relative to the optically passive circuit 20. For example, the optically active circuit 10 is moved while the optically passive circuit 20 remains stationary to bring the alignment marks 15, 20 into a predefined lateral registration. The predefined lateral registration is achieved when the distance between the marks 15, 20 in the x-direction and the y-direction falls within a predefined range, for example between 45 and 48 micrometers. In a particular embodiment, the range in both the x-direction and the y-direction is centered around 0.

[0054] By comparison, a perspective view is shown illustrating the non-coupled state of Figure 2A and a perspective view is shown illustrating the coupled state of Figure 2B It can be observed that the ends 22A, 22B have been brought into close proximity to the waveguide 12. This has been achieved by the ends 22A, 22B being pushed by the bottom surface of the recesses 13A, 13B and at least one of their side surfaces into the position on the end surface of the tapered recesses 13A, 13B where the spring force is exerted. This is shown in more detail in Figure 2C .

[0055] Figure 2C A perspective view is shown illustrating an embodiment of the tapered recess 13A. Here, the recess 13A comprises a first portion formed by a bottom surface 131 and a side surface 132. The facet 12A of the waveguide 12 is present in this first portion. The recess 13A further comprises a second portion formed by the bottom surface 131, side surfaces 133, 134 and a back surface 135.

[0056] Figure 2D A top view is shown illustrating the recess 13A in which the flexible end 22A has been arranged. Here, it can be seen that the side surface 133 forming the border between the first portion and the second portion of the recess 13A extends substantially parallel to the end facet 12A. Furthermore, the width of the first portion, as measured in the x-direction, is larger than the width of the waveguide 12 but smaller than the width of the flexible end 22A. On the other hand, the width of the second portion, also measured in the x-direction, is larger than the width of the flexible end 22A. Only at the corners between the surfaces 133, 134, the width of the second portion can be substantially the same as the width of the flexible end 22A. As shown in Figure 2D , the side surface 133 forms the end surface against which the flexible end 22A is pressed. Furthermore, as seen from theFigure 2D It can be seen that light from the waveguide 12 will pass through air before entering the flexible end 22A.

[0057] Figure 3A A hybrid photonic integrated circuit "PIC" 200 is shown, in which a plurality of optically active circuits 10 are coupled to a single optically passive circuit 20. Here, each of the optically active circuits 10 performs a function such as light generation, amplification, switching, modulation, or detection on each of two optical waveguides arranged on an optically active substrate. On the optically passive circuit 20, the waveguides 22 direct the various optical signals to the filters 30. It will be clear to the skilled person that the PIC 200 is merely an example of how various active functions can be implemented on the optically active circuits 10 and how various passive functions can be implemented on the optically passive circuit 20. The present invention is therefore not limited to the example shown. Furthermore, active functions typically require electrical power and / or require and / or produce electrical signals. These can be transmitted between the optically passive circuit 20 and the optically active circuits 10 using the contact pads 14, 24.

[0058] Figure 3B A wafer 26 is shown, of which the optically passive substrate 21 is a part. As shown, the optically active circuits 10 can be mounted on the optically passive circuit 20 while their substrates are still connected to each other. After final processing, the PIC 200 can be separated using known dicing techniques, such as sawing.

[0059] Figures 4A to 4D A manufacturing process of the optically active circuit 10 according to the present invention is shown. Here, it is noted that for Figures 4A to 4D the upper part shows a cross-sectional view, while the lower part shows a schematic top view. In each lower part, a line is indicated, which indicates the position at which the cross-sectional view is taken.

[0060] Figures 4A to 4D The process shown in Fig. 1 is based on indium phosphide technology and uses a semiconductor wafer comprising a layer stack as indicated in Fig. 2. The layer stack comprises an indium phosphide (InP) substrate 10 with several epitaxial layers 1-9, as indicated in Table 1. Figure 4A

[0061] Table 1

[0062] Layer number in Figure 3A ​ Description layer Layer number in Figure 3A ​ Description layer 1 InGaAs 7 InP n-doped 3 InP p-doped 8 Etch stop layer 4 Etch stop layer 9 InP n-doped 5 InP p-doped 10 InP substrate 6 Active layer

[0063] Figure 4B A first step is shown, in which the contact layer 1 is partially removed, and in which waveguides have been formed by etching down to the etch stop layer 4. More specifically, an etching technique is used to etch through a substantial part of the layer 3. This etching step can be well controlled, as it stops when layer 4 is reached. ​

[0064] A similar approach is followed to define the contact to the n-doped InP layer. This is shown in Figure 4C The same etching step, until the etch stop layer 8, is used to define the tapered groove 303.

[0065] As shown in Figure 4D , an anti-reflective "AR" coating 304 is applied, which is subsequently opened at the locations where an ohmic contact should be realized. A patterned metal layer 308 is used to realize the contact to the InGaAs layer 1, in the following referred to as P-contact, and another patterned metal layer 307 realizes the contact to the n-doped InP layer 9, in the following referred to as N-contact. As a last step, an interconnect metal is applied to the N- and P-contacts. This can be performed using a sputtering process.

[0066] Figures 5A to 5D Different processing steps for the fabrication of an optical passive circuit based on a silicon-on-insulator technology are shown. The process uses a layer stack as shown in Figure 5A Here, a high resistivity silicon substrate 52 covered by a buried oxide silicon 51 is preferably used. A single crystalline silicon layer 50 is provided on top of the buried oxide silicon. Further, it is noted that for Figures 5A to 5D , the upper part shows a cross-sectional view, while the lower part shows a schematic top view. In each lower part, a line is indicated, which indicates the position at which the cross-sectional view is taken.

[0067] As a first step, shown in Figure 5B , a silicon nitride Si x N y layer 400 is deposited to introduce a sufficient tensile strain to bend the end of the waveguide to be formed upwards. This layer is removed from the rest of the circuit.

[0068] As a second step, also shown in Figure 5B , a shallow etch is performed on the silicon layer 50 to define a ridge waveguide therein. More specifically, a shallow groove 53 is defined. As a next step, also shown in Figure 5B , a relatively deep groove 54 is etched within the groove 53, down to the buried oxide silicon layer 51. Thereafter, as shown in Figure 5C , an anti-reflective coating 401 is applied. A metallization 402 will be applied to route power and / or electrical signals to and from an optical active circuit to be connected to the optical passive circuit. Next, a relatively thick electroplated metal 403 is applied to realize a low ohmic resistance and to allow eutectic bonding. Next, in Figure 5D , openings 55A are defined, followed by a dry etch step to etch through the anti-reflective coating 401 and the buried oxide layer 51. As a last step, an anisotropic etch is performed to remove the silicon substrate 52 under the waveguide end. The anisotropic etch will result in a cavity 55. Due to the tensile strain introduced by the silicon nitride layer 400, the end of the waveguide will bend upwards.Figure 5D The residual strain in the waveguide formed using the layers 400, 50, 51 will cause the end to bend upwards and away from the silicon substrate 52, similar to Figure 1A and Figure 1B as shown in Figure 1A , Figure 1B , Figure 2A , Figure 2B twisting in the end of the waveguide caused by the engagement with the tapered groove during the process of aligning the end of the waveguide with the waveguide on the optically active circuit as shown in

[0069] Figures 6A to 6D Different process steps for manufacturing an optically passive circuit are shown, in which an optical waveguide is formed using a silicon nitride layer embedded in a silicon oxide layer. It is noted that for Figures 6A to 6D the upper part shows a sectional view, while the bottom part shows a schematic top view. In each bottom part, a line is indicated, which indicates the position at which the sectional view is taken.

[0070] As shown in Figure 6A , a silicon substrate 71 is used, on top of which a silicon oxide Si x O y layer 70A is thermally grown, which will act as a cladding layer. Thereafter, a silicon nitride Si x N y layer 72 is deposited, which will act as a guiding layer. After the waveguide is defined using etching, a further silicon oxide Si x O y layer 70B is grown. In this way, the guiding layer 72 is completely embedded in the cladding layers 70A, 70B.

[0071] As a next step, as shown in Figure 6B , a tensile-strained Si x N y layer 73 is deposited only at those locations where the upwardly curved end of the waveguide needs to be defined. Furthermore, the optical waveguide is defined by etching through the cladding layers 70A, 70B until the silicon substrate 71.

[0072] As a next step, as shown in Figure 6C , an AR layer 74 and metals 75, 76 are applied. As shown in Figure 6D , openings 77A are made in the AR layer 74 for the subsequent anisotropic etching of the substrate 71. After etching away the substrate 71, a cavity 77 is formed and the end of the waveguide will bend upwards.

[0073] Figure 7A A disassembly is shown of the flexible end extending away from the direction of the second optical substrate, and Figure 7BAn example is shown in which the forces applied by the flexible ends in different directions substantially cancel each other out.

[0074] In Figure 7A which a flexible end 80 is shown extending with respect to an x-y plane, which is parallel to the optically passive substrate 21. The direction along which the flexible end 80 extends can be decomposed into a component c2 parallel to the x-y plane and a component c1 perpendicular to the x-y plane. The parallel component c2 can in turn be decomposed into a second component c2_1 along the x-axis and a third component c2_2 along the y-axis.

[0075] Figure 7A The decomposition shown can be performed for each flexible end. For example, Figure 7B An optically active circuit 86 is shown mounted on an optically passive circuit 85. The optically passive circuit 85 comprises four flexible ends 80 corresponding to four waveguides 81. In addition, the optically passive circuit 86 comprises four tapered grooves 84 through which the flexible ends 80 are optically coupled to waveguides 82 on the optically active circuit 86. The optically active circuit 86 further comprises active components 83 which generate or act on optical signals in the waveguides 82.

[0076] Each of the flexible ends 80 extends away from the optically passive substrate along a different direction. By applying Figure 7A the decomposition scheme described above, it can be verified that the parallel components corresponding to these different directions will substantially cancel each other out when added together. In this way, it can be ensured that the optically active circuit 86 is clamped by the optically passive circuit 85 in a substantially symmetrical manner.

[0077] In the foregoing, the application has been explained using a detailed implementation thereof. It will be obvious that the application is not limited to these implementations, but that various modifications are possible without departing from the scope of the application as defined by the appended claims.

Claims

1. An optical assembly (100), comprising: a first optical circuit (10), the first optical circuit comprising: a first optical substrate (11); at least one first optical waveguide (12) formed in the first optical substrate (11); and a first tapered recess (13A) in the first optical substrate (11), the first tapered recess being arranged proximate to a first end of a first one of the at least one first optical waveguide (12), the first tapered recess having a bottom surface (131), an end surface (133), and a pair of side surfaces (134) extending from the bottom surface (131); and a second optical circuit (20) configured to be fixedly coupled to the first optical circuit (10) and comprising: a second optical substrate (21); at least one second optical waveguide (22) formed in the second optical substrate (21); a first recess (23A) below a first end of a first one of the at least one second optical waveguide (22), wherein the first end of the second optical waveguide (22) forms a first flexible end (22A); wherein the first optical circuit (10) further comprises a second tapered recess (13B) arranged proximate to a second end of a first one of the at least one first optical waveguide (12), the second tapered recess (13B) having a bottom surface, an end surface, and a pair of side surfaces extending from the bottom surface, and the second optical circuit (20) further comprises a second recess (23B) below a second end of a first one of the at least one second optical waveguide (22); wherein the second end of the second optical waveguide (22) forms a first flexible end (22A) and a second flexible end (22B), respectively, wherein the first flexible end (22A) and the second flexible end (22B) extend away from the second optical substrate (21) in at least partially opposite directions in the absence of an external force acting thereon; wherein the first flexible end (22A) and the second flexible end (22B) are configured to be received in the first tapered recess (13A) and the second tapered recess (13B), respectively, for the purpose of allowing the first flexible end (22A) and the second flexible end (22B) to exert a spring force onto the end surface of the first tapered recess (13A) and the second tapered recess (13B), respectively, when the first optical circuit (10) and the second optical circuit (20) are fixedly coupled; wherein, when the first optical circuit (10) and the second optical circuit (20) are fixedly connected, the first optical waveguide (12) having the first tapered groove (13A) arranged near its first end is optically coupled to the second optical waveguide (22) having the first groove (23A) arranged near its first end via the end surface (133) of the first tapered groove (13A), and the first optical waveguide (12) having the second tapered groove (13B) arranged near its second end is optically coupled to the second optical waveguide (22) having the second groove (23B) arranged near its second end via the end surface of the second tapered groove (13B); and wherein the first optical circuit (10) is one of an optically active circuit and an optically passive circuit, and wherein the second optical circuit (20) is the other of the optically active circuit and the optically passive circuit.

2. The optical assembly of claim 1, wherein the first optical circuit is an optically active circuit, and wherein the second optical circuit is an optically passive circuit.

3. The optical assembly of claim 1, wherein the first end of a first optical waveguide and the second end of a first optical waveguide are ends at opposite sides of the same first optical waveguide; or wherein the first end of a first optical waveguide and the second end of a first optical waveguide are ends of different first optical waveguides.

4. The optical assembly of claim 1, wherein the first optical circuit further comprises one or more first contact pads (14) and first alignment marks (15), and wherein the second optical circuit further comprises one or more second contact pads (24) and second alignment marks (25); wherein the one or more first contact pads are configured to be fixedly connected to the one or more second contact pads; wherein the one or more first contact pads and the one or more second contact pads each comprise one or more metal layers to allow them to be connected by soldering or bonding using simultaneous application of heat and pressure; wherein the first optical circuit and the second optical circuit are configured such that, when the first optical circuit and the second optical circuit have been moved relative to each other to bring the first alignment marks and the second alignment marks into a predefined lateral registration and the first optical circuit and the second optical circuit are subsequently brought closer together without substantially changing the lateral registration, the first flexible end and the second flexible end are guided by at least one of the bottom surface and the side surfaces of the first tapered groove and the second tapered groove, respectively, towards the first end of the first optical waveguide and the second end of the first optical waveguide, thereby enabling optical coupling between the first flexible end and the first end of the first optical waveguide and between the second flexible end and the second end of the first optical waveguide.

5. The optical assembly of claim 1, wherein a distance between the first flexible end and the second flexible end is greater than a distance between the end surface of the first tapered groove and the end surface of the second tapered groove before the first optical circuit and the second optical circuit are joined to each other, and wherein the distance between the first flexible end and the second flexible end is less than the distance between the end surface of the first tapered groove and the end surface of the second tapered groove if the first flexible end and the second flexible end are bent to be in line with a corresponding second optical waveguide.

6. The optical assembly of claim 1, wherein the first flexible end and the second flexible end extend away from the second optical substrate along a first direction and a second direction, respectively, each of the first direction and the second direction having a component parallel to the second optical substrate and a component perpendicular to the second optical substrate, wherein parallel components corresponding to the first flexible end and the second flexible end point in at least partially opposite directions; wherein each parallel component can be split into a second component along a third direction and a third component along a fourth direction perpendicular to the third direction, wherein the second component corresponding to the first direction is opposite to the second component corresponding to the second direction, and / or wherein the third component corresponding to the first direction is opposite to the third component corresponding to the second direction.

7. The optical assembly of claim 1, comprising a plurality of the first tapered grooves of a corresponding plurality of first optical waveguides, a plurality of the second tapered grooves of a corresponding plurality of first optical waveguides, a plurality of the first flexible ends of a corresponding plurality of second optical waveguides, and a plurality of the second flexible ends of a corresponding plurality of second optical waveguides, each first flexible end and second flexible end configured to be received in a respective first tapered groove and second tapered groove, respectively, the first flexible ends and the second flexible ends each extending away from the second optical substrate along a respective first direction and a respective second direction, respectively, each of the respective first direction and the respective second direction having a component parallel to the second optical substrate and a component perpendicular to the second optical substrate; wherein each respective parallel component can be split into a second component along a third direction and a third component along a fourth direction perpendicular to the third direction; wherein a sum of the second components and / or a sum of the third components is substantially zero.

8. The optical assembly of claim 1, wherein the at least one second optical waveguide comprises a first layer disposed on a second layer; wherein, the first layer is disposed between the second layer and the first optical substrate when the first optical circuit and the second optical circuit are fixedly connected; wherein the first layer is under tensile strain relative to the second layer, or wherein the second layer is under compressive strain relative to the first layer; wherein the first layer is disposed on the second layer only in regions corresponding to the first flexible end and the second flexible end.

9. The optical assembly according to any of the preceding claims, wherein a maximum angle of inclination of the first and second flexible ends relative to a plane parallel to the second optical substrate is in a range between 1 degree and 45 degrees; wherein a length of the first and second flexible ends can be in a range between 50 micrometers and 1000 micrometers; wherein a width of the first and second flexible ends is in a range between 0.5 micrometers and 20 micrometers, and wherein a thickness of the first and second flexible ends is in a range between 50% and 150% of the width of the first and second flexible ends, respectively; wherein a width of the at least one first optical waveguide is in a range between 0.1 micrometers and 10 micrometers, and wherein a thickness of the at least one first optical waveguide is in a range between 0.1 micrometers and 5 micrometers.

10. The optical assembly according to claim 1, wherein at least one passive element of a group of passive elements consisting of waveguides, filters, combiners, polarization converters, and splitters is implemented in the optically passive circuit.

11. The optical assembly according to claim 1, wherein at least one active element of a group of active elements consisting of lasers, semiconductor optical amplifiers, phase modulators, amplitude modulators, and photodetectors is implemented in the optically active circuit.

12. The optical assembly of claim 4, wherein at least one active element of the group of active elements consisting of a laser, a semiconductor optical amplifier, a phase modulator, an amplitude modulator, and a photodetector is implemented in the optically active circuit, wherein, The one or more first contact pads are connected to the optically active circuit and are configured for receiving or outputting electrical signals.

13. The optical assembly according to claim 1, wherein, the first optical waveguide has a first tapered recess arranged near a first end of the first optical waveguide, the first optical waveguide terminating in the first tapered recess, wherein an end facet of the first optical waveguide arranged near the first tapered recess forms the end surface of the first tapered recess; or wherein the first tapered recess comprises: a first section in which the first optical waveguide terminates near the first tapered recess, the first section being wider than a last portion of the first optical waveguide, but narrower than the first flexible end of the second optical waveguide to be received in the corresponding first tapered recess; a second section connected to the first section, the second section being wider than the first section and the first flexible end of the second optical waveguide to be received in the first tapered recess, wherein a boundary between the first section and the second section is configured to form the end surface of the first tapered recess against which the first flexible end of the second optical waveguide to be received in the first tapered recess abuts after the one or more first and second contact pads have been fixedly connected; and / or the first optical waveguide has a second tapered recess arranged near a second end of the first optical waveguide, the first optical waveguide terminating in the second tapered recess, wherein an end facet of the first optical waveguide arranged near the second tapered recess forms an end surface of the second tapered recess; or wherein the second tapered recess comprises: a first section in which the first optical waveguide arranged near the second tapered recess terminates, the first section being wider than a last portion of the first optical waveguide but narrower than a second flexible end of the second optical waveguide to be received in the corresponding second tapered recess; a second section connected to the first section, the second section being wider than both the first section and the second flexible end of the second optical waveguide to be received in the second tapered recess, wherein a boundary between the first section and the second section is configured to form an end surface of the second tapered recess, the second flexible end of the second optical waveguide to be received in the second tapered recess resting against the end surface after having fixedly connected one or more first and second contact pads.

14. The optical assembly of claim 1, wherein the optically active circuit further comprises a first contact ring arranged around the at least one first optical waveguide, and wherein the optically passive circuit further comprises a second contact ring arranged around the at least one second optical waveguide, wherein the first and second contact rings are configured to be fixedly connected to each other so as to provide a seal for the at least one first and second optical waveguide; wherein the first and second contact rings each comprise one or more metal layers to allow them to be connected using soldering or bonding with simultaneous application of heat and pressure.

15. The optical assembly of claim 1, wherein the first or second optical substrate corresponding to the optically active circuit comprises a first cladding layer having a first doping type, a second cladding layer having a second doping type different from the first doping type, and an active layer arranged between the first and second cladding layers; wherein the at least one first or second optical waveguide as part of the optically active circuit is formed by etching through at least a portion of the first cladding layer; and / or wherein the optically active circuit comprises a first metal contact to the first cladding layer and a second metal contact to the second cladding layer, the first cladding layer being connected to a respective one of the one or more first or second contact pads, the second cladding layer being connected to a respective one of the one or more first or second contact pads.

16. The optical assembly of claim 15, wherein, the first cladding layer comprises a first sub-layer, a second sub-layer, and a first etch stop layer arranged between the first and second sub-layers, wherein the at least one first or second optical waveguide as part of the optically active circuit is formed by etching up to the first etch stop layer; wherein the second cladding layer comprises a first sub-layer, a second sub-layer and a second etch stop layer arranged between the first sub-layer and the two sub-layers, wherein the second metal contact has been formed by etching up to the second etch stop layer and a metal layer arranged in a cavity formed by the etching.

17. The optical assembly according to claim 15, wherein the first cladding layer and the second cladding layer are made of indium phosphide, gallium arsenide or gallium antimonide, and / or wherein the second cladding layer is arranged on an indium phosphide, gallium arsenide or gallium antimonide substrate having the same doping type as the second cladding layer.

18. The optical assembly according to claim 1, wherein the optically passive substrate comprises a guiding layer, a carrier substrate and a cladding layer arranged between the guiding layer and the carrier substrate; wherein the at least one first or second optical waveguide as part of the optically passive circuit has been formed by etching through the cladding layer and the guiding layer up to the carrier substrate.

19. The optical assembly according to claim 18, wherein the first optical circuit is an optically active circuit, and wherein the second optical circuit is an optically passive circuit; wherein the first and second flexible ends of the second optical waveguide of the at least one second optical waveguide have been formed by partially etching the carrier substrate below the cladding layer.

20. The optical assembly according to claim 19, further comprising a stress layer arranged on the guiding layer, the stress layer being under tensile strain and the stress layer being configured to at least partially cause an upward bending of the first and second flexible waveguides.

21. The optical assembly of claim 20, wherein the stress layer is a silicon nitride layer Si x N y ; wherein the guiding layer is a silicon layer, wherein the cladding layer is a silicon oxide layer Si x O y and wherein the carrier substrate comprises a silicon substrate; or wherein the guiding layer is embedded in the cladding layer, wherein the guiding layer is a silicon nitride layer SiNx and wherein the cladding layer is a silicon oxide layer SiOx; wherein the at least one first or second optical waveguide as part of the optically passive circuit has been formed by etching through the cladding layer up to the carrier substrate.

22. The optical assembly according to claim 1, further comprising a further optically active circuit configured as the optically active circuit, wherein the optically passive circuit is configured to be coupled to the further optically active circuit in the same way as the optically active circuit.

23. The optical assembly according to claim 4 or 14, wherein the soldering comprises eutectic soldering.

24. The optical assembly according to claim 4 or 14, wherein the bonding comprises eutectic bonding.

25. An optical device comprising the optical assembly according to claim 1, wherein the optically active circuit is fixedly connected to the optically passive circuit.

26. The optical device according to claim 25, wherein the first optical circuit further comprises one or more first contact pads and first alignment marks, and wherein the second optical circuit further comprises one or more second contact pads and second alignment marks; wherein the one or more first contact pads are configured to be fixedly connected to the one or more second contact pads; wherein the one or more first contact pads are configured to be fixedly connected to the one or more second contact pads; wherein the one or more first contact pads and the one or more second contact pads each comprise one or more metal layers to allow them to be connected by soldering or bonding using simultaneous application of heat and pressure; wherein the first optical circuit and the second optical circuit are configured such that when the first optical circuit and the second optical circuit have been moved relative to each other to bring the first alignment mark and the second alignment mark into a predefined lateral registration and the first optical circuit and the second optical circuit are subsequently brought closer together without substantially changing the lateral registration, the first flexible end and the second flexible end are guided by at least one of the bottom surface and the side surfaces of the first tapered groove and the second tapered groove, respectively, towards the first end of the first optical waveguide and the second end of the first optical waveguide, thereby enabling optical coupling between the first flexible end and the first end of the first optical waveguide and between the second flexible end and the second end of the first optical waveguide.

27. The optical device of claim 26, wherein the soldering comprises eutectic soldering.

28. The optical device of claim 26, wherein the bonding comprises eutectic bonding.

29. A method for manufacturing the optical device of claim 26, comprising: moving the first optical circuit and the second optical circuit relative to each other to bring the first alignment mark and the second alignment mark into a predefined lateral registration; and after the predefined lateral registration has been obtained, bringing the first optical circuit and the second optical circuit closer together without substantially changing the lateral registration, during the moving, the first flexible end and the second flexible end are guided by at least one of the bottom surface and the side surfaces of the first tapered groove and the second tapered groove, respectively, towards the first end of the first optical waveguide and the second end of the first optical waveguide, thereby enabling optical coupling between the first flexible end and the first end of the first optical waveguide and between the second flexible end and the second end of the first optical waveguide; and fixedly connecting the first optical circuit and the second optical circuit by fixedly connecting the one or more first contact pads to the one or more second contact pads.

30. The method of claim 29, wherein the soldering comprises eutectic soldering.

31. The method of claim 29, wherein the bonding comprises eutectic bonding.

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