Patent Description:
Optical isolation is a typical optical function used in polarized and unpolarized light circuits to avoid reflected light disturbing the performance of the circuits. In essence an optical isolator ensures a unidirectional transmission of light. Isolators of the prior art have been developed comprising typically two polarizers and in between a Faraday rotator (a crystal) and are typically assembled together as a bulk component. To feed light to such a bulk optical isolator, two GRIN lenses (gradient-index lenses) have been used to collimate light in and out of the isolator, their flat surfaces being arranged in close proximity with the interface surfaces of the isolator. Such lenses have to be polished by hand, assembled individually with the isolator on a board as well as aligned with said isolator. The fabrication of such lenses as well as the assembling of such lenses on a board with the isolator, due to their dimensions, are thus both cumbersome processes, suffering drawbacks in terms of scalability, accuracy and costs.

<CIT> discloses a semiconductor laser module with an optical isolator. A semiconductor laser module includes a substrate provided with a first optical waveguide and a second optical waveguide, a semiconductor laser diode mounted in front of the first optical waveguide on the substrate, and an optical isolator between the first optical waveguide and the second optical waveguide.

The publication "<NPL>, discloses Hybrid photonic integration that combines complementary advantages of different material platforms, offering superior performance and flexibility compared with monolithic approaches. This applies in particular to multi-chip concepts, where components can be individually optimized and tested. The assembly of such systems, however, requires expensive high-precision alignment and adaptation of optical mode profiles. This publication shows that these challenges can be overcome by in situ printing of facet-attached beam-shaping elements. The approach in this publication allows precise adaptation of vastly dissimilar mode profiles and permits alignment tolerances compatible with cost-efficient passive assembly techniques. This publication demonstrates a selection of beam-shaping elements at chip and fibre facets, achieving coupling efficiencies of up to <NUM>% between edge-emitting lasers and single-mode fibres. This publication also realizes printed free-form mirrors that simultaneously adapt beam shape and propagation direction, and explores multi-lens systems for beam expansion. The concept paves the way to automated assembly of photonic multi-chip systems with unprecedented performance and versatility.

The object of the invention is to provide an optical integrated circuit for integrating an optical component capable of overcoming the above-mentioned drawbacks of the prior art.

According to a first aspect of the invention, an optical integrated circuit comprises a substrate, at least one open cavity provided in said substrate, at least one set of optical waveguides for each open cavity. Each set comprises a first optical waveguide and a second optical waveguide, wherein the first optical waveguide and the second optical waveguide of a set are each arranged in the substrate and each comprise a first end facet ending in the open cavity of that set. The optical integrated circuit further comprises a first collimating element for each set, the first collimating element of a set being arranged in the open cavity of that set at or near the first end facet of the first optical waveguide of that set to collimate light from that first optical waveguide, and a second collimating element for each set, the second collimating element of a set being arranged in the open cavity of that set at or near the first end facet of the second optical waveguide of that set to collimate light into that second optical waveguide.

In this way, the collimating elements are incorporated at substrate level in an optical integrated circuit, while a cavity is provided for an optical component, leading to a more scalable design and reduced costs. The alignment between the optical waveguides and the collimating elements is in particular dealt with at substrate level.

In the art, an optical integrated circuit may also be referred to as a photonic integrated circuit (PIC). By optical integrated circuit or photonic integrated circuit is meant a device that integrates multiple (at least two) photonic functions on a substrate (or chip). By substrate (or wafer) is typically meant the base layer of a structure such as a chip or a printed circuit board. Semiconductor technologies are typically used to integrate on one substrate (or chip) the multiple functions of the circuit. In that sense, a photonic integrated circuit (PIC) or integrated optical circuit is similar to an electronic integrated circuit. The fabrication techniques are similar to those used in electronic integrated circuits in which photolithography is used to pattern wafers for etching and material deposition. The major difference between photonic integrated circuits and electronic integrated circuits is that a photonic integrated circuit provides functions for information signals imposed on optical wavelengths typically in the visible spectrum or near infrared.

According to a preferred embodiment, arranged in the cavity cured but not (fully) processed material is provided acting as a mechanical support for supporting the first and second collimating elements. In this way, the first and second collimating elements may be protected for further processing, increasing thus the yield of production.

According to a preferred embodiment, the first optical waveguide and the second optical waveguide of a set are different segments of an optical waveguide that is interrupted by the open cavity of that set. In this way, the accuracy of the alignment of the first and the second optical waveguides can be improved. Alternatively, the optical paths of the first optical waveguide and of the second optical waveguide may be intersecting. In particular the optical path of an optical waveguide may substantially have an angle with an axis perpendicular to the end facet of that optical waveguide. The angle between the optical line and an axis perpendicular to the end facet of an optical waveguide may reduce reflections between that optical waveguide and the collimating element at or near the end facet of that optical waveguide. The angle may be dependent on the mode field diameter and the numerical aperture of an optical waveguide.

According to a preferred embodiment, each of the first and second collimating elements comprises one or more micro lenses. In this way, an optical waveguide can be efficiently connected to the optical component without losing light in the cavity. The one or more micro lenses on each side collimate the light coming out of the end facet of the first optical waveguide to a bundle reaching the optical component, respectively coming out of the isolator to a bundle reaching the end facet of the second optical waveguide. In particular, the one or more micro lenses are adapted to the wavelength of the light to be received.

According to a preferred embodiment, the first and second collimating elements are made of processed and cured photo-sensitive material, preferably the first and second collimating elements are made of polymerized and cured photo-sensitive material, or irradiated and cured photosensitive material, more preferably the first and second collimating elements are made of polymerized and cured epoxy resin, or irradiated and cured glass. In this way, accuracy of the positioning may be ensured while the ease of fabrication may be improved. By photo-sensitive material is understood a material that is sensitive to the action of light, and in particular a material having optical properties, in particular a refractive index, which can be modified by the action of light.

According to a preferred embodiment, the optical integrated circuit further comprises at least one optical component arranged in each open cavity between the first and the second collimating elements of the at least one set of that open cavity, said optical component being configured to perform an optical function. In this way, the collimating elements and the optical component are incorporated at substrate level in an optical integrated circuit, leading to a more scalable design and reduced costs. The alignment between the optical waveguides, the collimating elements and the optical component is in particular dealt with at substrate level.

According to a preferred embodiment, the optical integrated circuit comprises at least two sets of optical waveguides for each open cavity, wherein the optical component of an open cavity is shared by the at least two sets of optical waveguides for that open cavity. In this way a multi-channels integrated circuit may be provided on a single substrate increasing the scalability of the design.

According to a preferred embodiment, the optical component is for performing one of the following functions: optical isolation, sensing or beam splitting, beam deflection or plasmonics. In particular the optical component is one of an optical isolator, a drop of fluid or a beam manipulator. The list mentioned is not exhaustive and a skilled person would, depending on circumstances, understand that the principle of the invention may be applied to other types of optical functions and for other types of optical components which would benefit from being integrated in a PIC environment.

According to a preferred embodiment, the optical component is a bulk optical component. By bulk element is understood a separate element which can be provided as a sub assembly typically having micro or mm dimensions. For example the bulk component may be an off the shelf component. In this way a hybrid level of integration is achieved with on the one hand the wave guiding and collimation being totally integrated at substrate level, while the optical function may remain at macro level.

According to a preferred embodiment, the optical integrated circuit comprises at least two open cavities provided in said substrate. In this way a multi-channels and multi-functions integrated circuit may be provided on a single substrate increasing the scalability of the design. It is noted that the optical components in the cavities may be the same or different. Further multi/channels for each cavity may as well be used for this embodiment.

According to a preferred embodiment, the at least one optical component is arranged in a recess created when removing non-cured material from the cavity after forming the first and second collimating element and the mechanical support thereof. In this way, the optical element may be easily inserted inside the remaining cavity and placed in close proximity to the collimating elements. The proximity between the collimating elements and the optical element may insure an efficient optical coupling.

According to a preferred embodiment, a binding material is provided in the cavity for the optical component to the cavity. In this way a mechanical binding is realized as well as an optical binding to the collimating elements. In particular, the binding material has a refractive index close to the refractive index of the material of the collimating elements. In this way, reflections are mitigated and an efficient optical integrated circuit is obtained. More in particular, the binding material may be a cured photo-sensitive material, for example a cured resin or cured glass. In particular the binding material may be the photo-sensitive material used for the collimating elements. When using the photo-sensitive material used for the collimating elements, a simple and cheaper optical integrated circuit is obtained, while when using another photo-sensitive material for binding only, the binding characteristics may be adapted to the environmental constraints of a specific application (vibrations, moisture, etc.).

According to a preferred embodiment, the substrate is made of one of the following: a photo-sensitive substrate or a semiconductor wafer, preferably the substrate is made of one of the following: a polymer substrate, a glass substrate, a silicon wafer, silicon dioxide wafer, a lithium niobate wafer, a Gallium Arsenide wafer or an indium phosphide wafer. In this way, the substrate and the optical integrated circuit based on such a substrate is directly compatible with other substrate technology circuits.

According to a further embodiment, an optical integrated module is provided comprising a packaging with input and output interfaces, and an optical integrated circuit according to any of the above claims, arranged inside said packaging, wherein at least a first optical waveguide is connected directly or indirectly to the input interface and/or at least a second optical waveguide is connected directly or indirectly to the output interface. In this way, a modular design and product may be realized to meet the needs of the intended use.

According to a further embodiment, the module further comprises a pre-processing stage for processing one or more light signals to be guided in the one or more first optical waveguides and/or a post-processing stage for processing one or more light signals from the one or more second optical waveguides, in particular wherein the pre-processing stage comprises a laser integrated circuit and/or a spotsize converter, and/or the post-processing stage comprises a spot-size converter for adapting the mode size. In this way a packaged element that can be used as a building block for a process design kit in photonics can be obtained.

According to another embodiment of the invention, a method for fabricating an optical integrated circuit according to the previous embodiments is provided. The method comprises the steps of providing a substrate with one or more optical waveguides arranged in the substrate; forming, for example by means of etching, at least one open cavity in the substrate thereby forming at least one set of optical waveguides for each open cavity, each set comprising a first optical waveguide and a second optical waveguide. The first optical waveguide and the second optical waveguide of a set are each arranged in the substrate and each comprise a first end facet ending in the open cavity of that set. The method further comprises forming in the open cavity of a set and for each set, a first collimating element at or near the first end facet of the first optical waveguide of that set and a second collimating element at the first end facet of the second optical waveguide of that set.

In this way, the precision of the alignment of all optical components is improved to the level of precision of the etching and the forming step of the collimating elements. The accuracy of the fabrication processed is thus increased.

According to a preferred embodiment, forming, for example by means of etching, at least one open cavity comprises etching the open cavity through at least one optical waveguide interrupting at least that optical waveguide in different segments thereby forming at least one set of optical waveguides for each open cavity. In this way, the accuracy of the alignment of the first and the second optical waveguides can be improved. Alternatively, etching at least one open cavity and forming at least one set of optical waveguides for each open cavity are two separate steps, and comprises the step of forming a first and second optical waveguides having optical paths intersecting each other. In particular, forming at least one set of optical waveguides may comprise forming an optical waveguide having an optical path substantially at an angle with an axis perpendicular to the end facet of that optical waveguide. The angle between the optical path and an axis perpendicular to the end facet of an optical waveguide may reduce reflections between that optical waveguide and the collimating element at or near the end facet of that optical waveguide. The angle may be dependent on the mode field diameter and the numerical aperture of an optical waveguide.

According to a preferred embodiment, forming the collimating elements comprises 3D printing the collimating elements. Alternatively, forming the collimating elements comprises 3D etching the collimating elements. In this way, manual manipulation is no longer required. Automatization of the fabrication method is thus rendered possible increasing efficiency, scalability while reducing costs and time of fabrication.

According to a preferred embodiment, forming the collimating elements comprises filling the cavity of a set with a first photo-sensitive material, locally processing said first photo- sensitive material and subsequently curing the processed photo-sensitive material to obtain the first and second collimating elements, preferably processing comprises polymerizing or irradiating to respectively obtain polymerized or irradiated first photo-sensitive material. In this way, a precise positioning of the collimating element at the end-face of the optical waveguide is rendered possible, avoiding diffraction into the cavity, and enabling thus the incorporation of the optical component in the cavity.

According to a preferred embodiment, the method further comprises the subsequent steps of removing at least part of the non-processed first photo-sensitive material from the open cavity of that set, refilling the open cavity of that set with a second photo-sensitive material and after the inserting step, curing said second photo-sensitive material. In this way, room can be made to insert the optical component and the mechanical characteristics of the second photo-sensitive material, acting as binding material, may be adapted to the environmental constraints of a specific application (vibrations, moisture, etc.).

According to a preferred embodiment, forming the collimating elements is performed by two photon absorption laser lithography. In this way, a fine structure may be created in the available cavity space.

According to a preferred embodiment, the method further comprises inserting at least one optical component in each open cavity between the first and the second collimating elements of the at least one set of that open cavity, said optical component being configured to perform an optical function. In this way, the collimating elements and the optical component are incorporated at substrate level in an optical integrated circuit, leading to a more scalable design and reduced costs. The alignment between the optical waveguides, the collimating elements and the optical component is in particular dealt with at substrate level.

According to a preferred embodiment, providing a substrate with one or more optical waveguides arranged in the substrate comprises providing a first substrate portion with one or more optical waveguides arranged in the substrate, and at least one second substrate portion incorporating at least one of a pre-processing stage for processing one or more light signals to be guided in the one or more first optical waveguides or a post-processing stage for processing one or more light signals from the one or more second optical waveguides. In this way the fabrication process of multiple functions may be simplified, leading to high scalability, better accuracy and reduced costs.

According to a preferred embodiment, the method further comprises packaging the optical integrated circuit as a module in a packaging comprising input and output interfaces, wherein at least a first optical waveguide is connected directly or indirectly to the input interface and/or at least a second optical waveguide is connected directly or indirectly to the output interface. In this way a practical product may be delivered, with multiple types of interfaces depending on the use intended. In particular packaging further comprises packaging the optical integrated circuit with additional separate elements, in particular with a separate spot size converter for adapting the optical mode size. In this way a packaged element that can be used as a building block for a process design kit.

It is further noted that although some steps may have been described in a certain order, the invention should be understood in a broader sense, and the method of the invention should be understood as covering other orders of steps as long as logically possible.

This and other aspects of the present invention will now be described in more detail, with reference to the appended drawings showing currently preferred embodiments of the invention. Like numbers refer to like features throughout the drawings.

<FIG> illustrates a schematic cross section view of an optical integrated circuit according to an embodiment. <FIG> shows an optical integrated circuit <NUM> for performing an optical function realized by an optical component <NUM> to incoming light represented by an arrow coming into the optical integrated circuit <NUM> and delivering out-going light represented by an arrow leaving the optical integrated circuit <NUM>. In particular the optical function may be optical isolation, such that the optical component <NUM> may be an optical isolator comprising typically a crystal and two polarizers, the crystal being arranged in between the two polarizers.

The optical integrated circuit <NUM> comprises a substrate <NUM>, an open cavity <NUM> provided in said substrate <NUM>, a set of a first optical waveguide <NUM> and a second optical waveguide <NUM>, each arranged in the substrate <NUM> to guide light. It is noted that the open cavity <NUM> may also be referred to in the rest of the text simply as the cavity. The first and second optical waveguides <NUM> and <NUM> may for example be buried in the substrate <NUM>. Alternatively the first and second optical waveguides <NUM> and <NUM> may be embedded in the substrate <NUM>. The skilled person would understand that other optical waveguide solutions may further be envisaged for as long as compatible with the concept behind the present invention.

The substrate <NUM> may be a semi-conductor wafer. It is noted that the term substrate may here refer to a multi-layer multi-material structure. Alternatively the substrate may refer to a single layer, single material structure, like for instance a glass substrate or a polymer substrate. The semi-conductor wafer <NUM> may for example be based on silicon or indium phosphide technology. The optical waveguides <NUM> and <NUM> may for example be made of an insulating material, such as silico nitride, at least partially surrounded by one or more cladding layers, such as silicon dioxide. The optical waveguides <NUM> and <NUM> may have a dimension c, said dimension being adapted to the wavelength of the light to be received by the optical integrated circuit <NUM>. The optical waveguides <NUM> and <NUM> may have a substantially circular section or may have the shape of a substantially flat stripe. Other embodiments for the optical waveguides may yet be envisaged. The first optical waveguide <NUM> and the second optical waveguide <NUM> may be different segments of an optical waveguide that is interrupted by the open cavity <NUM>. In other words, the first and second optical waveguides <NUM> and <NUM> may be formed by the etching of the cavity <NUM> through an optical waveguide initially provided in the substrate <NUM>.

The first optical waveguide <NUM> and the second optical waveguide <NUM> each comprise a respective first end facet <NUM> and <NUM> ending in said cavity <NUM>. The end facets <NUM> and <NUM> may be arranged at opposite side faces of the cavity <NUM>. A first collimating element <NUM> is arranged in the cavity <NUM> at the first end facet <NUM> of the first optical waveguide <NUM> to collimate light from the first optical waveguide <NUM>, while a second collimating element <NUM> is arranged in the cavity <NUM> at the first end facet <NUM> of the second optical waveguide <NUM> to collimate light into the second optical waveguide <NUM>. The first collimating element <NUM> may collect all the light coming out of the end facet <NUM> and shape the beam of light to create a bundle of collimated light rays. Similarly the second collimating element <NUM> may collect all the light coming out of the optical component <NUM> and shape the beam of light to create a bundle of collimated light rays entering the end facet <NUM> of the second optical waveguide <NUM>.

Alternatively in an embodiment not represented, the end facets <NUM> and <NUM> may be arranged on the same side face of the open cavity <NUM>, while the opposite side face of the open cavity may be reflecting surface. Light coming from the first waveguide may travel through the cavity, be reflected back and coupled then in a second waveguide on the same side face of the open cavity as the first optical waveguide. This embodiment may be used with wavelength selective filters for devices like add drop multiplexers.

In a further not represented embodiment, at least one of the first or the second optical waveguide <NUM>,<NUM> may comprise at or near the open cavity <NUM> at least one taper, such that at least one end facet <NUM>, 41may then be arranged at the extremity of the at least one taper.

The optical component <NUM>, for example an optical isolator, may be arranged in the cavity <NUM> between the first and the second collimating elements <NUM> and <NUM>. The optical component <NUM> may be configured to perform a given optical function, for example to perform optical isolation. The first and second collimating elements <NUM> and <NUM> as well as the optical waveguides <NUM> and <NUM> may be aligned such that light entering the integrated circuit may travel along one substantially straight optical path. The side faces of the optical component <NUM> may be substantially perpendicular to the optical path defined by the first and second collimating elements <NUM> and <NUM> as well as the optical waveguides <NUM> and <NUM>. The optical component <NUM> may be a bulk element, with a height f in the millimeter range. The optical component <NUM> may have a depth g in the direction of the optical path, defined by the distance between an input surface <NUM> oriented towards the incoming light, i.e. towards the end facet <NUM>,<NUM> and an output surface <NUM> oriented towards out-going light, i.e. towards the end facet <NUM>.

The open cavity <NUM> may be an opening etched from an upper surface <NUM> of the substrate <NUM>, and may also be qualified as a recess in the substrate. The cavity <NUM> may have a length a, in between opposite side faces housing the end facets <NUM> and <NUM>, and a depth b from the upper surface <NUM> of the substrate <NUM>, each adapted to the dimensions of the optical component <NUM> and the collimating elements <NUM> and <NUM>. In an example, the depth b may be one to several hundreds of micrometers while the length a may be in the order of one to several millimeters. The depth b may for example be smaller than the height f of the optical component <NUM>, which may protrude above the upper surface <NUM> of the substrate <NUM>. The selection of the different proportions for the distance a, b, c and d may come within the scope of the customary practice based on considerations including among other the dimension e of the optical component <NUM>, the materials used for the optical waveguides <NUM>, <NUM> and the substrate <NUM>. As explained above, the first and second optical waveguides <NUM> and <NUM> may be formed by the etching of the cavity <NUM> through an optical waveguide initially provided in the substrate <NUM>. In other words, the open cavity <NUM> may interrupt an optical waveguide, creating two segments labelled as first and second optical waveguides <NUM> and <NUM>.

The first and second collimating elements <NUM> and <NUM> may be made of polymerized and cured photo-sensitive material. The first and second collimating elements <NUM> and <NUM> may be manufactured by additive manufacturing, also referred to as 3D printing, directly in situ, i.e. in the open cavity <NUM>. These elements <NUM> and <NUM> may be manufactured by two-photon laser lithography directly in the cavity <NUM>. Two-photon laser lithography uses laser for creating small features in a photo-sensitive material without the use of complex optical systems or photomasks. This method relies on a multi-photon absorption process in a material that is transparent at the wavelength of the light used for creating the pattern. By scanning and properly modulating the laser, polymerization occurs at the focal spot of the laser and can be controlled to create arbitrary three-dimensional periodic or non-periodic patterns.

Each of the first and second collimating elements <NUM> and <NUM> may have a diameter d at the first end facets <NUM> and <NUM> substantially larger than the dimension c of the optical waveguides <NUM>, <NUM> and a length e in the direction of the optical path to collimate efficiently. Each of the first and second collimating elements <NUM> and <NUM> may comprise one or more micro lenses. By micro-lenses are understood lenses having a diameter smaller than a millimeter, preferably between one micron and <NUM> microns, more preferably between one micron and <NUM> microns, typically in the range of one to several tens of microns. The one or more micro-lenses may have dimensions and/or shapes depending on the wavelength of the light to be received.

The length a of the cavity <NUM> may be larger than the sum of the depth g of the optical component <NUM> plus two times the length e of the collimating elements <NUM> and <NUM>. During the fabrication process a gap larger than the depth g of the optical component <NUM> may be provided between the collimating elements <NUM> and <NUM> to allow the insertion of the optical component <NUM> after the fabrication of the collimating elements <NUM> and <NUM>. A binding material <NUM> may, after insertion of the optical component <NUM>, be provided in the cavity <NUM> in between the elements <NUM>, <NUM>, <NUM> and the edges of the cavity to bind all elements <NUM>-<NUM> in place inside the cavity <NUM>. In particular a gap in between the collimating elements <NUM>, <NUM> and the surfaces <NUM> and <NUM> of the optical component <NUM> may be filled with a material <NUM> having a reflective index close to the reflective index of the material used to form the collimating elements <NUM> and <NUM>. The substantially small difference in reflective indexes between the material <NUM> and the material of the collimating elements <NUM> and <NUM> may be selected in order to avoid reflections at the interface between these materials. Typically the difference in reflective indexes is between <NUM> and <NUM>,<NUM>. In this way, no anti-reflection coating is needed. The forming of the collimating elements <NUM> and <NUM> using such a technique therefore additionally solves the problem of applying antireflective coating precisely on small structures. Alternatively a binding material <NUM> may, before insertion of the optical component <NUM>, be provided in the cavity <NUM> in between the elements <NUM>, <NUM>, <NUM> and the edges of the cavity to bind all elements <NUM>-<NUM> in place inside the cavity <NUM>. A skilled person would know depending on the selected binding material, whether to fill the cavity with the binding material before or after the insertion of the optical component.

<FIG> illustrates an enlarged view of a schematic cross section view of an optical integrated circuit according to another embodiment. It is noted that in this view only the first collimating element <NUM> is detailed as the enlargement focusses on the part of the optical integrated circuit arranged before the optical component <NUM> in the optical path. Yet a similar arrangement is to be understood to be present symmetrically on the other side of the optical component <NUM> for the second collimating element <NUM>. In this embodiment, the first collimating element <NUM> may comprise a plurality of curved and/or straight lens surfaces 50a-50e arranged in a stack for producing a collimated output light beam towards the receiving surface <NUM> of the optical component <NUM>. The collimating element <NUM> may be made of a photo-sensitive material, which has been polymerized locally using a laser and subsequently cured to solidify. A first photo-sensitive material may be filled into the cavity <NUM> prior to fabrication of the collimating elements <NUM> and <NUM>. The first photosensitive material may in particular be an epoxy resin. After polymerization, a selective curing may be performed using a mask and light, such that the initial first photosensitive material may then become a polymerized and cured material 82a forming one or more optical surfaces 50a-<NUM>. The material 82a may have interfaces with a cured but non-polymerized (or at least not fully polymerized) material 82b. The cured but non-polymerized material 82b may be obtained by directly curing the initial first photosensitive material without a polymerization step. Cured but non-polymerized material 82b may be present in closed spaces inside the collimating elements <NUM> and <NUM>, for example in between surfaces 50b and 50c, and/or in part of cavity <NUM> in the immediate proximity of the collimating elements <NUM> and <NUM>. To allow insertion of the optical component <NUM>, a portion of the first material <NUM> may have not been cured in the center of the cavity <NUM> and may thus be removed, using a solvent like for instance acetone. In the provided recess, the optical component <NUM> may then be inserted into place, followed by the addition of a second photo-sensitive material. The second photo-sensitive material may be filled in between the material 82b and the surfaces <NUM> and <NUM> of the optical component <NUM>. The second photosensitive material may in particular be another epoxy resin than the first epoxy resin used as the first photosensitive material. Alternatively the second photo-sensitive material may be any one of the following material: acrylates, polyurethane, silicones (PDMS), organic ceramic materials like ORMOCER. An additional curing of said second photo-sensitive material may be performed to obtain a cured material 83b for binding the optical component <NUM> to the collimating elements <NUM>, and <NUM> via the cured but not polymerized material 82b. Alternatively, the second photo-sensitive material may be same material as the first photo-sensitive material.

It is noted that the collimating element <NUM> of <FIG> is not to be understood as limitative but purely as illustrative of a possible arrangement of a plurality of micro-lenses. The number of surfaces, the curvatures of these surfaces and the dimensions of a collimating element <NUM> or <NUM> may be selected based on design parameters of a specific optical integrated circuit <NUM>, including the wavelength of the light to be received and the diameter of the optical waveguides <NUM> and <NUM> among others. A skilled person would know based on common general knowledge how to design a collimating element <NUM> and <NUM> for each situation based on a set of design parameters (radius of curvature, refractive index, wavelength, size of optical waveguide, birefringence, aberration).

The cured but non-polymerized first and second photo-sensitive materials 82b and 83b may act as binding (adhesive) material for binding the elements <NUM>, <NUM> and <NUM> together inside the cavity <NUM>. The materials 82b and/or 83b may have a reflective index close to the reflective index of the material 82a of the collimating elements <NUM> and <NUM> in order to minimize reflections at the interface surfaces between the two materials 82a and 82b, and/or 83b. The materials 82b and/or 83b may have a reflective index preferably between <NUM> and <NUM>.

In an alternative embodiment (not illustrated), the material 82b remaining accessible in the cavity (i.e. not captive in closed spaces inside the collimating elements) may be entirely removed after curing and replaced by the second binding photo-sensitive material 83b. In still another embodiment, the optical component <NUM> may be inserted while the first photo-sensitive material <NUM> is still liquid, such that after laser printing, i.e. after polymerization of the areas representing the collimating elements, the whole cavity <NUM> including the polymerized elements and the optical component <NUM> is cured, to solidify the elements <NUM> and <NUM> in the same step as the binding of all elements <NUM>-<NUM> in the cavity <NUM>. In particular, when using the photo-sensitive material used for the collimating element as unique binding material, a simple and cheaper optical integrated circuit is obtained, while when using a second photo-sensitive material for binding, the binding characteristics may be adapted to the environmental constraints of a specific application (vibrations, moisture, etc.).

<FIG> illustrates schematic views from above of other embodiments of the invention. In <FIG>, an optical integrated circuit <NUM> is shown where three sets of first and second optical waveguides (<NUM>, <NUM>), (<NUM>, <NUM>), (<NUM>, <NUM>) are arranged to end in the same cavity <NUM> of the substrate <NUM>. Three optical paths, also called optical channels, may in this way be arranged close to each other. A minimum distance between two sets of optical waveguides may for instance be in the range of tens of microns. Each set of optical waveguides (<NUM>, <NUM>), (<NUM>, <NUM>), (<NUM>, <NUM>) is provided with dedicated collimated first and second collimated elements (<NUM>, <NUM>), (<NUM>, <NUM>), (<NUM>, <NUM>) but share the same optical component <NUM>. In this way, a compact multi-channel integrated circuit sharing the same optical function may be achieved.

In <FIG>, an optical integrated circuit <NUM> is shown where three sets of first and second optical waveguides (<NUM>, <NUM>), (<NUM>, <NUM>), (<NUM>, <NUM>), may be arranged in the same substrate <NUM> but end into different respective cavities <NUM>, <NUM>, <NUM> arranged in the substrate <NUM>. Each set of optical waveguides (<NUM>, <NUM>), (<NUM>, <NUM>), (<NUM>, <NUM>) may be provided with dedicated collimated first and second collimated elements (<NUM>, <NUM>), (<NUM>, <NUM>), (<NUM>, <NUM>) and a dedicated optical component <NUM>, <NUM>, <NUM>. The optical integrated circuit <NUM> of <FIG> may accommodate a plurality of identical or different optical functions <NUM>, <NUM>, <NUM> on the same substrate <NUM>. The optical integrated circuit <NUM> may then be cut after fabrication into smaller optical integrated circuits <NUM>, each comprising one function only.

<FIG> illustrate the fabrication method according to an embodiment. The method for fabricating an optical integrated circuit <NUM> comprises the step <NUM> of providing a substrate with one or more optical waveguides arranged in the substrate, followed by step <NUM> of etching an open cavity in the substrate forming thereby a set of a first and a second waveguides each with first end facets ending in the cavity. The following step <NUM> comprises forming a first and second collimating element in the cavity at or near the first end facets of the first and second optical waveguides. Step <NUM> is optional and may comprise inserting an optical component between the first and the second collimating elements. It is noted that some further intermediate steps may be present, and that the last two steps may be inverted.

<FIG> illustrate schematically fabrication steps of an optical integrated circuit <NUM> according to an embodiment of the invention. Although explained for an optical integrated circuit <NUM> as in <FIG>, a skilled person would know how to decline similar fabrications steps for the optical integrated circuits <NUM> and <NUM> of <FIG>.

<FIG> illustrates a step [S501] in which an optical waveguide labelled <NUM>, <NUM> may be provided in the substrate <NUM>.

<FIG> illustrates a subsequent step [S502] in which a cavity <NUM> may be provided, for example etched, in the substrate <NUM> through the optical waveguide <NUM>, <NUM> interrupting said waveguide <NUM>, <NUM> in two segments thereby forming a set of a first waveguide <NUM> and a second waveguide <NUM>. The first and second waveguides <NUM> and <NUM> may then be intrinsically aligned as they would derive from a single optical waveguide.

<FIG> illustrates a subsequent step [S503] in which the cavity <NUM> may be filled with a first photo-sensitive material <NUM>, for example a liquid material may be applied to the cavity <NUM> by spin-coating or by a needle dispenser.

<FIG> illustrates a subsequent step [S504] in which a laser, labelled L, may be used to locally polymerize the first photo-sensitive material <NUM> to form the first and second collimating elements <NUM> and <NUM>. This step [S504] may for example be performed using two photon absorption laser lithography as known in the art.

<FIG> illustrates a subsequent step [S505] in which the first photo-sensitive material <NUM> is selectively cured using a mask to finally obtain the first and second collimating elements <NUM> and <NUM> made of cured and polymerized material 82a, while the cured but not (fully) polymerized material 82b may act as a mechanical support for supporting the first and second collimating elements inside the cavity <NUM>. This step [S505] may be performed using curing methods known in the art.

<FIG> illustrates a subsequent step [S506]in which the non cured material <NUM> is removed from the cavity, using a solvent (for example acetone), creating a recess <NUM>. The walls of the recess <NUM> may be formed by cured and polymerized material 82a and cured but not (fully) polymerized material 82b. The recess <NUM> may be dimensioned to receive an optical element <NUM>. The walls of the recess <NUM> may be arranged such that the optical element <NUM> may be arranged in close proximity to the first and second collimating elements <NUM> and <NUM>.

<FIG> illustrates a subsequent step [S507] in which an optical component <NUM> may be inserted in the recess <NUM>. This step may be performed by a machine suitable for handling a bulk optical component <NUM>.

<FIG> illustrates a subsequent step [S508] in which the remaining space in the recess <NUM>, once the optical component <NUM> has been inserted, may be filled with a second photo-sensitive material <NUM>, for example a liquid material may be applied by a needle dispenser.

<FIG> illustrates a step [S509] in which the second photo-sensitive material <NUM> is cured to create a binding material 83b, binding, via the material 82b, together the elements <NUM>-<NUM> inside the cavity <NUM>.

<FIG> illustrate schematic views from above of different packaging embodiments for an optical integrated circuit of the invention.

<FIG> illustrates a schematic perspective view from above of an optical integrated module A. The module A may comprise an optical integrated circuit <NUM> according to the embodiment of <FIG> as illustrated, or alternatively (not illustrated) an optical integrated circuit <NUM> or <NUM> according to embodiments of <FIG>. The first and second optical waveguides <NUM> and <NUM> may comprise second end facets <NUM> and <NUM> at the opposite end of the first and second optical waveguides <NUM>, <NUM> with respect to the first end facets <NUM> and <NUM>. The module A may be packaged as a chip having an upper surface <NUM>, a lower surface (not labelled), a first set of two opposite side surfaces including a surface <NUM> illustrated in <FIG>, at or near which the second end facets are provided, for instance end facet <NUM> may be provide at the surface <NUM>, and a second set of opposite side surfaces including a surface <NUM> illustrated in <FIG>. Module A may be arranged in between two chips, for instance in between a laser chip and an SiN chip. Connection to other chips may be performed by putting the side surfaces of the module A in contact with side surfaces of the other chips(s).

<FIG> illustrates a schematic view from above of an optical integrated module B. The module B may comprise a packaging <NUM> with an input interface <NUM> and an output interface <NUM>. Arranged inside said packaging <NUM>, a module A according to <FIG> may be provided, as well as at least one spot-size converter (SSC) <NUM>, and possibly one SSC on each side of the module A for adapting the mode field from optical waveguide to fiber and/or from fiber to optical waveguide. The input interface <NUM> may be, if an input SSC <NUM> is present, a fiber interface for one or more channels (fiber array interface), otherwise an optical waveguide interface for one or more channels suitable for interfacing with another input optical integrated circuit, and the output interface <NUM> may be, if an output SSC is present, a fiber interface for one or more channels (fiber array interface) otherwise an optical waveguide interface for one or more channels suitable for interfacing with another input optical integrated circuit.

<FIG> illustrates a schematic view from above of an optical integrated module C. The module C may comprise a packaging <NUM> with an input interface <NUM> and an output interface <NUM>. Arranged inside said packaging <NUM>, an optical integrated circuit <NUM>, <NUM> or <NUM> etched on the same substrate <NUM> as an additional optical integrated circuit <NUM> may be provided together with a spot-size converter <NUM> for adapting the mode field from optical waveguide to fiber. The optical integrated circuit may for instance be a laser circuit for generating the light signal to be transmitted to the optical integrated circuit <NUM>, <NUM> or <NUM>. The input interface <NUM> may be an interface for interfacing with the circuit <NUM>, and the output interface <NUM> may be a fiber interface for one or more channels (fiber array interface).

<FIG> illustrates a schematic view from above of an optical integrated module D. The module D may comprise a packaging <NUM> with an input interface <NUM> and an output interface <NUM>. Arranged inside said packaging <NUM>, an optical integrated circuit <NUM>, <NUM> or <NUM> etched on the same substrate <NUM> as an additional optical integrated circuit <NUM> and etched on the same substrate as a spot-size converter <NUM> for adapting the mode field from optical waveguide to fiber. The input interface <NUM> may be an interface for interfacing with the circuit <NUM>, and the output interface <NUM> may be a fiber interface for one or more channels (fiber array interface).

The packagings <NUM>, <NUM> and <NUM> may comprise a metallic housing, for instance a gold housing.

Claim 1:
An optical integrated circuit (<NUM>, <NUM>, <NUM>), comprising:
- a substrate (<NUM>),
- at least one open cavity (<NUM>, <NUM>, <NUM>) provided in said substrate (<NUM>),
- at least one set of optical waveguides (<NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>) for each open cavity, each set comprising a first optical waveguide (<NUM>, <NUM>, <NUM>) and a second optical waveguide (<NUM>, <NUM>, <NUM>), wherein the first optical waveguide (<NUM>, <NUM>, <NUM>) and the second optical waveguide (<NUM>, <NUM>, <NUM>) of a set are each arranged in the substrate (<NUM>) and each comprise a first end facet (<NUM>, <NUM>) ending in the open cavity (<NUM>, <NUM>, <NUM>) of that set,
- a first collimating element (<NUM>, <NUM>, <NUM>) for each set, the first collimating element (<NUM>, <NUM>, <NUM>) of a set being arranged in the open cavity (<NUM>, <NUM>, <NUM>) of that set at or near the first end facet (<NUM>) of the first optical waveguide (<NUM>, <NUM>, <NUM>) of that set to collimate light from that first optical waveguide (<NUM>, <NUM>, <NUM>), and
- a second collimating element (<NUM>, <NUM>, <NUM>) for each set, the second collimating element (<NUM>, <NUM>, <NUM>) of a set being arranged in the open cavity (<NUM>, <NUM>, <NUM>) of that set at or near the first end facet (<NUM>) of the second optical waveguide (<NUM>, <NUM>, <NUM>) of that set to collimate light into that second optical waveguide (<NUM>, <NUM>, <NUM>),
- wherein the first optical waveguide (<NUM>, <NUM>, <NUM>) and the second optical waveguide (<NUM>) of a set are different segments of an optical waveguide that is interrupted by the open cavity (<NUM>, <NUM>, <NUM>) of that set,
characterized in that
the first and second collimating elements (<NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>) are made of polymerized and cured photo-sensitive material (82a) having a first refractive index, and further comprising arranged in the cavity (<NUM>, <NUM>, <NUM>) cured but not polymerized material (82b), having a second refractive index different from the first refractive index and acting as a mechanical support for supporting the first and second collimating elements (<NUM>, <NUM>).