Fiber array unit with improved thermal stability

Abstract

A fiber array unit, FAU, [400] comprising a beam transformer [401] and a fiber ferrule [410], the fiber ferrule [410] comprising a protrusion [413] and an opening for receiving at least one optical fiber [411], the beam transformer [401] comprising a cavity [406] and an optical interface [402], the cavity [406] being configured for receiving the protrusion [413] of the fiber ferrule [410] and comprising a transmissive surface [404] configured for receiving or transmitting a light beam [300] from or to the optical fiber [411] when the protrusion [413] is inserted into the cavity [406], wherein the beam transformer [401] is configured for forwarding the light beam [300] to or from the optical interface [402], respectively. An optical interposer [100] for mating to the FAU [400], and an optical arrangement with the optical interposer [100] and the FAU [400].

Description

[0001] Our ref.: A19276WO / AS Munich, 14 May 2025

[0002] Fiber array unit with improved thermal stability

[0003] Field of the invention

[0004] The invention relates to fiber array units, FAU, for coupling light between optical fibers on the one hand and photonic circuits on the other hand.

[0005] Background

[0006] Fiber array units, FAU, are used to couple light between optical fibers on the one hand and photonic circuits on the other hand. To this end, optical fibers are permanently and rigidly mounted in the FAU that can then in turn be fixed relative to the photonic circuit.

[0007] The photonic circuit inevitably generates heat. Also, losses in the FAU may be a further source of heat. Thermal stress may generate high forces, up to the point that fibers attached to the FAU break.

[0008] The reason for this is that the coefficient of thermal expansion (CTE) of commercial fiber-optic ferrules made of injection molded plastic exceeds by far that of silicon or other materials used in-package, such as other semiconductors or ceramic submounts. Silicon has a CTE of 2.6 part per million (ppm) per degree Kelvin, ceramic submounts typically have a CTE in the order of 4.2 ppm / K, while injected molded plastic ferrules have a typical CTE of 17 ppm / K even when a substantial amount of glass beads are dispersed into the plastic to reduce its CTE. This creates difficulties when attaching a plastic molded part directly to an optical submount or to a PIC, since these systems have to function and stay reliable across a wide temperature range exceeding 70°C, while at the same time the alignment at the optical interface needs to stay very accurate, at a submicron or micron level depending on the interface type, to not loose light. It is thus advantageous to use a material with a closer matched CTE for the direct optical interface to the PIC or submount. For example, a micro-optical element made out of molded glass can be used to couple light between a PIC or an optical subassembly and a fiber. This micro-optical element is further referred to as an interposer. For compactness, the term optical subassembly is understood as also comprising PIC based solutions, even if it consists of a single chip.

[0009] Silica glasses are a preferred material class, as their mechanical and optical properties can be adjusted in the required range by adding various other chemical elements. In particular, the CTE of glass can be made to match that of silicon or ceramic submounts, or to be very close to it. However, other moldable glasses, such as for example chalcogenide glasses, can also be used. The term glass is thus more general and encompasses a wide range of moldable amorphous inorganic materials. In the following, silica glasses refer to glasses containing silicon and oxygen, but that may also contain further chemical elements.

[0010] However, the problem of matching CTEs is then not fully solved, but transferred from the interface between the subassembly and the interposer to the interface between the interposer and the array of fibers serving to couple light to or from the subassembly. This can be solved by encasing the fibers in a material whose CTE is also matched to that of the interposer, such as in a silica-glass based fiber array unit (FAU). This, however, is also problematic for a number of reasons. Glass based FAUs are more expensive than plastic molded fiber ferrules. Moreover, glass cannot be as easily molded as plastic, restricting the shapes that can be formed. It is also a brittle material, so that application of mechanical stress can lead to breakage. These last two aspects in particular make it difficult to develop a connectorized solution, in which the fiber array can be reversibly attached or removed from the interposer, that relies entirely on glass on both sides of the connector. In particular, guide holes commonly used in fiber optic connectors require a very high aspect ratio that is prohibitively difficult to mold in glass. Inserting guide pins into guide holes defined in glass is also likely to result in chipping of the material, creating significant reliability concerns. Objective of the invention

[0011] It is therefore an objective of the present invention to provide a fiber array unit, FAU, that allows a releasable optical connection between one or more optical fibers and a photonic circuit and has a better thermal stability.

[0012] This objective is achieved by a fiber array unit according to a first independent claim, an optical interposer for use with this FAU according to a second independent claim, and an optical arrangement with an optical subassembly and the optical interposer according to a third independent claim. Further advantageous embodiments are detailed in the respective dependent claims.

[0013] Summary of the invention

[0014] In the following, we disclose a connectorized interposer and a matched FAU configuration that addresses the difficulties mentioned above.

[0015] A large number of interposers, or glass building blocks out of which the interposers are formed, can be molded in parallel into a glass plate, so that the interposers can be cost-effectively fabricated out of a material with a CTE close to that of the optical subassembly. The interposer enables the incorporation of guide pins and may be fabricated out of a combination of glass molded and plastic molded building blocks. Moreover, the interposer may also transform the light beams emitted or received by the subassembly, so as to relax the alignment tolerances at the interface between the interposer and the FAU.

[0016] The FAU comprises matching guide holes formed in a suitable material such as injection molded plastic. The FAU may comprise a structure formed out of glass, whose CTE is matched or close to that of the base of the interposer, that constrains the thermal expansion of the FAU for better matching to that of the interposer and of the optical subassembly. The glass part of the FAU may also comprise optical elements transforming the beams between the fibers and the interface between the FAU and the interposer. This further enables using wide beams at the optical interface between the FAU and the interposer, whose beam diameter may be significantly larger than that of the optical fibers, relaxing spatial alignment tolerances. This also allows to split the optical elements required to transform beams

[0017] Disclosure of the invention

[0018] In a first aspect, the invention provides a fiber array unit, FAU, with a beam transformer and a fiber ferrule. The fiber ferrule comprises a protrusion and an opening for receiving at least one optical fiber. The beam transformer comprises a cavity and an optical interface. The cavity is configured for receiving the protrusion of the fiber ferrule and comprises a transmissive surface configured for receiving or transmitting a light beam from or to the optical fiber when the protrusion is inserted into the cavity. The beam transformer is configured for forwarding the light beam to or from the optical interface, respectively.

[0019] It was found that, in this manner, the FAU may be realized in glass as the main material. As discussed above, the advantage of glass is that its CTE is smaller than that of plastic and closer to the CTE of silicon than the CTE of plastic is. The difficulties discussed above and associated with glass as a material are circumvented. First, the optical fiber may be mounted rigidly in good alignment with the transmissive surface in the cavity, so that this alignment is not jeopardized as the FAU heats up for whatever reason. Second, the FAU consists only of structures that may be fabricated by molding of glass. That is, the FAU may easily be fabricated by molding even though, as discussed above, the set of manufacturable shapes is restricted compared to the set of shapes that may be molded from plastic.

[0020] In a particularly advantageous embodiment, the opening for receiving the optical fiber extends through the protrusion along a longitudinal axis, such that the optical fiber when inserted into the opening terminates at an end surface of the protrusion. In this manner, there is always a good lateral alignment (perpendicular to the direction of propagation) between the end of the optical fiber on the one hand, and the transmissive surface on the other hand. There is also good alignment in the direction of propagation. Thus, in total, when the FAU is assembled, the end of the optical fiber will come to rest in a position that ensures a good coupling efficiency, without a need for an active alignment.

[0021] In a further particularly advantageous embodiment, the beam transformer comprises a focusing optical element configured for focusing the light beam along at least one axis of divergence of the light beam. For example, the beam transformer may comprise a lens structure that focuses a light beam into the end of the optical fiber, or accepts a light beam that emanates from the optical fiber at a given angle.

[0022] In a further particularly advantageous embodiment, the optical interface of the beam transformer is configured for receiving or transmitting the light beam from or to an interposer. In this manner, the interposer may serve as a further intermediate for coupling the light beam between a photonic integrated circuit on the one hand, and the optical fiber on the other hand.

[0023] In a further particularly advantageous embodiment, the fiber ferrule is polished at a surface terminating the optical fiber. This ensures that all the fiber facets are smooth and at the targeted angle before assembly of the beam transformer with the fiber ferrule.

[0024] In a further particularly advantageous embodiment, the cavity is configured for limiting at least a lateral thermal expansion of the protrusion when the protrusion is inserted into the cavity. This helps to preserve the lateral alignment of the optical fiber when the FAU heats up.

[0025] In a further particularly advantageous embodiment, the fiber ferrule comprises a guide hole with an axis of insertion. The guide hole runs along that axis of insertion that does not intersect the beam transformer when the protrusion is inserted into the cavity. In particular, the axis of insertion of the guide hole of the fiber ferrule may form an angle with the optical interface of the beam transformer that is between 20 and 70 degrees. In this manner, guide pins that are attached to the interpose may be inserted into the guide holes when the interposer is mated with the FAU. The placement of the axis of insertion allows for an easier fabrication. The beam transformer is preferentially formed by molding a glass plate with a top and bottom mold. In such a molding process, it is very difficult to punch a hole through the glass plate. As a consequence, there is no hole in the glass through which the guide pin can extend to reach the guide hole. As a consequence, the guide hole is formed in a region of the ferrule outside of the protrusion. A guide pin may also be permanently inserted into the guide hole, so that its other end may then be inserted into a corresponding guide hole on another entity, such as the interposer. In a further particularly advantageous embodiment, the beam transformer comprises a reflector configured for deflecting the light beam. This facilitate changing the direction of the light beam as it passes from a photonic integrated circuit via an optional interposer into the beam transformer and from there into the optical fiber. For example, the light beam may be coupled to or from the photonic integrated circuit in a direction normal to a substrate carrying this photonic integrated circuit, and it may be coupled to or from an optical fiber a direction parallel to this substrate. To this end, in particular, the axis of insertion of the guide hole may form an angle with the reflector that is between 30 and 60 degrees.

[0026] In a further particularly advantageous embodiment, the fiber ferrule comprises at least two openings for receiving an optical fiber. The fiber ferrule and / or the protrusion is divided into two sections. Each section comprises at least one of the openings for receiving an optical fiber, and the fiber ferrule and / or the protrusion comprises a strain relief section. The strain relief section connects the two sections of the fiber ferrule and / or the protrusion. In this manner, stress accumulating across the ferrule, and in particular thermal expansion stress when the temperature rises, may be reduced. A thickness of the fiber ferrule and / or the protrusion is smaller in the strain relief section than in the two sections of the fiber ferrule and / or the protrusion comprising the openings for receiving an optical fiber.

[0027] In a further particularly advantageous embodiment, the cavity of the beam transformer comprises an opening in a side wall of the cavity. An anchor section of the fiber ferrule traverses the opening and connects the protrusion of the fiber ferrule with a section of the fiber ferrule outside of the cavity in which the guide hole has been fabricated. In this manner, the guide hole may be placed in a section that is more convenient to reach when inserting a guide pin during assembly.

[0028] In a second aspect, the invention provides an optical interposer for mating with the fiber array unit, FAU, described above. This optical interposer comprises a guide hole or a guide pin. A guide hole of the FAU mates with the guide pin of the interposer, or a guide pin of the FAU mates with the guide hole of the interposer. The position and / or orientation of the guide hole or guide pin are chosen such that, when the guide hole mates with the guide pin of the FAU, or the guide pin of the interposer mates with the guide hole of the FAU, the light beam is coupled between the optical interface and the optical fiber n a predetermined direction. In particular, the light beam may be coupled to or from a photonic integrated circuit where light can be coupled in or out only in certain predetermined directions.

[0029] In a particularly advantageous embodiment, the optical interposer comprises a base. The base comprises a beam transforming element and an optical interface. The optical interface of the base is configured for mating with the optical interface of the beam transformer of the FAU. In this manner, the structures on the optical interposer on the one hand, and the structured on the beam transformer of the FAU on the other hand, are of a kind and of a complexity that may be fabricated by glass molding of the respective part. A single unit with the functionality of both the base and the beam transformer could not be fabricated in this manner due to the inherent limitations of the molding process.

[0030] In a further particularly advantageous embodiment, the guide hole or the guide pin has an axis of insertion. The optical interface of the base forms an angle with the axis of insertion that is between 20 and 70 degrees. As discussed above, this facilitates changing, by means of a reflector in the beam transformer, the direction of the light beam between a direction normal to a substrate carrying a photonic integrated circuit and a direction parallel to such a substrate.

[0031] In a further particularly advantageous embodiment, the optical interface of the optical interposer and the optical interface of the FAU are substantially parallel when the two are mated. In this manner, the direction of the light beam is not changed when passing from the one unit to the other, and there is only little, if any, unwanted reflection at the interface.

[0032] In a further particularly advantageous embodiment, the optical interposer comprises a base, and the base comprises a groove. The guide pin is inserted in and attached to the groove. For example, the guide pin may be a permanent part of the base. A groove molded out of glass is brittle, so it may not be suitable for a guide hole in which the guide pin can be re-inserted. However, permanent attachment by means of epoxy or other adhesives would be fine. Therefore, it may be advantageous to form the guide hole to be formed in a polymer / plastic type material, in this case on the ferrule side. However, there are also applications where it is more advantageous to have the guide pin on the ferrule side and the guide hole on the base side. Analogous advantages apply in a further particularly advantageous embodiment in which the optical interposer comprises a base and a lid. The lid comprises a groove. The guide pin, which may be part of the base, is inserted in and attached to the groove.

[0033] That is, while there is typically a lid, since the guide pin is attached, there is not necessarily a lid. The pin can also be attached to the groove without it.

[0034] In a further particularly advantageous embodiment, the optical interposer comprises a base and a mechanical guide unit. The mechanical guide unit comprises the guide pin or the guide hole. In particular, the mechanical guide unit may be fabricated out of a non-brittle material, such as injection molded plastic. When a guide pin is inserted into a guide hole, some force needs to be applied, which might damage glass or another brittle material. Furthermore, a displacement of the mechanical guide unit against the base allows for an alignment of the base against the entity to which the guide unit will be mated, such as the ferrule.

[0035] Thus, in particular, the mechanical guide unit may be made out of a different material than the interposer base. A coefficient of thermal expansion of the material of the mechanical guide unit may be larger than a coefficient of thermal expansion of the material of the base of the interposer. To avoid that this difference in thermal expansion coefficients affects the alignment between the optical elements on the two sides of the connector, in a further particularly advantageous embodiment, the base comprises a cavity, and the mechanical guide unit comprises a part with the guide hole or the guide pin. This part of the mechanical guide unit form-fits the cavity of the base of the interposer. In this manner, the cavity of the base constrains a thermal expansion of the part with the guide hole or the guide pin when the part is inserted into the cavity of the base.

[0036] In a further particularly advantageous embodiment, a guide pin is permanently inserted into the guide hole of the interposer. This guide pin is then meant to be inserted into the guide holes of the FAU when the interposer is mated with the FAU. That is, if there is a risk of damage when inserting the guide pin, this risk is shifted to the FAU that may be easier to replace than the interposer. In a further particularly advantageous embodiment, the optical interposer comprises a base, and the base comprises a top-sided reflector. This facilitates the optical coupling of optical fibers which are typically routed out with a direction parallel to the plane of the photonic integrated circuit, or of the substrate carrying this circuit. Therefore, advantageously, the top-sided reflector and an axis of insertion of the guide hole of the interposer or of the guide pin may form an angle between 30 and 60 degrees. The top-sided reflector may be implemented as a curved reflector that focuses the light or reduces its angle of divergence, i.e., as a converging optical element. In that case, it may not be required to implement a lens or a converging reflector on the bottom side of the base.

[0037] In a third aspect, the invention provides an optical arrangement. This optical arrangement comprises an optical subassembly that is to be optically coupled to the outside world, e.g., to optical fibers. This coupling is achieved by the optical interposer according to the second aspect, and by the FAU according to the first aspect, which are also part of the optical arrangement. The FAU is mated with the optical interposer, and the optical interposer is attached to the optical subassembly, such that the light beam is coupled between the optical subassembly and the FAU. As discussed above, the advantage of this optical arrangement is that it has a much higher thermal stability. That is, when the temperature rises, e.g., due to heat generated in the optical subassembly, the components are subject to less mechanical stress, and the alignment is preserved better.

[0038] In a particularly advantageous embodiment, the optical interposer comprises an optical interface to the FAU and a lens. The lens is facing towards the optical subassembly and configured for coupling or receiving the light beam to or from a surface coupler or a surface emitting or surface receiving element of the optical subassembly. Such surface couplers are in particular present in photonic integrated circuits, PIC, as optical subassemblies.

[0039] Therefore, in a further particularly advantageous embodiment, the optical subassembly comprises a photonic integrated circuit, PIC. The optical interposer comprises an optical interface to the FAU and a protrusion. The protrusion is located on a side of the interposer facing towards the optical subassembly and configured for coupling or receiving the light beam to or from an edge coupler of the PIC. Such a protrusion may, in particular, be integrated into the fabrication of the interposer by glass molding. That is, the optical interface to the FAU on one side of the interposer may be defined in a first mold, the protrusion may be defined in a second mold, and a glass plate may be pressed between these two molds.

[0040] In a further particularly advantageous embodiment, the optical interposer comprises a base. The base comprises an optical interface to the FAU and a top-sided reflector. The top-sided reflector is located on a side of the base facing away from the optical subassembly. The top-sided reflector is curved and configured for focusing or reducing an angle of divergence of the light beam along at least one axis of divergence of the light beam. The relative lateral positioning between this topsided reflector on one side of the interpose, and a protrusion or other beamtransforming element on the other side of the interposer, can then be very well controlled. An uncertainty as to the thickness of the glass plate after the molding process does not matter.

[0041] In a further particularly advantageous embodiment, the optical subassembly comprises a surface coupler, or is surface-emitting or surface-receiving. The optical arrangement is configured to couple the light beam to or from the optical subassembly to the optical interface of the FAU, such that the light beam is transformed in between the optical subassembly and the optical interface of the FAU in a manner that the light beam has a substantially larger beam diameter at the optical interface of the FAU than at the optical subassembly. At least one of the interposer or the beam transformer comprises a reflector. The larger beam diameter helps to relax spatial alignment tolerances, which in turn is beneficial for splitting the optical elements required to transform beams between the optical subassembly and the fibers between the interposer and the FAU.

[0042] In a further particularly advantageous embodiment, the optical arrangement is configured to constrain a position of the FAU relative to the interposer in two directions by mating of a guide hole with a guide pin. A surface of the FAU is configured to abut against a surface of the interposer, thereby constraining the position of the FAU relative to the interposer in the third direction. An alignment in this manner can be assembled well using pick-and-place assembly techniques.

[0043] In a further particularly advantageous embodiment, the optical interposer is configured for coupling a first light beam between an optical subassembly and the FAU. The optical interposer comprises a reflector and an optical interface to the FAU. The FAU comprises an optical interface to the interposer, a reflector, and two openings each configured for receiving an optical fiber stacked on top of each other. The interposer is configured to couple the light beam between the optical subassembly and a first optical fiber to be received in one of the two openings, as well as a second light beam between the optical subassembly and a second optical fiber to be received in the other one of the two openings.

[0044] The first light beam is coupled from the optical subassembly to the reflector of the interposer, from the reflector of the interposer to the optical interface of the interposer, from the optical interface of the interposer to the optical interface of the FAU, from the optical interface of the FAU to the first optical fiber to be received in one of the two openings, or in inverse direction.

[0045] The second light beam is coupled from the optical subassembly to the optical interface of the interposer, from the optical interface of the interposer to the optical interface of the FAU, from the optical interface of the FAU to the reflector of the FAU, from the reflector of the FAU to the second optical fiber to be received in the other one of the two openings, or in inverse direction.

[0046] In this manner, multiple optical elements on the optical subassembly can be coupled to multiple fibers. To further increase the density of fibers, it is possible to stack two or more arrays of fibers in another direction, such as a vertical direction.

[0047] In a further particularly advantageous embodiment, the light beam and the second light beam cross the optical interfaces of the interposer and of the FAU with directions that form an angle to each other of at least 50 degrees. This facilitates the use of low-rise connectors, which in turn improves compatibility with existing form factors.

[0048] Description of the Figures

[0049] In the following, the invention will be described using Figures without any intention to limit the scope of the invention. The Figures show:

[0050] Figure 1 : Different ways of optically connecting an optical interposer 100 to an optical subassembly 200; Figure 2: Optical connection between a base 101 of the interposer 100 and a beam transformer 401 of the FAU 400;

[0051] Figure 3: Optical path from the base 101 of the interposer 100 via the beam transformer 401 of the FAU 400 to the optical fiber 411 in the ferrule 410;

[0052] Figure 4: Coupling of multiple light beams 300, 300B through the interposer 100 and FAU 400;

[0053] Figure 5: Exemplary way of reducing the probability of misalignment between the base 101 of the interposer and the beam transformer 401 of the FAU 400;

[0054] Figure 6: Exemplary configurations for coupling a light beam 300 between an optical fiber 411 and the beam transformer 401 of the FAU 400 via the bottom of the cavity 406;

[0055] Figure 7: Exemplary embodiment of the FAU 400 with guide holes 415 having an axis of insertion that does not intersect the beam transformer 401 ;

[0056] Figure 8: Exemplary ways of integrating guide pins 500 into the base 101 of the interposer 100.

[0057] The base of the interposer is a building block that can be directly attached to a PIC or an optical subassembly. It is molded in a material that is preferentially glass. Glass preferentially refers to silica glass combined with various other chemical elements that allow to set its coefficient of thermal expansion, its refractive index, its glass transition temperature and its adhesion to molds during the molding process, but can also refer to other amorphous materials, such as chalcogenide glasses. Generally, molding processes involve several molds that compress the material in between. The process for molding the glass is preferentially applied by compressing a glass plate that already has two surfaces with optical grade quality, i.e., with sufficiently low roughness that has a root mean square (rms) typically below a few nanometers, between two molds, a top and a bottom mold. During the molding process, these two molds are pressed together along an axis that is typically substantially perpendicular to the surface of the glass plate, i.e., along its surface normal. This compression is done by a glass press. Several interposers, or several glass building blocks meant to be assembled into interposers, are preferentially molded in parallel in a glass plate that can be diced after the molding. Optionally, additional process steps such as in-line testing, cleaning or thin-film coating can be applied to entire molded glass plates prior to dicing. The glass molding process is preferentially isothermal glass molding, in which the molds and the glass are heated up and cooled down together, since isothermal glass molding has a higher precision than non-isothermal glass molding. A typical number of interposers or glass building blocks that can be molded in parallel in a single glass wafer ranges between 30 and 500, depending on the number of fibers that are coupled to.

[0058] The base of the interposer comprises a coupling and beam transforming element for coupling light between the interposer and the optical subassembly. This coupling and beam transforming element may take the form of a lens receiving a beam emitted upwards from the optical subassembly and reducing the divergence of that beam, collimating, or refocusing it, i.e., the lens may be a converging lens. The coupling and beam transforming element may also comprise a curved surface acting as a converging reflector, that focuses or collimates the light, or reduces its angle of divergence. In the inverse direction, the coupling and beam transforming element may couple light from the interposer to the optical subassembly and focus a beam onto it.

[0059] The coupling and beam transforming element is preferably defined by a first mold. After assembly of the interposer with the optical subassembly, the coupling and beam transforming element preferentially faces towards the optical subassembly and is a bottom facing element of the interposer.

[0060] This is illustrated by Figures 1(a)-1 (c).

[0061] Figure 1(a) describes an interposer 100 whose base 101 is attached to the substrate 201 of an optical subassembly 200. A light beam 300 is coupled between a surface emitting optical element such as a VCSEL 202 or a surface receiving optical element such as a surface photodetector 202 of the optical subassembly 200 and the interposer 100 via a lens 102 of the base 101 of the interposer. The arrows in Figures 1(a)-1(c) represent the light as being coupled from the optical assemblies to the interposer, but it may also be coupled in the inverse direction from the interposer to the optical assemblies.

[0062] In Figure 1(b), the optical subassembly consists in a PIC 203 that comprises a waveguide 204. Light is coupled to or from the waveguide 204 via a surface emitting coupler 205, that may be a grating coupler. The PIC 203 may be a silicon photonics chip.

[0063] In Figure 1(c), light is coupled to or from the waveguide 204 of the PIC 203 via an edge coupler 206. The PIC 203 may be a silicon photonics chip or an EML. The base 101 of the interposer 100 comprises a protrusion 103. The protrusion 103 comprises a curved reflector 104. Like the lens 103 if Figure 1 (a) and 1(b), the curved reflector 104 is a converging optical element that reduces the angle of divergence of the light or focuses it.

[0064] On the other side of the interposer base 101 , that is formed by the second mold during the molding process and is facing away from the optical subassembly (the top side), a transmissive surface is formed that serves as the optical interface 106 to the FAU. This is shown in Figure 2(a). Since fibers are typically routed out with a direction parallel to the plane of the PIC 203 or substrate 201 , it is advantageous to deflect the light beam 300 either inside the interposer or inside the attached FAU. Inside the interposer, this can be accomplished by a top-sided reflector 105. This is shown in Figure 2(b). This top-sided reflector may be implemented as a curved reflector that focuses the light or reduces its angle of divergence, i.e., as a converging optical element. In that case, it may not be required to implement a lens 102 or a converging reflector 104 on the bottom side of the base. This is shown in Figure 2(c). In any case, by implementing a beam transforming element on the interposer side in the base of the interposer 101 , that is preferentially molded out of a glass with a CTE that is matched or close to that of the optical subassembly 200, an accurate alignment can be maintained between that beam transforming element and the optical subassembly, even if the temperature is cycled.

[0065] The base 101 of the interposer 100 can be optically interfaced to a beam transformer 401 on the side of the FAU 400. This beam transformer is also preferentially molded out of glass with a CTE matched or close to that of the interposer base and of the optically subassembly. This beam transformer also comprises a converging optical element such as a lens 404 or a converging reflector 403. This is shown in Figure 3. Figure 3(a) shows a configuration in which the light is deflected into the plane of PIC / substrate via a reflector 105 that is part of the base 101 of the interposer 100. In Figure 3(b), the light is deflected into the plane of the PIC / substrate via a reflector 403 that is part of the beam transformer 401 of the FAU 400. Figure 3(c) shows a configuration in which the reflector 403 is a converging reflector, so that the lens 404 is no longer required in the beam transformer 401 and is replaced by a transmissive surface 404.

[0066] In some cases, it may be sufficient to implement a converging optical element in the FAU 400, without requiring one in the base 101 of the interposer 100. This is in particular the case of light is coupled to or from a PIC 203 with a surface emitting coupler 205 that is able to emit or receive beams with a relatively wide diameter, exceeding that of a single mode fiber. In such a case, it is also possible for the beam 300 to transit between the FAU 400 and the optical subassembly 200 without transiting through the base 101 of the interposer 100. The reflector 403 then needs to be implemented in the beam transformed 401 of the FAU 400. The reflector 403 can then also serve as a mechanical contact surface that is brought in mechanical contact with the base 101 when the FAU 400 is mated with the interposer 100, defining its position in at least one axis. This is shown in Figure 3(d). While in Figure 3(d) the interposer 100 does not comprise a beam transforming element such as a lens 102 or a protrusion 103, here the beam transformation is done by the surface coupler 205 of a PIC 203 to which a light beam 300 is coupled to or from. While in other embodiments the relative position between the FAU 400 and the interposer 100 along the axis of insertion of the guide pins 500 may be determined by a mechanical contact between the optical interfaces 106, 402 of the FAU and of the interposer during mating, here it may be determined by a mechanical contact between a reflector 403 of the FAU 400 and a stopper surface 115 of the interposer 100. The mechanical stopper surface 115 may be fabricated in the base 101 of the interposer 100 or in a lid 111 or a mechanical guide unit 112 of the interposer.

[0067] In preferred embodiments of the invention, there is a beam transforming element on either side of the connector, in in the FAU 400 and one in the interposer 100 or in the optical subassembly 200, such that the light beam 300 transmits through the interfaces 106, 402 as a collimated or near collimated beam (i.e., with an angle of divergence much less than without the beam transforming elements) with an increased diameter, so that spatial alignment tolerances are increased at the interface. This enables, in particular, the use of plastic molded building blocks together with glass molded building blocks. Increased requirements in regard to angular alignment are maintained with sufficiently long guide pins. The diameter of the light beam 300 at the optical interfaces 106, 402 is typically larger than 30 pm. In a preferred embodiment, the diameter of the light beam is larger than 50 pm to further reduce alignment tolerances. In a preferred embodiment, this diameter is measured between points at which the intensity of the light beam 300 drops to 1 / e2of its maximum. In another preferred embodiment, it is calculated as four times the square root of the second moment of the light intensity distribution (D4cr method).

[0068] Multiple optical elements on the optical subassembly can be coupled to multiple fibers by fabricating arrays along the y-direction in the labeling of Figures 1-3. To further increase the density of fibers, it is possible to stack two or more arrays of fibers in the vertical direction. In the configuration shown in Figure 4(a), the light beam 300 is deflected by a reflector 105 inside the base 101 of the interposer 100 for one row of fibers 411 . A second light beam 300B is deflected by a reflector 403 inside the FAU 400 for another row of fibers 411 B. This facilitates the manufacturing of a compact connector that does not rise much above the surface of the optical subassembly. In particular, the beam transformer 401 is typically molded such that the surface in which the optical interface 402 is formed is V-shaped. One side of the V-shaped surface can then be used as the optical interface 402, while the other can be used as the reflector 403. In a configuration where the reflectors for both row of fibers are on either the base 101 of the interposer or the beam transformer 401 of the FAU, one side of this V-shaped structure would not be used, increasing the total height of the connector. This is shown in Figure 4(b). The beam transformed 401 cannot be stopped right on the dashed line, as this would require the molding of a surface parallel to the axis of compression of the press. Since low-rise connectors are essential for compatibility with existing form factors, the configuration shown in Figure 4(a) is advantageous. The beams 300 and 300B cross the optical interfaces between the interposer 100 and the FAU 400 with directions that are preferably 90 degrees from each other. In a preferred embodiment, the angle formed by these two directions is at least 50 degrees.

[0069] By expanding and collimating the beams between the optical subassembly 200 and the optical interface 106 of the interposer 100 and by expanding and collimating the beams between the optical fibers 411 and the optical interface 402 of the FAU 200, the required alignment accuracy between these two optical interfaces is highly relaxed in terms of positioning along the three spatial coordinates, z, y and z. However, the required angular alignment tolerance becomes higher. Dust particles on the optical interfaces 106 and 402 might lead to an angular misalignment when they are brought together. This can be remedied by reducing the surface along which the two interfaces are brought in mechanical contact with each other, to reduce the probability of an interposed dust particle. This is shown in Figure 5(a), in which pedestals 107 are formed on the interface 106 of the interposer bases 101 , that act as a mechanical contact. Alternatively, such pedestals 405 can be formed on the interface 402 of the beam transformer 401 of the FAU 400, as shown in Figure 5(b). In a preferred embodiment, the contact area created by the mechanical contacts is less than 20% of the total area of the interface surfaces 106, 402.

[0070] The base 101 of the interposer is preferentially molded out of glass, with an attachment surface service to attach it to the optical subassembly optionally defined by a first mold, a lens 102 or a protrusion 103 and a converging reflector 104 of such protrusion also defined by the first mold on the bottom side of the base 101 , the reflector 105 optionally defined by a second mold and the interface 106 also defined by that second mold on the top side of the base. The direction with which the light beam 300 propagates between the bottom and the top side of the base is preferentially along the direction with which the two molds are pressed together - the direction of compression of the press. This ensures that variations in the thickness of the glass plate after molding minimally impact the optical properties of the interposer, since light beam departs and arrives at the same point on both sides irrespectively of how thick it is. Depending on the acceptable insertion losses and thus on the required accuracy, the direction with which the light beam propagates between the two sides of the base should be within 10°, 20° or 30° of the axis of the press.

[0071] Similar considerations hold for the beam transformer 401 of the FAU 400. The lens 404 of the beam transformer 401 is preferentially defined by a first mold. The interface 402 of the beam transformer 401 and optionally the reflector 403 of the beam transformer 401 are preferentially defined by a second mold. The direction with which the light beam propagates between the two sides of the beam transformer should be within 10°, 20° or 30° of the axis of the press. The FAU 400 is preferentially formed out of the beam transformer 401 and a fiber ferrule 410 that may be fabricated out of injection molded plastic. This allows polishing of the fiber ferrule to ensure that all the fiber facets are smooth and at the targeted angle before assembly of the beam transformer 401 with the fiber ferrule 410, forming a polished facet 412 of the ferrule 410. Typical configurations with a straight polish or an angled polish reducing back-reflections into the fiber or into the optical subassembly are shown in Figure 6. The optical axis 411 a of the fiber 411 and the optical axis 404a of the lens 404 typically chosen to coincide, so that the optical beam 300 inside the beam transformer 401 propagates horizontally relative to the plane of the PIC 203 or of the substrate 201 forming the optical subassembly 200. An example where this is not the case is shown in Figure 6(c). A misalignment between the optical axis of the fiber and of the optical axis of the lens leads to the beam 300 to propagate off-axis inside the beam transformer. Since the relative angular alignment of the beams on the two sides of the optical interface between the interposer 100 and the FAU 400 is essential to maintain good insertion losses, as a consequence of the expanded beam sizes at the interfaces 106, 402, it is also essential to maintain a very good alignment between the optical axis of the fiber and the optical axis of the lens. This alignment needs to be a fraction of the fiber mode and should be typically better than 1 or 2 microns. This, however, is a difficult technical problem since the beam transformer 401 and the ferrule 410 forming the FAU 400 are made out of different materials with mismatched CTE.

[0072] In order to maintain the required alignment, it is necessary to mechanically constrain part of the ferrule 410 such that it cannot expand faster than the beam transformer 401. This can be accomplished by forming the ferrule 410 such that it comprises a protrusion 413 and inserting that protrusion into a cavity 406 of the beam transformer 401 . The protrusion is formed such that its sides walls are form a snug fit with the cavity at a low temperature of operation. As the temperature is increased, the protrusion that is preferentially made out of injection molded plastic with glass bead filling, would expand faster than the cavity, due to its higher CTE. However, insertion of the protrusion 413 into the cavity 406 prevents it from expanding faster. Since fibers 411 are terminated inside the protrusion, their facet position is also constrained by the cavity 406 as the temperature is cycled.

[0073] When a lens 404 is formed as part of the beam transformer, there needs to be a sufficiently long optical path length between the lens and the facet of the fiber 411 for the light beam 300 to be focused onto the fiber facet, or, in the reverse direction, for the light beam 300 to be collimated or near collimated inside the beam transformer 401. A cavity 406 is thus required in any case and using the beam transformer 401 as a mechanical frame for the protrusion 413 of the ferrule 410 does thus not increase the manufacturing complexity. The cavity 406 can be used to serve both purposes.

[0074] The cavity 406 is preferentially molded into one of the two sides of a glass plate. Since it is very difficult to form vertical or near vertical side walls with an angle of less than 20° relative to the axis of compression of the press, the cavity 406 is formed with slanted sidewalls. Preferably, these sidewalls form an angle of more than 20° relative to the angle of compression of the press. The side walls of the protrusion 413 of the ferrule 410 are angled accordingly. In order to maintain small components, compatible with existing form factors, it is also advantageous to define the cavity 406 to have a depth of less than a mm, typically of a few hundred pm. The area of the sidewalls of the cavity 406 is thus typically not sufficient to reliably attach the ferrule 410 to the beam transformer 401 with an adhesive layer such as UV or thermally curable epoxy. To solve this problem, larger attachment surfaces 407 and

[0075] 414 are provided on the beam transformer 401 and on the ferrule 410 outside of the cavity 406 and of the protrusion 413.

[0076] The ferrule 410 may comprise guide holes 415 in which guide pins 500 may be inserted with precise positioning. The guide pins may be permanently attached to the ferrule, in which case they are meant to be inserted into corresponding guide holes 109 of the interposer 100 when the FAU 400 is mated with the interposer. The guide pins 500 may also be permanently attached to the interposer 100, in which case they are meant to be inserted into the guide holes 415 of the FAU 400 when the interposer is mated with the FAU. The guide holes 415 have a main axis that is typically parallel to the optical axis of the optical fibers 411. This makes is difficult to configure the FAU 400 such that guide pins 500 can be inserted into the guide holes

[0077] 415 if the guide holes 415 are defined in the protrusion 413 that is inserted into the cavity 406 of the beam transformer 401. The beam transformer 401 is preferentially formed by molding a glass plate with a top and bottom mold. In such a molding process, it is very difficult to punch a hole through the glass plate. As a consequence, there is no hole in the glass through which the guide pin 500 can extend to reach the guide hole 415. As a consequence, the guide hole 415 is formed in a region of the ferrule 401 outside of the protrusion 413. Figure 7(a) shows an exemplary configuration of the beam transformer 401 and of the ferrule 410 in the plane at which the ferrule is terminated after polishing. The guide holes 415 are located outside of the outline of the beam transformer 401 . The ferrule may also terminate before this plane outside of the protrusion 413. In that case, the label 415 refers to the cross-section of the guide holes if they were extended to this plane, or to a cross-section of the guide pins 500 when they are inserted into the guide holes 415. In order to increase the accuracy over the position at which the polishing of the ferrule stops, it may be advantageous to first mold the ferrule 410 such that the protrusion 413 extends beyond this plane, but such that regions of the ferrule 410 outside of the outline of the beam transformer 401 stop at this plane, are extend less beyond this plane. This ensures that during polishing, the area that is being polished suddenly increases when the target position is reached, slowing down the rate with which the material is removed from the ferrule. This makes it easier to stop the polishing at the right time.

[0078] The protrusion 413 and the part of the ferrule in which the guide holes 415 is defined merge in another plane of the ferrule 410. In figure 7, an exemplary outline 416 of the ferrule 410 is also shown.

[0079] As the temperature of the FAU is increased, the ferrule 410 typically expands at a faster rate than the beam transformer 401 . This is in particular the case when the ferrule 410 is made out of injection molded plastic and the beam transformer 401 is made out of glass. Inside the cavity 406 the protrusion 413 is constrained, so that it ends up being compressively stressed and applies a force onto the walls of the protrusion 413. This may limit the width of the protrusion 413 and thus the number of fibers 411 that can be fitted into the cavity 406, since the wider the protrusion, the higher the force that ends up being applied on the side walls. Excessive force may lead to the sidewalls of the cavity to crack, or, since they are typically slanted with an angle exceeding 20 degrees due to the constrained of the glass molding process, it may lead to the ferrule 410 popping out of the beam transformer 401 or misaligning relative to it. To reduce the force applied on the cavity side walls, while at the same time allowing more fibers to be fitted inside the FAU 400, several cavities 406 and several protrusions 413 may be defined, each with a smaller number of fibers. To reduce the stress accumulating across the ferrule 410, its cross-section may be locally reduced in between groups of fibers, with a small bridge 417 holding them together while providing strain relief. This strain relief section 417 may be in between cavities 406 or within a cavity 406 as shown in Figure 7(b). It may be applied to the protrusion 413 but may also extend or be applied to regions of the ferrule not inserted into the cavity, as shown by the outline 416. It is characterized by there being a cross-section of the ferrule 410 in a plane perpendicular to the optical axis of the fibers 411 in which two sections of the ferrule 410, each comprising a group of fibers 411 , are connected by a strain-relief section that is thinned down relative to the sections of the ferrule in which the fibers are inserted. For some cross-sections, the thickness of that strain relief section may also be zero. In Figure 7(b), this thickness is shown to be non-zero in the plane where the ferrule terminates after polishing, to provide sufficient rigidity to the ferrule for the polishing step.

[0080] To further increase the precision with which each group of fibers is positioned relative to the converging elements on the beam former 401 , guide rails 408 may be provided inside a cavity 406 at places where the height of the ferrule has been reduced to form a strain relief section 417. It may be advantageous to keep the gap between guide rails 408 facing each other across the cavity 406 to be larger than the thickness of the strain relief section 417, so that the strain relief section can expand (in the z-direction) when it is being compressed (in the y-direction).

[0081] In Figure 7(a), the section of the ferrule 410 in which the guide holes 415 are defined is disjoint from the protrusion 413 in the plane where the ferrule 410 is terminated by the polish. This may reduce the accuracy of the relative alignment between these two sections, as they protrude separately from the base of the ferrule. In order to increase the rigidity between these two sections, it may be advantageous to form a bridge 418 that anchors them together. In order for the ferrule 410 to remain insertable into the cavity 406 of the beam transformer 401 , an opening 409 has to be provided in the side walls of the cavity 406. It should be noted that while the concept of the anchor 418 and the strain relief section 417 are illustrated in the same figure, these two aspects are independent from each other and can be separately implemented. There are several possibilities to integrated a matching guide hole 109 or guide pin 500 to the interposer 100 so that it can be mated with the FAU 400. V-shaped, ll- shaped, or otherwise shaped grooves 110 can be molded into the base 101 of the interposer, in which the guide pins 500 can be placed with high precision. Such grooves can me defined by the same mold as the optical interface 106 of the FAU and can be positioned with a higher precision relative to optical elements on the interposer base 101. A lid 111 can then be further attached to the interposer, over the positions where the guide pins 500, in order to increase the mechanical stability of the assembly. The lid 111 may be molded out of the same material as the interposer bases 101 or out of another material with a similar CTE. This is shown in Figure 8(a).

[0082] It is advantageous to make these guide pins out of a material that is non-brittle, so that they can be reliably repeatedly inserted into the guide holes, and whose CTE is close to that of the material out of which the interposer is made, to maintain alignment over temperature cycles and to avoid reliability problems such as cracking or breaking of the interposer. An exemplary material that satisfies these properties is tungsten carbide, that has a CTE of 4.9 ppm / K. Iron-nickel alloys and iron-nickel- cobalt alloys can also have low CTEs that can be adjusted according to their composition. The FeNiCo alloy named Kovar or Dilver P, in particular, has the same coefficient of thermal expansion as borosilicate glasses.

[0083] Alternatively, the grooves 110 can be defined in the lid 111 , as shown in Figure 8(b), or the guide pins 500 can be encased in a separate mechanical guide unit 112 comprising both a base and a lid, that is attached onto the bases 101 of the interposer, as shown in Figure 8(c). In such cases, the position of the guide pins is determined by how the lid 111 or the mechanical guide unit 112 are positioned and attached onto the base 101 of the interposer. This positioning can be constrained by providing matching mechanical guiding structures on the bases 101 of the interposer 100 and on the lid 111 or the mechanical guide unit 112. This can for example be accomplished by defining a protrusion 114 in the lid 111 or in the mechanical guide unit 112 as well as a matching cavity 113 in the interposer base 101 , the opposite is however also possible.

[0084] It may also be beneficial to leave the position of the lid 111 or of the mechanical guide unit 112 unconstrained, so that their position, and consequently the position of the guide pins, can be adjusted during an active alignment step, during which the coupling efficiency of the light across the two sides of the connector is used as a feedback signal. This may for example be beneficial when the base 101 of the interposer is first attached to the optical subassembly 200 and the lid 111 or the mechanical guide unit 112 are attached to the base 101 in a second step using active alignment. Misalignment between the base 101 and the optical subassembly 200 may then be partially compensated by the second alignment. In such an active alignment step, the lid 111 or the mechanical guide unit 112 may first be mated with an FAU 400 prior to attachment of the lid 111 or mechanical guide unit 112 to the interposer base 101. In particular, a displacement of the interposer base 101 relative to the optical subassembly 200 in the y-direction may be compensated by a displacement of the lid 111 or the mechanical guide unit 112 relative to the interposer base 101 also in the y-direction.

[0085] Instead of fabricating the mechanical guide unit 112 out of a brittle material, it can be fabricated out of a non-brittle material such as injection molded plastic. A guide hole 109 can then be fabricated into the mechanical guide unit 112 with well-established injection molding processes. This guide pin 500 can then be permanently inserted into the guide hole 109 to provide the interposer 100 with a guide pin to be mated with a guide hole 415 of the FAU. Instead, a guide pin can also be permanently attached to the FAU 400 to be mated with a guide hole 109 of the interposer 100.

[0086] When using a mismatched material such as injection molded plastic to fabricate the mechanical guide unit, the problem of mismatched CTE occurs. In particular, a mechanical guide unit 112 fabricated out of injection molded plastic will expand faster than the base 101 of the interposer when the temperature is increased. This can lead to misalignment of the optical elements on the two sides of the connector. To mitigate this problem, it can be advantageous to encase the part of the mechanical guide unit that contains the guide hole 109, optionally populated with the guide pin 500, into a cavity 113 of the interposer base 101 that constrains its thermal expansion to be matched to that of the interposer base. In that case, the mechanical guide unit 112 comprises a protrusion 114 that is inserted into the cavity 113 of the interposer base 101. The protrusion 114 comprises a guide hole 109.

[0087] From a mechanical perspective, it is advantageous for the sidewalls of the cavity 113 of the interposer base 101 to be as close as possible to being vertical, as this will ensure the most precise insertion of the mechanical guide unit 112 as well as the highest mechanical robustness against thermal expansion of the mechanical guide unit 112. However, it is very difficult to mold vertical side walls oriented parallel to the axis of compression of the press when molding the interposer base 101 out of glass. For this reason, the sidewalls of the cavity 113 preferably form an angle larger than 20 degrees relative to the axis of compression of the press or relative to the surface normal of the surface of the PIC 203 or of the substate 201 .

[0088] When the interposer base 101 or the beam transformer 401 of the FAU 400 are fabricated by molding a glass plate, there is some level of manufacturing repeatability in the thickness of the molded glass plate that is in the order of 10 pm. To reduce its effect on the performance of the interposer, it is advantageous for the light beam 300 to transit between the two surfaces in a direction that is parallel to the axis of compression of the press. This ensures that the light beam 300 is incident at the same position of these optical elements irrespectively of variations in the thickness of the molded glass plate. In the interposer base 101 , these optical elements may be a coupling and beam transforming element 102, 103 on one side and a top-sided reflector 105 or the optical interface 106 to the FAU on the other side. In the beam transformer 401 , these may be the optical interface 402 or a reflector 403 on the one side and a lens 404 on the other side. In a preferred embodiment, the angle with which the light beam 300 propagates between the two surfaces of either the interposer bases 101 or the FAU beam transformer 401 is less than 30 degrees relative to the direction of compression with which these building blocks were formed. In another preferred embodiment, this angle is less than 20 degrees. In a further preferred embodiment, it is less than 10 degrees. These different embodiments impose different requirements on the repeatability of the molding process.

[0089] The interposer may have an attachment surface 108 which forms the mechanical contact with the optical subassembly 200 or with which it is attached to the optical subassembly. This attachment surface is typically on the same side of the interposer base 101 as a coupling and beam transforming element 102, 103 and defined by the same mold. It is typically in mechanical contact with or attached to the top surface of a PIC 203 or the top surface of a substrate 201 of the subassembly 200. The interposer bases 101 is typically attached to the optical subassembly 200 such that the direction with which the light beam 300 propagates between the coupling and beam transforming element 102, 103 and the top surface of the interposer base 101 is within 30 degrees of the surface normal of the top surface of the PIC 203 or substrate 201. In a preferred embodiment, this direction is within 20 degrees of this surface normal. In another preferred embodiment, this direction is within 10 degrees of this surface normal. These different embodiments impose different requirements on the repeatability of the molding process.

[0090] Grooves 110 for attachment of the guide pins 500 or guide holes 109 also preferably have an axis that is parallel to the surface of the unmolded glass plate and that is thus substantially orthogonal to the direction of compression of the press. Consequently, the angle between the direction with which the light propagates between the two surfaces of the interposer base 101 and the axis of the pins is preferably above 60 degrees. In a preferred embodiment, this angle is above 70 degrees. In another preferred embodiment, this angle is above 80 degrees.

[0091] The interposer 100 comprises a surface through which light exits or enters when it is coupled to or from the FAU 400, which is the optical interface of the connector on the interposer side / the optical interface to the FAU. It is very difficult to mold surfaces that form an angle of less than 20 degrees relative to the direction of compression of the press. The grooves 110 for attachment of the guide pins 500 or the guide hole 109 also preferably have an axis that is parallel to the surface of the unmolded glass plate and thus substantially orthogonal to the direction of compression of the press used to form the interposer base 101. Consequently, the angle formed by the optical interface of the connector on the interposer side (the optical interface to the FAU) with the axis of the guide pins 500 or the guide holes 109 is preferably less than 70 degrees.

[0092] The FAU 400 comprises a surface through which light exits or enters when it is coupled to or from the interposer 100, which is the optical interface of the connector on the FAU side / the optical interface to the interposer. It is very difficult to mold surfaces that form an angle of less than 20 degrees relative to the direction of compression of the press. The guide pin 500 or the guide hole 415 preferably have an axis that is perpendicular the direction of compression of the press used to form the beam transformer 401 . Consequently, the angle formed by the optical interface of the connector on the FAU side (the optical interface to the interposer) with the axis of the guide pins 500 or the guide holes 415 is preferably more than 20 degrees.

[0093] The optical interface of the FAU and the optical interface of the interposer are preferably substantially parallel to each other. Consequently, to satisfy the constraints of the glass molding process, the angles formed by the optical interfaces 106, 402 of both the FAU 400 and the interposer 100 are preferably between 20 degrees and 70 degrees. In another preferred embodiment that facilitates manufacturability, these angles are between 30 degrees and 60 degrees.

[0094] It is much easier to mold lenses 102, 404 if their optical axis is parallel to the compression axis of the press, or form a small angle with it. In a preferred embodiment, the beam transformer 401 of the FAU 400 comprises converging lenses whose optical axis, defined as the direction with which a light beam propagates between the lens and an optical fiber 411 , is within 30 degrees of the axis of compression with which the beam transformer 401 has been molded. In another preferred embodiment, this angle is within 20 degrees. In a further preferred embodiment, this angle is within 10 degrees.

[0095] A difficulty arises from the fact that the interfaces 106, 402 on both the interposer base 101 and on the FAU beam transformer 401 need to be transmissive, while the reflector 105 or 403, that is on the same surface as either the interface 106 or the interface 402, needs to be transmissive. To increase the transmission through the interface surfaces, it may be advantageous to provide them with an anti-reflective coating (ARC) in the form of a stack of thin dielectric films. To prevent the ARC coating to compromise the reflectivity of the reflective surfaces, it may be advantageous to first selectively coat them with a reflective layer, or to selectively deposit a reflective layer on top of the ARC. Such reflective layer can be selectively deposited with inkjet printing of a metal precursor followed by annealing to obtain a high-quality metal layer. The ARC can be applied across an entire molded glass piece before dicing it into individual glass building blocks.

[0096] Alternatively, a stack of thin film layers can be applied across the entire surface of a molded glass plate comprising reflective and transmissive surfaces and engineered to be an ARC at the transmissive surfaces and to be a reflective stack at the reflective surfaces. This can be accomplished since the properties of a thin film stack depend on the angle of incidence of the light. Since the interface surfaces of the interposer and the FAU are substantially parallel to each other, the beam also traverses the thin film stack with substantially the same angle through both, so that it can be made to be an ARC for both. On the other hand, the interposer 100 or FAU 400 can be configured such that the light beam 300 is incident on the reflecting surfaces 105 or 403 with a substantially different angle, so that for these the coating can be made to be reflective, see for example Figure 2(a) in which than angle of the reflector 403 of the FAU 400 is preferably 45 degrees relative to the axis of the guide pins 500, while the angle of the interfaces 106, 402 is preferably chosen between 20 and 70 degrees. This is for example the case if the phase gained by the light across the thin layers forming the thin film stack is an odd multiple of TT / 2 for the reflective surfaces and an even multiple of !2 for the transmissive surfaces. The thickness of deposited layers also depends on the angle of the surfaces in non- confomnal deposition methods such as physical vapor deposition, so that this too can be used to tune the property of the coating across the different surfaces.

[0097] The glass building block of the FAU also has a surface that acts as an optical interface of the connector. Expanding the beam on both sides of this interface has the advantage of increasing spatial alignment tolerances, the scale with the size of the beam. At the same time, this requires more precise angular alignment, but that is easier to maintain in the disclosed configurations. The angular alignment is a function of the length over which the guide pins can be inserted into the guide holes, and can be maintained and even improved relative to commonly used connectors. On the other hand, the spatial alignment accuracy is worse due to the guide holes being outside the cavity and not constrained by it. The ferrule does thus expand in that region at a different rate than the interposer as the temperature is cycled. Nonetheless, the constraining effect of the cavity continues to play an essential role, as it constrains the alignment between the optical fibers of the FAU and the converging optical element of the FAU that generates the collimated or near collimated beam. In particular, it constrains the optical axes of the fibers and the optical axes of the converging elements of the FAU to remain aligned relative to each other. This is essential, as a displacement between these axes would lead to the phase front of the beam arriving at the connector interface to be distorted (the beam would have a modified axis of propagation at that point), which would lead to significant insertion losses. Consequently, the mechanical frame formed by the cavity ensures that the generated beams at the connector interface have a well- controlled direction, while placement of the guide holes outside of the cavity, as constrained by the cavity fabrication process, reduces the control on the spatial positioning. Expanding the beam on both sides of the interface is ideal to deal with these constraints, and is the function fulfilled by the converging elements in both the interposer and the FAU.

[0098] List of reference signs:

[0099] 100 optical interposer

[0100] 101 base of optical interposer 100

[0101] 102 lens in base 101

[0102] 103 protrusion in base 101

[0103] 104 curved reflector in protrusion 103

[0104] 105 top-sided reflector of base 101

[0105] 106 optical interface of base 101

[0106] 107 pedestals on interface 106

[0107] 108 attachment surface of interposer 100

[0108] 109 guide holes of base 101

[0109] 110 groove of base 101

[0110] 111 lid of interposer 100

[0111] 112 mechanical guide unit of interposer 100

[0112] 113 cavity of base 101

[0113] 114 part of guide unit 112 with guide hole 109 or guide pin 500

[0114] 115 mechanical stopper surface of interposer 100

[0115] 200 optical subassembly

[0116] 201 substrate of optical subassembly 200

[0117] 202 surface receiving element of optical subassembly 200

[0118] 203 photonic integrated circuit, PIC, as optical subassembly 200

[0119] 204 waveguide of PIC 203

[0120] 205 surface emitting coupler of optical subassembly 200

[0121] 206 edge coupler of PIC 203

[0122] 300 (first) light beam

[0123] 300B second light beam

[0124] 400 fiber array unit, FAU

[0125] 401 beam transformer of FAU 400

[0126] 402 optical interface of beam transformer 401

[0127] 403 reflector of beam transformer 401

[0128] 404 transmissive surface (lens) of cavity 406

[0129] 404a optical axis of lens 404

[0130] 405 pedestals on optical interface 402

[0131] 406 cavity of beam transformer 401 407 larger attachment surfaces on beam transformer 401

[0132] 408 guide rails in cavity 406

[0133] 409 opening in side wall of cavity 406

[0134] 410 fiber ferrule 411 optical fiber

[0135] 411a optical axis of optical fiber 411

[0136] 412 polished facet of ferrule 410

[0137] 413 protrusion of ferrule 410

[0138] 414 larger attachment surface of ferrule 410 415 guide holes of ferrule 410

[0139] 416 outline of ferrule 410

[0140] 417 strain relief section of ferrule 410, protrusion 413

[0141] 418 anchor section of ferrule 410

[0142] 500 guide pin

Claims

Claims:1 . A fiber array unit, FAU, [400] comprising a beam transformer [401] and a fiber ferrule [410], the fiber ferrule [410] comprising a protrusion [413] and an opening for receiving at least one optical fiber [411], the beam transformer [401] comprising a cavity [406] and an optical interface [402], the cavity [406] being configured for receiving the protrusion [413] of the fiber ferrule [410] and comprising a transmissive surface [404] configured for receiving or transmitting a light beam [300] from or to the optical fiber [411] when the protrusion [413] is inserted into the cavity [406], wherein the beam transformer [401] is configured for forwarding the light beam [300] to or from the optical interface [402], respectively.

2. The FAU of claim 1 , wherein the opening for receiving the optical fiber [411] extends through the protrusion [413] along a longitudinal axis, such that the optical fiber [411] when inserted into the opening terminates at an end surface of the protrusion [413].

3. The FAU of any one of the preceding claims, wherein the beam transformer[401] comprises a focusing optical element configured for focusing the light beam [300] along at least one axis of divergence of the light beam [300],4. The FAU of any one of the preceding claims, wherein the optical interface[402] is configured for receiving or transmitting the light beam [300] from or to an interposer [100],5. The FAU of any of the preceding claims, wherein the fiber ferrule [410] is polished at a surface terminating the optical fiber [411],6. The FAU of any one of claims 1 to 5, wherein the cavity [406] is configured for limiting at least a lateral thermal expansion of the protrusion [413] when the protrusion [413] is inserted into the cavity [406],7. The FAU of claims 1 to 6, wherein the fiber ferrule [410] comprises a guide hole [415] with an axis of insertion, wherein the guide hole runs along that axis of insertion that does not intersect the beam transformer [401] when the protrusion[413] is inserted into the cavity [406].

8. The FAU of claim 7, wherein the axis of insertion of the guide hole [415] of the fiber ferrule [410] forms an angle with the optical interface [402] of the beam transformer [401]that is between 20 and 70 degrees.

9. The FAU of claims 1 to 8, wherein the beam transformer [401] comprises a reflector [403] configured for deflecting the light beam [300].

10. The FAU of claim 9, wherein the axis of insertion of the guide hole [415] forms an angle with the reflector [403] that is between 30 and 60 degrees.11 . The FAU of claims 7 to 10, wherein a guide pin [500] is permanently inserted into the guide hole [415].

12. The FAU of claims 1 to 11 , wherein the fiber ferrule [410] comprises at least two openings for receiving an optical fiber [411], wherein the fiber ferrule [410] and / or the protrusion [413] is divided into two sections, each section comprising at least one of the openings for receiving an optical fiber, and wherein the fiber ferrule [410] and / or the protrusion [413] comprises a strain relief section [417], the strain relief section [417] connecting the two sections of the fiber ferrule [410] and / or the protrusion [413], and wherein a thickness of the fiber ferrule [410] and / or the protrusion [413] is smaller in the strain relief section [417] than in the two sections of the fiber ferrule and / or the protrusion [413] comprising the openings for receiving an optical fiber [411],13. The FAU of claims 7 to 12, wherein the cavity [406] of the beam transformer [401] comprises an opening [409] in a side wall of the cavity [406] , an anchor section [418] of the fiber ferrule [410] traverses the opening [409] and connects the protrusion [413] of the fiber ferrule [410] with a section of the fiber ferrule [410] outside of the cavity [406] in which the guide hole [415] has been fabricated.

14. An optical interposer [100] for mating to the FAU [400] of any one of claims 1 to 13, wherein the optical interposer [100] comprises a guide hole [109] or a guide pin [500], a guide hole [415] of the FAU mates with the guide pin [500] of the interposer or a guide pin [500] of the FAU mates with the guide hole [109] of theinterposer, the position and / or orientation of the guide hole [109] or guide pin [500] being chosen such that, when the guide hole [109] mates with the guide pin [500] of the FAU, or the guide pin [500] of the interposer mates with the guide hole [415] of the FAU, the light beam [300] is coupled between the optical interface [402] and the optical fiber [411] in a predetermined direction .

15. The optical interposer [100] of claim 14, wherein the optical interposer comprises a base [101], the base [101] comprises a beam transforming element [102], [104], [105] and an optical interface [106], the optical interface [106] of the base [101] being configured for mating with the optical interface [402] of the beam transformer [401] of the FAU [400].

16. The optical interposer of claim 15, wherein the guide hole [109] or the guide pin [500] has an axis of insertion, the optical interface [106] of the base [101] forms an angle with the axis of insertion that is between 20 and 70 degrees.

17. The optical interposer of claim 16, wherein the optical interface [106] of the optical interposer [100] and the optical interface [402] of the FAU [400] are substantially parallel when the two are mated.

18. The optical interposer of any one of claims 14 to 17, wherein the optical interposer [100] comprises a base [101], the base [101] comprises a groove [110], the guide pin [500] is inserted in and attached to the groove [110],19. The optical interposer of any one of claims 14 to 17, wherein the optical interposer [100] comprises a base [101] and a lid [111], the lid [111] comprises a groove [110], the guide pin [500] is inserted in and attached to the groove [110],20. The optical interposer of any one of claims 14 to 17, wherein the optical interposer [100] comprises a base [101] and a mechanical guide unit [112], the mechanical guide unit [112] comprise the guide pin [500] or the guide hole [109],21. The optical interposer of claim 20, wherein the mechanical guide unit [112] is made out of a different material than the interposer base [101], a coefficient of thermal expansion of the material of the mechanical guide unit [112] is larger than a coefficient of thermal expansion of the material of the base [101] of the interposer[100],22. The optical interposer [100] of any one of claims 20 or 21 , wherein the base [101] comprises a cavity [113] and the mechanical guide unit [112] comprises a part [114] with the guide hole [109] or the guide pin [500], this part [114] of the mechanical guide unit [112] form-fits the cavity [113] of the base [101] of the interposer [100], so that the cavity [113] of the base [101] constrains a thermal expansion of the part [114] with the guide hole [109] or the guide pin [500] when the part [114] is inserted into the cavity [113] of the base [101].

23. The optical interposer of any one of claims 20 to 22, wherein a guide pin [500] is permanently inserted into the guide hole [109] of the interposer [100].

24. The optical interposer of any one of claims 14 to 23, wherein the optical interposer [100] comprises a base [101], the base [101] comprising a top-sided reflector [105].

25. The optical interposer [100] of claim 24, wherein the top-sided reflector [105] and an axis of insertion of the guide hole [109] of the interposer [100] or of the guide pin [500] form an angle between 30 and 60 degrees.

26. An optical arrangement comprising an optical subassembly [200], the optical interposer [100] of any one of claims 14 to 25, and the FAU [400] of any one of claims 1 to 13, wherein the FAU [400] is mated with the optical interposer [100] and the optical interposer [100] is attached to the optical subassembly [200], such that the light beam [300] is coupled between the optical subassembly [200] and the FAU [400],27. The optical arrangement of claim 26, wherein the optical interposer [100] comprises an optical interface [106] to the FAU [400] and a lens [102], the lens [102] is facing towards the optical subassembly [200] and configured for coupling or receiving the light beam [300] to or from a surface coupler [205] or a surface emitting or surface receiving element [202] of the optical subassembly [200],28. The optical arrangement of claim 26, wherein the optical subassembly [200] comprises a photonic integrated circuit, PIC, [203], the optical interposer [100]comprising an optical interface [106] to the FAU [400] and a protrusion [103], the protrusion [103] being located on a side of the interposer [100] facing towards the optical subassembly [200] and configured for coupling or receiving the light beam [300] to or from an edge coupler [206] of the PIC [203].

29. The optical arrangement of claim 26, wherein the optical interposer [100] comprises a base [101], the base [101] comprising an optical interface [106] to the FAU [400] and a top-sided reflector [105], the top-sided reflector [105] being located on a side of the base [101] facing away from the optical subassembly [200], wherein the top-sided reflector [105] is curved and configured for focusing or reducing an angle of divergence of the light beam [300] along at least one axis of divergence of the light beam [300].

30. The optical arrangement of claim 26, wherein the optical subassembly [200] comprises a surface coupler [205], or is surface-emitting or surface-receiving, wherein the optical arrangement is configured to couple the light beam [300] to or from the optical subassembly [200] to the optical interface [402] of the FAU [400], such that the light beam [300] is transformed in between the optical subassembly [200] and the optical interface [402] of the FAU [400] such that the light beam [300] has a substantially larger beam diameter at the optical interface [402] of the FAU [400] than at the optical subassembly [200], and wherein at least one of the interposer [100] or the beam transformer [402] comprises a reflector [105, 403],31 . The optical arrangement of any one of claims 26 to 30, wherein the optical arrangement is configured to constrain a position of the FAU [400] relative to the interposer [100] in two directions by mating of a guide hole [109], [415] with a guide pin [500], and wherein a surface [402], [403] of the FAU [400] is configured to abut against a surface [106], [115] of the interposer [100], thereby constraining the position of the FAU [400] relative to the interposer [100] in the third direction.

32. The optical interposer [100] of any one of claims 14 to 25, wherein the optical interposer [100] is configured for coupling the light beam [300] between an optical subassembly [200] and the FAU [400], wherein the optical interposer [100] comprises a reflector [105] and an optical interface [106] to the FAU [400], wherein the FAU [400] comprises an optical interface [402] to the interposer, a reflector [403], and two openings each configured for receiving an optical fiber [411], [411B]stacked on top of each other, wherein the interposer [100] is configured to couple the light beam [300] between the optical subassembly [200] and a first optical fiber [411 ] to be received in one of the two openings, as well as a second light beam [300B] between the optical subassembly [200] and a second optical fiber [411B] to be received in the other one of the two openings, such that• the light beam [300] is coupled from the optical subassembly [200] to the reflector [105] of the interposer [100], from the reflector [105] of the interposer [100] to the optical interface [106] of the interposer [100], from the optical interface [106] of the interposer [100] to the optical interface [402] of the FAU [400], from the optical interface [402] of the FAU [400] to the first optical fiber [411] to be received in one of the two openings, or in inverse direction; and• the second light beam [300B] is coupled from the optical subassembly [200] to the optical interface [106] of the interposer [100], from the optical interface [106] of the interposer [100] to the optical interface [402] of the FAU [400], from the optical interface [402] of the FAU [400] to the reflector [403] of the FAU [400], from the reflector [403] of the FAU [400] to the second optical fiber [411 B] to be received in the other one of the two openings, or in inverse direction.

33. The interposer of claim 32, wherein the light beam [300] and the second light beam [300B] cross the optical interfaces [106], [402] of the interposer [100] and of the FAU [400] with directions that form an angle to each other of at least 50 degrees.

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