WO2007076888A1

WO2007076888A1 - Optical coupling device

Info

Publication number: WO2007076888A1
Application number: PCT/EP2005/014238
Authority: WO
Inventors: Bogdan Rosinski; Gnitaboure Yabre
Original assignee: Fci
Priority date: 2005-12-30
Filing date: 2005-12-30
Publication date: 2007-07-12

Abstract

The invention relates to an optical connector assembly for optically connecting at least one waveguide structure (3) comprised in a layer stack (1) to an external waveguide structure, said connector assembly comprising a coupling device (8) contributing to at least one first optical path (6), said waveguide structure (3) comprising at least one optical waveguide (2) providing at least one second optical path (7) deflecting from said first optical path (6) through deflecting means (14, 17, 46), wherein at least three raised contact areas (9) are provided on said coupling device so that said coupling device lies on said at least three raised contact areas when said coupling device is assembled with said layer stack.

Description

OPTICAL COUPLING DEVICE

The invention relates to an optical connector assembly for optically connecting at least one waveguide structure comprised in a layer stack, such as a PCB, to an external waveguide structure.

The invention further relates to connector assembly comprising a coupling device providing at least one first optical path, the waveguide structure comprising at least one optical waveguide providing at least one second optical path deflecting from the first optical path through deflecting means, the coupling device comprising first positioning means adapted to cooperate with second positioning means in the layer stack when the coupling device is assembled with the layer stack.

The invention further relates to a coupling device for use in such an optical connector assembly and to a method for assembling a coupling device to a waveguide structure.

In a backplane such as a printed circuit board (PCB), optical waveguides are generally used for providing an optical path to transmit optical signals over the backplane. Such waveguides are generally integrated or embedded in the backplane. The embedded waveguide comprises a core in which the optical signal propagates. The core may be obtained by etching of a thin glass layer, when glass is used. The core may also be a transparent polymer core. The core itself is surrounded by a polymer or glass cladding. The cladding has a proper refractive index compared to the core in order to let the light confined within the core. The PCB is then laminated over the waveguide with an opening or cutout left to access the embedded waveguides and position the coupling device.

Optical backplane interconnects may be needed to optically couple the PCB to optical, optoelectrical devices or other PCB's such as daughterboards, or any other optical components with their own optical path. Optical backplane interconnects consist in a connector assembly that ensures continuity between optical paths so that optical signals can be transmitted from the PCB to the optical component or vice versa. One major difficulty in optical coupling comes from the necessity to transfer the optical signals out of the waveguide within the PCB to the optical path from the other optical component. That latter optical path has in most instances a different direction. In order to reduce optical loss resulting from the coupling, proper alignment of the optical paths is of the utmost importance.

A connector assembly is already known from WO2004/015474, with deflecting means such as a mirror mount or a deflecting layer facing the waveguides. The positioning means, provided to align the coupling device with the PCB, consist of a plurality of small guide pins provided on the coupling devices cooperating with corresponding guide holes provided on a cutout formed in the PCB or layer stack.

The coupling device is generally assembled within the PCB cutout through an automated process. After the coupling device and the PCB cutout are aligned, e.g. by means of pre-alignment marks, the guide pins are assembled within the guide holes through applying a given pressure to the assembly.

With the known connector assembly, the external diameter of the pins is small, and must support the coupling device weight, and applied pressure from the assembly process. This may generate plastic deformation in the pins, and significant local stress within the layer stack, resulting in irreversible damage to the waveguide structure. Furthermore the plurality of pins can cause misalignment of one or more pins with its corresponding guide hole when tolerances are not reached during manufacturing. This misalignment can also generate damage when the misaligned pin is forced into its guide hole during the assembly process.

Another problem with the known assemblies is the contact surface between the coupling device and the layer stack. The coupling surface ought to comply with high constraints regarding surface roughness and flatness on its whole mating surface. It is an object of the invention to provide an optical connector assembly that improves the optical coupling between a waveguide structure and a coupling device, generating reduced stress levels within the layer stack, and thus ensuring the waveguide integrity during assembly. It is another object of the present invention to limit, at least partially, the problem of misalignment of the positioning means.

It is a further object of the present invention to provide an optical connector assembly that is easy to manufacture and assemble, with limited constraints on the coupling device surface roughness and flatness.

These objects are achieved by providing an optical connector assembly according to claim 1. These objects are also achieved by providing an assembly method according to claim 15.

The invention takes advantage of the plurality of raised contact areas, at least three, to distribute the coupling device weight as well as the assembly stresses over known contact surfaces. Furthermore, the number of raised contact areas allows a balanced and isostatic solution.

The embodiments of the invention will be described into more detail below with reference to the attached drawing of which Figs. 1A, B show_. the optical connector assembly according to the invention;

Figs. 2 shows a 3D perspective view of the coupling device of the connector assembly according to the invention;

Figs. 3 show the assembled optical connector assembly according to the invention;

Figs. 4A-G schematically illustrate various approaches to deflect an optical signal between optical paths; along with the positioning of the enlarged locating pin;

Fig. 5 illustrates an exemplary use of the connector assembly according to the invention to connect a PCB to daughter boards.

As can be seen in Fig. 1 B and Fig. 4A-F, an optical backplane or PCB 1 is a layer stack comprising several layers. Hereinafter this backplane 1 will also be referred to as layer stack 1.

Referring to Fig. 5, the layer stack 1 comprises at least one optical waveguide 2 which may be part of a waveguide structure 3 comprising multiple waveguides 2 (see Fig. 1A and 1B). The waveguide 2, as well as the waveguide structure 3 extend in at least one x-y plane of the layer stack, which is the plane perpendicular to Fig. 1B. Waveguide 2 or waveguide structure 3 is integrated or embedded in at least one of the layers of the layer stack 1. Embedded waveguides 2 may be polymer waveguides, glass sheet waveguides or waveguides obtained by embedded fiber technology.

Optical signals, transferred to or from a mating optical device, such as an optical device or optoelectrical device 4 or an other PCB 5, generally called daughter board, are provided over a first optical path 6 (and oriented in the z direction on Fig. 5) to the waveguide 2 of the layer stack 1. Waveguide 2 provides a second optical path 7 (oriented in the x direction) to the optical signal. In order to achieve an optimal optical coupling between the first and second optical paths - perpendicular to each other in different embodiments illustrated here after - a coupling device 8 is provided for alignment purposes. Coupling device 8 is part of the first optical path 6.

The coupling device 8 is used to receive an external optical connector 84 or 85 on Fig. 5 to respectively optically link device 4 and PCB 5 to layer stack 1. To that effect, coupling device 8 further comprises third positioning means 22 in order to align an optical connector such as external connector 84 or 85 with said coupling device. As seen in Fig. 1A, third positioning means comprises guiding holes 22 that may extend in the layer stack 1 through recesses 23. The mating optical connector is then provided with fourth positioning means such as pins adapted to cooperate with said third positioning means to align the coupling device with the external connector. Other possible positioning means may be used such as latching means or else.

In Fig. 1A and 1 B, an exemplary embodiment of the optical connector assembly according to the invention is displayed. Layer stack 1 comprises a waveguide structure 3 having various waveguides 2. Waveguides 2 comprise a top cladding layer 10, a waveguide core 11 and a bottom cladding layer 12. Cladding layers 10 and 12 may be present outside the waveguide structure 3 or may as well be a part of the layer stack 1. Waveguides 2 present a discontinuity consisting in facets that comprise deflecting means such as a deflecting layer 14 to deflect the optical signal from the first to the second optical path, or the other way around. Other possible deflecting means are illustrated in Fig. 4A-G and will be detailed later on. The optical connector assembly further comprises a coupling device 8 providing or contributing to a first optical path via an area 18. The area 18 may comprise a cavity and may be used to employ one or more optical components 15, such as a lens array for optimizing the optical coupling of the optical signals of the waveguides 2. The lens or lens array may be provided in the cavity as a separate component or may form an integral part of the coupling device 8. The area 18 may as well be a region transparent to the optical signals transmitted over the first optical path. The application of one or more optical components 15 may enhance or optimize the optical coupling between the two optical paths. Lenses or lens arrays can e.g. be used to collimate diverging light beams thereby avoiding optical loss.

As can be seen in Fig. 4C to 4G, the layer stack may comprise a top insulating layer 5 covering the waveguide structure, and cladding 10. A cutout is then provided around the deflecting means to receive the coupling device in said cutout. The insulating layer may not be needed as seen in Fig. 1 A and 1 B. The coupling device is generally manufactured using a micro-molding process, using a transparent material, such as cyclo-olefine copolymer (COC) or Polymethylmethacrylat (PMMA).

In the embodiment depicted in Fig. 1A and 1B, the coupling device 8 comprises a first positioning means 19 used for assembling coupling device 8 in the x-y plane with the waveguide structure 3 of the layer stack 1. A second positioning means 20 is provided on the layer stack 1 , and is adapted to cooperate with first positioning means 19 when the coupling device 8 and the layer stack 1 are assembled. In order to reduce the stress levels generated on the layer stack 1 from the coupling device weight as well as the assembly process, a plurality of raised contact areas 9, as seen on Fig. 2, is provided on the lower face - or mating face - of the coupling device 8. As the raised contact areas are to be laid upon the layer stack 1 , they are of the same height measured from the coupling device lower face, and present a flat top section with a low surface roughness. The contact areas top section may be of a rectangular or circular shape. Their dimensions will be detailed later on. To avoid any buckling or unwanted deformation of the coupling device, at least three raised contact areas are provided on coupling device 8, so that the coupling device 8 lies on the at least three raised contact areas when the coupling device is assembled with the layer stack 1. When three contact areas are provided, one may be located on the middle - or in the vicinity of the middle - of one edge of the coupling device lower face (opposite the first positioning means 19 from the cavity 18 for example), while the two others are locating on the opposite edge close to the lower face corners. When four contact areas are provided, as in the example of Fig. 2, two may be located next to the first positioning means 19, i.e. in the vicinity of the middle of the coupling device first edge, while the two others are located on the opposite edge next to the lower face corners.

On top of reducing the stress levels generated in the contact areas of the assembly, the raised contact areas allows to reduce the surface constraints on the coupling device. As the rest of the coupling device lower face does not come into contact with the layer stack 1 , the surface roughness of the lower face may be high as the constraint is transferred to smaller surfaces of the raised contact areas 9. The same comment may be made about the flatness constraint that may only be very high for the limited surfaces of the raised contact areas while the constraint is reduced for the coupling device lower face.

Thanks to these contact areas, the stress levels, the contact points on the layer stack, as well as the deformation and risk of buckling are kept within desired levels.

Optionally, as an additional way to reduce the stress levels generated on the stack layer when the positioning means 19 and 20 cooperate, either one of said positioning means comprises an enlarged locating pin. This enlarged locating pin 19 can be provided on the coupling device 8 as seen on Fig. 1A or

2. The corresponding positioning means 20 in the layer stack comprises an enlarged guiding hole, as seen in Fig. 1A, with a form adapted to receive the enlarged locating means.

In an alternate embodiment according to the present invention, the enlarged locating pin can be provided on the layer stack 1. As forming the waveguide is the subtractive process when glass waveguides are used (etching), copper growth on glass can be used to form the enlarged locating pin. The enlarged guiding hole is then provided on the coupling, device.

Whether the enlarged locating means is provided on the coupling device or the layer stack, the second positioning means on the layer stack 1 preferably extends in the z direction, i.e. the direction perpendicular to the x-y plane. The first positioning means extend accordingly, i.e. perpendicular to the coupling device lower face as seen in Fig. 1A and B in particular.

Furthermore, the enlarged locating pin is preferably provided at the vicinity of the deflecting means, i.e. close to the waveguide discontinuity in the assembly configuration, where the deflection takes place. In the exemplary embodiment of Fig. 1A and 1B, the enlarged locating pin is located next to the deflecting layer 14 opposite the waveguides discontinuity. Locating the enlarged locating pin close to the deflecting means, 'allow an improved alignment of the optical paths. In an advantageous embodiment of the present invention, the enlarged locating pin presents a substantially beveled or rounded edge (as seen on Fig. 1B) at the tip of the pin, in the x-y plane, i.e. extending perpendicular to the pin axis. The corresponding enlarged guiding hole extends further into the coupling device 8 or the layer stack so that the tip of the pin and the bottom of the enlarged guiding hole do not come into contact. Thus the weight of the coupling device and the pressure force exerted during assembly is not distributed over the bottom of the guiding hole 20. When the guiding hole 20 is provided in the layer stack 1 as in the example of Fig. 1B, the guiding hole 20 may run through the upper cladding layer 10 well into the waveguide core 11. The beveled or the rounded edge of the tip of the locating pin allows an easy insertion of the enlarged pin into the guiding hole 20, as the locating pin can slide on the lateral edges of the guiding hole in the early stage of the insertion. This characteristic is especially useful when the positioning means 19 and 20 are slightly misaligned with each other. Stress on the layer stack 1 through the guiding hole edges is thus reduced as the insertion is facilitated.

As seen in Fig. 2, the enlarged locating pin has preferably a substantially circular section. Such a shape may allow the coupling device and the layer stack to rotate with regard to each other. To avoid this degree of freedom, first anti-rotation means 31 are provided on the coupling device along with second anti-rotation means 30 provided in said layer stack, the first and second anti-rotation means being adapted to cooperate with each other when said coupling device 8 is assembled with the layer stack 1. The anti-rotation means may comprise at least one small pin and a fitting hole, provided apart from the positioning means. When the coupling device 8 is of a rectangular shape, it may be provided on one corner of the device, as seen on Fig. 2. As before, the small pin may be provided either on the coupling device or the layer stack. The resulting assembly can be seen on Fig. 3. Coupling devices are generally of a rectangular shape, with sides of a few mm to a few tens of mm long. A typical coupling device may present a 5mm x 8mm rectangular face (that carries either the first or second positioning means) and a 1-2mm thickness.

For such a coupling device, the enlarged locating pin may have a diameter of 0.8 to 1 mm. More generally the enlarged locating pin ought to have a diameter Φ of about 1/20 to 1/5 of the diagonal length of the coupling device rectangular face. The enlarged locating pin is generally of a few tens of μm long, typically 5 to 15 μm, and does not necessarily need to run deep into the layer stack 1 in the assembly configuration. The guiding hole is slightly longer than the locating pin so that the tip of the pin does not come into contact, as seen before, with the bottom of the guiding hole.

Regarding the raised contact areas, their top section may be of a rectangular or circular shape, with respectively sides or diameter of about 1mm. More generally, the contact areas sides or diameter may be of about 1/20 to 1/5 of the diagonal length of the coupling device, as for the enlarged locating pin. As for the raised contact areas may be of a few μm to a few tens of μm long, typically 2 to 10 μm.

A more accurate positioning of the coupling device with regard to the layer stack is also achieved thanks to the optical connector assembly according to the invention. Pre-alignment means are indicated with 29 on the coupling device 8 and with 29' on the layer stack 1 , also shown in Fig. 1A and 2. Pre- alignment of the coupling device with the PCB can be achieved through i.e. visual marks or chamfers provided on these two parts. In the example illustrated in Fig. 1B and 2, the pre-alignment means appear as visual marks in the form of a cross and four dots.

With optical connector assemblies, aligning precisely the coupling device with the layer stack is a costly process. The more precise the assembly machine is, the more significant the investment will be. With the connector assembly according to the invention, as the coupling device may be made with a transparent material, the coupling device and the layer stack are first aligned with each other through substantially matching the pre-alignment marks 29 with their corresponding marks 29'. A camera can be used to supervise the alignment of the marks 29 and 29' through the transparent coupling device.

This first step is the x-y alignment, in the plane of the layer stack and coupling device (see Fig. 1a).

When a first and a second positioning means are implemented, a not so precise assembly machine may be sufficient to achieve such a pre- alignment of the coupling device in front of the layer stack or PCB within a 10- 50 μm range, and generally 10-20 μm. The remaining misalignment in the x-y plane can be overcome thanks to the beveled or rounded edge of the tip of the locating pin 19. Indeed it allows an easy insertion - in the z direction - of the enlarged locating pin into the guiding hole as the tip of the pin 19 can slide into said hole. The dimensions of the beveled or rounded tip may be adapted according to the assembly machine alignment precision.

The association of the pre-alignment marks with the beveled or rounded tip of the locating pin allows an easy assembly with limited stress levels supported by the layer stack during assembly. Thanks to raised contact areas according to the invention, accurate positioning of less than 1 μm on z-axis can be achieved. With the pre-alignment marks and locating pin, a positioning of less than 5 μm on x-y plane can be reached.

A gluing step may be carried out to allow the attachment of both parts of the assembly and protection of the optical pathway from dust and humidity.

The coupling device 8 may be glued to the layer stack, using for example an adhesive transparent resin. The glue may be laid out on the outline of the coupling device mating face, for example along groove 51 provided at the periphery of the coupling device (as seen in FIG. 2). It may also be laid out on the layer stack. An anti-glue barrier (not shown in the figures) may be provided on the layer stack so as to avoid any migration, through i.e. capillarity, of the glue towards the waveguide that could result in an optical path pollution. In order to enable the optical signal to be deflected between the first optical path oriented in the z-direction and the second optical path oriented in the x-direction, various approaches can be taken. Fig. 1B illustrates an exemplary approach. Other examples of such approaches are schematically illustrated in Figs. 4A-G. It is noted that although only perpendicular deflection is illustrated, other deflection angles are possible as well. The arrows indicate the optical signals.

The waveguide 2, being part of layer stack 1, comprises a top cladding layer 10, a waveguide core 11 and a bottom cladding layer 12. Layer stack 1 may comprise various other layers below and/or above the waveguide 2. Such a layer is e. g. copper layer 13, deposited below the waveguide 2. Moreover the layer stack 1 may comprise several waveguides 2 or waveguide structures 3 in the z-direction. The man skilled in the art can easily adapt solutions of Fig. 4A- F to a layer stack with multiple waveguides in the z direction.

In Figs. 4A-D which are illustrations of the possible locations for the deflecting means, the deflection of optical signals between the first optical path in the z direction and the second perpendicular optical path in the x direction may be achieved by employing a deflecting layer 14 on a facet of the waveguide structure 3. Although, for sake of clarity, the raised contact areas are not represented, the coupling device shown comprises said raised areas. Figs. 4A and 4C show that deflecting layer 14 can be applied such that optical signals exhibit internal reflection, i.e. the optical signals are substantially confined in the waveguide 2 upon deflection at the deflecting layer 14. In Fig.

4C the internally layer 14 is combined with a coupling device 8 according to the invention, and comprising an optical component 15 such as a lens or lens array. As can be seen on Fig. 1B, optical component 15 can be provided on both sides of the coupling device 8. An enlarged locating pin -19 is on the coupling device face that is mated with the layer stack 1 , so that the pin 19 is next to the waveguide 2 transmitting the optical signal. On the right hand side of Fig. 4C₁ waveguide 2 is not part of the optical path as the optical signal has been diverted beforehand thanks to deflecting layer 14. In an alternate embodiment, a locating pin 19' - instead of locating pin 19 - can be provided on the side of the coupling device 8 opposite the waveguide portion that is not part of the optical path. As that waveguide portion does not participate in the optical signal propagation, the reduced stress generated by the assembly will have little effect on the optical path integrity.

Figs. 4B and 4D show the variant wherein the deflecting layer 14 is applied such that the optical signals exhibit external reflection, i.e. reflection takes place outside of the waveguide 2. The space 16 is preferably filled with a material having the same refractive index as the waveguide core. Again an optical component 15 can be applied as a part of the coupling device 8, as shown in Fig. 4D. Additionally, an enlarged locating pin 19 is provided on the coupling device 8 and faces in mated position the waveguide portion that does not participate in the optical path, as in Fig. 4C.

Figs. 4E and 4F illustrate another approach for deflecting optical signals between a first optical path (in the z direction) and a second optical path

(in the x direction) over ninety degrees. Here, a mirror mount 17 is used that can be placed within the first and second optical path. Mirror mount 17 refers to any component able to deflect an optical signal and thus may refer e.g. to a grating or a prism as well. Waveguide 2 is adapted to accommodate this mirror mount 17 through an opening 160 provided on said waveguide. The mirror mount 17 may lean upon the bottom of opening 160. The space 16 through which the optical signal travels preferably is filled with an index matching material once more. The mirror mount 17 can be integrated with the coupling device 8 comprising an optical component 15 such as a lens or lens array, as shown in Fig. 4E. An enlarged locating pin 19 may be provided on top of the second optical path, like in Fig. 4C. It may also be provided on the face of mirror mount 17 in contact with the bottom of the opening 160 as shown in Fig. 4E.

As shown in Fig. 4F, the mirror mount 17 can be a separate component integrated to the layer stack 1. An enlarged locating pin 19 can be provided on the part of coupling device 8 that lies upon the mirror mount 17. In both instances of Fig. 4E and 4F, the mirror mount 17 may also be used without the optical component 15.

Fig. 4G illustrates a waveguide structure comprising in the z direction at least 2 stacked waveguides, with their core 11 and respective claddings 10, 10' and 10". Cladding 10' may be common to both cores 11. An opening 160 is provided within the layer stack 1 to accommodate the coupling device 8 that comprises a lower part extending in said opening 160. Lower part of coupling device is made of a material transparent to the transmitted optical signals. It also comprises deflecting curved surfaces 46. Thus, optical signals that exit cores 11 can be transmitted through faces 45 to impact a deflecting surface 46 within the lower part. A 90° deflection occurs on deflecting surfaces 46 so that optical signals can reach lens arrays 15 of coupling device 8. To each core corresponds a deflecting surface 46 and a lens array 15 so that all waveguides in the z direction can transmit optical signals to as many optical paths in the z direction. As in Fig. 4D, an enlarged locating pin 19 is provided on the portion of coupling device 8 that lies upon the waveguide structure that does not transmit the optical signals.

In all instances of embodiments of Fig. 4, when an enlarged locating pin is provided, it is preferably located in the vicinity of either the first or second optical path. If all illustrations include a coupling device carrying the enlarged locating pin, the man skilled in the art can easily transpose these embodiments to a layer stack carrying the enlarged locating pin.

Claims

1. An optical connector assembly for optically connecting at least one waveguide structure (3) comprised in a layer stack (1 ) to an external waveguide structure, said connector assembly comprising a coupling device (8) contributing to at least one first optical path (6), said waveguide structure (3) comprising at least one optical waveguide (2) providing at least one second optical path (7) deflecting from said first optical path through deflecting means (14, 17, 46), wherein at least three raised contact areas (9) are provided on said coupling device so that said coupling device lies on said at least three raised contact areas when said coupling device is assembled with said layer stack.

2. A connector assembly according to the previous claim, wherein the coupling device presents a mating face with a substantially rectangular shape, a first raised contact area being located in the vicinity of the middle of a first edge of said coupling device, while two raised contact areas are provided on a second edge opposite said first edge close to the corners of said mating face.

3. A connector assembly according to the previous claim, wherein a fourth raised contact area is provided on the coupling device, in the vicinity of the first raised contact area.

4. A connector assembly according to one of the previous claims, wherein the raised contact areas are of the same height measured from the mating face of the coupling device, and present a flat top surface with a low surface roughness.

5. A connector assembly according to one of the previous claims, wherein the raised contact area height is 2 to 10 μm.

6. A connector assembly according to one of the previous claims, wherein the layer stack comprises an insulating layer covering the waveguide structure, a cutout being formed in said insulating layer around the deflective means, said cutout being adapted to receive the coupling device.

7. A connector assembly according to one of the previous claims, wherein the coupling device comprising first positioning means (19) adapted to cooperate with second positioning means (20) in the layer stack when said coupling device (8) is assembled with said layer stack (1 ), either one of said first and second positioning means comprising an enlarged locating pin within the vicinity of the deflecting means.

8. A connector assembly according to claim 7, wherein the enlarged locating pin has a substantially circular section, and wherein the coupling device comprises first anti-rotation means adapted to cooperate with second anti- rotation means in said layer stack when said coupling device is assembled with said layer stack, so that said coupling device and said layer stack cannot rotate with regard to each other.

9. A connector assembly according to claims 7 or 8, wherein the enlarged locating pin is formed on said coupling device, the second positioning means comprising an enlarged guiding hole formed in the stack layer and adapted to cooperate with said locating pin.

10. A connector assembly according to claims 7 or 8, wherein the enlarged locating pin is formed in the layer stack, the first positioning means comprising an enlarged guiding hole formed on said coupling device and adapted to cooperate with said locating pin.

11. A connector assembly according to one of the previous claims 7 to 10, wherein the waveguide structure is comprised in at least one x-y plane of the layer stack, and the second positioning means extends in a direction perpendicular to said x-y plane.

12. A connector assembly according to one of the claims 7 to 11, wherein the enlarged locating pin presents a substantially beveled or rounded edge at the tip of the pin.

13. A connector assembly according to one of the claims 7 to 12, wherein the enlarged guiding hole extends into the coupling device or the layer stack deep enough so that the tip of the coupling device does not come into contact with the bottom of said guiding hole when said coupling device is assembled with said layer stack

14. A connector assembly according to one of the previous claims 7 to 13, wherein the coupling device presents a substantially rectangular face carrying the first positioning means, and wherein the enlarged locating pin has a diameter that is 1/20 to 1/5 the diagonal length of said rectangular face.

15. A method for optically assembling a connector assembly according to one of the claims 1 to 6, comprising the steps of: a) aligning together pre-alignment marks (29) provided on the coupling device with pre-alignment marks (29') provided on said layer stack, b) mounting the coupling device on the layer stack c) attaching said coupling device on said layer stack.

16. A method according to previous claim, wherein the pre-alignment marks are either visual marks or chamfers.

17. A method according to claims 15 or 16 for optically assembling a connector assembly according to claims 7 to 14, comprising the step of aligning said first positioning means with said second positioning means prior to the step b).