Patent Application
20110013870
This application claims priority to European Patent Application No. 09 165 438.4 filed with the European Patent Office on Jul. 14, 2009, the entirety of which is incorporated by this reference.
1. Field of the Invention
The present invention relates to the field of optical fibres. It concerns more particularly a bidimensional device for coupling a bundle of optical fibres.
2. State of the Art
Coupling devices of this type are known to the person skilled in the art. They are intended for transmitting a light signal between one bundle of optical fibres and another bundle of optical fibres, or one or more, active or passive, optical or optoelectronic components such as connectors, light emitting or laser diodes, etc.
In order to minimise signal losses at the input or at the output of the optical fibres caused by optical misalignment, by retroreflection or another optogeometrical effect, the coupling device has to precisely align the axis of the optical fibres and avoid refractive index discontinuity at the interface between the core of the optical fibre and the external environment. The coupling device has, accordingly, a mechanical function of positioning and fixing the optical fibres and an optical function of continuity of the index of refraction. Moreover, the overall size of the coupling device must be as small as possible for obvious practical reasons and in order to comply with the increasing miniaturisation of optoelectronic devices.
Document U.S. Pat. No. 6,328,482 discloses a coupling device meeting these various requirements. Said device is formed by a first sheet made of silicon intended for positioning and for fixing a plurality of optical fibres in a plane and by a second sheet which is made of borosilicate glass and joined to the first by a face. The first sheet comprises a bidimensional network of through-holes, having a diameter adjusted to the diameter of the optical fibres. Typically, the diameter of the holes is of the order of 127 micrometres for a diameter of the optical fibres of 125±0.5 micrometres. The second sheet is equipped with a bidimensional network of lenses on its free face, the lenses being aligned with the holes. The compactness of the device is imparted by the bidimensional character of the coupling device.
The networks of holes and lenses are produced by processes well known to the person skilled in the art, of photolithography and reactive ion etching. The two sheets are aligned relative to each other with the aid of alignment structures etched at the same time as the holes and the lenses, then joined together by anode welding. The optical fibres are then engaged with the holes until their bevelled end enters into contact with the second sheet. The residual space around the fibre is filled up with an optical glue having an index of refraction close to that of the core of the fibre.
The device thus described ensures the positioning of the fibre owing to the hole, and the fixing thereof and also the refractive index continuity, with the aid of the glue.
The precision of the positioning of the fibre is provided by the maximum angle formed between the axis of the fibre and the normal to the second sheet. This angle is dependent on the height of the hole, on its diameter and on the error on the diameter. As mentioned hereinbefore, the diameter of the hole is 127 micrometres and the thickness of a sheet of silicon is typically 400 or 625 micrometres. With these values, it would appear that the precision of the positioning of the fibre in the device according to the prior art is, at best, 0.2 degrees. This value is satisfactory, but in practice, given the limits imposed by reactive ion etching techniques, it is unachievable. That is to say, a hole having a constant diameter of 127 micrometres can be formed only over a thickness of about 50 micrometres. Beyond that, the hole is flared and its lateral dimensions are not preserved. The useful height of the hole is then just 50 micrometres. As a result, the precision of axial positioning of the fibre according to the prior art is closer to 3! This precision is wholly insufficient.
The low useful height of the hole relative to the diameter of the fibre also produces a second potentially very serious limitation: The small gluing surface area gives rise to low resistance to the lateral stresses which can be exerted on the fibre during or after gluing, for example during handling. Thus, a lateral force applied, for example, at 1 centimetre from the gluing point of the fibre exerts, at this point, a shear stress higher than that withstood by the best optical glue! This weakness relative to lateral forces is amplified when use is made of fibres preassembled in a ferrule, of the sort of a metal, resin or ceramic tube intended to mechanically protect the fibre. A ferrule has a diameter of typically 1.25 mm; thus, the ratio between the diameter of the parts to be positioned and the useful height of the hole becomes even more disadvantageous.
Moreover, it will be noted that the use of optical glue for the double function of fixing the fibre and of continuity of the index of refraction can potentially cause signal losses. That is to say, the coefficient of thermal expansion of the silicon, which forms the first sheet, and of the borosilicate glass, which forms the second sheet, is 3.5 μm.deg−1.m−1. This same coefficient has a value of 0.5 μm.deg−1.m−1 for the quartz forming the cladding of the optical fibre. Such a difference of coefficients of thermal expansion generates major stresses on the glue in the case of variations in temperature. Over time, these stresses can cause delamination of the optical fibre and rupture of the index of refraction at the interface between the core of the fibre and the external environment. The signal losses are then considerable.
The present invention improves the prior art by providing a bidimensional coupling device for optical fibres that is capable of positioning optical fibres with a precision and a reliability higher than those conventionally obtained.
More particularly, the invention concerns a coupling device for optical fibres comprising a first sheet equipped with a plurality of through-holes, of which a portion, at least, has a diameter adjusted to the diameter of the optical fibres, and a second sheet joined to the first sheet by a face. According to the invention, the coupling device also comprises a third sheet, joined to the first sheet, and equipped with a plurality of through-holes, of which a portion, at least, has a diameter adjusted to the diameter of the optical fibres, aligned relative to the holes of the first sheet, so as to form recesses with which the optical fibres are engaged.
Owing to the third sheet joined to the first sheet, the positioning of the optical fibres, relative to the normal to the second sheet, is improved due to a more favourable ratio of the diameter of the recesses to their height.
In an advantageous embodiment, the device also comprises a spacer, arranged integrally between the first and third sheets, so as to hold them apart and parallel, the recesses then comprising an initial section and a terminal section set apart from each other by the height of the spacer.
Owing to the spacer, the aforementioned ratio is even more favourable, allowing the precision of the positioning of the optical fibre in its recess to be improved still further.
In a particularly advantageous embodiment, the spacer forms a tightly closed volume with the first and third sheets, the optical fibres are fixed in the recesses with the aid of a dose of glue injected into the initial section so as to form, with the optical fibre, a tight-fitting stopper in the initial section and the volume is filled with an optical fluid compound having an index of refraction close to that of the core of the fibre.
Owing to this configuration, the function of fixing the fibre is performed by glue and the function of continuity of the index of refraction is performed by the fluid which can absorb the expansions of the fibres and sheets without being subjected to stress. The device is accordingly exempt from the problem of optical losses due to the delamination of the optical fibre.
Other features and advantages of the present invention will become clearer from the following detailed description of an exemplary embodiment of a coupling device according to the invention, this example being given purely for the sake of illustration and merely without limitation and with reference to the appended drawings, in which:
It will be noted first and foremost that throughout
The coupling device for optical fibres shown in
Said first sheet 11 is made of double face-polished, monocrystalline silicon. It comprises a plurality of through-holes 13 aligned in a series of rows and columns. Said holes 13 are formed by photolithography and reactive ion etching techniques well known to the person skilled in the art. They comprise a first portion 13a having a height of about 50 micrometres and a diameter adjusted to the diameter of the optical fibres. This means that the diameter of the portion 13a is larger than that of the optical fibres, so as to be able to forcelessly introduce the fibres into the holes 13, but very close to the manner in which the optical fibres are positioned with precision once internally engaged. The question of the precision of the positioning of the fibres will be addressed in greater detail with reference to
The sheet 11 comprises, furthermore, at least one alignment mark 14 along two axes contained in the plane of the sheet 11, the alignment mark being etched through the sheet 11 concomitantly with the holes 13. In the case presented in
The second sheet 12 is made of glass, for example of double face-polished, borosilicate glass and does not comprise any structure. In a variant, the sheet 12 can display optical structures such as a network of lenses that is aligned with the network of holes 13. In this case, the sheet 12 comprises an alignment structure which is compatible with the alignment mark 14.
The sheets 11 and 12 are joined together by an anode welding process well known to the person skilled in the art or by any other process imparting high cohesion between the two sheets 11 and 12.
According to the invention, the coupling device 1 also comprises a third sheet 15 which is made of monocrystalline silicon and is joined to the first sheet 11 by a face. Said third sheet 15 is globally identical to the first sheet 11. It comprises a network of holes 16 in rows and columns that is identical to the network of holes 13 that is formed by the same processes and at least one alignment mark 17 which is compatible with the alignment mark 14. Advantageously, the sheets 11 and 15 are structured with the aid of the same photolithography masks.
The sheets 11 and 15 are mechanically aligned relative to each other, in such a way that the holes 13 and the holes 16 are situated in one another's extension and form recesses 18 for optical fibres. For this purpose, use is made of a wedge which is equipped with two reference surfaces and against which the alignment marks 14 and 17 will be positioned in abutment. If the alignment mark consists of one or more, cross-shaped or circular, holes, use is made of one or more suitably shaped reference pins which are engaged with the alignment holes. The alignment can also be carried out by permanently mounting said sheets 11 and 15 in a frame equipped with reference surfaces. Such mechanical alignment against reference surfaces provides an alignment precision of the order of 0.1 micrometres. It will be noted, moreover, that the useful height h of the recesses 18 thus formed is equivalent to one times the thickness of a sheet of silicon plus one times the height of the portion 13a, i.e. 675 micrometres.
The sheets 11 and 15 thus aligned are joined together by anode welding.
Optical fibres 19 are engaged with the recesses 18 formed by the holes 13 and 16, as shown in
Owing to the third sheet 15, and to the mechanical alignment thereof relative to the sheet 11, the coupling device according to the invention provides a gain in precision of the alignment of the optical fibre 19 relative to the prior art. Mention will be made of the fact that positioning precision may be expressed as the angle formed between the axis of the optical fibre 19 and the normal N to the sheet 12, the radiation issuing from the fibre 19 having to penetrate the sheet 12, ideally, with an angle of 90°.
Reference will now be made to
As for the first embodiment, the sheets 11 and 15 comprise a plurality of holes 13 and 16 respectively. Said holes 13 and 16 are formed either by reactive ion etching alone, as shown in
Optical fibres 19 are engaged with the recesses 25 consisting of the holes 13 and 16, as shown in
Said volume 24 and also the residual volume around the optical fibres 19 in the terminal sections 25b of the recesses 25 are filled with a fluid optical compound 26 such as an optical gel, an optical oil or another fluid compound having an index of refraction close to that of the core of the optical fibres 19. The optical compound 26 is injected into the volume 24 through a hole 27 which is made in the sheet 15 for this purpose and subsequently reclosed. This operation is carried out under vacuum so as to carefully fill up all of the inner volume 24 and the residual volume around the optical fibres 19. The fluid optical compound 26 is thus accommodated, furthermore, between the end of the optical fibres 19 and the surface 28 of the sheet 12 and ensures the continuity of the index of refraction at the output of the optical fibres 19. It will be noted that, for a fluid optical compound 26 formed by a gel, it is not necessary for the inner volume 24 to be tightly closed by the spacer 21, as the gel remains in place without running or evaporating. An air bubble 30 is added after introduction of the fluid 26 in order to absorb any thermal expansion thereof.
Reference will now be made to
It will be noted that, owing to the configuration of the coupling device 2 described hereinbefore, the mechanical function, of fixing the optical fibres 19, and the optical function respectively, of continuity of the index of refraction, are advantageously separate. The first is assured by the initial section 25a of the recess 25 and the glue 20, whereas the second is performed by the terminal section 25b of the recess 25 and the optical fluid 26. This separation is made possible by the presence of the spacer 21 which physically separates the recesses 25 into two distinct sections 25a and 25b which CaO be filled with a fluid independently of each other. The terminal section 25b is thus filled with a fluid optical compound 26, capable of absorbing the expansions or contractions of the coupling device 2 without the delamination effect as observed with an optical glue. The continuity of the index of refraction is then ensured under all circumstances and radiation losses at the output of the optical fibres 19 are avoided.
It will be noted, furthermore, that, owing to the spacer 21, the alignment of the optical fibre 19 with reference to the normal N to the sheet 12 is considerably improved. Reference is made in this regard to
A coupling device 1, 2 has thus been described, the performance levels of which are improved relative to a conventional coupling device. Of course, the coupling device according to the invention is not limited to the embodiments which have just been described and a broad range of simple modifications and variants may be conceived of by the person skilled in the art without departing from the scope of the invention as defined by the appended claims.
It will be noted, in particular, that in the embodiments presented the optical fibres are introduced bare into the recesses 18 and 25 or protected by a ferrule. In a variant, the fibre could be equipped with a connector at its end. In this case, the end of the connector would be mounted in the coupling device instead of the end of the optical fibre. For this variant, the size of the holes 13 and 16 would have to be adapted to the diameter of the end of the connector as illustrated in
| Number | Date | Country | Kind |
|---|---|---|---|
| 09 165 438.4 | Jul 2009 | EP | regional |
