WO2000065391A1

WO2000065391A1 - An optical fiber coupler

Info

Publication number: WO2000065391A1
Application number: PCT/SE2000/000824
Authority: WO
Inventors: Tarja Volotinen
Original assignee: Ofcon Optical Fiber Consultants Ab
Priority date: 1999-04-28
Filing date: 2000-04-28
Publication date: 2000-11-02
Also published as: SE9901540L; SE514477C2; AU4636300A; SE9901540D0

Abstract

An optical coupling assembly contains two or more individual, isolated, fused fiber couplers of type 1x2 or 2x2. The couplers are made from juxtaposed segments (15r, 15'r; 15b, 15'b) of individual fibers in two pieces of two-fiber ribbons (1, 1'). The juxtaposed segments have been fused to each other and simultaneously drawn in the conventional way. The coupling segments are placed inside an elongated protection element (21) having a groove or channel (19). The segments are secured in to the groove or channel and can be separated from each other either by being located at some distance of each other in the fiber longitudinal direction and by some suitable isolating material (23). Such a coupling element is particularly suited for building networks in which two-fiber ribbons are used, such as branched passive optical networks, in private homes, for duplex or bidirectional communication, but can also be used in any kind of coupler application instead of two or more separate couplers.

Description

AN OPTICAL FIBER COUPLER FIELD OF THE INVENTION

The present invention relates to an optical fiber coupler, in particular an optical fiber splitter or combiner, and a method of fabricating it. BACKGROUND AND PRIOR ART

Telecommunication systems using light signals propagating in different types of optical waveguides are used more and more today. Thus, optical fiber networks are nowadays built in all countries worldwide and are widely expanded and there is a large interest in extending the optical networks, in addition to branches to large companies, institutions, etc. also to small business offices and private subscribers, i.e. to apartments, private homes and estates, etc. The so called optical fiber access network which is also called "Fiber To (In/From) The Home" (FTTH), "Fiber To (In/From) The Customer (Business)" (FTTB), etc. will thus be expanded. Also, there is a large interest in extending the use of optical networks in LANs, i.e. local area networks, used for interconnecting computers, telephone sets, television sets, video cameras and peripheral devices such as printers etc. in a business estate and furthermore for making ordinary telephone calls or picture telephone calls, such as in using the Internet. In order to achieve this expansion, the costs of the components of the optical networks of course have to be reduced as much as possible. However, in optical fiber networks the most costly components generally are the cables, the optical switches and the laser transmitter devices used for injecting and modulating light signals in optical fibers and the receiver devices detecting light signals propagating in optical fibers. Also, the installation of cables generally is rather costly.

A very common optical component used in fiber optical devices, networks and systems are optical couplers, also called combiners or splitters, which combine light propagating on at least two optical fibers or split light propagating on an optical fiber to propagate on at least two fibers. Also, the use of fiber ribbons is common. In for instance connecting a subscriber a two-fiber ribbon can be suitably used, one fiber for transmitting light signals downstream, i.e. to the subscriber, and another fiber for transmitting light signals upstream, i.e. from the subscriber, for so called duplex communication. Optical couplers are also used for combining light of different wavelength such as in couplers used in optical fiber amplifiers to feed pump light into the active fiber and in add/drop nodes in wavelength multiplexed networks for adding and dropping channels. A common type of optical couplers are called fused optical fiber couplers or fused fiber couplers and are generally manufactured from standard telecommunication optical fibers made of glass, typically silica glass, by heating them to a near melting temperature, pulling the fibers and fusing them to each other. Many methods of manufacturing such fused fiber couplers, also called bi-directional optical power dividers, have been disclosed in the prior art. One of the known manufacturing methods comprises that e.g. two single mode, silica glass optical fibers are bonded to each other by retaining bare segments of the two fibers in retainers to place the segments in contact with and at the side of each other or twisted around each other and then heating the segments or 5 portions thereof to fusioning temperatures so that the fibers are glued to each other while simultaneously moving the retainers to exert a pulling or tensioning force on the portions of the fibers held between the retainers. In the pulling operation the heated segments of the two fibers become thinner resulting in that also the cores of the two fibers become thinner and that the cores also become positioned closer to each other. The refractive o index profiles of the two fibers become deformed. Then the optical power of a fundamental mode of light propagating along one of the fibers will be shared by the two fibers at the fused section, thus making the light wave being divided at the fused section to propagate along the two fibers. The high fusioning temperature of typically above 1800°C can be obtained from a flame, an electric arc, a laser beam, heat from light s beams concentrated by lens systems or from an oven comprising electric resistance heat elements or similar equipment, etc.

The sharing of optical power at the fusioned section is called optical coupling and can be explained owing to a partial overlap of the evanescent part of mode fields of light propagating along the two optical fibers. The coupling ratio between the power of the o light wave propagating on the mcoming fiber and the power of the light wave propagating on the other fiber depends on the overlap of the mode fields, i.e. on the separation distance between the center lines of the cores of the two fibers and to some extent also on the length of the coupling section or fused section and on the length of the tapering sections. The coupling ratio can be typically set to 50/50 or 70/30, 10/90, 1/99, etc. by 5 suitably designing the fused section. Basically, such a coupler including two fibers fused to each other has four ends and is thus a 2x2 coupler and furthermore it is symmetric as taken in longitudinal directions of the coupler, i.e. it has about the same coupling ratios and transmission characteristics for light waves entering in both directions. Typically fused couplers of this type are usable, i.e. have good transmission characteristics such as 0 a low insertion loss, very high signal bandwidth, up to 10 THz, in a wide wavelength range such as in a range including 1270 - 1340 nm and/or 1520 - 1570 nm. Fused couplers made of single-mode fibers furthermore have a low polarization dependent loss, except in the case of low coupling ratio such as below 10/90. In addition, provided that they have been given an appropriate mechanical protection, they are reliable at various 5 service environments such as that they are not sensitive to temperature changes or humidity.

In the case where one of the two fibers is cut off at place near the fused section, so that it does not reflect lightwaves, the 2x2 coupler becomes a 1x2 coupler. Furthermore, a similar method can be used to manufacture 3x3 and therefrom 1x3 or 2x3 couplers using three optical fibers which are pulled and fusioned simultaneously or even NxN couplers (or lxN, 2xn, etc.) can be manufactured. A common feature of optical fiber couplers is that the optical power is coupled from an input fiber, which can be any of the fibers, to every of the fibers on the opposite side of the coupling segment, the coupling segment being indicated by the "x" in the designations 2x2, 1x2, etc.

A typical length of the fused, tapering section of a fused fiber coupler is 5 - 70 mm depending on coupling ratio, wavelength range, etc. and the total length of 1x2 and 2x2 couplers is 20 - 100 mm. The fused section is conventionally protected by securing, by a UN-curable synthetic resin material, the fused segments of the fibers and adjacent bare fiber portions to the bottom of a U-groove of an elongated element made of a glass material such as silica glass, the groove e.g. having a cross-section of about 500x500 μm² and a length of about 20 - 100 mm. The elongated glass element and thus the U-groove can then be covered with some lid and all around a plastic tube is placed which is sealed at its ends by means of a glue or a UN-light curable resin and the whole package is finally enclosed by a tube made of stainless steel or any other non-permeable, hermetically sealing material which is sealed at the ends using also a UN-light curable, chemically resistant synthetic resin material.

The mechanical properties of a fused coupler as described above are obviously poor when no protection is used due to the thin bare fused segment. However, using a protection sleeve as described above having three layers enclosing the coupler segment of the fibers makes the fiber coupler mechanically stable and protects the coupler to the surrounding environment, i.e. to mechanical stress, temperature variations, humidity, water, chemicals, etc.

In order to facilitate the use of two-fiber ribbons a double coupler has been proposed as disclosed in the published Japanese patent application 01295211A for Furokawa. Thus, two fused fiber couplers are placed in the grooves of two semicylindrical reinforcing members which are secured to each other.

In the published Japanese patent application 01120510A for ΝTT an efficient method of producing optical fiber couplers is disclosed. Two five-fiber ribbons are used from which the protective coating is removed over predetermined lengths, the ribbons are heated and stretched within the bare fiber lengths, the ribbons are placed on top of each other with the thinned lengths of fibers from each ribbon placed in contact with each other in pairs, the ribbons are heated at one time and welded to each other in a central portion of the thinned lengths and finally the welded portions are heated and stretched. Thus five couplers are produced at the same time.

In U.S. patent 5,627,930 for Ishiguro et al. an optical fiber coupler component is disclosed which comprises a base element having a plurality of parallel grooves which probably uses couplers produced from ribbons as disclosed the cited Japanese application. SUMMARY

It is an object of the invention to provide an optical coupler assembly particularly suited to be connected to two-fiber or bidirectional links such as two-fiber ribbons used for branched, passive or arrayed optical networks. It is another object of the invention to provide an optical coupler assembly comprising a plurality of individual couplers which are securely optically isolated from each other.

It is another object of the invention to provide an optical coupler assembly comprising a plurality of individual couplers mounted in a protection element which is easily mounted.

It is another object of the invention to provide an optical coupler assembly comprising a plurality of individual couplers mounted in a protection element giving a secure protection.

Thus an optical fiber coupler or coupler assembly is proposed containing in one package two or more, e.g. four, individual couplers of the same type. Preferably the couplers are symmetric and bidirectional 1x2 or 2x2 couplers and are suited to be connected to two-fiber ribbons or four- fiber ribbons. The couplers are located, at the sides of each other or stacked on top of each other, in a groove or grooves such as U- grooves in a protective and/or reinforcing element such as an elongated glass element and are all enclosed by an outermost protection sleeve. The separate couplers are optically isolated from each other, so that substantially no significant overcoupling between the couplers can occur in any direction, in particular any existing overcoupling or cross- coupling between the couplers always typically being negligible, i.e. smaller than -40 dB. The two or more fiber couplers of a package can be used for lightwaves having the same propagation direction or having opposite propagation directions. The input terminals and output terminals of a coupler assembly comprising two individual couplers are arranged in pairs suitable to be connected to the ends of two-fiber ribbons but of course they can be also connected to separate optical fibers and any other fiber ribbon or ribbonized fiber. In the corresponding way, the input terminals and output terminals of a coupler assembly comprising four individual couplers are arranged in sets of four to be comfortably connected to the ends of four-fiber ribbons. The isolation between the fiber couplers in an assembly is obtained by fusing them in pairs from separate fibres, and is if necessary the isolation can be enhanced by arranging the fused sections in the protective element separated by a distance in the longitudinal direction of the fibers and/or by arranging a separating, partitioning wall or a light-absorbing, reflecting or non-transparent material layer between the two fused sections and/or by arranging the fiber couplers at some lateral distance of each other only separated by air.

The fused optical couplers as described herein are designed for any kind of optical fiber application using loose fibers or fiber ribbons or ribbonized fibers and have been specially developed for Fiber-To-The Home and local network applications, where incoming and outgoing signals are to be separated or a bidirectional communication line is divided into two fibers suitably composed to one two-fiber ribbon. The fused double ribbon couplers as described herein are easily spliced using standard ribbon splicing procedures to any two-fiber-ribbon or any other fiber ribbon or ribbonized fibers comprising optical fibers of the same mode type as that used in the couplers or even to loose optical fibers of the same mode type, or connected by means of two fiber connectors/single fiber connectors to any other optical fiber or fiber ribbon or O/E-E/O terminal. By using the double couplers as described herein, the installation costs, both in terms of material and the manual installation work required, of local networks can be reduced compared to the standard case where no couplers at all are used (star type network configuration instead of a branched single line or annular line) or simple fiber couplers are used. The need for many types of specific passive components, such as wavelength selective fiber optic filters and/or directional fiber optic isolators are minimized in the local networks by using separate isolated fibers for up-stream and down-stream signals. The very good transmission properties including a wide wavelength range, an extremely high bandwidth, low PLD (Polarization Dependent Losses), a good mechanical stability, a good temperature stability and other features of fiber couplers made of loose, separate fibers are preserved.

Thus, generally a coupler assembly is provided for combining or splitting light propagating along waveguides comprising two optical fiber assemblies, each of the two optical fiber assemblies including a plurality of parallel optical fibers. The fiber assemblies are typically ribbon fibers or ribbonized fibers. The two optical fiber assemblies have been rid of their protective coatings and layers along predetermined portions to expose the bare surfaces of the optical fibers included in the assemblies. The optical fibers of the two optical fiber assemblies are composed to form pairs so that the two optical fibers of a pair belong to different ones of the two optical fiber assemblies. The optical fibers of each pair are fused to each other in a fused segment within the predetermined portions to form an optical fiber coupler. The channel or groove of a reinforcing and protective element receives the predetermined portions of the two optical fiber assemblies.

Particularly, the fused segments of the optical fibers of a first one of the pairs are located at a distance, as seen in the longitudinal direction of the optical fibers, of the fused segments of the optical fibers of a second, different one of the pairs to provide an optical isolation between the fused segments of the first and second ones of the pairs. The bare surfaces of the optical fibers of the first one of the pairs are thus preferably located at a longitudinal distance of and not overlapping in the longitudinal direction the bare surface of the optical fibers of the second one of the pairs. A wall or partition can separate the bare surfaces of the optical fibers of a first one of the pairs from the bare surfaces of the optical fibers of a second, different one of the pairs, and can be configured to have an outer surface adapted to the channel or groove of the reinforcing and protective element and an inner opposite surface having a longitudinal groove receiving the bare surfaces of the optical fibers of one of the first and second ones of the pairs. Termed somewhat differently, the reinforcing and protective element can comprise at least two elongated members each having a groove, so that in the groove of each elongated member the bare surfaces of the optical fibers of a one of the pairs are located, and at least a first one of the elongated members is located at least partly inside the groove of a second one of the elongated members.

The reinforcing and protective element can in another embodiment comprise at least two elongated members each having a groove receiving the bare surfaces of the optical fibers of a one of the pairs, and it then further comprises a holding member. This holding member has a web portion and outer portions designed to hold the elongated members making the web portion cover the grooves. The web portion can be substantially flat or have a cross-section configured as radii extending from a center and can all have substantially the same shape. The elongated members can each have a cross-section configured substantially as a sector of a circle, the cross-section also comprising a recess extending from the center of or located at the circle. This recess corresponds to the groove of the elongated member.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described by way of non-limiting embodiments with reference to the accompanying drawings, in which:

Fig. la is a schematic view of a double 1x2 coupler, Fig. lb is a schematic view of a two-fiber ribbon,

Fig. 2 is schematic view of two two-fiber ribbons, at the middle sections of which a matrix layer has been removed,

Fig. 3 is a schematic view illustrating the juxtaposition of the two ribbons of Fig. 2,

Fig. 4a is schematic view of the two ribbons of Fig. 2 after protective coatings according to a first embodiment have been removed from the individual fibers,

Fig. 4b is a schematic view similar to that of Fig. 4b but for another location of places of the removed protective coatings,

Fig. 5 is a schematic view illustrating the manufacture of a first fiber optic coupler from ribbons according to Fig. 4a, Fig. 6 is schematic view illustrating a protective element placed around the manufactured optical couplers,

Fig. 7a is schematic cross-sectional view illustrating a protective element placed around the manufactured optical couplers and a partition placed between the couplers,

Fig. 7b is a schematic perspective view illustrating the protective element and the partition of Fig. 7a before attaching a second coupler and a lid,

Fig. 8a is a schematic perspective view illustrating two protective elements having optical fibers placed therein,

Fig. 8b is schematic cross-sectional view illustrating the two protective elements shown in Fig. 8a placed on top of each other enclosing coupling segments of the optical fibers, and

Figs. 8c and 8d are a schematic perspective view and a schematic cross-sectional view respectively of optical fiber couplers mounted in protective elements hold by a metal member. DETAILED DESCRIPTION

The manufacture of a double optical fiber coupler intended for single-mode transmission will now be described. The double coupler is preferably manufactured from two pieces 1, 1' of about 2 m each of a two-fiber optical fiber ribbon, see Fig. 2 and the cross-sectional view of Fig. lb. The lengths of the two ribbon pieces can be substantially the same or at least they must be sufficient to allow the processing which will be described hereinafter. The two individual fibers of a two-fiber ribbon are for the sake of clarity called the red fiber and the blue fiber corresponding to the reference letters r, b. Thus, the individual fibers of the first piece of fiber ribbon are denoted by 3r, 3b and the fiber of the second piece by 3'r, 3'b. The optical fibers in the two-fiber ribbon used are preferably standard single mode fibers (ITU G.652, G.653, G.654, G.655, etc.) or can be some modified type of such fibers or even standard multimode fibers (ITU g.651, etc.) can be used. The fibers contain a silica glass cladding having an outer diameter of typically 125 μm and a core of a diameter of typically about 5 - 10 μm or in the multimode case 50 - 62.5 μm or even larger. The individual fibers are coated with a standard primary protective coating 5 comprising two polymer, e.g. polyacrylate, layers and having an outer diameter of typically 240 - 250 μm and thereon a thin coloured layer 7 giving each of the individual fibers together with their protective layers a total outer diameter of 250 μm. Two individual fibers are attached in parallel to each other to form a two-fiber ribbon of encapsulated type having a matrix layer 9 of a thickness of typically about 10 - 30 μm in common, the two-fiber ribbon formed having typically a thickness of 280 - 350 μm and a width of about 550 - 700 μm.

As is illustrated in Fig. 2, the ribbon matrix 9 of the two fiber ribbon pieces 1, 1 ' is stripped over a length of about 100 mm if location separation is not used, as will be described hereinafter, or about 150 mm if location separation is used. The stripped portions of the fiber ribbons, where thus there are free portions llr, l ib, U'r, ll'b of the individual fibers still having their primary protective coating, are located somewhere at the middle of the fiber ribbon pieces 1, 1 ' and preferably at the same location, i.e. they can be located between left and right portions of unstripped ribbon, the left portions having the same length and the right portions having the same length and the length of the left and right portions being about equal to each other. The stripped portions are given such a length that when individual couplers have been formed, as will be described hereinafter, they then each have a total length of about 10 mm less than the length of a U-groove in a protective and reinforcing element.

5 The fiber ribbon pieces 1 , 1' are attached to each other at places distant of the stripped portions so that the red fibers 3r, 3'r are located at the side of each other and the blue fibers 3b, 3'b are located at each other and so that all of the stripped portion of the first ribbon is located at the stripped portion of the second ribbon, see Fig. 3.

The colour layer 7 and the primary coating 5 are then removed from adjacent o portions of the red fibers 3r, 3'r over a length of about 20 mm to expose portions 15r, 15'r of the bare fibers, see Figs. 4a and 4b. The colour layer 7 and the primary coating 5 are in the same way removed from adjacent portions of the blue fibers 3b, 3'b over a length of about 20 mm to expose portions 15b, 15'b of the bare fibers. The bare portions 15r, 15'r of the red fibers can be located at the same place as the bare portions 15b, 15'b s on the blue fibers, in the portions stripped from the matrix and as seen in the longitudinal direction of the fibers, see Fig. 4b. They can also be located not having any part located adjacent each other, i.e. on separate locations, as is illustrated in Fig. 4a, the latter case being called location separation. The bare portions are carefully cleansed by e.g. wiping with cotton material containing an alcohol and/or by being inserted in an ultrasonic 0 cleaning bath.

The ribbon lengths are then mounted in fusioning equipment, as indicated by the flame 17 in Fig. 5 and having suitably adapted, movable fiber retainers or clamps, not shown, so that the free portions of the blue fibers 3b, 3'b are first bent to have their bare portions 15b, 15'b be located in contact with each other and the red fibers freely pass 5 around the bare portions of the blue fibers. The blue fibers are pulled when fusioning them to each other in the middle of their bare portions to form a common bare portion constituting an optical coupler. The common bare portion or fused segment has after the fusioning operation a length of at least 15 mm.

The free portions of the blue fibers including the common fused portion are then o secured to the bottom of the U-groove 19 of a protective and reinforcing elongated element 21, see Fig. 6, the U-groove having a length exceeding the length of the free portions and the protective element 21 also generally having a U-shape. The protective element 21 is made of a material having substantially the same coefficient of thermal dilatation as the glass material of the optical fibers used. Thus it can be made of a glass 5 material and particularly of silica glass having the same temperature elongation coefficient as the glass of the fibers. The groove of the element 21 can be covered by ,a lid of a material similar to that of the base protective element and the U-groove and the inner space be sealed as will be described for the embodiment of Figs. 7a and 7b.

Thereupon the red fibers 3r, 3'r are bent to have their bare portions 15r, 15'r come in contact with each other and they are then pulled and fused to form an optical coupler in the common bare portions. The free portions of the red fibers including their common bare portion are then glued to the U-groove of the protective element. They can be placed directly on top of the already attached blue fibers provided that the fused segments are locationally separated, see Fig. 4a.

Otherwise an optical isolation must be arranged such as arranging an air gap, not shown, between the two fused coupler segments. In the embodiment of Figs. 7a and 7b the red fibers are placed in a separate space in the protective U-shaped element 21 by arranging a separating material such as a partitioning wall 23 on top of the already glued blue fibers. This optically isolating wall 23 is advantageously of the same material as the protective element 21. The wall can also have the shape of an elongated element having a longitudinal groove as seen in Figs. 7a, 7b, such as a U-shape with lateral projections extending from the top of the legs of the U-shape, these projections resting at the top end surfaces of the legs of the U-shape of the base protective element 21. The U-shape of the partitioning element 23 then preferably has dimensions making the element fit into the groove of the base protective element 21. The partitioning element 23 has a length smaller than the protective base element 21 and is glued on top of the blue fibers. The red fibers are glued into the groove of the partitioning wall.

Finally the groove of the partitioning element 23 can be covered by a lid 25 of a material e.g. similar to that of the base protective element 21 and the partitioning element. The lid 25 can be glued on top of the red fibers. The space in the total protective element where the individual fiber couplers or the very coupling segments are located is sealed at its ends as described above. In the case where the ribbon matrix layer does not reach the protective element, it can be fixed by an UN curable polyacrylate on both sides of the protective element or by using additional reinforcing protective tapering devices at the ends of the protective element 21. For a double 1x2 optical coupler, the blue and red fibers of one of the fiber ribbons are cut-off at a suitable distance from the fusioned segment and are terminated in a suitable manner not to reflect any light propagating towards the cut-off ends, these ends also being located inside the groove of the protective element and inside said space.

Two complementary U-shaped protective elements 27, 27' can also be used, as illustrated in Figs. 8a and 8b. Here the blue fibers and the red fibers are secured by a suitable adhesive 29 in the U-grooves of the elements 27, 27' and then the elements are turned to have their grooves directed towards each other and placed at each other and are attached to each other in this position by e.g. a UN-curable resin 31. The elements 27, 27' are finally enclosed and sealed by a protective sleeve 33. In order to obtain an optical isolation between the couplers a metal element 35 can be used as illustrated in Fig. 8c. It has a flat web portion 37 placed at and adhesively bonded to the surfaces connecting the elements 27, 27' and cylindrical surfaces 39 extending from the edges of the web portion to give the metal element an outer near-circular cylindrical shape, the shape having longitudinal slots 41 to give the holding element 35 a cross-section like an H having upper and lower legs curved towards each other. The metal element 35 can be sealed by a protective, polymer sleeve 33 as in Fig. 8b. The design of Fig. 8c can also be used for more couplers, see Fig. 8d. There, four couplers are placed in the four central grooves or recesses of four protective elements 27" , each protective element having a cross- section corresponding substantially to a sector of circle with a recess at the center of the circle. The web portion 37" of the holding metal element 35" then has a cross-section comprising radii extending from the center of the circle. In Fig. 1 a finished double 1x2 optical coupler is illustrated. It has a rigid middle segment housing the two individual couplers. From the rigid segment extend at one end two pieces of two-fiber ribbons, which can easily, using a splicing device adapted for such ribbons, be connected to other lengths of e.g. similar two-fiber ribbons. At the opposite end there is only one extending piece of a two-fiber ribbon, the other piece then being cut-off in a non-reflecting way at a place inside the rigid reinforced segment.

The double coupler as described herein can in some cases also be manufactured from two ribbonized optical fibers, such fibers behaving basically as a two-fiber ribbon. Also, the double coupler can in some cases be manufactured from separate optical fibers and, after producing the couplers and enclosing them by some reinforcing and protective element, ribbonizing the fiber pieces extending from the element. The term "ribbon fiber" or only "ribbon" as used above can thus be taken to include, in a general sense, also optical fibers in some way, more or less permanently, retained to form an assembly of parallel fibers.

A coupler assembly containing more than two individual couplers such as four couplers can be manufactured in the same way as described above for a double coupler. For optical isolation of the couplers more partitions may be used in the case where couplers are placed at the sides of each other.

Claims

1. A coupler assembly for combining or splitting light propagating along waveguides comprising two optical fiber assemblies, each of the two optical fiber assemblies including a plurality of parallel optical fibers, the two optical fiber assemblies being rid of protective coatings along predetermined portions to expose the bare surfaces of the optical fibers and the optical fibers of the two optical fiber assemblies being collected to pairs, the optical fibers of a pair belonging to different ones of the two optical fiber assemblies, and the optical fibers of each pair fused to each other in a fused segment within the predetermined portions to form an optical fiber coupler, and a reinforcing and protective element having a channel or groove, the predetermined portions of the two optical fiber assemblies being located in the channel or groove, characterized in that the fused segments of the optical fibers of a first one of the pairs are located at a distance, in the longitudinal direction of the optical fibers, of the fused segments of the optical fibers of a second, different one of the pairs to provide an optical isolation between the fused segments of the first and second ones of the pairs.

2. A coupler assembly according to claim 1 , characterized in that the bare surfaces of the optical fibers of the first one of the pairs are located at a longitudinal distance of and not overlapping in the longitudinal direction the bare surface of the optical fibers of the second one of the pairs.

3. A coupler assembly according to any of claims 1 - 2, characterized in that in the fused segment of the optical fiber coupler formed by each pair the optical fibers of the pair is also drawn to a smaller diameter than other portions of the optical fibers.

4. A coupler assembly according to any of claims 1 - 3, characterized by a wall separating the bare surfaces of the optical fibers of a first one of the pairs from the bare surfaces of the optical fibers of a second, different one of the pairs, the wall having an outer surface adapted to the channel or groove of the reinforcing and protective element and an inner opposite surface having a longitudinal groove receiving the bare surfaces of the optical fibers of one of the first and second ones of the pairs.

5. A coupler assembly according to any of claims 1 - 3, characterized in that the reinforcing and protective element comprises at least two elongated members each having a groove, the groove of each elongated member receiving the bare surfaces of the optical fibers of a one of the pairs, at least a first one of the elongated members being located at least partly inside the groove of a second one of the elongated members.

6. A coupler assembly according to claim 5, characterized in that the reinforcing and protective element comprises a lid covering the groove of said first one of the elongated members.

7. A coupler assembly according to any of claims 1 - 3, characterized in that the reinforcing and protective element comprises at least two elongated members each having a groove, the groove of each elongated member receiving the bare surfaces of the optical fibers of a one of the pairs, and that the reinforcing and protective element further comprises a holding member, the holding member having a web portion and outer portions, outer portions holding the elongated members to cover the grooves of the elongated members by the web portion. 5

8. A coupler assembly according to claim 7, characterized in that the web portion is substantially flat or has a cross-section configured as a radii extending from a center.

9. A coupler assembly according to any of claim 7 - 8, characterized in that the elongated members have substantially the same shape.

10. A coupler assembly according to any of claim 7 - 8, characterized in that the o elongated members each have a cross-section substantially as a sector of a circle and a recess extending from the center of the circle and forming the groove of the elongated member.

11. A method of fabricating a coupler assembly for combining or splitting light propagating along waveguides, the method comprising the steps of: 5 providing two pieces of optical fiber assemblies, each of the two optical fiber assemblies including a plurality of parallel optical fibers and the two optical fiber assemblies and the optical fibers included therein having substantially the same length, removing from the optical fibers of each of the two pieces a protective coating along predetermined portions of the optical fibers, o collecting the optical fibers of the pieces to pairs, the optical fibers of a pair belonging to different ones of the two optical fiber assemblies, fusing the optical fibers of each pair to each other in a fused segment located within the predetermined portions to form an optical fiber coupler, and protecting the predetermined portions of the two optical fiber assemblies by placing 5 at least the predetermined portions of the two optical fiber assemblies in a longitudinal channel or groove of a reinforcing and protective elongated element, characterized in that in the step of removing the protective coating, the protective coating of the optical fibers of a first one of the pairs are removed in predetermined portions separated from or located at a distance of, taken in a longitudinal direction of the 0 optical fibers, the predetermined portions of the optical fibers of a second, different one of the pairs.

12. A method according to claim 11 , characterized in that in the step of fusing, the optical fibers are pulled or drawn in the fused segments so that in the fused segments the optical fibers obtain a smaller diameter than other portions of the optical fibers. 5

13. A method according to any of claims 11 - 12, characterized in that in the step of protecting, a wall is placed in the channel or groove to separate the predetermined portions of the optical fibers of a first one of the pairs from the predetermined portions of the optical fibers of a second, different one of the pairs, the wall including an elongated element adapted to the channel or groove of the reinforcing and protective element and to also have a longitudinal groove for receiving the optical fibers of and the fused segment of one of the pairs.

14. A method according to any of claims 11 - 12, characterized in that in the step of protecting, the reinforcing and protective element is made to comprise at least two elongated members each having a groove, and placing the bare surfaces of the optical fibers of a one of the pairs in each of the grooves, and placing at least a first one of the elongated members at least partly inside the groove of a second one of the elongated members.

15. A method according to claim 14, characterized in that in the step of protecting, the reinforcing and protective element is made to comprise a lid and placing the lid to cover the groove of said first one of the elongated members.

16. A method according to any of claims 11 - 12, characterized in that in the step of protecting, the reinforcing and protective element is made to comprise at least two elongated members each having a groove, and placing in the groove of each elongated member the bare surfaces of the optical fibers of the pairs, and making the reinforcing and protective element to further comprise a holding member, the holding member having a web portion and outer portions, and placing the outer portions to hold the elongated members to make the web portion cover the grooves of the elongated members.

17. A coupler assembly for combining or splitting light propagating along waveguides comprising two optical fiber assemblies, each of the two optical fiber assemblies including a plurality of parallel optical fibers, the two optical fiber assemblies being rid of protective coatings along predetermined portions to expose the bare surfaces of the optical fibers and the optical fibers of the two optical fiber assemblies being collected to pairs, the optical fibers of a pair belonging to different ones of the two optical fiber assemblies, and the optical fibers of each pair fused to each other in a fused segment within the predetermined portions to form an optical fiber coupler, and a reinforcing and protective element having a channel or groove, the predetermined portions of the two optical fiber assemblies being located in the channel or groove, characterized in that the reinforcing and protective element comprises at least two elongated members each having a groove, the groove of each elongated member receiving the bare surfaces of the optical fibers of a one of the pairs, at least a first one of the elongated members being located at least partly inside the groove of a second one of the elongated members.

18. A coupler assembly for combining or splitting light propagating along waveguides comprising two optical fiber assemblies, each of the two optical fiber assemblies including a plurality of parallel optical fibers, the two optical fiber assemblies being rid of protective coatings along predetermined portions to expose the bare surfaces of the optical fibers and the optical fibers of the two optical fiber assemblies being collected to pairs, the optical fibers of a pair belonging to different ones of the two optical fiber assemblies, and the optical fibers of each pair fused to each other in a fused segment within the predetermined portions to form an optical fiber coupler, and a reinforcing and protective element having a channel or groove, the predetermined portions of the two optical fiber assemblies being located in the channel or groove, characterized in that the reinforcing and protective element comprises at least two elongated members each having a groove, the groove of each elongated member receiving the bare surfaces of the optical fibers of a one of the pairs, and that the reinforcing and protective element further comprises a holding member, the holding member having a web portion and outer portions, outer portions holding the elongated members to cover the grooves of the elongated members by the web portion.