WO2000065390A1

WO2000065390A1 - A fused fiber coupler

Info

Publication number: WO2000065390A1
Application number: PCT/SE2000/000823
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: SE9901541D0; AU4636200A; SE514479C2; SE9901541L

Abstract

A fused optical fiber coupler comprises as conventional two or more optical fibers (1) fused to each other and thinned within a short segment. The fibers used have an extra doping of especially selected atoms such as nitrogen in the cores (11). When heating the segments to fusion them to each other, these extra dopant atoms have diffused out of the original core regions and have even been eliminated from the fibers reducing the refractive index of the cores. New cores are formed enclosing the former core regions, making the cores within the heated segments also wider. When pulling the fibers within the heated segments for thinning them these new cores are also being thinned. The redistribution of dopants changes the refractive index profile so that the modefields of light propagating in the segments will be wider than without any diffusion or releasing of dopant. The wider modefields give wider evanescent fields in the fused segments making light interact more easily in the segments, resulting in that the heated segments must not be drawn to such a large extent or to be as thin or narrow as conventional. This gives fused segments considerably thicker than the fused segments of a conventional fused coupler. Such an optical coupler can be more easily handled and is less subjected to breaks or ruptures. In particular, the optical coupler will have a greater fracture load, longer lifetime and a better temperature stability than conventional fused couplers.

Description

A FUSED 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 today more and more used. Thus, optical fiber networks are nowadays built in all countries world-wide and 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 and telephone sets, video cameras, television sets 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, optical switches and lasers, transmitter devices used for injecting and modulating light signals in optical fibers and receiver devices detecting light signals propagating in optical fibers. Also, the installation of cables generally is costly.

A very common optical component used in fiber optical networks are optical couplers, also called optical combiners or splitters, which combine light propagating in at least two optical fibers or split light propagating in an optical fiber to propagate in at least two optical 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 commumcation. 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 typically silica glass by heating them to a near melting temperature, pulling the fibers and making them be fused 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 or clamps to place the segments in contact with and at the side of or twisted around each other and then heating the segments or 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 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 about 1800°C can be obtained from a flame, an electric arc, a laser beam, heat from light 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 as resulting from 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 light wave propagating on the incoming 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 also on the angle between tapering fibers. The coupling ratio can be typically set to 50/50 or 70/30, 10/90, 1/99, etc. by 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 a low insertion loss and a 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 a 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 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 in the similar way. 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.

In the pulling operation the heated segments of two fibers are thinned producing a rather thin and delicate common segment, see e.g. Fig. 1 of the U.S. patent for Murphy et al. discussed hereinafter. This can give couplers which cannot be easily handled.

In U.S. patent 5,448,673 for Murphy et al. a fiber optic coupler is disclosed in which, by an additional heat treatment after fusioning the fibers to each other along pulled and twisted segments, dopants in the cores are made to diffuse into the thin claddings of the optical fibers which are used in the manufacture of the coupler and all have thick cores. In the region of contact between two fibers the cladding is eliminated so that there a constant refractive index is obtained. This will result in a uniformity of optical coupling. The dopants mentioned in this document are Ge in the core and possibly

P in the cladding. However, in the temperature range used, 1350°C and some interval thereabove, the diffusion of standard dopants will be low and will not appreciably affect the properties a coupler made from single mode or few mode fibers having relatively thin cores, the diameter of the cores in such fibers being a small fracture of the diameter of the cladding such as less than 10 % or at least less than 50% . A heating for such fibers at the suggested temperatures and performed during a reasonable time period would not make the core regions contact each other and would not eliminate the cladding at the contact segment because the silica glass in the cladding is still solid at temperatures up to

1750°C and in an interval thereabove. Furthermore, the heat treatment at elevated but not too high temperatures such as mentioned in the cited patent is associated with a risk of crystal formation since Si0₂ crystallizes rather easily in a temperature interval below

1700°C.

SUMMARY

It is an object of the present invention to provide an optical fiber coupler having a good temperature stability.

It is another object of the present invention to provide an optical fiber coupler having a good mechanical strength allowing it to be handled without risk of breaking it and thereby having a long lifetime.

It is another object of the present invention to provide a simple method of manufacturing an optical fiber coupler to give it a good mechanical strength, the method not requiring any additional processing steps in the fusioning step.

Thus, generally in a fused fiber coupler, optical quartz fibers similar to or resembling standard optical fibers, see ITU G.651, G.652, G.653, G.654, G.655, etc., are used as a starting material. At least one of the optical fibers used contains a specially selected doping material. In particular, only the core of at least one of the optical fibers used can contain the specially selected doping material which can be N (nitrogen). The cores may normally also contain doping materials such as Ge, B, F, etc., all of the doping materials being added to totally make, in the common way, the refractive index of the core larger than the refractive index of the surrounding cladding material which thus has a lower refractive index and e.g. can be substantially undoped or have a refractive index lowered by suitable dopings. In a fusioning operation when manufacturing the fused fiber coupler the special dopant rearranges inside the region heated in the fusioning operation. The rearrangement causes a change of the refractive index in the region resulting in a reduced absolute difference between the refractive index of the core region and the region surrounding it. In particular the special dopant, such as N-atoms/-mo- lecules, of the core of said one of the optical fibers can partly diffuse to the surrounding fiber cladding and to some extent be released as a gas, the diffusion and possibly the elimination of the special dopant causing a decrease of the refractive index of the core and a slight increase of the refractive index in a layer around the core, e.g. resulting in a widening of the cores or at least a decrease of the refractive index of the cores. In the fusioning operation, simultaneously, the cores of all the fibers fused to each other are made narrower owing to a pulling of the fibers.

The reduction of the refractive index difference or generally the change of the refractive index in turn results in an effective widening of the mode fields of the fundamental modes or guided modes in said one of the optical fibers considered as an optical wave guide, the widening being taken in comparison with the width of the modefields extending into the cladding of an optical fiber having the same original refractive index profile in a cross-section and being pulled in the same way but in which no redistribution of dopants has occurred in the heating process and thus the refractive index profile is similar to that of the original fiber, i.e. in which the refractive index profile is a uniform reduction of the dimensions of the original profile, the changed index profile considered as a function of the distance to e.g. the core of the considered fiber being obtained from the corresponding function for the original index profile by multiplying the latter function by a constant.

The special dopant is selected so that a rearrangement and/or decomposition or breaks of the chemical bonds thereof, in particular a diffusion thereof, preferably occurs only at a very high temperature, near the melting temperature of the silica glass used in optical fibers. For a nitrogen doping when siliconoxynitride is formed this decomposition and diffusion start to occur about 1500 °C and is effective at temperatures about and above 1800°C. This means that e.g. in the heating process when the two silica glass fibers having a nitrogen doping are fused to each other, the bonds of nitrogen will be broken and nitrogen will diffuse to be partly released at the surfaces of the fibers.

Also, the special dopant could be originally added to only the cladding of an optical fiber, the special dopant diffusing in the heating process also into the core, reducing the refractive index thereof. More complex doping schemes could also be obtained by combining special dopants both in the core and the cladding of the optical fiber, the dopants possibly also reacting chemically in the heating process changing the refractive 5 index in the desired way to widen the mode fields.

The widening of the modefields caused by particularly the reduction of the refractive index of the cores and to some extent also by the swelling of the cores, i.e. the increase of the effective diameters of the cores, by the rearrangement and diffusion during the fusioning operation results in that it is not necessary to draw the fibers to the o very thin diameters, such as 30 to 50 μm used in the prior art. Owing to the effect of the widening of mode fields by deforming the refractive index profiles, the final thickness of the thinnest sections of the coupler can be at least 20 - 50 % thicker than in similar optical fiber couplers made without a widening of mode fields such as a swelling of the core in single mode fibers and even in multimode fibers some significant increase of the s thickness of the fiber portions within the coupler will be obtained compared to conventionally manufactured fused fiber couplers. The increased diameters give the fused couplers an increased strength, in particular an increased load at fracture in the coupler portion. Thus, the strength, i.e. the fracture load of the weakest flaw, of the bare coupler is thus at least about 40 % higher than for standard couplers made from optical fibers o without any significant change of the refractive index profile in the heating process, i.e. without a diffusion based swelling of the core, provided that the weakest flaws have the same sizes.

Using N as the dopant instead of or as supplement to Ge has two general advantages: 5 1. The good mechanical properties of the original optical fibers are conserved in the coupler and in particular the coupler has, compared to conventional couplers:

- A higher breaking load due to a larger diameter of the fused thin sections.

- A higher tensile strength (higher tensile stress at fracture) due to higher fictive temperature of the silica material resulting from the fact that the fusion is made quicker 0 and at a higher temperature, thereby avoiding crystallization of the silica material occurring at temperatures in the range of about 1200 - 1750°C and avoiding water diffusing into the silica material and further avoiding a too large diffusion of Ge and crystallisation of GeO₂ at temperatures in the range of 1000 - 1300°C what would change the refractive index. 5 2. The transmission properties are improved compared to Ge-doped fiber couplers:

- An effective local mode field widening due to effective index decrease, due to diffusion and release of N at high temperature fusion and due to widening of the core owing to diffusion and due to the higher viscosity and melting temperature of the core material.

- Avoidance of refractive index modifications both in the core and the cladding due to the crystallization of GeO₂ and SiO₂ owing to a sufficiently high, i.e. above 1750°C, fusion temperature.

- Compactness of the coupling length, which can typically be only about 5 - 15 mm which implies a low insertion loss and a wider wavelength range of transmission. 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:

Figs. 1 and 2 are schematic views illustrating the manufacture of a fused optical fiber coupler using a flame and an electric arc respectively in a combined fusing and pulling operation,

Fig. 3 is a schematic view of a fused optical fiber coupler,

Figs. 4a, 4b and 4c are plots illustrating the refractive index profile and the concentration profiles of Ge and N respectively in an optical fiber of step index type used in manufacturing an optical fiber coupler, Figs. 5a, 5b and 5c are plots of the refractive index profile and the concentration profiles of Ge and N respectively of a heated segment of the optical fiber having the profiles illustrated in Figs. 4a - 4c, and

Figs. 6a, 6b and 6c are plots of the refractive index profile and the concentration profiles of Ge and N respectively in an optical fiber of the type having the profiles shown in Figs. 4a - 4c after a heating and pulling operation used in manufacturing an optical fiber coupler.

DETAILED DESCRIPTION

In Figs. 1 and 2 the manufacture of a fused optical fiber coupler is illustrated.

Thus, as seen in Fig. 1, two optical fibers 1 are placed in contact with and in parallel to each other along some suitable length, the fibers being held by retainer devices, not shown. At a place or region within this length the two fibers are intensely heated by means of a flame 3 from a micro burner 5 or some other heater giving a localized heating. The flame 3 heats a region of the two fibers 1 to a very high temperature of the order of the magnitude of e.g. 1800°C to make the fibers melt or at least be semi-melted. The flame can be laterally moved to make the heated region shorter or longer. At the same time the two fibers 1 are subjected to a tensioning or pulling force as indicated by the arrows 6. Then the heated segments of the two fibers are fused so that the claddings of the fibers adhere to and are melted into each other forming one body or a single fused segment due to the surface tension and at the same time they become thinner in the heated region owing to the pulling force. Finally the two fibers are allowed to cool. The heat can for instance also be obtained from an electric arc 7 generated between electrodes

9 as is illustrated in Fig. 2.

In the heated segment the cores 11 of the two fibers 1 , owing to the simultaneous pulling operation, will become located closer to each other and also a thinning of the cores will normally be obtained in the heated segment. Of course, the refractive indices of the portions of the two fibers within the heated segment can be also changed by the heating process and thus the propagation characteristics of light propagating in the fibers, this effect being for instance derived by diffusion of dopant atoms which can be accompanied by a release of some dopant at the free surfaces of fibers, or by chemical changes such as by a decomposition of some compound accompanied by a diffusion of one of the products released by the decomposition. The finished coupler is illustrated in Fig. 3.

However, the two optical fibers 1 used in the fusioning and pulling process are preferably similar to a standard single mode optical fiber, i.e. they have an index profile allowing single-mode propagation of light for a considered wavelength band, but also few-moded fibers and even multimode type fibers can be used, as specified by ITU G.651, G.652, G.653, G.654, G.655, etc. In the method as disclosed herein optical fiber having not to thick cores are preferably used, i.e. optical fibers having relatively thin cores such as e.g. fibers having an effective core diameter not larger than 50 % and advantageously smaller than 20 % or even better smaller than 10 % of the diameter of the cladding. For such fibers, the resulting coupling will have characteristics clearly surpassing conventional couplers. However, the optical fibers used have been modified to contain a doping of a specially selected material, here preferably nitrogen (N), in their cores. In the standard manner the fibers 1 have a silica glass cladding which normally has an exterior diameter of 125 μm and the cores 11 typically have an effective diameter of 5 -

10 μm for single mode optical fibers or a diameter of about 50 - 62.5 μm for multimode optical fibers. The fibers can in the usual way be coated with a standard dual layer of a polyacrylate primary coating having for example an exterior diameter of 240 - 250 μm and thereon a thin colour layer having an exterior diameter of 250 μm, which can also be the diameter of the optical fibers as taken together with their protective coating.

In the manufacture then, such coated fibers are stripped or rid of their protective coatings along segments having e.g. a length of about 20 mm. The two fibers are then mounted in some fusion and transporting/pulling equipment, here symbolized by the micro burner 5 or the electrodes 9 respectively as described above. The fibers are held the ends of the bare segments by retainers or clamps, not shown, and an aligning device, not shown, guides the portions between the clamps, so that the middle portions of the bare segments of the two optical fibers will be located at the side of and in contact with each other. At a suitable place within the middle portions at some distance from the aligning device the heating source 5 or 9 respectively is located. Then the heating is started applying heat in the juxtaposed middle portions of the bare segments, as illustrated in Figs. 1 and 2. At the same time a pulling or tensioning force is applied to the bare segments by moving the clamps correspondingly. The heated place of the fibers can be moved some distance along the juxtaposed portion by moving the clamps and or the heating source and then the aligning device is also moved correspondingly while the pulling of the fibers is continued. The heating, the movement of the heated place and the pulling operation are continued until a fused coupling segment having a desired length and thickness and thereby a desired coupling ratio is attained. The temperature of the heated portion can be estimated to be somewhere in the range of 1500 - 2100°C. After the heating the fibers 1, which are now fused to each other in the segments, are allowed to cool and the coupling segment and the coupler is finished. Finally a protective sleeve or similar protective member can be applied around the bare segments of the two optical fibers in order to protect the rather sensitive bare fiber surfaces. The length of the bare segments will after the fusioning and pulling operation be about 25 - 70 mm.

As disclosed in the paper by N.I. Karpov, M.N. Grekov, E.M. Dianov, K.M. Golant, R.R. Khrapko, "Ultra-thermostable long period gratings written in nitrogen- doped silica fibers" , Mat. Res. Assoc. Symp. Proc. Vol. 531, 1998, it appears that at elevated temperatures, above 1500°C, nitrogen atoms diffuse rather easily in silica material. For germanium atoms, normally used in the cores of optical fibers to give them an increased refractive index, at temperatures around or above 2000 °C there may be a noticeable diffusion, particularly if the heating is continued for some time period, this effect being conventionally used in widening the cores when splicing optical fibers to other optical fibers having different core diameters or when connecting optical fibers to special devices.

In Figs. 4a, 4b and 4c the refractive index and the dopant concentrations in the fibers 1 before heating are illustrated. The fibers are for reasons of simplicity here assumed to be step index fibers, as shown by the diagram of Fig. 4a, but of course also for instance graded-index fibers can be used. In the core thus, there are some rather uniform concentrations of only Ν-atoms or of Ge- and Ν-atoms, the concentration of the Ν-atoms often having a dip, i.e. is reduced, at the center of the fibers owing the special manufacturing process required for producing Ν-doped fibers. After subjecting the fibers to a heating process, as applied in making an optical fiber coupler as described above but without any pulling of the fibers the index and concentration profiles will look as illustrated in the diagrams of Figs. 5a, 5b and 5c. The cores will thus be widened, as seen in the diagram of Fig. 5a. The main effect is the diffusion and elimination of Ν- atoms as seen in Fig. 5c. The Ν-atoms will after the heating process occupy a region having a considerably lower refractive index in the core and a larger diameter than the diameter of the core before the heating. When also the pulling of the fibers during the fusioning is considered, the diagrams of Figs. 6a, 6b and 6c are obtained. Thus the special optical fibers have obtained a reduced diameter, but generally only a reduction such that their width of their thinnest parts is about 5 - 20% larger than the width of the thinnest parts in conventional couplers. At the same time the diameters of the respective regions inside the optical fibers are also reduced so that the cores after the pulling operation e.g. can have a diameter approximately equal to that of the original optical fibers, as seen in Fig. 6a.

The relative concentrations of N- and Ge-atoms in the core are chosen to give the finished coupler the desired core diameter, considering the refractive index increasing ε characteristics of the atoms of these two kinds.

By the fact that an extra dopant is used which positively contributes to the refractive index or optical density and is located in the core and which has such diffusion properties, that it will diffuse to a considerable extent during the heating operation when heating the fibers, at temperatures at or near the melting point of the silica material, i.e. ιo above 1700 °C, to form the fused segment constituting the very coupler, the optical fibers have to be pulled less than when making fused optical fibers in the conventional way from standard fibers. This in turn means that the finished coupler will totally have a larger diameter and will therefore have a higher mechanical strength such as a 40 - 200% higher strength for a single mode optical coupler. Also, the pulling operation will not be is as sensitive to mistakes since the fibers are not pulled to as small diameters and also the lifetime of the fiber optical couplers as described herein will be increased, in particular under tensile stress and in harsh environments. Also the fiber optical couplers are very stable since the diffusion of N-atoms only occurs at temperatures about or above 1800°C resulting in couplers which have very good stability below temperatures of say at least

20 1000°C.

The very good transmission characteristics including a wide wavelength range and extremely high bandwidth, low PDL (Polarization Dependent Losses) and a very good temperature stability are the same as or are better than for standard fused fiber couplers. They have a higher breaking load and a better mechanical yield in the fabrication

25 process. They can resist high temperatures and ionizing radiation, see the cited paper by V.I. Karpov et al. The couplers as described herein are particularly designed for fiber-to- the-home applications and local network applications, for WDM and DWDM systems and devices and for optical fiber amplifier devices. The couplers can be easily spliced using standard fusion splicing techniques to any other single mode or multimode fiber or con-

30 nected using any type of single mode or multimode fiber connector or other connector to any other fiber or to any device or O/E-E/O-terminal.

Claims

1. A method of manufacturing a fused optical fiber coupler, the method comprising the steps of: providing at least two optical fibers, each having a core and a cladding surrounding the core, placing the optical fibers in contact with each other along a segment, heating a region within the segment to a fusioning temperature such that the optical fibers are melted or semi-melted within the region and are fused to each other within the region and at the same time pulling or tensioning the optical fibers, so that the optical fibers are stretched within the region to become thinner within the region, characterized in that in the step of providing the at least two optical fibers, at least one of the at least two optical fibers is doped with at least one dopant to give the core an original refractive index profile, at least one of the at least one dopant being selected, so that in the heating step at the fusioning temperature said at least one dopant will rearrange and/or be at least partly eliminated to change the refractive index and to give the optical fiber a changed refractive index profile within part of the region to make modefields of light propagating in said at least one of the at least two optical fibers wider within the region than within a region of an optical fiber which originally has the same original refractive index profile and which is heated and pulled in the same way but in which any dopant does not significantly rearrange or is not significantly eliminated in the heating step and/or to change the refractive index within the region to reduce a difference between the refractive index of the core and the refractive index of the cladding.

2. A method according to claim 1 , characterized in that in the step of placing the optical fibers in contact with each other, the optical fibers are placed parallel to other or are twisted around each other.

3. A method according to claim 1, characterized in that the step of heating, the heating is made to a temperature above 1700°C.

4. A method according to claim 1 , characterized in that the rearrangement of said at least one dopant is made by diffusion.

5. A method according to claim 4, characterized in that the diffusion is accompanied by an elimination of said at least one dopant at free surfaces of the optical fibers.

6. A method according to claim 1, characterized in that in the step of providing the at least two optical fibers, said at least one of the at least two optical fibers is doped with at least said at least one of the least one dopant in the core of the optical fiber, so that in the heating step at the fusioning temperature said one of the at least one dopant will diffuse out of the core to thereby change the refractive index and to give the optical fiber a changed refractive index profile.

7. A method according to claim 6, characterized in that the diffusion out of the core is accompanied by an elimination of said at least one dopant at free surfaces of the optical fibers.

8. A method according to any of claims 6 - 7, characterized in that in the step of providing the at least two optical fibers, in the diffusion of said at least one of the at least one dopant, a widened core is formed within the region, the widened core having a diameter larger than the diameter of a core within a region of an optical fiber which originally has the same original refractive index profile and which is heated and pulled in the same way but in which any dopant does not significantly diffuse in the heating step.

9. A method according to claim 1, characterized in that in the step of providing the at least two optical fibers said at least one of the at least one dopant is chosen to comprise nitrogen.

10. A method according to claim 1, characterized in that in the step of providing the at least two optical fibers, said at least one of the at least two optical fibers is provided with a core having in addition to nitrogen a doping not containing nitrogen and increasing the refractive index in the cores.

11. A method according to claim 1 , characterized in that in the step of providing the at least two optical fibers, said at least one of the at least two optical fibers is provided with a core also having a doping of at least one material selected among Ge and B.

12. A fused optical fiber coupler comprising: at least two optical fibers, each of the at least two optical fibers having a core with a doping and a cladding surrounding the core, a region of the at least two optical fibers in which the claddings of the two optical fibers are fused to each other and in which the claddings have a diameter less than the diameter of the at least two optical fibers outside the region, characterized in that at least one of the at least two fibers, which has an original refractive index profile, contains dopant atoms capable of rearranging and/or be at least partly eliminated at temperatures used in a heating process when manufacturing the coupler and during the time period used for the heating, the rearranging being such that a considerable rearranging of the dopant atoms takes place to change the refractive index and to give the optical fiber a changed refractive index profile within part of the region to make modefields of light propagating in said one of the at least two optical fibers wider within the region than within a region of an optical fiber which originally has the same original refractive index profile and which is heated and pulled in the same way but in which any dopant does not significantly rearrange or is not significantly eliminated in the heating step and/or to change the refractive index within the region to reduce a difference between the refractive index of the core and the refractive index of the cladding.

13. A fused optical fiber coupler according to claim 12, characterized in that the core of said at least one of the at least two fibers contain dopant atoms capable of diffusing at temperatures used in the heating process when manufacturing the coupler and during the time period used for the heating, the diffusion properties being such that a considerable diffusion of the dopant atoms takes place to change the refractive index within the region.

14. A fused optical fiber coupler according to claim 13, characterized in that the dopant atoms are capable of diffusing significantly at a temperature in the range of 1500 - 2100°C, preferably at temperatures higher than substantially 1700°C and particularly at temperatures higher than 1800°C.

15. A fused optical fiber coupler according to claim 12, characterized in that the dopant atoms comprise nitrogen forming within the region deformed cores in which the nitrogen atoms have diffused out of the original core, the nitrogen atoms being in a diffused state forming a new core surrounding the original core within the region.