Classifications

• • G—PHYSICS
  • G02—OPTICS
  • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
  • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
  • G02B6/24—Coupling light guides
  • G02B6/36—Mechanical coupling means
  • G02B6/38—Mechanical coupling means having fibre to fibre mating means
  • G02B6/3801—Permanent connections, i.e. wherein fibres are kept aligned by mechanical means
• • G—PHYSICS
  • G02—OPTICS
  • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
  • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
  • G02B6/24—Coupling light guides
  • G02B6/26—Optical coupling means
  • G02B6/262—Optical details of coupling light into, or out of, or between fibre ends, e.g. special fibre end shapes or associated optical elements
• • G—PHYSICS
  • G02—OPTICS
  • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
  • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
  • G02B6/24—Coupling light guides
  • G02B6/36—Mechanical coupling means
  • G02B6/38—Mechanical coupling means having fibre to fibre mating means
  • G02B6/3801—Permanent connections, i.e. wherein fibres are kept aligned by mechanical means
  • G02B6/3802—Assembly tools, e.g. crimping tool or pressing bench

Definitions

• This invention relates to a method of connecting optical fibres and apparatus for making optical fibre connections. 5 Optical fibres are now very widely used in telecommunications networks.
• Optical fibres are well suited for such use as they enable the transmission of high bit-rate data streams over long distances.
• One of the relative disadvantages of optical fibres is the relatively high losses that can occur when optical fibres are jointed.
• Optical fibres are typically 250 ⁇ m in diameter with a composite glass
• the glass centre which carries the optical signal, surrounded by a layer, or layers, of mechanically protective plastics material.
• the glass centre is typically 1 25 ⁇ m in diameter and comprises two regions, the core, where the bulk of the optical radiation is guided, and the cladding which surrounds the core.
• the core In multi-mode optical fibres the core has a diameter of 40-80 ⁇ m, typically 50 ⁇ m or 62.5 ⁇ m.
• Single-mode fibre is of greater interest as it can support higher bandwidth transmission over longer distances.
• the core of a single-mode fibre is typically 8-1 O ⁇ m in diameter.
• the glass is typically silica based, with index increasing dopants, such as germania, in the core and/or index reducing dopants in the cladding.
• Fluoride glass optical fibres also
• the plastics layer, or layers, which surround the cladding are commonly referred to as the primary coating, even when there are multiple plastics layers.
• the primary coating is to provide mechanical protection to the optical fibre.
• the primary coating has multiple layers the innermost layer, which is adjacent the glass of the fibre, will typically be a 'soft' material, i.e. have a low elastic modulus, while the outer layer will typically be a 'hard' material with a higher elastic modulus.
• the electric moduli of the primary costing are orders of magnitude lower than those of the glass portion of the fibre. Typical values for the
• elastic modulus of inner primary coating layers are a few megapascals (MPa), for outer primary layers approximately several hundred MPa and approximately 70,000 MPa for the glass region of an optical fibre.
• the optical fibre When the optical fibre is transmitting an optical signal, the light propagates primarily within the core of the fibre.
• the refractive index of the core is chosen to be greater than the refractive index of the cladding, so that total internal reflection constrains the light within the fibre's core.
• the cores of the fibres When it is desired to joint two optical fibres together the cores of the fibres must be aligned so that the light being transmitted in the core of the first fibre is coupled into the core of the second fibre. Therefore when jointing optical fibres, especially, single mode fibres, it is necessary to position the two fibres with great precision in order to couple sufficient power between the cores of the fibres.
• the two optical fibres that are to be joined should have end faces that have been accurately cleaved so as to prevent the transmitted light being scattered by the irregular geometry of the end face.
• a 5/vm lateral offset between the two fibres can introduce an additional loss of 0.5 dB; a 25 ⁇ m longitudinal offset can introduce an additional loss of 1 dB and the failure to accurately cleave the two fibre ends can increase the connection loss by up to 20 dB.
• optical fibre jointing techniques can be very time consuming. There are occasions, e.g.
• optical fibre cables can contain hundreds of fibres, each of them transmitting data at rates of up to 1 0 gigabits per second. If a mechanical digger were to sever such a cable then an enormous data flow would be disrupted.
• an optical fibre connector unit for connecting optical fibres including a primary coating, the unit comprising:
• anchoring means configured to secure, in use, an end of the optical fibre within the groove, characterised in that the anchoring means apply, in use, a radial compressive force to the optical fibre secured within the groove so as to deform the primary coating of the optical fibre, the deformation of the optical fibre being constrained by the groove.
• the groove and the anchoring means co-operate to provide a clamping force which is sufficient to deform the fibre's primary coating, thereby improving alignment and hence reducing coupling loss.
• an optical fibre connector device comprising first and second optical fibre connector units as described above, optically coupled by a connecting optical fibre, the first optical fibre connector unit accepting, in use, a first optical fibre end and the second optical fibre connector unit accepting, in use, a second optical fibre end, the first optical fibre end and the second optical fibre end being secured, in use, in the groove of the respective optical fibre connector unit, so that optical radiation can pass from the core of the first optical fibre end to the core of the second optical fibre end, via the core of the connecting optical fibre.
• the optical fibre connector device comprises an optical amplifier arranged, in use, to amplify the optical radiation transmitted by the connecting optical fibre.
• a third aspect of the present invention there is provided a method of connecting first and second optical fibres, at least the second optical fibre including a primary coating, the method comprising the steps of:
• optical fibres may be connected using apparatus as described above.
• a fourth aspect of the present invention there is provided a method of restoring communications services carried by an optical fibre transmission link following the breaking of said optical fibre transmission link, the method comprising the steps of;
• each optical fibre connector unit comprising anchoring means and a groove, the first and second anchoring means securing respectively the first and second optical fibres of the optical fibre transmission link within respective first and second grooves such that the optical fibres are arranged to couple light from the core of the first optical fibre of the optical fibre transmission link to the core of the second optical fibre of the optical fibre transmission link via the core of the connecting optical fibre of the optical fibre connection device.
• the first and second anchoring means respectively apply a compressive radial force to the first and second optical fibres of the optical fibre transmission link, the deformation of the primary coating of first and second optical fibres being limited by the geometry of the respective first and second grooves.
• the light being coupled from the core of the first optical fibre of the optical fibre transmission link to the core of the second optical fibre of the. optical fibre transmission link may be amplified intermediate the first optical fibre of the optical fibre transmission link and the second optical fibre of the optical fibre transmission link.
• Figure 1 shows a schematic depiction of the optical fibre connection device according to the present invention
• Figure 2 shows a schematic depiction of an optical fibre connection unit according to the present invention
• Figure 3(a) shows a schematic depiction of the profile of an optical fibre when located within a groove
• Figure 3(b) shows a schematic depiction of the profile of an optical fibre when located within a groove and subjected to a compressive force
• Figure 4 shows a graph indicating the theoretical variation of fibre-to-fibre coupling loss with lateral core offset
• Figure 5 is a plan view of an example of a groove suitable for use with the present invention.
• Figure 6(a) shows a schematic depiction of the structure of a standard telecommunications optical fibre
• FIG. 6(b) shows a schematic depiction of the structure of a thermally expanded core (TEC) optical fibre
• Figure 7 is a graph indicating the measured variation of fibre-to-fibre coupling loss with lateral core offset for both standard telecommunications optical fibre and thermally expanded core (TEC) optical fibre.
• Figure 1 shows a schematic depiction of the optical fibre connection device 1 which comprises an optical amplifier 2 and two optical fibre connector units 3.
• the optical amplifier 2 is connected in series with the two optical fibre connector units 3 using optical fibre 4.
• the aim of the optical fibre connection device is to connect the two fibre ends that have been formed following the severing of an optical fibre cable.
• Figure 2 shows a schematic depiction of an optical fibre connection unit 3.
• the optical connection unit 3 comprises a block 1 0 with a precision formed groove 5, first anchoring means 6, a prepared end 7 of optical fibre 4 and a second anchoring means 8.
• the accidentally severed fibre 9 is placed in the groove 5 and positioned such that the fibre ends 7 and 1 1 are in close proximity.
• the severed fibre 9 is then secured using first anchoring means 6.
• the severed optical fibre does not need to be specially prepared prior to its use with the optical fibre connection device 1 .
• the end 1 1 of the severed fibre can be cut with a sharp knife or a pair of scissors without affecting the functioning of the optical fibre connection device.
• refractive index matching liquid 1 2 is placed on the end of the severed fibre. This index matching liquid also reduces the reflections between the end faces of the two optical fibres, further reducing the insertion loss of the optical fibre connection device 1 .
• first anchoring means 6 is shown in an open position. After severed fibre end 1 1 has been prepared and suitably positioned in groove 5, the first anchoring means 6 is closed to secure severed fibre 9 in position.
• the amount of light that is coupled from fibre 9 to fibre 4 will depend upon the alignment of the cores of the two different fibres.
• the core, cladding and primary coating all have a circular cross-section and are concentric.
• the fibre components may have a degree of ovality and eccentricity, introducing an element of variability into the position of the core with respect to the outer surface of the fibre.
• the maximum extent of ovality and eccentricity will be specified by purchasers of fibre and fibre manufacturers will produce fibres having ovality and eccentricity values that are statistically distributed within the limits of that specification.
• fibre manufacturing processes provide better control for the core and the cladding than for the primary coating so that the core and the cladding tend to be less oval than the primary coating and the concentricity of the core and cladding tends to be greater than the concentricity of the core and primary coating.
• the primary coating of a fibre is removed there is an increased probability of locating the core at the centre of the fibre.
• both cores are at the centre of the respective fibres then it is a simple matter to align them. However the variation of the . manufacturing processes mean that it is unlikely that both cores will be at the centre of the fibre. If a fusion splice were to be used to connect the two fibres then some form of alignment, either core alignment or cladding alignment, would be used to reduce the loss of the splice. However, this is time consuming and requires expensive equipment. For a rapid restoration device neither of these limitations are acceptable, it is, however, possible that the optical fibre 4 can have its primary coating removed before it is secured in the optical fibre connector unit, and this will remove one source of variability in the position of one of the fibre cores, helping to reduce the coupling loss of the connector.
• the fibres can be connected without having to perform this step (as optical fibre 4 will be secured in the optical connector unit within a manufacturing environment the additional time required to remove the coating is not important) .
• the time saving accrued from not having to prepare the fibre ends is about 1 minute per fibre end when compared with mechanical fibre splices such as 3M's FibrlokTM and even more when compared with fusion splicing. For a severed 240 fibre cable repaired using the present invention rather than 3M's FibrlokTM the time taken to restore service on all fibres can be reduced by up to 8 hours.
• the length of optical fibre 4 should be sufficient so as to allow the fibres to be temporarily connected until an additional length of cable can be installed.
• Figure 3(a) shows schematically the profile of an optical fibre 9 which is located in a groove 5.
• Figure 3(a) shows that the core 1 3, cladding 1 4 and coating 1 5 are not entirely concentric, as discussed above.
• the likely position of the fibre core when the fibre occupies the groove can be calculated. From this information the optical connector unit can be formed so that the two fibre cores are in alignment.
• first anchoring means 6 we have found that by applying a sufficient compressive force with first anchoring means 6, that the fibre (in fact the primary coating of the fibre) can be deformed subject to the geometry of groove 5, reducing the variation in the position of the fibre core.
• anchoring means 6 is used to apply a compressive force to the fibre, deforming it within groove 5.
• This applied force reduces the eccentricity of the core within the fibre and this reduction in variability can be used to change the design of the optical connector unit to reduce the coupling loss between the two fibres.
• Table 1 shows the variation in vertical offset of the core for a number of standard telecommunications fibres measured with respect to the nadir of the V-groove. In this case the groove was a V-groove which had a depth of 1 35 ⁇ m and an angle of 80°.
• the same fibre samples were used in testing with both the grooves.
• the fibre samples used in the test had a soft, innermost primary coating layer and a harder, outermost primary coating layer. Each average was obtained from 3 different measurements. In between each measurement the fibre sample was removed from the groove, its end face was cleaved, the sample was rotated by 1 20° and then returned to the groove. The off set of the fibre core from the nadir of the groove was measured both before and whilst the fibre was compressed by the action of the anchoring means. The standard deviation of the fibre core offset with the groove having a depth of 1 35 ⁇ m was 14.5 ⁇ m. When the fibre was compressed the standard deviation of the core offset was only 9.4 ⁇ m.
• FIG. 4 is a graph showing the theoretical variation of fibre-to-fibre coupling loss with core lateral offset, which indicates that even reducing the core offset by a few microns will lead to a significant reduction (even as much as a few dBs) in the light that is lost between the two fibres.
• V-groove when the fibre is compressed the geometry of the V-groove will act to centre the core of the fibre so that both the horizontal and the vertical core offsets are reduced.
• the use of a V-groove which is of comparable width to the fibre will also act to reduce the likelihood of the fibres having an angular misalignment, which will prevent further increases in coupling loss.
• Figure 5 is a plan view of an example of a groove 5 that could be used in the optical fibre connection unit 3 of the invention
• the groove has three regions 51 , 52 & 53 which accommodate the two optical fibres 4 and 9.
• Optical fibre 4, which is part of the optical fibre connection device 1 has been stripped of its primary coating and its end face has been accurately cleaved.
• the stripped portion of optical fibre 4 is secured within region 52 of the groove 5.
• Region 52 has a V-shaped cross- section with a maximum depth of 1 56 /m and a maximum width of 1 56 ⁇ m.
• Region 51 holds the coated portion of fibre 4 and has a rectangular profile with a width of 260 ⁇ m and a depth of 1 25 ⁇ m.
• Fibre 9 is held in region 53 of the groove 5, which has a width of 300 ⁇ m and a depth of 1 50/vm.
• regions 51 , 52 and 53 it is necessary to form the groove to a very tight tolerance, e.g. + 1 ⁇ m. Any method which meets the above dimensional tolerances can be used to profile the regions of the groove, for example, excimer laser profiling.
• the block 10 within which the grooves 51 , 52 and 53 are formed can be made from any material that is compatible with the method used to form the grooves and is sufficiently rugged for temporary deployment in an external network environment. It will be clearly understood that the grooves may be formed either directly within the block or within an additional piece that is subsequently placed on or within such a block. Also, it will be understood that the material from which the grooves are formed must be sufficiently rigid to withstand the forces caused by the clamping of the fibre and thus cause the coating of the fibre to deform. Examples of such materials are, etched silicon, stainless steel, aluminium (or an aluminium alloy) or polymeric materials, such as, for example, polymethyl methacrylate (PMMA)
• the anchoring means may be clamps or any other suitable form of restraint.
• a preferred anchoring means is a sprung clamp.
• the anchoring means may be designed to impose a constant force on the optical fibre or to compress the fibre by a constant displacement.
• a constant displacement anchoring means is preferred as this results in a smaller variation of fibre core position with respect to the nadir of the groove. If a constant force anchoring means is used then the fibre core position when under compression will be dependent upon the compression modulus of the fibre coating(s) leading to unwanted core position variations that will lead to unacceptable coupling losses.
• a favoured constant displacement anchoring means is one that is designed, in conjunction with the dimensions of the groove, to compress the fibre so that its effective diameter is midway between the diameter of the outer coating layer and the cladding diameter.
• the diameter of the outer coating layer is 250 ⁇ m and the cladding diameter is 1 25 ⁇ m, giving a target effective diameter of approximately 1 87.5 ⁇ m. Attempts to use a much smaller effective diameter involved the application of such high compression forces that, for some materials, physical damage was sustained by the groove and the block within which the groove if formed.
• the insertion loss of the optical fibre connection device 1 can be further reduced by the use of thermally expanded core (TEC) fibre.
• TEC fibre is manufactured by heating standard telecommunications fibre so that the dopants that define the size of the core region diffuse to give a larger core size. This diffusion process can be controlled to give a tapered core, as shown in Figure 6.
• Figure 6(a) shows a schematic representation of the core 1 3 and the cladding 14 of a standard optical fibre.
• Figure 6(b) shows a schematic representation of a TEC . , fibre which has been processed so that the core size 1 3 tapers from an expanded size, e.g. a diameter of approximately 30-50 ⁇ m, to substantially the same size as the core of a standard telecommunications fibre, e.g.
• optical fibre 4 is a TEC fibre with a suitably tapered core then the larger core size decreases the fibre-to-fibre coupling loss between fibres 4 and 9.
• Figure 7 shows a graph that indicates the variation of fibre-to-fibre coupling loss with lateral offset for both standard optical fibre (represented by triangular symbols) and TEC fibre (represented by square symbols).
• Figure 7 shows that for larger lateral offsets the use of TEC fibre gives a lower insertion loss for an optical fibre connection unit 3.
• the processing required to give a tapered core as shown in Figure 6(b) also cause an increase in the attenuation of the fibre, i.e. the attenuation of TEC fibre tends to be higher than the attenuation of standard telecommunications fibre.
• the fibre-to-fibre coupling loss may be greater for TEC fibre when compared with standard fibre, but TEC fibre can withstand a greater lateral offset before the fibre-to-fibre coupling loss becomes too great.
• the TEC fibre used was obtained from Kyocera and it had a maximum core diameter of approximately 40 ⁇ m (as compared with 8-1 O ⁇ m for standard telecommunications fibre) . It should be understood that fibre with a core that has been expanded using other processing techniques, i.e. selective fibre doping, would be equally suitable for use in the invention.
• a typical insertion loss for the fibre to fibre coupling in an optical fibre connection unit 3 is 5 - 10 dB.
• an optical amplifier 2 can supply typically 1 0-25 dB of gain, which should compensate for the typical losses within the optical fibre connection device 1 , without significantly effecting the system margin of the transmission system.
• Either a semiconductor optical amplifier or an optical fibre amplifier may be used.
• Semiconductor optical amplifiers have the advantage of occupying a significantly smaller volume than a fibre amplifier, which consideration would become important when a large number, i.e. a few hundred, of amplifiers were needed to be used at a single site. The amount of power required to operate the amplifiers is not high and could be met by a low capacity generator if the amplifiers must be operated in a remote location, away from mains electricity supplies.
• connection devices and rapid restoration kits described above have been designed with single-mode fibre in mind. However, it will be readily understood that the same principles can be applied to connection devices for multi- mode fibres. Because of the relatively larger core size of multi-mode fibres the alignment of the fibre cores to promote efficient light coupling is somewhat easier and should lead to much lower connection losses. Thus it would be possible to omit some of the steps that are taken when connecting single-mode fibres, e.g. optical amplifiers might not be needed, expanded core fibres might not be necessary, lower-tolerance manufacturing processes could be used to form the grooves in the fibre connection unit, etc., leading to a cheap and simple multi- mode fibre connector that could be of great use for in-building applications, such as fibre to the desktop (FTTD) .
• FTTD fibre to the desktop

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• General Physics & Mathematics (AREA)
• Optics & Photonics (AREA)
• Mechanical Coupling Of Light Guides (AREA)

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• 1998
  • 1998-10-29 EP EP98950216A patent/EP1046072A1/de not_active Withdrawn
  • 1998-10-29 AU AU96372/98A patent/AU9637298A/en not_active Abandoned
  • 1998-10-29 WO PCT/GB1998/003233 patent/WO1999024857A1/en not_active Application Discontinuation

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