GB1602360A

GB1602360A - Coupling device for coupling optical fibres

Info

Publication number: GB1602360A
Application number: GB21730/78A
Authority: GB
Original assignee: Philips Gloeilampenfabrieken NV
Current assignee: Koninklijke Philips NV
Priority date: 1977-05-27
Filing date: 1978-05-24
Publication date: 1981-11-11
Also published as: DE2822211A1; BR7803320A; AU3644578A; SE7805905L; FR2396318A1; NL7705841A

Description

(54) COUPLING DEVICE FOR COUPLING OPTICAL FIBRES (71) We. N.V. PHILIPS' GLOEILAM PENFABRIEKEN. a limited liability Com pany organised and established under the laws of the Kingdom of the Netherlands. of Emmasingel 99. Eindhoven, the Netherlands. do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed. to be particularly described in and bv the following statement: The invention relates to a coupling device arranged to couple at least one pair of optical fibres to one another, comprising support means arranged to support the adjacent end portions of the respective optical fibres making up the or each said pair so that the end faces thereof oppose each other and the optical axes thereof substantiallv coincide, and a coupling medium provided between the end faces of said optical fibres.

The term "optical" as used herein. is to be understood to include not only light radiation in the visible spectrum. but also in the ultra-violet. infrared and far-infrared spectral regions.

A coupling device of the kind hereinbefore referred to. is known from U.S. Patent Specification 3,912,574. It is known that in devices of this kind use is always made of a coupling medium whose refractive index is (substantially) equal to the refractive index of the light conducting core of the optical fibres, in order to avoid loss of light by reflection at the end faces of the optical fibres. It is also known that during the coupling of pairs of optical fibres. the fibres must be accurately aligned with a common optical axis at the junction and the distance between the end faces of the fibres must be minimal. The greater the distance between the end faces, the smaller the amount of light which will be transmitted from the one fibre to the other because the light beam emerging from the end of a fibre will be divergent, and the other fibre will only intercept and conduct a part of the emergent light, the remaining light being lost as stray-radiation leading to an undesirably low coupling efficiency.

The invention has for an object to provide an improved coupling method and device for optical fibres.

According to the invention there is provided a method of coupling a pair of optical fibres to one another including supporting adjacent ends of the respective optical fibres so that the end faces therof oppose each other and the optical axes thereof substantially coincide, and applying a coupling medium having a refractive index greater than that of the respective cores of said optical fibres, to occupy the space separating the opposing end faces of said optical fibres.

According to the invention there is further provided a coupling device arranged in combination with at least one pair of optical fibres to couple the optical fibres to one another, said coupling device comprising support means arranged to support the adjacent end regions of the optical fibres making up the or each said pair so that the end faces thereof oppose each other and the optical axes thereof substantially coincide, and a coupling medium provided between the end faces of said optical fibres, wherein the refractive index of said coupling medium is made greater than the refractive index of the light conducting core of said optical fibres.

As a result of the use of a coupling medium having a refractive index greater than that of the light-conducting core of the individual optical fibres, light rays emerging from the end face of a fibre are refracted towards the direction of the optical axis of the core of the other fibre. The emergent light beam thus diverges to a smaller extent, with the result that losses due to stray radiation can be reduced. It has been found that any increase in the reflection loss which occurs at the end faces due to the difference between the refractive index of the coupling medium and that of the core of the fibre can be more than offset by the reduction in the loss caused by stray radiation. As a result of the use of a coupling medium having a refractive index greater than that of the core of the optical fibre, the coupling efficiency can be improved. It is also possible to allow a greater separation to occur between the end faces of the respective fibres or the fibres to be slightly less accurately aligned with respect to each other, while still maintaining a coupling efficiency which is equal to or greater than that achieved in the case of a coupling between two optical fibres when a coupling medium is employed having a refractive index which is the same as that of the core of the respective fibres.

In an embodiment of the invention in which the refractive index of the core varies as a function of the distance from the optical axis, the refractive index of the coupling medium is made greater than the highest refractive index of the light-conducting core of the optical fibre.

In an optical fibre whose light conducting core has a refractive index which decreases as the distance from the optical axis increases, the angle of incidence at which the core will still accept and conduct light, will decrease as the distance from the optical axis is increased. The said angle of incidence will be greatest on the optical axis and will be zero for the outermost region of the light-conducting core. It is therefore beneficial to minimize the divergence of the beam emerging from an end face. This is effected by selecting the refractive index of the coupling medium so that it is greater than the refractive index in the axial region of the light-conducting core of the fibre.

In an embodiment of the invention the refractive index of the coupling medium is more than 2% and less than 14arc greater than the refractive index of the core of the respective optical fibres in the vicinity of the optical axis.

A significant improvement in the coupling efficiency can already be provided in the case of a difference of 2% between the said refractive indices. However, when a difference of more than 14% is provided between the said refractive indices, the increase in the reflection losses may start to become greater than the corresponding reduction in the losses due to stray radiation for a given, customarily small distance between the end faces of the respective optical fibres.

In order than the invention may be clearly understood and readily carried into effect certain embodiments thereof will now be described by way of example with reference to the accompanying diagrammatic drawings, of which: Figure 1 illustrates the effect of a coupling medium having a high refractive index, Figure 2 is a graph indicating the coupling efficiency for different coupling media, Figure 3 shows a coupling device embodying the invention Figure 4 shows a further coupling device embodying the invention, and Figure 5 shows an exploded view of a coupling device embodying the invention and employing solid coupling medium.

Figure 1 shows two optical fibres la and 1b which have a common optical axis 2. The optical fibres la and 1b comprise respective light-conducting cores 3a and 3b, each having a refractive index n1, and corresponding cladding 5a and 5b, each having a refractive index n2. The distance d between the end faces 7a and 7b amounts in practice to some tens of Fm (Figure 1 is not drawn to scale). When the space 9 between the end faces 7a and 7b is filled with a coupling medium (for example, glycerine) having a refractive index n = 1.46 = n1, a light ray 11 will not be refracted when it emerges from the end face 7a; this ray will continue to travel along a path 13 inclined at the same angle 0 to the axis 2, as in the core 3a, and will be incident on the end face 7b of the fibre 1b outside the region of the core 3b.

The light ray 11 will thus be lost and will not be conducted by the core 3b. If the space 9 is filled with a coupling medium, for example, bromonaphthalene, having a refractive index n = 1.66, the light ray 11 will be refracted towards the direction of the optical axis 2 when it emerges from the end face 7a. The light ray will then continue to travel along a path inclined at an angle 02 < 01) with respect to the optical axis 2, and will be incident on the end face 7b of the fibre Ib within the boundary of the core 3b and will be conducted by the core 3b along a path 15.

It will thus be apparent that the use of a coupling medium between the end faces of fibres to be coupled can result in an improved coupling efficiency when the refractive index of the coupling medium is made greater than the refractive index of the light-conducting core of the optical fibres to be coupled.

The graph depicted in Figure 2 shows the coupling efficiency r. as defined hereinafter, as a function of the distance d between the end faces of a pair of coupled optical fibres. The refractive indices of the various coupling media serve as a parameter. The points plotted in the graph have been obtained from measurements performed on optical fibres having a core whose refractive index is a decreasing function of the distance between the optical axis and the core (so-termed graded-index fibres).

The measurement values shown in the graph have been normalised to define a coupling efficiency n = 100% measured for a coupling between two fibres where the distance between the end faces is 511m and "air" is present between the end faces. The refractive index of the light-conducting core is =1.473 on the optical axis and =1.458 at the transition between the core and the cladding. The diameter of the lightconducting core is =501lem. The refractive index of air is taken as unity. As a result, relatively large reflection losses will occur at the end faces of the fibres.

The curve I of the graph shown in Figure 2 illustrates the variation in the efficiency n with the distance d between the adjacent end faces of the coupled optical fibres using air as the coupling medium, and it will be apparent that loss due to an increase in stray radiation rises rapidly with increased separation.

The curve II in Figure 2 illustrates the result of a series of measurements carried out when glycerine. n = 1.46 was introduced as the coupling medium between the end faces of the fibres. Reflections at the end faces are in this case substantially completely avoided as is known from prior art. and this will be apparent from the improvement in the coupling efficiency, as herein defined, to 106%, measured for a distance of 5Rm between the end faces. As the distance d is increased, the coupling efficiency decreases as shown by the curve II due to increasing losses from stray radiation.

The curve III in Figure 2 illustrates the results of a series of measurements carried out using bromo-naphthalene as the coupling liquid having a refractive index n = 1.66 which can be regarded as considerably greater than the refractive index of the fibre core. Thus, significant reflection losses occurred at the end faces of the optical fibres due to the difference in refractive index. However, the light beam emerging from the end face diverged rather less than during the previous series of measurements relating to the curve II. This leads to a slight reduction in the efficiency. for a small distance between the end faces ( = 104% for a distance of 511m between the end faces) but an improvement in efficiency for greater distances. (Compare the curve II and the curve III, measured for a coupling refractive index n = 1.46 and n = 1.66, respectively).

The curve IV illustrates the results of a further series of measurements using a liquid coupling medium having a refractive index n = 1.58 (in the present example, a transparent oil such as vacuum pump oil DC-705). In comparison with the measurements relating to the curve III, the light beam emerging from an end face will diverge slightly more, with the result that the losses due to stray radiation will be slightly greater, however, reflection losses will be reduced due to the smaller difference in refractive index. It will be apparent from the curve IV that a significant improvement in efficiency was achieved for any customary distance between the end faces of fibres thus coupled over the arrangements used in respect of the curves I, II and III.

Figure 3 shows a device embodying the invention for the pair-wise coupling of optical fibres 17a and 17b, whereby optical fibres 17a and 17b to be coupled are combined in an envelope of synthetic material in order to form tapes 19a and 19b in which the fibres 17a and 17b extend parallel to one another. The coupling device comprises an injection moulding of synthetic material which is formed as two trapeziumshaped portions 21 and 22 and a flat intermediate plate-like portion 23. The faces 24 and 25 of the portions 21 and 22 slope down towards the intermediate platelike portion 23, are provided with wide guide grooves 26 and 28 for guiding the tapes 19a and 19b. The intermediate plate 23 is provided with grooves 27 in which the fibres 17a and 17b, projecting from the tapes 19a and 19b, can be guided towards each other in order to achieve a required axial alignment of corresponding pairs of fibres 17a and 17b. In the grooves 27 in the face 29 of the intermediate plate-like portion 23, a coupling medium is provided which has a refraction index which is greater than the refractive index n1 of the light conducting cores of the fibres 17a and 17b to be coupled. Preferably, the difference in refractive index amounts to approximately 7%. After provision of the coupling medium, the fibres 17a and 17b are slid as far as possible towards each other in the grooves 27. The distance between the end faces of the various fibre pairs 17a and 17b is determined by the difference in the lengths of the individual fibres resulting from cut- ting the fibres.

In order to make a permanent coupling, a coupling medium can be used in the form of a transparent coupling liquid which sets after application (for example, a setting polymer, such as Stycast-35D, n = 1.59). In the case of a detachable coupling between the fibres 17a and 17b, use can be made of an adhesive coupling liquid, for example, the transparent vacuum pump oil DC-705.

In either case it is beneficial to clamp the fibres 17a and 17b in the grooves 27 in known manner by means of a resilient cover plate (not shown), for example, made of rubber, which is clamped onto the intermediate plate-like portion 23.

Figure 4 shows a further coupling device embodying the invention, whereby two optical fibres have been coupled to each other.

The coupling device comprises a body 30 wherethrough a capillary passage 31 extends, the capillary passage having funnellike end portions at both ends. A coupling liquid 36 is forced via a nozzle 33 into the capillary passage 31 by way of a subsidiary duct 35 which opens into the capillary passage 31. An injection syringe (not shown) is connected to the nozzle 33, and a liquid coupling medium 36 is thereby forced into the capillary passage 31. The refractive index of the liquid coupling medium 36 is greater than that of the cores of the respective optical fibres 37a and 37b which are then slid or drawn into the capillary passage 31. When the fibres 37a and 37b are slid into the capillary passage 31 part of the liquid coupling medium 36 will be forced out of the capillary passage 31 via the subsidiary duct 35. If a further part of the liquid coupling medium 36 is then withdrawn via the subsidiary duct 35, for example, by means of said injection syringe the fibres 37a and 37b then will be drawn together into the capillary passage 31 and towards the subsidiary duct 35, by the further removal of coupling medium 36. After completion of the coupling of the fibres 37a and 37b. the injection syringe is removed and the nozzle 33 is sealed by means of a cap 39 of synthetic material, if the coupling liquid is not a setting coupling medium.

A further embodiment is illustrated in an exploded view in Figure 5 and comprises two coupling members 40a and 40b of a synthetic material. for example, polycarbonate, in which the optical fibres 41a and 41b to be pair-wise coupled are moulded.

The fibres 41a and 41b extend as far as a wedge-shaped front face 43a, 43b of the respective coupling members 41a and 41b and emerge at the tip of the projecting wedge shape. Both coupling members 40a and 40b are mounted on a base 45 in which a wide guide groove 47 is recessed, the coupling members 40a and 40b being pressed towards each other by means of resilient clamps 49 connected to the base 45. A solid coupling medium, for example. a foil 51 of a resilient silicon rubber having a thickness of some tens of Rm. is clamped between the coupling members 40a and 40b.

The refractive index of the foil 51 is greater than that of the light-conducting core of the fibres 41a and 41b. The foil 51 must be very supple and flexible. because it must connect with the end faces of the fibres 41a and 41b. Furthermore. the foil 51 can also be used to protect the end faces of the fibres 41a and 41b in the unassembled condition of the device to reduce the chance of dirt or dust adhering to the end faces. Just before mounting, the "protective" foil is replaced by a clean foil 51 which then serves as the coupling medium.

WHAT WE CLAIM IS: 1. A method of coupling a pair of optical fibres to one another including supporting adjacent ends of the respective optical fibres so that the end faces thereof oppose each other and the optical axes thereof substantially coincide, and applying a coupling medium having a refractive index greater than that of the respective cores of said optical fibres, to occupy the space separating the opposing end faces of said optical fibres.

2. A method of coupling a pair of optical fibres to one another substantially as herein described with reference to Figures 3 or 4 or 5 of the accompanying drawings.

3. A coupling device arranged in combination with at least one pair of optical fibres to couple the optical fibres to one another, said coupling device comprising support means arranged to support the adjacent end regions of the optical fibres making up the or each said pair so that the end faces thereof oppose each other and the optical axes thereof substantially coincide, and a coupling medium provided between the end faces of said optical fibres, wherein the refractive index of said coupling medium is made greater than the refractive index of the light-conducting core of said optical fibres.

4. A coupling device combination as claimed in Claim 3, wherein the refractive index of the core of each said optical fibre varies as a function of the distance from the optical axis of said optical fibre, and the refractive index of said coupling medium is made greater than the highest refractive index of the light-conducting core of said optical fibre.

5. A coupling device combination as claimed in Claim 3 or 4, wherein the refractive index of said coupling medium is more than 2% and less than 14% greater than the refractive index of the core of the respective optical fibres in the vicinity of the optical axis.

6. A coupling device combination as claimed in any one of Claims 3, 4 and 5 wherein said coupling medium is a transparent setting polymer.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. The coupling device comprises a body 30 wherethrough a capillary passage 31 extends, the capillary passage having funnellike end portions at both ends. A coupling liquid 36 is forced via a nozzle 33 into the capillary passage 31 by way of a subsidiary duct 35 which opens into the capillary passage 31. An injection syringe (not shown) is connected to the nozzle 33, and a liquid coupling medium 36 is thereby forced into the capillary passage 31. The refractive index of the liquid coupling medium 36 is greater than that of the cores of the respective optical fibres 37a and 37b which are then slid or drawn into the capillary passage 31. When the fibres 37a and 37b are slid into the capillary passage 31 part of the liquid coupling medium 36 will be forced out of the capillary passage 31 via the subsidiary duct 35. If a further part of the liquid coupling medium 36 is then withdrawn via the subsidiary duct 35, for example, by means of said injection syringe the fibres 37a and 37b then will be drawn together into the capillary passage 31 and towards the subsidiary duct 35, by the further removal of coupling medium 36. After completion of the coupling of the fibres 37a and 37b. the injection syringe is removed and the nozzle 33 is sealed by means of a cap 39 of synthetic material, if the coupling liquid is not a setting coupling medium. A further embodiment is illustrated in an exploded view in Figure 5 and comprises two coupling members 40a and 40b of a synthetic material. for example, polycarbonate, in which the optical fibres 41a and 41b to be pair-wise coupled are moulded. The fibres 41a and 41b extend as far as a wedge-shaped front face 43a, 43b of the respective coupling members 41a and 41b and emerge at the tip of the projecting wedge shape. Both coupling members 40a and 40b are mounted on a base 45 in which a wide guide groove 47 is recessed, the coupling members 40a and 40b being pressed towards each other by means of resilient clamps 49 connected to the base 45. A solid coupling medium, for example. a foil 51 of a resilient silicon rubber having a thickness of some tens of Rm. is clamped between the coupling members 40a and 40b. The refractive index of the foil 51 is greater than that of the light-conducting core of the fibres 41a and 41b. The foil 51 must be very supple and flexible. because it must connect with the end faces of the fibres 41a and 41b. Furthermore. the foil 51 can also be used to protect the end faces of the fibres 41a and 41b in the unassembled condition of the device to reduce the chance of dirt or dust adhering to the end faces. Just before mounting, the "protective" foil is replaced by a clean foil 51 which then serves as the coupling medium. WHAT WE CLAIM IS:

1. A method of coupling a pair of optical fibres to one another including supporting adjacent ends of the respective optical fibres so that the end faces thereof oppose each other and the optical axes thereof substantially coincide, and applying a coupling medium having a refractive index greater than that of the respective cores of said optical fibres, to occupy the space separating the opposing end faces of said optical fibres.

7. A coupling device combination as

claimed in any one of Claims 3, 4 and 5, wherein said coupling medium is a resilient, silicon rubber foil.

8. A coupling device arranged in combination with at least one pair of optical fibres to couple the optical fibres to one another, substantially as herein described with reference to any one of Figures 3.4 and 5 of the accompanying drawings.