GB1599885A

GB1599885A - Optical fibre terminations

Info

Publication number: GB1599885A
Application number: GB2806777A
Authority: GB
Original assignee: Standard Telephone and Cables PLC
Current assignee: STC PLC
Priority date: 1977-07-05
Filing date: 1977-07-05
Publication date: 1981-10-07
Also published as: CH639496A5; AU522394B2; JPS5445148A; JPS5649331B2; FR2396982A1; NL7806775A; AU3761578A; FR2396982B1; DE2828989A1

Abstract

The beam-expanding graded-index lens is provided in the end piece (11) for an optical fibre (10) either as a discrete or as an integrated part of the end piece. The optical fibre (10) is inserted into the end of the end piece opposite the lens, it being possible for said optical fibre to be fastened in the end piece by bonding or by the coincidence of the end piece sleeve in the end piece. The expansion which the lens causes in the light beam emerging from the fibre (10) permits a less precise lateral alignment of two end pieces (11, 12) in the optical coupling of a pair (10, 13) of optical fibres than would be the case without using the lens. <IMAGE>

Description

(54) OPTICAL FIBRE TERMINATIONS (71) We, STANDARD TELEPHONES AND CABLES LIMITED, a British Company of 190 Strand, London, W.C.2, England, 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 by the following statement:- This invention relates to graded index expanded beam optical fibre terminations.

According to the present invention there is provided a graded index expanded beam optical fibre termination including a socket having a blind hole for accepting the end of an optical fibre, which hole is aligned with the axis of a radially graded index collimating lens whose length is equal to an odd integral number of quarters of the characteristic sinusoidal wavelength of the lens. The above follows from the fact that with a radially graded refractive index medium whose refractive index decreases parabolically with distance from the axis, if light rays are launched from a point on the axis of the medium they follow a sinusoidal course that intersects the axis at regular intervals. Following from this, such a medium functions as a lens, and in particular, if the length of the lens is equal to an odd integral number of quarters of the characteristic sinusoidal wavelength, the lens acts as a collimating lens for the two points where the axis intersects the end faces of the lens. Use is made of this fact to form an expanded beam optical fibre termination in which an appropriate length of radially graded index fibre which acts as a lens is butted against the end of an optical fibre to transform the emergent diverging beam of light into a collimated beam of light. To couple two fibres, the lens then directs the light on to a similar lens which focusses the collected light, the collimated beam, on to the face of a further fibre. This effects an optical coupling between the two fibres which is less sensitive to small lateral misalignment of the two lens-terminated parts than would be the case if the fibres were directly butted together. It is however necessary to have the lenses accurately aligned with their respective fibres.

The invention also provides an expanded beam terminated optical fibre wherein an end of the fibre terminates inside a socket member with its axis aligned with that of a radially graded index collimating lens forming part of said socket member, wherein the length of the lens is equal to an odd integral number of quarters of the characteristic sinusoidal wavelength of the lens.

Preferably the index grading of the lens extends radially from its axis at least as far or further than the radial dimensions of the hole or fibre.

A. preferred way of making such terminations is by a 'collapse sleeve' technique to be described hereinafter. In one form of this technique a portion of the sleeve is collapsed on to a suitable length of graded index lens. In another, a portion of the sleeve is collapsed on to itself, and in this instance the sleeve is one having an index grading that provides the collapsed portion with the properties of a graded index lens.

It should be noted that, as in the case of other expanded beam terminations the arrangements to be described herein can be used for beam splitting and as couplers.

Thus a termination such as referred to in the second statement of invention can be provided with aligned socketing for two fibres, thus producing a coupler.

There follows a description of preferred embodiments of the invention, which description is with reference to the drawings accompanying the Provisional Specification in which: Fig. 1 shows a pair of optically coupled graded index lens terminated optical fibres, Figs. 2 and 3 show successive stages in the manufacture of a graded index lens terminated optical fibre, and Figs. 4, 5, and 6 show three different types of graded index lens termination for optical fibres.

Fig. 1 shows a length of optical fibre 10 terminating inside a socket member 11 whose end portion is constructed by a graded index.lens the length of which is assumed to be one quarter of the characteristic sinusoidal wavelength of the lens. The lens therefore converts the divergent beam of light emerging from the fibre end into a collimated beam. This beam is intercepted by a similar socket 12 whose graded index lens focusses the collected light on to the end of a second fibre 13.

The parabolic index grading of the lens may be described by the equation:- n=nO (lm,BZr2), where 2/ is the characteristic sinusoidal wavelength of the lens. In this case the radius of the beam launched through a point on the axis of the lens at an angle to the axis is described by the equation:- r=h sin Az where z is the distance along the axis, and h is the maximum radius of the beam.

The relation between 'h' and the launch angle 'a' (measured with respect to the axis) can be derived from this second equation by differentiating it with respect to 'z' and setting 'z' to zero, giving the equation:

If for a typical fibre having a numerical aperture of 0.18 (a=0.12) the diameter of the expanded beam may be required to be about 400y (2h=400y), in which case mm~l, and a quarter of the characteristic wavelength (L) is approximately 2.6 mm.

Typical tolerances for this would be 1020y in the length of the lens, 4y in lateral misalignment of each fibre with respect to its lens axis, 0.1 angular misalignment, and 25p in lateral misalignment between the axes of the two lenses.

Figures 2 and 3 show stages in a preferred method of making a termination using a collapsed sleeve technique employing a sleeve 20 which has a tubular substrate portion 20a whose bore is lined with a lining 20b the refractive index of which is graded.

This sleeve may be made by the method used in an early stage of optical fibre preform manufacture described in our Patent Specification No. 1,427,327 in which a succession of layers of progressively changing composition are deposited by a hydrogen free chemical vapour reaction upon the bore of a substrate tube. In particular in the termination of graded index fibre made by this chemical vapour reaction deposition method, it is convenient Rto derive the fibre and the sleeve from the same or similar stock in order that their numerical apertures may be conveniently matched. The sleeve will not normally be as large in diameter as the original tubular preform, but can be made from the preform by heating the preform so as to cause its bore to shrink under the action of surface tension while it is rotated about its axis to assist preserving its symmetry.

In a typical example a graded index lens termination approximately 400,um in diameter was required for a graded index optical silica fibre having an external diameter of 110 ym and a graded index core 30 ,um in diameter. For this purpose a sleeve having a 120 zm diameter bore to fit over the fibre end requires the thickness of the lining to be about 150 ,um. If the sleeve is prepared from the same tubular preform stock as the fibre, then the external diameter of the sleeve will be about 1.47 mm.

Normally a drawing operation will be required to prepare the sleeve 12 from the tubular preform since mere partial collapse of its bore at constant length will not provide the required dimensions.

The end of fibre 21 is inserted into the bore of the sleeve 20 to the required depth (Figure 2) and then the assembly is heated to cause the bore of the sleeve to collapse on to the fibre in the region occupied by the fibre, and to collapse on to itself in the region beyond the fibre (Fig. 3), the proper location of the fibre end within the sleeve may be observed through the sleeve wall with the aid of a microscope, or it may be located by spacing it the appropriate distance from a reference surface with the aid of some form of slip gauge which is then removed before the sleeve end is brought up against the reference surface.

It will be appreciated that the flow of glass round the fibre end will produce a localised region where the refractive index grading is liable to be rather poorly defined.

The effect of this will be to make the appropriate depth of insertion of the fibre end to be slightly different from that produced by calculation based on the assumption that the index grading is uniform from the face of the lens right up to the fibre end. The requisite 'correction' factor may be found by experiment and this will normally be substantially constant for similar terminations provided that the collapse conditions are not significantly changed. Fine adjustment may be made with a final polish of the collapsed end face.

The fibre around which the sleeve is collapsed may be a relatively short tail suitable for splicing on to the end of a longer length of optical fibre.

Alternatively, the fibre around which the sleeve is collapsed may be, as shown in Fig.

4, a stub 41 that is so short that neither end protrudes from the sleeve. In this instance the sleeve bore is collapsed on to itself beyond only one end of the stub so that a socket 43 is left at the other end into which a fibre (not shown) can subsequently be secured, either with a suitable adhesive such as an ultra voilet light curing index matching adhesive, or by heating this end of the sleeve to collapse it on the fibre.

To provide a coupler, either two devices each as shown in Fig. 3 and 4 are used with their end faces aligned, or a single such device is used with two aligned fibres terminated therein, the distance between the fibre ends after collapse being that appropriate an even number of quarterwavelength.

In the arrangement depicted in Fig. 5, the stub of optical fibre has been dispensed with, and instead the sleeve is collapsed on to a mandrel (not shown), for instance of metal, which is then subsequently removed to leave a socket 53. The socket 53 serves the same function as the socket 43 of the previously described embodiment.

Although previous reference has been made to the use of silica for the sleeve, it should be appreciated that such sleeves may alternatively be made of more conventional multicomponent glasses. Since the length of the lens is short compared with the lengths of fibre with which it will normally be used, it will not be necessary for its optical loss per unit length to be as low as that of the fibre. For connecting silica fibre the use of silica sleeve will have the advantages of thermal expansion match, stability, and of a suitably high softening point to facilitate deposition of the graded index lining 20b by a direct oxidation chemical vapour reaction. But the high temperature of the softening point makes the collapsing of the sleeve more difficult, particularly, if this is to be effected in the field. However, since the optical loss per unit length can be higher than that of the fibre it is not necessary to employ a hydrogen-free deposition reaction and so a lower temperature reaction can be chosen than that of the direct oxidation reaction preferred for the manufacture of the fibre.

In each of the embodiments so far described, the graded index lens has been formed by a portion of the sleeve itself. In an alternative method of construction the lens is originally a discrete integer which is fitted into the bore of a sleeve and then secured in position by collapsing the surrounding sleeve on to the curved surface of the lens. This produces a structure of the type depicted in Fig. 6 in which a graded index lens 60 is secured at one end of a glass sleeve 61. The lens diameter maybe slightly smaller or slightly larger than the fibre diameter, but needs to be more closely matched than is the case in the previously described embodiment.

The more restricted beam expansion requires a steeper grading of index and hence a shorter characteristic wavelength of the lens in order to retain the same numerical aperture. Thus, in order to obtain the same numerical aperture for a graded index lens of only 120 ,um diameter it is necessary to strengthen the lens so that a quarter of its characteristic wavelength is reduced from about 2.6 mm to about 0.8 mm. This may be considered rather short to ensure adequate angular alignment of the lens within the collapsed portion of the sleeve, and therefore it may be preferred to use a lens whose length is a small odd integral number of quarters of the characteristic wavelength in order to increase the slenderness ratio of the lens.

Thus in a typical example of termination for a 100 ,um external diameter fibre with a 30 ,am diameter graded index core whose numerical aperture is 0.18, the sleeve may be about 15 mm long while the lens is made 3/4 or 5/4 of the characteristic wavelength of refractive index profile. These dimensions must be kept to a strict tolerance in order to preserve the same quality of imaging.

Reflection loss at the end face of the termination can be reduced by providing it with a reflection-reducing interference coating, but this will not be required if terminated fibres are to be optically coupled by butting their end faces together to form an interface quality join.

Alternatively, if the fibres are to be coupled in a manner involving the spacing of the end faces from each other, this space can advantageously be filled with an index matching medium.

Any form of connector for effecting an optical coupling between one or more pairs of these terminated fibres can make use of known methods of achieving butt joints.

The expansion of the beam diameter has the effect of making the lateral alignment tolerance less severe at the expense of a more stringent angular alignment requirement. Providing that the termination end faces are accurately normal to their axes, the necessary angular alignment can be achieved simply by butting their end faces together to form a fit of interface quality.

In the above described examples the numerical aperture of each lens termination was designed to match that of its fibre but for some applications it may be desirable to choose a smaller numerical aperture for the lens in order that it shall act as a filter to reject certain of the higher order modes that would otherwise be launched into the next fibre.

WHAT WE CLAIM IS: 1. A graded index expanded beam optical fibre termination, including a socket having a blind hole for accepting the end of an optical fibre, which hole is aligned with the axis of a radially graded index collimating lens whose length is equal to an odd integral number of quarters of the characteristic sinusoidal wavelength of the lens.

2. A termination as claimed in claim 1, wherein the socket is in a substantially cylindrical member of a transparent material with the light from the fibre leaving the inner end of the blind hole so that the portion of the cylindrical member beyond said inner end acts as the lens, the collimation thus occurring within the cylindrical member, and wherein the end face of the cylinder opposite from the outer end of the blind hole is normal to the axis of the cylindrical member.

3. An optical fibre coupler which includes two terminations each as claimed in claim 2 with their end faces parallel to and adjacent to each other.

4. A termination as claimed in claim 1, and wherein the socket is formed by collapsing a sleeve of a radially graded index material about the end of a fibre to be terminated, the radially graded index material beyond the end of the fibre end forming the lens.

5. A termination as claimed in claim 4, and wherein the radially graded index material sleeve is on the inside of a substrate sleeve which is collapsed with the firstmentioned sleeve.

6. A termination as claimed in claim 4 or 5, modified in that two fibres are fitted into opposite ends of a said sleeve of said radially graded index material, the length-of the sleeve being such that the distance between the fibre ends after the collapse is substantially equal to an even number of said quarter wavelength.

7. A termination as claimed in claim 1, wherein the socket is formed by collapsing a sleeve of a radially graded index material about a stub of an optical fibre in such manner that the blind hole is left at one end of the stub and axially aligned therewith, and wherein the material of the sleeve at the other end of the stub forms the lens.

8. A termination as claimed in claim 7, and wherein the radially graded index material is on the inside of a substrate sleeve which is collapsed with the first-mentioned sleeve.

9. A termination as claimed in claim 1, wherein the lens is a stub of a radially graded index material where length is equal to an odd integral number of said quarterwavelength about which there is collapsed a sleeve of a suitable glass, such that a socket is left at one end of the sleeve, which socket is axially aligned with the stub, and wherein the opposite end face of the stub and the associated end of the sleeve are co-planar.

10. An optical fibre coupled which includes two terminations each as claimed in claim 4, 5, 7 or 8 with their non-blind hole end faces parallel with and adjacent to each other.

11. An expanded beam terminated optical fibre wherein one end of the fibre terminates inside a socket member with its axis aligned with that of a radially graded index collimating lens forming part of said socket member, wherein the length of the lens is equal to an odd number of quarter of the characteristic sinusoidal wavelength of the lens.

12. A graded index expanded beam optical fibre termination, substantially as described with reference to Fig. 1, Figs. 2 and 3, Fig. 4, Fig. 5 or Fig. 6 of the drawings accompanying the provisional specification.

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

Claims

**WARNING** start of CLMS field may overlap end of DESC **. reject certain of the higher order modes that would otherwise be launched into the next fibre. WHAT WE CLAIM IS:

1. A graded index expanded beam optical fibre termination, including a socket having a blind hole for accepting the end of an optical fibre, which hole is aligned with the axis of a radially graded index collimating lens whose length is equal to an odd integral number of quarters of the characteristic sinusoidal wavelength of the lens.