GB2238396A - Optical waveguide taper having core, interlayer and cladding - Google Patents

Abstract

An optical waveguide adiabatic taper includes an interlayer 32 between its optical core 30 and its optical cladding 31 in order to "square-off" the optical field profile at the larger end of the taper while leaving the profile at the smaller end substantially unchanged. This finds particular application as a component of a 1 x N single mode coupler. The smaller end of the taper can make a low-loss coupling with a single single mode fibre input, and the larger end will distribute the available power more evenly between the members of a close-packed array of N single mode output fibres than would be the case in the absence of the interlayer 32. The refractive index of the interlayer 32 is greater than that of the core 30 whose refractive index is greater than that of the cladding 31. <IMAGE>

Description

Optical Waveguide Tapers This invention relates to optical waveguide tapers, particularly adiabatic tapers. One application for such tapers is in the construction of 1 x N single mode optical fibre couplers. For this purpose the single input fibre is optically coupled with the small end of the taper while the N output fibres are arranged in side-by-side close-packed array optically coupled with the large end of the taper.

One of the problems associated with the design of such a 1 x N coupler is the loss of coupling efficiency associated with packing fraction considerations. In conventional constructions of single mode fibre the spot size of the single mode is usually quite a small proportion of the total area of the end face of the fibre. Therefore, even with close-packing of the fibres, much of the light launched from the taper fails to couple into the fundamental core modes of the N output fibres. This situation can be improved by providing each of the N output fibres with its own adiabatic taper to increase its spot size so as to match more nearly the size of its end face.

A second problem associated with the design of such a 1 x N coupler concerns the uniformity with which the available optical power is shared between the individual members of the set of N output fibres.

Hitherto adiabatic tapers for 1 x N couplers have been made with a step-index profile in which the optical core of the taper is directly surrounded by an optical cladding of a material of lower refractive index. (Such a cladding may itself be surrounded by further material that provides mechanical support for the fibre but, because the optical field of the propagating fundamental mode does not noticeably penetrate into this supporting region, the value of its refractive index is immaterial to the propagation of the light).The distribution of optical power of the fundamental mode of a step-index taper exhibits a bell-shaped, centrally peaked, profile across the end face of the core, and hence at the large end considerably more power is launched into those of the output fibres that register with the central region of this end face than is launched into those of the fibres that register with peripheral regions of the end face of the core.

The present invention is concerned with the provision of a structure of adiabatic taper providing a more nearly uniform distribution of fundamental mode optical power across the core region of its larger end face.

According to the present invention there is provided an optical waveguide adiabatic taper having an interlayer of a first material between an adiabatically tapered optical core of a second material and an optical cladding of a third material, wherein the refractive index of the second material is greater than that of the third material and the refractive index of the first material is greater than that of the second material, and wherein the thickness of the interlayer in relation to the size of the optical core and the relative refractive indices of the first, second and third materials is such as to provide, for light launched in fundamental mode into the smaller end face of the taper, a more nearly uniform optical power distribution across the larger end face of the optical core than would result in the absence of the interlayer.

There follows a description of optical waveguide tapers embodying the invention in preferred forms. Also described, is a 1 X N single mode optical fibre coupler employing such an optical waveguide taper. The description refers to the accompanying drawings, in which: Figures la and lb depict refractive index and fundamental mode optical field profiles for two optical waveguides having a 500 pm core diameter, respectively without and with an interlayer between the core and cladding.

Figures 2a and 2b depict similar profiles for waveguides having a 200 pm core diameter.

Figure 3 depicts a taper drawn from a waveguide like those described with reference to Figures lb and 2b.

Figure 4 depicts a 1 x N single mode fibre coupler, and Figure 5 schematically depicts apparatus employed to form adiabatic tapers in optical fibre waveguides.

In Figure la there is plotted at 10a the refractive index profile of a large diameter cylindrical optical waveguide comprising a 500 pm diameter fused silica optical core surrounded by a 30 Fm thickness doped fused silica optical cladding which in its turn is surrounded by fused silica sleeving. The level of doping in the optical cladding is arranged to depress its refractive index relative to undoped fused silica by 1 x 10 3. Using the waveguide equation for cylindrical optical waveguides and inserting the appropriate boundary conditions for this particular structure of waveguide, the corresponding plot ila describes the optical field distribution of the fundamental mode of that waveguide at a wavelength of 1300 nm.Plot Ila indicates that there is a marked reduction in optical field as one proceeds from the centre of the core of the waveguide towards the periphery of the core.

In the plots 10b and 11b of Figure Ib there are provided equivalent plots in respect of a structure differing from that of Figure la only in that an 11.2 pm thickness interlayer of doped silica has been interposed between core and cladding with a corresponding reduction in core radius of 11.2 pm. The doping of the interlayer is to provide it with a refractive index raised by 1.4 x 10 4 above that of undoped fused silica. From plot llb it is seen that the inclusion of the relatively thin interlayer of slightly raised refractive index has produced a marked change in optical field distribution of the fundamental mode at a wavelength of 1300 nm, producing an almost uniform field distribution over substantially the whole area of the core and then a rapid decay.

From an examination of the waveguide equation it may be ascertained that, to achieve a substantially equivalent squaring-off of fundamental mode optical field profile for a waveguide with a somewhat smaller core diameter but the same core/cladding refractive index difference, the thickness of the interlayer may be reduced in proportion with the reduction in core diameter while its refractive index is raised still further above that of the core. Plots 20a and 21b of Figure 2a respectively depict the refractive index and fundamental mode optical field profiles for a 200 pm diameter optical fibre with no interlayer between its optical core and its optical cladding.Similarly plots 20b and 21b of Figure 2b respectively depict the profiles when a 4.4 pm thickness interlayer with a refractive index 6 x 10 above that of the fused silica of the core is included between the core and the cladding. As before, the optical field profile is plotted for a wavelength of 1300 nm, and the optical cladding has a refractive index 1 x 10 3 below that of the undoped silica of the core. The optical cladding thickness has also been reduced in proportion with the reduction of core diameter.

A large diameter waveguide as described above with reference to Figure lb or 2b may be subjected to a controlled drawing operation to produce a taper as schematically depicted in Figure 3. In Figure 3 the undoped fused silica optical core is depicted at 30, its doped fused silica optical cladding at 31, its doped fused silica interlayer at 32, and its fused silica sleeving at 33. In respect of a particular example of taper, at the large end 34 of the taper, the core 30 is approximately 500 pm in diameter, the thicknesses of the interlayer 32 and optical cladding layer are respectively 11.2 pm and 30 pm, and the wall thickness of the sleeving such as to provide at this end an overall diameter (OD) of 800 pm. The refractive index of the interlayer is 1.4 x 10 4 above that of the fused silica of the optical core 30, and that of the optical cladding is 1 x 10 3 below that of the core.

There is no need for the taper to be linear, but at all points along its length the taper angle must be small enough for the taper to be adiabatic (i.e. the taper angle must be small enough at every point along the length not to induce mode mixing at that point).

If the taper is to be directly coupled with the end of a conventional 125 pm OD single mode fibre, the OD of the taper at its small end 35 may be as small as 10 to 20 pm in order to provide a modal spot size suitably matched with that of the single mode fibre.

(Modal spot size is defined as the diameter at which the optical field of the fundamental mode has dropped to l/e of its peak value at the centre of the spot). At this small end of the taper the thicknesses of the optical cladding layer 31 and the interlayer 32 are far too small to provide any noticeable waveguiding effect, and instead light will be guided by the interface between the external curved surface of the taper and the medium in which the taper is immersed, which may be air or a transparent low refractive index plastics protective material.

Generally it is found more easy to provide a direct optical coupling between fibres of substantially equal size than between fibres of widely different size, and so it is usually preferred to make the coupling between the small end 35 of the taper and the end of a standard 125 pm OD single mode fibre that has been drawn down in an adiabatically tapered region to provide a reduced diameter end section with an enlarged modal spot size. For operation at a wavelength of about 1300 nm, an acceptable match of modal spot size between that of the fibre and that of the small end of the taper is provided when both have been drawn down to about 40 to 50 pm OD.

At the large end 34 of the taper the core 30 is large enough to register with a close-packed array of twelve or thirteen 125 pm diameter single mode fibres, and hence the taper can be used for forming a 1 x 12 or 1 x 13 single mode coupler. However such a coupler would not be very efficient because the modal spot size of a 125 pm OD single mode fibre is much less than 125 pm, and thus output fibre would receive much less than 1/13 of the power launched into the input with the result that much more than half the available power would be wasted by being launched into cladding modes of the output fibres. Such wastage of optical power can be reduced by drawing down the ends of the individual output fibres to form adiabatically tapered regions that provide reduced diameter end sections with enlarged modal spot sizes.This has the dual effect of increasing the proportion of the light in the taper of Figure 3 that is launched into the core mode of any individual output fibre, and of increasing the number of output fibres enabled to register with the core 30 at its large end 34. Thus by tapering the output fibres down to 50 Xum, the number of fibres that can be close-packed into an array registering with a 500 pm diameter core is increased to eighty-five.

In Figure 4 there is depicted a 1 x N coupler comprising a single single mode input fibre 40 provided with an adiabatically tapered region 41 to provide a reduced diameter end section 42 with an enlarged modal spot size satisfactorily matched with that of the small end 43 of an adiabatic taper 44. This adiabatic taper 44 is constructed as described above with reference to Figure 3 so as to include an interlayer between core and cladding. Registering with the core of the adiabatic taper 44 at its large end 46 is a close-packed array of N 125 pm single mode output fibres 47, each provided with an adiabatically tapered region 48 to provide a reduced diameter end section 49 with an enlarged modal spot size.

The taper of Figure 3 is made from a large diameter cylindrical fibre waveguide and, in its turn this waveguide is prepared from still larger diameter optical fibre preform using the drawing technique conventionally used for drawing optical fibre from optical fibre preform, but drawing to a larger diameter than is usual for optical fibres. Conveniently the preform is made by a rod-in-tube process. In this process the rod may consist solely of the material of the optical core 30, while the tube comprises a substrate tube sleeving 33 upon whose bore has been deposited first the material of the optical cladding 31 and then the material of the interlayer 32.

Alternatively these materials may be deposited upon the rod, in which case the material of the interlayer 32 is deposited before the material of the optical cladding layer 31. Preferably the material is deposited by a vapour deposition process.

In performing the rod-in-tube sleeving process, the rod is assembled inside its tube, and a localised hot zone is traversed along the assembly to promote local heat-softening of the tube. In its softened state, surface tension forces cause the tube to shrink down on to the rod. Alternatively the rod-in-tube assembly may be lowered axially into the hot zone where surface tension forces in the heat-softened material similarly promote the collapse of the tube on to the rod, and at the same time the heat-softened material is drawn into fibre. In this way the rod-in-tube assembly is converted directly to the large diameter cylindrical fibre waveguide without going through the intermediate step of forming an optical fibre preform.

Tapers are made from the large diameter cylindrical fibre waveguide by a controlled drawing operation adapted from the progressive stretching method of making fused fibre tapered couplers that is described in United Kingdom Patent Specification GB 2,150,703 A.

Instead of simultaneously stretching a pair of optical fibre that have been stranded together, the large diameter cylindrical fibre waveguide is stretched on its own. Referring to Figure 5, the large diameter cylindrical fibre waveguide 50 is secured by clamps 51 and 52 which are mounted on independently driven linear movement carriages (not shown) that operate along a common direction aligned with the axial extent of the fibre waveguide 50. A micro-torch 53 is located beneath the fibre waveguide 50 between the two clamps.This provides a short localised zone of the fibre waveguide lying, within the flame of the micro-torch, where the temperature is high enough to produce sufficient heat-softening of the fibre waveguide to allow it to deform plastically under tension provided by moving the two carriages in the same direction with the leading carriage being constrained to move slightly faster than the trailing carriage. A traverse of this kind produces a controlled drawing-down of the fibre waveguide by an amount determined by the relative speeds of the two carriages. The location of the drawing-down region is determined by the movement of the two carriages relative to the micro-torch. A tandem arrangement of a pair of oppositely directed tapers is produced with their small diameter ends linked by an intervening length of (small diameter) fibre by performing a succession of traverses, each symmetrically disposed with respect to its predecessor, but covering a shorter range. Each traverse produces a small neck at the beginning of the traverse, and another at the end. By diminishing the range there is produced a succession of necks whose spacing is such as to approximate to a pair of oppositely directed tapers. In practice, the necks are so gradual that the approximation is close. Typically, but not necessarily, successive traverses are performed in opposite directions.

The resulting drawn fibre waveguide is then parted in the middle to provide two tapers, each having a short section of parallel-sided fibre, at both its small diameter end and its large diameter end. The adiabatically tapered regions of single mode input and output fibres of the 1 x N coupler of Figure 4 may be made in the same way using the same, or similar, drawing apparatus.

Claims (5)

Claims.

1. An optical waveguide adiabatic taper having an interlayer of a first material between an adiabatically tapered optical core of a second material and an optical cladding of a third material, wherein the refractive index of the second material is greater than that of the third material and the refractive index of the first material is greater than that of the second material, and wherein the thickness of the interlayer in relation to the size of the optical core and the relative refractive indices of the first, second and third materials is such as to provide, for light launched in fundamental mode into the smaller end face of the taper, a more nearly uniform optical power distribution across the larger end face of the optical core than would result in the absence of the interlayer.

2. An optical waveguide adiabatic taper substantially as hereinbefore described with reference to Figure 3 of the accompanying drawings.

3. A 1 x N single mode optical fibre coupler incorporating an optical waveguide adiabatic taper as claimed in claim 1 or 2.

4. A 1 x N single mode optical fibre coupler as claimed in claim 3, wherein the larger end face end of the taper is optically coupled with a set of N single mode fibres in a region where those fibres are arranged in close-packed array, and wherein each member of the set of N single mode fibres includes an adiabatic taper which provides for that member in said close-packed array a larger ratio of modal spot size to overall fibre size than is provided for that member on the side of its taper remote from the close-packed array.

5. A 1 x N single mode optical fibre coupler substantially as hereinbefore described with reference to Figure 4 of the accompanying drawings.