GB2215079A - Coated optical fibre for use with non-intrusive taps - Google Patents

Abstract

A plastics protective coated optical fibre suitable for use with non-intrusive taps by means of which optical power may be coupled into or out of the fibre intermediate its ends is provided with a Fresnel reflection reducing layer (12a) of thickness 0.2 - 3.0 mu m intermediate the optical cladding (11) of the fibre (10) and its plastics protective coating (12). The cladding, reflection reducing layer and protective coating have lower, intermediate and higher reflective indices, respectively. <IMAGE>

Description

OPTICAL FIBRE This invention relates to optical fibre suited for use in networks incorporating non-intrusive taps by which light may be injected into, or extracted from a length of fibre intermediate its ends. Optical fibre for use in such networks possesses an optical core surrounded by a lower refractive index optical cladding and the resulting fibre is itself surrounded by a plastics protective coating whose refractive index is greater than that of the optical cladding, and which does not absorb strongly at the wavelengths of interest. Examples of materials of such coatings include epoxy-acrylate, urethan, and some silicone resins.

The present invention is preferably applied to fibres which are single moded or few moded at the wavelengths of interest. In multimode fibres the performance of taps based on bending or microbend induced coupling is sensitive to the initial modal power distribution between the bound modes of the fibre. As a result, the performance of such taps on multimode fibres may vary widely, depending on the power distribution launched into the fibre, the degree of mode coupling in the fibre, and the spacing between taps in a system in which more than one tap is applied. In single mode fibre such problems are avoided.

It is convenient to consider the extraction of light from the bound modes of the fibre in three stages as follows. First, the fibre is perturbed so as to induce coupling between one or more of the bound modes and one or more of the cladding modes. Second, the cladding power is transmitted across the boundary between the cladding and the protective coating. Third, the radiated light is collected and focussed onto a suitable detector. Light injection is achieved by a reciprocal process, i.e. all the rays may be reversed in direction and the detector replaced by a light source.

The principal difference is that if a coherent source such as a semiconductor laser is used for injection, it is advantageous to minimise the aberrations in the imaging optics as far as possible in order to maximise the power transferred to a particular mode. The sensitive area of a semiconductor detector will typically be much larger than the emitting area of a laser, and good collection efficiency is achievable with less highly corrected optics.

Coupling between the bound and cladding modes is achieved by applying a suitable perturbation to the fibre. For coupling between the fundamental Lip01 mode and a higher order mode with azimuthal mode number unity, this is achieved most conveniently by a periodic bending of the fibre core. The optimum bending pitch for resonant coupling is inversely related to the difference in propagation constants, or axial components of the wavevector, for the two modes, and should be close to the period of beats between the two modes, as described in UK Patent Application GB 2182516A. The pitch can be calculated to acceptable accuracy by solving the scalar wave equation for the two waveguide modes at the wavelengths of interest, or by measuring the attenuation induced in the fibre by periodic bends of known pitch.

The bend amplitude needed to transfer a useful fraction of the available power by resonant coupling between two modes is very small, typically a fraction of one micrometre for a periodic bend of pitch 0.2 to 1.0mum applied over a few millimetres length of the fibre.

The cladding modes to which power is coupled from the bound modes are not true eigenmodes of the core/cladding/coating structure because power is lost at each reflection at the cladding/coating boundary. For typical telecommunications fibres, the wavevector of the lowest order cladding modes that are most readily excited are nearly parallel to the fibre axis.

Consequently, the Fresnel reflection coefficient at the boundary with the coating is almost unity, and power may propagate for several centimetres or more with relatively low losses if the fibre is straight. Low losses enhance the resonant coupling of power between the bound and cladding modes, but inhibit the transfer of power across the cladding/coating boundary.

This problem can be somewhat aleviated by applying a periodic perturbation of higher spatial frequency than is required for resonant coupling to low order cladding modes so that higher order cladding modes may be selectively excited, or coupled into the fundamental bound mode. This may be achieved by pressing a periodic undulation or grating of shorter pitch against the fibre. The higher order modes propagate at a greater angle to the interface, and so have a lower Fresnel reflection coefficient, and higher transmission coefficient.

In some circumstances, it may not be convenient or practical to adopt a configuration employing such higher order modes. In other circumstances, even though higher order modes are being employed, the Fresnel reflection is still inconveniently high.

According to the present invention there is provided a plastics coated optical fibre which includes an optical core surrounded by a lower refractive index optical core which is encased in a plastics protective coating of higher refractive index than that of the optical cladding wherein, in contact with the optical cladding and the plastics protective coating, lies an intermediate Fresnel reflection reducing layer whose refractive index is intermediate that of the protective coating and that of the optical cladding and whose thickness lies in the range from 0.2 to 3.0um.

Conveniently the Fresnel reflection reducing layer may be a plastics layer applied in the same way as the plastics protection layer. Alternatively, in the case of an optical fibre with a glass optical cladding, this coating may take the form of an inorganic glassy layer on the outer surface of the optical cladding.

Such a coating may for instance be provided by forming a surface layer on the material of the optical cladding in which the nature of the material of the cladding has been modified, for instance by a nitriding process, in such a way as to change its refractive index. In this instance the layer will be required to be rather thicker for its Fresnel reflection reducing attributes than the thickness of nitride layers described in the literature for improving the mechanical properties of the glass.

There follows a description of a plastics protective coated optical fibre embodying the invention in a preferred form and of illustrative non-intrusive taps employing such fibre. The description refers to the accompanying drawings in which: Figure 1 depicts a schematic cross-section of the plastics protective coated fibre; Figures 2 and 3 depict means for effecting an optical coupling between the fundamental bound mode of the optical fibre of Figure 1 and a cladding mode; Figure 4 depicts a modified form of the means of Figures 2 and 3, and Figures 5, 6, and 7 depict different embodiments of coupling element.

Referring to Figure 1, a plastics protective coated fibre consists of an optical core 10 surrounded by an optical cladding 11 whose refractive index is less than that of the optical core 10. Surrounding the optical cladding is a plastics protective layer 12 whose refractive index is greater than that of the optical cladding 11. Thus far in the description of this coated fibre it has not been distinguished from known forms of coated fibre. Its distinction lies in the presence of a thin Fresnel reflection reducing layer 12a intermediate the coating 12 and the optical cladding 11. This Fresnel reflection reducing layer 12a has a refractive index intermediate that of the coating layer 12 and that of the optical cladding 11. It thickness lies in the range 0.2 to 3jam, being typically at least 0.5)ism thick.

The optimum thickness and refractive index of this antireflection coating layer may be calculated as follows: Index of antireflection layer

where ne is the refractive index of the cladding and flp is the refractive index of the Thickness of antireflection layer

coating. 1 coating. a 1 t=/;mna2 t z 42 nc(np~ nc) where > is the wavelength in-vacuo of the light.

For a useful reduction in Fresnel reflection in a non-intrusive tap neither the thickness nor the value of the refractive index are critical, and a substantial improvement in transfer of optical power between the optical cladding 11 and the coating layer 12 will result for a range of thicknesses or wavelengths. Where a very wide spectral range must be accommodated it will generally be beneficial to optimise at shorter wavelengths within the operating spectral range.

By way of specific example the optical core and optical cladding may be that of a conventional fused silica optical fibre designed for single mode operation and constructed using a vapour deposition process. The refractive index of the cladding layer of such a fibre, nc, is 1.46, while the refractive index, np, of a typical acrylate protective coating layer 12 suitable for use with such a fibre is 1.54. Such an acrylate may be for instance that marketed by DeSoto Inc under the designation 950131. Therefore the optimum refractive index, na, of the antireflection coating 12a is 1.50.

For an operating wavelength of l.3jim the optimum coating thickness is 0.84pm. A substantial reduction in Fresnel reflection can therefore be provided by the use of a lum thickness Fresnel reflection reducing layer 12a made of the acrylate resin marketed by DeSoto under the designation 3471U2-9 whose refractive index is 1.52.

This coating can be applied at this thickness to a silica optical fibre using the type of coating apparatus presently used for the provision of conventional acrylate protective coatings on such fibre, such as for instance a coating applicator substantially as described in UK Patent Application GB 2173708A.

Turning attention now to illustrative examples of non-intrusive taps which may employ this fibre, a preferred form of coupling device for a non-intrusive tap introduces deliberate spatially periodic microbending by arranging for the protective coated fibre to be pressed against a ribbed surface. In principle it could grip the fibre between two ribbed surfaces of complementary form, but instead it is preferred to grip it between a ribbed surface and a plane surface. This arrangement makes use of the compliance of the coating and reduces the close mechanical tolerances that would be required of the gripping surfaces in order to achieve acceptable uniformity of bending in the case of a silica fibre directly gripped between a pair of substantially non-compliant surfaces.Thus the preferred way of producing the requisite microbending is by means of a structure as schematically illustrated in Figures 2 and 3. Here the glass optical fibre constituted by the optical core 10 surrounded by its lower refractive index optical cladding 11 and encased in a lower modulus plastics protective coating 12 provided with Fresnel reflection reducing undercoating 12a, and is gripped between upper and lower jaws 13 and 14 respectively provided with a plane surface and a ribbed surface.

Under appropriate clamping conditions the microbending that is caused by the ribbed surface causes light to leak from the fibre into its protective coating 12 with an azimuthal distribution indicated by the arrows 21 and 22 of Figure 3. This light then leaks into the upper jaw 13, which also constitutes the coupling element of the device, most efficiently where the coating is in direct contact with the coupling element, as depicted by arrows 23. The width of this area of contact determines the azimuthal angular divergence of the light entering the coupling element.

Too wide an area of contact, and the spread is so great that an excessive proportion of the light fails to be usefully collected; too narrow an area of contact, and diffraction effects will again broaden the angular spread excessively. Diffraction effects may also produce side lobes such as those depicted at 24.

In the preceding paragraph the coupling element 13 has been described as having a plane surface for clamping against the protective coating 12, but it is generally preferred to provide this surface with a single longitudinal channel for providing positive location of the fibre. In Figure 3 the sides of such a channel are shown in broken outline at 25. In Figure 2 the ribbed surface of the lower jaw 14 is represented as being provided by an undulating surface of a monolithic block. An alternative way of providing these ribs is as depicted in Figure 4 in which an array of uniform diameter fibres 14a, made for instance of fused silica, are secured in side contact with each other on the plane face of a block 14b.

Having regard to the fact that the light coupled out of the fibre by the microbending into a particular cladding mode is launched into the coupling element centred at a particular angle e to the fibre axis, and to the fact that this light has an azimuthal distribution of the general form depicted in Figure 3, a preferred geometry of coupling element 13 as depicted in Figure 5 has a conical 40 reflecting surface with a semi-vertical angle of + = 8/2 The fibre 41 (comprising core 10 cladding 11, protective coating 12 and Fresnel reflection reducing coating 12a not separately shown) extends along the axis of the cone.

Reflection at the curved surface may be produced by arranging conditions to ensure total internal reflection at this interface or by metallising this surface.

Geometrical analysis reveals that light launched into the coupling element at an angle e to its axis becomes collimated after making a single reflection at the curved surface. If, after making this single reflection, the light emerges from a plane facet it can be brought to a focus on a detector (not shown) by means of a single lens (not shown). Figure 6 depicts an arrangement that employs a first ribbed surface 50a for inducing microbending in order to extract light from the plastics protection coated fibre 41 for detection at a detector 51, and a second ribbed surface 50b for inducing microbending in order to allow light from a source 52 to be injected into the fibre. In principle the same ribbed surface can be used for both purposes but the use of separate ribbed surfaces allows the use of a shorter optical coupling element 54. Furthermore the use of two ribbed surfaces instead of only one enables different pitches to be used for injection and extraction thereby facilitating the possibility of using a system in which light of one wavelength is extracted from the fibre and fed to the detector 51 while light of a significantly different wavelength emitted by the source 52 is employed for injection into the fibre. The coupling element 54 has first and second conical reflecting surfaces 53a and 53b both having their axis extending collinearly along the axis of the fibre 41.

The pitches of the ribbed surfaces 50a and 50b are chosen respectively to couple light from the fibre into the coupling element at an angle 1 and to couple light into the fibre from the coupling element at an angle a2 in the coupling element. Hence the semivertical angle of the cones of conical surfaces are respectively 81/2 and 92/2. (Generally but not necessarily 82 is made the same as 81). The coupling element is further provided with two plane reflecting facets 55a and 55b, a plane exit window 56a and a plane entrance window 56b.A first lens 57 is positioned to bring the collimated light emerging from exit window to a focus at the detector 51, while a second lens 58 which may be a graded index lens is positioned to collimate light from source 52 and direct that light at the appropriate angle into the plane entrance window 56b.

It is found that little loss of optical efficiency is suffered if, instead of providing conical reflecting surfaces 53a and 53b as depicted in Figure 6, a single cylindrical reflecting surface 63 is used in their place as depicted in Figure 7. The fibre 41 now extends along the focal line of this reflecting surface. In the case of this coupling element 64 of Figure 7 light extracted from the fibre 41 that is launched into the coupling element and is reflected in the cylindrical surface 63 is directed by that reflection back towards the fibre, whereas in the case of coupling element 54 of Figure 6 that reflected light is directed parallel with the fibre axis. Accordingly it is convenient to orient the reflecting facets 55a and window 56a slightly differently in the two instances.

Similar considerations apply also to the orientation of reflecting facet 55b and window 56b.

Claims (10)

CLAIMS:

1. A plastics coated optical fibre which includes an optical core surrounded by a lower refractive index optical core which is encased in a plastics protective coating of higher refractive index than that of the optical cladding wherein, in contact with the optical cladding and the plastics protective coating, lies an intermediate Fresnel reflection reducing layer whose refractive index is intermediate that of the protective coating and that of the optical cladding and whose thickness lies in the range from 0.2 to 3.0um.

2. A plastics coated optical fibre as claimed in claim 1 wherein the optical fibre cladding is made of glass and the Fresnel reflection reducing layer is made of an inorganic glassy material.

3. A plastics coated optical fibre as claimed in claim 1 wherein the Fresnel reflection reducing layer is made of a plastics material.

4. A plastics coated optical fibre substantially as hereinbefore described with reference to Figure 1 of the accompanying drawings.

5. A non-intrusive optical fibre tap including a length of plastics coated optical fibre as claimed in any preceding claim which tap includes a coupling device for coupling light into or out of the fibre, said device including means for gripping the fibre in a manner that induces microbending of the optical fibre with a spatial periodicity to effect optical coupling between the fundamental bound mode of the fibre and a cladding mode, which gripping means includes an optical coupling element provided with a reflective cylindrical or conical surface positioned such that light launched into the coupling element from said bound mode via said cladding mode and making a direct reflection in said surface is substantially collimated by making that reflection.

6. A non-intrusive tap as claimed in claim 5 wherein the coupling element is provided with a reflective conical surface and with a channel for locating the fibre on the axis of conical surface.

7. A non-intrusive tap as claimed in claim 5 wherein the coupling element is provided with two reflective conical surfaces with a common axis and oppositely directed tapers, wherein the element is also provided with a channel for locating the fibre on the common axis of said two reflective conical surfaces.

8. A non-intrusive tap as claimed in claim 5 wherein the coupling element is provided with a reflective cylindrical surface and with a channel for locating the fibre on the focal line of that cylindrical surface.

9. A non-intrusive tap as claimed in claim 5 whereing the coupling element is substantially as described with reference to Figures 2 or 4, Figure 3 and Figures 5, 6 or 7 of the accompanying drawings.

10. An optical fibre transmission system including a plurality of non-intrusive taps as claimed in claim 5, 6, 7, 8 or 9.