GB2166885A - Single mode fibre adjustable attenuator - Google Patents

Abstract

An adjustable attenuator for single mode fibre in which the attenuation is adjusted by varying the spacing between the opposed ends of two single mode optical fibres (16, 17) held in alignment by an index matching liquid containing, prismatic apertured, alignment member formed between two elastomeric strips (10, 11). <IMAGE>

Description

SPECIFICATION Single mode fibre adjustable attenuator This invention relates to attenuating the light propagating in a single mode fibre. Variable attenuators have been constructed for multimode fibre which rely upon placing a gradu ated density neutral filter between the two halves of an expanded beam type connector, or rely upon adjusting the angular misalign ment of the two halves of a connector. The disadvantage of using either of these types of approach for a single mode attenuator is that they are liable to be too polarisation state sensitive for a number of applications. A de sign is required which provides an attenuation that is substantially independent of polarisation orientation.

One such design is described in our Patent Specification No. 2114769 A. In that design the opposed ends of two optical fibres are held in alignment in a Vee-groove formed in one surface of a block. The efficiency of opti cal coupling between these two fibres is changed by moving one relative to the other in the direction of the groove. The fibres are held against the groove by virtue of their own resilience upon being lightly flexed. Fresnel reflections are suppressed by arranging for the groove to be immersed in an index matching liquid contained in a vessel.

The present invention is concerned with a design in which the fibres do not need flexing to secure their alignment, and in which the quantity of index matching liquid required can be very much reduced.

According to the present invention there is provided a single mode fibre adjustable optical attenuator in which two single mode fibres are held by clamps near their opposed ends so that these ends are caused to enter the oppo site ends of a prismatic aperture formed in an elastomeric alignment member, the size of the aperture being such that both fibres are urged by a first side of the aperture against second and third sides that cooperate to form a Vee groove, by which urging, the ends are con strained to lie on a common axis, wherein the aperture contains a quantity of an index matching liquid in which the fibre ends are immersed, and wherein the two clamps are mounted on a slide mechanism by means of which one clamp may be moved in a con trolled manner relative to the other in a direc tion that is substantially in alignment with the groove direction so as to adjust the distance separating the fibre ends.

There follows a description of a single mode fibre adjustable optical attenuator embo dying the invention in a preferred form. The description refers to the accompanying draw ings in which: Figures 1, 2 and 3 are respectively perspec tive, end, and longitudinal sectional views of the attenuator alignment member; Figure 4 is a plan view of the attenuator, and Figure 5 is a section view of the attenuator on the line A-A of Fig. 4.

The principle of operation of the attenuator is that two single mode fibres are constrained to lie in alignment in a prismatic aperture formed in an alignment member. One fibre remains stationary in the aperture while the other is advanced along the aperture towards the first, or is retracted from it, so as to increase, or decrease, the optical coupling between the fibre ends. The aperture, which is formed in elastomeric material, holds the fibres in axial alignment at all times and thus avoids the needs for complicated micro-positioning means to retain lateral positioning of the fibres during this translational movement in the axial direction. Lateral movement is undesirable because it is liable to be inconsistent and excessively sensitive to polarision orientation.

Referring now particularly to Figs. 1, 2 and 3, the alignment member is constituted by two strips 10, 11 of elastomeric material contained within a metal sleeve 12. In the central section of the alignment member the upper surface of the lower strip 11 is formed with a Vee-groove 13 which co-operates with the lower surface of the upper strip 10, which is flat, to define a prismatic aperture with an equailateral triangular cross-section. This aperture is flared out at both ends 14 and 15 to facilitate insertion of the two single mode optical fibres 16 and 17 which are inserted to a depth to leave a small gap 18 between their opposed ends. Normally these ends are prepared by cleaving.Freshly cleaved fibre ends have particularly sharp edges, and hence it is generally preferred to flame polish the cleaved ends sufficiently to round off their edges and thereby reduce the rate of wear of the elastomeric strips by movement of the fibres. The interior of the aperture is filled with an index matching liquid in order to suppress Fresnel reflections at the fibre ends. For silica fibre this liquid may be for instance a paraffin or a suitable silicone.

The alignment member may conveniently be provided by the splicing element marketed G.T.E. Products Corporation and described in their European Patent Specification No. 27818.

Referring now to Figs. 4 and 5, a block 20 housing the sleeve 12 of the alignment member is mounted on a slide mechanism constituted by a commercially available translation stage 21. The fibres 16, 17 are secured respectively by clamps 22 and 23. Clamp 22 is rigidly secured to the attenuator housing 24 by way of a pillar 25, while clamp 23 is rigidly secured to a flange 26 itself secured to the slide of the translation stage so as to be moveable relative to clamp 22 under the control of a knob 27 mechanically linked with the translation stage lead screw. (The housing is completed by a lid that is not shown in any of the drawings.) Optionally a screw 28 is provided to act as a stop to limit the slide movement to prevent the end of fibre 17 from being urged into actual physical contact with the end of fibre 16.

Each fibre enters the attenuator housing by way of a feed-through 29 from where it is led to one of the clamps 22, 23, optionally via a cladding mode stripper 30. Initially the fibres are protected by plastics coatings typically constituted by a thin primary coating covered by a thicker extrusion coating. Each cladding mode stripper 30 consists essentially of a plate provided with circular channei 31, which, for 125 micron single mode fibre, is typically about 60mm in diameter.One turn of the fibre is accommodated within this channel 31, and vver < ~s length the plastics coatings are removed to expose the bare fibre, and the channel is filled with a higher refractive index potting compound, such as an epoxy resin, so that any optical energy guided by the optical cladding of fibre is stripped from the fibre to be absorbed in the potting compound. In the case of a fibre with a typical silicone resin primary coating and a nylon extrusion coating, the extrusion coating can conveniently be removed over the required length with the aid of a hot air blower or a soldering iron, and then the primary coating by knotting a nylon fibre around the optical fibre and using the knot to strip the seating from the fibre. The knot itself is then conveniently removed with the soldering iron.The coatings are also stripped from the region of each fibre protruding into the alignment member so that it aligns the fibres by the fibres themselves rather than by their coatings.

It will be appreciated that this attenuator functions by launching a greater or lesser proportion of the light emerging from the end of one of the fibres, not into the core mode of the other fibre, but into its cladding modes.

This light launched into cladding modes needs to be attenuated. In the case of a silica fibre encased in a silicone resin primary coating, the refractive indices of the fibre and coating are such as to provide a waveguiding structure for these cladding modes, and hence it will generally be necessary to employ a cladding mode stripper on the output fibre. On the other hand, if instead the silica fibre is encased in an acrylate coating, the relative refractive indices of fibre and coating provide an anti-waveguiding structure, and hence in general the cladding modes will be adequately attenuated without recourse to specific cladding mode stripper elements.

Claims (4)

1. A single mode fibre adjustable optical attenuator in which two single mode fibres are held by clamps near their opposed ends so that these ends are caused to enter the opposite ends of a prismatic aperture formed in an elastomeric alignment member, the size of the aperture being such that both fibres are urged by a first side of the aperture against second and third sides that- cooperate to form a Vee groove, by which urging, the ends are constrained to lie on a common axis, wherein the aperture contains a quantity of an index matching liquid in which the fibre ends are immersed, and wherein the two clamps are mounted on a slide mechanism by means of which one clamp may be moved in a controlled manner relative to the other in a direction that is substantially in alignment with the groove direction so as to adjust the distance separating the fibre ends.

2. An attenuator as claimed in claim 1, wherein each optical fibre passes through a cladding mode stripper.

3. A single mode fibre adjustable optical attenuator substantially as hereinbefore described with reference to the accompanying drawings.

4. An optical coupling device in which two optical fibres are located at their opposed ends in an elastomeric alignment member with the ends constrained to lie substantially on a common axis, wherein optical coupling between the fibres is provided by an optical interfacing material and wherein the distance separating the fibre ends determines the attenuation in the coupling.