GB2114769A

GB2114769A - Single mode fibre adjustable attenuator

Info

Publication number: GB2114769A
Application number: GB8203647A
Authority: GB
Inventors: Andrew Baxter Harding
Original assignee: Standard Telephone and Cables PLC
Current assignee: STC PLC
Priority date: 1982-02-09
Filing date: 1982-02-09
Publication date: 1983-08-24
Also published as: GB2114769B; AU1097783A; NZ203079A

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 (10, 11) held in alignment by a Vee-groove in an alignment block (12) positioned in a vessel optionally containing an index matching liquid. <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 graduated density neutral filter between the two halves of an expanded beam type connector, or rely upon adjusting the angular misalignment of the two halves of a detector. 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 design is required which provides an attenuation that is substantially independent of polarisation orientation.

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 deflected by the walls of a Vee groove formed in an alignment member so as to constrain the ends to lie on a common axis, wherein the alignment member is contained in or forms part of a vessel for containing an index matching liquid in which to immerse the fibre ends, 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.

There follows a description of a single mode fibre adjustable optical attenuator embodying the invention in a preferred form. The description refers to the accompanying drawings in which: Figure 1 is a perspective view of the attenuator alignment block Figure 2 is a plan view of the attenuator Figure 3 is a section view of the attenuator on the line A-A and Figure 4 is a sectional view of one of the diaphragm glands preventing egress of index matching fluid from the vessel containing the alignment block in the regions where the optical fibres enter the vessel.

The principle of operation of the attenuator is that two single mode fibres 10, 11 are constrained by their natural flexural resilience to lie in alignment in a Vee-groove formed in an alignment block 12. One fibre, 10, remains stationary in the groove while the other, 11 is advanced along the groove towards the first, or is retracted from it, so as to increase, or decrease, the optical coupling between the fibre ends. The Vee-groove 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. The alignment block 12 is located in a vessel 20 (not shown in Figure 1) which can be 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.

Referring now to Figure 2 and 3, the vessel 20 containing the alignment block 12 is mounted on a slide mechanism constituted by a commercially available translation stage 21. The fibres 10, 11 are secured respectively by clamps 22 and 23.

Clamp 22 is rigidly secured to the vessel 20, while clamp 23 is rigidly secured to the slide of the translation stage so as to be moveable relative to clamp 22 under the control of a knob 25 mechanically linked with the translation stage lead screw. (The housing is completed by a lid that is not shown in any of the drawings). The fibres enter the vessel through apertures 26 closed off by diaphragm moulded glands whose shape is shown in cross section in Figure 4. These glands are required to be highly compliant so as to minimise the mechanical effect upon the fibres, and may be made for instance of a silicone rubber. In its relaxed state the central stub of each giand is inclined at an angle roughly corresponding to the angle at which the two clamps are secured so as to cause the fibre ends to bed satisfactorily in the Vee-groove of the alignment block.Typically the angle between the two clamps is about 1400 so that each fibre is bent through an arc of about 200. The vessel 20 is completed by a lid 27.

Optionally a screw 28 is provided to act as a stop to limit the slide movement to prevent one fibre end from being urged into actual physical contact with the other.

Each fibre enters the attenuator housing by way of a feed-through 29 from where it is led via a cladding mode stripper 30 to one of the clamps 22, 23. 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 channel 31, which, for 125 micron single mode fibre, is typically about 60 mm in diameter. One turn of the fibre is accommodated within this channel 31, and over this 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 coating 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 inwardly from the gland inside the vessel 20 so that the Vee-groove aligns the fibres by the fibres themselves rather than by their coatings.

If the maximum attenuation required is relatively small the alignment block 12 may conveniently be made of hardened ground steel, but this can give rise to reflection problems at large fibre separations with a significant proportion of the light coupled from one fibre to the other being coupled by way of reflection in the surface of the Vee-groove. Typically this begins to become a problem for attenuations in excess of 15 dB. The problem can be alleviated by providing a hole (not shown) in the block intersecting the Vee-groove between the fibre ends. For 125 micron diameter fibre a 0.5 mm diameter hole (not shown) positioned approximately one fibre diameter in front of the end of the transmitting fibre has proved satisfactory. An alternative approach to the problem is to suppress the reflections by constructing the block of an index matching substance, such as silica.

Claims

1. A single mode fibre adjustable optical attentuator in which two single mode fibres are held by clamps near their opposed ends so that these ends are deflected by the walls of a Vee groove formed in an alignment member so as to constrain the ends to lie on a common axis, wherein the alignment member is contained in or forms part of a vessel for containing an index matching liquid in which to immerse the fibre ends, 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. An attenuator as claimed in claim 1 or 2, wherein the alignment member includes an aperture intersecting the Vee-groove between the opposed ends of the two optical fibres.

4. An attenuator as claimed in claim 1 or 2, wherein the alignment member is constructed of a material whose refractive index matches that of the cores of the two optical fibres.

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