GB2228585A

GB2228585A - Silica optical fibre having two cladding layers

Info

Publication number: GB2228585A
Application number: GB8904483A
Authority: GB
Inventors: Terry Bricheno; Kevin John Warbrick
Original assignee: STC PLC
Current assignee: STC PLC
Priority date: 1989-02-28
Filing date: 1989-02-28
Publication date: 1990-08-29
Also published as: GB8904483D0

Abstract

Silica optical fibre, particularly for use in fibre taper structures, includes a core section of refractive index n1, a substantially thick first cladding layer of refractive index n2 surrounding the core and a relatively thin outer cladding layer of refractive index n3 (n1>n2>n3). When such a fibre is tapered the thin outer cladding layers serves to confine a mode to the guide and prevents power loss. Such tapered fibres can be encapsulated directly by material, such as an epoxy resin, which has a higher refractive index than n2, without power loss. <IMAGE>

Description

OPTICAL FIBRE.

This invention relates to optical fibre and in particular to fibre for optical fibre tapers and optical devices and structures including optical fibre tapers.

Single mode optical fibre (1 - Fig. 1) is frequently provided with an adiabatic taper 2 adjacent its end which is to be coupled to other fibres or devices, and a reduced diameter parallel-sided section 3. Such tapering is performed in order to reduce coupling losses between, for example, spatially separated fibres, which fibres are separated to allow for the inclusion of a device or other material, for example, in the optical path therebetween. The effect of tapering is to increase the field radius of the transmitted mode together with increased collimation of the unguided light (in the optical path between the fibre ends, for example).For typical single mode telecommunication fibre of 125jim OD the mode spot size diameter is 10cm. By adiabatically tapering the fibre down to 5(m OD, the spot size is increased to 25cm.

Adiabatic tapering may be achieved using the basic progressive stretching (differential pulling) technique of GB 2150703B.

One limitation of this tapering approach is that the larger propagating mode has a significant power density external to the taper which makes it sensitive to changes in the refractive index external to the taper. In particular power can be lost as a result of encapsulation using epoxy resins since they have a higher refractive index than silica.

According to one aspect of the present invention there is provided a silica optical fibre including a core section of refractive index nl, a substantially thick first cladding layer of refractive index n2 surrounding the core and a relatively thin outer cladding layer of refractive index n3, and wherein n1n2n3.

According to another aspect of the present invention there is provided an optical fibre element comprising a length of optical fibre according to the preceding paragraph and which has an adiabatic taper whereby one length section of the fibre is of a reduced diameter in comparison with another length section, the outer cladding layer of the fibre serving to increase the confinement of a guided mode in the reduced diameter length section in comparison with a tapered fibre without such an outer cladding layer.

Embodiments of the invention will now be described with reference to the accompanying drawings, in which: Fig. 1 illustrates a fibre taper as referred to above; Fig. 2 illustrates for a conventional fibre the mode field amplitude between the centre of a tapered fibre and the edge of the fibre as well the portion thereof which extends outside of the fibre; Fig. 3 is similar to Fig. 2 but for a fibre having a depressed external glass cladding, and Fig. 4 illustrates an encapsulated fibre taper.

In Fig. 2, is shown the mode field amplitude 5 of a conventional single-mode fibre whose outer protective plastics coating, e.g. of acrylate, has been stripped and the fibre tapered by the differential pulling technique referred to above. As can be seen there is significant power (shaded area 6) in the region outside of the fibre, which power will be lost if the taper is encapsulated in a material having a higher refractive index than the material of the fibre (silica). To overcome this a previous suggestion involved encapsulating (coating) the taper at least over the length from which the acrylate was removed with a thin layer of a low index polymer prior to use of an epoxy encapsulant.In one previous arrangement the tapered fibre end coated with the low index polymer is inserted in a length of tube of glass or metal, for example, and then the space between the tube and the polymer filled with an epoxy, thus providing a handleable structure. Whilst this approach should reduce power losses as a result of the low index polymer confining the mode more tightly to the fibre guide, in practice there are a number of difficulties. The polymer is very soft and its adhesion to the silica fibre is poor. Furthermore the optical properties of the polymer are variable depending on how it was treated/applied and contaminents thereof.

The mode may be more tightly confined to the guide by an alternative approach which is that proposed by the present invention, namely that the optical fibre itself is provided with a thin depressed refractive index silica cladding which confines the power to the guide after tapering. Fig. 3 illustrates the effect of an extra cladding 10 which is only some 5Jum thick prior to tapering and 2.5,utn thick after tapering and is depressed in refractive index relative to the normal silica cladding by an amount of the order of 0.05. As can be seen this depressed external silica cladding has the effect of confining the power 7 within the guide (fibre).Thus the silica fibre has a core section of refractive index n1, a substantially thick first index n, and a relatively cladding layer of refractive index n2 and a relatively thin outer cladding layer 10 of refractive index n3 (n1 > n2n3). Typically for single-mode fibre the core diameter is in the range 9-lOpm, the first cladding layer outer diameter is in the range 1l5-120,um and the outer cladding layer is of the order of 5-lOpm. thick.

The thin depressed refractive index cladding approach may also be applicable to large cored multimode fibres in which case the core diameter will be larger than that quoted.

Such a depressed external silica cladding can be achieved in various ways. For example it can be achieved at the fibre preform stage by the deposition of doped silica, fluorine being a suitable dopant, either directly on the preform or on the internal bore of a silica support tube which is then collapsed around the preform, before having the support tube removed in a reducing flame. The refractive index profile in Figure 3 relates to the latter method of manufacture, the step at the edge of the fibre being due to incomplete removal of the support tube. Alternatively a predepressed support tube of a suitable thickness may be used. What ever method is used to produce the preform, fibre will be drawn therefrom as normal to a size compatible with, for example, the standard single-mode telecommunications fibre.

Tapers can be provided in such optical fibre with a depressed refractive index silica cladding in the conventional manner referred to above. Instead of having first to coat them with polymer as described above, the fibre taper and end 12 (Fig. 4) can inserted directly into a reinstatement tube 13 and the space therebetween filled with epoxy resin 14. Thus the epoxy is applied directly to the fibre, i.e. to the low refractive index cladding thereof. This obviates the problems associated with the use of low refractive index polymers. Thus the fibre taper end is easily reinstated using a standard epoxy technique. Whereas reinstatement of a single fibre taper has so far been described, the principle is also applicable to arrays of tapers such as in 1 x N couplers. See for example our GB Applications Nos.

8718055 (Serial No. 2207525) (T. Bricheno 18-3-1) and 8816896.8 (Serial No. ) (T. Bricheno 23-62).

The removal of the low refractive index polymer for the epoxy encapsulation process, simplifies the manufacture of arrays of tapers, in particular. By providing optical fibre with a thin depressed refractive index silica cladding, fibre tapers which are insensitive to the refractive index of the surrounding medium are achieved.

One use for reinstated fibre tapers is in optical isolators, which are optical "one-way valves" used to protect laser diodes from back reflected light.

Tapered optical fibres are used to feed light to and from the Faraday rotator (active component) in the isolator.

Claims

CLAIMS.

1. A silica optical fibre including a core section of refractive index nl, a substantially thick first cladding layer of refractive index n2 surrounding the core and a relatively thin outer cladding layer of refractive index n3, and wherein n1n2 > n3.

2. A fibre as claimed in claim 1, wherein for single-mode fibre the core diameter is in the range 9-lOpm, the first cladding layer outer diameter is in the range 115-120Fm and the outer cladding layer is of the order of 5-lOpm thick.

3. An optical fibre element comprising a length of fibre as claimed in claim 1 or claim 2 and which has an adiabatic taper whereby one length section of the fibre is of a reduced diameter in comparison with another length section, the outer cladding layer of the fibre serving to increase the confinement of a guided mode in the reduced diameter length section in comparison with a tapered fibre without such an outer cladding layer.

4. An optical fibre element as claimed in claim 3 wherein the taper and at least a portion of the fibre on either side thereof is disposed within a tube, the space between the tube and the fibre being filled with an encapsulant.

5. An element as claimed in claim 4 wherein the encapsulant is an epoxy resin.

6. An optical device including at least one optical fibre element as claimed in any one of claims 3 to 5.

7. An optical fibre for optical fibre tapers substantially as herein described with reference to the accompanying drawings.

8. An optical device including an optical fibre taper substantially as herein described with reference to the accompanying drawings.