GB2166257A - Optical attenuator - Google Patents

Abstract

An attenuator is provided by a length of optical fibre 10, part of which is bent to form one or more turns. Attenuation provided will depend on the turns radius and may be varied by change of this radius using for example a split former 14, parts 15 and 17 of which are located each side of a spacer 19. The spacer 14 may be a bellows, or split plates adjusted by jacking screws, or a piezo-electric crystal. <IMAGE>

Description

SPECIFICATION Optical attenuator The present invention concerns an optical attenuator, and in particular an attenuator applicable to optical systems incorporating optical fibre.

In such systems, as aforesaid, it has been common practice to introduce attenuation by means of dedicated lumped components coupled to optical fibre.

In accordance with this invention there is provided an optical attenuator comprising a length of optical fibre, a part thereof being conformed to one or more turns, the radius of which is such that a substantial fraction of light propagated therethrough is coupled out through the fibre cladding.

A classical interpretation of this phenomenon is given as follows:- In the case of a monomode fibre, light is confined within the core of a straight length of the fibre, total internal reflection occurring at the interface between the core and cladding material of lower refractive index.

As the fibre is bent however, the incidence of light at angles less than critical angle is increased and thus losses by refraction into the cladding material is likewise increased. It is noted that fibre dimensions are at least comparable with optical wavelength. A more precise analysis, therefore, must take account of the extent of electro-magnetic field and the boundary conditions applicable.

The attenuator thus is of especially simple construction, and relatively inexpensive to produce.

Advantageously, it may be introduced using existing system fibre, or if desired it may be inserted and coupled as a lumped component. In the former instance, coupling losses are obviated. Furthermore, for fine tuning of attenuation, modification is relatively trivial - only the turns radius need be altered to produce the desired result.

Preferably, the attenuator utilises monomode optical fibre. However, it is not restricted to fibres of this form. The fibre could instead be of multimode type. In this latter instance, macrobending of the fibre has tendency to couple out higher order modes, changing power distribution within the fibre. For certain applications this may not be desirable.

This invention will be more clearly understood from the description that follows. In this description, embodiments of the invention will be described, by way of example only, reference being made to the accompanying drawings wherein: Figure 1 is a simple illustration of a fibre conformed to a single turn, a turn in the form of a knot; Figure 2 is a simple illustration of a fibre conformed to a single turn, the fibre being wound around a cylindrical former; Figure 3 is a plan view of a fibre, the fibre being wound around a cylindrical former of adjustable radius; and, Figure 4 is a perspective view of a fibre, the fibre being wound around a split mandrel assembly.

Figure 1 shows an optical fibre 10 tied to form a knot 11. The fibre 10 can be tied into a multiple knot so as to prevent the possibility of the knot expanding or becoming unravelled. The knot 11 can be reduced in size by tensioning the optical fibre 10. The knot radius can then be reduced until the required degree of light attenuation results. The fibre is then bonded, and may be anchored or potted to provide additional strength to the optical fibre 10 in the region of the knot 11 and to ensure reproducible attenuation.

Alternatively, as shown in Figure 2, one or more turns of the fibre 10 may be wound around a cylindrical former 12 of a given diameter. Whilst attenuation is dependant upon the number of turns and turns diameter, it is also dependant upon choice of fibre. Optical properties of fibres vary significantly, even amongst nominally similar fibres supplied from a single proprietary source. Thus where precise attenuation is sought, an emperical approach is practically requisite. Using, for example, a cylindrical former of relatively large diameter and using many turns, fine adjustment may be achieved by incrementing the number of turns. This may then be followed by bonding and potting, once the precise degree of attenuation is achieved.

As a specific example, a doped glass monomode optical fibre of nominal outer diameter 125 micron and core diameter 8 micron, conformed to a single turn of 1cm. diameter, will produce an attenuation of the order 20dB's.

A variant of this structure is shown in Figure 3.

The fibre 10 is, in this example, wrapped around a mandrel 14 and tensioned such that as the mandrel is adjusted the fibre is allowed to follow such changes and the turns radius increased or reduced to adjust the resultant attenuation.

In the alternative variant, shown in Figure 4, the mandrel 14 is comprised of two spaced components 15 and 17 located each side of a spacer 19.

The spacing provided between mandrel components 15 and 17 may then be varied by means of bellows action - the spacer 19 being of hollow construction and in the form of a bellows. Alternatively, the spacer may be of split structure and the walls of the spacer 19 may be separated by means of jacking screws. As further alternative the spacer 19 may be formed of a crystal slice of piezo-electric material and the spacing controlled by applying electrical field across the crystal and varying this field to provide the desired change in attenuation.

Attenuators, as described above, may with appropriate modification serve as optical sensors. In one example described above the spacer 19 was in the form of bellows. Providing in addition to this attenuator, a light source and a light detector, it can be seen that the same may be used to monitor the pressure within the bellows. Other physical variables - for example linear expansion, motion or the like, - provided that they can be translated into a change of bending radius - may likewise be monitored.

The invention is applicable to optical transmission and control systems.

Claims (8)

1. An optical attenuator comprising a length of optical fibre, a part thereof being conformed to one or more turns, the radius of which is such that a substantial fraction of light propagated therethrough is coupled out through the fibre cladding.

2. An attenuator, as claimed in claim 1, the fibre being conformed to a single turn having the form of a knot, parts of the fibre being bonded together, the single turn being potted.

3. An attenuator, as claimed in claim 1, the fibre being wound about a cylindrical former.

4. An attenuator, as claimed in claim 2, the former being of adjustable radius.

5. An attenuator, as claimed in claim 2, the former comprising split components the spacing between which being adjustable.

6. An attenuator, as claimed in claim 5, wherein the components are located one each side of a split spacer, the spacing of spacer parts being adjustable by jacking screws.

7. An attenuator, as claimed in claim 5, the former components being arranged one each side of a bellows spacer.

8. An attenuator, as claimed in any one of the preceding claims, in combination with: : a light source; and, a light detector; and, wherein the adjustment means is responsive to change of a sensed physical variable.

8. An attenuator, as claimed in claim 5, the former components being arranged one each side of a piezo-electric material spacer.

9. A sensor comprising: a light source; a light detector; and, an optical attenuator interposed in the path of light between the source and detector, the attenuator comprising a length of optical fibre part of which is conformed to one or more turns, the turns radius being adjustable and responsive to change of a sensed physical variable.

10. A sensor, as claimed in claim 9, including as attenuator one wherein the fibre is wound about split former components one located each side of a bellows spacer.

11. An optical transmission or control system including an optical fibre link, an integral part of said link being conformed to one or more turns to serve as an attenuator.

12. An attenuator constructed, arranged and adapted to perform substantially as described hereinbefore with reference to and as shown in the accompanying drawings.

New claims or amendments to claims filed on 30 April 1985 Supersed claims 1 to 12 New or amended claims:

1. An optical attenuator comprising: a cylindrical former including a plurality of movable component parts; adjustment means co-operative with said component parts, for varying the radius and/or circumference of the former; and, a length of optical fibre wound about the periphery of said former and conformed thereto.

2. An attenuator, as claimed in claim 1, wherein the former is segmented, the segments thereof being movable co-operatively in a radial direction.

3. An attenuator, as claimed in claim 1, wherein the former is split, including thus a pair of component parts, these parts being spaced apart, the spacing therebetween being variable by said adjustment means.

4. An attenuator, as claimed in claim 3, wherein the adjustment means comprises jacking screws arranged to vary said spacing.

5. An attenuator, as claimed in claim 3, wherein the adjustment means comprises an electroded piezoelectric spacer.

6. An attenuator, as claimed in claim 3, wherein the adjustment means comprises a bellows located in the space between the pair of component parts.

7. An attenuator constructed, arranged and adapted to perform substantially as described hereinbefore with reference to and as shown in either Figure 3 or Figure 4 of the accompanying drawings.