WO2012080715A1

WO2012080715A1 - Method and apparatus for controlling distributed attenuation in an optical fibre

Info

Publication number: WO2012080715A1
Application number: PCT/GB2011/052424
Authority: WO
Inventors: Ian Peter Mcclean; Peter Wigley; Martin Williams
Original assignee: Oclaro Technology Limited
Priority date: 2010-12-15
Filing date: 2011-12-08
Publication date: 2012-06-21
Also published as: GB201021298D0; GB2486460A

Abstract

A variable optical attenuator comprising an optical fibre and a means for applying controllable longitudinally varying external stress to at least part of the length of the fibre so as to attenuate light passing through the fibre.

Description

Method and Apparatus for Controlling distributed attenuation in an optical fibre

Background of the Invention The invention relates to a method and an apparatus for controlling distributed attenuation in an optical fibre.

Optical networks comprise one or more spans of optical fibre bounded by nodes through which data signals are transmitted and received. The length of the spans may vary depending on the physical location of the nodes, and therefore the data signal attenuation associated with each span may vary. I n addition many different optical components are required in the network with a distribution of loss, thus providing an additional variation in the total node to node loss. Traditionally, control of loss variation in an optical network is achieved with a bulk loss component, such as a Variable Optical Attenuator (VOA) that can have its optical loss set to a value matching the local requirement. VOAs may be used in many positions of a network, such as for loss padding in hubs or within an Erbium Doped Fibre Amplifier (EDFA) to satisfy optical signal gain requirements. Many variants of VOA components are available including Microelectromechanical systems (MEMS), liquid crystal and Si systems. However, these loss components are bulk loss devices providing a loss at a fixed point in the network which has certain drawbacks, especially within an EDFA. In addition, these components are costly, have environmental variations and require additional components to work optimally.

It is therefore clear that it would be desirable to provide variable optical attenuation in specific parts of an optical network. The attenuation means should be efficient in their use of resources. Summary of the Invention

In accordance with one aspect of the present invention there is provided a variable optical attenuator comprising an optical fibre having a plurality of windings and a means for applying controllable longitudinally varying external stress to at least part of each of the windings so as to attenuate light passing through the fibre. The stress may be applied at discrete locations distributed along the longitudinal direction of the fibre and the stress may induce loss. The stress may be applied by placing the fibre between a plurality of surfaces and the surfaces facing each other may be pressed together, thereby applying pressure on the fibre in the transverse direction of the fibre. The surfaces may be substantially flat. The stress may induce strain in the fibre, or the stress may induce deformations in the fibre. The stress may be applied by placing the fibre between corrugated surfaces. The external stress may be applied by placing part or all of the fibre between a plurality of ridges. The stress may be distributed periodically or aperiodically along the longitudinal direction of the fibre. The light propagating through said fibre may be attenuated by an amount depending on the wavelength of the light. The fibre may be suitable for use as the amplifying medium in a fibre based optical amplifier. The fibre may be a doped fibre, such as an erbium-doped fibre. The fibre may be a single mode fibre, or a multimode fibre.

The variable optical attenuator may further comprise a substantially cylindrical body for winding the fibre, and/or a sleeve for containing the cylindrical body and for applying stress on part or all of the fibre. One or both of the cylindrical body and the sleeve may comprise a corrugated or irregularly corrugated surface.

The variable optical attenuator may further comprise a heating element for temperature stabilisation of the optical fibre and/or for controlling the loss of the VOA function within the optical fibre. The sleeve and cylindrical body may comprise materials with different thermal expansion coefficients.

Electro-mechanical means may be provided for inducing distributed loss to part or all of the fibre. Th e variable optical attenuator may further comprise thermo-mechanical means for inducing distributed loss to part or all of the fibre.

The attenuation may be wavelength-dependent so that longer wavelength light is attenuated more than shorter wavelength light. A closed-loop or open-loop control method may be provided for controlling the attenuation.

The fibre may have a high doping concentration for reducing the length of the fibre.

According to another aspect of the invention, a fibre based optical amplifier is provided comprising a gain section, wherein the gain section comprises a means for applying controllable external stress to at least part of the length of the fibre so as to attenuate light passing through the fibre.

According to another aspect of the invention there is provided a fibre based optical amplifier comprising a gain section, wherein the gain section comprises a variable optical amplifier. According to another aspect of the invention there is provided a variable optical attenuator, comprising applying controllable longitudinally varying external stress to part or all of the fibre so as to attenuate light passing through the fibre. The fibre may be part of a fibre based optical amplifier. According to another aspect of the invention there is provided a variable optical attenuator, wherein the attenuation is distributed along the length of an optical fibre.

Brief Description of the Drawings Some preferred embodiments of the invention will now be described by way of example only and with reference to the accompanying drawings, in which:

Figure 1 is a vertical cross section of a schematic drawing of a fibre attenuator;

Figure 2 is a top plan view of a schematic drawing of a fibre attenuator;

Figure 3 is a vertical cross section of a schematic drawing of a fibre attenuator;

Figure 4 is a schematic cross section of a distributed fibre optical attenuator;

Figure 5 is a schematic cross section of a distributed fibre optical attenuator with heating elements.

Detailed Description of the Drawings Fibres are used as optical connections between components and as optical processing elements in themselves, for example as optical amplification media. Light may be guided in an optical fibre within a fibre core. If the fibre is bent or otherwise deformed, the optical mode of the core will be affected. Light which would otherwise travel through a straight fibre without significant losses will experience an increase of loss due to the bending or deformation, because the mode of the light does not match the mode of the fibre at those locations where the fibre is bent or deformed.

In addition, if stress is applied to a part of the fibre (even if the fibre is not physically deformed), the refractive index will change in that part of the fibre. A local change in refractive index may cause Rayleigh scattering and therefore loss of light travelling through the fibre. The stress may be applied by placing the fibre between surfaces which are pressed together. The stress may also be applied to the fibre by bending the fibre or by twisting the fibre. The fibre may become birefringent when stress is applied due to a change in refractive index of one optical axis, while the refractive index of the other optical axis remains unchanged. What has now been appreciated is that the losses associated with stress or deformities in the fibre can be turned to advantage. A variable optical attenuator may be created by deformations in a fibre with a controllable shape by applying stress to the fibre so as to cause strain in order to achieve a desired attenuation of the light travelling through the fibre. A variable optical attenuator may also be created by applying stress to a fibre, even if the stress does not lead to deformations or strain . The amount of attenuation may be controlled by controlling the stress applied to the fibre.

Figure 1 shows a vertical cross section of an illustration of one way of producing a variable optical attenuator. An optical fibre 12 is placed between two sets of parallel ridges 14, 16, whereby the plates of each set are spaced apart and wherein the edges of the ridges define a plane. The fibre is placed on the plane defined by one set of ridges, while the second set of ridges is placed against the fibre such that the plane of the second set faces the plane of the first set and such that the ridges of the first set are opposite the spacings between the ridges of the second set. When the two sets of ridges are pressed together, the fibre is deformed around the ends to the two sets of ridges. The more pressure is applied, the more the fibre is deformed. Controlling the pressure applied to the ridges enables loss from the fibre to be controlled, resulting in a variable optical attenuator. In general, any corrugated surface would suffice to create a device with a similar function.

The radius of curvature or size of the bends or deformations determines the amount of light lost at each bend or deformation, and thus the attenuation of the light travelling through that fibre section. A small radius of curvature of the bend or a sudden sharp deformation causes a large loss of light. A larger radius of curvature or a smaller and smoother deformation results in a smaller loss of light. The dimensions of the parallel plates 14, 16 in Figure 1 are therefore preferably small: the spacing between the plates is of the order of millimetres. Conventional structures for holding fibres, such as spools for winding fibres, are typically several centimetres. Figure 2 shows a top plan view of the system of Figure 1 . The fibre 12 has several windings in the horizontal direction, parallel to the plane defined be the ends of the two sets of ridges. The windings may be placed entirely between the two sets of ridges, or only part of the wi nd ing may be placed between the two sets of ridges. The deformations in the fibre caused by the two sets of ridges may be quasi-periodic or aperiodic. The losses due to the deformations may be distributed over a longer part of the fibre by increasing the number of windings.

The variable optical attenuator illustrated in Figure 1 may suffice for some applications, but the periodic structure of the stress and deformations may cause additional physical effects which may be a disadvantage in some applications. In particular, a periodic disturbance of the optical mode of the fibre may cause resonances for light travelling through the fibre.

One example of a physical effect obtained by a periodic variation of the refractive index of the fibre core, in addition to the physical effect of attenuation, is an effect similar to that of a Bragg grating. The periodic variation of the refractive index of the fibre core could generate a wavelength dependent dielectric mirror: light with some wavelengths travelling through the fibre will be reflected and light with other wavelengths will be transmitted. I n applications where additional physical effects caused by resonances are not desirable, an aperiodic or random distribution of stress points may be used. Figure 3 illustrates an aperiodic distribution of stress points. Both the spacing and the longitudinal extension of the stress is different between the locations at which stress is applied. A fibre 12 is placed between a surface 23 and stress-providing object 15. The object 15 comprises extensions 17, 19 and 21 facing the surface 23 such that the fibre may be pressed between the extensions and the surface 23. A first extension 17 comprises a flat surface facing surface 23. A second extension 19 terminates in a tip facing surface 23 and may apply stress on the fibre along a much shorter length of the fibre than extension 17. A third extension 21 is provided with a surface facing surface 23. The surface of extension 21 extends along a larger longitudinal section of the fibre 12 than the surface of extension 17. The three extensions are separated from each other by spaces of a different size from each other. In this illustration, both the extent of the longitudinal section in which pressure may be applied and the separation between those sections is irregular, such that any physical effects due to periodicity are avoided. The illustration of Figure 3 also shows how stress may be applied in order to create losses, where the stress does not deform the fibre and does not cause strain. The fibre shown in Figure 3 is substantially straight in the section where the stress is applied by the external objects. It will be appreciated that corresponding indentations (not shown) may be provided in the surface 23 opposite the extensions 17, 19, 21 to allow for strain in the fibre.

Loss induced in a fibre using the technique described by this disclosure may be distributed over a large set of stress locations or confined to a few or even a single stress location. Requirements for sensitivity control over the loss will define the actual design that is implemented.

In an optical network, losses may vary from node to node due to variation in fibre length and variation in losses of components. Schemes that compensate for loss variation are commonly used and generally require the need to provide control of optical losses. One embodiment of the disclosed method of inducing a controlled loss into a fibre is to utilise the control of loss in a bulk component where the loss can be controlled to the required level for the particular location where it is used. Another embodiment is the use of controlled fibre loss in an optical amplifier such as an erbium doped fibre amplifier (EDFA). Erbium Doped Fibre (EDF) is used as an amplification medium in EDFAs and is conventionally wound in relatively large diameter (>25 mm) coils. EDFAs sometimes have integrated heaters. Examples of setups are a flex circuit heater in a racetrack device or a PCB heater, with either being installed next to a compact insulated coil housing. Loosely wound fibre coils are typically used to prevent damage to fibres from tem peratu re effects or add itional bend loss. Temperature control is typically used to minimize the adverse impact of variable spectral data signal gain as a function of changes in ambient temperature (so called Thermal Wiggle). Erbium doped fibre spools are conventionally designed to prevent bend loss. Bend loss is usually considered undesirable, resulting in careful deployment of fibre with minimum stress and on diameters designed to avoid loss and minimize bend induced stress to avoid degrading the long term reliability of the fibre by minimizing induced strain. The method for inducing distributed loss disclosed herein can be beneficial to such an EDFA in several ways.

The fibres used in applications such as EDFAs are typically single mode fibres, but losses may also be induced by applying stress in multimode fibres. The losses in the variable distributed attenuator disclosed herein are not uniform over the optical spectru m . Larger wavelengths are attenuated more than smaller wavelengths. The wavelength dependence of the distributed loss attenuator means that in, for example, an EDFA, the intensity of a pump of the amplifier, which has a typical wavelength of less than 1 μηι, is much less attenuated than the signal wavelength, which has a typical wavelength of around 1.5 μηι. So, the efficiency of the EDFA is higher than in the case of a bulk loss VOA component where pump light is attenuated either by it passing through the bulk VOA or in a path bypassing the bulk VOA. Within an EDFA this embodiment allows use of different control schemes for the variable loss required in the EDFA such as an open loop loss control scheme (for example which is now incorporated into a overall gain control scheme) which does not need additional components specifically required to control loss which is often required in a bulk VOA component that is generally designed in closed loop loss control with optical power taps and monitors either side of the VOA. Because the pump power is not affected by the attenuation and the removed need for optical monitoring the overall pump power can be reduced to achieve the same gain An EDFA is often designed with variable external gain which requires wavelength dependent equalisation of gain for each external gain setting which requires matching of fixed losses plus variable loss(es) to gain in the erbium doped fibre so as to achieve correct total gain and spectral gain. Conventionally a variable loss is used to ensure the gain in the Er doped fi bre is always the same to ensure equal wavelength dependent external gain. The variable loss is achieved in a bulk optical component positioned at a fixed point or points within the EDFA optical topology. In Figure 3 the fibre 12 may be a doped fibre, such as an EDF and loss is induced in the fibre in the same manner as a transmission fibre. Thus the distributed loss attenuation can be combined with the EDF gain medium in which case a separate VOA is no longer required. This results in a lower cost EDFA as well as reduced space requirement.

A further advantage is achieved through reduction of the noise figure (NF) of an EDFA with distributed loss compared to an EDFA with a bulk loss such as a MEMs VOA. This occurs because, as the signal channels travel along a pumped EDF which has a controlled amount of distributed loss, the signal channels have both gai n and attenuation. The gain increases the instantaneous optical power and the attenuation reduces the instantaneous optical power at any point in the fibre keeping the optical power lower than a design with a lumped loss. The effect is to reduce gain saturation of the optical powers resulting in higher pump efficiency within the EDF, a higher signal to noise ratio and thus lower N F. This effect was observed by A. H . Liang et al ("Improvement of gain and noise figure by periodic bending of an erbium-doped fiber", Optics Letters 22(23) P1766 1997) where bend loss was introduced into part of the EDF within an EDFA breadboard and measurements showed the improvement in NF. The work by Liang, although showing the NF benefits, had a fixed distributed loss by providing a bend in a defined length of EDF. A change of loss was introduced by changing the length of the fibre within the fixed bend spool. This is not practical in an EDFA and the ability to provide variable distributed loss was not considered. The spool used for applying loss was also large and fixed, again not providing a practical solution for variable loss, required as described elsewhere here.

Within any optical amplifier there are many components that make up the final design and the loss of individual components within the amplifier will vary. I n the process of manufacturing an amplifier such as an EDFA, the loss is controlled on a per amplifier basis to obtain the best spectral performance, generally via changing the length of EDF or use of a loss padding method. This latter method can be achieved using the loss from a conventional VOA, or techniques such as adding controlled attenuation in a fibre splice. However all the above methods require an added manufacturing process and loss padding requires a new loss at a fixed point within the amplifier topology. By use of the distributed loss technique within the EDF itself, loss padding can be achieved without the need for an additional process or part. Thus, the distributed loss also simplifies the manufacturing of optical amplifiers. The use of the distributed loss is not limited to EDFA, but can be applied to any amplifier that requires control of a loss. It is also not restricted to a single amplifier or EDFA, but can be used in any combinations of amplifier, for example such as one or both E D FA i n a M id-Span Access structure or where combinations of different amplifiers are used together, such as a Raman-EDFA hybrid amplifier.

An alternative approach to creating a variable optical attenuator is illustrated in Figure 4. In order to reduce the size of the variable attenuator, a fibre 26 is wound around a cylinder 18. The cylinder 18 is provided with an outside surface comprising a periodic (or aperiodic) structure of protruding ridges 20. The ridges 20 are formed parallel to each other and parallel to the axis of the cylinder 18. A sleeve 22 is provided comprising a bore, whereby the diameter of the bore is larger than the outer diameter of the cylinder 18. Ridges 24 are provided on the inside of the sleeve and the ridges have substantially the same spacing, shape and orientation as the ridges on the outside of the cylinder. The fibre 26 is wound around the cylinder 18, and then the sleeve is fitted around the cylinder with the fibre. The orientation of the sleeve 22 around the cylinder 18 is such that the ridges 24 on the inside of the sleeve 18 face the spacings between ridges 20 on the cylinder. The cylinder 18 and sleeve 22 can be moved relative to each other to create a pattern of bends in the fibre. There are many designs that can be used to achieve a pattern of bends in the fibre such as sinusoidal corrugations on the inside of the sleeve and the outside of the cylinder, non-systematic corrugations such as a roughened surface with a random or pseudo-random surface structure, or corrugations either on the sleeve or on the cylinder. In order to induce stress in the fibre coil between the interfaces of the inner cylinder and the outer sleeve, the corrugated surfaces should be moved relative to each other. Many options exist that could be used to achieve this. Heat could be applied to either the sleeve or the cylinder to expand either part or both parts. Mechanical movement of either section through an electro-mechanical, piezoelectric or magnetostrictive induced movement are other examples. Rotation of the two parts relative to each other could also be used to apply pressure to the fibre. The embodiment shown in figure 4 could be adapted in many ways, for example a fibre coil could be attached externally to a tube or a planar structure where the coil is spiral, wound or coiled.

The losses may be induced by applying pressure on the fibre, but the diameter of the cylinder 18 is preferably small, for example 15 to 20 mm, and therefore additional controllable losses may be induced by winding the fibre around the cylinder 18. It is important in the design to manage losses induced from any of the sources to ensure accurate control during operation.

The length of the fibre may also be varied. A longer fibre gives the possibility of distributing the loss more when compared to a short fibre. Methods to set the loss to a desired level include an open loop control in which a known loss is achieved by setting a control parameter. Alternatively, a closed loop scheme may be applied where measurements of one or multiple wavelengths, in or outside the gain wavelength band, are monitored, and where a control parameter is adapted in response to the measurement to achieve a desired amount of loss. An electronic circuit will be required for analysis and monitoring, which may be an analogue or digital design.

In an EDFA used for DWDM transmission many channels of different wavelengths are transmitted at the same time and all wavelengths are amplified. The amount of amplification or gain of each wavelength is aimed to be the same, but varies when the temperature of the erbium fibre is varied. The temperature is therefore an important parameter that needs to be controlled in a practical realisation of an amplifier.

Often, Er fibre is heated and controlled to a fixed temperature to achieve constant gain and gain tilt performance. Heaters that keep the coil close to 65 °C or close to the maximum operating ambient temperature are used to provide control simplicity. Since bend loss is avoided in conventional designs, the Er fibre is held loosely and therefore requires a lot of space for storing and is not always in direct contact with the heater. As a consequence of one or all of these points, heat transfer is inefficient. The design that is required to keep fibre bends loose, combined with long fibre lengths mean that an efficient design is difficult. Power consumption of the heating element of 12W or more is not atypical to maintain the Er doped fibre at low ambient temperature conditions

Figure 5 illustrates how temperature control may be provided. Heating elements 28 are integrated in a cylinder 16, around which the fibre 10 is wound. This embodiment improves the efficiency of the heating process due to the closer proximity of the optical fibre to the heating element. In the example of figure 5 the cylinder is heated using integral resistive heater elements 28 having current passing through. A significant portion of the fibre will be in contact with large sections of the heater, resulting in better thermal efficiency, and the inherent enclosed design will ensure more heat is trapped within the device so that the overall electrical power needed to heat the fibre is significantly reduced. An insulating sleeve could be mounted on the outside of the outer former for even higher efficiency, but there will be a resultant elevation of the thermal response time of the composite structure, which might prove undesirable.

Heating elements provided inside the cylinder may be used to control the temperature of the optical fibre only. However, the material of the cylinder and the sleeve expands when they are heated up. When the material used for the cylinder is different from the material used for the sleeve, the coefficient of thermal expansion of the two parts will also be different. The different expansion rates of the two parts may be used to apply pressure to the optical fibre, which is placed between the two parts. The heating may therefore have both the function of controlling the temperature of the fibre and the function of controlling the optical attenuation in the fibre. Clearly, many options exist that can achieve the same result. For example, both corrugated parts may contain heating elements or only one of the elements may contain heating elements. If a preferential design requires a large coefficient of thermal expansion (CTE) mismatch, one or both formers could employ a material with a very high CTE. Thermoplastic resins and waxes exist with coefficients of thermal expansion which are a hundred times greater than some metals, making the temperature change required to induce a unit of stress many times smaller than that associated with metals. Memory metals also possess properties that might make them ideal for this purpose.

Constraints in size of the elements within optical modules and on optical cards in networking apparatus is significant, a n d th i s is d rivi n g the d esig n of sm al ler components which increase features and functionality while simultaneously reducing size. Minimization of fibre bend loss in the prior art is a key issue that limits the size reduction capability of modules while guaranteeing performance. For example, in an EDFA a large portion of the module is taken up with extraneous optical fibre, and the necessary lengths of Er doped fibre, which are typically wound inside a racetrack containing a coil heater. These conventional designs use loosely coiled Er in a large diameter elliptical racetrack ring. By using the apparatus described in this application, on the other hand, a much smaller sized Er coil can be implemented, for example in a cylindrical packaging format that is much more compatible with micro optic components used elsewhere in a typical module. By the use of controlled winding processes and careful control of induced bend loss within the device, a smaller device can be achieved. Moreover, by careful selection of Er doping concentrations and fibre design (for example using high concentration Aluminosilicate, Phosphate or Ytterbium co- doped glass) very short lengths of fibre with low loss when deployed with a very small bend radius is achievable. Applying this fibre to the apparatus described in this application will allow very small size devices to be made, enabling significant reduction in total size of the overall EDFA module. Similar methods could be applied to any fibre type, doping concentration or dopant material to achieve benefits. Many standard micro optic components are cylindrical, about 5-6mm diameter and about 25-40mm long, making them very convenient for packaging integration.

A typical diameter of a fibre that may be used in the embodiments disclosed herein is 125 μηη. The diameter of such a fibre including coating may be 250 μηη. A diameter of 80 μηη may also be used, which may reduce stress when compared to a fibre of 125 μηη. The fibre may be wrapped around a cylindrical mandrel, which mandrel may have a diameter of less than 10mm, but a 15 to 20 mm diameter may make it easier to wrap the fibre around the mandrel. The size of the deformations used to induce losses may be 5 to 10 μηη. To realise a very small Er cylinder it may be possible to use Er fibre that has very high concentrations of Er to reduce the overall fibre size. To minimise bend losses the fibre diameter may be smaller than δθμηη. These reductions in length and diameter must be matched with no degradation to reliability, and optical performance such as polarisation mode d ispersion or wavelength dependent loss. The exact fi bre length , E r concentration and fi bre size wi l l be ch osen to su it the specific appl ication . Development of a fibre to meet all requirements is a key factor in this disclosure.

It will be clear to anyone with knowledge of optical fibre and optical component design that there can be many other designs for holding and inducing loss in the fibre to achieve the same goal as described in this disclosure. For example a racetrack design could be used rather than a cylinder. In this design the same techniques as above may be used as well as others such as use of micro electromechanical elements to push bends into the fibre. In addition it is also clear that bends can be induced in all of the fibre or only parts of the fibre or in segments that cover some or all of the fibre.

Claims

CLAIMS:

1 . A variable optical attenuator comprising an optical fibre having a plurality of windings and a means for applying controllable longitudinally varying stress to at least part of each of the windings so as to attenuate light passing through the fibre.

2. A variable optical attenuator as claimed in claim 1 , wherein the stress is applied at discrete locations distributed along the longitudinal direction of the fibre.

3. A variable optical attenuator as claimed in claim 1 or 2, wherein the stress induces loss in the fibre.

4. A variable optical attenuator as claimed in any preceding claim, wherein the stress is applied by placing the fibre between a plurality of surfaces.

5. A variable optical attenuator as claimed in claim 4, the stress is applied by pressing together the surfaces facing each other, thereby applying pressure on the fibre in a transverse direction of the fibre.

6. A variable optical attenuator as claimed in any preceding claim, wherein the stress induces strain in the fibre.

7. A variable optical attenuator as claimed in any of claims 1 to 5, wherein the stress induces deformations in the fibre.

8. A variable optical attenuator as claimed in any preceding claim, wherein the stress is applied by placing the fibre between corrugated surfaces.

9. A variable optical attenuator as claimed in any preceding claim, wherein the stress is applied by placing part or all of the fibre between a plurality of ridges.

10. A variable optical attenuator as claimed in any preceding claim, wherein the stress is distributed periodically along the longitudinal direction of the fibre.

1 1 . A variable optical attenuator as claimed in any of claims 1 to 9, wherein the stress is distributed aperiodically along the longitudinal direction of the fibre.

12. A variable optical attenuator as claimed in any preceding claim, wherein light propagating through said fibre is attenuated by an amount depending on the wavelength of the light.

13. A variable optical attenuator as claimed in any preceding claim, wherein the fibre is suitable for use as the amplifying medium in a fibre based optical amplifier.

14. A variable optical attenuator as claimed in any preceding claim, wherein the fibre is a doped fibre.

15. A variable optical attenuator as claimed in claim 14, wherein the fibre is an erbium-doped fibre.

16. A variable optical attenuator as claimed in any preceding claim, wherein the fibre is a single mode fibre.

17. A variable optical attenuator as claimed in any one of claims 1 to 15, wherein the fibre is a multimode fibre.

18. A variable optical attenuator as claimed in any preceding claim, further comprising a substantially cylindrical body for winding the fibre.

19. A variable optical attenuator as claimed in claim 18, further comprising a sleeve for containing the cylindrical body and for applying stress to part or all of the fibre.

20. A variable optical attenuator as claimed in claim 19, wherein one or both of the cylindrical body and the sleeve comprise a corrugated or irregularly corrugated surface.

21 . A variable optical attenuator as claimed in any one of claims 18 to 20, further comprising a heating element for temperature stabilisation of the optical fibre and/or for controlling the loss of the VOA function within the optical fibre.

22. A variable optical attenuator as claimed in any one of claims 19 to 21 , wherein the sleeve and cylindrical body comprise materials with different thermal expansion coefficients.

23. A variable optical attenuator as claimed in any preceding claim, further comprising electro-mechanical means for inducing distributed loss in part or all of the fibre

24. A variable optical attenuator as claimed in any preceding claim, fu rther comprising thermo-mechanical means for inducing distributed loss in part or all of the fibre.

25. A variable optical attenuator according to any preceding claim, wherein the attenuation is wavelength-dependent so that longer wavelength light is attenuated more than shorter wavelength light.

26. A variable optical attenuator as claimed in any preceding claim, further comprising a closed-loop control system for controlling the attenuation.

27. A variable optical attenuator as claimed in any preceding claim, further comprising an open-loop control system for controlling the attenuation.

28. A variable optical attenuator as claimed in any preceding claim, wherein the fibre is wound according to a racetrack design.

29. A fibre based optical amplifier comprising a gain section and a means for applying controllable external stress to at least part of the length of the gain section so as to attenuate light passing therethrough.

30. A fibre based optical amplifier comprising a gain section, wherein the gain section comprises a variable optical attenuator as claimed in any of claims 1 to 28.

31 . A fibre based optical amplifier as claimed in claim 29 or 30, comprising one or more EDFA sections and/or one or more Raman gain sections.

32. A method of attenuating light passing through an optical fibre, comprising applying controllable longitudinally varying external stress to part or all of the fibre so as to attenuate light passing through the fibre.

33. A method as claimed in claim 32, wherein the fibre is part of a fibre based optical amplifier.

34. A variable optical attenuator, comprising means for inducing controllable distributed loss along the length of an optical fibre so as to generate a controllable distributed attenuation.