WO2003048844A2

WO2003048844A2 - Optical fibre switch and method of construction of such a switch

Info

Publication number: WO2003048844A2
Application number: PCT/GB2002/005495
Authority: WO
Inventors: Ian Peter Giles; Rodney Badcock; Gareth Colin Watson Jones
Original assignee: British Technology Group Inter-Corporate Licensing Limited
Priority date: 2001-12-04
Filing date: 2002-12-04
Publication date: 2003-06-12
Also published as: AU2002365722A8; AU2002365722A1; WO2003048844A3; GB0128987D0

Abstract

An optical fibre switch comprises first and second optical fibres (21, 31) each having region in which the core cladding is reduced to provide access to the fibre's evanescent field, these regions being placed in juxtaposition, a layer of control material having a variable refractive index between the two optical fibres at the juxtaposed regions, securing means for securing the fibres in their juxtaposed position and control means (7) for controlling the refractive index of the control material. The invention also includes a method of constructing such a switch.

Description

OPTICAL FIBRE SWITCH AND METHOD OF CONSTRUCTION OF SUCH

A SWITCH

This invention relates to an optical fibre switch and a method of constructing such a switch

One of the key functions in an optical network is directing the optical carriers between fibres. This requires switches with an n x n switching capability. The ideal method to obtain optimum performance is to switch directly between optical fibres selecting only the required channel. In practice, however, the passive nature of silica fibre makes it extremely difficult to implement a direct fibre to fibre switching process. Usually, an alternative medium is used to switch the signals to the desired outputs. This can be achieved with integrated ^* planar waveguide solutions or miniature bulk optics solutions such as MEMS (Micro Electro Mechanical Systems), but, in ail cases, fibre interfacing is necessary requiring accurate alignment and this introduces a performance penalty due to signal loss.

It is therefore clear that an all fibre low footprint switching will provide an optimum solution for optical fibre systems.

Optical fibre switches are not, in themselves, new. Thus optical fibre switches have been demonstrated, based on optical fibre coupler technology. In a fused taper coupler, the evanescent field extends beyond the fibre, due to the tapering, to provide the desired power coupling. With such a device, the coupling ratio of the coupling can be varied by changing the refractive index of the surrounding material.

Another method of accessing the evanescent field comprises grinding and polishing an optical fibre which has been set in an arc in a substrate block. If two such ground and polished fibres are brought info close proximity provides power splitting between the fibres. If, further, a thin layer of material of variable refractive index is incorporated between the two processed fibres, varying the refractive index of this material allows modification of the coupling ratio between the two fibres an, as a consequence, the power can be directed between the two output fibres in a controlled manner by controlling the interfacing medium.

For example, a precision refractive index oil could be used, its refractive index being controlled by a heater formed onto the surface of one of the blocks carrying the fibres. Thus, when the oil is heated by application of the heater in the interaction region, power is passed from one optical fibre to the other. Such a 2 x 2 switch is effective and meets the basic requirements for an all fibre solution, it is too bulky to cascade into n x n switch configurations.

The invention, therefore seeks, in particular embodiments of the invention, to provide a multi-way all optical fibre switch having an n x n configuration which is of a compact nature and reasonably simple to manufacture.

According to a first aspect of the invention, there is provided an optical fibre switch comprising first and second optical fibres each having region in which the core cladding is reduced to provide access to the fibre's evanescent field, these regions being placed in juxtaposition, a layer of control material having a variable refractive index between the two optical fibres at the juxtaposed regions, securing means for securing the fibres in their juxtaposed position and control means for controlling the refractive index of the control material.

Preferably the regions of reduced cladding are produced by polishing individual fibres.

The control means may comprise one or more electrodes formed on the surface of one or both of the fibres in the regions of reduced cladding. The electrode(s) may comprise means for heating the control material, and may be resistive material or may comprise means for applying an electric current to the control material.

A plurality of first optical fibres may be used, each of said first fibres being arranged in parallel to each other to provide a multi-way switch. A plurality of second optical fibres may also be used, each of said second fibres being arranged in parallel to each other. The or each plurality of fibres may be mounted in a substrate.

The substrate may comprise a block of material in which V-grooves have been precision etched.

According to a second aspect of the invention, there is provided a method of constructing an optical fibre switch comprising reducing the core cladding in a region of each of first and second optical fibres to provide access to the fibre's evanescent field, placing these regions in juxtaposition, providing a layer of control material having a variable refractive index between the two optical fibres at the juxtaposed regions, securing the fibres in their juxtaposed position and providing control means for controlling the refractive Index of the control material.

Preferably the method also comprises producing the regions of reduced cladding by polishing individual fibres,

The control means may be provided by forming one or more electrodes on the surface of one or both of the fibres in the regions of reduced cladding.

The electrode(s) may be formed to heat the control material and may comprise resistive material or may be formed to apply an electric current to the control material.

A plurality of first optical fibres may be arranged in parallel to each other to provide a multi way switch. A plurality of second optical fibres may be- arranged in parallel to each other.

The method may also comprise mounting the or each plurality of fibres in a substrate and the substrate may comprise a block of material in which V- grooves have been precision etched. The invention will now be described in greater detail, by way of example, with reference to the drawings,- in which:-

Figure 1 is a schematic cross section of a fibre which has been polished to bring the core close to the surface of the fibre.

Figure 2 is a schematic plan view of the fibre shown in figure 1 ;

Figure 3 is a view similar to figure 2 showing the formation of electrodes formed on the polished surface;

Figure 4 is a schematic plan view of a basic form of switch in accordance with the invention;

Figure 5 is a graph of a typical coupling ratio of a switch with two output ports;

Figure 6 is a schematic plan view of a multi-fibre electrode configuration for an integrated switch;

Figure 7 is a schematic of an optical add/drop module using a switch in accordance with the invention;

Figure δ is a plan view of multiple fibres set in a base substrate so as to couple light from one set of fires to a second set;

Figure 9 is a schematic plan view of a single fibre to multi-fibre coupler or switch.

Referring firstly to figures 1 and 2, there is shown a fibre 1 which has been polished on a polishing wheel so as to bring the core 3 of the fibre relatively close to the surface of the fibre, as shown a distance d over a length L. This operation allows the evanescent field of the fibre to be accessed. In order to enable the fibre 1 to react with a second fibre, the two fibres must be placed in juxtaposition with their reduced polished areas in close proximity. To enable control of this coupling, a material with a variable refractive index is placed as a thin film between the two fibres, the variation of the refractive index is produced, depending on the material of the film, by providing an electrical or magnetic field impinging on the film, passage of an electric current through the film, Applying an optical signal in a predetermined direction or by heating the film. The variable refractive index material may be a suitable polymer (electro-optic) or a precision refractive index oil or other polymers. Thus the variable refractive index material is an optical material, with low optical loss, which can be applied to the fibre with carefully defined physical dimensions. The refractive index of the material is typically but not necessarily close to that of the optical fibre core. The index of such materials can be varied by changing temperature, applying an electric field, passing a current, application of a magnetic field or an optical signal. The material may be a suitable polymer, crystal, oil, glass or a material with similar properties. However, the invention is not limited to the use of any particular material.

To control the variable refractive index of this material, it is necessary to introduce electrodes in contact with the film by means of which an electrical current or heat can be passed into the film. A typical example of one such electrode pattern used with a heat sensitive film is shown at 7 in figure 3. As can be seen, the electrode has two portions 9 and 11 extending along each side of the core 3 in the polished area, ending in two terminal portions 13 and 15 to which wires can be connected.

Figure 4 shows the arrangement of a pair of optical fibres 21 and 31 arranged to provide an optical fibre switch. As can be seen, the electrode 7 is formed on the fibre 31 and is formed of resistive material to form a heating element for the variable refractive index film (not shown) laid thereon. Control wire contacts 23 and 25 are bonded to opposite ends of the electrode 7 by any conventional wire bonding method. The whole is then fixed in a final package, indicated in figure 4 by the dotted line 27. This switch has four ports indicated in the figure 4 as port 1 , port 2 etc. Two of these ports will be input ports and two of them will be output ports. Which two are which depends on the direction of the light flow. Figure 5 shows a typical coupling ratio between two output ports as a function of the changing refractive index of the control film.

Figure 6 shows one example of a multi-fibre configuration for a multi-channel integrated switch design. Here, nine fibres 31 are aligned in parallel in a substrate block 33 in V-grooves 35 produced by precision etching. Also seen _{nere are} the nine individual control electrodes 7. To complete the switching arrangement, a film of variable refractive index control material is placed on the substrate over the electrodes 7 and a second set of optical fibres in its own substrate is superimposed over the substrate 33 and fixed in place.

The method of construction of a multi-way optical fibre switch in accordance with the invention will now be considered:

A multi-way optical switch can be constructed, using the following stages:

First, a series of fibres corresponding to the number of channels required are processed by polishing the individual fibres on a polishing wheel to access the evanescent field.

Then the fibres are accurately positioned relative to one another, side by side on a base to create a multi-fibre substrate.

The substrate can be a series of parallel v-grooves in a suitable substrate material or can be formed from a block of appropriate material with accurate slots machined into the surface. One embodiment of the invention involves using a silicon micro-machined substrate with V-grooves produced by precision etching. The substrate can be processed using conventional electronic processing methods. This facilitates the accurate alignment of processing masks to create electrodes.

The electrode design is formed on the surface using the required material and contacts are bonded by conventional wire bonding methods. An example of a possible design is shown in figure 6. The surface is covered with a thin layer of the optical control material and a second fibre substrate block is brought into close proximity and adjusted to provide the required coupling between each of the fibres. The two sections are fixed to form a robust series of switches.

Each individual switch can be adjusted by applying a voltage across the two associated electrodes enabling complete coupler tuning and full switching-

Large numbers of fibre switches in a small substrate footprint can be fabricated by this method.

Up to now, there has been described switching in which the intermediate variable refractive index film has bee controlled thermally.

In the case of electro optic interaction, this requires the use of an electric field across the material in the appropriate direction.

The electrode design for electro-optic materϋal will depend on the specific interaction utilised, the direction of the field and the speed at which the device will operate. For example the requirement for high speed switching demands low capacitance electrodes and designs would be made specific to the requirements-

There is a range of applications for multi-channel switches, one application is the optical add/drop multiplexer (OADM) in which an individual wavelength channel can be dropped from the main highway or added to it. Figure^' 7 is a schematic illustration of the basic components for such an OADM, indicating the position for the all fibre, multi-channel switch.

The multi-channel switch provides a switch for each of the wavelength channels separated by a wavelength division multiplexer 45 (WDM). The main transport fibre input 41 carrying all of the channel wavelengths is split into a series of individual fibres 43, each carrying one wavelength. These are recombined by a second WDM 47 to the main fibre output line 49. Each channel is now accessible, when the in-line switch 51 is in the transmission State the signal continues through without loss. When switched to the add/drop state, the signal is coupled out and redirected, through add drop fibres 53, possibly to a receiver. In the sa e switch condition one or more wavelengths can be added by add/drop fibres 53 to continue through the main transmission link.

Channel balancing is achieved by the use of variable optical attenuators 55. Power monitors for each of the fibres produce an electronic signal, which can be used to control the attenuator and maintain the power throughput in each channel using power taps 59. The second WDM 47 recombines the individual channels onto a single fibre 49.

This form of OADM is known, but the multi-channel all-fibre switching unit 51 enhances the performance of the OADM.

The flexibility of the fibre substrate approach enables cross connect switching to be achieved. The length of fibre over whicn the evanescent field is exposed can be controlled through the fibre processing. This leads to a high aspect ratio of about 100 for the exposed region, typically 10's mm long and 0.125mm wide. Coupling can be achieved between the fibres when they are crossed as shown schematically in figure 8. It is to be noted that the fibres of the upper block 61 are drawn so that the polished regions can be seen so as to indicate the position of these regions in relation to the lower block 63

In this application of the invention two multi-fibre substrates 61 and 63 are fabricated as described earlier and aligned at an angle such that each of the fibres 65 from one substrate 61 crosses the fibres 67 of the second substrate 63. The angles might range from 0 to 90 degrees according to the desired performance requirements. Electrodes (not shown) are formed close to each of the cross over points (interaction sites) to provide the necessary electro- optic interaction. In the case described here the interaction is thermo-optic. The electric current to each of the electrodels is adjusted to control the level of coupling between the fibres at that interaction point. This will allow the optical signal in any of the fibres from one substrate to be coupled to any of the fibres in the second.

It is important that each of the "cross over" points can be addressed independently to ensure individual switch control. The electrodes may be fabricated to provide heating of a thermo-optic material or, if electro-Optic materials are used, develop an electric field for electro-optic materials.

The number of fibres in each block may be different. For example, as shown in figure 9. one block 63 may contain n fibres (here shown as five) whilst the second block 61 contains only one fibre 65. In practice, the block 61 would not be a block as such but merely a single treated optical fibre. This would allow power from the single fibre to be switched to any of the n fibres or conversely power from any or all of the n fibres to be switched onto the single fibre (figure 9)-

Further advantages can be achieved if polarisation maintaining optical fibres are used. Polarisation maintaining optical fibres are asymmetrical fibres designed to provide preferentially, essentially linear polarisation axes in an optical fibre. The asymmetry is produced during manufacture by shaping either the core or cladding (typically elliptically) or introducing stress members into the cladding, which create a strain on the core causing a high birefringence across the core. A linearly polarised optical signal launched such that the polarisation is parallel to one of the orthogonal fibre axes will maintain that linear state of polarisation along the fibre. This fibre type is becoming increasingly more important to - optical fibre telecommunication networks.

The two axes are referred to as the fast and slow axes, which indicates that one has a higher effective refractive index than the other. These fibres can be aligned to be ground and polished such that the resulting flat is parallel to either axis.

This fibre substrate can be used in all Of the proposed embodiments to provide a polarization maintaining switching operation.

In addition it is possible to polish one set of fibres parallel to the fast ax[s, prior to mounting in the substrate block and a second set parallel to the slow axis. In this way it is possible to switch from the fast axis of the fibre to the slow axis of a second fibre.

It will be appreciated that various modifications of or additions to the above described embodiments may be made without departing from the scope of the invention.. For example, while two basic applications of the invention have been described, the skilled man will be aware of many other such applications requiring multi-switching techniques.

It should be observed that one of the strengths of the approach used in the present invention is that any optical fibre type can be used. This means that the switches can be used at a wide range of wavelengths. The most important aspect of this is that these switches can be made with polarisation maintaining fibre.

Claims

CLAIMS:

1. An optical fibre switch comprising first and second optical fibres each having region in which the core cladding is reduced to provide access to the fibre's evanescent field, these regions being placed in juxtaposition, a layer of control material having a variable refractive index between the two optical fibres at the juxtaposed regions, securing means for securing the fibres in their juxtaposed position and control means For controlling the refractive index of the control material.

2. A switch as claimed in claim 1, where;ιn the regions of reduced cladding are produced by polishing individual fibres.

3. A switch as claimed in claim 1 or 2, wherein the control means comprise one or more electrodes formed on the surface of one or both of the fibres in the regions of reduced cladding

4. A switch as claimed in claim 3, wherein the electrode(s) comprise means for heating the control material.

5. A switch as claimed in claim 4, wherein the eiectrode(s) comprise resistive material.

6. A switch as claimed in claim 3, wherein the electrode(s) comprise means for applying an electric current to the'^, control material.

7. A switch as claimed in claim 3, wherein the electrode(s) comprise means for applying an electric field to the control material.

8. A switch as claimed in claim 1 or 2, wherein the control means comprise means for applying an optical signal to the layer of control material.

9. A switch as claimed in claim 1 or 2, wherein the control means comprise means for applying an magnetic field to the layer of control material.

10. A switch as claimed in any preceding claim, comprising a plurality of first optical fibres, each of said first fibres being arranged in parallel to each other to provide a ulti way switch.

11. A switch as claimed in claim 10 and comprising a plurality of second optical fibres, each of said second fibres being arranged in parallel to each other.

12. A switch as claimed in claim 10 or 11 , wherein the or each plurality of fibres are mounted in a substrate.

13. A switch as claimed in claim 12, wherein the substrate comprises a block of material in which V-grooves have been precision etched,

14. A method of constructing an optical fibre switch comprising reducing he core cladding in a region of each of first and second optical fibres to provide access to the fibre's evanescent field, placing these regions in juxtaposition, providing a layer of control material having a variable refractive index between the two optical fibres at the juxtaposed regions, securing the fibres in their juxtaposed position and providing control means for controlling the refractive index of the control material.

15. A method as claimed in claim 14, and comprising producing the regions of reduced cladding by polishing individual fibres.

16. A method as claimed in claim 14 or 15, wherein the control means is provided by forming one or more electrodes on the surface of one or both of the fibres in the regions of reduced cladding

17. A method as claimed in claim 16, wherein the electrode(s) are formed to heat the control material.

18. A method as claimed in claim 17, wherein the electrode(s) comprise resistive material.

19. A method as claimed in claim 16, wherein the electrode(s) are formed to apply an electric current to the control material.

20. A method as claimed in claim 16, wherein the electrode(s) comprise means for applying an electric field to the control material.

21. A method as claimed in claim 14 or 15, wherein the control means comprise means for applying an optical signal to the layer of control material.

22. A method as claimed in claim 14 or 15, wherein the control means comprise means for applying an magnetic field to the layer of control material.

23. A method as claimed in any one of claims 14 to 22, wherein a plurality of first optical fibres are arranged in parallel to each other to provide a multi way switch.

24. A method as claimed in claim 23, wherein a plurality of second optical fibres are arranged in parallel to each other.

25. A switch as claimed in claim 23 or 24. and comprising mounting the or each plurality of fibres in a substrate.

26. A method as claimed in claim 25, wherein the substrate comprises a block of material in which V-grooves have been precision etched,

27. An optical fibre switch substantially as described herein with reference to the drawings.

28. A method of constructing an optical fibre switch substantially as described herein with reference to the drawings.