GB2407055A

GB2407055A - Method and apparatus for processing optical fibre

Info

Publication number: GB2407055A
Application number: GB0323972A
Authority: GB
Inventors: Saeed Rehman
Original assignee: FIBERLOGIX Ltd
Current assignee: FIBERLOGIX Ltd
Priority date: 2003-10-13
Filing date: 2003-10-13
Publication date: 2005-04-20
Anticipated expiration: 2023-10-13
Also published as: GB0323972D0; GB2407055B

Abstract

A method and apparatus for selectively removing material from an optical fibre using a femtosecond or fluorine laser and means for causing light emitted by the laser to be incident on a part of the optical fibre from which material is to be removed.

Description

METHOD AND APPARATUS FOR PROCESSING OPTICAL FIBRE

The present invention relates to a method for processing optical fibre and an apparatus for carrying out the method.

In this application the term light is used to refer to electromagnetic radiation suitable for propagation along an optical fibre. The term light is used in this context in this application as not being limited to visible light but is intended to also include other parts of the electromagnetic spectrum to include at least infrared, ultraviolet and visible light.

In general optical fibres comprise a central core region surrounded by an outer cladding region having different properties.

It is known that the propagation of an optical signal through an optical fibre can be modified or sensed in a variety of ways by interacting with the evanescent field of the optical signal extending out of the core region of the optical fibre and into the cladding region of the optical fibre.

There are two main known methods of processing optical fibres in order to allow access to the evanescent field. The first method is to reduce the diameter of the optical fibre until the evanescent field extends beyond the cladding by heating and drawing the fibre. The second method is to remove the cladding in a region of the optical fibre by abrading, such as by grinding and polishing. One such example of this second method is disclosed in W000/49439.

There are problems with both of these known methods of processing an optical fibre in order to allow access to and interaction with the evanescent field.

The method of heating and drawing the optical fibre has the problem that both the core and the cladding of the optical fibre are thinned by the heating and drawing process.

This thinning of the optical fibre core can have undesirable effects on light signals passing along the optical fibre. A further disadvantage of the heating and drawings approach is that the reduction in cladding thickness takes place substantially symmetrically around the circumference of the fibre. As a result, where a fibre supports asymmetric modes of the light passing along it it is not possible to selectively thin the cladding on only a part of the circumference of the optical fibre to provide an axially asymmetric region of reduced cladding thickness having a desired geometric relationship to the asymmetric modes.

The second technique of grinding and polishing optical fibres in order to remove the cladding also has problems. When using a grinding and polishing technique to physically abrade the optical fibre cladding to reduce the cladding thickness it is necessary to continuously pass a light signal through the optical fibre being ground and polished and detect changes in the signal which is emitted at the opposite end of the fibre after passing through the region being processed. The changes in this emitted signal are used to determine when the cladding has been reduced to the desired thickness so that the grinding and polishing process should be stopped. This requirement to pass a light signal along the optical fibre during processing and to measure the emitted signal for use as a feedback signal to control the progress of the processing makes this technique slow and expensive to carry out because of the extra equipment required to produce, monitor and respond to the emitted light signal and the additional processing steps of linking the optical fibre to a suitable light source and light sensor. Further, the grinding and polishing technique of producing a region of reduced cladding thickness cannot process optical fibres supporting asymmetric modes, for example a polarization maintaining fibre such as a Panda or Bow- tie optical fibre to produce an axially asymmetric region of reduced thickness cladding having a predetermined geometric relationship to the asymmetric mode reliably. Use of the grinding and polishing operation to remove the cladding from the fibre will produce a fibre having an axially asymmetric region of reduced thickness having a particular geometric relationship to the fibre. However, due to twisting of the optical fibre during the processing operation, where the optical fibre supports an asymmetric mode the geometric relationship between the asymmetric mode and the region of reduced cladding thickness is highly variable. As a result, the production of an axially asymmetric region of reduced cladding thickness having a predetermined geometric relationship to an asymmetric mode supported by the optical fibre cannot be guaranteed and the processing of asymmetric mode optical fibres by this method results in an extremely high and undesirable level of wastage.

Both the heating and drawing and grinding and polishing techniques for reducing the cladding thickness of an optical fibre suffer from the further disadvantages that they can only be used to reduce the cladding thickness over relatively long sections of optical fibre and that they can only be used to process individual optical fibres in isolation.

There are many situations in which it would be advantageous to be able to attach one or more optical fibres to a substrate before carrying out processing to reduce the cladding thickness of the optical fibres. However, the known processing methods cannot be used to do this. The method of heating and drawing an optical fibre clearly cannot be applied to an optical fibre secured to a substrate. Regarding the grinding and polishing method, as is explained in WOOO/49439, in order to carry out the cladding reduction operation by grinding and polishing, a single optical fibre is held under tension against a wheel having an abrasive surface so that the fibre is bent around the wheel. Such an operation of bending an optical fibre around an abrasive wheel under tension cannot be carried out if the optical fibre is attached to a substrate.

The present invention is intended to provide a method and apparatus for processing optical fibres overcoming these problems at least in part.

In a first aspect, this invention provides apparatus for selectively removing material from an optical fibre comprising a femtosecond or fluorine laser and means for causing light emitted by the laser to be incident on a part of the optical fibre from which material is to be removed.

In a second aspect, this invention provides a method of removing material from an optical fibre using a femtosecond laser or a fluorine laser including the steps of causing light emitted from the laser to be incident on a part of the optical fibre from which the material is to be removed.

Preferred embodiments of this invention will now be described, by way of example only, with reference to the accompanying diagrammatic Figures, in which: Figure 1 shows apparatus for processing an optical fibre according to the invention and; Figure 2 shows an optical fibre processed by the invention.

This invention is based upon the particular concept of removing the cladding of an optical fibre in a desired area in order to expose the evanescent wave using lasers, and in particular using femtosecond lasers or fluorine lasers.

Femtosecond lasers operate at a wave length of about 800 nm with pulse durations of about 100 fs. Fluorine (F2) lasers operate at a wave length of 157 rim with pulse durations of around 20 ns. Both femtosecond lasers and fluorine lasers are well known in their own right and are readily available. Accordingly, the construction and operation of femtosecond lasers and fluorine lasers will not be discussed in detail in this application.

An example of apparatus for processing optical fibres according to the invention is shown diagrammatically in Figure 1.

In Figure 1 an optical fibre 1 is located on a stage 2 by clamping means 10. A laser 3, which may be a femtosecond laser or fluorine laser, is arranged to emit a beam of light which passes through a homogeniser 4, a focussing lens 5 and a mask 6. The homogenised and focussed light beam is then directed by a mirror 7 into a projection lens 8. The projection lens 8 projects the light beam onto a desired part of the optical fibre 1 on the stage 2. The light beam incident on the optical fibre 1 then removes material from the surface of the optical fibre in a predictable manner.

It should be understood that although the term light beam is used above for simplicity to designate the path of the light emitted by the laser 3, both femtosecond lasers and fluorine lasers are pulse lasers and not continuous beam lasers.

An example of an optical fibre which has been subject to laser micromachining according to the invention is shown in Figure 2 where a series of rectangular pits 9 have been cut into the cladding of the optical fibre 1, the pits 9 being spaced apart along the length of the optical fibre 1.

In Figure 1 the homogenizer 4, focussing lens 5 and mask 6 are shown as single discreet components for simplicity. It should be understood that many other optical arrangements can be used in order to provide a suitable beam for laser machining from a femtosecond laser or fluorine laser. Further, the use of a single beam directing mirror to direct the light beam from the laser 3 through the projection optics 8 onto the optical fibre 1 is shown in Figure 1. The number of beam directing mirrors used in any apparatus embodying the invention can be varied as convenient depending upon the desired beam path. The use of beam directing mirrors 7 is not essential. However, in practice it is usually inconvenient to arrange the laser 3 and all of the optical components along a single straight beam path so that the use of at least one beam directing mirror 7 is usually preferred.

It will usually be necessary to carry out the laser machining in a controlled non- ambient atmosphere. The necessary components to do this are well know in their own right and have been omitted from Figure 1 for clarity.

The apparatus according to Figure 1 can be used to micro-machine the surface of an optical fibre 1 in order to accurately and reliably remove a predetermined thickness of the cladding layer over a predetermined region of the outer surface of the optical fibre 1 to achieve a desired three-dimensional cladding thickness profile.

In order to remove the exterior cladding of an optical fibre to form a desired three dimensional cladding thickness profile there are two possible approaches.

The first approach is to use direct writing of the laser, in which the laser is focussed onto a small area of the optical fibre surface and this focus point is moved over the optical fibre surface. Cladding material is removed from the surface of the optical fibre at the focus point at a predictable rate over time or rate per number of pulses so that the movement of the focus point across the surface of the optical fibre can be used to remove the cladding material to provide a desired three dimensional cladding thickness profile.

The movement of the focus point across the optical fibre can be carried out by movement of the optical fibre or by movement of some or all of the optical components in order to provide the desired relative movement, as is well known in the field of optical machining.

An alternative technique to provide the desired three dimensional profile is to pass the laser beam through a mask projection system so that a pattern of different beam intensities is formed across an area of the optical fibre surface. Cladding material will then be removed from the optical fibre cladding surface at different rates in different areas of the projected pattern in dependence on the beam intensity at the different points in the pattern.

Removal of the cladding material at each point in the pattern is predictable as a rate over time or rate per number of pulses. Accordingly, if the optical fibre has the projected pattern incident on it for a predetermined time or number of pulses a desired three dimensional cladding thickness pattern will be produced.

In principle femtosecond lasers and fluorine lasers can both be used with either of these two methods of forming three dimensional patterns. However, in practice the different characteristics of femtosecond lasers and fluorine lasers mean that it is usually advantageous to use different pattern forming methods for the different types of laser.

Femtosecond lasers provide high precision thermal damage free machining and are better suited to use in the direct writing mode of producing the desired thickness pattern. Typically in such a direct writing mode the femtosecond laser beam is focussed into a small spot having a diameter of around 10 micro-metres and then moved over the optical fibre to produce the desired thickness pattern.

Fluorine (F2) lasers are best used with a mask projection system to control the generated cladding thickness pattern.

The method of reducing cladding thickness of an optical fibre according to the present invention overcomes the problems encountered in the known techniques described above.

The rate of removal of cladding material from the surface of the optical fibre can be reliably predicted in both the direct writing and mask projection techniques. This rate may be expressed as a rate over time or over a number of pulses as convenient.

Accordingly, the process of laser machining using a femtosecond laser or a fluorine laser according to the invention can be used to form a desired cladding thickness profile on an optical fibre with a high level of accuracy and repeatability without any requirement to monitor and control the process using a light signal passing along the optical fibre.

Further, the laser machining technique according to the present invention can be used to form an axially asymmetric region of reduced cladding thickness on an optical fibre. Also, the laser machining technique according to the present invention requires only a very short length of optical fibre to be exposed and does not exert any significant physical forces on the optical fibre, so that where the optical fibre has an asymmetric core the alignment of the optical fibre during the machining process can be controlled so that a predetermined geometric relationship exists between the asymmetric core and the asymmetric region of reduced cladding thickness. The optical fibre having an asymmetric core could for example be a polarization maintaining fibre such as a Panda or Bow-tie optical fibre. Accordingly, where the optical fibre has an asymmetric core which can support asymmetric modes of the light passing through it the machining technique of the present invention can reliably produce an axially asymmetric region of reduced cladding thickness having a predetermined geometric relationship to the asymmetric core and/or asymmetric light modes. Thus, the problem encountered in the known grinding and polishing process of W000/49439 of excessive wastage can be overcome.

Further, using the laser machining technique of the present invention there is no requirement to stretch or bend the optical fibre being processed during the machining operation, unlike the techniques described above. As a result, the machining techniques of the present invention can be used to process optical fibres which are secured to a substrate.

Provided that the cladding surface of the optical fibre is accessible to the laser beam from at least one direction the laser machining of the optical fibre according to the present invention can be carried out regardless of whether the optical fibre is secured to a substrate and regardless of whether further optical fibres are also secured to the same substrate. As a result, machining according to the present invention can be applied to single or multiple optical fibres secured to a substrate.

In Figure 1 the optical fibre 1 is shown as being flat and straight and as being supported directly on the flat surface of the stage 2. As explained above, it is not essential for the fibre 1 to be mounted directly on the upper surface of the stage 2 and the fibre 1 may be located on a substrate which is in turn directly or indirectly supported by the stage 2. Further, the optical fibre may be arranged in a curved shape. If the optical fibre lies in a curve in a plane substantially parallel to the stage 2 or, to put it another way, perpendicular to the laser beam, this will have no effect on processing of the fibre using the machining method of the present invention. However, if the optical fibre is arranged in a curve extending outside this plane so that the position of the optical fibre along the optical axis of the projection lens 8 varies it may be necessary to compensate for this when moving the optical fibre 1 relative to the projection lens 8 so as to keep the part of the optical fibre 1 being machined by the laser at desired, approximately fixed, distance along the light beam axis from the projection lens 8.

Preferably the clamping means 10 is a clamping and tensioning means able to apply a predetermined tension to the optical fibre 1 during the cladding removal operation.

It has been found that holding the optical fibre under a predetermined tension during the cladding removal operation can give a better surface finish to the optical fibre. The optimum level of tension will vary from fibre to fibre depending upon the fibre composition and diameter.

In practice it may not be possible to apply a tension to an optical fibre during the processing if the optical fibre is attached to a substrate. Nevertheless, it is preferred to apply a tension to the optical fibre during the cladding removal process if possible.

A further advantage of the present invention is that the optical fibre cladding material can be removed to allow access to the evanescent wave with minimal disturbance to any protective coating.

As explained in the introduction of the present application, optical fibres in general comprise a central core region surrounded by an outer cladding region having different properties. However, in addition to these parts of the optical fibre which control its optical characteristics optical fibres commonly also comprise a further exterior protective layer or coating on the outside of the cladding. The exterior protective layer does not have any influence on the optical characteristics of the optical fibre but provides physical and chemical protection for the optical fibre.

Where the present invention is used to move cladding material from an optical fibre it is only necessary for the laser beam to be able to be incident on the cladding in the region where the cladding material is to be removed. Accordingly, in the present invention it is necessary only for any external protective layer to be removed in a small area where the cladding material is to be removed, although in practice it will usually be desirable to remove the protective layer over an at least slightly larger area than the area from which the cladding material is to be removed.

In contrast, using the known heating and stretching or grinding and polishing techniques to thin the cladding material it is usually necessary to remove any protective layer from the optical fibre cladding over a large length of the optical fibre before beginning to thin the cladding. Further, even if such a protective layer removal over a large length of the optical fibre is not necessary the known heating and stretching or grinding and polishing techniques for cladding thickness reduction will inevitably remove or disrupt and disturb any protective layer over a large length of the optical fibre.

Thus, if an optical fibre having an external protective layer is to be processed use of the present invention minimises the disruption to this protective layer.

In the present invention a protective layer could be removed from the optical fibre as a separate operation before beginning the laser machining of the optical fibre. However, for simplicity it is preferred to remove any protective layer as part of the laser machining operation. The removal of the protective layer and cladding may be a single continuous process or a two stage process in which the protective layer is removed in a first stage and the cladding in removed in a second stage. Where a two stage process is used it is preferred to remove the protective layer in a first stage using a first low power setting of the laser and then to remove the cladding material in a second stage using a second higher power setting of the laser. Such a technique of removing the protective layer at a lower laser power setting than the cladding removal prevents any variation in the protective layer composition and thickness causing variations in the three-dimensional cladding and thickness profile which could be produced by the coupling of the two stages of the process.

This preferred two stage two power approach will not always be possible. Further alternative arrangements include use of a separate laser for removal of the protective layer or coating. The best technique in any particular application will depend upon the material and properties of the protective layer and cladding.

A further advantage of the present invention is that the removal of cladding material from the optical fibre using a femtosecond or fluorine laser has no effect on the dimensions or properties of the optical fibre outside the region where the cladding material is removed. As a result, multiple regions of reduced cladding thickness allowing access to the evanescent wave of light passing along the optical fibre can be formed at closely spaced intervals along the optical fibre in order to allow different devices to interact with the evanescent wave. Although multiple regions of reduced thickness can be produced allowing multiple optical devices to be spaced along the optical fibre using the known cladding reduction techniques described above, the regions of reduced cladding thickness produced using the known techniques must be placed much further apart. This is because the known techniques for reducing cladding material thickness affect the properties of the optical fibre along a relatively large length of the optical fibre. As a result, where multiple regions of reduced cladding thickness are to be formed along a single optical fibre the regions must be spaced apart by a greater length of optical fibre in order to avoid the heating and stretching or grinding and polishing operation of the prior art being carried out on a part of the optical fibre which has already been heated and stretched or ground and polished. As a result, the present invention allows multiple regions of reduced cladding thickness to be formed on a single optical fibre with a closer spacing than can be achieved by know techniques.

A further advantage of the present invention is that the regions of reduced cladding thickness formed by the femtosecond or fluorine laser can be controlled to be holes having a predetermined shaped cross section and depth, for example as shown in Figure 2. Semiconductor elements or other electrical elements can then be grown or formed directly onto the area of reduced cladding thickness. For example devices such as lasers or photodiodes could be grown in the areas of reduced cladding thickness. These could then be used to allow side pumping or other interaction with the light passing along the optical fibre to take place.

The size of such semiconductive devices is variable, but one typical example would be a photodiode having a width of approximately 250 micrometers. A femtosecond laser of fluorine laser can easily be arranged to be focussed onto an illuminated spot or region on the surface of the optical fibre having a diameter of around 10 micrometers would be adequate. With a laser focussed to produce a spot size of 10 micrometers, it is straightforward to form a region of reduced cladding thickness on the surface of the optical fibre having the desired width of 250 micrometers.

This technique of growing semiconductive devices within a region of reduced cladding thickness on a surface of an optical fibre can not be carried out for know techniques of cladding removal because the known techniques reduce the cladding thickness over a great length of the optical fibre, this length being orders of magnitude larger than the practical size at which a semiconductive device could be grown.

This technique of forming or growing semiconductive or electrical elements within recesses in the cladding of the present invention can be used to make parts of electrical devices inside the region of reduced cladding thickness. The elements formed within the area of reduced cladding thickness may be parts of electrical or and electrical device or a complete electrical or electro-optical device.

Where the optical fibre is mounted on a substrate and electro-optical or electrical components are formed in the region of reduced cladding thickness these components may be connected to Farther electro-optical or electrical components mounted on the substrate.

A farther advantage of the present invention is that the invention can be used to form a region of reduced cladding thickness of an optical fibre to which another optical fibre cleaved at an angle can be attached. Provided that the fibre is cleaved at the correct angle the resulting optical fibre connection can be used to introduce light into the optical fibre or allow side pumping of the optical fibre or to allow mode mixing in the optical fibre, for example in a double clad fibre or a rare-earth doped fibre.

A farther embodiment of the invention is that the technique of removal of cladding material can be used to form a series of holes in the optical fibre in order to produce a Bragg grating structure. - 1 1

In the description the invention is described with reference to an optical fibre having a core, a single cladding layer and optionally a single protective coating. The invention can also be applied to more complex structures having multiple core, cladding or protective layers.

The embodiments and examples of the invention described above are not intended to be exhaustive and the scope of the invention is as defined in the appended claims.

Claims

1. Apparatus for selectively removing material from an optical fibre comprising a femtosecond or fluorine laser and means for causing light emitted by the laser to be incident on a part of the optical fibre from which material is to be removed.

2. Apparatus according to claim 1, and further comprising a stage adapted to retain an optical fibre in a predetermined position.

3. Apparatus according to claim 2, and further comprising tensioning means arranged to apply a predetermined tension to an optical fibre retained on the stage.

4. Apparatus according to any preceding claim and further comprising moving means arranged to change the part of the optical fibre on which light emitted from the laser is incident.

5. Apparatus according to claim 4, in which the moving means are arranged to move the stage.

6. Apparatus according to claim 4, in which the moving means is arranged to change the path of light emitted from the laser.

7. Apparatus according to any preceding claim, and further comprising an optical projection means adapted to project light emitted by the laser onto the optical fibre with a predetermined intensity pattern.

8. A method of removing material from an optical fibre using a femtosecond laser or a fluorine laser including the steps of causing light emitted from the laser to be incident on a part of the optical fibre from which the material is to be removed.

9. A method according to claim 8, in which the material is removed from the cladding of the optical fibre.

10. A method according to claim 9, in which light from the laser is stopped being incident on said part of the optical fibre when a predetermined amount of material has been removed.

1 1. A method according to claim 9 or claim 10 and including the further step of holding the optical fibre under tension while the material is being removed.

12. A method according to claim 9 and including a further step of removing material from a protective layer outside the cladding before removing material from the cladding.

13. A method according to claim 12, in which the removal of material from the protective layer is carried out using light incident on the optical fibre with a first intensity and the removal of material from the cladding is carried out using light incident on the optical fibre at a second intensity higher than the first.

14. A method according to any one of claim 9 to 13, in which the thickness of the cladding of the optical fibre is reduced to allow access to an evanescent wave of light passing along the fibre.

15. A method of forming an optical fibre having an integral electrical component comprising the steps of: producing a recess in the cladding of the optical fibre using a method according to any one of claims 9 to 14; and forming the electrical component within the recess.

16. A method according to claim 15 in which the component comprises a semiconductor material grown within the recess.

17. An optical fibre produced according to the method of any one of claim 8 to 16.

18. Apparatus for processing an optical fibre substantially as shown in or as described with reference to Figure 1 of the accompanying drawings.