This invention relates to a method of coupling an electromagnetic signal between an electromagnetic signal source and an electromagnetic waveguide, in particular for an optical signal.
The use of optical fibres to carry information is becoming increasingly common. There are a number of applications where the information must be transferred between two parts of a system that have a relative motion. A typical situation is where the information is being generated on a rotating element (e.g. a sensor on a rotating wheel) and needs to be transferred to the stationary part of the system for processing and display/storage. In many circumstances the transfer can be achieved by using a length of optical fibre and allowing the flexibility of the fibre to enable the rotation of the elements whilst maintaining the required connection. However, clearly for a system such as wheel where the wheel can rotate many times the limited flexibility of the fibre will limit the number of revolutions before the fibre will break.
Currently, the solution for this requirement is to use an optical rotating joint (ORJ). These are well known devices that can be used to transfer the information across a rotating interface. However, this type of device has a severe limitation in that it must be mounted on the axis of rotation of the system and there are many applications where it is not possible to use this axis. For example for sensors on a wheel the axle of the wheel may be a solid body that occupies the axis of rotation and although space can be made available on the axis this may weaken the mechanical strength of the systems and may thus be undesirable. There are also applications where other services, such as fluids, must be passed across the rotating interface and these are most effectively achieved by mounting a rotating coupling on the axis of rotation. Thus, there is very strong competition for space on the axis of rotation and this can make the use of an ORJ inconvenient or impossible in many applications.
In accordance with a first aspect of the present invention, a method of coupling an electromagnetic signal between an electromagnetic signal source and an electromagnetic waveguide, wherein, in use, there is relative movement between the source and the waveguide comprises, at each site where the source is launched into the waveguide, modifying the properties of the waveguide to permit the signal to be coupled into the waveguide; launching the signal into the waveguide; reversing the modification at that site, such that a signal once launched propagates along the waveguide; and repeating the process for each launch site along the waveguide.
In accordance with a second aspect of the present invention, an electromagnetic signal coupler comprises a waveguide and a controller associated with an electromagnetic signal source; wherein, in use, there is relative movement between the source and the waveguide; wherein the controller causes selective modification of the waveguide at a site where the source is being launched into the waveguide, to permit the signal to be coupled into the waveguide; reverses the modification as the source moves away from that site; and repeats the process for each launch site along the waveguide.
Preferably, the waveguide is stationary and the source is moving.
Preferably, electromagnetic energy that has propagated along the waveguide, exits through the end of the waveguide.
Preferably, at least one gap is provided in a substantially continuous waveguide ring to enable data to be extracted.
Preferably, loss of data from the signal source passing over the at least one gap is avoided by increasing the rate of transmission at all other sites and preventing transmission in the gap.
Preferably, the number of sources equals the number of gaps; arranged such that only one source is over a gap at any one time; and wherein data extracted from the waveguide is combined to provide a continuous data stream.
Preferably, the electromagnetic signal source is an optical signal source.
Preferably, the signal is output through an optical fibre coupled to the end of the waveguide.
Preferably, the waveguide comprises a surface incorporating a variable grating or surface coating.
Preferably, the grating or surface coating comprises one of a magneto-optic, electro-optic, acousto-optic or light sensitive material.
Preferably, the waveguide comprises an optical fibre having a fluorescent core or cladding.
An example of a method of coupling an electromagnetic signal between an electromagnetic signal source and an electromagnetic waveguide according to the present invention and an example of an electromagnetic signal coupler will now be described and contrasted with the prior art with reference to the accompanying drawings in which:
If the axis of rotation is not available, such as shown in
In addition to rotating systems there are applications where the movement is linear, such as transferring data from a moving train to a trackside receiving system. An ORJ cannot address this type of requirement and the optical equivalent of a slip ring is required.
The present invention addresses these problems and in one embodiment, it provides an optical equivalent of an electrical slip ring. This has the advantage of being deployable on rotating systems in circumstances where ORJs cannot be and has wider applications to non-rotating systems.
The basic approach of the present invention is illustrated in
This approach can be used to form a circular slip ring for rotating applications, as illustrated in
A fundamental consideration of the present invention is that if it is possible to launch light into a waveguide, then light will also tend to leak out of the waveguide. This causes unacceptable loss in the waveguide. The total loss between the launch point and the output fibre also varies with the launch position, which causes additional complications. In principle, the more efficient the launch of the light, the greater the loss in the waveguide. Overcoming this limitation is an important requirement for implementing the present invention.
One known technique that can be used to optimise launch efficiency vs. waveguide loss is described by Chen et al in “Fully Embedded Board-Level Guided-Wave Optoelectronic Interconnects”—Proc. IEEE Vol. 88, No. 6, June 2000. This describes a number of techniques that can be used to launch light into waveguides embedded into planar materials. One of the properties of the coupling structure described is that efficient coupling (35%) of the light into the waveguide is possible, but that light already travelling in the waveguide and passing the coupler couples out at a very low efficiency of <1%. Thus it is possible to arrange a continuous line of these couplers along the length of the waveguide so that light can be coupled in at any point along the waveguide. The coupling is achieved using gratings fabricated into the surface of the waveguides.
To improve the coupling/waveguide loss performance of the invention, a number of techniques are available. These are based on modifying the properties of the waveguide at the launch point. Thus the coupler can be made to have good coupling performance at the launch point. This can be at the expense of higher loss at this point. Because this higher loss occurs only at the coupling point it will be more acceptable.
An implementation of this is illustrated in
Equally, other physical phenomena could be applied to affect the waveguide locally. For example, by use of an electro-optic effect, in which an electrostatic field replaces the magnetic field and locally alters the coupling via the electro-optic effect or, by evanescent coupling, where the gap between the material carrying the light in Part A 10 and the waveguide 15 could be made sufficiently small that presence of the material itself would locally affect the coupling. Alternatively, optical interactions, such as local illumination of the waveguide, typically with a wavelength different from the information-carrying wavelength, can be arranged to change the properties of the waveguide or acousto-optic means using sound injected locally to change the waveguide properties can be utilised.
Alternative mechanisms to provided coupling which do not require a grating structure, include the use of a coating on the surface of the waveguide that would normally reflect the light, thus keeping the light within the waveguide, but that could be locally modified to allow light to pass though at the launch site, or using fluorescence. An example of coupling using fluorescence is given in
The present invention provides a slip ring that enables an electromagnetic signal, in particular an optical one, to be transmitted between a waveguide on one part and a waveguide on a second part where the two parts are in relative motion. Efficient transfer of the signal is obtained by the use of coupling techniques that maximise the coupling of the signal between the waveguides whilst minimising any unwanted coupling out of the signal during its transmission along the remainder of the guide. A slip ring of this type enables communication between moving parts in a wide range of systems, such as between a moving vehicle and a road or track-side receiver.
An important application of the present invention is the transfer of data across a rotating interface in a machine where a standard rotating joint cannot be used because the axis of rotation is not available. A particular example of this is in computerised axial tomography (CAT) or computed tomography (CT) scanners for medical applications, where an X-ray tube and radiation detectors rotate around a body and data received at the rotating radiation detectors must be transferred to a computer for calculation of what was in the path of the X-rays, so that this can be displayed that for a medical professional to interpret. Use of an optical rotating joint is not practical because it would need to be mounted on the axis, which is where the body is positioned for scanning. Conventionally, the data transfer has been by electrical slip rings or Radio Frequency techniques, but the problems with this are these techniques are running out of data transfer capacity, require large mechanical assemblies and are prone to electromagnetic interference problems.
These examples of the present invention have been described in terms of optical signals. However, it should be noted that a number of the implementations are also applicable to other parts of the electromagnetic spectrum, in particular to microwaves.
Number | Date | Country | Kind |
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0405682.6 | Mar 2004 | GB | national |
Filing Document | Filing Date | Country | Kind | 371c Date |
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PCT/GB05/00185 | 1/17/2005 | WO | 9/8/2006 |