GB2423589A

GB2423589A - Wireless communication receiver with optical diverging element and filter

Info

Publication number: GB2423589A
Application number: GB0503667A
Authority: GB
Inventors: Roger Green
Original assignee: University of Warwick
Current assignee: University of Warwick
Priority date: 2005-02-23
Filing date: 2005-02-23
Publication date: 2006-08-30
Also published as: GB0503667D0

Abstract

A wireless communication receiver 10 comprises a detector 20 for detecting a signal, a filter 16, having a filter surface, a first optical collecting device 12 which converges incident radiation R towards the filter and an optical diverging device 14 disposed between the collecting device and the filter which diverges radiation from the first collecting device and narrow the angle of the radiation incident on the filter relative to the normal to the filter surface. Lens 12 may be be a converging Fresnel lens.

Description

Optical Device This invention relates to an optical device and is

concerned more particularly but not exclusively with optical wireless communication receivers such as infrared communication receivers.

The term "optical" is used in this specification to denote not only those wavelengths in the visible spectrum but also infrared and ultraviolet wavelengths, that is a complete wavelength range from about 1 nm to about 1mm.

In recent years there have been attempts to improve optical wireless communication systems for indoor and outdoor applications. This is because the optical part of the spectrum offers significant advantages compared to radio as a medium for short range communication. In particular, infrared communication provides high bandwidth at low cost, reduces radio interference and is in a spectrum that is freely available. However, it is well known that atmospheric transmission of light is effected by ambient illumination, and this has a number of undesirable effects. Firstly, the ambient illumination tends to mask the weak infrared or optical signals which are being detected. Secondly, the presence of the ambient illumination produces shot noise in the front end amplifier of the receiver which is greater the higher the overall signal level being received.

Various solutions to the problem of ambient illumination have been proposed. An example is the use of a narrow band optical filter placed between a collection region of the device and a detector. A problem with such a narrow band filter is that its optical transmission characteristics are dependent upon the angle of incidence of the signal. In particular, in order to deliver increased benefit for optical wireless applications in reducing ambient illumination effects, the filter bandwidth should be as narrow as possible. However, the narrower the bandwidth the more sensitive the device to the angle of incident illumination.

In figure 1 is shown a graph of angle sensitivity of a typical narrow band optical filter against the angle of incidence with bandwidth (nm) on the vertical axis and angle (degrees) on the horizontal axis. As can be seen, if the filter is designed for an optical bandwidth of 820 nm to 850 nm then its tolerance of incident radiation is typically only within 10 degrees of the normal. This can often result in some or much of the light collected by the collecting device to be blocked by the filter thereby preventing all of the signal from being transmitted to the detector. Accordingly, the use of a filter whilst greatly helping to cut down ambient radiation also means that the signal received may be cut as well.

In Figure 2 is shown typical point-to-point free space optical wireless link. This link has an input lens L and a detector D. Incoming rays R from a distant source are focussed by the input lens L on to the detector D. The input lens can be a conventional lens, Fresnel lens or reflecting mirror. The detector therefore receives rays on its surface over a wide range of angles, especially if the lens has a short focal length, which may be necessary for the physical requirements of the size of both the lens and collecting unit. Clearly, many of these incident rays are at too great an angle to pass through a narrow band optical filter, particularly when for optimal performance the bandwidth is particularly narrow.

It is also known to use an optical antenna in wireless communication detection. The antenna is placed between the lens L and the detector D and helps with the concentration of light to the detector thus enabling the detector to be smaller. However, most concentrators still present light to the detector at a large range of angles and therefore still suffer the same problem of some of the signal being cut out by narrow band filters as well as ambient light.

It is an object of the present invention to provide improvements on the above designs in particular to help with the problem of the signal being cut out by a filter.

According to the first aspect of the invention there is provided a wireless communication receiver comprising a detector for detecting a signal, a filter, having a filter surface, a first optical collecting device which converges incident radiation towards the filter and an optical diverging device disposed between the collecting device and the filter which diverges radiation from the first collecting device thereby to narrow the angle of the radiation incident on the filter relative to the normal to the filter surface.

Preferably the optical diverging device comprises a diverging lens which collimates the radiation so that radiation incident on a filter is substantially parallel and normal to the filter surface. Additionally, a second collecting optical device disposed between the filter and the detector which converges radiation from the filter towards the detector can be provided.

The optical collecting device can comprise a converging lens such as a convex lens and the optical diverging device can comprise a diverging lens such as a concave lens.

Preferably the virtual focus of the diverging device is at substantially the same point as the real focus of the collecting device. Preferably the second collecting device comprises an optical concentrator such as a dielectric total internally reflecting concentrator.

Preferably the receiver comprises a bandpass filter which is ideally a narrowband filter having a band width less than about 100 nm, preferably less than about 20 nm such as in the order of 5 nm or 10 nm. Additionally, the receiver can comprise a filter such as a polarising filter and/or a liquid polarisation selector. Moreover, the filter can comprise one or more of a narrow band filter, a polarising filter, liquid crystal polarisation selector, and a multilayer interference filter. Preferably the filter is adhered to or deposited on the diverging device. The filter may be adhered to or deposited on the second collecting optical device. Additionally, it is noted that the focal point of the first optical collecting device and the virtual focal point of the optical diverging device are preferably substantially superposed.

The embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings in which: Figure 1 is a graph of the angle sensitivity of narrow band optical filters, Figure 2 is a schematic view of typical known point-to-point free space optical wireless link, Figure 3 is a schematic view of a wireless communication detection device in accordance with the invention, and Figure 4 is a second embodiment of wireless communication detecting device according to the invention.

Referring to figure 3, there is shown a receiver 10 comprising first collecting lens 12, divergence lens 14, filter 16, second collecting lens 18 and detector 20. Also depicted are rays of optical radiation R. The first collecting lens 12 is a convex lens (shown here as a biconvex lens) or alternatively a converging Fresnel lens, which has its real focus at point F in Figure 3.

The diverging lens 14 is a plano-concave lens. As depicted in Figure 3 it can be seen that the collecting surface 22 which is nearest to lens 12 is significantly concave whilst the opposite surface 24 is substantially straight, and parallel with the filter 16. The virtual focus of diverging lens 14 is also at point F. Filter 16 in this embodiment is a narrow band filter which allows particular wavelengths of radiation through, while blocking other wavelengths outside the band.

Alternatively, the filter can be a polarising filter selected at particular incident polarisation, this can be used in conjunction with a polarised signal to allow discrimination from the ambient background predominant polarisation. Filter 16 can also be a combination of wavelength filter and polarisation selection or could be a combination of polarising filter and liquid crystal polarisation selector to permit adaptation to a different optical input polarisation to be achieved, and include the possibility of a wavelength filtering at the same time.

The band pass of the narrow band filter 16 shown in Figure 3 is preferably very narrow; that is, less than about 100 nm, more preferably less than 20 nm such as in the order of nm or 10 nm. As described below, the angular sensitivity of the filter is not a significant concern with the present invention and therefore this does not have to be an important consideration when choosing the narrowness of the bandwidth. The main limitation to bandwidth is in practical terms the bandwidth of the signal at which the transmitter operates. If the source transmits at a very narrow bandwidth of wave lengths then the filter can be extremely narrow minimising the amount of ambient radiation which gets through the filter 16. The filter 16 can be made from a multi-layered structure in the form of an interference filter.

Filter 16 can be a separate component or can be adhered to, or deposited on or otherwise attached to diverging lens 14 to allow the device 10 to be smaller overall.

Beneficially, by having the filter 16 placed downstream of the convergence done by the first collecting lens 12, the filter can be small in size and therefore cheap to manufacture. Indeed, as shown in Figure 3 using this arrangement a filter 16 can be significantly smaller size than the collecting lens 12.

A second collecting lens 18 is provided in the embodiment shown in Figure 3 in the form of a converging lens with flat surface 26 facing the filter 16 and a concave surface 28 fiucing the detector 20. The second collecting lens 18 can be spaced from filter 16 or can be adhered or connected to it. Filter 16 can also be deposited directly on surface 26 of the lens 18. The real focus of the lens 18 point G is located on the surface of detector 20.

The detector is a conventional detector and can be any type as is commonly used in wireless communication receivers. The arrangement device 10 however, allows detector 20 to be of a very small size thus permitting a low capacitor device to be used which in turn permits a high speed operation.

Input rays approaching device 10 first pass through concave collecting lens 12, and then converge towards its real focus F. Before reaching this point however, the rays reach surface 22 of diverging lens 14. Since the virtual focus of the divergence lens 14 is at point F to which the incoming rays are converging, the rays diverge as they pass through lens 14 and exit as a parallel beam of rays R'. Parallel rays R' are incident on the optical filter 16 normal to its surface 17, as shown in Figure 3. Thus the rays are depicted through the filter 16 at a 0 degree angle to the normal of surface 17 and therefore angular sensitivity of the filter is not an issue. Of course some rays R may enter the input collecting lens 12 at a slightly different angle and therefore this light would not necessarily end up being exactly parallel before entering the filter 16.

However, even in these circumstances the range of angles which the light approaches surface 17 of filter 16 is often very small if correctly aligned, so that nearly all of the intended signal should pass easily through the filter 16. Much ambient radiation also meets the filter 16 at a small or no angle. However, most of the ambient radiation will be at the wrong wavelength (or polarisation) and therefore will not pass through the filter 16.

Once the rays R' have passed through the filter 16 they are collected by collecting lens 1 8 which converges the light to focus G on detector 20. Since the focus of lens 18 is coincident with the detector 20, the detector 20 can of a very small size.

Accordingly, device 10 can be used with filter 16 of narrow bandwidth and therefore can cut out a very large percentage of ambient radiation but still collect nearly all of the desired signal to be collected.

In Figure 4 is shown a second embodiment of the invention in the form of device 110.

Components which are very similar or identical to components in device 10 are denoted with the same two digit reference prefixed with a 1. Device 110 is essentially the same as device 10, except that instead of a second collecting lens 18 an optical concentrator is provided. The type of concentrator could be used, such as an optical antenna, hemispherical, parabolic, or hyperbolic concentrator. Preferably concentrator 130 is of a type described in patent application WO 02/21734 and is a dielectric totally internally reflecting concentrator. Concentrator 130 gives device 110 a greater tolerance to optical misalignment with the incoming rays R from a distance. Concentrator 130 can be any design with a capture angle of 5 - 10 degrees and also of a large optical gain of around fifty. The optical gain refers to the ratio of the areas of the input surface 32, (facing filter 16) to the output surface 134 which can face or be adhered to the detector 120. Such a large optical gain allows all of the rays R' which pass through the filter to be collected by the concentrator and to be concentrated down to a small area on the detector 120 allowing the detector 120 to be small. Unlike the focus of a lens the outlet area 134 of concentrator 130 is not dependent on the angle at which light hits its front surface 132 and therefore detector 120 need not be any bigger than the output area 134 and still capture light in the case of optical misalignment of the incoming rays from a distance. Thus with device 110 the detector 120 can be even smaller and hence have lower capacitance and thereby producing an even faster detection system. Beneficially device 110 need not be precisely aligned.

Claims

Claims 1. A wireless communication receiver comprising a detector for

detecting a signal, a filter, having a filter surface, a first optical collecting device which converges incident radiation towards the filter and an optical diverging device disposed between the collecting device and the filter which diverges radiation from the first collecting device and narrow the angle of the radiation incident on the filter relative to the normal to the filter surface.

2. A wireless communication receiver in which the optical diverging device comprises a diverging lens which collimates the radiation so that radiation incident on the filter is substantially parallel and normal to the filter surface.

3. A wireless communication receiver according to claim 1 or 2 comprising a second collecting optical device disposed between the filter and the detector which converges radiation from the filter towards the detector.

4. A wireless communication receiver according to any preceding claim in which the optical collecting device comprises a converging lens and preferably a convex lens.

5. A wireless communication receiver according to any preceding claim in which the optical diverging device comprises a diverging lens such as a concave lens.

6. A wireless communication receiver according to any preceding claim is arranged so that the virtual focus of the diverging device is at the substantially same point as the real focus of the collecting device.

8. A wireless communication receiver according to any preceding claim in which the second collecting device comprises an optical concentrator, and preferably a dielectric total internally reflecting concentrator.

9. A wireless communication receiver according to any preceding claim in which the filter comprises a bandpass filter.

1 0. A wireless communication receiver according to claim 9 in which the filter comprises a narrowband filter, which preferably has a bandwidth of less than about 100 nm, more preferably less than about 20 nm such as in the order of 5 nm or 10 nm.

Ii. A wireless communication receiver according to any preceding claim in which the filter comprises a polarising filter which discriminates based on polarisation.

12. A wireless communication receiver according to any preceding claim in which the filter comprises a liquid polarisation selector.

13. A wireless communication receiver according to any preceding claim in which the filter comprises one or more of a narrowband filter, a polarising filter, liquid crystal polarisation selector, and a multilayer interference filter.

14. A wireless communication receiver according to any preceding claim in which the filter is adhered to or deposited on the diverging device.

15. A wireless communication receiver according to any of claims 3 to 14 when dependent on claim 3 in which the filter is adhered to or deposited on the second collecting optical device.

16. A wireless communication receiver according to any preceding claim in which the focal point of the first optical collecting device and the virtual focal point of the optical diverging device are substantially superposed.