AND/OR COLLECTION OF OPTICAL SIGNALS

Field of the Invention

The present invention relates to an optical device for the transmission/collection of optical signals, and to a method for the same. In particular, in its preferred form at least, the present invention relates to a device for the collection and concentration of free-space optical signals received over a substantial angular range.

The invention, in conjunction with a suitable detector, may have application in transceivers of communications systems making use of free-space optical signals. However, the invention is not restricted to the visible region, and the principles of the invention may be employed with any wavelengths from the hard ultraviolet (from about 50 nm) upwards. In practice, the longest wavelengths that are likely to be used are in the cm-wave RF band (above 30-40 GHz). This being understood, in the rest of this document, the electromagnetic radiation that the invention is designed to transmit or collect will be referred to as "light", or "optical".

Background to the Invention

Devices to collect light for various purposes are well known. For example Ramirez-Iniguez et al., in WO0221734, describe an "optical antenna" in which light enters a curved conical transparent horn and is reflected towards a detector by means of total internal reflection within a homogeneous medium.

Another arrangement, presented in US 4,265,515 to C K Kao, is a graded refractive index (RJ) optical fibre comprising a set of concentric optical waveguides of differing refractive index and radial thickness. This arrangement employs a cylindrical optical fibre to transmit rays (more strictly specific waveguide propagation modes) over lengths of many km.

However, the known devices suffer from a number of significant disadvantages. In particular, the prior art devices are generally incapable of efficiently collecting and concentrating optical signals from a wide range of angles simultaneously. In some cases, this can be achieved through requiring physical re-orientation of the device. However, a product incorporating a device (or multiple devices or detectors in order to cover the desired angular range) requiring physical reorientation leads to higher product weight and to increased design and manufacturing costs, and reliability may be reduced.

When an optical medium is used for commercial wireless communication purposes :-

1. As much signal light as possible needs to reach any optical detector(s),

2. Extraneous light ingress must be minimised,

3. To ensure that high data rates can be supported, optical temporal dispersion in the apparatus must be minimised, and the detector(s) must have as small a sensitive area as possible (to ensure a low device capacitance and hence fast response time),

4. In many practical applications, the need to align carefully transmitter and detector (either manually or automatically) should not be required.

5. The detectors need to be as cheap and spatially compact as possible.

6. Unlike the majority of optical systems, it should not, in principle, be necessary to image the incoming signals, merely to ensure that they all impinge on as small a spatial area as possible (a so-called "non-imaging system").

The practical meaning of these requirements is that the device must:-

1. Have maximum sensitivity to signal light, ie collect as much light as possible. In practice, this means that the device must have a large input aperture, but with minimal overall dimensions.

2. Deliver light to single low-cost, small aperture, fast electro-optical detectors.

3. Be sensitive to signal light impinging from as wide an angular range as possible.

Achieving simultaneously all these desirable features is difficult, if not impossible with conventional systems. For example, consider the following conventional optical systems as shown in Figure 1.

Converging lens

A converging lens 10 (either thin spherical as illustrated in Figure Ia below, or in Fresnel form) gathers light from a given direction impinging on its front aperture 12 and focuses it on to a small point F (in principle of zero dimension) on a curved image plane P (a distance f behind the lens for parallel incident rays) as shown in Figure Ib. However, light rays from differing directions (e.g. from Δθ below the x-axis) are focussed onto separate points of the image plane, the deviation being Δz ~ f Δθ. Clearly, to avoid multiple detectors, or having to move the detector, the detector dimensions must be less than Δz, and hence the angular sensitivity is restricted to a range of angles +/-tan'1(Δz/f). [For Δz = lmm, and f = 100mm, this means the angular sensitivity of a fixed detector is just +/- 0.6 deg of arc] Parabolic Mirror

A parabolic mirror 14 (illustrated in Figures Ic and Id) has an analogous performance to a converging lens, and similar constraints apply to the angular range and sensitivity.

Reflective Pipe

An internally reflecting cylindrical mirror ("light pipe") 16 is good at accepting and transporting light from a wide-range of angles, but has no concentration effect. Rays exiting the pipe have substantially the same spatial and angular distribution as those entering it, as illustrated in Figure Ie, which shows a vertical section through a light pipe.

Conical reflecting surface

It might be imagined that, to improve the concentrating effect of a cylindrical light pipe, it is merely necessary to make the diameter of the light pipe become smaller with the distance along its axis, as in a conical or "horn" mirror 18 shown in Figure If. Any light entering the larger aperture of the cone would then be successively "squeezed" down to exit the smaller aperture as desired. This is not quite the case in practice. Depending on the cone semi-angle (ψ), more or less light actually gets reflected back out of the cone before reaching the smaller aperture - as illustrated in Figure If.

To decrease the likelihood of rejection by reflection, the angle ψ must be small. However, for a given sensitivity, this either makes the cone very long, or increases the size of the output aperture - thus decreasing the concentrating power of the geometry. Of course, in the limit ψ -> 0, this cone becomes a non- concentrating pipe.

In summary, the converging lens and parabolic mirror optical systems are very efficient at collecting light. However, their angular sensitivity to light is very directional. Practical systems using these types of optics will require careful (manual or automatic) alignment in order to collect light impinging from a range of angles. In contrast, optical systems that are not particularly directionally sensitive such as the conical reflectors are also not particularly efficient at collecting light.

This is summarised in Figure 2, which shows a 2d polar plot of amounts of light gathered (r) versus angle (θ) for various optical systems. Plot A is characteristic of conventional light pipe, or conical-section mirror, geometries (small r, large θ). Plot B is characteristic of converging lens/parabolic mirror geometries (large r, small θ). Plot C is an example of an ideal curve where a large amount of light is gathered from many angles simultaneously (large r and large θ).

Summary of the Invention

The present invention seeks to overcome the above disadvantages of the prior art.

According to one aspect of the present invention, there is provided an optical transmission/collection device comprising a composite optical element having a first light transmission aperture and a second light transmission aperture and an optical axis x extending between the first aperture and the second aperture, the optical element having plural optical regions, which are located along the axis x and which have respectively varying refractive powers, such that the deviation of a ray of light at any point of the optical element varies with the distance along the axis x and/or with the displacement from the axis x, wherehy a bundle of rays collected at one aperture and travelling along the optical axis towards the other aperture is concentrated into a cross section of increasingly small area when travelling in one direction relative to the axis x or is expanded into a cross section of increasingly large area when travelling in the other direction relative to the axis x.

According to another aspect of the invention, there is provided a method of light transmission/collection by means of an optical device comprising a composite optical element having a first light transmission aperture and a second light transmission aperture and an optical axis x extending between the first aperture and the second aperture, the method comprising the steps of:

transmitting the light through plural optical regions, which are located along the axis x and which have respectively varying refractive powers; bending the light such that the deviation of a ray of light at any point of the optical lens varies with the distance along the axis x and/or with the displacement from the axis x; and

for a bundle of rays collected at one aperture and travelling along the optical axis towards the other aperture, effecting one of: concentrating the rays into a cross section of increasingly small area when travelling in one direction relative to the axis x, and expanding the rays into a cross section of increasingly large area when travelling in the other direction relative to the axis x.

Brief Description of the Drawings The invention will be described further, by way of example, with reference to the accompanying drawings, in which: Figures Ia and Ib are schematic diagrams of a conventional optical system in the form of a converging lens; Figures Ic and Id are schematic diagrams of a conventional optical system in the form of a parabolic mirror; Figure Ie is a schematic diagram of a conventional optical system in the form of a reflective pipe; Figure If is a schematic diagram of a conventional optical system in the form of a conical reflecting surface; Figure 2 is a graph showing polar plots representing the amount of light collected in two conventional optical systems and in the present invention; Figure 3 is a schematic diagram representing a ray of light traversing an arbitrary heterogeneous medium in order to illustrate a principle of the present invention; Figures 4a to 4c are schematic diagrams representing a bundle of rays of light traversing a composite medium in order further to illustrate the principle of the present invention; Figure 5 is a diagrammatic view of a first embodiment of optical device according to the present invention; Figures 6a and 6b are schematic diagrams representing a bundle of rays of light traversing the optical device of Figure 5 ; Figures 7a and 7b are views of a realization of the embodiment of the invention shown in Figure 5; Figures 8 and 9 are diagrammatic views of a second embodiment of optical device according to the present invention; Figures 10a and 10b are views of a realization of the embodiment of the invention shown in Figures 8 and 9; Figure 11 is a diagrammatic view of a variation on the first embodiment of optical device according to the present invention; and Figure 12 is a diagrammatic view of another variation on the first embodiment of optical device according to the present invention. Description of the Preferred Embodiments

Turning now to Figures 2 and 3, the principles of ray propagation in a medium may be considered to see how Plot C of Figure 2 might be achieved in practice. Figure 3 shows a ray of light R traversing an arbitrary heterogeneous medium modelled as a number of discrete planes 20 separated by a constant distance (δx).

By applying Snell's law at each plane 20 to compute the deviation of each short element of a ray's path, we can compute the "equation of motion" of each ray traversing the medium. An important aspect of this invention therefore is to arrange that the "bending power" of a medium, or its refractive power, varies with the distance x along a horizontal axis of symmetry (x) and the displacement (Z) from the horizontal axis of symmetry (x). If the variation is proportional to Z, then, the total path of each ray can be represented by the second-order difference equation:-

δ2Z(x)/δx2 = -Z(x)/f(x,Z)

where x is the horizontal distance into the medium and Z(x) is the vertical position of the point of crossing of the ray with a plane 20. This equation is well-known in physics and engineering, it is a form of the "wave equation", and in this case gives rise to potentially oscillatory paths (in Z) of the rays through the medium.

In the above equation T is a function related to the bending power of each layer, or its (reciprocal) focal length, and 'f depends on the local physical curvature or slope of the layers as well as their local refractive index.

For clarity and simplicity, the above treatment has considered 2 dimensions only. It is obviously applicable to real 3 dimensional systems.

The present invention relates to methods of organising a composite heterogeneous medium with associated values of f(x, Z) so that rays are not only gathered efficiently, from a wide range of angles, but concentrated into substantially the same small region as they traverse the medium. Of course, it will be appreciated that the invention can also be employed to operate in the reverse sense as a light transmitter in which concentrated light rays are expanded during transmission.

However, the following description will refer primarily to the use of the invention for the purposes of collecting and concentrating light, and the principles of the invention when employed for this purpose will be described with reference to Figure 4.

A composite medium as shown in Figure 4a consists of planes 22, 24 of material of alternating properties such that the deviation of a ray R is proportional to its displacement (Z) from the central axis (x). This material propagates a bundle B of rays with oscillatory paths as shown. However, if off-axis ray bundles Bl, as in Figure 4b, are considered it is seen that the oscillatory pattern of rays is skewed by displacing the nodes N of the distribution alternately above and below the central axis. The ray bundle Bl is not significantly compressed, nor is the deviation of the bundle nodes N substantially less than in a conventional system. In addition the node spacing or "bundle wavelength" (λB) is unaffected. However, if we arrange that the optical deviating properties, summarised by the change of f(x, Z) with x, so that the deviation power becomes greater with x, then as well as the oscillatory behaviour, we see a concentration (or "damping") effect - illustrated in Figure 4c. This effect is also seen over a wide-range of incident angles.

This is exactly the behaviour that is required for an efficient, wide-angle optical collector.

If it were possible to have negative refractive indices then such a collector could be constructed from planar layers of alternating positive and negative refractive index materials of increasing magnitude. However, if the possibility of negative refractive index material is excluded for now, the same effect can be achieved in practice in two main ways:

1) By arranging that the layers are suitably shaped and have alternating positive and negative slopes, and differing refractive indices, referred to below as the "Geometrical approach"; or

2) By arranging that the deviation power of a planar layer (proportional to 1/f) decreases with Z and x, referred to below as the "Optical Density approach". This variation can be smooth or step-wise.

Geometric Approach A first embodiment of the invention is illustrated in Figure 5 and comprises a number of similar dielectric layers 26, 28 of increasing deviation or refractive power (1/f) and respectively of positive and negative gradient relative to the axis x. The shaping or profiling of each layer 26, 28 can be prismatic (linear) as shown in Figure 5, or can be spherical or aspherical lenticular. For simplicity in the following description only the prismatic profile is mentioned, but, as stated above, any positive/negative slope profile can be used in principle. It will be noted that the increase in deviation power in the present instance is achieved by gradually reducing the transverse dimensions of the layers 26, 28 with respect to an increase in the distance at which they are positioned along the axis x.

The effect of this geometry is to provide an angular deviation to all rays at each layer 26, 28, which is dependent on the displacement (Z) from the axis x and on the distance travelled along the axis x.

In a simulation of such a geometry to trace the paths of a bundle B2 of rays, the results shown in Figures 6a and 6b were obtained. Figure 6a clearly shows the oscillatory and damped nature of the ray paths for rays impinging with θ = Odeg. As demonstrated, the bundle B2 of rays becomes increasingly concentrated as the rays travel along the axis x, in a manner similar to that described with reference to Figure 4c. Turning to Figure 6b, the situation for θ = 9 deg (160 mrad) is illustrated, and again the bundle B2 of rays becomes increasingly concentrated as the rays travel along the axis x, but without significant deviation from the axis x in the region where the bundle is most concentrated. By comparison, in a convex lens of comparable length the separation of the foci would be about 10 times greater.

One effect of this geometry that might tend to reduce the overall light collection efficiency is due to the finite reflection coefficient of each dielectric boundary. This effect can be mitigated by the well-known technique of a quarter-(optical) wavelength (λ/4) dielectric coating of suitable refractive index being added at each layer. For example, 99.5% transmission coatings are available in the near infrared, and this means that the overall reflection loss through many layers would be negligible.

In a practical realisation of the device illustrated in Figure 5, there are two key considerations that must be reconciled.

1) If the design tries to concentrate the input light over too compact a spatial distance (Z or x), then light can be lost through the sides of the device or can be reflected back out of the input aperture due to total internal reflection, because of ray angles on incidence at a layer being too steep. This critical incidence angle (θcrit) is simply given by the well-known (Snell's law) relationship: θcrit > where n2 is the lower refractive index material, and ni is the higher.

2) If, however, the device is too long, this will entail a penalty in terms of excessive reflective loss at each dielectric boundary (as noted above), together with increased manufacturing costs.

A further consideration for a practical device is that it should be as geometrically simple as possible, subject to satisfactory performance, to keep manufacturing costs down, and to ensure straightforward integration into products employing the device.

An example of a practical device is shown in Figure 7a and 7b.

The device shown in Figures 7a and 7b comprises a composite optical element 30 of circular cross-section and tapering diameter, having a light entrance aperture 32 and a light exit aperture 34 separated by a length 1. The composite optical element 30 includes alternate clusters 36, 38 of optical members 40. For example, the optical members 40 may be optical lenses formed from a transparent dielectric material, such as glass or a polymer, or they may be substrates bearing dielectric layers 26, 28 as in Figure 5. As stated, the members 40 may be prismatic, spherical or aspherical in profile. The members 40 in the clusters 36 are of relatively large diameter whereas the members 40 in the 5 clusters 38 are of relatively small diameter. However, the members within each cluster 36 or 38 are of successively decreasing diameter in the direction of the exit aperture 34. More particularly, the clusters 36 are located in positions along the axis x designed to receive rays of light at maximum displacement from the axis x, whereas the clusters 38 are designed to be located at positions along the i o axis x for receiving the nodes in the bundle of rays.

In addition, the composite optical element 30 is provided with a diverging element 42 situated in the entrance aperture 32. The function of the diverging element 42 is to disperse light impinging on the entrance aperture 32 and is entering the composite element 30, and in the present instance the diverging element 42 comprises an array of circular diverging lenses 44 equi-spaced about a central diverging lens 46. However, alternative geometries may be employed.

A preferred feature of the embodiment of Figure 7, as described, is the periodic 20 diameter variation, related to the bundle wavelength, of the clusters 36, 38, which is designed to increase the compression of the ray bundle. This arrangement has a very high convergence where the ray bundles are diverging downstream from a node position towards an extremum (see Figure 4a), interspersed with more gentle convergence from the extrema to the nodes, and 25 ensures that convergence is efficiently achieved throughout the length of the device. In addition, this arrangement of clusters of converging members, in which the members in the vicinity of nodes are physically more compact, in terms of the displacement Z, than the members in the extrema zones, can be used to save on material and hence reduce cost and weight. It should be noted in the device of Figure 7 that the form of the ray bundle and the design of the device are correlated variables, and so the design process must take into account the intended application of the device and find a design of device that is satisfactory for the particular application envisaged.

Also shown in the device of Figure 7, as mentioned, is the diverging layer 42 that consists of one or more diverging elements 44, 46. The purpose of this is to improve the angular acceptance of the system even further, albeit at the cost of some light loss at the front aperture. This layer 42 can be used in applications where angular acceptance is more important than absolute sensitivity. The effect of the diverging layer 42 is shown in Figure 7b. Light impinging from a wide range of input angles is refracted by the diverging layer into a new set of rays of differing angles. Some of the rays in this new set are within the angular acceptance of the remainder of the device and can thus be detected. A subset of this new set of rays will be paraxial and will thus form a node pattern located on the x-axis as shown in Figure 7b. This means that the node pattern is effectively fixed independent of the angle of incidence. Clearly, this improved angular acceptance is achieved at the expense of the loss of some of the incident light. In this way, a finite angular acceptance of over +/-1,200 mrad can be achieved for a single device.

The device shown in Figure 7 consists of a moderate number of layers or members to achieve adequate compression whilst minimising cumulative boundary losses. In addition, the periodic shape of the array resulting from the use of the clusters 36, 38 minimises ray slopes to prevent total internal reflection losses mentioned above. A specific example of the device shown in Figure 7 for use in an optical communications system, or in a security system, in a simple, non-video based long-range motion detector may comprise spherical lenticular members 36, 38 having smooth/polished surfaces and formed from materials such as a transparent glass (or polymer) of refractive index 1.8 interspersed with layers of air (refractive index ~ 1.0), the refractive indices used for this configuration being constant. A preferred method of manufacture for the members 36, 38 in this instance would be injection moulding. Typically, the device might have 15 - 35 layers or members depending on the required performance, and a length of 80 - 120 mm. The diameters of the entrance aperture and the exit aperture might then typically be 10- 100 mm and 1-10 mm, respectively.

It is envisaged that such a design may achieve an angular sensitivity of +/- 200 mrad, an overall collection loss of approx -5 dB, a concentration factor of +20 dB , and an overall gain of + 15 dB .

The example just given is based on a "macro" size device with dimensions of the order of centimetres. However, "micro" scale devices are also possible in which the layers are very thin (potentially of the order of a fraction of a millimetre). Such a micro device could be used for wide-angle light collection for one or more very small light detectors.

Optical Density Approach

A second embodiment of the invention is shown schematically in Figure 8 and comprises a series of coaxial conical regions 50, 52, 54 extending between a relatively large entrance aperture 56 and a relatively small exit aperture 58. The refractive index (RI) of the successive these regions 50 to 54 effectively decreases with Z, so that Rl > R2 > R3. A significant feature is that the zones of differing refractive index are physically compressed with increasing x, forming a number of concentric cones of constant RI5 in order to concentrate light gathered from the large entrance aperture 56 onto the small area of the exit aperture 58 substantially independently of the incident angle.

The behaviour of some of the light rays traversing a device with this geometry is shown in Figure 9, which also shows a notional (or practical) transverse cross- sectional disc in which the refractive index (RI) varies with radial distance from the x-axis. It can be seen that the rays R of a large angle of incidence at the entrance aperture 56 are gradually deviated so that they eventually exceed the critical incidence angle in a region, here 54, and are totally internally reflected towards the (smaller) exit aperture 58.

Such a device could be realised by similar methods that are used to "draw" graded index fibre optic cable, the main difference being that instead of drawing the original radially-varying refractive index billet into a long, constant diameter fibre, the drawing process would stop when the lower end of the molten billet had been drawn off into a cone of the required dimensions. This cone would then be detached from the billet.

Another way of realising such a device is to assemble it from a number of planar discs 60 with different effective diameters as illustrated in Figure 10. Each disc 60 is formed from radially- varying refractive index material as described below, and the discs 60 are stacked in slabs in intimate contact with one another, eg by mechanically clamping the discs together or by fusing them together by a suitable heat treatment.

As shown in Figure 10a, the present embodiment comprises a composite optical element 62 having a relatively large entrance aperture 64 and a relatively small exit aperture 66. A series of the planar discs 60 are arranged successively along the axis x in the direction from the entrance aperture 64 to the exit aperture 66 in a first cluster 68 of initially decreasing diameter followed by a second cluster 70 of discs 60 of increasing and then decreasing diameter. Each disc 60, irrespective of its diameter is formed from a plurality of annular regions 72, 74, 76, 78 etc, respectively, of different refractive index, and each such region of one disc is aligned with the corresponding region of the adjacent disc or discs such that the profile of all the combined regions 72 or 74 etc. decreases or increases in outer diameter with the decrease or increase in diameter of the discs 60.

As in the case of the first embodiment shown in Figure 7, the entrance aperture 64 is provided with a diverging element 42 whose features have already been described.

Figure 10b shows the effect of the diverging element 42 and the paths of various rays of light as previously.

This arrangement is suitable where a "re-entrant" x-axis profiled device is required in an analogous manner to the device illustrated in Figure 7a/7b.

Two specific embodiments of the invention have been described with reference to Figures 5 to 7, and Figures 8 to 10, respectively. It will be apparent that a number of modifications are possible within the scope of the invention.

In particular, in place of the prismatic or spherical lenticular surfaces described for the members 40 in the embodiment of Figure 7, fresnal profiled layers 90 as shown in Figure 11 could be employed.

Likewise, both of the described embodiments feature successive generally circular optical regions located at a series of cross-sectional planes along the axis x. It is alternatively possible for the optical regions to be located along a helical path. It is also possible for the optical regions to be generally elliptical in section, or to have some other section.

The two embodiments, described as the geometrical arrangement and the optical density arrangement respectively, have been discussed as if they were entirely separate solutions. It is, of course, possible to combine the two types of approach in the same device.

Various supplementary features may also be employed. For example, to achieve a selection of certain wavelengths, a separate filter could be added, or the respective optical regions could be constructed from a material designed to pass the desired waveband. Likewise, a polarising filter or filters could be added.

Other supplementary features include the possibility of combining plural devices together in order to increase the angular sensitivity, for example. A particular such combined device is shown in Figure 12 and features a plurality of input heads 92 for supplying light to the main device 94.

As stated previously, although the device has been described in terms of the collection and concentration of light, it could also be employed in reverse such that light is introduced into the smaller aperture, here described as the exit aperture, and is expanded towards the larger aperture, described as the entrance aperture. Such an arrangement could be employed as a light transmitter for expanding light from a small source, such as an LED or laser or fibre waveguide, for output through a large aperture with a large visibility angle.

The invention offers significant advantages in a wide range of applications. In particular, the invention may be employed with a variety of electro-optical detectors, and can be connected to a suitable waveguide at its exit aperture to carry signals to a detector at a remote location. The invention may also be used in conjunction with a variety of conventional optical systems, e.g. mirrors, reflectors, converging lenses, optical fibres etc.