GB2352829A - Infrared diffractive focusing mirror - Google Patents

Abstract

A mirror structure 24 for an infrared detector device 28 includes focussing means formed by a diffractive optical element 7. There may be several such elements carried on a single support each element having a surface comprising an array of fine grooves. The mirror structure 24 may be formed of a moulded plastics composition and it may be of a shallow profile which enables a detector device 28 of compact shape to be constructed. The element may be holographic, and may act as a spatial filter. Methods of making the diffractive optical element are described. The diffractive optical element may have a symbol encrypted in it as an anti-counterfeiting device.

Description

2352829 INFRARED MIRROR STRUCTURE This invention relates to an infrared

mirror structure. It relates particularly to means and methods for producing such a structure and devices where the structure may be employed.

Optically reflecting mirrors with flat, spherical or aspheric profiles, either as a single mirror or in arrays of multiple mirrors, are used as a means of focussing passive infrared radiation for example, for intruder detection within security devices, switching of lighting for domestic and industrial applications and other applications that require sensing by passive infrared radiation. Arrays consisting of one or more mirrors are used to provide a plurality of sensing zones from a smaller number of electronic infrared sensing components.

These mirrors, either as single mirrors or in arrays can also be used in active infrared detector devices. In such devices, a suitable light source emits infrared radiation and one or more sensors detect the infrared radiation reflected back from a target object to be detected. The mirror, or arrays of mirrors, are used to reflect the infrared radiation in both the emitting and sensing parts of the device.

Typically, when produced in large volume, the mirror arrays are made by injection moulding, or any similar process to accurately replicate the mirror forrns as an inexpensive plastics component.

A convenient plastics composition to use is acrylonitrile butadiene styrene (ABS) because it has moulding properties and the required mechanical properties suitable for producing complex mirror arrays. Another consideration is the suitability of the ABS composition to accept thin vacuum coated layers to improve the reflection efficiency of the mirror surfaces. Similarly, the mirror surfaces may be electroplated with metal layers to provide an improved reflection efficiency in the infrared wavelength region.

The mirror forms used for infrared detection devices are typically deeply concave curved forms. Since it is desirable to make the detection devices as small and compact as possible, the mirrors must have short focal lengths and therefore deeply concave curved forms are -2required.

A problem can arise in that if the mirror surfaces are made more deeply curved, the task of depositing a vacuum coated or metal plating layer on to the deep concave mirror surfaces becomes more difficult.

A further problem arises in that deeply curved mirror segments are more difficult to manufacture since the construction of the moulding tooling is complex. Typically, the complex geometry of each mirror segment in an array has to be made as a separate inserted section which is assembled into the mould tool. Complex mould tools with these inserted mirror segments are often very costly to make.

The invention was devised in an attempt to provide a focussing optical element for use in infrared detector devices, and a method of manufacturing the same which enables the abovernentioned problems to be reduced.

According to the invention, there is provided a focussing element for an infrared detector device said element comprising a diffractive optical element.

The invention further comprises an infrared detector device with an infrared detector and a focussing element, said element comprising a diffractive optical element.

Preferably, there is provided a method of detecting an object comprising the steps of emitting infrared radiation and focussing said emitted radiation by diffracting it in order to direct it towards an object to be detected.

One advantage of the invention is that a diffractive optical element with focussing optical power can be formed on a planar surface. The overall volume needed for a flat diffractive optical element can be much smaller than that needed for an array of mirrors capable of performing the same function. Therefore, the diffractive optical element can allow the detection device to be of a more compact design.

The flat planar component geometry this invention allows gives improved characteristics -3for the large volume manufacture of components in a plastics composition such as ABS. In order to replicate the very fine groove detail of the diffractive optical element by the preferred method of injection moulding, it is usually necessary to employ higher pressures and plastics melt temperatures than those that would be typically used for conventional devices. This also allows the final injection moulded component design to be more complex, for example with additional features of frames, holes, protrusions, snap-fit features, push-fit features and a wide variety of other features for securing the diffractive optical element array into the detector device, to be produced at the same time from the same moulding cavity.

The d if fractive optical element preferably comprises a segmented surface microrel ief structure where the active planar surface is divided into individual segments. A wavefront incident on the surface is reflected and is split into secondary wavelets by each of the segments where each segment is characterised by its surface relief profile and its segment boundary. The surface relief structure is designed in such a way that the desired optical function in the far field is performed by the superposition of all the reflected secondary wavelets produced by the surface segments. The required constructive or destructive interference of the secondary wavelets is achieved by choosing the segment boundaries, depths and shapes to ensure that the optical phase difference of two rays crossing neighbouring segments and meeting at the desired far field position is an integer multiple of 2n. The various computerised mathematical methods that can be used to determine and optimise the surface microrelief structure have been well documented.

Preferably, the focussing element comprises an array of diffractive optical elements. The array may be formed from a single piece of material. The method may include the additional steps of receiving infrared radiation reflected from or emitted from the object, focussing the reflected or emitted radiation by diffraction and detecting said focussed radiation.

The or each diffractive optical element comprises a holographic optical element, but it could comprise other types of diffractive optical element, for example binary optics, kinoforms or diffraction gratings.

The or each diffractive optical element may comprise a plurality of fine grooves, for example of approximately 5 micrometres in depth, disposed on the optical surface of a mirror. The grooves on the optical surface impose a change in phase of the wavefront reflecting from the surface which can be designed to focus the light reflected from the surface by diffraction.

It will be appreciated that the grooves can be formed by a wide variety of different means, for example, recording in photosensitive media, single-point diamond machining, ion beam etching, chemical etching, laser machining, laser writing, electron beam writing or photomasking. Some of the various means of producing the grooves of the diffractive optical element allow the whole mirror area of a closely packed array of diffractive optical elements to be made in one single piece without the need for assembling together the individual mirror elements of the array. This allows better positional accuracy to be achieved between each element in the array and it removes the cost of separately assembling each element of the array to produce the final components or tooling for moulding, embossing, stamping or the like for large volumes of mirror arrays.

Since the suggested means of making the fine diffractive optical element groove structures can produce features as small as in the order of 0.5 micrometres, it is possible to encrypt a unique symbol or some other marking strategically placed within the diffractive optical element's structure which identifies the diffractive optical element structure uniquely, for example to a particular design or manufacturer, or to designate some other property of the element. This 'anti-counterfeiting' device may be made so small that it does not adversely affect the optical properties of the diffractive optical element and cannot be seen with the naked human eye, but can be observed under high powered magnification. Any attempt to copy the diffractive optical element design by a replication method would similarly replicate -5the anti-counterfeiting device allowing the origins of the original to be traced.

In one embodiment of the invention, at least a part of the diffractive optical element is formed from groove patterns that are non-rotationally symmetric. In particular, this means that the grooves can be something other that concentric rings centred on the optical axis of the element. Using groove patterns that are non-rotationally symmetric allows the optical power of the element to be different in each axis across the surface of the element passing through the optical centre. This enables a range of geometric optical aberrations including astigmatism and coma to be corrected, or alternatively to be introduced into the optical elements in a controlled manner. This may be used to provide a different vertical sensing zone size in relation to a horizontal sensing zone size, or to change the shape of the sensing zone, for each element of an array in an infrared detection device. This enables the focussed image shape to match the actual shape of the detector surface which are quite often not circular and so improve the signal to noise ratio of the detected signal.

In another embodiment, the or each diffractive optical element in the array has spatial filtering properties to provide some initial processing of the detected image for each of a number of detection zones. This may be arranged to enable each optical element of the array to provide the infrared detector with a different detecting sensitivity to large objects than to small objects, thus enabling some distinction to be made in the detection between, say, a human being shape and a small animal shape and therefore reducing or removing the need for this processing to be done electronically. For instance, in a simple form, the detection zones could be shaped to be tall narrow rectangular zones so that a human target would fill most of the zone and give a strong signal at the detector, whereas a small animal target will only fill a very small part of the zone and so give a weak signal. A more elaborate method may be to shape the detection zones to be trapezoidal in shape, so the detection zones are wider at the top of the zone and narrow at the bottom. In this way, a human target will fill more of the -6wider part of the zone all the top and so produce a large signal at the detector, whereas a small animal target will fill only a small part of the zone at the narrow bottom region of the zone and so produce a small signal. Additionally, these trapezoidal zones will give some spatial filtering effect in that a small animal target will be more likely to move between the narrow, wider spaced areas of the zones nearer the floor level. In a further embodiment, the chromatic properties of the diffractive optical element or elements may be selected to diffusely scatter light in the visible to near infrared wavelength range while maintaining good optical performance in the required mid-infrared wavelength range. This property can be used to improve an infrared detector system's immunity to false sensing of light sources. As an example, this can reduce the susceptibility to false alarms by extraneous radiation sources in passive infrared intruder alarm detectors.

A further possible embodiment may be an optical device having only one diffractive optical element in the form of a holographic optical element or hologram which is capable of reconstructing multiple detection zones to an electronic detector when infrared radiation reflects from the diffractive optical element.

The diffractive optical element may be comprised in a flat or curved reflective optical device. The diffractive optical element may be applied to a curved surface to help the mechanical construction of the device or to produce a combined effect in which both the diffraction reflection and curved form reflection properties provide some optical effect in the reflected beam.

Some particular embodiments of the invention will now be described with reference to the accompanying drawings, in which:

Figure I is a perspective view of a typical prior art rnirror array,

Figure 2 is a cross-sectional view of a diffractive optical mirror element of the invention, Figure 3 is a perspective view of an array of diffractive optical elements of the invention as used in a passive infrared detector device, Figure 4 is a similar view of a different embodiment, and, Figure 5 shows an infrared detector including the array of Figure 4.

As depicted in Figure 1, a mirror array 1 has a frame 2 which supports a number of mirror 3 arranged in a row. The mirror segments 3 are mounted at particular off- axis segments distances and angles to the vertical and horizontal planes of the frame 2 and the mirror segments 3 each have different surface forms so that each mirror segment produces a different field of view. Additional rows 4 and 6 of mirror segments 3 are included to provide further multiple fields of view in the vertical and horizontal planes of the frame 2.

Figure 2 illustrates a cross-sectional view of a diffractive optical element 7 of the invention which has a thin body 8 substantially rectangular in plan. In a different embodiment, the body alternatively could be circular, quadrilateral or some other shape in plan as necessary, depending on the means used to produce the body 8. One surface 9 of the body 8 is flat. The opposite surface 11 includes grooves 12. Each groove 12 is formed by a substantially vertical surface 13 (when viewed in the orientation shown) and a curved surface 14. The height of the vertical surfaces 13 and the width and curvature of the curved surfaces 14 will depend on the change in phase they are required to impart to a wavefront reflected from the diffractive element 7. Therefore, the heights of the vertical surfaces 13 and the widths of the curved surfaces 14 will vary and may not follow a uniform trend across the entire diffractive element surface 11. The grooves 12 may be rotationally symmetric and concentric about a centre 16. However, in an alternative embodiment this might have grooves 12 that are not rotationally symmetric and are not placed concentrically about the centre 16 to provide a different optical power in each axis across the surface of the element 7 in order to change the shape of the field of view of a detector in which the mirror is fitted.

Figure 3 shows a perspective view of a diffractive optical mirror 17 consisting of one -8element 18 fon-ned from an infrared reflecting material, such as an ABS plastics composition carrying a thin vacuum deposited gold layer. The element 18 in this case has grooves 12 which are non-rotationally symmetric about the centre 16. In this embodiment, the nonrotationally symmetric 'grooves 12 of the element 18 provide a different optical power in each radial orientation about the centre 16 across the surface of the element. Primary light paths 19 from a rectangular detector area 21 reflect from the element 18 to form secondary light paths 22 which form the projected trapezoidal beam cross-section 23. The primary light paths 19 are re-directed by a different angular amount' dependant on the radial position of the groove 12 from the centre 16 they intercept with to form the secondary light paths 22. The nonrotationally symmetric grooves 12 therefore transform a rectangular bundle of light paths 19 projected from the rectangular detector area 21 into secondary light paths 22 which form a trapezoidal beam cross-section 23. The transformed trapezoidal beam cross-section 23 defines the field of view the detector would achieve through the element 18.

Figure 4 shows a perspective view of an array 24 with non-rotationally symmetric grooves which provide different optical powers in each axis across the surface and produce a trapezoidal field of view. The array 24 includes diffractive optical e lernents 18 and it is capable of being used in a passive infrared detector device. The elements 18 provide for a plurality of detection zones in the detector device. The array 24 may be made in one piece regardless of the number of elements 18 in the array, provided, of course, that the array is within the size constraints of the particular means used to produce it. Alternatively, the elements 18 could be assembled together to form the array. The array 24 is formed from an infrared reflecting material, such as ABS plastics composition vacuum coated with a thin layer of gold, or the array can be etched or replicated by some method on to tooling surfaces for moulding, embossing, stamping etc. The array 24 may have a frame 26 with a flat surface for mounting the array in a detector or sensor device.

Figure 5 is an exploded view of an infrared detector 28 comprising a housing formed from a first portion 29 and a second portion 31. The first portion 29 houses an electronic circuit, in particular a printed circuit board 32 on which is mounted an infrared sensor 33. Tile first portion 29 also houses an array 24 of diffractive optical elements. The second portion 31 uses -in infrared transparent window 34 to sea] the housing. In use, infrared radiation enters by the infrared transparent window 34 and falls on the array 24 of diffractive optical elements. This is focussed by the array 24 in a desired manner onto the sensor 33 to produce a detection zone. The sensor 33 generates a signal in response to received radiation. This signal is subsequently processed by the electronic circuit to determine the presence of objects in the detection field.

Some possible methods by which diffractive optical elements of the invention may be produced will now be described. Example 1: Laser Writing A substrate sample is coated in a photoresist, light sensitive, material to a controlled thickness. This is then placed on an XY scanning motion stage under a focussed laser beam, whose intensity is synchronously modulated as the photoresist coated sample is scanned, to write a continuous exposure in the photoresist layer. This enables the fine relief pattern of the diffractive optical element structure to be written with a wide range of feature sizes and this is produced in one continuous writing operation. The area size that can be patterned in this way is only limited by the length of travel of the motion stage XY axes, the accuracy of the XY motions over longer travels, and the duration of the total writing time. A typical focussed beam size used is 1.5 microns, although by overlapping the Gaussian beam profiles of each successive pass of the writing beam. features of the order of one micron can be made with good accuracy. The depth of the structures that can be made is limited by the thickness of the photoresist layer and the depth of focus properties of the laser beam, however, diffractive _10optical element structures of up to 20 microns in depth can be made for use at mid-infrared wavelengths. Example 2: Electron Beam Writing A substrate sample is coated in photoresist material and scanned under an exposing beam of focussed electrons. The process of electron beam writing is required to be carried out under vacuum conditions. The electron beam can be focussed to a spot of 50-100 nanornetres in diameter to produce very small spatial features in the diffractive optical element structures. However, the resolution of the process is quite often limited by a scattering of the electrons in the photoresist layer, therefore as the depth of the diffractive optical element structure written increases, the lateral resolution of the structure can be degraded. Typically, areas of only one millimetre square can be written at one time, so large area diffractive optical element structures have to be made in stages by the curnulative effect of 'tiling' a number of scanned areas. Example 3: Photo Masking Again, a photoresist coated substrate sample is prepared. The diffractive optical element structure is exposed by using a mask placed in an optical projection system and the mask image is then projected onto the sample. The whole area of the diffractive optical element structure may be exposed at the same time, depending on its size. By using a series of masks, each aligned in accurate registration to the last, the diffractive optical element structure depth and profile is exposed in a series of exposure steps. The masks can have grey scale gradations located within them to smooth the steps between successive exposure levels. The overall resolution of the process is limited by the accuracy of the masks used which can be as good as of the order of 50 nanometres, the alignment accuracy of each mask and the performance of the optical imaging system used.

Once one of the above methods has been employed to produce a diffractive optical element -11structure in a photoresist material this master surface is transferred on to a mould tool surface to provide a robust working surface that will resist the harsh conditions to which the surface will be exposed in a moulding process. This operation is done by using an electroforming method. The surface of the photoresist material is made electrically conductive by depositing on this a thin layer of gold, nickel or some other suitable metal under vacuum. The vacuum coated component is then electroplated with a thi ck layer of nickel to produce a robust tooling surface for the mould tooling.

The tool may then be used to mould large numbers of mirrors. The mirrors are typically moulded from an opaque ABS composition, one grade used is Cycolac EP 3510. The grade of ABS composition is chosen as being suitable for accepting a vacuum coating with thin metal or dielectric layers to provide a highly reflective mirror surface in the infrared and/or visible wavelength range required.

Some typical moulding conditions for the diffractive optical element mirror components moulded in ABS composition are as follows:

Melt temperature 230-2500C Tool temperature 800C Injection pressure 80bar Hold pressure 60bar Cycle time 30 seconds The present invention has been found to provide a practical mirror construction which can enable an infrared detector device to be built in a compact and robust form. In a modification of the mirror construction, a small area of the infrared diffractive optical mirror array can be patterned to perform at visible wavelengths and to act as an optical alignment feature. The visible wavelength elements would be formed in such a way that they would mimic the zones of the infrared mirror array when a suitable light source, such as a laser, was projected onto -12them. The visible wavelength mirror elements would project the light from the visible light source over the same detection zones as the infrared mirror detection zones and form bright spots of visible light on the walls, floor, ceiling or other surroundings to indicate the limits of the areas which the infrared detection zones are viewing. In infrared detection devices available at the present time, it is not possible to know exactly where the infrared detection zones are looking and the device typically has to be tested by the installation engineer walking around in the detection field of view to ascertain the detection zone positions and floor area coverage, which is a time consuming task. The provision of visible wavelength patterns on the mirror will ease the operation of setting up the detection unit. In addition, the alignment of the visible wavelength mirror elements to the infrared mirror elements is easily achieved since they are accurately produced on the mirror array in the same manufacturing process.

The present invention is not suitable only for use as a detector device. One alternative possibility is in a Gas Detector construction. One form of gas detector available at the present time uses curved reflective mirror optics to collimate the light from an infrared light source, transfer it through a gas detection chamber, and then onto a similar mirror to focus the light onto a detection device. The mirrors used are frequently deeply curved and bulky and they could be replaced by simple flat diffractive optical element mirror optics which will be more compact. Additionally, the diffractive optical element mirror may be made up of an array of elements to provide a number of simultaneous detection channels which make use of a single light source and a single detection device.

The foregoing description of embodiments of the invention has been given by way of example only and a number of modifications may be made without departing from the scope of the invention as defined in the appended claims. For example, in the diffractive element 7 shown in Figure 2, the grooves 12 could be formed on both the upper and lower surfaces of the element.

Claims (9)

1. I A focussing mirror for an infrared detector device, in which the mirror is a diffractive optical element.
2. 2 A mirror as claimed in Claim 1, in which the diffractive optical element is a holographic optical element, a binary optics device, a kinoform or a diffraction grating.
3. 3 A mirror as claimed in Claim I or 2, in which each diffractive optical element comprises an array of fine grooves on a substrate.
4. 4 A mirror as claimed in Claim 1, 2 or 3, which comprises a single body supporting several diffractive optical elements.
5. A mirror as claimed in any one of Claims 1 to 4, in which the reflective surface has a segmented microrelief structure with an active planar surface divided into individual segments.
6. 6 A mirror as claimed in any one of Claims I to 5, in which the element includes a groove pattern where the grooves are located in a non -rotational ly symmetric arrangement.
7. 7 A mirror as claimed in any one of Claims 1 to 6, in which the or each diffractive optical element has spatial filtering characteristics effective to give some initial processing of a detected image for different detection zones.
8. 8 A mirror as claimed in any one of Claims I to 7, in which the mirror is formed from a synthetic plastics composition which has been shaped by a plastics moulding technique.
9. 9 An infrared detector device including a focussing mirror as claimed in any one of Claims I to 8.

   An infrared detector device substantially as hereinbefore described with reference to any one of the accompanying drawings.