WO2000062267A1

WO2000062267A1 - Diffractive focusing lens for infrared detector

Info

Publication number: WO2000062267A1
Application number: PCT/GB2000/001318
Authority: WO
Inventors: Mark Gregory Pitts
Original assignee: Texecom Limited
Priority date: 1999-04-09
Filing date: 2000-04-07
Publication date: 2000-10-19
Also published as: GB9908073D0

Abstract

A focussing element (50) is incorporated in an infrared detector, the focussing element (50) being in the form of a series of grooves (54) which form one or more diffractive optical elements. The diffractive optical elements may have spatial filtering properties and may also be arranged to correct chromatic aberrations. Alternatively, the focussing element may comprise a holographic optical element.

Description

DIFFRACTIVE FOCUSING LENS FOR INFRARED DETECTOR

This invention relates to lenses used in infrared detector devices, their

use and to methods of their manufacture.

It is well know that optically refracting Fresnel lenses with prismatic,

spherical or aspheric profiles, either as a single lens or in arrays of multiple

lenses, are used as a means of focusing 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

Fresnel lenses are used to provide a plurality of sensing zones from a fewer

number of electronic infrared sensing components.

U.S. Patent No. 4,787,722 discloses Fresnel lenses with prismatic,

spherical or aspheric groove profiles of the type mentioned above.

It is also known that such Fresnel lenses, either as single lenses or in

arrays can be used in 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

lenses, or arrays of lenses, are used to refract the infrared radiation in both

the emitting and sensing parts of the device.

Typically, when produced in large volume, the Fresnel lens arrays are

made by moulding, embossing, stamping or any similar process to

accurately replicate the Fresnel lens forms in an in-expensive thin plastic component. Polyethylene is a convenient plastic material to use because it

has reasonable transmission properties in the infrared spectrum.

Nevertheless transmission losses are suffered through thick sections, so the

arrays have to be made as thin as possible to maintain adequate

transmission of infrared radiation. This means that the Fresnel lens grooves

have to be made as fine as possible to enable thinner lens array sections to

be made. A problem with this is that, as the Fresnel grooves are made

finer, it becomes more difficult to achieve accuracy in the form of the

Fresnel lens and, as accuracy decreases, the performance of the lens array

suffers.

The Fresnel lens forms used for infrared detection devices are

typically made by single-point diamond machining to enable Fresnel lens

groove profiles of the order of approximately 0.2 millimetres depth to be

made with sufficient accuracy and an optical surface finish. The Fresnel

lens forms can either be directly machined in the final component or, more

usually, into tooling surfaces used for moulding, embossing or stamping,

lenses or arrays in large volume. Arrays of lenses are usually formed by

assembling together a number of individual lenses.

There are a number of other problems with known infrared detection

lens arrays made with prismatic, spherical or aspheric Fresnel lens elements.

Firstly, each lens element in an array of more than one lens has to be

individually made or replicated from a master lens form and a different master Fresnel lens form has to be made for each different focal length, or

other property, of the lens array.

Furthermore, the single-point diamond machining typically used can

be expensive, especially when used to produce a large number of individual

Fresnel lens forms of different properties to be later assembled together into

an array.

A further problem arises due to the fact that only Fresnel lens forms

that are rotationally symmetric about their optical axis can be made in

accordance with the methods mentioned above which limits the optical

properties of the Fresnel lens or arrays.

A still further problem arises in that a Fresnel lens can be corrected

only for spherical aberrations in its focus properties, but cannot be corrected

for other lens aberrations such as astigmatism, coma or chromatic

aberrations.

An object of this invention is to provide focusing optical element for

use in infrared detector devices, and a method of manufacturing the same

which overcomes, or at least minimises, the above mentioned problems.

According to a first aspect of the present invention there is provided

a focusing element for an infrared detector device said element comprising

a diffractive optical element.

According to a second aspect of the present invention there is provided an infrared detector device comprising an infrared source, an infrared detector and a focusing element, said element comprising a

diffractive optical element.

According to a third aspect of the present invention there is provided

a method of detecting an object comprising the step of:

emitting infrared radiation and focusing said emitted radiation by

diffracting said radiation in order to direct said radiation towards an object

to be detected.

One advantage of this invention is that the size of the groove pattern

typically used for the diffraction optical element is much smaller than the

grooves used in a Fresnel lens, typically by a factor of 10 or more.

Therefore, the overall section thickness of the diffractive optical element

which could be moulded, embossed or stamped in a typical polyethylene

plastic material, can be smaller than that needed for an array of Fresnel

lenses to perform the same function. The smaller section thickness allows

better infrared transmission to be achieved, as compared to a conventional

Fresnel lens.

The relatively small groove depth to overall section thickness ratio

this invention allows gives improved characteristics for the large volume

manufacture of components in a plastic material such as polyethylene. In

order to replicate the very fine groove detail of the diffractive optical

element by the preferred method of injection moulding, unusually high pressures and plastics melt temperatures have to be used than 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 diffractive optical element preferably comprises a segmented

surface microrelief structure where the active planar surface is divided into

individual segments. A wavefront incident on the surface 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

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 2π. Various computerised mathematical methods can be used to

determine and optimise the surface microrelief structure which are well

documented.

Preferably, the focusing element comprises an array of diffractive optical elements. Preferably, the array is formed from a single piece of

material.

Preferably the method includes the additional steps of: receiving

infrared radiation reflected from the object, focusing the reflected radiation

by diffraction and detecting said focused radiation.

Preferably, 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 preferably comprises a plurality

of fine grooves, for example of approximately 20 micrometers depth,

disposed on the optical surface of a lens or mirror. The grooves on the

optical surface impose a change in phase of the wavefront passing through,

or reflecting from, the surface, which can be designed to focus the light

transmitted through 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 photo-sensitive media,

single-point diamond machining, ion beam etching, chemical etching, laser

machining, laser writing, electron beam writing or photo-masking. Some of

the various means of producing the grooves of the diffractive optical

element allow the whole lens 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 lens elements of the array. This allows better positional accuracy to be achieved between each element in the array

and removes the cost of assembling together each element of the array to

produce the final components or tooling for moulding, embossing, stamping

or the like of large volumes of lens arrays.

Since the suggested means of making the fine diffractive optical

element element groove structures can produce features as small as in the

order of 0.5 micrometers, 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 element and cannot be seen with the naked

human eye, but can be seen under high powered magnification. Any

attempt to copy the diffractive optical element element design by a

replication method would similarly replicate the 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 than 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 aberration including astigmatism and coma to be

corrected, or introduced into the optical elements in a controller manner.

This may be used to provide a different vertical sensing zone size in relation

to the horizontal sensing zone size, or to change the shape of the sensing

zone, of each element in an array for an infrared detection device. This

enables the focused 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 small

objects, thus enabling some distinction in the detection between, say, a

human being and a small animal and therefore reducing or removing the

need for this processing to be done electronically. For instance, in the

simplest 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 wider part

of the zone at 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 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, other optical corrections are performed to

correct chromatic aberrations typically in the passive infrared wavelength

detection range of 7 to 14 micrometers but possibly at other wavelength

ranges to suit other applications in infrared sensing. 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 outside the

wavelength range of the intended infrared 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 passes through the diffractive

optical element element.

The diffractive optical element may be comprised in a flat or curved

transmission 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 and transmission

device provides some optical effect in the transmitted beam.

In order that the invention may be more clearly understood

embodiments thereof will now described, by way of example, with reference

to the accompanying drawings in which:

Figure 1 shows a plan view of a typical prior art aspheric Fresnel lens

form;

Figure 2 illustrates a cross-sectional view of the lens of Figure 1 taken

along the line ll-ll;

Figure 3 shows a perspective view of an array of prior art Fresnel lens

elements as typically used in a passive infrared detector

device.

Figure 4 illustrates cross-sectional view of an embodiment of a

diffractive optical element according to the invention;

Figure 5 illustrates a perspective view of another embodiment of a diffractive optical element according to the present invention

with non-rotationally symmetric grooves and representing the

different optical powers in each axis across the surface;

Figure 6 illustrates a perspective view of an embodiment of an array of

diffractive optical elements according to the invention; and

Figure 7 illustrates a schematic exploded perspective view of an

infrared detector including the array shown in Figure 6.

Figures 1 to 3 show prior art. Referring to Figures 1 and 2 a Fresnel

lens 10 has a thin circular body 1 1. One face surface 12 of the body 1 1 is

flat. The other face 13 has a plurality of concentric grooves 14. Other

types of prior art lenses have grooves on both faces. Each groove 14 may

have a triangular, spherical or aspherical cross section 17. The grooves 14

may be of constant depth and a varying width. The width of each groove

14 is the radial distance between each pair of successive vertical sides 15.

The width of each groove 14 decreases as the distance between the centre

16 and the groove 14 increases. Thus the grooves 14 nearer the edge of

the lens 10 are narrower than the grooves 14 nearer the centre 16. Some

prior art designs may instead have grooves of constant width and therefore

varying depth.

Figure 3 shows a perspective view of an embodiment of an array 20

of prior art Fresnel lens elements 10 as typically used in passive infrared

detector devices. The lenses 10 provide a plurality of detection zones in the detector device. The array 20 is typically made by moulding an infrared

transmitting plastic, such as polyethylene. The array has a frame 21 with

a flat surface 22 for mounting the array 20 in to a detector device.

Figure 4 illustrates one embodiment of a cross-sectional view of a

diffractive optical element 50. Referring to this figure the element 50 has

a thin, substantially rectangular in plan, body 51 . It will be appreciated,

however, that the body could be circular, rectangular or some other shape

in plan as desired or as appropriate and depending on the means used to

produce the body 51 . One surface 52 of the body 51 is flat. The opposite

surface 53 has a plurality of grooves 54. The element 50 could, however,

have grooves 54 on both surfaces. Each groove 54 is formed by a

substantially vertical surface 55 (when viewed in the orientation shown) and

a curved surface 56. The height of the vertical surfaces 55 and the width

and curvature of the curved surfaces 56 will depend on the change in phase

they are to impart on a wavefront transmitted through the diffractive

element 50. Therefore, the heights of the faces 55 and the widths of the

curved surfaces 56 will vary and may not follow a uniform trend across the

entire diffractive element structure surface 53. The grooves 54 may be

rotationally symmetric and concentric about the centre 57. However, it may

be preferential to have grooves 54 that are not rotationally symmetric and

are not placed concentric about the centre 57 to provide a different optical

power in each axis across the surface of the element 50 in order to change the shape of the field of view of a detector in which the lens is fitted.

Referring to Figure 5 this is shown a perspective view of an

embodiment of a diffractive optical lens 60 consisting of one element 50

formed from an infrared transmitting plastic, such as polyethylene. The

element 50 in this case has grooves 54 which are non-rotationally

symmetric about the centre 61 . In this embodiment the non-rotationally

symmetric grooves 54 of the element 50 provide a different optical power

in each axis across the surface of the element 50. Light paths 62 passing

through the element 50 from the focal point 63 to form the projected beam

cross-section 64 are re-directed by a different angular amount dependant on

the radial position of the groove 54 from the centre 61 they intercept with.

The non-rotationally symmetric grooves 54 therefore transform a circular

bundle of light paths 62 projected from the focal point 63 into a non-circular

beam cross-section 64. Similarly, if the focal point 63 is considered to be

a detector the non-circular beam cross-section 64 defines the field of view

the detector would achieve through the element 50.

Referring to Figure 6 there is shown a perspective view of an array

70 of diffractive optical elements 50 as typically may be used in a passive

infrared detector device. The elements 50 provide for a plurality of

detection zones in the detector device. The array 70 may be made in one

piece regardless of the number of elements 50 in the array 70, provided, of

course, that the array 70 is within the size constraints of the particular means used to produce it. Alternatively, the elements 50 could be

assembled together to form the array 70 the array 70 is formed from an

infrared transmitting plastic, such as polyethylene, or the array 70 can be

etched or replicated by some method on to tooling surfaces for moulding,

embossing, stamping etc. The array 70 may have a frame 21 with a flat

surface 22 for mounting the array 70 in a detector or sensor device, similar

to the prior art Fresnel lens arrays.

Referring to Figure 7 there is shown an exploded view of an infrared

detector 80 comprising a housing formed from two portions 81 and 82.

The first portion 81 houses electronics, particularly a printed circuit board

85 on which is mounted an infrared emitter 86 and sensor 84. The second

portion 82 having an array 83 of diffractive optical elements. In use the

emitter 86 produces infrared radiation. This is focused by the array 83 in

a desired manner to produce a detection zone. Any radiation reflected back

by objects in the detection zone is re-focused by the array 83 onto the

sensor 84. The sensor 84 generates a signal in response to received

radiation. This signal is subsequently processed by the electronics to

determine the state of objects in the detection field.

A number of methods by which diffractive optical elements according

to 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 a XY scanning motion

stage under a focused 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 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 XY axes, the accuracy of the XY motions over longer

travels, and the duration of the total writing time. The typical focused 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 1 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 optical

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 and scanned under the

exposing beam of focused electrons. Electron beam writing has to be

carried out under vacuum. The electron beam can be focused to a spot of

50-100 nanometers to produce very small spatial features in the diffractive

optical element structures. However, the resolution of the process is quite often limited by the 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 degrades. Typically, areas

of only 1 mm x 1 mm are written, so larger area diffractive optical element

structures are made by "tiling" of 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 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

mask can have grey scale gradations 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 nanometers, 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 structure in a photoresist material this master

surface is transferred on to a mould tool surface to provide a robust surface

that will resist the harsh moulding conditions. This is done by using an electroforming method. The surface of the photoresist material is made

electrically conductive by depositing a thin layer of gold, nickel or some

other metal under vacuum. The vacuum coated component is then

electroplated with a thick layer of nickel to produce a robust tooling surface

for the mould tooling.

The tool may then be used to mould large number of lenses. The

lenses are moulded for high density polyethylenes (HDPE), a typical grade

used is Rigidex 6070. These are used either in their natural form or

pigmented with zinc sulphide, zinc selenide, titanium oxide or some mixture

of these to provide a white coloured material or iron oxides or carbon to

provide a black coloured material. Other pigment materials and mixtures

can possibly be used to produce other colours. The pigments and their

relative concentrations are chosen to provide the required visible colour and

to, as much as possible, maintain the mid-infrared transmission of the base

HDPE. UV inhibitor additives are also typically added to the formulations to

reduce the UV degradation of the polymers and so enable the lenses to be

used out-doors.

The typical moulding conditions used for the diffractive optical

element lens components are as follows:

Melt temperature - 250 to 270°C

Tool temperature - 40°C

Injection pressure - 120bar Hold pressure - 40bar

Cycle time - ^"8 seconds

The above embodiments are described by way of example only, many

variations are possible without deporting of the invention.

Claims

1 . A focussing element for an infrared detector device, said element

comprising a diffractive optical element.

2. An infrared detection device comprising an infrared source, an

infrared detector and a focussing element, said element comprising

a diffractive optical element.

3. A method of detecting an object comprising the step of emitting

infrared radiation and focussing said radiation in order to direct said

radiation towards an object to be detected.

4. A focussing element according to claim 1 , wherein the diffractive

optical element comprises a segmented surface micro relief structure

in which a active planer surface is divided into individual segments.

5. A focussing element according to claim 1 or claim 4, wherein the

element comprises an array of diffractive optical elements.

6. A method according to claim 3 further including the steps of

receiving infrared radiation reflected from the object, focussing the

object reflected radiation by diffraction and detecting said infrared

radiation.

7. A focussing element according to any one of claims 1 , 4 or 5,

wherein the or each diffractive optical element comprises a

holographic optical element.

8. A focussing element according to any one of claims 1 , 4, 5 or 7, wherein the or each diffractive optical element comprises a plurality

of fine grooves disposed on the optical surface of a lense or mirror.

9. A focussing element according to claim 8, wherein the grooves are

of approximately 20 μm in depth.

10. A focussing element according to any one of claims 1 , 4, 5, 7, 8 or

9 in which a unique symbol is encrypted within the structure of the

diffractive optical element.

1 1 . A focussing element according to any one of claims 8 to 10, wherein

at least a part of the diffractive optical element is formed from a

groove pattern which is non-rotationally symmetrical.

1 2. A focussing element according to any one of claims 1 ,4,5,7 or 8 to

1 1 , wherein the or each diffractive optical element has spatial filtering

properties to provide initial processing of the detected image for each

of a number of detection zones.

1 3. A focussing element according to any one of claims 1 , 4, 5, 7 or 8

to 12, wherein the or each diffractive optical element is arranged to

correct chromatic aberrations.

14. A focussing element according to claim 13, wherein the diffractive

optical element is arranged to correct chromatic aberrations in the

passive infrared wavelength detection range of up to 7 to 14 μm.

1 5. A focussing element according to claim 1 3 or claim 14, wherein the

chromatic properties of the diffractive optical element or elements which diffusely scatter light in the visible to near infrared wavelength

range while maintaining good optical performance in the required mid-

infrared wavelength range.

16. A focussing element according to claim 7, wherein the element

comprises 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 passes through the diffractive optical element.

17. A focussing element according to claim 1 , wherein the diffractive

optical element is comprised in a flat or curved transmission optical

device.