WO1999061941A1

WO1999061941A1 - Diffractive optical element

Info

Publication number: WO1999061941A1
Application number: PCT/GB1999/001608
Authority: WO
Inventors: Daniel Richard Lobb; Kenneth Jay Weible
Original assignee: NEW WAVE OPTICAL TECHNOLOGY Sarl; Sira Ltd
Current assignee: NEW WAVE OPTICAL TECHNOLOGY Sarl; Sira Ltd
Priority date: 1998-05-22
Filing date: 1999-05-21
Publication date: 1999-12-02
Anticipated expiration: 2000-11-22
Also published as: AU3947499A; GB9811195D0

Abstract

A diffractive optical element (DOE) comprising at least two layers of materials in close proximity or in contact with at least one optical interface therebetween; an optical path through the DOE leading through the first and second layers; the first layer being configured to provide a first optically diffracting pattern (405, 602) of phase retardation; the second layer being configured to provide a second optically diffracting pattern (404, 604) of phase retardation; the first and second diffracting patterns being selected to optimise the DOE's diffraction efficiency at two predetermined design wavelengths. Preferably the first and second diffracting pattern satisfy the condition that the sum of the phase retardations of the two patterns at a first design wavelength is substantially equal to the sum of their phase retardations at a second design wavelength.

Description

DIFFRACTIVE OPTICAL ELEMENT

The present invention relates to diffractive optical elements (DOEs).

A diffractive optical element can be described as an element that exploits the wave nature of light, and other electro-magnetic radiation, to deflect the direction of the radiation using the effect of diffraction. In this specification 'optical' includes regions of the electromagnetic spectrum outside those visible to the human eye including, but not limited to, ultraviolet, infrared and microwave regions of the spectrum. Diffraction is produced by structure in the DOE, which generates a spatially-varying modulation in the incident radiation. The structure is generally spatially-periodic; however, the periodicity can be constant or variable over the DOE aperture, depending upon the application. The modulation depth produced by the structure can also be constant or variable over the DOE, depending upon the application. The invention discussed here applies to DOEs in general, independently of whether the periodic modulation and modulation depth are constant or variable.

There are several types of cyclic structures used in different kinds of DOE. The structure may be a surface profile on a refracting element, as indicated in Figure 1, or a similar surface profile on a reflecting surface. Alternatively, the structure may be a pattern of refractive index variation produced in the bulk of the DOE material. Refractive index variations may be produced, for example, by an acoustic wave passing through a transmitting medium, or by photographic processes, as in some forms of hologram. All these DOEs may be considered to work by modifying the spatial phase distribution of the transmitted optical waves, and in this document, they are called phase DOEs

In the simplest form of DOEs, the cyclic pattern may be a spatial variation in transmission or reflection, produced for example by etching a thin metal layer on a glass surface to produce alternate transmitting and reflecting bands. These DOEs work by modifying the spatial amplitude or intensity distribution of the transmitted or reflected optical waves. In this document they are called amplitude DOEs.

In practice, many DOEs combine aspects of surface profile, refractive index variation and variations in transmission and reflection. We will describe phase DOEs, although some will in practice also show some amplitude effects.

The function of DOEs is in general to provide controlled deflections of radiation, as indicated simply in Figure 1. A monochromatic parallel beam of radiation, falling on the DOE, is indicated in Figure 1 by the ray 105. The effect of the cyclic pattern is to diffract the beam, splitting it into several components travelling on paths represented in Figure 1 by the rays numbered 106, 107 and 108. The direction of the diffracted rays 106, 107 etc. is dependent on the spacing of the cyclic pattern, s, in relation to the wavelength of the radiation λ. In a simple case, where the incident beam direction is orthogonal to the mean plane of the DOE, the angles θ between the incident ray direction and the diffracted ray directions are given by the formula:

sinθ = N.λ/s

where N takes positive and negative integer values, for example -2, -1, 0, 1, 2, 3. The beam transmitted without deflection, corresponding to the value N = 0, is called the zero-order beam. The beams in directions given by N = 1 and N = -1 are the first order diffracted beams, N = 2 and N = -2 give the second order diffracted beams etc.

One application of diffraction is in spectrometers. These are instruments that provide measurements of the spectral distribution of radiation from selected sources. Some spectrometers use DOEs in the form of diffraction gratings to separate the radiation into its spectral components. Most commonly, a diffraction grating has a cyclic pattern with a constant spacing s in one direction. Typically, radiation from the source is passed through a slit, and the radiation is then collimated and transmitted or reflected at a diffraction grating. It will be apparent from the above formula that the direction of radiation leaving the grating is dependent on the radiation wavelength. When refocused, therefore, the radiation forms a spectrum image, at which a detector array may be placed to measure the relative spectral distribution of the radiation.

Another application of DOEs is in image formation, as indicated schematically in Figure 2. In this case, the DOE, 200, has a surface profile in which the locus of constant depth is a set of concentric circles, represented on the diagram by circles numbered 201, 202 and 203. The spacing of the circles varies with separation from the common centre 204, according to the function of the DOE. Typically, the spacing of circles decreases progressively with increasing radius from a central point, so that the first-order diffracted beam is deflected through larger angles at larger off-centre distances; this produces an effect similar to that of a conventional converging or diverging lens element.

DOEs of this kind can be used as alternatives to refracting lens elements and curved mirrors in image-forming systems, such as collimators for display systems and special camera lenses. The diffraction effect is strongly wavelength- dependent, so that individual DOEs have strong chromatic aberrations. One use of DOEs is to provide a method for correcting the weaker chromatic aberrations of refracting elements, in systems that combine both refracting and diffracting elements.

DOEs are also used in a range of other applications. These include, but are not limited to, diffusers, Fresnel lenses, beam shaping, beam splitters, and phase compensation plates. They are used for example as scanning elements in point-of- sale scanners, and as special beam combiners in head-up and helmet-mounted displays. Holograms are examples of DOEs, in which very complex diffracting patterns are recorded, to reproduce complex images in suitable illumination. In most applications of DOEs, it is desirable to concentrate diffracted power into a single selected diffraction order. In spectrometers, for example, optimum signals are obtained when diffracted power is concentrated in one selected diffraction order. Where DOEs are used for imaging, typically only one diffracted order provides the image that is required, while other diffraction orders reduce the useful image contrast.

The efficiency of a DOE in diffracting radiation into a specified diffraction order is known as the diffraction efficiency. The diffraction efficiencies of DOEs are controlled by their amplitude and/or phase patterns. The efficiencies of amplitude DOEs, for the first and higher diffraction orders, are generally low, with a large proportion of the total incident power absorbed and a large proportion also appearing in the zero order in reflection or transmission. Where high efficiency is needed, phase DOEs are used, since they can provide much higher diffraction efficiencies, in selected first or higher diffraction orders.

The diffraction efficiency of a phase DOE depends on several factors, including the shape of the surface profile or the spatial-distribution of refractive index variations, and the angle of incidence of radiation on the DOE. However, the most significant factor in deciding diffraction efficiency of a phase DOE is the range of perturbations produced by the cyclic structure of the device. This range of perturbations is called the phase modulation of the DOE. Phase modulation is quantified as a phase angle, in radians, where 2π radians corresponds to one full wave of perturbation-range, for the defined wavelength of the radiation beam.

For example, the saw-tooth surface profile shown in Figure 1 has a maximum depth d in each fine-structure cycle. This produces a range of phase modulation [in radians] on the transmitted wavefront from 0 to a maximum of:

2π d [_{n ι} (λ) - n₂ (λ)] . where nι(λ) and n₂(λ) are the refractive indices of the media before and after diffraction, at the radiation wavelength λ.

A maximum diffraction efficiency in the Nth diffraction order is typically achieved when the phase modulation is close to 2.π.N - that is, when the depth of the DOE structure produces whole numbers of wavelengths of perturbation in the radiation beam. The saw-tooth profile shown in Figure 1 can in theory provide 100% diffraction efficiency in this condition.

In the event that one of the media is air, it is usually reasonable to assume that its refractive index can be approximated to unity for all wavelengths (as for vacuum), so that for a single material of index n(λ) used in air, the maximum phase retardation can be written as:

For phase DOEs using internal refractive index variations, a similar formula may be applied. The maximum phase modulation at wavelength λ can be written as:

2 π d' f (λ)

where f(λ) is a wavelength-dependent function of the glass refractive index and variation of refractive index with wavelength, and d' is a notional surface profile depth that would produce the same phase retardation as the actual retardation range produced by refractive index variations. For a reflecting DOE with profile depth d, operating in a medium of refractive index n(λ), the maximum phase retardation is given by:

^ d n(λ) If the medium is air or vacuum, this formula for a reflecting DOE can be simplified to:

4π , — d λ

In general, the phase modulation may vary over the DOE surface, with variations in the profile depth d, or of the equivalent profile depth d' produced by internal refractive index variations. However, for a diffraction grating, d and s are typically near-constant over the DOE surface, giving near-constant diffraction efficiency and near-constant deflection angle, for a given radiation wavelength. A DOE used as a lens, as shown in Figure 2 will also typically have a constant maximum phase modulation over its area, although the spacing s of the cyclic pattern varies.

In practice for any phase DOE that is made in a single real material and operated in air or vacuum, the phase modulation of the DOE, always varies monotonically with the radiation wavelength (over wavelength ranges within which the material transmits). This is because the refractive index functions n(λ), n(λ)-l and f(λ) of real materials vary more slowly than wavelength itself. This means that it is normally not possible, using a single DOE material, to achieve the same value of phase modulation for two radiation wavelengths in the same diffraction order. In general, an ideal phase function can be produced in a phase DOE only for a single chosen wavelength of illuminating radiation. For other radiation wavelengths, the phase retardation induced will deviate from an ideal phase function; it is not possible to achieve the same phase retardation for all wavelengths in a broad wavelength range.

One consequence of this fact is that it is not possible to achieve high diffraction efficiency for all wavelengths in a broad wavelength range. This is a significant disadvantage for DOEs used in many applications, for example in spectrometers that are required to cover large wavelength ranges and in imaging systems working in white light.

A DOE consisting of a sandwich of two materials of nearly matched indices of refraction On <0.01), with an interfacial surface whose relief height impresses the desired optical phase, is know from US 5,734,502 Ebstein. Using nearly matched refractive indices allows the interfacial surface profile to be figured with less precision than would otherwise be necessary. However, Ebstein teaches that the sum of the phase retardations produced by the DOE, on a transmitted beam of radiation, must be stationary with respect to radiation wavelength at a selected wavelength, and equal to 2π or a multiple of 2π at this single selected wavelength.

There therefore arises a need to reduce the variations in phase modulation with respect to radiation wavelength.

According to a first aspect of the present invention there is provided a diffractive optical element (DOE) comprising at least two layers of materials in close proximity or in contact with at least one optical interface therebetween; an optical path through the DOE leading through the first and second layers; the first layer being configured to provide a first optically diffracting pattern of phase retardation; the second layer being configured to provide a second optically diffracting pattern of phase retardation; the first and second diffracting patterns being selected to optimise the DOE's diffraction efficiency at two predetermined design wavelengths. Preferably the diffraction efficiency is maximised by selecting the first and second diffracting patterns to satisfy the condition that the sum of the phase retardations of the two patterns at a first design wavelength is substantially equal to the sum of their phase retardations at a second design wavelength. Preferably the sum is substantially equal to an integral number of wavelengths for both design wavelengths.

The design wavelengths may be chosen to differ by at least 5% of the smaller of the two wavelengths. The materials of the first and second layers may be chosen to have respective refractive indices differing by at least 0.1 at one of the design wavelengths and/or at least one of the phase retarding diffracting patterns may comprise an optical surface profile with features of physical depth of a comparable size to a design wavelength.

Advantageously, the first and second diffracting patterns are patterns of physical surface profile variations on surfaces of the two layers, and the DOE is configured so that the shape of the surface profile of one pattern does not physically determine the shape of the surface profile of the other pattern. This gives the optical design an additional degree of freedom for optimisation purposes. Preferably the first diffracting pattern comprises a first surface profile on a surface of the first layer adjacent to the second layer and the second diffracting pattern comprises a second surface profile, matched to the first but of different physical depth, on a surface of the second layer. In this configuration the second surface profile is preferably on a surface of the second layer which is not directly adjacent the surface of the first layer bearing the first surface profile.

Preferably the first and second layers are in optical contact with each other and have respective refractive indices μi and μ₂, and the second layer is in optical contact with a third layer of medium of refractive index μ₃. In one embodiment the first and second surface profile both have a generally sawtooth shape of maximum respective physical depth di and d₂, the phase retardation di (μi - μ₂) + d₂(μ₂ - μ3) at each design wavelength is equal to Nλ, N being an integer.

According to a second aspect of the invention there is provided a method for optimising the diffraction efficiency of a diffractive optical element (DOE), the DOE comprising two adjoining layers of optically-transmitting first and second media with respective refractive indices at wavelength λ of ni (λ) and n₂ (λ) and meeting at a common boundary, the boundary being physically patterned with a boundary profile to define an optical diffraction pattern, the method comprising the steps of: 1) selecting first and second wavelengths λi and λ₂; and 2) selecting the first and second media having regard to the condition (ni (λi) - n (λ₇))lλ_λ = (n_x (λ₂) - n₂ (λ_))lλ₂.

According to a third aspect of the invention there is provided a diffractive optical element comprising first and second layers of radiation-transmitting materials, in close proximity or contact, each layer profiled, in substantially the same pattern of peaks and troughs, on at least a part of one surface, the profile patterns being aligned such that the peaks on a first profiled surface are adjacent to either the peaks or the troughs of each other profiled surface, the profile depths, d, being selected such that, the sum of phase retardations introduced by the profiled surfaces of the first and second layers at two selected wavelengths of a transmitted beam of radiation, are substantially equal.

Preferably the materials of the layers have different refractive indices μ, the sum of the products d.(μ-l), for the profiled surfaces, being substantially equal to an integral number of wavelengths, taking account of the variation of refractive indices μ with wavelength, profile depth d being considered positive for said first profiled surface, positive for any other profiled surface having profiled peaks adjacent the peaks of said first profiled surface, and negative for any other profiled surface having profile troughs adjacent the peaks of said first profiled surface.

According to a fourth aspect of the invention there is provided an optical structure comprising first second and third media, the second medium having a refractive index different to that of the first and third media, an optical path through the structure leading from the first medium, through a first optical interface between the first and second media, into the second medium and through a second optical interface (404, 604) between the second and third media into the third medium, the first optical interface having a first surface profile forming a first diffracting pattern; the second optical interface having a second surface profile forming a second diffracting pattern. According to a further aspect of the invention there is provided a DOE comprising two materials with differing refractive indices, abutting at a mutual optical interface, the interface having a first surface profile forming a first diffracting pattern, at least one of the materials having a second optical surface with a second surface profile forming a second diffracting pattern, an optical path through the DOE leading through the interface and the second surface.

According to another further aspect of the invention there is provided an optical device comprising two elements of differing refractive indices, a first element having a first diffracting pattern and a second element having two further diffracting patterns substantially matching the lateral spatial variations of the first diffracting pattern but introducing differing phase retardations into an optical path traversing the device whereby the phase modulations introduced by the device at two different wavelengths are substantially the same.

We will also describe a DOE constructed of at least two layers in close proximity, all layers treated to provide phase retardation on a radiation beam transmitted through the layers, having the same lateral spatial pattern for each layer although the depth of each layer may vary, the materials of the layers having different refractive properties such that the sum of the phase modulations produced by all layers shows a reduced variation with respect to radiation wavelength than would be achieved using a DOE made in a single material.

Preferably the sum of the phase modulations produced by all layers for one radiation wavelength is equal to the sum of the phase modulations produced by all layers for at least one other radiation wavelength. In some configurations the phase retardation provided by the layers is cyclical.

The technique applies to any application of a DOE where the illumination source is polychromatic, or monochromatic but unstable. Furthermore, it is independent of the application or shape of the phase function. These and other aspects of the DOE will now be described, by way of example only, with reference to the accompanying Figures in which

Figure 1 shows a cross section through the thickness of a conventional diffractive optical element;

Figure 2 shows a conventional image forming diffractive optical element,

Figure 3 shows a DOE comprising a layer of glass and a layer of optical cement,

Figure 4 shows a DOE comprising two layers of different glasses separated by a layer of air,

Figure 5 shows the typical variation in diffraction efficiency achieved using a single material DOE, and the reduced variation in diffraction efficiency achieved using a layer of glass and a layer of optical cement,

Figure 6 shows a DOE comprising a plate with an etched profile, filled by a thin film of a different material that is also profiled on its outer surface, and

Figure 7 shows an embodiment of a DOE according to the present invention

Referring first to Figure 3, this shows a DOE comprising two glass sheets, 301 and 302 separated by a cement layer 303 In this case the important layers are the glass sheet 301 and the cement layer 303 The glass sheet 301 is etched on the surface 304 in a cyclic saw-tooth profile Elements of the profile pattern are indicated by numerals 305, 306 and 307 The depth of the profile is d, and the spacing of profile pattern elements is s The etched face 304 is cemented to a flat face of 308 of the glass sheet 302, so that the etched profile is filled with the cement 303.

The refractive index of the glass sheet 301 is nι(λ), and the refractive index of the cement is n (λ). The phase modulation produced on a beam of radiation transmitted through the DOE, represented by rays 309 and 310, is given by:

2.π.d.(nι(λ)-n₂(λ))/λ

for the radiation wavelength λ.

For two selected wavelengths λi and λ₂, the materials of the etched plate 301 and the cement layer 303 are selected such that:

(n₁(λ₁)-n₂(λι))/λ₁ = (n₁(λ )-n₂(λ₂))/λ2

In this condition, the DOE has equal phase retardations or modulations at the two (different) selected wavelengths. The phase modulation at the same wavelengths may then be adjusted to be close to N2π, by selection of the profile depth d, so that the DOE gives high diffraction efficiency in the N^th diffraction order at the two selected wavelengths.

Provision of two separated peaks of diffraction efficiency, as described above, in general provides high diffraction efficiency over a broader spectral band. Moreover, in some applications, it is necessary to provide good diffraction efficiency in two quite distinct spectral bands. For example, some infrared imaging systems, that include DOEs to assist in correction for chromatic aberration, are required to work in two separate wavebands: the atmosphere transmission windows between wavelengths 3 and 5 microns, and the window between wavelengths 8 and 12 microns. For use in infrared radiation, the DOEs described here would of course use infrared transmitting materials. Figure 4 shows a DOE comprising two glass sheets, 401 and 402 separated by an air layer 403. In this case the important layers are the glass sheets 401 and 402. The glass sheets are etched on the facing surfaces 404 and 405 in a cyclic sawtooth profile. Lines of maximum depth in the profile formed on surface 404 are aligned opposite lines of minimum depth in the profile formed on surface 405, and vice versa.

The depth of the profile on surface 404 is di, and the depth of the profile on surface 405 is d₂. The refractive index of the glass sheet 401 is nι(λ), and the refractive index of the glass sheet 402 is n₂(λ). The phase modulation produced on a beam of radiation transmitted through the DOE, represented by rays 409 and 410, is given by:

2.π.dι.(mι(λ)-α.m₂(λ))/λ

where mi (λ) = nι(λ)-l, m₂(λ) = n₂(λ)-l and α = d₂/αY

The glass of sheet 401, which has a deeper profile, is selected to have a lower refractive dispersion (variation of refractive index with wavelength), and the glass of sheet 402 is selected to have a higher refractive dispersion. The glasses may for example be Schott BK7 glass for sheet 401 and Schott F2 glass for sheet 402. The ratio of profile depths, α, is selected such that, for two selected radiation wavelengths, λi and λ₂:

(mι(λι)-α.m₂(λι))/λ i = (mι(λ₂)-α.m₂(λ₂))/λ ₂

In this condition, the DOE has equal phase modulations at the two selected wavelengths. The phase modulation at the same wavelengths may then be adjusted to be close to N2π, by selection of the profile depth of sheet 401 (di), so that the DOE gives high diffraction efficiency in the N diffraction order at the two selected wavelengths. As an alternative, the space 403 between the two layers 401 and 402 in Figure 4 may be filled by a solid or liquid medium, for example an optical cement. In this case the formulae for phase modulation are modified to include the refractive index of the filling layer.

A standard DOE consisting of a single material-air interface has a substantial variation in phase modulation with radiation wavelength. To illustrate this point, the solid line in Figure 5 shows the theoretical diffraction efficiency of a conventional DOE made of an optical glass, and used in air. The graph shows efficiency, over the spectral bandwidth 400 nm to 900 nm. Maximum efficiency is achieved only at one wavelength, in this case 600nm. The dashed line in Figure 5 shows a theoretical efficiency of a DOE using glass and cement in contact, when the cement has lower refractive index, but higher dispersion than the glass. In the illustrated case, a theoretical maximum efficiency is achieved at two wavelengths, 470nm and 750nm, and the DOE has high efficiency over a much broader spectral range.

Figure 6 shows a DOE comprising a first layer 601, etched to a cyclic sawtooth profile on the surface 602. A layer 603 of different solid material is formed in contact with the sheet 601, on the side of the profiled face 602. The layer 603 therefore acquires a complementary cyclic sawtooth profile also indicated by numeral 602. In practice, the layer 603 may be formed by several methods, depending on the materials required for specific spectral bands; it may be an optical cement or a moulded plastic film, or it may be formed by electrolytic deposition.

The outer surface 604 of the layer 603 is also profiled, preferably in a pattern matching that of the surfaces 602 and aligned with it. The pattern on the outer surface 604 may again be produced by a variety of processes depending on the materials used, including etching, replication and compression moulding. The depth of the inner profiles on face 602 is di, and the depth of the profile on the outer surface 604 of layer 603 is d₂. The refractive index of layer 601 is μι,ι at a first selected wavelength λi, and μ_lj at a second selected wavelength λ₂. The refractive index of layer 603 is μ₂.ι at wavelength λi, and μ₂, at wavelength λ₂. The DOE introduces phase modulations on a beam of transmitted radiation indicated by numerals 605 and 606. To equalise the phase modulations at the two wavelengths, the profile depths are selected such that:

(dι.(μ - μ₂.ι) + d₂.(μ₂.ι - l))/λι = (dι.(μ - μ_2>2) + d₂.(μ₂,₂ - l))/λ₂

To achieve high diffraction efficiency at both of these two wavelengths, the profile depths should be selected such that:

(dι.(μ - μ₂,ι) + d₂.(μ₂,ι - l))/λι = (dι.(μι.₂ - μ₂,₂) + d₂.(μ₂ - l))/λ₂ = N

where N is an integer.

One advantage of including two different profile depths, as indicated in Figures 4 and 6, is that a wide range of materials can be used to make efficient DOEs. This helps to overcome a potential drawback of the arrangement of Figure 3 in which, for it to operate at its best, it is necessary to select two materials such that the difference of their refractive indices is approximately linearly with wavelength over the spectral band of interest - there are relatively few pairs of materials that meet this condition and are also suitable in other respects.

The structure illustrated in Figure 6, in which the two layers are integrally formed as a single DOE, can be manufactured in a variety of different ways, as has been suggested above. In one method of manufacture layer 601 is patterned on, for example, glass, fused silica or, for the infrared, silicon. Then a space above the layer is filled with a second material to form layer 603, which can then be patterned by compression (for example, stamping), replication or a polish and etch process. The initial deeper profiled surface 602 of layer 601 can be formed by, for example, etching or diamond turning. In another technique the shallower profile of face 604 is first formed on the interior surface of a mould, another surface of which is formed directly by face 602, plastic then being injected into the space between these two surfaces to provide layer 603 with the correct bounding surface profile.

Suitable materials for the visible region of the spectrum include optical glasses, fused silica, optical cements such as epoxies and clear plastics materials such as acrylic and polycarbonate plastics. Infrared materials include silicon, germanium, zinc sulphide, zinc selunide, sodium chloride, silver chloride, calcium fluoride (fluorite), potassium chloride and sapphire, as well as other materials which are known to those skilled in the art. The exact choice of materials for any particular design will need to take account of the refractive indices of the materials at the design wavelengths, the physical properties of the materials and the intended working environment, cost and other factors. For this information reference can be made to any standard data book, such as "The Infrared Handbook".

Although any generally sawtooth-shaped diffraction pattern has been illustrated, the invention is not limited to such a diffraction pattern. However, a sawtooth profile is commonly used as it assists in diffracting light into a single order. Other profiles which can be used, include a rounded sawtooth profile, and a sinusoidal profile (although in this latter case the lack of asymmetry normally results in light being deflected in at least two directions symmetrically disposed around the zero order direction).

It is preferable that the two diffracting profiles (602 and 604 in Figure 6) have the same lateral spatial pattern (but different profile depths), that is, that they are laterally spatially aligned and have profiles of the same shape but scaled in depth with respect to one another. However some beneficial effect can be obtained with patterns which do not exactly match. Furthermore, although diffraction patterns comprising a physical surface or interface profile have been described, as will be understood by those skilled in the art, diffraction patterns may also be produced by refractive index variations within a layer with a physically flat bounding surface. This technique is commonly used to form volume holograms from dichromated gelatine.

Although a diffractive optical element employing two layers with two separate diffraction patterns has been described, the principle may be extended to include further layers bearing further spatially aligned diffraction patterns. In principle, each additional layer should add an additional degree of freedom to the system and thus allow diffraction efficiency to be optimised at an additional wavelength.

Figure 7 shows a DOE, constructed as a lens element, using the method described with reference to Figure 6. The inner profile is produced on the right-hand side surface of layer 701, and filled by the material of layer 702. The outer surface of layer 702 is profiled, in a pattern matching that made on layer 701 (but not shown in the diagram). Ridges of the sawtooth profile produced on the outer surface of layer 702 are indicated by numerals 703 to 705. Incident rays 706 and 707 are indicated, which are deflected by the DOE onto ray paths 706' and 707'.

Other variations will occur to those skilled in the art and it should be understood that the invention is not limited to the described embodiments.

Claims

1. A diffractive optical element (DOE) comprising at least two layers of materials in close proximity or in contact with at least one optical interface therebetween; an optical path through the DOE leading through the first and second layers; the first layer being configured to provide a first optically diffracting pattern (405, 602) of phase retardation; the second layer being configured to provide a second optically diffracting pattern (404, 604) of phase retardation; the first and second diffracting patterns being selected to optimise the DOE's diffraction efficiency at two predetermined design wavelengths.

2. A DOE as claimed in claim 1 wherein the first and second diffracting patterns satisfy the condition that the sum of the phase retardations of the two patterns at a first design wavelength is substantially equal to the sum of their phase retardations at a second design wavelength.

3. A DOE as claimed in claim 1 or 2 wherein the sum is substantially equal to an integral number of wavelengths for both design wavelengths.

4. A DOE as claimed in claim 1, 2 or 3 wherein the design wavelengths differ by at least 5% of the smaller of the two wavelengths.

5. A DOE as claimed in claim 1, 2, 3 or 4 wherein the first and second layers have respective first and second refractive indices differing by at least 0.1 at one of the design wavelengths.

6. A DOE as claimed in claim 1, 2, 3 or 4 wherein at least one phase retarding diffracting pattern comprises an optical surface profile on one of the layers with features of physical depth of a comparable size to a design wavelength.

7. A DOE as claimed in any preceding claim wherein the first and second diffracting patterns are patterns of physical surface profile variation on surfaces of the two layers and wherein the shape of the surface profile of one pattern does not physically determine the shape of the surface profile of the other pattern.

8. A DOE as claimed in any preceding claim wherein the first diffracting pattern comprises a first surface profile on a surface (602) of the first layer adjacent to the second layer and wherein the second diffracting pattern comprises a second surface profile (404, 604), matched to the first but of different physical depth, on a surface of the second layer.

9. A DOE as claimed in claim 8 wherein the second surface profile is on a surface of the second layer which is not directly adjacent the surface of the first layer bearing the first surface profile.

10. A multilayer DOE as claimed in claim 9 wherein the first and second layers are in optical contact with each other and have respective refractive indices μi and μ₂, wherein the second layer is in optical contact with a third layer of medium of refractive index μ₃, wherein the first and second surface profile both have a generally sawtooth shape of maximum respective physical depths di and d₂, and wherein the phase retardation

at each design wavelength is equal to Nλ, where N is an integer.

11. A method for optimising the diffraction efficiency of a diffractive optical element (DOE), the DOE comprising two adjoining layers of optically-transmitting first and second media with respective refractive indices at wavelength λ of ni (λ) and n (λ) and meeting at a common boundary, the boundary being physically patterned with a boundary profile to define an optical diffraction pattern, the method comprising the steps of:

1) selecting first and second wavelengths λi and λ₂; and 2) selecting the first and second media having regard to the condition

(ni (λi) - n₂ (λ₂))/λ< = (m (λ₂) - n₂ (λ₂))/λ₂.

12. A diffractive optical element comprising first and second layers of radiation-transmitting materials, in close proximity or contact, each layer profiled, in substantially the same pattern of peaks and troughs, on at least a part of one surface, the profile patterns aligned such that the peaks on a first profiled surface are adjacent to either the peaks or the troughs of each other profiled surface, the profile depths, d, being selected such that, the sum of phase retardations introduced by the profiled surfaces of the first and second layers at two selected wavelengths of a transmitted beam of radiation, are substantially equal.

13. A diffractive optical element as claimed in claim 12 wherein the materials of the layers have different refractive indices μ, and wherein the sum of the products d.(μ-l), for the profiled surfaces, is substantially equal to an integral number of wavelengths, taking account of the variation of refractive indices μ with wavelength, profile depth d being considered positive for said first profiled surface, positive for any other profiled surface having profile peaks adjacent the peaks of said first profiled surface, and negative for any other profiled surface having profile troughs adjacent the peaks of said first profiled surface.

14. A diffractive optical element according to Claim 12 or 13 in which the difference between the two selected wavelengths is at least 5% of the smaller of the two wavelengths.

15. A diffractive optical element as claimed in Claim 12, 13 or 14 in which facing surfaces of the first and second layers are profiled, the profile depths being of different modulus and opposite sign.

16. A diffractive optical element as claimed in Claim 15 in which the profiled facing surfaces are separated by an air or vacuum gap.

17. A diffractive optical element as claimed in Claim 15 in which the profiled facing surfaces are separated by a solid or liquid film, which forms a third layer profiled on both surfaces, the third layer having profile depths d on its two surfaces that are complementary to those of the profiled surfaces of the first and second layers, the profile depths being selected such that, for two selected wavelengths of a transmitted beam of radiation, the sum of the products d.(μ-l), for all profiled surfaces of all three layers, is equal to an integral number of wavelengths, taking account of the variation of refractive indices μ with wavelength, profile depth being considered positive for said first profiled surface, positive for any other profiled surface having profile peaks adjacent the peaks of said first profiled surface, and negative for any other profiled surface having profile troughs adjacent the peaks of said first profiled surface.

18. A diffractive optical element as claimed in Claim 12, 13 or 14 in which facing surfaces of the first and second layers are profiled in depths of substantially the same modulus and opposite sign, and another surface of the second layer is also profiled.

19. A diffractive optical element as claimed in Claim 18 in which the profile of the facing surface of the second layer is formed by contact with the profiled face of the first laver.

20. An optical structure comprising first (402, 601), second (403, 603) and third (401) media, the second medium having a refractive index different to that of the first and third media, an optical path through the structure leading from the first medium, through a first optical interface (405, 602) between the first and second media, into the second medium and through a second optical interface (404, 604) between the second and third media into the third medium, the first optical interface having a first surface profile forming a first diffracting pattern; the second optical interface having a second surface profile forming a second diffracting pattern.

21. An optical structure as claimed in claim 20 wherein the first and second diffracting patterns have substantially the same lateral spatial pattern but introduce mutually differing optical phase modulation.

22. An optical structure as claimed in claim 20 or 21 wherein the third medium is air or a vacuum.

23. An optical structure as claimed in claim 20 or 21 wherein the second medium is air or a vacuum.

24. An optical structure as claimed in claim 22 or 23 configured for high efficiency diffraction of radiation at two different predetermined wavelengths.

25. An optical structure as claimed in any one of claims 20 to 24 wherein the first and second media are layers of a single diffractive optical element.

26. An optical structure as claimed in any one of claims 20 to 25 wherein the first and second surface profiles include matching regions of generally sawtooth surface profile.

27. A DOE comprising two materials with differing refractive indices, abutting at a mutual optical interface, the interface having a first surface profile forming a first diffracting pattern, at least one of the materials having a second optical surface with a second surface profile forming a second diffracting pattern, an optical path through the DOE leading through the interface and the second surface.

28. A DOE as claimed in claim 27 wherein the two materials comprise two layers of a lens.

29. An optical device comprising two elements of differing refractive indices, a first element having a first diffracting pattern and a second element having two further diffracting patterns substantially matching the lateral spatial variations of the first diffracting pattern but introducing differing phase retardations into an optical path traversing the device whereby the phase modulations introduced by the device at two different wavelengths are substantially the same.

30. An optical device as claimed in claim 29 wherein the diffracting patterns comprise surface profile patterns on optical surfaces of the optical elements in the optical path.

31. A diffractive optical element substantially as hereinbefore described with reference to any one of Figures 3, 4 and 6.