WO2006031643A1

WO2006031643A1 - Diffractive optical element, optical system including the same and associated methods

Info

Publication number: WO2006031643A1
Application number: PCT/US2005/032125
Authority: WO
Inventors: Michael R. Feldman
Original assignee: Digital Optics Corporation
Priority date: 2004-09-10
Filing date: 2005-09-12
Publication date: 2006-03-23

Abstract

A diffractive optical element (DOE) for use with first, second and third wavelengths includes a first diffractive element on a first surface of a first material, the first diffractive element including first features for diffracting the third wavelength, all of the first features having an etch depth of about λ1/(n1-1) or integer multiples thereof, where λ1 is the first wavelength and n1 is the refractive index of the first material for the first wavelength, the first features providing a first phase function for the second wavelength having a high efficiency in the first order and a second phase function for the third wavelength having a high efficiency in the first order. A diffractive optical element includes a first diffractive element in a first material and a second diffractive element in a second material, the second material having a different dispersion than the first material, the first and second diffractive elements providing a combined phase function to a design wavelength having higher diffraction efficiency than provided by the first and second diffractive elements alone.

Description

DIFFRACTIVE OPTICAL ELEMENT, OPTICAL SYSTEM INCLUDING THE SAME AND ASSOCIATED METHODS

BACKGROUND OF THE INVENTION

1. Field of the Invention

[0001] The present invention is directed to a diffractive optical element (DOE).

2. Description of Related Art

[0002] Numerous applications require a single objective lens to be used for multiple wavelengths. In many such cases, there are three wavelengths for which the lens is to be used. For example, in blue laser based digital video disc (DVD) systems, it is desirable that these systems remain backwards compatible with red laser DVD systems and compact disc (CD) systems, which use infrared (IR) lasers. Each different color may require different focal lengths and/or different numerical apertures, depending on the corresponding media.

[0003] FIGS. 5A-5C illustrate the different requirements for the three different systems. FIG. 5A illustrates a schematic side view of the blue-ray system, FIG. 5B illustrates a schematic side view of the blue-ray system, and FIG. 5C illustrates a schematic side view of the blue-ray system. Here, only cover portions of the media are shown for ease of illustration, so the beam will be focused beyond these cover portions. As can be seen therein, the different systems use the same objective lens 50 and the same DOE corrector 58, while requiring different numerical apertures (NAs) and having different media thicknesses. To get the NAs correct while maintaining reasonable working distances and minimizing differences in the focal lengths, the beam diameter may be changed across the systems. More particularly, the blue-ray system has a beam 51 that is roughly 3 mm which is to be focused onto a blue-ray media 52, the DVD system has a beam 53 that is roughly 2.5 mm which is to be focused onto a DVD media 54, and CD system has a beam 55 that is roughly 2 mm which is to be focused onto a CD media 56.

[0004] Other DOE correctors do not provide very high diffraction efficiency for all three wavelengths, especially when the input beams are collimated, are sensitive to temperature changes and are difficult to manufacture. SUMMARY OF THE INVENTION [0005] The present invention is therefore directed to a diffractive optical element

(DOE), a DOE corrector, an optical system using the DOE corrector and methods of making the DOE, DOE corrector and system, which substantially overcome one or more of the problems due to the limitations and disadvantages of the related art. [0006] It is a feature of an embodiment of the present invention having a high efficiency in a first order for two wavelengths and a high efficiency in the zero order for a third wavelength. [0007] It is another feature of an embodiment of the present invention to provide a thin DOE corrector. [0008] It is still another feature of an embodiment of the present invention to provide a DOE corrector having multiple levels. [0009] It is yet another feature of an embodiment of the present invention to provide a DOE corrector having which is relatively insensitive to changes in temperature. [0010] It is still another feature of an embodiment of the present invention to provide a DOE corrector having which is relatively easy to manufacture. [0011] It is yet another feature of the present invention to provide a DOE corrector on two surfaces. [0012] It is yet another feature of an embodiment of the present invention to provide a DOE having a diffractive portion in two materials having different dispersions providing a single phase function.

BRIEF DESCRIPTION OF THE DRAWINGS [0013] The above and other features and advantages of the present invention will become readily apparent to those of skill in the art by describing in detail embodiments thereof with reference to the attached drawings, in which [0014] FIG. 1A is a schematic side view of a DOE corrector in accordance with a first embodiment of the present invention; [0015] FIG. 1 B is a schematic side view of a DOE corrector in accordance with a second embodiment of the present invention; [0016] FIG. 1C is a schematic side view of a DOE corrector in accordance with a third embodiment of the present invention;

[0017] FIG. 1 D is a schematic side view of a DOE corrector in accordance with a fourth embodiment of the present invention;

[0018] FIG. 2 is a plot of the structure of the diffractive element for red light on the fused silica side, from the center of the element outwards;

[0019] FIG. 3 is a schematic side view of a DOE corrector of the present invention aligned with a lens to be corrected according to an embodiment of the present invention;

[0020] FIG. 4 is a schematic perspective view of a DOE corrector of the present invention aligned with a lens to be corrected according to another embodiment of the present invention; and

[0021] FIG. 5A-5C are schematic side views illustrating a problem to be solved by the present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0022] The present invention will now be described more fully hereinafter with reference to the accompanying drawings, in which preferred embodiments of the invention are shown. The invention may, however, be embodied in different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the concept of the invention to those skilled in the art. In the drawings, the thickness of layers and regions are exaggerated for clarity. It will also be understood that when a layer is referred to as being "on" another layer or substrate, it may be directly on the other layer or substrate, or intervening layers may also be present. Further, it will be understood that when a layer is referred to as being "under" another layer, it may be directly under, or one or more intervening layers may also be present. In addition, it will also be understood that when a layer is referred to as being "between" two layers, it may be the only layer between the two layers, or one or more intervening layers may also be present. Like numbers refer to like elements throughout. [0023] According to a first embodiment of the present invention as shown in FIG.

1A, a DOE corrector 3 has first diffractive element 2 on a single surface 1 thereof. The diffractive element 2 includes a first phase function providing a high first order efficiency for red light and a second phase function providing a high first order efficiency for IR light, while providing high zeroth order efficiency for blue light. In order to achieve this, a thick DOE needs to be used. For example, to make phase levels that are multiples of 2π for the blue wavelength, the phase delay for a transmission DOE is given by:

2π(n-1 )d/λ (1 ) where n is the index of refraction of the DOE for blue light, d is the thickness of the DOE and lambda is the wavelength of the blue light. The 2π thickness D for each wavelength and corresponding refractive index is given by:

D= λ/(n-1) (2)

[0024] Thus, for example, if a DOE is designed to transmit 407n m (blue light), impart the first phase function on 650 nm (red light) and impart the second phase function on 785 nm (IR), since 785 nm is nearly twice 407 nm, levels which effect 785 nm but would not effect 407 nm need to be determined. The phase levels would be determined from integer multiples M of D that do not effect the blue light. For most materials this results in very thick elements with relatively low efficiency, especially in the IR, e.g., less than 50%.

[0025] Thus, when using fused silica, the first embodiment is limited to a binary

DOE for IR light, unless a very thick diffractive structure, e.g., much thicker than 65 microns, is used. Such a binary DOE has very low efficiency, roughly 40%, compared with roughly 80% for a four-level DOE. Thicker DOEs are a problem, as they are more difficult to fabricate, and generally don't perform as well due to shadowing. Shadowing is due to the relative aspect ratios of the etch depth and the period. For manufacturability, this aspect ratio should be less than about two, and the etch depth should less than about 35 microns. Materials other than fused silica, such as plastic, may be used, as these materials have a larger dispersion than for fused silica, allowing the phase delay ratio to exceed 2.0 and move further from the harmonic. However, in these higher dispersion materials, the proper operation of the first phase function for the red light becomes a problem, especially while achieving proper operation of the second phase function.

[0026] As noted above, the use of fused silica for DOE correctors may not work well for beams at 405nm and 785nm, since the phase delay ratio of these wavelengths in fused silica if very close to 1 :2, making the only manufacturable harmonic structure practical a binary lens, which is very inefficient. This structure in fused silica alone may provide sufficient diffraction efficiencies for certain applications. For example, a minimum diffraction efficiency for only reading from a media may be about 25%, while a minimum diffraction efficiency for writing to a media may be about 60-70%. As used herein "high diffraction efficiency" means a minimum diffraction efficiency required for the desired use.

[0027] In a second embodiment, shown in FIG. 1B, two diffractive elements 12 and 14 are used. The first diffractive element 12 diffracts two of the three wavelengths, phase levels for a first phase function at a first wavelength, e.g., 650 nm, are selected that correspond to a zero phase delay (modulo 2ττ) or about zero phase delay for the other two wavelengths, e.g., 407 nm and 785 nm. For a second phase function at a second wavelength, e.g., 785 nm, phase levels are chosen to correspond to zero for the other two wavelengths, e.g., 407 nm and 650 nm. Assume the phase levels are provided in a material having no dispersion and a refractive index of 1.46. For simplification, consider only solutions MD for blue light. In designing the second phase function and restricting the multiple of D to M<IO, and then looking for values of M within this range where the phase angle for the red light is less than ±20°, then there are five values for M which satisfy this condition. However, these phase levels also need to provide phase angles close to 0°, 90°, 180° and 270° for a four phase level diffractive for the IR light. Only three of the five values are within ±20° of these target values. A diffractive other than a binary diffractive would thus need to be made with more than a thickness of M=40 at 407 nm, i.e., more than 35 microns thick.

[0028] Since the refractive index of fused silica actually decreases as wavelength increases, i.e., positive dispersion, the refractive index of fused silica is actually 1.470 at 405 nm, 1.457 at 650 nm, and 1.453 at 785 nm. This dispersion results in the blue and IR light becoming even more closely harmonic, as can be seen with reference to the following phase delay ratio of Equation (3):

Without dispersion, i.e., when n_B = ΠIR, this phase delay ratio is 1.93, while in fused silica, it becomes 2.01. With these refractive indices, when M is selected to be an integer for the blue light, then phase values for the IR light will all be within ±10° of either 0° or 180° for all values of M<75, resulting in a DOE having a thickness of at least 65 microns to realize even a four level DOE.

[0029] In order to address the potential problems of harmonic influence associated with using fused silica alone the second embodiment, a DOE corrector 5 in accordance with the second embodiment of the present invention is shown in FIG. 1 B. The DOE corrector 5 includes a substrate 10 having a first diffractive 12 in a first material providing a harmonic phase delay, i.e., the phase delay ratio of equation 3 is approximately an integer, and a second diffractive 14 in a second material providing a non-harmonic phase delay, i.e., so that the first and third wavelengths are treated substantially differently. For example, the substrate 10 may be a harmonic phase delay material, such as fused silica, into which the first diffractive 12 is etched, and then a non-harmonic phase delay material 16 may be provided on an opposite side of the substrate 10 in which the second diffractive 14 is formed. For example, an embossable material, such as a polymer, may be used as the non-harmonic phase delay material 16, and the second diffractive 14 may be stamped into the embossable material.

[0030] Conventionally, when designing a diffractive which is to provide a high efficiency zero-th order beam for a particular wavelength, the etch depths in the diffractive are set to be 2π multiples for that wavelength, so the diffractive structure essentially does not effect light at that wavelength, i.e., the phase delay will be negligible. In accordance with the second embodiment of the present invention, in designing the first diffractive 12 for use with the red light, the diffractive etch depths are limited to be 2τr multiples of the IR light, rather than the blue light, since the IR light is practically a harmonic of the blue light. In other words, the diffractive etch depths are limited to be 4π multiples of the blue light. After determining thickness values that are close to those multiples of 4ττ, those that also have phase values at or near fractional phase values of 2π for the red light are chosen. For example, if a sixteen phase level structure is to be provided in fused silica, then the target (modulo 2ττ) phase values for the red light are given by:

2τr*i/16 (4) where i varies from 0 to 15.

[0031] On the non-harmonic phase delay side, the second diffractive element is designed to provide a high efficiency first order for the IR light. The second diffractive element is designed by selecting a maximum phase error for each wavelength not to be effected by the second diffractive element, here the blue and red light. Then, all levels that are equal to integer multiples of 2ττ, within the maximum phase error, are determined for the blue light. The maximum phase error for each wavelength may be the same. Then those levels that are not also within a maximum phase error of 2π for the red light are eliminated. Finally, the remaining levels are then selected in accordance with equation (1 ) for the IR light. The non-harmonic phase delay material may be Tiθ₂, SU-8, ultra-violet (UV) curable polymers, thermally curable polymers, or semi-fluorinated polymer, such as Perfluorocyclobutane (PFCB) polymer, e.g., BPVE polymer from Tetramer Technologies, having an appropriate dispersion.

[0032] Numerous levels satisfying the above conditions are available for creating both diffractive elements, allowing efficient DOE corrector to be created. For example, if using fused silica and only diffracting 660 nm into the first order, while 407 nm and 785 nm are substantially directed into the zero-th order, i.e., the etch depths are at 2π multiples of 785 nm, within a 20 degree error and restricting M to less than twenty, four levels satisfy these requirement, i.e., M=O, M=2, M=14 and M=16 for 407 nm. Better performance may be realized in practice by also considering etch depths that are not exact 2π multiples of blue light, e.g., within a 20 degree error as for the IR. Using this method, if the maximum etch depth of the fused silica material is nine microns, a practical diffractive optical element may be formed in the fused silica having between four and twelve levels. If the maximum etch depth of the thin film, e.g., a UV curable polymer noted above, is fifteen microns, a diffractive optical element formed therein may have between four and eight levels. Again, the limitations on the etch depth is due to shadowing and vector diffraction effects due to the aspect ratio.

[0033] A specific example of a structure for the first diffractive element according to the second embodiment is shown in FIG. 2. As can be seen therein, the period and etch depth across the diffractive element may be varied.

[0034] A third embodiment of the present invention is shown in FIG. 1C. A DOE corrector 30 includes a first diffractive element 32 on a surface of a substrate 31 and a second diffractive element 34 on a surface of a non-harmonic phase delay material 36 on an opposite side of the substrate 31 from the first diffractive element. The non-harmonic phase delay material 36 may be any of the polymers noted above. In the third embodiment, the two diffractive elements 32 and 34 are used to create a single phase function for at least one of the wavelengths, as opposed to the dedicated phase functions of diffractive elements 12 and 14 in the second embodiment.

[0035] Here, the low dispersion material, e.g., the fused silica, may be used to diffract one of the harmonic wavelengths, partially diffract another wavelength and transmit the other harmonic wavelength. Examples of available phase levels in fused silica are provided below in the following Table. All phase delays are given in waves (modulo 2pi) and the phase delay is 0 or 1 wave (2pi) for 405 nm.

This resultant effective diffractive is equivalent to a five level, evenly spaced diffractive for 660 nm and a binary diffractive for 785 nm. [0036] In the above example, the scalar diffraction efficiency for the element only in fused silica is 100% for 405 nm, 87% for 660 nm and 40% for 785 nm. This low efficiency for the IR light is further compounded due to the presence of 40% in the -1 order as well. Light outside a desired diffraction order, here +1 for 660 nm and 785 nm, and 0^th for 405 nm, should be minimized as well as maximizing the light in the desired diffraction order. If a higher diffraction efficiency for 785 nm is desired, since the 0 and π levels for 785 nm are now provided in the fused silica element, only a few levels are required in the polymer side to increase the diffraction efficiency for 785 nm. For example, the polymer side may have two levels, e.g., 0 and 0.25 waves or three levels, e.g., evenly spaced between 0 and 0.5 wave, i.e., 0, 1/6, and 1/3 waves. This would then provide a resultant effective diffractive for 785 nm having a total of four levels or six levels, respectively.

[0037] By providing 0 and π levels in the fused silica for 785 nm, the polymer may be made much thinner. For example, in accordance with the second embodiment, using fused silica and a BPVE polymer from Tetramer Technologies, the fused silica part is 8.7 microns thick and the polymer part is over 16 microns thick. This resulted in a combined diffraction efficiency for the part to be 94%, 81 % and 69% for 405 nm, 660 nm and 785 nm, respectively. In accordance with the third embodiment, using the same materials, the fused silica part is 7.78 microns and the polymer part was reduced to binary structure of 4.98 microns, which provides a phase value of approximately zero radians for both 405 nm and 660 nm, and a phase delay of approximately 0.25 waves for 785 nm. The resultant combined diffraction efficiency is then 97%, 85% and 80% for 405 nm, 660 nm and 785 nm, respectively. When using three levels in the polymer, now having a thickness of 6.34, the resultant combined diffraction efficiency is then 97%, 85% and 88% for 405 nm, 660 nm and 785 nm, respectively.

[0038] Since both diffractive elements 32 and 34 in the third embodiment diffract a common wavelength, the alignment between the two elements is more critical than in the second embodiment, and the spacing between the diffractive elements must be tightly controlled. One manner of achieving this control is shown in FIG. 1 D.

[0039] A fourth embodiment of the present invention is shown in FIG. 1 D. Here, a DOE corrector 40 includes a first diffractive element 42 on a first substrate 41 and a second diffractive element 44 on a layer 46 on a second substrate 43. A securing element 45 may secure the first and second substrates 42, 43 together. In the fourth embodiment, the first and second diffractive elements are on opposing surfaces of different substrates which have been secured together, rather than on opposite sides of the same substrate as in the second and third embodiments. This allows the diffractive elements to be made closer together. The spacing between the two diffractive elements is particularly important when the two surfaces create a single phase function, e.g., in accordance with the third embodiment, as opposed to two independent phase functions. For the third embodiment, the two diffractive elements need to be close enough so that there are no significant diffractive effects between them. The smaller the feature sizes, the closer the two diffractive elements will need to be. The alignment may be realized in numerous manners, e.g., wafer-to-wafer bonding, injection molding the polymer part including alignment features to mate with corresponding features on the fused silica part, insert injection mold the polymer part around the fused silica part, or replicate the polymer part directly on top of the fused silica part. When providing two independent phase functions on the two substrates, the alignment and spacing is not very sensitive, and the separate substrates do not even need to be immediately adjacent.

[0040] A further alternative using two substrates would include securing a surface of a wafer having the first diffractive element to a second wafer, thinning the second wafer to a desired thickness, e.g., 20 microns, and depositing the second material in which the second diffractive element is to be formed.

[0041] To adjust Numerical Aperture (NA) in any of the embodiments, scattering regions can be created to scatter light at 785 nm or at both 785 and 660 nm, to reduce the effective aperture for these wavelengths. For example, in order to reduce the effective aperture for 785 nm, the above design approach can still be used, but the desired phase function for 785 nm becomes a binary grating with at a radius larger than a desired effective aperture for 785 nm at a high enough spatial frequency to cause light at 785 nm to be diffracted in to a region large enough to significantly reduce the signal power at the disk.

[0042] Further, for any of the embodiments, an anti-reflective coating may be provided on the DOE corrector, the input beams may be collimated and the DOE corrector may also correct for aberrations and dispersion.

[0043] FIG. 3 illustrates the DOE corrector 5 of FIG. 1 B aligned with a lens 24 to be corrected, when the lens 24 is roughly a sphere. As can be seen in FIG. 3, a substrate 28 is patterned and etched to form a hole 22 therein. This hole 22 receives the lens 24, which may be secured in the hole by using an adhesive 26, e.g., solder. The lens 24 may be polished to flatten a surface 25 thereof to be about even with a surface of the substrate 28, as shown in FIG. 3, or may remain in its original form. The substrate 28 is then aligned with the DOE corrector 5 and these components may be secured together, e.g., using a bonding material 21 , as shown in FIG. 3. The DOE corrector 5 and the substrate 28 may be aligned and secured as a plurality of elements, e.g., on a wafer level. Then, a resultant optical element 20 may be realized by separating the wafer containing multiple resultant optical elements 20 along lines 138.

[0044] FIG. 4 illustrates the DOE corrector 5 of FIG. 1 B aligned with a lens 48 to be corrected, when the lens 48 is provided on a surface of a substrate 49. The DOE corrector 5 and the substrate 49 may be die bonded together on a multiple scale or individually.

[0045] Thus, in accordance with the present invention, a DOE corrector for use with three wavelengths may be provided. The DOE corrector may be formed by providing a first diffractive element in a harmonic phase delay material and a second diffractive element in a non-harmonic phase delay material. For example, assuming the harmonic relationships between the wavelengths is two, the phase delay ratio may be less than 1.95 or greater than 2.05 in the non- harmonic phase delay material, and within these bounds for the harmonic phase delay material. The DOE corrector may face either direction. Further, both the harmonic and non-harmonic phase delay materials may be provided on opposite sides of a substrate, or the substrate may be the non-harmonic phase delay material. Preferably, the diffraction efficiency for a desired order is greater than about 70%. Finally, a diffractive optical element having two diffractive surfaces providing a single phase function may be realized in two materials having different dispersions, such that the combined efficiency is greater than either efficiency alone. Embodiments of the present invention have been disclosed herein and, although specific terms are employed, they are used and are to be interpreted in a generic and descriptive sense only and not for purpose of limitation. For example, while a spherical lens has been illustrated, other shapes, using different alignment mechanisms, may be used. Accordingly, it will be understood by those of ordinary skill in the art that various changes in form and details may be made without departing from the spirit and scope of the present invention as set forth in the following claims.

Claims

What is claimed is:

1. A diffractive optical element (DOE) for use with first, second and third wavelengths, comprising: a first diffractive element on a first surface of a first material, the first diffractive element including first features for diffracting the third wavelength, all of the first features having an etch depth of about λ-ι/(ni-1 ) or integer multiples thereof, where A₁ is the first wavelength and ni is the refractive index of the first material for the first wavelength, the first features providing a first phase function for the second wavelength having a high efficiency in the first order and a second phase function for the third wavelength having a high efficiency in the first order.

2. The DOE corrector as claimed in claim 1 , further comprising a second diffractive element on a second surface of a second material.

3. The DOE corrector as claimed in claim 2, wherein each of the first and second diffractive elements includes a difference between phase levels of more than 2π for at least one of the three different wavelengths.

4. The DOE corrector as claimed in claim 2, wherein the second diffractive element only diffracts the second wavelength.

5. The DOE corrector as claimed in claim 2, wherein the first features also diffract the second wavelength.

6. The DOE corrector as claimed in claim 5, wherein the second diffractive element diffracts the third wavelength.

7. The DOE corrector as claimed in claim 6, wherein a total diffractive efficiency of the DOE corrector for the third wavelength is greater than a first diffractive efficiency of the first diffractive element for the third wavelength and a second diffractive efficiency of the second diffractive element for the third wavelength.

8. The DOE corrector as claimed in claim 6, wherein the second diffractive element reduces light other than in a desired diffractive order for the third wavelength.

9. The DOE corrector as claimed in claim 6, wherein the second diffractive element only diffracts the third wavelength.

10. The DOE corrector as claimed in claim 2, wherein the first material and the second material are on different substrates secured together, the first and second diffractive elements facing each other.

11. The DOE corrector as claimed in claim 1 , wherein the second material is provided directly on the first material.

12. The DOE corrector as claimed in claim 1 , wherein the second material is secured to the first material.

13. The DOE corrector as claimed in claim 1 , wherein the second material is more dispersive than the first material.

14. The DOE corrector as claimed in claim 1 , wherein the first features have a higher spatial frequency towards an outer edge of the DOE corrector.

15. The DOE corrector as claimed in claim 14, wherein the first features at the higher spatial frequency scatter light at the third wavelength.

16. A diffractive optical element, comprising: a first diffractive element in a first material; and a second diffractive element in a second material, the second material having a different dispersion than the first material, the first and second diffractive elements providing a combined phase function to a design wavelength having higher diffraction efficiency than provided by the first and second diffractive elements alone.

17. An optical system, comprising: a refractive optical element; and a diffractive optical element (DOE) corrector for use with three different wavelengths and aligned with the refractive optical element, the DOE corrector including a first diffractive element on a first surface of a first material, the first diffractive element including first features for diffracting the third wavelength, all of the first features having an etch depth of about λi/(n-ι-1 ) or integer multiples thereof, where λi is the first wavelength and ni is the refractive index of the first material for the first wavelength, the first features providing a first phase function for the second wavelength having a high efficiency in the first order and a second phase function for the third wavelength having a high efficiency in the first order.

18. A method of creating an optical apparatus, comprising: forming a first diffractive element in a first material; and forming a second diffractive element in a second material, the second material having a different dispersion than the first material, the first and second diffractive elements providing a combined phase function to a design wavelength having higher diffraction efficiency than provided by the first and second diffractive elements alone.

19. The method as claimed in claim 18, wherein the first material is on a different substrate than the second material.

20. The method as claimed in claim 19, wherein the first material is on a same substrate as the second material.

21. The method as claimed in claim 20, wherein the first material is the substrate.

22. The method as claimed in claim 18, further comprising: aligning and securing a refractive optical element to the first and second diffractive elements.