Abstract
The invention relates to optical devices comprising a transparent substrate and a first transparent grating layer on the substrate, the grating layer comprising periodically alternating zones having different refractive indices. According to the invention, the device comprises a second transparent grating layer located on top of the first grating layer and also comprising periodically alternating zones having different refractive indices so that the zones of the first grating layer having higher refractive index are at least partly aligned with the zones of the second grating layer having lower refractive index and vice versa, the second grating layer reducing the amount of light diffracted to non-zero transmission orders. The invention allows for reducing the so-called rainbow effect for example in head-up displays (HUDs).
Claims
1. An optical device wherein the device is a near-to-eye display (NED) or an exit pupil expander (EPE) or part thereof comprising: a transparent substrate, a grating on or within the substrate, the grating comprising a first transparent grating layer which further comprises periodically alternating zones having different refractive indices, wherein the grating has a second transparent grating layer located on the first grating layer and also has periodically alternating zones having different refractive indices so that the zones of the first grating layer having higher refractive index are at least partly aligned with the zones of the second grating layer having lower refractive index and vice versa, wherein said first and second grating layers diffract light to produce a first transmission order having a diffraction efficiency and a first reflection order having a diffraction efficiency, wherein the period, layer thicknesses and refractive indices of the first and second grating layers are adapted to allow diffraction of light by the grating to non-zero reflection orders and to make the diffraction efficiency of the first transmission order lower than the diffraction efficiency of the first reflection order over the wavelength range of 450-650 nm for preventing visible rainbow effect caused by transmitted light; wherein said grating is an out-coupling grating adapted to diffract light directed to the out-coupling grating from the substrate on the surface of which or within which the out-coupling grating is located; and an in-coupling grating adapted to diffract light from the outside of the substrate into the substrate and further towards the out-coupling grating so that light propagates in the light-guiding substrate via total internal reflections.
2. The optical device according to claim 1, wherein the diffraction efficiency of the first transmission order is no more than 0.4% and the diffraction efficiency of the first reflection order at least 3% over the wavelength range of 450-650 nm.
3. The optical device according to claim 1, wherein the first and second grating layers have the same grating period and each comprise two types of zones having different refractive indices with a single grating period.
4. The optical device according to claim 1, wherein the first and second grating layers are of equal thickness.
5. The optical device according to claim 1, wherein the first and second grating layers are non-identical in their material properties and have different thicknesses.
6. The optical device according to claim 1, wherein at least one of the refractive indices of the second grating layer is the same as in the first grating layer.
7. The optical device according to claim 1, wherein the first and second grating layers are periodic in the same direction or directions.
8. The optical device according to claim 1, wherein the zones of the first grating layer having higher refractive index are fully aligned with the zones of the second grating layer having lower refractive index and vice versa.
9. The optical device according to claim 1, wherein the second grating layer has a similar internal structure as the first grating layer but being laterally shifted by half of the grating period in the periodic direction of the grating.
10. The optical device according to claim 1, wherein the first and the second grating layers are separated by a uniform dielectric layer.
11. The optical device according to claim 1, wherein at least some of the zones in the first and/or second grating layer comprise the same material as the substrate or material that has nearly the same refractive index as the substrate.
12. The optical device according to claim 1, wherein the grating is provided on a surface of the substrate and has a coating layer on the other side of the grating, whereby at least some of the zones in the first and/or second grating layer comprise the same material as the coating layer.
13. The optical device according to claim 1, wherein a period of the first and second grating layers is between 300 nm and 1500 nm and the layer thicknesses of the first and second grating layers are between 5 nm and 200 nm.
14. The optical device according to claim 1, wherein the lower refractive index in each of the first and second grating layer is between 1.3 and 1.7 and the higher refractive index in each of the first and second grating layer is between 1.5 and 2.2.
15. The optical device according to claim 1, further comprising a light projector capable of illuminating the in-coupling grating on the surface the substrate.
16. The optical device according to claim 1, wherein the two layer grating structure is doubly periodic.
17. The optical device according to claim 1, wherein the device is a transparent element used in the construction industry, as a lighting apparatus or as a visual aid such as eyewear.
Description
BRIEF DESCRIPTION OF THE DRAWINGS
(1) FIG. 1 shows a general representation of a grating structure according to the invention.
(2) FIG. 2 shows a grating structure according to one embodiment.
(3) FIG. 3a shows a grating structure according to another embodiment.
(4) FIG. 3b shows calculated diffraction efficiency of the first reflection (R.sub.+1) and transmission (T.sub.+1) order as a function of free space wavelength for a double layer grating structure according to FIG. 3a with exemplary dimensions and refraction indices.
(5) FIG. 3c shows calculated diffraction efficiency of the first reflection (R.sub.+1) and transmission (T.sub.+1) order as a function of free space wavelength for a single layer grating structure that is otherwise identical than the structure modeled in FIG. 3b but it contains only one grating layer.
(6) FIG. 4a shows a grating structure according to another embodiment.
(7) FIG. 4b shows calculated diffraction efficiency of the first reflection (R.sub.+1) and transmission (T.sub.+1) order as a function of free space wavelength for the structure according to FIG. 4a with exemplary dimensions and refraction indices.
(8) FIG. 5a shows a grating structure realized using metal plating according to another embodiment.
(9) FIG. 5b shows calculated diffraction efficiency of the first reflection (R.sub.+1) and transmission (T.sub.+1) order as a function of free space wavelength for the structure according to FIG. 5a with exemplary dimensions and refraction indices.
(10) FIG. 5c shows calculated diffraction efficiency of the zeroth transmission order (T.sub.0) as a function of free space wavelength for the structure modeled in FIG. 5b.
(11) FIG. 5d shows calculated diffraction efficiency of the first reflection (R.sub.+1) and transmission (T.sub.+1) order as a function of free space wavelength for the structure that is otherwise identical than the structure modeled in FIG. 5b, but it contains only a single grating layer.
(12) FIG. 6 shows a general representation of a doubly periodic grating structure according to the invention.
DETAILED DESCRIPTION OF EMBODIMENTS
(13) FIG. 1 illustrates a general structure of the two-layer grating according to the invention. The grating comprises a first grating layer 11 and a second grating layer 12. The both grating layers have the same grating period () and are binary. The first grating layer is composed of a periodic pattern of alternating material zones 11A and 11B having different refractive indices n.sub.11 and n.sub.12, respectively. Likewise, the second grating layer is composed of a periodic pattern of alternating material zones 12A and 12B having different refractive indices n.sub.21 and n.sub.22, respectively. On the first side of the two-layer grating there is provided a first optically transparent material layer 10 having a refractive index n.sub.1 and on the second side of the grating there is provided a second optically transparent material layer 13 having a refractive index n.sub.2. The layers 10, 13 on one or both sides of the grating may comprise also air (or vacuum) layers, i.e. lack any solid material.
(14) A simplified and practically more feasible structure is shown in FIG. 2. The structure comprises a first grating layer 21 and a second grating layer 22, like in FIG. 1. Further, the first grating layer is composed of a periodic pattern of alternating material zones 21A and 21B having different refractive indices n.sub.11 and n.sub.1, respectively. Likewise, the second grating layer is composed of a periodic pattern of alternating material zones 22A and 22B having different refractive indices n.sub.21 and n.sub.2, respectively. The essential difference to FIG. 1 is that the material layers 20, 23 on each side of the grating layers 21, 22 continue seamlessly from the grating zones 21A and 22A, respectively.
(15) A still more simplified structure is shown in FIG. 3a. The structure comprises a first grating layer 31 and a second grating layer 32, like in FIGS. 1 and 2. The grating layers are composed of periodic patterns of alternating material zones 31A, 31B; 32A, 32B having different (within each layer) refractive indices n, n.sub.1; n, n.sub.2, respectively. Also in this embodiment, the material layers 30, 33 on each side of the grating layers 31, 32 continue seamlessly from the grating zones 31A and 32A, respectively. In this configuration, the materials at one zone 31B, 32B of each of the grating layers 31, 32 are the same and therefore the zones 31B, 32B have the same refractive index n.
(16) It is not excluded that the material zones 31A and 32A would also be made of the same material, i.e., that n.sub.1=n.sub.2, whereby only two different materials would be needed to make the proposed structure. The same holds for other structures described herein. Referring to FIG. 1, according to one embodiment, the n.sub.11 zone (n.sub.12 zone) of the first grating layer has the same refractive index as the n.sub.21 zone (n.sub.22 zone) of the second grating layer. This embodiment provides for optimal suppression of the odd transmitted diffraction orders when the grating layers are of equal thickness. If n.sub.11n.sub.21 or n.sub.12n.sub.22, then the optimal suppression may be obtained with grating layers having unequal thicknesses.
(17) FIG. 3b shows the diffraction efficiency of the first transmission (T.sub.+1) and the first reflection (R.sub.+1) order as a function of the free space wavelength (.sub.0) for the structure according to FIG. 3a with the following parameters: n.sub.1=n.sub.2=1.7, n=1.3, h.sub.1=h.sub.2=50 nm and =450 nm. The structure is illuminated by a normally incident plane wave. FIG. 3c shows the same results for the structure that is otherwise identical than the structure modeled in FIG. 3b but it contains only one grating layer. Clearly, T.sub.+1 is much weaker in FIG. 3b than in FIG. 3c. All the modeling results presented in this patent application were obtained with the Fourier modal method (also known as rigorous coupled wave analysis) that utilizes the correct Fourier factorization rules to obtain good convergence also with metallic grating structures.
(18) FIG. 4a shows a modified structure of the grating in which the alternating material zones material zones 41B, 42B overlap each other in the direction normal to the grating. Thus, there is a unified layer of material with refractive index n between the actual grating layers 41, 42. Also in this embodiment, the material layers 40, 43, having refractive indices n.sub.1 and n.sub.2, respectively, on each side of the grating layers 41, 42 continue seamlessly from the grating zones 41A and 42A, respectively.
(19) FIG. 4b shows the diffraction efficiency of the first transmission (T.sub.+1) and the first reflection (R.sub.+1) order as a function of the free space wavelength (.sub.0) for the structure according to FIG. 4a with the following parameters: n.sub.1=n.sub.2=1.7, n=1.3, h.sub.1=50 nm, h.sub.2=80 nm and =450 nm. The structure is illuminated by a normally incident plane wave. As compared to FIG. 3b, the 30 nm thick uniform dielectric layer between the 50 nm thick grating layers enhances R.sub.+1. Also T.sub.+1 slightly increases but it is still significantly lower than in FIG. 3c.
(20) FIG. 5a shows still another embodiment. In this embodiment, the desired double grating is formed by a substrate 50 provided with ridges 51A and having thin layers 54B of metal, such as gold or silver, or some high refractive index material, such as indium tin oxide (ITO), provided in the bottom of each groove 52A and on each ridge 51A formed between the grooves 52A. On the second side of the structure, there is provided an inversely shaped layer 53, 52A.
(21) FIG. 5b shows the diffraction efficiency of the first transmission (T.sub.+1) and the first reflection (R.sub.+1) order as a function of the free space wavelength (.sub.0) for the structure according to FIG. 5a with the following parameters: n.sub.1=n.sub.2=1.5, n=wavelength dependent refractive index of silver (CRC handbook of Chemistry and Physics, 83.sup.rd edition), t=50 nm, and =450 nm. The silver zones are 10 nm thick and the structure is illuminated by a normally incident plane wave. FIG. 5b shows the diffraction efficiency of the zeroth transmission order for the same structure. The mean spectral zeroth order transmittance is over 60%. FIG. 5c shows the same results for the structure that contains only single silver grating layer (as in the structure modeled in FIG. 5b, n.sub.1=n.sub.2, the structure can be considered to consist of two metallic grating layers separated by a uniform dielectric layer). By comparing FIGS. 5b and 5c, it can be clearly seen that the two layer grating structure diffracts significantly less light into the first transmitted order than the single layer structure.
(22) The embodiments presented so far have been periodic only in one direction. All the presented embodiments can be implemented also as doubly periodic (also called as biperiodic) structures. A doubly periodic version of the structure of FIG. 3a is shown in FIG. 6. It should be noted that only one unit cell of the doubly periodic grating is illustrated in FIG. 6. The grating consists of two grating layers. The unit cell of each grating layer contains four rectangular zones with equal height, width and depth. Each layer consists of two materials with different refractive indeces. The rectangular material zones in the unit cell of each grating layer are arranged in a checkerboard pattern. The grating layers are aligned so that the zones of the first grating layer having higher refractive index are aligned with the zones of the second grating layer having lower refractive index and vice versa.
(23) In all of the above the ridge and groove regions in both gratings layers are preferably of equal width. In all of the above examples, the alternating zones of the grating layers are shown as fully aligned with each other in the lateral direction of the grating, providing an optimal performance. However, the structure is expected to work also when the zones are partly aligned, e.g. if there is a displacement less than quarter of the period of the grating from the optimal situation.
(24) The two grating layers may be directly superimposed or be separated by a distance, which is typically less than the width of the ridges and grooves.
(25) The structures of FIGS. 2, 3a, 4a and 5a can all be manufactured by a) providing an optically transparent bottom substrate having a refractive index n.sub.1, b) manufacturing a sequence of grooves and ridges to the bottom substrate, c) depositing into the grooves first zones of optically transparent material having a refractive index n.sub.11 or n to complete the first grating layer, d) depositing on the ridges second zones of optically transparent material having a refractive index n.sub.21 or n, e) depositing between the second zones, and, optionally also on top of the second zones as a uniform coating layer, optically transparent material having a refractive index n.sub.2 (which can be but does not need to be equal to n.sub.1).
(26) In the case of the structure of FIG. 3, the manufacturing steps (c) and (d) can be accomplished by a single deposition. That is, when the grooves of the first grating are filled by a material having refractive index n, the ridge zones of the second grating layer are simultaneously formed.
(27) The grooves and ridges to the substrate may be provided using any known microfabrication technique, such as mechanical engraving, (hot) embossing, laser (e-beam) fabrication, etching or material deposition technique such as nanoimprinting.
(28) Deposition of the material zones of the grating layers with a refractive index different from the substrate and top layer preferably takes place using printing methods, such as gravure, reverse-gravure, flexographic and screen printing, coating methods, spraying methods, or commonly known thin film deposition methods such as thermal evaporation, sputtering and atomic layer deposition.
(29) The top layer may be provided by a suitable coating, spraying or printing method.
(30) The substrate and top layer materials may comprise e.g. glass, polystyrene (PS), Polyethylene terephthalate (PET), Poly(methyl methacrylate) (PMMA), polycarbonate, cellulose acetate, polyvinylpyrrolidone, or ethylcellulose.
(31) The alternative material zones may comprise e.g. sulfonated fluoropolymers like Nafion.
(32) The materials and refractive indices may also be interchanged.
(33) In the case of FIG. 5a, the metal-containing layer may be deposited using e.g. vapour-deposition methods such as chemical vapour deposition (CVD), atomic layer deposition (ALD) or any modification thereof. The thickness of the metal-containing layer may be e.g. 1-50 nm, preferably 5-20 nm.
