Micro-projection system based on hybrid refractive-diffractive lens

Abstract

A micro-projection system based on a hybrid refractive-diffractive lens, relating to the technical field of near-eye display devices. A micro-projection lens set sequentially comprises, along an optical axis and from an exit pupil side (6) to an image source side (7), a first lens (1), a second lens (2), a third lens (3), and a fourth lens (4), wherein the first lens (1) is a plano-convex lens and protrudes toward the exit pupil side (6), the second lens (2) is a biconcave lens, the third lens (3) is a meniscus convex lens and protrudes towards the exit pupil side (6), and the fourth lens (4) is a biconvex lens. The first lens (1) has positive focal power, and is used for deflecting large-angle light rays. The second lens (2), the third lens (3), and the fourth lens (4) are cemented by means of an optical adhesive to form a hybrid refractive-diffractive lens; and the cemented surfaces of the second lens (2), the third lens (3), and the fourth lens (4) are each a superposition of an even asphere and a diffractive surface.

Description

A micro-projection system based on compound diffractive lens Technical Field

[0001] The present invention relates to the technical field of near-eye display devices, and in particular to a micro-projection system based on a compound diffractive lens. Background Art

[0002] With the rapid development of technology in recent years, AR glasses based on near-eye display devices have attracted widespread attention and are showing great potential in various fields, including military, medical, education, and entertainment. Near-eye display devices primarily consist of two parts: a micro-projection system and a near-eye display optical system. The micro-projection system first outputs a virtual image, which is then merged with the real scene by the near-eye display optical system and presented to the user.

[0003] To make near-eye display devices comfortable to wear, micro-projection systems need to be miniaturized and thinner. Micro-projection systems generally consist of a display chip and a projection optical path. However, the projection optical path is limited by the large field of view and high projection quality. Using traditional spherical and aspherical lenses makes system miniaturization difficult, which is not conducive to the integration of near-eye display devices.

[0004] Summary of the Invention

[0005] To this end, an embodiment of the present invention provides a micro-projection system based on a compound diffractive lens, which is used to solve the problem that the micro-projection system of the near-eye display device in the prior art is difficult to meet the requirements of large field of view, high projection quality and miniaturization.

[0006] To address the above-mentioned issues, an embodiment of the present invention provides a micro-projection system based on a compound diffractive lens, comprising: a liquid crystal on silicon display chip, a polarization beam splitter cube, and a micro-projection lens assembly. Light emitted by the liquid crystal on silicon display chip passes through the polarization beam splitter cube and then is projected out through the micro-projection lens assembly, ultimately exiting from the exit pupil and entering the human eye as parallel light at different angles.

[0007] The side closest to the human eye when worn is the exit pupil side, and the side of the silicon-based liquid crystal display chip is the image source side. The micro-projection lens assembly is arranged in order along the optical axis from the exit pupil side to the image source side, including a first lens, a second lens, a third lens, and a fourth lens.

[0008] Among them, the first lens is a plano-convex lens, convex to the exit pupil side, the second lens is a biconcave lens, the third lens is a meniscus convex lens, convex to the exit pupil side, and the fourth lens is a biconvex lens; the optical focal length of the first lens is positive and is used to deflect light at large angles; the second lens, the third lens and the fourth lens are bonded together to form a composite diffractive lens through optical glue; the bonded surface of the second lens, the third lens and the fourth lens is a superposition of an even-order aspheric surface and a diffractive surface.

[0009] Preferably, the focal length of the first lens and the focal length of the micro-projection lens satisfy the relationship: 0≤f1 / f≤2

[0010] Wherein, f1 is the focal length of the first lens, and f is the focal length of the micro-projection lens group.

[0011] Preferably, the focal length of the second lens and the focal length of the micro-projection lens satisfy the relationship: -2≤f2 / f≤2

[0012] Wherein, f2 is the focal length of the second lens, and f is the focal length of the micro-projection lens assembly.

[0013] Preferably, the focal length of the third lens and the focal length of the micro-projection lens satisfy the relationship: -2≤f3 / f≤-2

[0014] Wherein, f3 is the focal length of the third lens, and f is the focal length of the micro-projection lens group.

[0015] Preferably, the focal length of the fourth lens and the focal length of the micro-projection lens satisfy the relationship: 0≤f4 / f≤2

[0016] Wherein, f4 is the focal length of the fourth lens, and f is the focal length of the micro-projection lens group.

[0017] Preferably, two optical surfaces of the first lens, the second lens, the third lens and the fourth lens are all even-order aspheric surfaces; the image source side of the second lens is a superposition of the even-order aspheric surface and the diffraction surface, the exit pupil side of the third lens is a superposition of the diffraction surface and the even-order aspheric surface, the image source side of the third lens is a superposition of the even-order aspheric surface and the diffraction surface, and the exit pupil side of the fourth lens is a superposition of the diffraction surface and the even-order aspheric surface.

[0018] Preferably, the refractive index of the first lens, the second lens and the fourth lens are all 1.81, and the Abbe constant is 25.5; the refractive index of the third lens is 1.50, and the Abbe constant is 81.6.

[0019] Preferably, the refractive index of the optical glue used to glue the second lens and the third lens, and the third lens and the fourth lens together is 1.62.

[0020] Preferably, the phase expression of the diffraction surface is:

[0021] Where M is the diffraction order, A2 and A4 are phase coefficients, r is the distance from a point on the lens to the center of the lens, and R is the lens aperture.

[0022] Preferably, the diffraction orders of the diffraction surface on the image source side of the second lens and the diffraction surface on the exit pupil side of the third lens are 8 and -7 respectively, and the phase coefficients A2 and A4 are -260.47 and 50.17 respectively; the diffraction orders of the diffraction surface on the image source side of the third lens and the diffraction surface on the exit pupil side of the fourth lens are 8 and -7 respectively, and the phase coefficients A2 and A4 are -2201.03 and 1411.65 respectively.

[0023] It can be seen from the above technical solutions that the present invention has the following advantages:

[0024] The present invention provides a micro-projection system based on a compound diffractive lens. This system utilizes a silicon-based liquid crystal display chip as the image source, with the side closest to the human eye as the exit pupil. The system consists of a single-element lens and a compound diffractive lens. The diffractive surface, combined with a high-order aspheric surface, not only achieves achromatic aberration but also balances and reduces other aberrations, achieving high-quality projection while also meeting system miniaturization requirements. Furthermore, because the diffraction efficiency of a single-layer diffractive surface decreases with deviation from the design wavelength and increasing incident angle, leading to a decrease in projected image quality, the present invention utilizes a multi-layer diffractive structure to improve diffraction efficiency. High-refractive-index optical adhesive is used to bond the two diffractive surfaces together to achieve the compound diffractive lens design. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] In order to more clearly illustrate the implementation cases of the present invention or the technical solutions in the prior art, the following is a brief description of the drawings required for use in the embodiments. By referring to the drawings, the features and advantages of the present invention will be more clearly understood. The drawings are schematic and should not be understood as limiting the present invention in any way. Those skilled in the art can derive other drawings based on these drawings without inventive effort. Among them:

[0026] FIG1 is a structural diagram of a micro-projection system based on a compound diffractive lens according to an embodiment;

[0027] FIG2 is a schematic diagram of a multilayer diffraction structure of the present invention;

[0028] FIG3 is a schematic diagram of an MTF curve of the present invention;

[0029] FIG4 is a schematic diagram of field curvature and distortion of the present invention;

[0030] FIG. 5 is a schematic diagram of a vertical axis chromatic aberration curve according to the present invention.

[0031] Reference numerals: 1. first lens; 2. second lens; 3. third lens; 4. fourth lens; 5. polarization beam splitting cube; 6. exit pupil side; 7. image source side. DETAILED DESCRIPTION

[0032] To further clarify the objectives, technical solutions, and advantages of the embodiments of the present invention, the following will provide a clear and complete description of the technical solutions in the embodiments of the present invention, in conjunction with the accompanying drawings. It should be understood that the described embodiments are only a portion of the embodiments of the present invention, not all of them. All other embodiments derived by persons of ordinary skill in the art based on the embodiments of the present invention without inventive effort are intended to fall within the scope of protection of the present invention.

[0033] Example 1

[0034] As shown in FIG1 , an embodiment of the present invention provides a micro-projection system based on a compound diffractive lens, comprising: a liquid crystal on silicon display chip, a polarization beam splitter cube 5, and a micro-projection lens assembly. Light emitted by the liquid crystal on silicon display chip passes through the polarization beam splitter cube 5 and then is projected out through the micro-projection lens assembly, ultimately exiting from the exit pupil as parallel light at different angles and entering the human eye.

[0035] The side closest to the human eye when worn is defined as the exit pupil side 6, and the side of the liquid crystal on silicon display chip is defined as the image source side 7. The micro-projection lens assembly is sequentially arranged along the optical axis from the exit pupil side 6 to the image source side 7, including a first lens 1, a second lens 2, a third lens 3, and a fourth lens 4.

[0036] Among them, the first lens 1 is a plano-convex lens, convex to the exit pupil side, the second lens 2 is a biconcave lens, the third lens 3 is a meniscus convex lens, convex to the exit pupil side, and the fourth lens 4 is a biconvex lens; the optical focal length of the first lens 1 is positive and is used to deflect light at large angles; the second lens 2, the third lens 3 and the fourth lens 4 are bonded together by optical glue to form a composite diffractive lens; the bonded surface of the second lens 2, the third lens 3 and the fourth lens 4 is a superposition of an even-order aspheric surface and a diffractive surface.

[0037] This invention uses a silicon-based liquid crystal display chip as the image source, with the side closest to the human eye as the exit pupil. It consists of a single lens and a compound refractive lens. The diffractive surface combined with a high-order aspheric surface not only achieves achromatic aberration but also balances and reduces other aberrations, achieving high-quality projection while also meeting the requirements of system miniaturization. Furthermore, because the diffraction efficiency of a single-layer diffractive surface decreases with deviation from the design wavelength and increasing incident angle, leading to a decrease in projected image quality, the present invention employs a multi-layer diffractive structure to improve diffraction efficiency. High-refractive-index optical adhesive is used to bond the two diffractive surfaces together to create the compound refractive lens design.

[0038] The micro-projection system in the embodiment of the present invention has an exit pupil diameter D=3.6mm, a field of view of 40°, an operating band of visible light, a selected silicon-based liquid crystal display chip with a resolution of 1280×720, a pixel size of 4.5μm, a diagonal length of 0.26 inches, and a system focal length of 9mm.

[0039] In this embodiment of the present invention, both optical surfaces of the first lens 1, second lens 2, third lens 3, and fourth lens 4 are even-order aspheric surfaces. The image source side of the second lens 2 is a superposition of an even-order aspheric surface and a diffractive surface, while the exit pupil side of the third lens 3 is a superposition of a diffractive surface and an even-order aspheric surface. These surfaces are bonded together using high-refractive-index optical adhesive to form a multilayer diffractive structure, improving the diffraction efficiency of the diffraction element. The image source side of the third lens 3 is a superposition of an even-order aspheric surface and a diffractive surface, while the exit pupil side of the fourth lens 4 is a superposition of a diffractive surface and an even-order aspheric surface. These surfaces are bonded using the same adhesive method. The refractive index of the optical adhesive used to bond the second lens 2 to the third lens 3, and the third lens 3 to the fourth lens 4, is 1.62. The second lens 2, third lens 3, and fourth lens 4 are bonded together using optical adhesive to form a composite diffractive lens. The composite diffractive lens is used to balance chromatic aberration, aberration, coma, field curvature, and distortion in a micro-projection system.

[0040] The lenses used in the present invention meet the following conditions: 0≤f1 / f≤2 -2≤f2 / f≤2 -2≤f3 / f≤-2 0≤f4 / f≤2

[0041] Wherein, f is the focal length of the micro-projection lens group, f1 is the focal length of the first lens 1 , f2 is the focal length of the second lens 2 , f3 is the focal length of the third lens 3 , and f4 is the focal length of the fourth lens 4 .

[0042] Specifically, f1 / f is 0.64, f2 / f is 0.21, f3 / f is 1.06, f4 / f is 1.41, and the distance between two adjacent diffraction surfaces of the second lens 2 and the third lens 3, and the third lens 3 and the fourth lens 4 is 0.02 mm.

[0043] In this embodiment, a total of four single lenses, two optical materials, and an optical adhesive with a refractive index of 1.62 are used. The total length of the micro-projection system is 17.34 mm, and the maximum aperture does not exceed 8 mm. The dimensions of the polarization beam splitter cube 5 are 7.6 mm × 7.6 mm × 7.6 mm. The refractive index of the material used in the polarization beam splitter cube 5 is 1.72, and the Abbe constant is 29.5.

[0044] The expression of even aspheric surface is as follows:

[0045] Where c = 1 / R, R is the radius of curvature of the aspheric vertex, k is the cone coefficient, and α1, α2, α3, α4, α5, and α6 are the coefficients corresponding to the 2nd, 4th, 6th, 8th, 10th, and 12th order terms of the even-order aspheric surface.

[0046] The phase expression of the diffraction surface is:

[0047] Where M is the diffraction order, A2 and A4 are phase coefficients, r is the distance from a point on the lens to the center of the lens, and R is the lens aperture.

[0048] In the embodiment of the present invention, the diffraction orders of the diffraction surface on the image source side of the second lens 2 and the diffraction surface on the exit pupil side of the third lens 3 are 8 and -7, respectively, and the phase coefficients A2 and A4 are -260.47 and 50.17, respectively; the diffraction orders of the diffraction surface on the image source side of the third lens 3 and the diffraction surface on the exit pupil side of the fourth lens 4 are 8 and -7, respectively, and the phase coefficients A2 and A4 are -2201.03 and 1411.65, respectively.

[0049] Table 1 and Table 2 below together show the specific parameters of each lens:

[0050] Table 1

[0051] Table 2

[0052] In Tables 1 and 2, surface number 2 represents the optical surface on the exit pupil side of the first lens 1, and surface number 3 represents the optical surface on the image source side of the first lens 1; surface number 4 represents the optical surface on the exit pupil side of the second lens 2, and surface number 5 represents the optical surface on the image source side of the second lens 2; surface number 6 represents the optical surface on the exit pupil side of the third lens 3, and surface number 7 represents the optical surface on the image source side of the third lens 3; surface number 8 represents the optical surface on the exit pupil side of the fourth lens 4, and surface number 9 represents the optical surface on the image source side of the fourth lens 4.

[0053] Figure 2 is a schematic diagram of the multi-layer diffraction structure of the present invention, where n1 and n3 are the refractive indices of the base material of the diffraction structure, and n2 is the refractive index of the bonding layer. Figure 3 is a schematic diagram of the MTF curve of the present invention. It can be seen that the MTF of the edge field of view is approximately 0.2 at 50lp / mm, and the image quality MTF of other parts except the edge field of view is greater than 0.3 at 50lp / mm. For human viewing, the image quality meets the requirements. Figure 4 is a schematic diagram of the field curvature and distortion of the present invention. The maximum distortion is approximately -2.2%, which is relatively small and does not affect the viewing experience. Figure 5 is a schematic diagram of the vertical axis chromatic aberration curve of the present invention. The maximum value is 4.12μm, which is less than the size of a pixel. The overall imaging quality is good.

[0054] Note that the above are only preferred embodiments of the present invention and the technical principles employed. Those skilled in the art will understand that the present invention is not limited to the specific embodiments described herein, and that various obvious changes, readjustments, and substitutions can be made by those skilled in the art without departing from the scope of protection of the present invention. Therefore, although the present invention has been described in detail through the above embodiments, the present invention is not limited to the above embodiments and may include many other equivalent embodiments without departing from the concept of the present invention. The scope of the present invention is determined by the scope of the appended claims.

[0055] The above embodiments are merely examples for clarity of explanation and are not intended to limit the embodiments. Those skilled in the art will appreciate that other variations or modifications based on the above descriptions are possible. It is not necessary and impossible to enumerate all embodiments here. Obvious variations or modifications arising therefrom remain within the scope of protection of the present invention.

Claims

1. A micro-projection system based on a compound diffractive lens, characterized in that: include: Liquid crystal on silicon display chip, polarization beam splitter cube, micro-projection lens group. The light emitted by the liquid crystal on silicon display chip passes through the polarization beam splitter cube and then is projected out through the micro-projection lens group, and finally enters the human eye from the exit pupil as parallel light at different angles. The side closer to the human eye when worn is the exit pupil side, the side of the silicon-based liquid crystal display chip is the image source side, and the micro-projection lens group is arranged in sequence along the optical axis from the exit pupil side to the image source side, including a first lens, a second lens, a third lens and a fourth lens; Among them, the first lens is a plano-convex lens, convex to the exit pupil side, the second lens is a biconcave lens, the third lens is a meniscus convex lens, convex to the exit pupil side, and the fourth lens is a biconvex lens; the optical focal length of the first lens is positive and is used to deflect light at large angles; the second lens, the third lens and the fourth lens are bonded together into a composite diffractive lens through optical glue; the bonded surface of the second lens, the third lens and the fourth lens is a superposition of an even-order aspheric surface and a diffractive surface.

2. The micro-projection system based on the compound diffractive lens according to claim 1, characterized in that: The focal length of the first lens and the focal length of the micro-projection lens satisfy the relationship: 0≤f1 / f≤2 Wherein, f1 is the focal length of the first lens, and f is the focal length of the micro-projection lens group.

3. The micro-projection system based on a compound diffractive lens according to claim 1, characterized in that: The focal length of the second lens and the focal length of the micro-projection lens satisfy the relationship: -2≤f2 / f≤2 Wherein, f2 is the focal length of the second lens, and f is the focal length of the micro-projection lens group.

4. The micro-projection system based on a compound diffractive lens according to claim 1, characterized in that: The focal length of the third lens and the focal length of the micro-projection lens satisfy the relationship: -2≤f3 / f≤-2 Wherein, f3 is the focal length of the third lens, and f is the focal length of the micro-projection lens group.

5. The micro-projection system based on a compound diffractive lens according to claim 1, characterized in that: The focal length of the fourth lens and the focal length of the micro-projection lens satisfy the relationship: 0≤f4 / f≤2 Wherein, f4 is the focal length of the fourth lens, and f is the focal length of the micro-projection lens group.

6. The micro-projection system based on a compound diffractive lens according to claim 1, characterized in that: Both optical surfaces of the first lens, the second lens, the third lens and the fourth lens are even-order aspheric surfaces; the image source side of the second lens is a superposition of an even-order aspheric surface and a diffraction surface, the exit pupil side of the third lens is a superposition of a diffraction surface and an even-order aspheric surface, the image source side of the third lens is a superposition of an even-order aspheric surface and a diffraction surface, and the exit pupil side of the fourth lens is a superposition of a diffraction surface and an even-order aspheric surface.

7. The micro-projection system based on a compound diffractive lens according to claim 1, characterized in that: The refractive indexes of the first lens, the second lens and the fourth lens are all 1.81, and the Abbe constants are all 25.5; the refractive index of the third lens is 1.50, and the Abbe constant is 81.

6.

8. The micro-projection system based on a compound diffractive lens according to claim 1, characterized in that: The refractive index of the optical glue used to glue the second lens and the third lens, and the third lens and the fourth lens together is 1.

62.

9. The micro-projection system based on a compound diffractive lens according to claim 1, characterized in that: The phase expression of the diffraction surface is: Among them, M is the diffraction order, A2 and A4 are phase coefficients, r is the distance from a point on the lens to the center of the lens, and R is the lens aperture.

10. The micro-projection system based on the compound diffractive lens according to claim 9, characterized in that: The diffraction orders of the diffraction surface on the image source side of the second lens and the diffraction surface on the exit pupil side of the third lens are 8 and -7 respectively, and the phase coefficients A2 and A4 are -260.47 and 50.17 respectively; the diffraction orders of the diffraction surface on the image source side of the third lens and the diffraction surface on the exit pupil side of the fourth lens are 8 and -7 respectively, and the phase coefficients A2 and A4 are -2201.03 and 1411.65 respectively.

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