Variable-focus optical system for eliminating chromatic aberration of diffractive lens and design method

Abstract

The invention discloses a variable-focus optical system capable of eliminating chromatic aberration of diffractive lenses and a design method, which utilize the characteristic that the chromatic dispersion characteristic of geometric phase liquid crystal lenses is opposite to that of common glass, and can utilize mutual compensation of a plurality of geometric phase liquid crystal variable-focus lenses to help the system to reduce chromatic dispersion. The system vertical axis chromatic aberration is reduced by about 1 / 2 compared with a common aspheric surface equal-surface type Pancake system; according to the invention, the geometric phase liquid crystal zoom lens can be matched with human eyes to quickly adjust the focal length, so that the convergence adjustment conflict problem is solved, and the uniformity of image quality is ensured while quick zooming is realized.

Description

Technical Field

[0001] The present invention relates to the field of optical display technology, and in particular to a variable focus optical system and a design method for eliminating chromatic aberration of a diffraction lens, which can solve the convergence adjustment conflict and chromatic aberration in a virtual reality display optical system and is suitable for near-eye display devices. Background Art

[0002] Virtual reality (VR) technology is becoming an important display platform for the next generation of human-computer interaction. With the development of VR devices, their optical systems have also undergone multiple evolutions. The earliest VR optical system used a single aspherical lens for imaging, but due to the large center thickness of the lens, the entire optical system was bulky, which was not conducive to the lightweight and wearing comfort of the device. In order to solve the thickness problem, subsequent VR devices introduced Fresnel lenses, which reduced the thickness of the optical system by etching concentric ring structures on the surface. However, while reducing the volume, the Fresnel lens introduced the problem of stray light, which reduced the image quality and affected the user experience.

[0003] In recent years, Pancake optical systems based on polarization optical elements have gradually become the mainstream solution for VR devices. Pancake optics uses multiple reflections of light between two optical surfaces to greatly shorten the system optical path, thereby significantly reducing the volume of the optical system. At the same time, multiple light path propagations enhance the light modulation capability and improve the imaging quality. Combined with high-resolution 4Kmicro-OLED display panels, Pancake optics can provide clear images close to the visual limit of the human eye, thereby gradually reducing the screen door effect in VR devices. The screen door effect is due to the visible black gaps between the pixels of the display screen, which causes users to see a screen door grid-like effect when watching, affecting the sense of immersion. With the development of Pancake optics and high-resolution display technology, the image quality of VR devices has been significantly improved.

[0004] Although VR devices have made progress in reducing the size of optical systems and improving image resolution, they still face the problem of convergence-accommodation conflict. Vergence-accommodation conflict is a long-standing visual conflict phenomenon in 3D display technology, which mainly comes from the mismatch between convergence cues and accommodation cues when the human eye views 3D images. Convergence cues are determined by the image parallax of the two eyes, that is, when the human eye focuses on objects at different distances, the visual axes of the two eyes will change accordingly. Accommodation cues are determined by the adjustment of the human eye lens. Usually, in a natural viewing environment, these two cues are matched. However, in VR devices, the position of the display screen is fixed and the optical imaging depth remains unchanged, making the accommodation cues fixed, while the vergence cues change with the 3D image content, causing the two to lose their matching relationship. This conflict can cause visual fatigue, dizziness and discomfort, seriously affecting the wearing experience of VR devices.

[0005] In order to solve the problem of convergence-accommodation conflict, researchers have proposed a variety of methods, including computational holography, integrated imaging display, and zoom / multifocal display. Computational holography generates the wavefront of a 3D scene through lasers and spatial light modulators to provide correct depth cues, but the hardware architecture of this solution is quite different from the existing VR optical system and is difficult to implement in current devices. Integrated imaging and light field display use optical elements such as microlens arrays or pinhole arrays to guide pixel light to different angles to simulate real light fields. However, this method has mutual constraints between the number of viewing angles and image resolution, and the resolution of current VR devices is not enough to support high-resolution light field display. In addition, zoom / multifocal display dynamically adjusts the focal length of the lens to make the optical imaging depth variable, and uses the visual persistence effect of the human eye to quickly switch multiple focal planes in a short time, thereby alleviating the problem of convergence-accommodation conflict. This solution can be well compatible with the existing VR optical system and is currently the most feasible solution to convergence-accommodation conflict.

[0006] In the zoom / multi-focal display solution, the zoom lens is the core component, which needs to have a large clear aperture (~40mm), a fast response time (<15ms) and a thin design to adapt to the compact structure of the virtual reality display device. However, traditional zoom lens solutions, such as liquid lenses and mechanical moving lenses, are difficult to meet the above requirements at the same time. Therefore, in recent years, a geometric phase liquid crystal zoom lens solution based on a geometric phase liquid crystal lens has been proposed. The geometric phase liquid crystal lens is based on photo-alignment and liquid crystal self-assembly technology, has an ultra-thin thickness of microns and a large clear aperture, and is combined with a fast-response polarization converter, which can meet the needs of VR devices for zoom lenses.

[0007] Although the application prospects of geometric phase liquid crystal lenses in VR devices are broad, they are essentially diffraction lenses, and their focal length is inversely proportional to the wavelength, which results in serious dispersion problems. The effective Abbe number of geometric phase liquid crystal lenses is about -3.45. Compared with traditional refractive lenses, their dispersion direction is opposite and the value is one order of magnitude larger. Therefore, in a dynamic zoom system, the dispersion effect of geometric phase liquid crystal lenses will cause focal length shifts of light of different colors, causing color distortion in the image and affecting the final display effect.

[0008] In recent years, in order to solve the dispersion problem of geometric phase liquid crystal lenses, researchers have proposed a variety of achromatic geometric phase liquid crystal lens schemes. Among them, one method uses high-order wave plates to adjust the diffraction efficiency of RGB wavelengths and achieves the achromatic effect by superimposing multiple wave plates. However, high-order wave plates are sensitive to the angle of incidence, which makes this scheme challenging to integrate in VR devices with a large field of view (~100°). Therefore, how to effectively integrate geometric phase liquid crystal lenses in VR devices and solve their dispersion problems remains a key issue in current research. Summary of the invention

[0009] In view of this, the present invention provides a variable focus optical system and a design method for eliminating chromatic aberration of a diffraction lens.

[0010] A variable focus optical system for eliminating chromatic aberration of a diffraction lens comprises at least two geometric phase liquid crystal variable focus lenses.

[0011] Preferably, the geometric phase liquid crystal variable focus lens is used in a virtual reality display system.

[0012] Preferably, the geometric phase liquid crystal variable focus lens is used in a Pancake optical system.

[0013] Preferably, the geometric phase liquid crystal zoom lens is composed of N pairs of dual-focus active lens modules (203) stacked together, each dual-focus active lens module (203) comprising a polarization converter (201) and a geometric phase liquid crystal lens (202); the value of N is greater than or equal to 1;

[0014] The Pancake optical system includes, in sequence from the observation end of the human eye (700) to the display, a first lens (702), a first geometric phase liquid crystal zoom lens (703), a quarter wave plate (704), a reflective polarizer (705), a quarter wave plate (706), a second geometric phase liquid crystal zoom lens (707), a second lens (708), a third lens (709), an input filter (710) and a display screen (711).

[0015] Preferably, the device parameters in the Pancake optical system are:

[0016]

[0017] A simple model design method for a variable focus optical system is proposed. When a pair of geometric phase liquid crystal variable focus lenses are used to form an imaging system, each geometric phase liquid crystal variable focus lens is represented as:

[0018]

[0019] Where λ represents the wavelength; i = 1, 2; the image distance v is represented as v(c 1 ,c 2 ,λ), where c 1 ,c 2 They represent the variable coefficients of a pair of geometric phase liquid crystal zoom lenses. When zooming to a distance v, the coefficient c 1 ,c 2 Among the multiple combinations, select at least one combination that can eliminate the color difference, and this combination meets the following conditions:

[0020]

[0021] Among them, λ 0 represents the central wavelength;

[0022] Based on the coefficient c 1 ,c 2 A simple model is designed to achieve chromatic aberration correction of the imaging system.

[0023] Preferably, a variable coefficient c is adjusted 1 or c 2 , then according to the formula Another variable coefficient is calculated to achieve zoom and correct chromatic aberration at the same time.

[0024] A design method for a variable focus optical system. In the actual design of a virtual reality display system, when light passes through a geometric phase liquid crystal lens in a geometric phase liquid crystal variable focus lens, the phase of the light is modulated:

[0025]

[0026] Among them, J HWP The Jones matrix representing the geometric phase of the liquid crystal lens, Represents the in-plane angle of the molecules in the geometric phase liquid crystal lens.

[0027] Modulated phase and geometric phase The angle of the molecules in the plane of the liquid crystal lens About, angle The distribution satisfies the conditions:

[0028]

[0029] Among them, f is the focal length of the geometric phase liquid crystal lens, λ is the wavelength, β(x,y) is the high-order phase term; (x,y) represents the local coordinates of the light on the surface of the geometric phase liquid crystal lens.

[0030] Preferably, the phase distribution of the geometric phase liquid crystal variable focus lens is:

[0031]

[0032] where λ 0 B is the wavelength of light used for liquid crystal exposure, r is the radial distance from the center of the exposure area to a point on the surface; i is the i-th order coefficient;

[0033] In the simulation at different imaging distances, the high-order parameters B in the phase distribution of the geometric phase liquid crystal zoom lens iThe lens parameters in the virtual reality display optical system are jointly optimized to achieve simultaneous achromatism at different imaging distances; thereafter, during actual imaging, the phases of the first geometric phase liquid crystal zoom lens (703) and the second geometric phase liquid crystal zoom lens (707) are modulated at the corresponding simulation parameters at different imaging distances to achieve the achromatism effect.

[0034] The present invention has the following beneficial effects:

[0035] 1. The prior art can only improve the image quality of the entire system by complicating the corresponding curve function of the optical surface, which has high requirements for precision processing technology; however, the present invention cleverly utilizes the dispersion characteristics of geometric phase liquid crystal lenses that are opposite to those of ordinary glass, and can use multiple geometric phase liquid crystal lenses to compensate each other to help the system reduce dispersion. In the examples listed in the present invention, the vertical axis chromatic aberration of the system is reduced by about 1 / 2 compared with the ordinary aspherical isosurface Pancake system.

[0036] 2. In order to solve the problem of convergence adjustment conflict, the existing technology can only use a mechanical module to adjust the back focus of the VR optical module to achieve zooming, but the efficiency of mechanical zooming is very low, which is not enough to match the human eye's rapid response to a specific position when observing an object in a fast movement; and the current Pancake optical system uses a mechanical zoom design scheme that greatly sacrifices image quality, and the image quality is very poor. The present invention uses a geometric phase liquid crystal zoom lens to cooperate with the human eye to quickly adjust the focal length, thereby solving the problem of convergence adjustment conflict, and ensuring the uniformity of image quality while rapidly zooming. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] Figure 1 Schematic diagram of the geometric phase liquid crystal lens characteristics.

[0038] Figure 2 Schematic diagram of the geometric phase liquid crystal zoom lens.

[0039] Figure 3 Schematic diagram of a simple model of imaging using a liquid crystal variable focus lens with two geometric phases.

[0040] Figure 4 Schematic diagram of the arrangement of the orientation layer of the geometric phase liquid crystal lens.

[0041] Figure 5 Schematic diagram of the geometric phase liquid crystal lens structure.

[0042] Figure 6 Schematic diagram of the polarization switch device structure.

[0043] Figure 7Schematic diagram of the zoom achromatic Pancake optical lens structure, wherein 700 is a human eye, 701 is an exit pupil, 702 is a first lens, 703 is a first geometric phase liquid crystal zoom lens, 704 is a quarter wave plate, 705 is a reflective polarizer, 706 is a quarter wave plate, 707 is a second geometric phase liquid crystal zoom lens, 708 is a second lens, 709 is a third lens, 710 is an input filter, and 711 is a display screen.

[0044] Figure 8 This is an example diagram of a variable focus achromatic Pancake optical lens corresponding to a 0.5D virtual image distance.

[0045] Figure 9 Schematic diagram of the MTF curve of an example of a variable focus achromatic Pancake optical lens corresponding to a 0.5D virtual image distance.

[0046] Figure 10 A schematic diagram of the vertical axis chromatic aberration curve of a variable focus achromatic Pancake optical lens corresponding to an example of a 0.5D virtual image distance.

[0047] Figure 11 This is an example diagram of the 1D virtual image distance corresponding to the variable focus achromatic Pancake optical lens.

[0048] Figure 12 Schematic diagram of the MTF curve of an example of a variable focus achromatic Pancake optical lens corresponding to 1D virtual image distance.

[0049] Figure 13 A schematic diagram of the vertical axis chromatic aberration curve of an example of a variable focus achromatic Pancake optical lens corresponding to a 1D virtual image distance.

[0050] Figure 14 This is an example diagram of the 5D virtual image distance corresponding to the variable focus achromatic Pancake optical lens.

[0051] Figure 15 Schematic diagram of the MTF curve of an example of a variable focus achromatic Pancake optical lens corresponding to a 5D virtual image distance.

[0052] Figure 16 A schematic diagram of the vertical axis chromatic aberration curve of a variable focus achromatic Pancake optical lens corresponding to an example of a 5D virtual image distance. DETAILED DESCRIPTION

[0053] like Figure 1 As shown, the geometric phase liquid crystal lens is a planar diffractive optical element. For incident parallel light in the visible light band, the focal length of red light is smaller than that of green light, and the focal length of green light is smaller than that of blue light. Its longitudinal chromatic aberration characteristics are opposite to those of traditional refractive optical elements.

[0054] Each geometric phase liquid crystal lens can achieve both convergence and divergence effects according to the polarization state when determining a phase distribution, that is, it has positive and negative refractive powers and focal lengths. At the same time, if the positive and negative sides of the geometric phase liquid crystal lens are reversed, its positive and negative refractive powers and focal length characteristics are also opposite. Without loss of generality, assuming that the incident light is right-handed circularly polarized light, it will become left-handed light after passing through the geometric phase liquid crystal lens and then converge. When the incident light is left-handed circularly polarized light, the left-handed light will be converted into right-handed light and diverge after passing through the geometric phase liquid crystal lens.

[0055] like Figure 2 As shown, 201 is a polarization converter, 202 is a geometric phase liquid crystal lens, if the properties of the input polarization state can be dynamically controlled by the polarization converter 201, we can have a dual-focus active lens 203; stacking N pairs of dual-focus lens modules (203) together, we can get a focal length of 2 N The geometric phase of the liquid crystal zoom lens can be regarded as a quasi-continuously variable focal length lens.

[0056] In VR display systems, a major problem with optical lenses is dispersion, which means that light of different colors cannot be completely focused on the same point, resulting in color deviation in the picture. This phenomenon reduces the clarity of the display and affects the sense of immersion. The core of our technology is to use two special geometric phase liquid crystal zoom lenses ( Figure 3 301 and 302 in the figure) to eliminate this dispersion and make the picture clearer.

[0057] The ratio of the focal length of the geometric phase liquid crystal variable focus lens to the dispersion wavelength is:

[0058]

[0059] Where λ is the wavelength, c i is the variable coefficient of the i-th geometric phase liquid crystal zoom lens, which is used to adjust the focal length.

[0060] A single geometric phase liquid crystal zoom lens is a special lens that can adjust the focal length, but its focal length changes with the wavelength (color) of light, which means that the focal positions of blue light, green light, and red light are different, resulting in chromatic aberration. Although this does not affect the zoom capability of the lens, in VR devices, users will see color stratification at the edge of the picture, affecting the visual experience.

[0061] Our solution is to use two geometric phase liquid crystal zoom lenses to work together, so that the dispersion of one geometric phase liquid crystal zoom lens is offset by the dispersion of the other geometric phase liquid crystal zoom lens. In this way, even during the zoom process, the image will not become blurred or distorted due to dispersion. To verify this method, we established a mathematical model, such as Figure 3As shown, two geometric phase liquid crystal zoom lenses act on the display screen at the same time, ultimately forming a clear image.

[0062] We can obtain the image distance v using the following equation:

[0063]

[0064] Among them, v 1 is the distance of the intermediate image after the first geometric phase liquid crystal zoom lens. 1 d 2 They represent the distance from the display to the first geometric phase liquid crystal variable focus lens, and the distance from the first and second geometric phase liquid crystal variable focus lenses, respectively; 1 、f 2 Respectively represent the focal lengths of the first and second geometric phase liquid crystal zoom lenses; c 1 、c 2 denote the variable coefficients corresponding to the first and second geometric phase liquid crystal zoom lenses respectively; the final solution of the image distance v depends on the focal lengths of the two lenses. Therefore, the image distance v can be expressed as v(c 1 ,c 2 ,λ).

[0065] In the above mathematical calculations, we found that the distance of the image depends on the focal length of the two lenses and the wavelength of light, that is, the focal length needs to satisfy a specific mathematical relationship, that is, c can be adjusted 1 and c 2 To change the distance of the image (to achieve zoom function):

[0066] v(c 1 ,c 2 ,λ 0 )=v 0 (4)

[0067] Among them, v 0 represents the distance at which the image is to be formed; 0 represents the central wavelength;

[0068] Theoretically, when zooming to a distance v, c 1 ,c 2 There are countless combinations, but at least one of them can eliminate the color difference. And this combination should meet the following conditions:

[0069]

[0070] Among them, the central wavelength λ 0 Typically set to the green wavelength in visible light.

[0071] This is the only way to accurately align different colors of light without causing chromatic aberration. We can just adjust a variable coefficient c1 or c 2 , and then automatically calculate another variable coefficient based on the above mathematical formula, so that chromatic aberration can be automatically corrected while achieving zoom.

[0072] In practical applications, in addition to the coefficient c, there are higher-order phases that can be expressed using the local azimuth angles of the liquid crystal molecules. Figure 4 As shown, the molecules at the bottom of the liquid crystal layer in contact with the photo-alignment layer of the geometric phase liquid crystal lens are arranged according to the photo-alignment molecules, and the angle of the molecule 401 in the plane The distribution satisfies the conditions:

[0073]

[0074] Where f is the focal length of the geometric phase liquid crystal lens, λ is the wavelength, and β(x,y) is the high-order phase term.

[0075] When designing a virtual reality display system, the structural parameters of the glass in the system (such as curvature, thickness, air gap, etc.) and the β(x, y) of the first and second geometric phase liquid crystal zoom lenses are usually jointly optimized at different imaging distances. The specific expression of β(x, y) can have different mathematical expressions according to actual design requirements. The optimized parameters are mainly the high-order coefficients of the corresponding mathematical expressions to meet the requirements of correcting chromatic aberration. After that, during actual imaging, it is only necessary to modulate the phase of the first and second geometric phase liquid crystal zoom lenses at different imaging distances to achieve the effect of achromatic aberration.

[0076] The configuration of the geometric phase liquid crystal lens is as follows Figure 5 As shown, the bottom layer 501 is a photo-alignment layer, and there are N layers of liquid crystal layers such as 502 and 503 on top. The molecules at the bottom of the liquid crystal layer in contact with the photo-alignment layer are arranged according to the photo-alignment molecules, and the molecules in contact between each layer of liquid crystal are arranged in the same direction. Each layer of liquid crystal has a spontaneous spiral structure 504 in the direction perpendicular to the substrate. In layer 502, the liquid crystal molecules rotate a certain angle α in the direction perpendicular to the substrate. 1 And has a thickness of d 1 The liquid crystal rotation angle α of each layer i (i = 1 ~ N) and thickness d i (i=1-N) should make the geometric phase liquid crystal lens have an efficiency greater than 90% in the visible light band.

[0077] The configuration of the polarization switch device is as follows Figure 6 As shown, the structure is divided into a TN type liquid crystal box (603) and a polarization compensation film (601). The polarization compensation film is composed of three A sheets (602) with different optical axis rotation angles, and the two TN type liquid crystal boxes also have a spontaneous spiral structure in the direction perpendicular to the substrate. The liquid crystal molecules rotate a certain angle α in the direction perpendicular to the substrate.i (i=1, 2) and has a thickness of d i (i=1, 2).

[0078] In order to show that the use of two geometric phase liquid crystal zoom lenses can solve the convergence adjustment conflict and chromatic aberration problems in virtual reality display, the Pancake optical system is taken as an example to demonstrate cases at 0.5D, 1D, and 5D virtual image distances.

[0079] like Figure 7 As shown, from the human eye observation end to the display, it mainly includes an aperture 701, a first lens 702, a first geometric phase liquid crystal zoom lens 703, a second geometric phase liquid crystal zoom lens 707, a second lens 708, a third lens 709, an input filter 710 and a display screen 711. In addition, it also includes a reflective polarizer and a quarter-wave plate.

[0080] By using two geometric phase liquid crystal variable focus lenses, which are respectively placed on the right side of the first lens (702) and the right side of the quarter wave plate (706), the input filter (710) receives the image light and converts it into left-handed circularly polarized light; after passing through the third lens (709) and the second lens (708) for the first time, it reaches the second geometric phase liquid crystal variable focus lens (707). At this time, the focal length of the geometric phase liquid crystal variable focus lens is set to f 2 ; Then it passes through the quarter wave plate (706) and becomes vertical linear polarized light. Then, the polarization direction of the light when it first arrives at this point is perpendicular to the polarization direction of the reflective polarizer (705), and is reflected back into the lens group, and the polarization state remains unchanged; After passing through the quarter wave plate (706) again, it becomes left-handed circular polarization and is incident on the second geometric phase liquid crystal zoom lens (707) again. At this time, the focal length of the geometric phase liquid crystal zoom lens is -f 2 After that, the light passes through the second lens (708) and the third lens (709) for the second time and reaches the semi-transparent and semi-reflective film to be reflected, and the polarization state becomes right-handed polarized light. After passing through the third lens (709) and the second lens (708) for the third time, it is incident on the second geometric phase liquid crystal zoom lens (707) again. At this time, the focal length of the geometric phase liquid crystal zoom lens is -f 2 ; After passing through the quarter wave plate (706), it becomes horizontally polarized light and is transmitted from the reflective polarizer (705); after passing through the quarter wave plate (704), it becomes right-handed polarized light and is incident on the first geometric phase liquid crystal zoom lens (703). At this time, the focal length of the geometric phase liquid crystal zoom lens is f 1 , and finally reaches the exit pupil (701) after passing through the first lens (702) and enters the human eye (700).

[0081] In the following three examples, the phase distribution of the first and second geometric phase liquid crystal zoom lenses satisfies:

[0082]

[0083] where λ 0 B is the wavelength of light used for liquid crystal exposure, and r is the radial distance from the center of the exposure area to a point on the surface. i are higher-order coefficients.

[0084] Example 1:

[0085] like Figure 8 As shown, it is Example 1 of the variable focus achromatic Pancake optical lens corresponding to a virtual image distance of 0.5D. Table 1 gives the diffraction coefficient of the geometric phase liquid crystal zoom lens, and Table 2 gives the surface type, curvature radius, thickness, material and cone coefficient of other components; wherein the units of curvature radius and thickness are both millimeters (mm).

[0086] The system has a field of view of 96° and uses a 2.1-inch display. The overall thickness is less than 20mm, the exit pupil diameter is 8mm, and the exit pupil distance is 12mm.

[0087] Table 1 Diffraction coefficient of liquid crystal lens

[0088]

[0089] Table 2 Parameters of other components

[0090]

[0091]

[0092] like Figure 9 The figure shows an MTF curve diagram of an example of a zoom achromatic Pancake optical lens corresponding to a 0.5D virtual image distance. At the cutoff frequency (30 line pairs / mm), the full-field MTF is greater than 0.12, and is greater than 0.2 except for the edge field. However, the MTF of the current Pancake optical design with mechanical zoom quickly drops to 0 at 10 line pairs / mm.

[0093] like Figure 10 The figure shows a schematic diagram of the vertical axis chromatic aberration curve of an example of a variable focus achromatic Pancake optical lens corresponding to a 0.5D virtual image distance. The vertical axis chromatic aberration is less than 0.015mm in the full field of view.

[0094] Example 2:

[0095] like Figure 11As shown, it is Example 2 of the variable focus achromatic Pancake optical lens corresponding to 1D virtual image distance. Table 3 gives the diffraction coefficient of the geometric phase liquid crystal zoom lens. The surface type, curvature radius, thickness, material and cone coefficient of other elements are the same as those of Example 1; wherein the units of curvature radius and thickness are both millimeters (mm).

[0096] The system has a field of view of 96° and uses a 2.1-inch display. The overall thickness is less than 20mm, the exit pupil diameter is 8mm, and the exit pupil distance is 12mm.

[0097] Table 3 Diffraction coefficient of liquid crystal lens

[0098]

[0099] like Figure 12 The figure shows an MTF curve diagram of an example of a zoom achromatic Pancake optical lens corresponding to a 1D virtual image distance. At the cutoff frequency (30 line pairs / mm), the MTF of most fields of view is greater than 0.1, while the MTF of the current Pancake optical design with mechanical zoom quickly drops to 0 at 10 line pairs / mm.

[0100] like Figure 13 The figure shows a schematic diagram of the vertical axis chromatic aberration curve of a variable focus achromatic Pancake optical lens corresponding to an example of 1D virtual image distance. The vertical axis chromatic aberration is less than 0.006mm in the full field of view.

[0101] Example 3:

[0102] like Figure 14 As shown, it is Example 3 of the variable focus achromatic Pancake optical lens corresponding to 5D virtual image distance. Table 4 gives the diffraction coefficient of the geometric phase liquid crystal zoom lens. The surface type, curvature radius, thickness, material and cone coefficient of other elements are the same as those in Example 1; wherein the units of curvature radius and thickness are both millimeters (mm).

[0103] The system has a field of view of 96° and uses a 2.1-inch display. The overall thickness is less than 20mm, the exit pupil diameter is 8mm, and the exit pupil distance is 12mm.

[0104] Table 4 Diffraction coefficient of liquid crystal lens

[0105]

[0106] like Figure 15The figure shows an MTF curve diagram of an example of a zoom achromatic Pancake optical lens corresponding to a 5D virtual image distance. At the cutoff frequency (30 line pairs / mm), the full-field MTF is greater than 0.1, and the MTF of most fields is greater than 0.18. However, the MTF of the current Pancake optical design with mechanical zoom quickly drops to 0 at 10 line pairs / mm.

[0107] like Figure 16 The figure shows a schematic diagram of the vertical axis chromatic aberration curve of a variable focus achromatic Pancake optical lens corresponding to an example of a 5D virtual image distance. The vertical axis chromatic aberration is less than 0.008mm in the full field of view.

[0108] The above specific embodiments only describe the design principle of the present invention. The shapes and names of the components in the description may be different and are not limited. Therefore, those skilled in the art in the field of the present invention may modify or replace the technical solutions recorded in the above embodiments; and these modifications and replacements do not deviate from the creative purpose and technical solutions of the present invention and should all fall within the protection scope of the present invention.

Claims

1. A variable focus optical system for eliminating chromatic aberration of a diffraction lens, characterized in that: The invention comprises at least two geometric phase liquid crystal variable focus lenses.

2. A variable focus optical system for eliminating chromatic aberration of a diffraction lens as claimed in claim 1, characterized in that: The geometric phase liquid crystal zoom lens is used in a virtual reality display system.

3. A variable focus optical system for eliminating chromatic aberration of a diffraction lens as claimed in claim 2, characterized in that: The geometric phase liquid crystal zoom lens is used in a Pancake optical system.

4. A variable focus optical system for eliminating chromatic aberration of a diffraction lens as claimed in claim 3, characterized in that: The geometric phase liquid crystal zoom lens is composed of N pairs of dual-focus active lens modules (203) stacked together, each dual-focus active lens module (203) comprising a polarization converter (201) and a geometric phase liquid crystal lens (202); the value of N is greater than or equal to 1; The Pancake optical system includes, in sequence from the observation end of the human eye (700) to the display, a first lens (702), a first geometric phase liquid crystal zoom lens (703), a quarter wave plate (704), a reflective polarizer (705), a quarter wave plate (706), a second geometric phase liquid crystal zoom lens (707), a second lens (708), a third lens (709), an input filter (710) and a display screen (711).

5. A variable focus optical system for eliminating chromatic aberration of a diffraction lens as claimed in claim 4, characterized in that: The device parameters in the Pancake optical system are:

6. A simple model design method for a variable focus optical system according to claim 1, 2, 3, 4 or 5, characterized in that: When a pair of geometric phase liquid crystal zoom lenses are used to form an imaging system, each geometric phase liquid crystal zoom lens is expressed as: Where λ represents the wavelength; i = 1, 2; the image distance v is represented as v(c1, c2, λ), where c1 and c2 represent the variable coefficients corresponding to a pair of geometric phase liquid crystal zoom lenses. When zooming to a distance v, among the multiple combinations of coefficients c1 and c2, at least one combination that can eliminate chromatic aberration is selected, and this combination satisfies the following conditions: Where λ0 represents the central wavelength; A simple model is designed based on coefficients c1 and c2 to achieve chromatic aberration correction of the imaging system.

7. The design method according to claim 6, characterized in that: Adjust a variable coefficient c1 or c2, and then according to the formula Another variable coefficient is calculated to achieve zoom and correct chromatic aberration at the same time.

8. A method for designing a variable focus optical system according to claim 1, 2, 3, 4 or 5, characterized in that: In the actual design of a virtual reality display system, when light passes through a geometric phase liquid crystal lens in a geometric phase liquid crystal variable focus lens, the phase of the light is modulated: Among them, J HWP The Jones matrix representing the geometric phase of the liquid crystal lens, Represents the in-plane angle of the molecules in the geometric phase liquid crystal lens. Modulated phase and geometric phase The angle of the molecules in the plane of the liquid crystal lens About, angle The distribution satisfies the conditions: Among them, f is the focal length of the geometric phase liquid crystal lens, λ is the wavelength, β(x,y) is the high-order phase term; (x,y) represents the local coordinates of the light on the surface of the geometric phase liquid crystal lens.

9. The method for designing a variable focus optical system according to claim 8, wherein: Phase distribution of geometric phase liquid crystal variable focus lens: Where λ0 is the wavelength of the light used for liquid crystal exposure, r is the radial distance from the center of the exposure area to a point on the surface; B i is the i-th order coefficient; in the simulation at different imaging distances, the high-order parameter B in the phase distribution of the geometric phase liquid crystal zoom lens i The lens parameters in the virtual reality display optical system are jointly optimized to achieve simultaneous achromatism at different imaging distances; thereafter, during actual imaging, the phases of the first geometric phase liquid crystal zoom lens (703) and the second geometric phase liquid crystal zoom lens (707) are modulated at the corresponding simulation parameters at different imaging distances to achieve the achromatism effect.

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