阅读说明：本技术 一种基于液晶偏振体光栅的二维扩瞳方法 (Two-dimensional pupil expanding method based on liquid crystal polarizer grating ) 是由 李海峰 罗豪 翁嘉诚 刘旭 于 2021-07-21 设计创作，主要内容包括：本发明公开了一种基于液晶偏振体光栅的二维扩瞳方法,二维扩瞳方法基于液晶偏振体光栅的波导显示器件,包括光波导,在光波导上设有均为液晶偏振体光栅的入耦合光栅、下转置光栅、上转置光栅和出耦合光栅；入耦合光栅将光束耦合进入光波导,在光波导中发生全反射进入到下转置光栅,完成一维方向的光束扩展；光束在下转置光栅完成一维方向的光束扩展后,同时将光束折射进入上转置光栅,完成另一方向上的出瞳扩展；光束折射进入上转置光栅完成另一方向上的出瞳扩展后,在光波导内继续全反射向前传播至出耦合光栅,在出耦合光栅处耦合出射。该方法解决了传统光栅波导系统由于传统衍射光栅折射率调制度限制下较窄的响应带宽所导致的较小FOV的问题。(The invention discloses a two-dimensional pupil expanding method based on a liquid crystal polarizer grating, which is a waveguide display device based on the liquid crystal polarizer grating and comprises an optical waveguide, wherein the optical waveguide is provided with an in-coupling grating, a lower transfer grating, an upper transfer grating and an out-coupling grating which are all the liquid crystal polarizer gratings; the in-coupling grating couples the light beam into the optical waveguide, and the light beam is totally reflected in the optical waveguide and enters the lower transfer grating to complete the light beam expansion in the one-dimensional direction; after the light beam completes the light beam expansion in one-dimensional direction on the lower transposition grating, the light beam is refracted to enter the upper transposition grating to complete the exit pupil expansion in the other direction; after the light beam is refracted and enters the upper transfer grating to complete the exit pupil expansion in the other direction, the light beam is continuously totally reflected in the optical waveguide and forwards transmitted to the out-coupling grating, and the light beam is coupled and emitted at the out-coupling grating. The method solves the problem of small FOV of the traditional grating waveguide system caused by narrow response bandwidth under the limitation of the refractive index modulation degree of the traditional diffraction grating.)

1. The two-dimensional pupil expanding method based on the liquid crystal polarizer grating is characterized in that the two-dimensional pupil expanding method is based on a waveguide display device of the liquid crystal polarizer grating, the waveguide display device comprises an optical waveguide (1), and an in-coupling grating (2), a lower transfer grating (3), an upper transfer grating (4) and an out-coupling grating (5) are arranged on the optical waveguide (1); the in-coupling grating (2), the lower transfer grating (3), the upper transfer grating (4) and the out-coupling grating (5) are all liquid crystal polarizer gratings;

the two-dimensional pupil expanding method comprises the following steps:

the in-coupling grating (2) couples the light beam into the optical waveguide (1), and the light beam is totally reflected in the optical waveguide (1) and enters the lower transfer grating (3) to complete the light beam expansion in the one-dimensional direction;

after the light beam completes the light beam expansion in one-dimensional direction in the lower transposed grating (3), the light beam is refracted to enter the upper transposed grating (4) to complete the exit pupil expansion in the other direction;

after light beams are refracted and enter the upper transposed grating (4) to complete exit pupil expansion in the other direction, the light beams are continuously and totally reflected in the optical waveguide (1) and are transmitted forwards to the outcoupling grating (5) and coupled and emergent at the outcoupling grating (5).

2. The two-dimensional pupil expanding method based on the lc polarizer grating of claim 1, wherein the incoupling grating (2) reflects and diffracts the light beam into the optical waveguide (1), the light beam is totally reflected in the optical waveguide (1) and enters the lower transposing grating (3), a part of the light beam is reflected and diffracted to the upper transposing grating (4) at the lower transposing grating (3), the other part of the light beam is continuously reflected and diffracted to the upper transposing grating (4) at the next position of the lower transposing grating (3), and the light beam is expanded in one dimension by repeating the above propagation modes.

3. The two-dimensional pupil expanding method based on liquid crystal polarizer grating according to claim 1, characterized in that the in-coupling grating (2) grating vectorDown-conversion grating (3) grating vectorUpper transposed grating (4) grating vectorAnd the out-coupling grating (5) grating vectorThe conditions to be satisfied are:

4. the liquid crystal polarizer grating-based two-dimensional pupil dilation method according to claim 1, characterized in that the liquid crystal molecular handedness of the in-coupling grating (2), the lower turning grating (3), the upper turning grating (4) and the out-coupling grating (5) remains the same.

5. The liquid crystal polarizer grating-based two-dimensional pupil dilation method according to claim 4, wherein the in-coupling grating (2), the lower turning grating (3), the upper turning grating (4) and the out-coupling grating (5) are left-handed or right-handed.

6. The liquid crystal polarizer grating based two-dimensional pupil dilation method according to claim 1, characterized in that the in-coupling grating (2), the lower turning grating (3), the upper turning grating (4) and the out-coupling grating (5) are PVG.

Technical Field

The invention relates to the technical field of waveguide display, in particular to a two-dimensional pupil expanding method based on a liquid crystal polarizer grating.

Background

As a hotspot in the field of information display technology at present, the development of head mounted augmented reality (HMD-AR) devices bears a great vision of people on future information interaction methods. As a widely recognized technical solution, a waveguide-type virtual Reality (AR) display device has certain potential and advantages in terms of volume weight, exit pupil size, and visibility. The principle of waveguide transmission is utilized, and the exit pupil replication and expansion which are difficult to realize by a traditional visual optical system can be realized, so that an AR system wearer can obtain a larger eye movement range.

In conventional visual optical systems, the FOV and exit pupil size are limited by the lagrange invariance, in an inverse relationship. The lagrange optical invariance is expressed as:

n·θ·y_p＝n'·θ'·y_p' (1)

where θ is the half field angle at the entire optical system TONG-in, y_pIs the TONG size of the object and n is the refractive index of the object. Corresponding theta ', y'_pAnd n' denote a half field size, an exit pupil size, and a refractive index of the image side, respectively. For AR optics, achieving better display requires a larger exit pupil size while achieving a larger field of view. An excessively small exit pupil size will result in the human eye seeing the image only to a small extent. As can be seen from equation (1), the lagrangian invariance determines the inverse relationship between the field range and the exit pupil size, which limits that we cannot optimize the field range and the exit pupil size at the same time, and further, the field range and the exit pupil size reach the maximum value at the same time.

In order to break through the limitation that the FOV and the size of the exit pupil are limited by Lagrangian invariants, the copying and the expansion of the exit pupil in the waveguide transmission process are utilized to realize. In the main waveguide coupling scheme at present, the copy and expansion of the exit pupil with a good effect can be realized, wherein the beam splitter prism and the free-form optical coupling scheme can be regarded as the off-axis deformation of the traditional visual system, so that the off-axis deformation is limited by Lagrangian invariants, and the difficulty in realizing two-dimensional pupil expansion in process realization is high.

Disclosure of Invention

The invention aims to provide a two-dimensional pupil expanding method based on a liquid crystal polarizer grating, which has the advantages of large field angle, high diffraction efficiency, wide wavelength bandwidth, large angular response range and the like, and mainly solves the problem of small FOV (field of view) of a traditional grating waveguide system caused by narrow response bandwidth under the limitation of the refractive index modulation degree of a traditional diffraction grating.

The invention provides the following technical scheme:

a two-dimensional pupil expanding method based on a liquid crystal polarizer grating obtains a larger exit pupil range, namely a two-dimensional pupil expansion, through copying and expanding of the exit pupil in the transmission process of a waveguide display device based on the liquid crystal polarizer grating, and can realize continuity between exit pupil beams.

The liquid crystal polarizer grating-based waveguide display device includes: the optical waveguide is provided with an in-coupling grating, a lower transfer grating, an upper transfer grating and an out-coupling grating; the in-coupling grating, the lower transfer grating, the upper transfer grating and the out-coupling grating are all liquid crystal polarizer gratings.

In the invention, an external light beam vertically enters the incoupling grating, the incoupling grating reflects and diffracts the light beam to be coupled into the optical waveguide, total reflection occurs in the optical waveguide and enters the lower transposing grating, one part of the light beam is reflected and diffracted to the upper transposing grating at the lower transposing grating, the other part of the light beam is continuously reflected and diffracted forward, the light beam is continuously reflected and diffracted to the upper transposing grating at the next position of the lower transposing grating, and the light beam can complete the light beam expansion in one-dimensional direction by repeating the above propagation modes.

And after the light beam is expanded in one-dimensional direction, the light beam is reflected and diffracted to the upper transposed grating, and the light beam is reflected and diffracted at the upper transposed grating to complete the exit pupil expansion in the other direction. After the light beam is refracted and enters the upper transfer grating to complete the exit pupil expansion in the other direction, the light beam is continuously totally reflected in the optical waveguide and forwards transmitted to the outcoupling grating, and is coupled out at the outcoupling grating.

The incoupling grating vectorLower transposed grating vectorUpper transposed grating vectorAnd out-coupling grating vectorThe conditions to be satisfied are:

the liquid crystal molecular rotation directions of the in-coupling grating, the lower transfer grating, the upper transfer grating and the out-coupling grating are kept the same.

The in-coupling grating, the lower transfer grating, the upper transfer grating and the out-coupling grating are in left-hand rotation direction or right-hand rotation direction.

The in-coupling grating, the lower transfer grating, the upper transfer grating and the out-coupling grating are PVG.

The invention uses the polarizer grating based on liquid crystal material as the in-out coupling element and the middle grating to realize the copy and expansion of the exit pupil, compared with the traditional holographic grating, the liquid crystal polarizer grating has the advantages of large field angle, high diffraction efficiency, wider wavelength bandwidth, large angle response range and the like, and mainly aims to break through the limitation that the FOV and the exit pupil size in the traditional AR waveguide display are not changed by Lagrangian, realize the copy and expansion of the exit pupil and obtain a larger FOV; the problem of a small FOV of a traditional grating waveguide system caused by a narrow response bandwidth under the limitation of the refractive index modulation degree of a traditional diffraction grating is mainly solved. In the preparation, the liquid crystal polarizer grating only needs to utilize a holographic exposure method and a coating process, and compared with other waveguide coupling elements, the liquid crystal polarizer grating is simple to prepare and lower in cost.

Drawings

FIG. 1 is a waveguide coupling structure based on a reflective diffraction grating;

FIG. 2 is a structure of a liquid crystal polarizer grating;

FIG. 3 is the polarization state distribution of the orthogonal circular polarization interference light field in one period;

FIG. 4 is a grating vector distribution diagram of a two-dimensional pupil expansion method based on a liquid crystal polarizer grating;

FIG. 5 is a schematic diagram of a two-dimensional pupil expansion method based on a liquid crystal polarizer grating;

wherein, the optical waveguide is 1-optical waveguide, the coupling grating is 2-in, the lower transfer grating is 3-lower, the upper transfer grating is 4-upper and the coupling grating is 5-out.

Detailed Description

For a more clear understanding of the technical features, objects and effects of the present invention, the present invention will be described in detail below with reference to the accompanying drawings and examples.

As shown in fig. 1, the schematic diagram of transmitting a light waveguide beam based on a reflection diffraction grating includes a light waveguide 1, an in-coupling grating 2 and an out-coupling grating 5, where the in-coupling grating 2 and the out-coupling grating 5 are both reflection diffraction gratings, and the light beam is reflected and diffracted at the in-coupling grating 2 to enter the waveguide, and is transmitted by total internal reflection in the light waveguide 1, and after reaching the out-coupling grating 5, a part of the light is diffracted and coupled out, and the other part of the light is continuously transmitted in the waveguide 1 in a total reflection manner, and is coupled out again when entering the out-coupling grating 5 next time. Transmitted in this way, the input beam will be continuously replicated on the outcoupling element and coupled out to finally achieve the expansion of the exit pupil.

As shown in fig. 2, it is a schematic diagram of the structure of the liquid crystal polarizer grating. The PVG has a two-dimensional periodic structure, and the substrate is processed by two orthogonal circularly polarized exposures to provide a periodic rotation of the optical axis in the xz plane, the rotation angle continuously varying along the x-axis with a period of the transverse period ax. Cholesteric liquid crystal materials exhibit a period on a substrateLinear pitch structure, longitudinal period length is ay (half pitch length p along the y-axis). Such a two-dimensional period can produce a tilted, periodic tilt angleOf the refractive index of (a). We specify that

The periodicity in the transverse direction (x direction) is realized by utilizing a light orientation layer under the polarization holographic exposure technology, namely two beams of circularly polarized light with completely orthogonal polarization states and coherence are realized, and the two beams of light are intersected and irradiated to the surface of a sample at an included angle of 2 theta. If a layer of liquid crystal optical orientation film (photo alignment layer) is coated on the substrate in a spinning mode, the substrate is placed in two circularly polarized light superposition fields with completely orthogonal and coherent polarization states, the wavelengths of the two circularly polarized light beams can trigger photochemical reaction on the liquid crystal orientation film to form the same photo-orientation effect as the superposition fields, and then the substrate with the photo-orientation film is contacted with liquid crystal, so that liquid crystal molecules can be induced to be oriented according to the distribution of the superposition fields, and the liquid crystal polarizer grating is manufactured. The interference light field is linearly polarized light with light intensity uniformly distributed and polarization direction periodically and linearly changed along an x axis, and the period is lambda ═ lambda/2 sin theta, wherein lambda is the wavelength of the recording light. The distribution of the polarization states within one period is shown in fig. 3.

The periodicity in the longitudinal direction (y direction) requires that the PVG liquid crystal layer is doped with a proper amount of chiral material, such as Reactive Mesogen (RM). The RM monomer molecular structure contains acrylic double bonds at the tail end of a main chain, and can perform free radical reaction under the initiation of certain light energy to ensure that the monomer is polymerized. At a certain concentration or temperature, RM can exhibit a liquid crystal phase and have similar optical properties as nematic liquid crystals. Like nematic liquid crystals, RM will be transformed into a cholesteric phase after the addition of chiral dopants, resulting in the liquid crystal periodic rotation in the longitudinal direction (perpendicular to the substrate direction) required for PVG. Wherein, the period of the x direction is Λ_xCan be changed by adjusting the exposure angle of two coherent light beams in holographic exposurePeriod Λ_yIt can be modulated by the concentration c according to the helical Twist force constant (Helix Twist Power, HTP) of the chiral material. Period Λ in y-direction_yExpressed as:

p＝(HTP·c)^-1

where p is the twist pitch of the liquid crystal (liquid crystal molecules rotated 360 °), i.e. twice the longitudinal period p ═ 2 Λ_y. For a normally incident beam (incident angle of 0 °), the bragg condition of PVG can be represented by the following formula:

λ_Bis the Bragg wavelength in vacuum, phi is the tilt angle of the plane of refractive index or the tilt angle expressed as the grating vector K, n_effIs the average refractive index, Λ, of the anisotropic medium_BIs the bragg period, the latter two can be defined as follows:

in the preparation of PVG, RM257 with good optical performance, which is widely used, is selected as RM material. RM257 is also a common material and is relatively inexpensive. While for chiral agents we selected R5011/S5011 (HTP. apprxeq.108/. mu.m) with a large twisting power (HTP). Wherein R5011 and S5011 correspond to left-handed and right-handed liquid crystal helical directions respectively.

In the experiment, the method for preparing PVG was as follows:

(1) cleaning a substrate:

(2) preparing an orientation layer solution;

(3) spin coating an orientation layer;

(4) drying the orientation layer;

(5) exposing the orientation layer;

(6) preparing a liquid crystal mixture solution;

(7) spin coating preparation of a liquid crystal layer;

(8) ultraviolet curing;

(10) the spin coating is repeated until a sufficient thickness is obtained.

According to the theory and the experimental steps, only the exposure angle and the doping concentration of the chiral molecules are changed, so that PVG with different central wavelengths and different diffraction angles can be obtained.

As shown in fig. 5, it is a schematic diagram of a two-dimensional pupil expanding method based on liquid crystal polarizer grating. A lower 3 and an upper 4 transpose grating are added on the basis of fig. 1. Wherein, the in-coupling grating 2, the lower transfer grating 3, the upper transfer grating 4 and the out-coupling grating 5 are all liquid crystal polarizer gratings, and the grating vector of the in-coupling grating 2Raster vector of lower transposed raster 3Raster vector of upper transpose raster 4And the grating vector of the out-coupling grating 5The conditions to be satisfied are: as shown in fig. 4.

Finally, a two-dimensional pupil expanding structure based on the liquid crystal polarizer grating is obtained: an external light beam vertically enters the incoupling grating 2, the incoupling grating 2 reflects, diffracts and couples the light beam into the optical waveguide, the light beam is totally reflected in the optical waveguide 1 and enters the lower transposition grating 3, one part of the light beam is reflected and diffracted to the upper transposition grating 4 at the lower transposition grating 3, the other part of the light beam is continuously reflected and diffracted forward, the light beam is continuously reflected and diffracted to the upper transposition grating 4 at the next position of the lower transposition grating 3, and the light beam can be expanded in one-dimensional direction by repeating the above propagation modes.

After the light beam is expanded in one-dimensional direction, the light beam is reflected and diffracted to the upper transposed grating 4, and the light beam is reflected and diffracted at the upper transposed grating 4 to complete the exit pupil expansion in the other direction. After the light beam is refracted and enters the upper transfer grating 4 to complete the exit pupil expansion in the other direction, the light beam is continuously totally reflected in the optical waveguide 1 and forwards transmitted to the outcoupling grating 5, and is coupled out at the outcoupling grating 5.

The above description is only of the preferred embodiments of the present invention, and it should be noted that: it will be apparent to those skilled in the art that various modifications and adaptations can be made without departing from the principles of the invention and these are intended to be within the scope of the invention.