阅读说明：本技术 全息波导显示系统出瞳均匀性的方法 (Method for exit pupil uniformity of holographic waveguide display system ) 是由 刘奡 张宇宁 沈忠文 翁一士 于 2019-09-16 设计创作，主要内容包括：本发明设计一种全息波导显示系统的出瞳均匀性设计方法,在全息波导近眼显示系统中,系统的出瞳经过扩展后,其显示亮度、色彩的均匀性是最重要的指标之一。通过对输出耦合全息体光栅区域效率的精细控制原理及方法深入分析与优化,对全息波导显示系统出瞳均匀性进行讨论与设计。(The invention designs an exit pupil uniformity design method of a holographic waveguide display system, in the holographic waveguide near-eye display system, the exit pupil of the system is expanded, and the uniformity of display brightness and color is one of the most important indexes. The exit pupil uniformity of the holographic waveguide display system is discussed and designed by deeply analyzing and optimizing the fine control principle and method of the output coupling holographic body grating area efficiency.)

1. A method of exit pupil uniformity for a holographic waveguide display system, the method comprising the steps of:

Step 1: analyzing the energy distribution of all light waves in the whole exit pupil range according to the propagation process of the image-bearing light waves coupled into the waveguide at different tracking angles in the waveguide and the coupling-out condition of the image-bearing light waves out of the output coupling grating,

Step 2: changing the peak efficiency of each local area of the output grating by programming to obtain the energy distribution of the corresponding output light intensity, and 3: selecting the output coupling grating efficiency distribution corresponding to the most uniform display effect in the whole exit pupil range by using a neural network algorithm;

And 4, step 4: by utilizing a holographic exposure technology, when the grating is recorded, the recording material is subjected to double-beam interference exposure by precisely controlling the energy ratio, polarization, exposure and pre-exposure time of object light and reference light of each part of an exposure area so as to prepare an output coupling grating with uniform efficiency distribution;

The above steps 1-4 achieve the effect of meeting the display output luminance uniformity under the full field of view over the entire exit pupil range.

2. The method of claim 1, wherein in step 1, the propagation paths of the image light waves under different FOVs in the waveguide are tracked and analyzed, the relationship between the energy of each light exit pupil and the local efficiency of the grating at the exit pupil is tracked, the appropriate optimization criterion is selected, and an optimization algorithm is programmed to optimize the efficiency distribution curve of the output coupling grating.

3. The method as claimed in claim 1, wherein in step 2, the coupled-in light is discretized according to the angular range of FOV, the coupled-out grating is discretized according to the spatial position, the propagation trajectory of the incident light in the waveguide at each angle is tracked until the coupled-out light is partially coupled out according to the local efficiency at the coupled-out grating, the coupled-out energy of the coupled-out light at the corresponding coupled-out grating position and the local efficiency relationship at the corresponding coupled-out grating position are recorded, and when the discretized number is large enough and the coupled-out light is sufficiently large, the corresponding coupled-out grating efficiency distribution set can be calculated according to the minimization of the mean variance of the coupled-out light.

4. The method of claim 1, wherein in step 3, the criterion of the uniformity of the outcoupled light is one of the minimization of mean square deviation or covariance, standard deviation, and coefficient of variation.

5. The method of claim 1, wherein in step 1, when the image-bearing light wave is incident into the waveguide at an angle θ, and diffracted by the input coupling grating, it propagates through the waveguide at an angle β, and its propagation period P is related to the waveguide thickness t and the beam propagation angle β by the following relation:

P(β,t)＝2t·tan(β) (6.1)

The incident angle is discretized in the range of the full field of view (FOV), the coupling output grating is discretized according to the propagation direction and the position coordinate, and the efficiency eta can be controlled by the output grating along different positions_mTracking the output intensity I of the full FOV over the entire exit pupil_diff(m)。

6. The method for exit pupil uniformity of a holographic waveguide display system of claim 1, wherein, in step 2,

the method comprises the following specific steps: the width of the incident beam in the waveguide depends on the pupil size of the collimating system and the size of the in-coupling grating, and the periodic step length of the light wave propagation in the waveguide is expressed as P, W_inDenotes the width of the incident light wave beam at an angle, when W_inP, there will be a gap Δ P-W between two adjacent output beams_infor the output beam to be continuous, the incident beam width W at different angles_inshould be greater than or equal to the propagation period length P after coupling into the waveguide at the angle of incidence;

As a standard of target optimization, the uniformity of the output light intensity is the minimization of the integral deviation of the output light intensity diffracted from each local area of the output grating under each propagation angle, and N different propagation angles are takenthe coefficient of variation (C.V) is selected as the optimization criterion to satisfy the output uniformity and further ensure the maximization of the system light effect:

representing angles of propagationThe average value of the local intensity of each diffraction light intensity of the lower output grating, and m is the diffraction and rebound times of the light wave in the output grating area in the waveguide under a specific propagation angle;

According to the distribution of the initial light intensity, one propagation period is divided into four regions: l1, L2, L3, L4,For the calculated average efficiency of the output grating local area, where m represents the bounce times of the beam propagation, L_kMeaning that such reflected light covers a certain L of the output grating_kRegion, the average efficiency being according to the efficiency η of the respective region_NAnd calculating the weighted average value of the values; the propagation period and the local area division corresponding to the optical waveguide are different under different FOV angles, and the optical waveguide is obtained in a large enough range in the FOV rangeWhen the number of samples is used for sampling calculation, enough characterization equations are obtained to deduce output grating efficiency distribution eta under the requirement of required uniformity₁～η_N。

Technical Field

The invention relates to a method for exit pupil uniformity of a holographic waveguide display system, and belongs to the technical field of augmented reality near-to-eye display.

Background

in recent years, rapid advances in integrated optics and microelectronics have led to the widespread use of holographic waveguide technology for image displays, liquid crystal lighting and optical interconnects. Holographic waveguide technology is a revolutionary new approach to head-mounted display design. In this way, a compact structure, light weight, large exit pupil, and excellent real-world transmission of the wearable near-eye display can be achieved. For a holographic waveguide display system, the holographic waveguide display system mainly comprises a micro-display image source, a collimation system, an optical waveguide, a driving circuit and a system circuit, wherein the holographic optical waveguide comprises three important parts: an input-coupled grating, a waveguide, and an output-coupled grating. The output coupling grating (holographic grating, holographic body grating, plane grating, relief grating, etc., thin microstructure optical device for realizing input and output coupling of light source image and binary expansion function) is the difficult part in the core component and system design of the optical waveguide display system. The holographic waveguide structure is used for near-eye display, has the obvious advantages of conveniently realizing the exit pupil expansion function, and solving the fundamental contradiction of the traditional optical display device on the principle of field angle, exit pupil size and device volume, thereby realizing a light, thin, large-field-of-view and large-exit pupil near-eye display device. With the exit pupil expanded, the brightness of the display output is closely related to the local efficiency of the out-coupling grating. The total internal reflection propagation path and exit pupil position of the waveguide are different at different wavelengths and different field angles. Thus, to ensure the consistency of the output display at a large exit pupil, different requirements are placed on the local efficiency of the output coupling grating. So overall, the efficiency distribution requirement of the output grating cannot get a perfect result, and the process can only be a result of a comprehensive compromise for all fields of view and all colors. The optimization process requires modeling the guiding process of the holographic waveguide and optimizing the efficiency distribution of the output coupling grating by a program algorithm.

Over the years, research related to holographic waveguide displays has mostly focused on improving grating peak efficiency and the expansion of the field of view (FOV) of the system, leaving less discussion of the continuity and uniformity of the exit pupil of holographic waveguide systems. RevitalShechter et al, Israel, in 2002 proposed a design for an out-coupling grating with a stepped diffraction efficiency profile. However, this design only considers the central viewing angle, ignoring the distribution of the out-coupled grating derived from other viewing angles in the FOV, with the result that it is less effective in full-field applications; in 2003, IosephGurwich et al attempted to discuss the uniformity of the exit pupil across the entire FOV, but in the actual optimization process, only three views, left and right edges and the middle field of view, were used for the calculations, which resulted in a slight improvement, but were not ideal for larger fields of view applications.

In the above schemes, the input and output optical coupling elements are all reflective volume type holographic gratings, but the optimization of the exit pupil uniformity of the holographic waveguide display system is not ideal, and the consideration of the problem is relatively simple.

disclosure of Invention

the invention aims to solve the technical problem that in order to overcome the defects of the prior art, the invention provides a design method of the field-of-view exit pupil uniformity of a holographic waveguide display system.

In order to achieve the purpose, the technical scheme of the invention is as follows: a method for exit pupil uniformity of a holographic waveguide display system, the method comprising: step 1: and analyzing the energy distribution of all light waves in the whole exit pupil range according to the propagation process of the image-bearing light waves coupled into the waveguide from different tracking angles in the waveguide and the coupling-out condition of the image-bearing light waves out of the output coupling grating.

further, when the image-bearing light wave is incident into the waveguide at an angle θ, the light wave is diffracted by the input coupling grating and then propagates in the waveguide at an angle β. The propagation period P is related to the waveguide thickness t and the light beam transmission angle beta, and the specific relationship is as follows:

P(β,t)＝2t·tan(β) (6.1)

The expansion process of the exit pupil of the system can be described as: the output coupling grating diffracts a portion of the energy-bearing image light wave out of the waveguide and leaves a portion of the energy to continue propagating forward so that light can be diffracted out at different locations along the waveguide on the output grating. The system can be conveniently designed to fully meet output uniformity requirements at least at one wavelength and angle of incidence. Discretizing the incident angle in the range of the full field of view (FOV), discretizing the coupled output grating in the propagation direction and in the position coordinate,The efficiency eta can be controlled by controlling the output grating along different positions_mTracking the output intensity I of the full FOV over the entire exit pupil_diff(m)。

Step 2: and changing the peak efficiency of each local area of the output grating by programming to obtain the energy distribution of the corresponding output light intensity.

Further, in addition to uniformity of output intensity, the continuity of the pupil of the exit pupil region is another key factor to improve display performance. In practice, the width of the incident beam in the waveguide depends on the pupil size of the collimating system and the size of the incoupling grating. The periodic step length of light wave propagation in the waveguide is recorded as P, W_inRepresenting the width of an incident lightwave beam at an angle. When W is_inP, there will be a gap Δ P-W between two adjacent output beams_in. The presence of these gaps affects the continuity of the exit pupil, giving a dark field on the display. For the output beam to be continuous, the incident beam width W at different angles_inShould be greater than or equal to the propagation period length P after coupling into the waveguide at that angle of incidence.

Further, for a relatively large FOV, corresponding to different propagation angles β_nThe propagation conditions of the light beams in the waveguide are very different. We need to help with more sophisticated numerical optimization procedures. First, as a criterion for target optimization, the uniformity of the output light intensity is the minimization of the integral deviation of the output light intensity diffracted from each local region of the output grating at each propagation angle. Taking N different propagation anglesThe coefficient of variation (C.V) is selected as the optimization criterion to satisfy the output uniformity and further ensure the maximization of the system light effect:

representing angles of propagationThe average value of the local intensity of each diffraction light intensity of the lower output grating, and m is the diffraction and rebound times of the light wave in the output grating area in the waveguide under a specific propagation angle.

Further, according to the distribution of the initial light intensity, a propagation period is divided into four regions: l1, L2, L3, L4.For the calculated average efficiency of the output grating local area, where m represents the bounce times of the beam propagation, L_kmeaning that such reflected light covers a certain L of the output grating_kRegion, the average efficiency being according to the efficiency η of the respective region_NAnd a weighted average thereof. The propagation period and the local area division corresponding to the optical waveguide are different under different FOV angles, and the optical waveguide is obtained in a large enough range in the FOV rangewhen the number of samples is used for sampling calculation, enough characterization equations can be obtained to derive the output grating efficiency distribution eta under the requirement of required uniformity₁～η_N。

And step 3: and (4) optimizing the output coupling grating efficiency distribution corresponding to the most uniform display effect in the whole exit pupil range by using a neural network algorithm.

furthermore, a genetic algorithm is utilized to carry out global intelligent search on the evaluation criterion to find the optimal sample, the process is a recursive iterative process at the same time, and the minimum C.V. of the system is obtained by approximating the optimal value of the efficiency distribution.

Further, in the above process, in order to prevent C.V from converging at the local extremum, a random scrambling process is added to the program to ensure the robustness of the algorithm.

Compared with the prior art, the invention has the following technical effects: the method for designing the uniformity of the exit pupil of the holographic waveguide display system in the field of view is based on a light ray tracing model of a waveguide image, combines a holographic waveguide structure, and performs global optimization on an efficiency distribution curve of an output coupling grating related to a spatial position according to the uniformity of the output coupling grating in the whole exit pupil range under the large-field-of-view display condition so as to solve the problem of the uniformity of the exit pupil of the traditional holographic waveguide display device.

Drawings

The technical scheme of the invention is further explained by combining the accompanying drawings as follows:

FIG. 1 is a schematic representation of image-bearing light waves propagating within a waveguide;

Figure 2 is a schematic of exit pupil expansion;

Figure 3 exit pupil continuity schematic;

FIG. 4 is a schematic diagram of an efficiency distribution curve of a one-dimensional exit pupil expansion output coupling grating;

FIG. 5 shows the convergence of the criterion coefficient of variation C.V in the optimization of the efficiency distribution curve;

FIG. 6 holographic optical waveguide system ray trace 3D model;

Fig. 7 shows the simulation of the output illumination of the optimized system.

Detailed Description