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Abstract
We develop a compact full-color augmented reality near-eye display system with a multicolor holographic optical combiner and a freeform relay system. The digital image is produced by a full-color micro organic light-emitting diode (Micro-OLED) display module. The freeform relay system includes four freeform optics and a holographic optical mirror, which are employed to correct both the monochromatic and chromatic aberrations caused by the holographic optical combiner. The two multicolor holographic mirrors have a three-layer laminated structure and are delicately fabricated to yield an improved diffractive efficiency and a reduced efficiency difference for red, green, and blue colors. The high degrees of freedom of freeform optics, and the thin and light nature of the holographic optical combiner yield a compact form factor near-eye display system with a diagonal field of view (FOV) of 20° and the eye-box of 5 mm × 5 mm. Two prototypes are built to demonstrate the feasibility of the proposed display system.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Augmented reality (AR), which aims to seamlessly superimpose virtual digital contents on a real-world scene to supplement reality, has been widely recognized as a next-generation interactive display technology [1]. Near-eye displays (NEDs), which are an enabling head-mounted technology capable of implementing augmented reality, have attracted a great deal of interest in both academia and industry [2]. A key component of an AR NED system is the image optical combiner, which delivers the displayed images to viewer’s eye and in the meantime transmits the environment light [3]. Generally, there are two typical types of combiners: waveguide combiner [4–10] and free-space combiner [11–19]. The waveguide combiner usually includes two couplers (an input coupler and an output coupler) and a waveguide that transmits the digital image via total internal reflection (TIR). Since the trapped light is repeatedly coupled out of the waveguide after each TIR, the waveguide-based AR NED systems could have a large eye-box without compromising the whole FOV, which enlarges the system etendue. According to the features of the couplers, the waveguide combiners can be divided into two types: geometric [4–7] and diffractive couplers [8–10]. For the geometric type, the partial mirror array is usually used as the output coupler. Due to the excellent optical properties of mirrors, the geometric waveguide usually yields higher efficiency and better color uniformity than its diffractive counterparts [3]. An inherent issue facing the geometric waveguide is light uniformity. To improve uniform light output, the efficiency of the out-coupler should be a gradient, which may result in quite complicated and costly fabrication [20]. For the diffractive type, surface relief gratings (SRGs) are usually employed as the couplers (e.g., Microsoft Hololens) [9]. However, the accurate fabrication of SRGs is a big challenge, and consequently high cost of fabrication cannot be avoided. Besides, the ghost issue caused by the out-coupler grating may degrade the display quality. Additionally, the full-color performance is also a major issue of the waveguide-based AR NED systems that need to be addressed [3]. Although great efforts have been made, a better and more practical solution is still needed for the waveguide combiners. For the free-space combiner, the light propagates freely in space [3]. The conventional AR NED systems usually use traditional geometric optics (e.g., a half mirror [11] or a freeform prism [12–14]) as combiners. These approaches could yield a relatively simple design with a decent FOV and eye-box. Since the thickness of the combiner is strongly determined by the FOV and eye relief of the display system, a larger FOV or eye relief usually yields a greater thickness of the combiner [12,13]. Thus, tradeoffs should be made between the system form factor and the key parameters (e.g., FOV, eye relief, and eye-box) [21]. Alternatively, volume holographic optical elements (VHOEs) which are flexible photopolymer films with a thickness of tens of microns can also be used as combiners. The diffractive behavior and unique features (e.g., strong selectivity on the wavelength and incident angle, and multiplexing ability) of the VHOEs could yield AR NED systems with a compact form factor. VHOEs are relatively conveniently fabricated by recording the interference pattern of a pair of coherent wavefronts in a photosensitive material, and can be easily laminated onto a glass plate or a curved surface [22]. These distinct properties allow the VHOEs to have wide applications in AR near-eye displays. Many reported AR NED systems [15–18] which employed a free-space VHOE combiner focused on the Maxwellian display. The major limitation of a typical Maxwellian system is the tiny eye-box which is theoretically just a point. It means that a slight deviation of viewer’s eye pupil will make the virtual image disappear completely, which is undesirable for NEDs systems.
In this paper, we develop a compact full-color AR NED system with a decent FOV and eye-box, using a freeform relay system and a free-space VHOE combiner. The digital image displayed on a full-color Micro-OLED display panel is relayed by the freeform relay system and further diffracted by the VHOE combiner to form a virtual image. The unique characteristics of VHOEs and the high degrees of design freedom offered by the freeform relay system yield a compact form factor AR NED system with high display performance. The rest of this paper is organized as follows. Section 2 introduces the basic architecture of the proposed full-color AR NED system and the optimization design of the display system. Section 3 introduces the fabrication of the VHOE mirrors and presents the optimal structure of the VHOE mirror. After that, we implement two prototypes of the proposed display system in Section 4, and the display performance of the two prototypes is also evaluated in this section. Then, the limitations of the proposed NED system are discussed in Section 5 before we conclude our work in Section 6.
2. Architecture and optical design of the proposed NED system
The architecture of the proposed full-color AR NED system is shown in Fig. 1. The display system includes a full-color Micro-OLED display module, a freeform relay system, and a VHOE combiner. Figure 2 shows the broadband spectrum of the Micro-OLED display, which indicates that large chromatic aberration cannot be avoided due to the diffractive nature of the VHOE combiner. Due to the chromatic behavior of VHOEs which is quite different from that of a refractive optical element, it is difficult to correct the chromatic aberration caused by the VHOE combiner with refractive lenses in the proposed AR NED system. In order to achieve a small form factor of the display system, a VHOE mirror (VHOE 2 in Fig. 1) is employed in the freeform relay system to correct the chromatic aberration caused by the VHOE combiner (VHOE 1 in Fig. 1), and four freeform optical elements are used here to correct the large monochromatic aberrations caused by the highly tilted architecture of the display system.
Fig. 1. Architecture of the proposed display system. The digital image displayed on the Micro-OLED (MOLED) is relayed by the freeform relay system (dotted box) and further diffracted by the VHOE combiner (VHOE 1) to form a virtual image. The freeform relay system is composed of three freeform lenses (FL1, FL2, and FL3), a freeform mirror (FM), and a VHOE mirror (VHOE 2).
Fig. 2. The spectrum of the Micro-OLED when a full-white image is displayed. The peak wavelengths are 452 nm, 525 nm, and 614 nm.
The freeform relay system relays the input image generated by the Micro-OLED image source to form an intermediate image between the relay system and the VHOE combiner with large optical aberrations. In the freeform relay system shown in Fig. 1, the light rays emitted from the Micro-OLED display sequentially pass through freeform lens 1 (FL1), freeform lens 2 (FL2), and a thin glass plate, on which the VHOE mirror is attached. After the light rays are reflected by the freeform mirror (FM) and VHOE 2, the light rays are further refracted by freeform lens 3 (FL3) to form an intermediate image between the relay system and the VHOE combiner. It is worth mentioning that the freeform mirror is used here to correct the monochromatic aberrations and fold the optical path to guarantee a compact form factor of the display system. Then, the light rays are reflected off the VHOE combiner which is also a VHOE mirror, and captured by the human eye, thereby forming a high-quality virtual image in front of the eye, as shown in Fig. 1. The specifications of the proposed AR NED system are given in Table 1. Due to the relatively narrow spectral bandwidth of the photopolymer-based VHOEs, the AR NED system is optimized at R/G/B color positions with central wavelengths of 614 nm, 525 nm, and 452 nm which coincide with the dominant wavelengths of the Micro-OLED shown in Fig. 2. The spectral bandwidth at the R/G/B color positions is 15 nm. One thing to note is that the VHOE mirrors are separately optimized for better correction of chromatic aberrations at each color position. The construction parameters of the two VHOEs have been presented in Section 3.
Table 1. Specifications of the proposed AR NED system
When designing this highly tilted and folded NED system, a big challenge facing us is to find a starting point for the optimization of the NED system. In order to find a good starting point, we start from a spherical imaging system with an on-axis configuration. Six field points are sampled over the FOV. The radius of curvature of each spherical surface, the lens thickness, and the air spacing are optimized to reduce the spot size of each field point and to maintain the desired first-order parameters. It should be mentioned that constraints should be imposed on the lens thickness to avoid weird lens shapes. For the optimization of the two VHOE mirrors, the positions of the centers of curvature of both the object and reference wavefronts are optimized, and the relatively narrow angular bandwidth of photopolymer-based VHOEs is also considered. After that, each optical element is tilted and decentered, and further optimized to shift the display system from an on-axis configuration to an off-axis configuration until the predefined structure constraints are satisfied. This optimization scheme can usually yield a good starting point for further optimization. When a starting point is obtained, more field points are sampled over the FOV, and all spherical surfaces are converted to conic surfaces for an intermediate state of optimization. After the intermediate optimization, both reflective and refractive optical surfaces are represented by XY polynomials (some other polynomial surfaces can also be employed), and the order of the polynomial is gradually increased and optimized till the optimization gets saturated. Figure 3 shows the modulation transfer function (MTF) curves and distortion at the R/G/B color positions. From Fig. 3(a) we clearly see that the MTF value at 80 lp/mm is greater than 20%. It should also be noted that it is not a simple task to eliminate distortion in a highly tilted imaging system. Thus, the distortion at the R/G/B color positions is reduced to below 10%, as shown in Fig. 3(b).
Fig. 3. Optical performance of the optimized full-color AR NED system: (a) the MTF curves and (b) the distortion at the R/G/B colors positions.
3. Fabrication of the two multicolor VHOE mirrors
In this section, we introduce the fabrication of the two multicolor VHOE mirrors. Figure 4 presents a schematic configuration of an experimental setup for recording the two multicolor VHOE mirrors. The recording is made at 639 nm (red), 532 nm (green), and 473 nm (blue). The R/G/B laser beam is first divided into two parts by means of a polarizing beam splitter (PBS) and a half-wave plate (HW1) which controls the intensity ratio of the two parts. The transmitted P-polarization beam passes through a spatial filter (SF) which is compounded of a 20 × microscope objective lens and a pinhole with a diameter of 15 μm, and then is converted into a parallel beam after passing through a collimating lens (CL1). After that, the R/G/B collimated laser beams are combined into a single beam via dichroic mirrors (DM1, DM2), and then the combined laser beam passes through a spherical lens (L1) with a focal length of 30 mm to form a divergent reference beam. Similarly, the reflected S-polarization beam passes through a spatial filter which also includes a 20 × microscope objective lens and a pinhole with a diameter of 15 µm. The filtered laser beam is converted into a parallel beam by use of a collimating lens (CL2), and the R/G/B collimated laser beams are combined into a single beam. Then, the combined laser beam passes through a spherical lens (L2) with a focal length of 300 mm to form a divergent object beam. It should be noted that the polarization of both the object and reference beams should be the same in order to enhance the contrast of interference fringes. Thus, another half-wave plate (HW2) is introduced into the laser beam path to rotate the P-polarization to S-polarization. Three electric shutters are employed here to control exposure time to enhance diffraction efficiencies when recording the R/G/B VHOE mirrors. The interference of the object beam and the reference beam is recorded in the photopolymer with refractive index modulation. The CRT20 holographic film is used to record the VHOE mirrors and is attached to a glass substrate. The beams are power balanced and the respective total R/G/B exposure dosage is set at 30-45 mJ/cm2. After recording, UV exposure (about 2 minutes) is required to consume the residual photosensitizer and fix the photopolymer.
As mentioned above, the AR NED system is optimized at R/G/B color positions. Consequently, the parameters of the recording beams at each color position are slightly different, as shown in Table 2. Figure 5 schematically shows the arrangement of the recording beams. A Cartesian coordinate system is placed at the VHOE plane. F1 and F2 are the construction points of the VHOE. α and β, respectively, denote the angle between the normal to the VHOE plane and the line OF1, and the angle between the normal to the VHOE plane and the line OF2. l1 and l2, respectively, denote the distance between F1 and O, and the distance between F2 and O.
Fig. 5. Schematic diagram of VHOE constructive parameters. O - axial point of the VHOE, F1, F2 - construction points of the VHOE, N - normal to the VHOE plane, α, β - the angle between OF1/OF2 and N, l1, l2 – the distance between F1/F2 and O.
Table 2. Parameters of the recording beams for the two VHOE mirrors
In the proposed full-color AR NED system, the brightness and color uniformity of the virtual image observed by the viewer is strongly determined by the diffraction efficiency of the VHOE mirrors. Here, the diffraction efficiency of the VHOE mirror is defined by
where, Iin and Idiff represent the intensity of the incident light beam and that of the diffracted beam, respectively. It is apparent that we need to improve the diffraction efficiency of the VHOE mirrors at R/G/B colors and meanwhile reduce the efficiency differences between R/G/B colors.Since the performance of a multicolor VHOE is strongly determined by the structure of the multicolor VHOE, we investigate three different structures of the VHOE combiner, as shown in Fig. 6. In Fig. 6(a), three color holograms are recorded in a single photopolymer film. This structure is simple; however, the diffraction efficiencies of R, G, and B colors are relatively low, which are equal to 35%, 38%, and 30%, respectively. Figure 6(b) shows a composited structure, where two-layer photopolymers are laminated. The green and blue holograms are recorded in a single photopolymer film which is placed on top of the VHOE combiner, and the red hologram is recorded in a single photopolymer film. The diffraction efficiencies of R, G, and B colors in this structure equal 70%, 50%, and 45%, respectively. Obviously, the diffraction efficiencies are significantly improved when compared to the first structure shown in Fig. 6(a); however, the efficiency differences between R/G/B colors are relatively large. We also record R, G, and B holograms in three separate photopolymer films, and laminate the three-layer photopolymers, as shown in Fig. 6(c). The diffraction efficiencies of R, G, and B colors in this structure are 60%, 62%, and 70%, respectively. It is apparent that the diffraction efficiencies of R, G, and B colors are improved and the efficiency differences between R/G/B colors are reduced. From the three structures, we know that the third structure shown in Fig. 6(c) can yield better performance. Thus, the third structure is employed to fabricate the two multicolor VHOE mirrors, as shown in Fig. 8(a). Exposure parameters are listed in Table 3. The average transmittance of the multicolor VHOE mirror equals 67.1% in the visible spectrum (from 400 nm to 780 nm), as shown in Fig. 7(a). We also fabricate two monochromatic VHOE mirrors, as shown in Fig. 8(a). The diffraction efficiency of the green VHOE mirror equals approximately 70%, and the average transmittance of the green VHOE mirror is equal to 84.5% in the visible spectrum, as shown in Fig. 7(b), indicating a good see-through capability.
Fig. 6. Three different structures of the VHOE mirror: (a) one-layer structure, (b) two-layer composited structure, and (c) three-layer laminated structure.
Fig. 8. Prototypes and testing setup. (a) The fabricated freeform elements, the fabricated VHOE mirrors, and the prototype; (b) the testing setup for performance evaluation.
Table 3. Exposure parameters of R/G/B color
4. Prototypes and experimental verification
The four freeform elements are fabricated by injection molding, as shown in Fig. 8(a). The freeform relay system is constructed by assembling the four freeform elements and VHOE 2. Then, we implement two wearable binocular prototypes: one with monochrome display and the other one with full-color display. The freeform optical components are identical in the two prototypes, except for the VHOE mirrors. The Micro-OLED display drivers are fixed on the glasses legs. Figure 8(a) shows the prototype of the proposed AR NED system. From this figure we can see a compact form factor of the prototype, indicating a comfortable wearing experience.
In order to evaluate the performance of the two prototypes, we build a testing setup shown in Fig. 8(b). An industrial camera (Image Source DFK 33UX250) is placed at the eye-box plane to simulate the eye pupil for capturing images. The focal length of the camera lens equals 8 mm. The digital image displayed on the Micro-OLED display module is relayed by the freeform relay system and projected to the VHOE combiner (VHOE 1). After reflection by the VHOE combiner, the virtual digital image is captured by the camera. An apple is placed in the real world scene for distance reference. The distance between the apple and the camera equals approximately 70 cm.
First, we evaluate the performance of the monochrome prototype. The display results of the monochrome prototype are given in Fig. 9. Figures 9(a) and 9(b) show the original digital images displayed on the Micro-OLED display panel, and the virtual images captured by the camera are given in Figs. 9(c) and 9(d). As mentioned in Section 2, both the monochromatic and chromatic aberrations are corrected at each color position. From Fig. 9 we can see that both the virtual images and the apple in the real world are clearly observed, indicating the good performance of the monochrome prototype and therefore the feasibility of the proposed system. One thing to note is that the brightness of the virtual images decreases from the center to the marginal part of the images. It is mainly due to the angular selectivity of the two VHOE mirrors. It also tells us that both diffraction efficiency and uniformity should be considered in the optical design of the display system. The eye-box size of the proposed system is verified as well. The experimental results in Fig. 10 show photographs of display results observed at different eye-box positions. For testing accurately, we set a “ZJU” pattern boxed in boundary lines. The relative location in the eye-box is indicated at the bottom of each picture. Figure 10(c) shows the result when the camera is placed in the middle of the eye-box. Move the camera horizontally and vertically until the edges of the displayed image disappear. Eventually, the eye-box size of the prototype approximately equals 4.5 mm (H) × 5 mm (V). Errors in prototype assembly primarily lead to differences with the design.
Fig. 9. Display results of the monochrome prototype: (a) and (b) the original digital images displayed on the Micro-OLED display panel; (c) and (d) the captured images.
Fig. 10. Observed images at different eye-box positions. The location is indicated at the bottom of each photograph.
Next, we also evaluate the performance of the full-color prototype. The display results of the full-color prototype are given in Fig. 11. As mentioned above, the proposed full-color AR NED system is individually optimized at R, G, and B color positions. Although the chromatic aberrations are well corrected at each color position, the chromatic aberrations caused by the R, G, and B color positions are not corrected. It means that the images at the three color positions cannot overlap properly, and position deviations cannot be avoided. It is worth mentioning that the position deviation can be reduced by reducing the chromatic aberrations caused by the R, G, and B color positions. Besides, both assembly and fabrication errors will cause the position deviations. Thus, the original images displayed on the Micro-OLED display panel should be pre-compensated to eliminate the position deviations of the virtual images, as shown in Figs. 11(a)–11(c). The virtual images captured by the camera are given in Figs. 11(d)–11(f). From Figs. 11(d) and 11(f) we can clearly see that white color images are properly reproduced, also indicating the feasibility of the proposed full-color AR NED system. It should be noted that from the captured virtual images we can also observe an image duplication effect, as shown in Figs. 11(d)–11(f). This image duplication effect is mainly caused by the Micro-OLED display module, and will be discussed in detail in the next section.
Fig. 11. Display results of the full-color prototype: (a), (b), and (c) the original digital images displayed on the Micro-OLED display panel; (e), (d), and (f) the captured images.
5. Discussions
Fig. 12. The image duplication effect in the monochromatic display: (a), (b), and (c) the original full red, green, and blue images displayed on the Micro-OLED display panel; (d), (e), and (f) the captured images with a black background.
As mentioned above, there is an image duplication effect in the full-color display. In order to analyze the image duplication effect, full red, green, and blue images are individually displayed with the full-color prototype. The captured images with a dark background are shown in Fig. 12. We can see a slight image duplication effect in Fig. 12(d), and an obvious image duplication effect in both Figs. 12(e) and 12(f), which tells us that the image duplication effect also exists when monochromatic images are displayed with the full-color prototype. Figure 13 gives the spectrum distributions of the Micro-OLED display module measured along three different directions. The direction 2 is perpendicular to the Micro-OLED panel. The angle between the directions 1 and 2 equals 60°, and that between the directions 2 and 3 also equals 60°. Figures 13(b)–13(d) show the measured spectrum distributions of the Micro-OLED panel when full red, green, and blue images are individually displayed. From the measured spectrum distributions, we can see that the color is correct at the direction 2 which is perpendicular to the Micro-OLED panel. However, the color shift takes place when the detector moves to the directions 1 and 3. This is a common issue in OLED display systems, which is known as angular color shift [23–25]. In order to improve optical efficiency and color purity, OLED with two metallic electrodes utilizing strong microcavity resonance has been widely adopted. Due to the microcavity resonance, the emission spectrum of each individual pixel would shift as viewing angles increase [23]. Consequently, the angular color shift inevitably yields the image duplication effect in the full-color display. However, the image duplication effect can be avoided in the monochrome display due to the narrow spectral bandwidth of the VHOE mirrors, as shown in Fig. 8. It is of great interest to mention that the image duplication effect can also be avoided in the full-color display by using a narrowband light source, such as a laser beam scanning (LBS) projector.
Fig. 13. The different spectrum distributions of the Micro-OLED measured along different directions. (a) Schematic diagram of spectrum test of the Micro OLED; the measured spectrum distributions of the Micro-OLED measured along different directions when full-red (b), full-green (c), and full-blue images (d) are displayed on the Micro-OLED display panel.
6. Conclusion
In summary, a compact full-color AR NED system which includes a multicolor VHOE combiner and a freeform relay system is proposed in this paper. The high degrees of design freedom of the freeform optical elements and the unique characteristics of the VHOE mirrors allow to achieve a compact form factor of the proposed AR NED system. The multicolor VHOE mirror includes three separate photopolymer films. The blue hologram is placed at the top of the VHOE mirror, and the green hologram is placed in between the red and blue holograms. This structure of the VHOE mirror can yield an improved diffractive efficiency and a reduced efficiency difference. Two wearable binocular prototypes are built to demonstrate the feasibility of the proposed system. The performance of the monochrome prototype is good and the image duplication effect caused by the angular color shift of the Micro-OLED display module is avoided in the monochrome display due to the narrow spectral bandwidth of the VHOE mirrors. In the full-color prototype, the chromatic aberrations caused by the R/G/B color positions need to be well corrected, and the image duplication effect can be avoided by using a narrowband light source. In future work, we will take the angular and wavelength selectivity, and shrinkage of the material into account. We expect the proposed system could facilitate the development of AR NED displays.
Funding
National Key Research and Development Program of China (2021YFB2802200); National Natural Science Foundation of China (62022071, 12074338).
Disclosures
The authors declare that there are no conflicts of interest related to this paper.
Data availability
No data were generated or analyzed in the presented research.
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