Waveguide-type see-through displays have been presented for augmented-reality (AR) and mixed-reality (MR) application [1–6]. In such kinds of systems, a planar waveguide with grating structures for light in-coupling and light outcoupling is used as a see-through combiner. The information light is first coupled into the thin planar waveguide via the in-coupling grating, and then after propagating within the waveguide, it is extracted from the waveguide at the location of the outcoupling structure. The outcoupling structure can be either a holographic optical element (HOE) or a surface relief grating (SRG). Although the outcoupling structure extracts information light from the waveguide, it keeps the uniformly transparent property for see-through information from the real scene. Optical path for the modulated information light is folded within the waveguide, and therefore a planar and compact system layout is provided.

Planar waveguides with pupil replication are basically designed for collimation light sources. Consequently, the final presented virtual images are also located infinity optically for the image observer, and the observer’s eye is forced to focus at infinity owing to eye accommodation. If the location of a real object is near to a user, visual conflict is induced owing to the depth difference between the real objects and display images. If we try to input an image not locating at infinity in this system, the in-coupled information will not be collimation lights anymore, and simultaneously cross talk noise induced by the outcoupling may arise and an unwanted ghost image may appear [7]. Therefore, the waveguide display cannot easily implement a tunable image distance.

Several approaches have been proposed to resolve the issue of depth mismatch between real objects and display images for the waveguide see-through display. The waveguide-type holographic display is one of the proposed solutions, which offers a holographic image with natural accommodation response [8–10]. However, the FOV of the waveguide-based holographic display implemented with a spatial light modulator (SLM) is inherently limited by its pixel size. Enhancing the FOV in this system can be achieved with phase modulation on the outcoupling HOE [11,12], but consequently results in a reduced eye-box. Another technique is the Maxwellian type waveguide display, which can provide always focused images within a finite depth range [13,14], but the eye-box for the system is very small. A dual-focal waveguide-type display has also been presented by using a polarization-dependent lens device to mitigate the depth mismatch issue [15]. The dual-focal technique provides an alternative possibility to design a waveguide display with proper eye-box and FOV.

In this paper, we propose a see-through display based on a holographic waveguide combiner with a continuous tunable focal plane. Function of the tunable focus is achieved by using an electrically modulated liquid crystal (LC) lens. The focal-tunable LC lens has been applied to various focal-tunable display systems [16–18]. Furthermore, combination of a focal-tunable LC lens and pupil-replicating waveguide has been much discussed. The paper of Ref. [18] seems to be the first report to combine a pupil-replicating waveguide and tunable LC lens. Nevertheless, a rotation system setup must be used to avoid a ghost image. In this study, our proposed system works for a pupil-replicating waveguide, and the waveguide is aligned normal to the visual axis of the eye. The LC lens used in this paper is a polarization-dependent optical device. The lens function of the LC device is only operated for a p-polarized wave. For the s-polarized wave, the LC device always acts as a transparent window regardless of the applied voltage. The LC lens is attached on the waveguide at the outcoupling location. Techniques for generating arbitrary image distance in waveguide-type display become possible with operation of the attached liquid crystal lens.

Figure 1(a) shows the structure of the tunable-focal waveguide-based see-through display. The see-through waveguide display is designed for the p-polarized wave. A glass waveguide with a pair of symmetry HOEs was used to guide the information from the image source to the observer. The in-coupling HOE and outcoupling HOE are both reflection type linear gratings, which are generated with interference of two collimation waves.

 

Fig. 1. (a) Structure of tunable-focal waveguide-based see-through display. (b) Photograph of holographic waveguide. (c) Photograph of LC lens device.

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The recording light source for the interference system is a diode-pumped solid-state (DPSS) laser with a wavelength of 532 nm. Both HOEs were generated with the same recording process with the same optical specification. The reference beam for both HOEs is a collimation wave which is normal incident on a photopolymer film. The signal beam is designed to propagate within the waveguide with 125° deviation from the incident direction of reference beam. The dimensions of the waveguide are 7.5 cm in width and 5 cm in height. The thickness of the waveguide is 2 mm and the refractive index is approximately 1.52. The dimensions of the in-coupling HOE and outcoupling HOE are both 1.4 cm in width and 2.1 cm in height. The distance between the two HOEs is approximately 4 cm. A photograph of the waveguide is shown in Fig. 1(b). Polarizer 1 in Fig. 1(a) was used to make the input information maintain p-polarization. Meanwhile, polarizer 2 was used to make the see-through light from the real scene keep s-polarization. The coupled-out information light will propagate to the eye by passing through the LC lens. Because the diffracted image from the waveguide display is designed to be located at infinity, the final diffraction image will be located at the focal plane of the LC lens. Figure 1(c) shows a photograph of the LC lens. From the photograph, the lens device shows high transmission quality to serve as a transparent window for the real scene.

Without the LC lens, the system is a conventional waveguide see-through display providing a floating virtual mage at infinity. In our presented system, the lighting source for the input image is a white LED with a broadband spectrum from 400 nm to 700 nm. A plane image pattern is used as the input information. A camera lens (Nikon, Nikkor 50 mm f1.4) with 46° FOV is used as the projection lens to generate a collimation virtual image source. Figure 2 shows an image with a horizontal FOV of 24.5° and vertical FOV of 14° can be achieved in this system without using the LC lens. The images were captured by a camera with a 20-mm eye-relief.

 

Fig. 2. Diffraction image observed from the waveguide display without using LC lens. (a) USAF 1951 resolution target, (b) see-through AR scenes, and (c) 24.5° × 14° FOV is achieved within the red rectangular area.

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Figure 3 shows the scheme diagram of the LC lens structure. Three indium tin oxide (ITO)-deposited glass substrates were used for the presented LC lens. The glass substrate was 0.55-mm thick, and the ITO layer deposited on the glass substrate was approximately 100-nm thick. As indicated in Fig. 3, the homogeneous polyimide (AL 1426 CA, Daily Polymer, Taiwan) was spin-coated on the inner surfaces of the middle and bottom substrates with mechanical rubbing in the antiparallel direction. The bottom and the middle substrates were separated by a 25-µm-thick Mylar spacer. The ITO layer of the middle substrate was etched as a hole electrode. The diameter of the hole was 4 mm. The nematic LC E7 mixture (Daily Polymer Kaohsiung, Taiwan) had a nematic-isotropic temperature of 59°C, dielectric anisotropy Δε of 14.1 at room temperature, and birefringence Δn of 0.216 (n_o = 1.522 and n_e = 1.738) at the wavelength λ = 589 nm. The LC mixture was initially heated up to the isotropic phase and then filled in the empty lens cell through the capillary action. After filling, the LC mixture was cooled down to the nematic phase.

 

Fig. 3. Scheme diagram of LC lens structure.

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As shown in Fig. 4(a), with the absence of a supplied voltage, the LC molecules were aligned homogeneously, and the LC lens could not diverge the incident light. Figure 4(b) shows that once the voltage was supplied between the top and bottom ITO electrodes, the middle hole electrode was kept at equipotential with the bottom ITO electrode. The LC molecules at the periphery of the hole were aligned parallel to the substrate surface, but those at the center of the hole were aligned vertically. The effective refractive index for light that was transmitted through the sample exhibited a gradient profile, wherein the LC molecules in the periphery of the hole had a larger refraction index than those in the center of the hole. The incident light was therefore diverged and the LC sample operated as a negative lens.

 

Fig. 4. Scheme diagrams of LC orientations (a) without and (b) with the supplied voltage across the top and bottom ITO electrodes, where the middle hole electrode is kept at equipotential with the bottom ITO electrode.

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A focal-length measurement system was designed in this study to confirm the focal length of the fabricated LC lens. Figure 5(a) shows the experimental setup of the measurement system, which was optically constructed by using a 4f system and a SLM. The SLM was located at the front focal plane of Lens 2, and a spatial high-pass filter was located at its back focal plane. The back focal plane of Lens 2 was also the front focal plane of Lens 3. The LC lens was located at the back focal plane of Lens 3. When a collimation beam is incident on the SLM, the computer-generated-hologram (CGH) loaded on the SLM will generate a virtual image locating at infinity behind the SLM. Without the applied voltage on the LC lens, the final diffraction image from the SLM will locate at infinity for the camera, and therefore the camera will focus at infinity to obtain the diffraction image clearly. Once the negative power is added on the LC device, the diffraction image becomes out of focus for the camera. If a positive lens phase with the same power is added to the SLM, the negative power will be compensated, and the image captured by the camera becomes in focus again. With the vision observation, the power of the LC lens was inferred from the adding power on the SLM. The relationship between focal power and the applied voltage is shown in Fig. 5(b). The results show the tunable range for the focal power is from −1.54 D to 0 D. Equivalently, the focal length of LC lens is from −65 cm to negative infinity. The applied electric field is an AC square wave with 1-kHz frequency.

 

Fig. 5. (a) Scheme diagram of focal length measurement system of LC lens. (b) Relationship between the focal power of LC lens and the applied voltage.

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Figure 6 shows the bench-top experimental system of the proposed waveguide display. Figure 7 shows the experimental results when the LC lens was attached to the waveguide with an electric field of 0 V and 130 V and with an eye-relief of 20 mm behind the LC lens. Figure 7(a) shows a diffraction image locating at infinity when the LC lens is operated with a 0-V electric field. The whole diffraction image can reach 24.5° × 14° but it is zoomed into a 14° × 14° FOV only for detail observation. Figures 7(b) and 7(c) show the diffraction results when the LC lens is operated with a 130-V electric field, but they are observed at different focus planes. As shown in Fig. 7(b), when the camera focus is at infinity, the image at the outer periphery is in focus and the image at inner periphery becomes out of focus. Figure 7(c) shows the contrary focus condition. When the camera focus at 650 mm, the image at the outer periphery is out of focus and the image at the inner periphery becomes in focus. The results prove the LC lens modulates the image distance for the inner periphery part. However, in Fig. 7(c), for the images near to the aperture edge, including the inner side and outer side, cross talk images appear. The reason for the cross talk is that the dimension of the outcoupling HOE is larger than the diameter of the LC lens. The other area of the HOE which exceeds the LC lens aperture also contributed diffraction signals, and they were collected by the camera lens or observer’s eye pupil simultaneously. The way to remove cross talk effect is to make a dimension match between the LC lens and outcoupling HOE.

 

Fig. 6. Constructed bench-top experimental system.

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Fig. 7. Experimental results for image with 14° × 14° FOV. (a) Focusing at infinity with LC lens is operated with 0 V. (b) Focusing at infinity with LC lens is operated with 130 V. (c) Focusing at 650 mm with LC lens is operated with 130 V.

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To remove the cross talk noise, the dimension of the outcoupling HOE in our demonstration system is reduced to be 4 mm × 4 mm. Accordingly, the FOV for distance-tunable images is limited by the LC lens aperture, and therefore, the FOV of the system can be given as

Figure 8 shows the display results with the matched outcoupling HOE when the LC lens was operated at different applied voltages. When the applied voltage is 0 V, 50 V, and 130 V, the corresponding image distance is at infinity, 1.5 m, and 0.65 m in front of the LC lens. The FOV in the demonstrated system is 10.5° only because there is an air gap between the LC lens and the outcoupling HOE. If the LC lens is attached to the outcoupling HOE without a gap, a theoretical FOV of 11.4° can be reached. We took photos for the diffraction images by focusing at its in-focus plane and focusing at the infinity plane. For images not locating at infinity, their photographs show somewhat blurring phenomena when the camera is focused at infinity. The results prove the image distance is successfully modulated by applying voltage to the LC lens.

 

Fig. 8. Diffraction images of experimental results for different applied voltage on LC lens.

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In conclusion, a see-through display based on planar holographic waveguide with tunable focal plane is presented. A focal tunable negative liquid crystal lens is adapted on the waveguide display to modulate the image distance. The system provides a 10.5° FOV for arbitrary image distance. Image depth can be controlled to locate in front of the LC lens from 650 mm to infinity by tuning the applied voltage on the LC lens. The system FOV can be enlarged by increasing the diameter of the LC lens. The image FOV in this system can reach to a maximum when the LC lens diameter matches the dimension of the outcoupling HOE. The LC lens diameter can be enlarged by controlling the diameter of the hole-patterned electrode, as shown in Fig. 3. Techniques to design a larger aperture LC lens has been previously published [19] and therefore a display system operated with a larger aperture LC lens becomes possible for practical application. The depth mismatch issue for AR display can be effectively resolved with the proposed technique.