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Abstract
An augmented reality display based on a planar waveguide made in photo-thermo-refractive glass had been demonstrated. The monolithic integration of multiplexed volume Bragg gratings with the waveguide platform provided in/out-coupling and image transmission from a portable projector. Design and calculation of the grating configuration was performed. The optical replication method was simulated and implemented for manufacturing the multiplexed coupling elements. Resulted field of view was limited by the projection system of the prototype, however can be expanded beyond 50o.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Augmented reality (AR) technology is highly anticipated as the next generation display devices and indicators. Still, a perfect AR platform is very challenging to develop. It requires unobstructed see-through, large eye box and field of view at eye resolution and, not less important, an AR device should be ergonomic, lightweight, and resistant to environment [1,2]. Planar diffractive waveguides are a good trade-off to satisfy these requirements [3,4]. These devices became the focus of extensive research for AR head mounted displays (HMD). For today, the development conditions relate to increase the efficiency [5], enlarge the eye box [6,7], enlarge field of view (FOV) [8], improve the user interaction [9] or simplify the fabrication process [10].
Grating based configuration utilize diffraction to guide the image through a glass plate with thickness not more than a couple millimeters. A group of surface or volume diffractive optical elements (DOE) input and output the light, at the same time providing pupil expansion to enlarge size of eye box and required FOV. Inside of waveguide, beams propagate with help of total internal reflection (TIR). However, phase surface gratings are limited in the diffraction efficiency (maximum 20% per one grating) and need protection coatings because of its fragility to environment impact [1,11]. In general case, due to the dispersive nature of gratings, layers of the planar waveguide with different DOEs provide coupling for different components of the color spectrum (red, green and blue). The structural parameters for each DOE are optimized to the corresponding wavelength [12]. These conditions result in low efficiency and are associated with manufacturing difficulties. Volume holographic gratings is another commonly used DOE for AR HMD utilizing Bragg diffraction what helps to increase efficiency of the planar waveguide system [3]. However, volume holographic gratings (VHG) have high angular and spectral selectivity and require photosensitive materials with superior phase modulation properties and includes space or time variant elements.
Presented in this paper planar waveguide relies heavily on photo-thermal-refractive glass (PTR glass) used as photosensitive volume media for multiplexed holographic input and output couplers integrated into waveguide platform for image transmitting. PTR glass is a very promising and competitive photosensitive material for recording of highly efficient volume phase holograms operating in visible and near IR spectral range [13,14]. Exposure to near UV radiation and following thermal treatment at temperatures close to the glass transition temperature results in refractive index change in exposed area up to 1.5 × 10−3. This feature provides 3D phase hologram recording in volume of PTR glass. Volume Bragg gratings and image holograms recorded on this glass reveal a unique combination of working characteristics such as the high angle and spectral selectivity, high diffraction efficiency (up to 99.9%), high mechanical and optical strength, as well as high thermal and chemical durability (close to optical BK7 glass). Based on PTR glasses a broad variety of optical elements and devices were developed and fabricated, and today they are commercially available. The variety includes extra narrow-band spectral filters, wavelength division multiplexing devices, combiners of high-intensity light beams, phase plates, chirped gratings for compressing the light pulses, filters for increasing the spectral brightness and thermal stabilization of wavelength of laser diodes, holographic prisms and sights, optical, plasmonic and luminescent waveguides and fibers, etc. [13,14].
PTR glass material offers a breakthrough for see-through displays because avoids various problems when exposed to environmental perturbations such as temperature, humidity and pressure. In comparison to other holographic materials (silver halide, dichromated gelatin, photopolymers, holographic polymer dispersed liquid crystals) PTR glass has lower refractive index modulation, but at the same time it allows recording holograms over the entire thickness of the substrate not being limited with few micrometers. When recording, a phase structure is formed directly inside the medium, so the waveguide doesn’t need to be laminated and resistant to any cleaning means. Indisputable advantage of PTR glass for holography is the absence of shrinkage. It implies that the recorded structure would be developed without distortions and fulfills the coupling conditions that have been set during the recording. It needs to be mentioned that it is possible to make volume holograms in nonphotosensitive glass by exposing it with high-power femtosecond laser pulses, but direct laser writing offers not enough resolution.
Our paper demonstrates the key features of PTR glass as competitive holographic medium for monolithic integration of multiplexed holograms and planar waveguide platform for AR display technology.
2. Experiment
2.1 PTR glass substrate preparation
For this research, the PTR glass based on Na2O-ZnO-Al2O3-SiO2-NaHal (Hal = F, Br) system doped with CeO2, Sb2O3 and Ag2O was used. The glass was synthesized in platinum crucible at 1500 °С in the environment air. Stirring with a platinum thimble was used to homogenize the liquid. Plane-parallel glass substrates with good optical quality and size of 40×40×2 mm were prepared by regular grinding and polishing technique.
In the fabricated PTR glass, the dispersion before recording and after heat treatment is of interest. The refractive index of the unexposed region after the baking corresponds to the refractive index before the recording. According to the interpolation results of experimental measurements, the refractive index of the glass is determined by the function n(λ) = 4332·λ–2 +1.484 before recording and n(λ) = 4353·λ-2 +1.483 after the baking treatment. The dispersion characteristic of the PTR glass before the recording is shown in Fig. 1(a). Based on the dispersion curve after exposition, we can calculate the amplitude of refractive index modulation shown in Fig. 1(b).
Fig. 1. Dispersion curves for (a) refractive index and (b) refractive index modulation amplitude of PTR glass.
The refractive indexes and periodic refractive index modulations are following: nB = 1,5035, ΔnB = 4,51·10–4 for the wavelength λB = 461 nm; nG = 1,4991, ΔnG = 4,61·10–4 for λG = 520 nm; nR = 1,4936, ΔnR = 4,74·10–4 for λR = 640 nm. Δn is responsible for the entire dynamic range of the refractive index modulation. It is divided during N-fold multiplexing resulting in n1, n2,…,nN.
2.2 Design holographic AR display
The designed monolithic integrated multiplexed VHGs and planar waveguide platform is presented in Fig. 2(a). Multiplexed volume holographic gratings are used as the optical couplers for the green part of the visible spectrum that is propagating inside waveguide plate. Each multiplexed volume grating, schematically shown at Fig. 2(b), is responsible for FOV stitched part defined as full width at half maximum (FWHM) of angular selectivity contour. contour. The in-coupling VHG is employed to modulate the propagation directions and diffraction efficiencies of the collimated beams from the projector. According to the in-coupler properties, light beams introduced into the waveguide travel along the corresponding direction. The out-coupling VHG that breaks TIR is located near eye helps to enlarge the size of eye box (to expand the pupil). Light beams go out from the waveguide in the same direction as the incident beams as long as the structure of the out-coupling VHG is appropriately designed. Demonstrated in the paper, in- and out-couplers are identical and recorded with the same phase mask. Symmetric in/out-coupler configuration is the straightforward technique to output the inputted beam.
Fig. 2. (a) Schematic diagram of the waveguide display with multiplexed volume holographic gratings. (b) Single Bragg grating that recorded in the full thickness.
Diffraction gratings are designed to work with collimated beams and to create the image in infinity. The planar optical waveguide increases the eye box without increasing the size of the optical part in AR system. This property led to the general use of such design. To fulfill the waveguiding conditions inside the plate in-coupler can diffract the incident beams in reflection or transmission mode. That is defined by the KR and KS vector relation (Fig. 2(b)). This relation is expressed with angular coefficients In Kogelnik theory [15].
2.3 Fabrication of the waveguide by optical replication
This section describes in detail a general theoretical guideline for optical replication [16] with multiple exposures. The main idea of proposed method is in application of one surface diffraction grating as a phase mask for sequential conical near-field recordings using a single coherent beam [17]. The thin structure of the phase mask is transferred to a thick-layer holographic material. This optical replication method is accompanied with no need in strong vibration isolation because of absence of interferometric branches of the optical scheme. To avoid problems with vibration during the long exposure time and problems with angular placement of phase mask, the phase mask is manufactured being right on the surface of PTR glass plates. The waveguide is a recording medium itself, thus, copying from master grating to the PTR glass waveguide eliminates significance of vibrations. Proposed manufacturing process considers few preparations, recording and post exposure steps, schematically presented in Fig. 3.
Fig. 3. Fabrication scheme including: (a) deposition of photoresist on PTR glass plate; (b) blackening of the back surface and facets of the glass plate; (c) phase mask recording in the photoresist with He-Cd laser light (λ = 442 nm); (d) photoresist wet development, (e) optical replication of the phase mask into the volume of PTR glass plate in He-Cd laser light (λ = 325 nm); (f) multiplexing by duplication of the previous step with the same mask but with another incident angle; (g) post exposure development, baking at the temperature T ≈ 500 °, (h) illustration of the recorded pattern in the volume of the plane waveguide resulted with the multiple exposures; (i) bleaching of the manufactured waveguide.
Preparation steps include photoresist deposition on the PTR glass plate for further phase mask recording and the blackening its back surface and facets, as presented in Fig. 3(a) and (b) respectively. The blackening is necessary to significantly reduce adverse effects of Fresnel reflections of the following holographic recording.
The phase mask in the photoresist was recorded by general interference of two beams with plane wavefronts, as Fig. 3(c) demonstrates. Photoresist exposed according to its sensitivity with He-Cd laser (λ = 442 nm). PTR glass is not photosensitive to this wavelength. The phase mask can also be manufactured by electron-beam lithography and then transferred to the waveguide plate by nanoimprint technique. After photoresist development, Fig. 3(d), the sample is ready for the recording of the waveguide couplers.
Waveguide couplers consist of several multiplexed volume Bragg gratings. First step of multiplexing is presented in Fig. 3(e) when phase mask is illuminated with He-Cd laser (λ = 325 nm) at the incident angle α1. Near-field distribution of the light intensity results in the periodic refractive index modulation n1. In Fig. 3(f) following step is shown. Illumination with the incident angle α2 results in the periodic refractive index modulation n2. Exposure on a single step should be chosen the way it allows to utilize the whole refractive index dynamic range of the PTR glass. Number of exposures depends of a target FOV. Each multiplexing provides sub field-of-view in accordance with Bragg condition and projector spectral range of green LED of the projector. During the multiplexing process at each subsequent exposure, it is necessary to rotate the sample with the phase mask in the recording beam, which simplifies the process. A surface-relief diffraction grating with specific spatial frequency is used as photomask. Figure 4(a) shows the optical scheme of a recording stand for optical replication.
Fig. 4. (a) Scheme of the recording setup: 1 — He-Cd laser, 2, 3 — lenses, 4 — diaphragm, 5 — waveguide sample in conical exposition, 6 — angular positioner, 7 — shutter. (b) Closer look at the near-field recording area.
Radiation with the wavelength of 0.325 μm (He-Cd laser 1) is expanded and collimated using lenses 2 и 3 as telescopic Kepler system. The diaphragm 4 cuts the central part of the recording beam with a satisfactory uniformity of radiation intensity. The PTR glass substrate 5 is mounted for exposure on motorized angular positioner 6 that controls angle of incidence α1. On the front surface the sample 5 has surface photoresist grating. The exposing area in the recording beam is controlled by the displacement in the xy plane. The exposure dosage is provided using an electromechanical shutter 7. The entire recording procedure with the necessary angular and planar positions is carried out using computer programmed control of the actuators of the recording setup. The exposure dose is calculated based on PTR glass sensitivity of 0.5–1 J/cm2[14].
The main recording step is presented in detail Fig. 4(b) with the configuration of VHGs. Coherent plane wave with wavelength of 0.325 μm falls onto surface diffraction grating under the angle of incidence α1. Plane of incidence is normal to the phase mask grooves. Diffraction on the mask results in three waves that are propagating inside the PTR glass substrate: transmitted wave (0th diffraction order) and two diffracted waves (+1st and –1st orders). Therefore three interference areas are formed in PTR glass substrate: area I corresponds to interference of +1st and 0th diffraction orders, area II with interference of 0th and –1st diffraction orders and area III with interference of all three +1st, 0th, and –1st diffraction orders. In practice area III occupies most of the recording volume due to relatively small waveguide thickness (≤ 2 mm).
Post-exposure development for PTR glass consists of high-temperature treatment (T≈500°C), Fig. 3(g). Figure 3(h) illustrates the resulting refractive index changes at the end of multiplexing procedure. Final step is required to increase transparency and diffraction efficiency of the manufactured diffractive waveguide plate. The bleaching reduces the scattering in the glass and, as a result, makes the image clearer. The recorded element is bleached [14] by the direct scanning using pulsed femtosecond high-power green laser radiation (λfs = 515 nm), Fig. 3(i). The bleaching is happening due to photodestruction of silver nanoparticles and does not damage the recorded structure. In our case, scanning rate was less than 0,03 mm/s.
The proposed optical replication provides sub-micrometer spatial period of VHGs. This method allows one to form volume structures, with the less periods than the phase mask relief. It needs to be mentioned that similar process in literature sometimes called holographic lithography or near-field patterning [18]. It is usually used to record photonic crystals, both in photoresist and in volume media [19]. Nevertheless, in this work we would like to emphasize that optical (or holographic) replication is occurring.
2.4 Replication process
Due to the optical replication from the surface phase mask, manipulation of waveguided ray paths in the glass plate is provided by the structure that corresponds to three-beam interference products. It includes Bragg fringes that are slanted in proper direction. Here, we perform mathematical modeling of this three-beam interference.
Let complex amplitudes of three waves inside PTR glass be equal to $\overrightarrow {{E_1}} \exp ({ - j\overrightarrow {{k_1}} \cdot \overrightarrow {{r_1}} } )$, $\overrightarrow {{E_2}} \exp ({ - j\overrightarrow {{k_2}} \cdot \overrightarrow {{r_2}} } )$ and $\overrightarrow {{E_3}} \exp ({ - j\overrightarrow {{k_3}} \cdot \overrightarrow {{r_3}} } )$which represent waves in +1st, 0th, and –1st diffraction orders respectively. Then light intensity in the area II (Fig. 3(b)) will be equal to
Each pair of interfering beams forms sinusoidal grating, with grating vectors $\overrightarrow {{K_1}} \ldots \overrightarrow {{K_3}}$, illustrated in Fig. 5(a)–(c). Resulting structure of the single exposure (without multiplexing) is shown in Fig. 5(d). Four periodicities with different orientations exist in three-beam interference pattern. In our case only one grating with the vector $\overrightarrow {{K_2}}$ is utilized for waveguide coupling. This grating results from interference of 0th and –1st orders diffraction on the phase mask. Other three structures $({\overrightarrow {{K_1}} ,\overrightarrow {{K_3}} ,\overrightarrow {{K_4}} } )$ are «parasitic» and reduce diffraction efficiency of the useful structure. However, due to significantly different spatial frequency there is no noise diffraction on parasitic structures in visible spectrum able form waveguide wave. Figure 5(e) shows the volume structure of the pattern recorded that is equivalent to woodpile-type photonic crystal [17]. The figure depicts elliptical-like isophase fringes, that corresponds to the refractive index change in the substrate of PTR glass. Each multiplexing step complicates the final structure, as illustrated Fig. 5(f)–(g).
Fig. 5. Illustration patterns inside a PTR glass wave provided by the single phase mask exposure due to (a) interference of 0th and +1st diffraction orders, (b) interference of 0th and –1st diffraction orders, (c) interference of +1st and –1st diffraction orders, (d) interference of all three diffraction orders. (e) 3D-simulation of the resulted “woodpile” structure for case (d). Illustration of the multiplexed volume interference structure with (f) two sequential exposures and (g) sequential five exposures.
The final structure is further governed by complex relations between laser exposure of a single grating and saturation exposure for PTR glass. The diffraction efficiency of the surface grating (phase mask) determines the visibility of the recorded pattern. The contrast needs to be maximized due to proper use of PTR glass dynamic range of the refractive index. The simulation showed that even with very low diffraction efficiency of the photomask (within 3%), the visibility of the three-wave interference pattern remains above 0.6. It should be noted that low photomask diffraction efficiency leads to the fact that the minimum intensity of the interference fringes in the recording region is close to the intensity of zero-order diffraction beam. This condition must be considered when selecting illumination dose. Three-beam interference leads to irrational utilization of the PTR glass dynamic range, however waveguide coupling can still be effective.
3. Calculation
3.1. Angular selectivity
For volume Bragg gratings diffraction efficiency is strongly related to the angular and spectral selectivity, if there is a slight mismatch in the wavelength or incident angle, the diffraction efficiency declines rapidly. Multiplexed VHGs must be carefully analyzed to balance the angular bandwidth and diffraction efficiency. Depending on the coupling condition, of operation the VHG is divided into transmission and reflection.
If the vector of the diffracted wave projected onto a straight line containing the incident wave vector is co-directed with the incident wave vector, then the grating works in the transmission mode. If they are oppositely directed, then the lattice operates in reflection mode. For the reflection mode, the spectral bandwidth is narrower, and the angular bandwidth is wider compared to the transmission mode. Considering application and relatively high thickness of a gratings, it is more suitable to use reflection ones to reduce angular selectivity. Thus, only multiplexing can expand the effective angular FOV. Each sequential multiplex can be explained as a static implementation of one condition for rolling K-vector gratings applied in switchable Bragg grating waveguides [10]. The structural parameters for each multiplexed VHG are optimized to the propagation direction that corresponds to the sub-FOV. General waveguide design and optimization procedure is supported by Kogelnik’s coupled wave theory, which is very effective in modelling Bragg gratings [15].
Figure 6(a) shows the angular-spectral selectivity for single Bragg grating recorded in the PTR glass. At first sight, it should set target conditions for sub-FOV corresponding to FWHM. A fundamental property of a Bragg gratings is that, for a given refractive index modulation, thinner gratings have larger angular bandwidth while thicker gratings have smaller angular bandwidth. On the other hand, the wider the spectral bandwidth the wider overall angular bandwidth due to Bragg condition. Therefore, in AR systems, unlike other applications of Bragg gratings, it is useful to apply radiation sources with wider spectrum. Figure 6(b), (c) present the angular bandwidth broadening due to wide wavelength spectrum, demonstrated in Fig. 6(d). The integral bandwidth of the single VHG is about ΔFWHM ≈ 3° and should be used as the multiplexing step to generate stitched FOV in the waveguide.
Fig. 6. Representation of the single Bragg diffraction grating without multiplexing: (a) diagram of angular-spectral selectivity, (b) angular selectivity profile for reflective mode, (c) integral angular bandwidth for wide green spectrum (d) of Mini Ray projector.
3.2 Design of the waveguide based on Bragg gratings
This section describes the parameter calculation of the multiplex gratings. As the proposed display is set in the symmetrical way, the in- and out-couplers have same configuration. The generated image is presented with a set of plane waves falling to the in-coupler. Figure 7(a) and (b) illustrates the general working geometry of in-coupling VHG1 and out-coupling VHG2. Bragg grating fringes are slanted to waveguide substrate at the angle γ. Each pair of couplers is responsible for one propagation direction from the overall FOV. Along with the function of image outputting, VHG2 expands the exit pupil along one coordinate. Any incident beam satisfies the Bragg condition for one grating from multiplex set. There are two main limitations on light propagating angle inside the waveguide. Along with the recording of diffraction gratings in the entire glass thickness, the direction of the isophase fringes is the same in the regions of the VHG1 and VHG2. Each grating pair determines the identical recording geometry resulting in incident recording angle, presented in Fig. 7(c).
Fig. 7. Geometry of Bragg grating corresponding to one pair of couplers (a) for in-coupler and (b) for out-coupler. (c) Scheme for recording a single diffraction grating by optical replication. The parameters: θ, θn — incident angles in the air and in the waveguide relative to the normal to the surface; θBr — Bragg angle; γ — slant angle of the interference fringes; βn — diffraction angle in the waveguide relative to the normal to the surface and the propagation angle in the same time; α1, α2 — two recording angles relative to the normal to the substrate surface; α1n, α2n — two recording angles in the waveguide relative to the normal to the substrate surface; α1n, α2n, α3n — diffraction angles in the waveguide corresponding to 0th, −1st and +1st diffraction orders.
The phase mask period dS is calculated to provide 0th and –1st diffraction orders in UV laser beam. Certain restrictions are imposed on the recording angle α1 associated with manufacturability of oblique illumination of the phase mask, which is α1 ≤ 45°. On the other hand, sufficient diffraction angle must exist in the medium, which is α2n ≤ 80°. The period of the phase mask is defined by following equation:
Then Bragg angles θBr and slant angles γ angle are calculated considering that the propagation geometry fulfills the Bragg condition for the parts of the input FOV. Incident angles in the waveguide are calculated as follows:
The direction of the diffracted beam in the waveguide is determined in accordance with the grating equation:
Thus, the key parameters of the Bragg grating can be defined as:
Then, the corresponding replication geometry (incident angle α1) is calculated for recording wavelength of λ = 0.325 μm. To find α1n and α2n, in Fig. 7(c), (a) system of two equations is solved, where one is the grating equation for the phase mask, the other defines the slant angle as the bisector between interfering beams (propagation directions of the 0th and –1st diffraction orders)
The analytical solution with respect to α1n is
The desired value of the angle α1 of the recording beam is
For the wavelength λ = 0.325 μm and corresponding refractive index n = 1.5242, from Eq. (2) ds = 0.409 μm. At this point, a photoresist layer is exposed in the interference field of two coherent laser beams with a wavelength of λ = 0,442 μm where incidence angles determined from the relation α = arcsin(λ/2dS) = 32.7°. Considering working conditions of the display, the critical angle of TIR βTIR = arcsin(1/n) =41.8° sets a limit on the shallowest angle light may travel within the waveguide, where n = 1.499 for λ = 0.520 μm is refractive index of the PTR glass waveguide. There are two main limitations on light propagating angle inside the waveguide. The maximum incident angle θmax is determined by the diffraction angle β2nsatisfying the condition β2n ≥ βTIR. Also, it is preferred to maintain the steepest angle less than ≈75° to prevent gaps between each two TIRs what defines θmin. In theory, because we have the entire medium volume suitable for recording, we can use the twice deployed angle inside the waveguide, limited on both sides by the TIR. However, in this case we will not implement the method of optical replication. Table 1 represents the recording parameters for the described multiplexed optical replication.
Table 1. Specifications of the multiplexed exposures
Preliminary experiments on the optical copying in PTR glass showed high-quality multiplexing implemented for seven exposures. Therefore, providing the pitch ΔFWHM ≈ 3° for angular FOV by single recording, the resulting multiplex VHG should ensure the image transmission with stitched FOV at least 20°. Presented replication method reduces the period of recorded optical structures by ≈1.5 times in comparison with the period of the phase mask.
4. Display prototype
To verify the design procedures, we experimentally recorded the designed waveguide configuration. For two-beam interference recording of the phase mask the He-Cd laser HCL-100V(I) (by JSP Plazma) with TEM00 single mode and maximum power 150 mW was used. Photoresist phase mask period is ds = 0.410 μm. Beam expansion in the setup is provided with the pair of lenses with focal lengths f’1 = 30 mm, f’2 = 1000 mm, a diameter of the incident beam is D = 50 mm. The optical multiplexed replication was carried out in diaphragmed wide beam using continuous wave He-Cd laser HCL-100U(I) (by JSP Plazma) with TEM00 single mode with the maximum power 30 mW. The VHG recording was carried out in in accordance optical replication process (in Section 2.1) using the parameters in Table 1. The separate areas for in- and out-couplers have been provided by the diaphragm. The baking took 10 hours at the temperature T ≈ 500°C.The bleaching was provided by pulsed 250 fs laser Avesta ATsG 1030C with pulse energy of 10 mJ at the wavelength 515 nm using direct beam scanning at the rate of 0.02 mm/s.
The prototype is based on the planar waveguide 40 × 40 mm2 with in-coupler of 10 × 20 mm2 and out-coupler 20 × 20 mm2, as presented in Fig. 8(a). A micro display 1 inside the image generation module forms the image introduced into planar waveguide by in-coupler 2 (this part is covered by the construction case and is not visible on the photo). AR-display construction includes commercial compact projector Mini Ray (resolution 640 × 360) as image generation module. This projector was chosen because it has the widest spectral band compared to other pocket versions. The observation of output virtual image is provided by out-coupler 3. As aforementioned, for this prototype we used no folded grating to expand the pupil in two directions. The pupil expansion is only provided in horizontal direction before projecting it into the eye box as illustrated at Fig. 8(b). At this step, prototype is just connected to the computer. Photos in Fig. 8(c)–(e) show monochrome augmented information «BMSTU» in different ambient light conditions that is demonstrating the possibility of simultaneous observation the surrounding scene. Photos were taken in a non-laboratory environment using a smartphone camera. A shortcoming in designed prototype relates to unobstructed observation, which is one of the general requirements. The images show that the left side of the field of view is overlapped due to the placement of the lighting system there. This situation can be remedied by development of the image generation module to replace a commercial projector.
Fig. 8. (a) Photograph of the prototype: (1) — DLP Mini Ray projector, (2) — in-coupling grating, (3) — in-coupling grating. (b) Illustration of the horizontal pupil expansion. (c)–(e) Photographs of the displayed green light images in different ambient light conditions.
Figure 9(a)–(b) illustrates continuity of the image in entire FOV. There is a good agreement between the experimental and calculated results in terms of angular selectivity. However, diffraction efficiency is lower than the calculated one. The originals of the green and color technical tables are shown in the upper left corners in Fig. 9(a) и 9(b), presented squares in the grid cover the FOV of 1 ° and 5 °, respectively. We attribute it mainly to woodpile-type structure of the single VHGs that prevents the effective refractive index modulation. Likewise, there is a noticeable horizontal brightness unevenness. This is associated with the bell-shaped selectivity contours and insufficient angular step 3° of the incident beam chosen for the multiplexing. In this regard, image quality can be improved by decreasing the step down to 2–2.5°, with a corresponding increase in the number of multiplexing steps. The fact that other wavelengths can also diffract on the Bragg grating, only at a different angle of incidence
Fig. 9. Illustration of the resulting field of view with (a) green light technical table and (b) RGB light technical table. (c) The photograph, illustrating the transparency of the waveguide. (d), (e) Photographs of the displayed multicolor images in different ambient light conditions.
Other wavelengths can also diffract on the recorded VHGs, only at a different angle of incidence. This fact can have a positive effect of for augmented reality representation, as illustrates Fig. 9(d), (e). For multicolor image, the symbolic information is presented in such a way that different colors have its own spatial position in the horizontal field of view. Even though the volume holographic elements are calculated for the green spectrum, the blue and red wavelengths are partially trapped in the waveguide too and appear in the output FOV. The wavelength change of the incoming radiation leads to the FOV shift without the image shift. Therefore, with a multicolor image in the input, the outputted image consists of three sub-FOVs (red, blue and green), shifted relative to each other.
Many techniques have been employed to improve performance of the designed planar waveguide, however, some drawbacks caused by imperfections during manufacturing and physical principle of the diffraction waveguides itself are present. Nonetheless, the horizontal FOV of 23° is achieved in monochromatic green light, which corresponds to the calculated value. Multicolor image allows utilizing of a full horizontal FOV provided by the compact projector. The vertical FOV observed from one point of the eye box is approximately 10°. The ability to observe other angular zones of the output image in the vertical direction (within 30°) is achieved by shifting the observation point in this direction.
5. Conclusions
AR display based on a planar waveguide made in photo-thermo-refractive glass had been demonstrated. The monolithic integration of multiplexed volume Bragg gratings with the waveguide platform provided in/out-coupling and transmission of the image. Multiplexed VHGs acting as an in- and out- couplers provide the stitched field of view. Adverse effect of high selectivity of volume gratings was negated by multiplexing and spectral wideness of the image generation module. The PTR glass having high mechanical, thermal, optical, and chemical resistance allowed us to manufacture waveguiding platform without cover laminating. Volume holographic gratings are manufactured and multiplexed by exposing the PTR glass plate with the UV laser light through the phase mask. Laser exposure conditions and VHG design parameters are presented.
Positive results were obtained regarding the field of view, transparency of the display, brightness and usability. Namely there was no attenuation of the input image by the designed system. Achieved FOV was limited by projection system and multiplexing technique. Nonetheless, PTR glass is capable of recording up to 20 gratings in a single volume with demanded diffraction efficiency [14]. Consider that, increasing the number of multiplexes in the couplers can provide FOV beyond 50o. Since the PTR glass has sufficient refractive index change and high transparency in the visible the main focus of the following work should be on the suitable image generation module and multiplexing technique whereas crosstalk between single grating may be an interesting topic itself.
Funding
Russian Science Foundation (18-79-00304).
Acknowledgments
The authors acknowledge the support by Ekaterina Drozdova, Pavel Khanevich and Alexander Betin for their help in technical issues.
Disclosures
The authors declare no conflicts of interest.
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