A staff of researchers on the College of Melbourne has developed a low-cost prototype of a liquid-based geometric waveguide combiner for augmented actuality (AR) purposes. The machine replaces complicated fabrication steps utilized in typical optical combiners by integrating silicone oil and PolyJet 3D printing in a simplified manufacturing technique. The work was revealed in a journal by Springer Nature.
Geometric waveguides are key elements in AR near-eye shows, permitting digital content material to be optically superimposed over the actual world. Conventional fabrication strategies contain dicing, layer bonding, and sharpening of stacked glass substrates with reflective coatings, all of which require excessive precision and vital labor. The liquid waveguide developed by the Melbourne staff avoids these steps fully by sealing a silicone oil-filled 3D printed body between cowl glasses, with dielectric reflectors inserted into pre-defined slots.
Dimensions of the prototype measure 38 mm in size, 28.5 mm in width, and three.2 mm in thickness. A triangular prism with a 50° slant angle is used to couple gentle into the waveguide, whereas three parallel dielectric reflectors, spaced 2.024 mm aside and angled at 25° relative to the bottom, redirect gentle towards the viewer. These structural options had been optimized utilizing COMSOL Multiphysics with the finite ingredient ray optics module. The optical simulation confirmed that complete inside reflection maintained gentle transmission effectivity, with a checkerboard picture projected through a collimating lens and reconstructed on a simulated retina.
Stratasys, a producer of commercial 3D printers, supplied the PolyJet platform utilized in fabrication. The researchers used its J826 system with clear RGD810 resin and water-soluble SUP707 assist materials. A skinny movie of fluorinated ethylene propylene or flat glass was utilized to the print mattress to make sure a easy end. Slots for reflector insertion had been printed at widths starting from 165 µm to 200 µm, with 180 µm decided to be optimum for alignment. The printed reflectors had been 175 µm thick. Venting holes of 0.5 mm diameter had been included to help silicone oil injection and air displacement throughout meeting.
Dielectric bandpass filters had been inserted utilizing precision tweezers below a microscope. Parallelism between reflectors was visually inspected utilizing ceiling gentle reflections, and distance measurements had been carried out at a number of factors utilizing a microscope stage with 1 µm decision. Though this course of is time-consuming, the researchers suggest omitting the inspection at early phases and changing it with visible inspection strategies. Extra correct alignment strategies, corresponding to laser autocollimators or Fizeau interferometers, had been famous as areas for future enchancment.
The body was sealed with three 0.1 mm-thick cowl glass items, cleaned by plasma and bonded with UV epoxy below managed stress. After sealing, silicone oil with a viscosity of 20 cSt was injected into the waveguide at a move price of 0.2 ml/min utilizing a 27-gauge needle and syringe pump. UV resin was utilized to seal the vent gap. The oil, chosen for its thermal and chemical stability, has a refractive index of 1.41 and displays solely a 0.28° change in important angle over a 20 °C temperature vary.
Optical characterization was carried out utilizing a Cytoviva hyperspectral microscope. Reflector transmittance ratios confirmed a 3% drop throughout the 400–700 nm seen spectrum at 0° incidence, and a 2% discount on common at 25°. Floor roughness was measured utilizing a Bruker Dimension Icon atomic drive microscope, with root imply sq. roughness (Rq) values of 1.4 nm for reflectors and 1.3 nm for canopy glass. Modulation switch operate (MTF) efficiency was assessed at 15 cycles per diploma, displaying 70% retention in see-through mode and 24% in digital picture mode.
Horizontal and vertical fields of view had been measured at 19.52° and 12.56°, respectively, by projecting onto a goal board positioned 25 cm from the attention pupil airplane. Attributable to the usage of solely three reflectors, the eyebox was restricted to roughly 1 mm horizontally and a couple of mm vertically. The researchers notice that rising the variety of reflectors might increase the eyebox and FOV however would lead to higher optical losses and better energy consumption.
Reflection effectivity measurements had been carried out not directly in a darkroom utilizing a monochrome CMOS sensor. The staff in contrast the projector’s unique gentle output with the redirected gentle from the waveguide. Recorded efficiencies had been 4.48% at 460 nm, 4.49% at 515 nm, and 4.56% at 625 nm.
Handbook meeting posed challenges in exactly chopping and inserting reflectors with out injury. Though labor-intensive, the present meeting course of permits for quick prototyping iterations. The total course of consists of roughly 3 hours of 3D printing, 2 hours of post-processing, and 4 hours of guide meeting. Complete materials value was calculated at $18, together with $12 for reflectors, $3.30 for tools use, $1.10 for resin, $0.70 for silicone oil, and $0.60 for canopy glasses.
To handle thermal enlargement of the liquid medium—estimated at 900 ppm/°C—the design consists of non-compulsory air pockets to stop stress buildup. These bubbles rise to the highest and will be hidden within the higher sealed area. Costlier high-index clear liquids corresponding to microscope immersion oils had been mentioned as alternate options however deemed cost-prohibitive for prototyping.
Waveguide efficiency might also be restricted by the selection of projection system. The staff used a normal LCoS projector, which can not align optimally with the prism’s pupil location, doubtlessly decreasing the efficient eyebox. Customized-designed projectors had been proposed for future improvement.
FiconTEC, a German photonics automation firm, collaborated with the staff to develop a {custom} waveguide meeting system. The equipment integrates a gantry system, pressure-sensing reflectors pickers, a UV glue preparation platform, and high-resolution imaging techniques for alignment. Its present analysis goals to assist scalable manufacturing of waveguides primarily based on the staff’s patented technique.
The authors chosen glass because the chamber sealing materials for simplicity, noting a secure transmittance ratio of 93%. They advocate sapphire as a possible future substitute because of its hardness and scratch resistance. Further enhancements might embrace anti-reflective coatings corresponding to MgF₂ or TiO₂/SiO₂ to scale back reflection losses at wider viewing angles.
Researchers concerned within the venture are affiliated with the College of Melbourne’s Division of Electrical and Digital Engineering, the Neural Dynamics Laboratory within the Division of Drugs, and KDH Superior Analysis Pty Ltd.
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Featured picture reveals optical efficiency of the fabricated waveguide. Picture through Springer Nature.
