A group 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 gadget replaces advanced 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 parts in AR near-eye shows, permitting digital content material to be optically superimposed over the true world. Conventional fabrication strategies contain dicing, layer bonding, and sprucing of stacked glass substrates with reflective coatings, all of which require excessive precision and vital labor. The liquid waveguide developed by the Melbourne group avoids these steps solely 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 mild into the waveguide, whereas three parallel dielectric reflectors, spaced 2.024 mm aside and angled at 25° relative to the bottom, redirect mild 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 inner reflection maintained mild transmission effectivity, with a checkerboard picture projected by way of a collimating lens and reconstructed on a simulated retina.
Stratasys, a producer of business 3D printers, offered the PolyJet platform utilized in fabrication. The researchers used its J826 system with clear RGD810 resin and water-soluble SUP707 help materials. A skinny movie of fluorinated ethylene propylene or flat glass was utilized to the print mattress to make sure a clean 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 underneath a microscope. Parallelism between reflectors was visually inspected utilizing ceiling mild 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 levels and changing it with visible inspection strategies. Extra correct alignment strategies, similar 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 underneath managed strain. After sealing, silicone oil with a viscosity of 20 cSt was injected into the waveguide at a stream charge 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 pressure microscope, with root imply sq. roughness (Rq) values of 1.4 nm for reflectors and 1.3 nm for canopy glass. Modulation switch perform (MTF) efficiency was assessed at 15 cycles per diploma, exhibiting 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. As a consequence of using solely three reflectors, the eyebox was restricted to roughly 1 mm horizontally and a pair of mm vertically. The researchers word that growing the variety of reflectors may broaden 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 group in contrast the projector’s unique mild output with the redirected mild 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 harm. 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 handbook 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 growth of the liquid medium—estimated at 900 ppm/°C—the design consists of elective air pockets to stop strain buildup. These bubbles rise to the highest and will be hidden within the higher sealed area. Costlier high-index clear liquids similar to microscope immersion oils had been mentioned as options however deemed cost-prohibitive for prototyping.
Waveguide efficiency may be restricted by the selection of projection system. The group used a typical LCoS projector, which can not align optimally with the prism’s pupil location, probably lowering the efficient eyebox. Customized-designed projectors had been proposed for future growth.
FiconTEC, a German photonics automation firm, collaborated with the group 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 help scalable manufacturing of waveguides based mostly on the group’s patented technique.
The authors chosen glass because the chamber sealing materials for simplicity, noting a steady transmittance ratio of 93%. They suggest sapphire as a possible future alternative on account of its hardness and scratch resistance. Extra enhancements may embody anti-reflective coatings similar to MgF₂ or TiO₂/SiO₂ to scale back reflection losses at wider viewing angles.
Researchers concerned within the challenge are affiliated with the College of Melbourne’s Division of Electrical and Digital Engineering, the Neural Dynamics Laboratory within the Division of Medication, and KDH Superior Analysis Pty Ltd.
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Featured picture exhibits optical efficiency of the fabricated waveguide. Picture by way of Springer Nature.