In a waveguide-type display for augmented reality, the image is injected in the waveguide and extracted in front of the eye appearing superimposed on the real world scene. An elegant and compact way of coupling these images in and out is by using blazed gratings, which can achieve high diffraction efficiencies, thereby reducing stray light and decreasing the required power levels. This study investigates the fabrication of blazed gratings with grayscale electron beam lithography and the subsequent replication of the realized 3D grating structures in a polymer material with ultraviolet nanoimprint lithography. As such, diffractive elements are realized on a waveguide sheet, with very good control over the dimensions and the profile of the printed features. Blazed gratings are designed for green light (λ= 543 nm) and a diffraction angle of 43°. Making use of a PMMA resist and by carefully optimizing the electron-beam parameters, electron dose distributions and development step, blazed gratings with a pitch of 508 nm and a fill factor of 0.66 are achieved. Finally, a master is realized with two blazed gratings, 3 cm apart, which are replicated using ultraviolet nanoimprint lithography onto a waveguide sheet. The in- and outcoupling of an image through these two blazed gratings is shown, appearing sharp and non-distorted in the environment, and a throughput efficiency of 17.4% is confirmed.
Waveguides with Bragg gratings realized on a flat polymer foil are promising candidates for advanced strain sensors since such a planar approach allows precise positioning of multiple sensors in various well-defined directions, in the same foil. As such, an optical version of an electrical strain gage can be realized. Herein, several parameters are discussed which define the behaviour of such sensor foils, in particular the grating design, including the wavelength of operation and mechanical and optical properties of the used polymers. Epoxy and Ormocer®-based Bragg grating sensors operating at 850 nm and 1550 nm wavelength were realized using nano-imprint lithography and laser direct-write lithography and their strain and temperature sensitivities were compared. Finally, it is demonstrated that optical strain gage rosettes can be realized by multiplexing 3 angularly displaced sensors in the same waveguide on a single foil.
In this paper, different guided mode resonance (GMR) grating sensors are studied with the focus on investigating the possibility of high-performance configurations with simplified and potentially low-cost fabrication methods. The gratings are fabricated in polymer using nano-imprint lithography (NIL). We have chosen Ormocer materials, as they allow fabrication at room temperature, are UV patternable and have good optical and dielectric properties. The GMR gratings have a pitch around 550 nm, which corresponds to a resonance response around 850 nm with a Q factor of 2200 in simulations. In experiment, gratings imprinted on glass and on foil are characterized, showing a successful imprinting process of both devices. The measured optical response of the GMR grating for a change of the refractive index unit (RIU) of the cover (Δλ=Δnc) is 100 nm/RIU. The measured temperature sensitivity is -0.07 nm/K.
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