The mismatch between positions of virtual images and a see-through view constitute a serious problem in virtual and augmented reality optical systems with a single projection plane. These issues may lead to a user’s discomfort: eye fatigue, headache and nausea. In order to solve these problems a tunable lens forming several projection planes at different locations can be used. Developed varifocal lens consists of two tunable liquid crystal cells. The first cell for fine adjustment varies optical power from 1 D to 3 D, the second cell for coarse adjustment varies power from 0.25 D to 1 D. The total dioptric range is -4 D … +4 D with an equidistant step of 0.25 D that forms 33 projection planes. Electrode pattern made of indium zinc oxide consists of rings corresponding to Fresnel zones, each zone is divided to multiple subzones. In order to minimize the number of control electrodes (bus lines) and keep high diffraction efficiency, the bus lines shunt together all of the corresponding sub-zones in all of the zones. Developed lens is tested with AR glasses based on a holographic waveguide. Displacement of virtual image from 250 mm to 1 meter is demonstrated.
Hollow-core fibers (HCFs) which guide light by an antiresonant reflection from arrays of silica walls have been attracting much interest due to their extraordinary optical properties and potential interdisciplinary applications including highly efficient laser-matter interaction, ultra-short pulse delivery, pulse compression and low-loss mid-infrared transmission. There are several types of HCFs having either a photonic crystal cladding, Kagome lattice or a single cycle of capillaries surrounding the core. In the latter case the antiresonant guidance properties depend strongly on the core size and the shape of the core/cladding boundary.
In this work, we focus on the capabilities of two HCF designs (negative curvature of the core/cladding boundary and nodeless capillary structure) to obtain a nearly single-mode guidance from the visible to the mid-infrared spectral regions.
The first HCF (Sample A) was drawn from the stack comprising a cycle of eight touching capillaries having the wall thickness 1.5 µm which provided a negative curvature of the core/cladding boundary. The fiber was intentionally manufactured with the trapezoidal shape of the capillaries in order to minimize the interaction between the surface modes, trapped amidst the touching trapezoids, and the fundamental mode in a hollow core. The negative curvature of the boundary resulting in the octagonal shape of the core was achieved by putting an excess gas pressure inside the capillaries during the drawing process. The second HCF (Sample B) was produced from the stack comprising a cycle of six non-touching capillaries having the wall thickness 2.5 µm with a view to restrict the abovementioned interaction via breaking the surface modes coupling between the adjacent capillaries. As in the first case, the gas pressure was controlled carefully to keep all capillaries separately from each other. In both samples the core diameter was equal to 50 µm ensuring a relatively large effective mode area.
Taking into account the periodic nature of HCFs transmission windows, we simulated and measured accurately transmission spectra and modal properties of the fibers. The simulations were performed using the finite element analysis. The transmission spectra were measured by passing light from the tungsten halogen lamp through the samples of 35 cm long and registering output signal applying three optical spectrometers covering the wavelength range 600-2500 nm.
We observed a good agreement between the simulation and the experiment. The Sample A has transmission windows at the wavelengths 650, 750, 850-900, 950-1050, 1150-1300, 1450-1700, 2000-2300 nm and the Sample B – at the wavelengths 600, 650-700, 750-800, 850-950, 1000-1100, 1150-1350, 1450-1750, 1900-2400 nm. The mid-infrared window for the Sample B is larger and more pronounced in terms of relative transmission due to the larger wall thickness at the core/cladding boundary. Moreover, the Sample B is predicted to be practically single-mode in the considered spectral region, as the losses of the most competitive higher-order modes are estimated to be much above 1 dB/m. A similar regime for the Sample A is expected only when operating at the long-wavelength limit of the spectral region, due to the increase in the fundamental and higher-order modes refractive index difference.