Recent research in augmented reality (AR) eyewear has prompted interest in using volume holographic optical elements for this application. However, many sensing operations in AR systems require the use of wavelengths in the near-infrared (NIR) (750 to 900 nm). These wavelengths typically exceed the sensitivity range of available commercial holographic recording materials (450 to 650 nm), which complicates the design of optical elements with power since significant aberrations result when the reconstruction wavelength differs from the construction wavelength. Several methodologies for designing a waveguide hologram imaging system in NIR are reviewed and evaluated. The design approach presented in our work integrates the most effective practices such as fabrication point source location optimization and aberration analysis to realize effective holographic waveguide couplers formed with visible wavelength light and reconstructed in the NIR. The technique is demonstrated by designing and fabricating an input waveguide hologram in conjunction with a multiplexed output coupling hologram. The resulting input/output waveguide holograms can achieve an image resolution of (∼3 lp / mm) with a 0.6-mm-thick glass substrate that has a refractive index of 1.8.
Conventional photovoltaic (PV) modules (exclude interdigitated back contact modules) with silicon or gallium arsenide PV cells often have significant inactive module surface area. This results from wafer cutting techniques and metal contacts that reduce the module’s collection area and the resultant power conversion efficiency. A holographic light collector (HLC) combining a low-cost holographic optical element and a diffuser into conventional PV modules is proposed and evaluated to collect the solar illumination over the inactive module area. The angular tolerance and extra annual energy yield (EY) of the HLC are analyzed. It is found that improvements in EY of 4.5%, 4.1%, and 3.8% can be obtained when PV panels are deployed with two-axis tracking systems, single-axis tracking systems, and without tracking systems, respectively.
Recent research in augmented reality (AR) eyewear has prompted interest in the use of volume holographic optical elements (VHOEs) for this application. This interest in VHOEs is due to a number of factors including: their formation in thin, lightweight films that can be deposited on a variety of substrate; high diffraction efficiency, transparency, and low scatter of the resulting elements; the ability to multiplex several elements in the same aperture; and the potential for mass production by using replication methods. However, a number of design issues must be taken into consideration when using VHOEs especially when used as input and output couplers that have optical power as required for AR eyewear. One such issue is the design of input and output couplers with optical power for use at wavelengths that differ from the construction wavelength. For instance, most photopolymers and dichromated gelating materials are sensitive in the blue-red (450-650 nm) wavelength range but not in the infrared (IR) (750-900nm) where sensing is desired for AR systems. Several methods have been suggested in the literature to address this problem for holographic lenses and vary in the degree of complexity. The problem of making holographic lenses for waveguide input and output couplers at different wavelengths is even more complex due to the need to exceed the critical angle for the construction beams. Fortunately, optical sensing functions frequently do not require high resolution, and this can be used to advantage in the design process. In this paper, a design method is presented that combines wavefront/diffraction efficiency optimization, nonsequential raytracing, and wavefront compensation to form waveguide couplers with an optical power that are formed with a construction wavelength of 532 nm and a reconstruction wavelength of 850 nm. The aberrations caused by Bragg mismatch and the contrast reduction introduced by ghost images are analyzed by simulation and experiment. The experimental results show that an image resolution of ~10 lp/mm can be achieved with the holographic lens with potential improvement to ~40 lp/mm by including a cylindrical lens in the reconstruction beams.
In this paper a spectrum-splitting photovoltaic system is proposed that uses bifacial silicon solar cells to maximize total energy yield. The system is unique in its ability to convert direct sunlight with high-efficiency (<30%) while simultaneously converting diffuse and rear-side irradiance. A volume holographic lens array is used to divide the solar spectrum into spectral bands optimized for conversion by wide-bandgap and bifacial silicon solar cells. An approach for simulating the energy yield, optimizing the holographic lens array, and analyzing the effect of concentration ratio, aspect ratio, and illumination characteristics is described. Design examples for two different solar cell combinations are provided. A GaAs and bifacial silicon combination achieves an energy conversion efficiency of 32.0% and a MgCdTe and bifacial silicon combination achieves a 31.0% energy conversion efficiency. Additional solutions are provided when constraints on concentration ratio and aspect ratio are applied, allowing the designer to balance energy yield with cost and size considerations. The performance of the proposed system is compared to conventional monofacial silicon, bifacial silicon, and monofacial spectrum-splitting modules, and show that improvements in energy yield of over 45%, 25%, and 10% can be achieved, respectively.
Traditional silicon photovoltaic modules have some inactive module surface area (IMSA) that are not covered by the solar cells due to the shading of bus bars, finger contacts, and the unused space between the octagonal solar cells. Collecting the solar power falling onto the IMSA will increase the overall energy yield and potentially decrease the $/kW-hr rating for the PV system. Therefore, we proposed a low-cost holographic light management technique which combines the holographic optical elements (HOEs) and a white Lambertian scatter surface. Simulation and comparison are performed for three different light collection systems. The results show that 6.40% more light collection efficiency can be achieved by using HOEs combined with a white scatter surface.
The most expensive electrical energy occurs during early morning and late afternoon time periods. This poses a problem for fixed latitude mounted photovoltaic (PV) systems since the sun is low in the sky. One potential solution is to use vertically mounted bifacial PV modules to increase the East-West collection area and solar energy production during high energy usage time periods. However, vertically mounted PV modules have reduced conversion efficiency during mid-day time periods. In this paper the use of a horizontally mounted collector with holographic elements is examined as a way of increasing the energy yield of vertically mounted bifacial PV (VMBP) modules during mid-day time periods. The design of a holographic `cap’ collector is evaluated that considers dimensional constraints, holographic diffraction efficiency characteristics, and system solar collection efficiency properties. The irradiance illuminating the vertical mount is modeled with and without the cap. The design process also includes the optimization of separation between rows of vertically mounted modules and the use of directional diffusers in the proximity of the modules to maximize system energy yield.
Conventional silicon Photovoltaic (PV) modules often have a significant surface obstruction that reduces the collection of incident solar illumination and energy conversion efficiency. In this paper, light management methods that combine low cost holographic optical elements and diffusers into conventional PV modules are evaluated to capture unused illumination. It is found that by using reflection volume holograms (RVHs) with 300 nm spectral bandwidth in combination with a diffuser on a PV module with 12% of its surface area not covered with active PV cell regions that an improvement in power collection efficiency of 9.36% is possible.
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