Design of volume holographic lenses for operation at 850nm in augmented reality eyewear applications

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.