利用波导技术开发增强高折射率玻璃薄片

Waveguide Technology is widely believed to constitute one of the  most promising approaches to realize affordable Augmented  Reality (AR) / Mixed Reality (MR) devices combining smallest  form-factor with uncompromised image quality allowing for full  immersion user experience. Optical waveguides are made from  special grade glass wafers. The characteristics of such wafers are  directly determining image properties, such as Field-of-View,  contrast, brightness, distortion of the image projected into the  user’s eye and many more. We are reporting latest advances in  measuring and controlling key quality parameters with focus on  total thickness variation, optical homogeneity and thickness  tolerance of the wafers. We discuss the impact of these parameters  on image quality using optical modelling of the waveguides.

Glass wafers are a key component for the optoelectronic industry,  commonly used in sensor applications as substrate or as material  for wafer level packaging. Other applications include temporary  carriers for processing and packaging of advanced semiconductors.  Such “conventional” wafers are produced by taking an asmanufactured glass sheet with the chemical composition of  alkaline-free TFT display glass or alumino-borosilicate glass.  Optically, these materials show a “normal” refractive index of  typically nd = 1.50 ± 0.03. A second key specification of glass  wafers is uniformity in thickness, characterized by total thickness  variation (TTV). Typical values for TTV are in the order of 10 µm;  special grades can be selected for some materials with values down  to 5 µm.

Emerging Augmented Reality (AR) and Mixed Reality (MR)  market is requiring a completely new class of glass wafers: devices  based on the so-called waveguide technology – commonly  regarded a most promising approach for high image quality but  small form factor devices. Devices based on waveguides are  developed and produced by companies such as Microsoft,  Magic Leap, Vusix, Waveoptics and many more. This technology  requires wafers with high refractive index (typically n ≥ 1.6) and  index uniformity and reproducibility much smaller than 10-4 .  Furthermore, extremely low total thickness variation is required for  AR-wafers with typical demands of TTV < 1 µm.

It is not possible to realize such materials using the standard manufacturing approach: the commonly used manufacturing  processes for sheet glass, such as overflow fusion, down-draw or  float technology are not compatible with the thermochemical  properties of a molten high refractive index glass. In addition, the  required TTV levels of AR waveguide technology targeting to  below 1 µm are not within the reach of these manufacturing  processes.

On the other hand, high refractive index glasses are well established  in the optical industry. In addition, the use of plano-plano-polishing  to manufacture glass substrates from optical glass was known in the  past. However, requirements on total thickness variation for wafers  used in Augmented Reality waveguides exceed the specifications  of “normal” glass wafers by a factor of 10 as shown by light-guide  modeling and experimental investigations.

Fig1

High quality modeling requires also the inclusion of detrimental  higher grating orders with accurate grating modeling. In the outcoupling region, the so-called eyebox, per FOV mode, that is a  plane wave with one direction, generates various partially overlapping modes, which are generated by splitting at gratings on  the path through the light guide. Dependent on the light source  these modes are partially coherent to each other, which must be  included in the modeling. That finally requires a correct modeling  of the interference of the split modes per FOV in the eyebox and  for the PSF/MTF calculation.

High index-glass wafers were produced out of blocks of optical  glasses using sawing techniques for slicing, and plano-plano  polishing processes for reaching specified thickness and surface  properties like roughness. The homogeneity of the bulk material is  investigated by interferometric means with high accuracy using  glass blocks utilizing “polished sample” method. Respective result are shown in Figure 3 obtained at a block of  optical glass.