1. Field of the Invention
The present invention relates to polarizing beam splitters and displays including the polarizing beam splitters.
2. Description of the Related Art
Among known polarizing beam splitters is a wire-grid polarizing beam splitter, which includes a metal diffraction grid.
This polarizing beam splitter includes conductors, for example metal conductors, arranged in parallel in a grid pattern. This structure has the effect of eliminating polarized light oscillating in a direction parallel to the grating and transmitting only polarized light oscillating in a direction substantially perpendicular to the grating. This polarizing beam splitter is intended for electromagnetic waves and infrared light, and has also been used for visible light.
Because the wire grid has a sub-wavelength pitch, the polarizing beam splitter is produced by a semiconductor process (e.g., photolithography). Accordingly, the wire grid is often formed on a plane-parallel plate. In addition, the wire grid is often disposed between plane-parallel plates to reduce the oxidation corrosion of the metal grid.
For use in an optical system, the polarization surface of the polarizing beam splitter is tilted with respect to the optical path. As a result, the plane-parallel plates become inclined in the optical path, thus causing astigmatism in the optical system.
Japanese Patent Laid-Open No. 2003-195223 (corresponding to US AA2003081317) discusses a polarizing beam splitter including a diffraction grid disposed between plane-parallel plates, as a pair of diffraction grid substrates. The plane-parallel plates are disposed between prism-shaped block substrates having substantially the same refractive index. To reduce astigmatism, the block substrates and the diffraction grid substrates are bonded with a flexible adhesive or a medium having substantially the same refractive index as the block substrates and the diffraction grid substrates.
On the other hand, the wire-grid polarizing beam splitter has difficulty in achieving sufficient optical performance because both reflected light and transmitted light undergo absorption.
Accordingly, for example, a polarizing beam splitter including stacked dielectric layers having a sub-wavelength structure (SWS) is conventionally used.
This polarizing beam splitter includes, for example, stacked one-dimensional dielectric gratings having the SWS to exploit the increased birefringence of the one-dimensional gratings. The dielectric gratings are arranged so that a chosen refractive index can be achieved in each polarization direction to provide increased polarization splitting performance over wide ranges of angles and wavelengths.
Japanese Patent Laid-Open No. 11-95027 discusses an SWS polarizing beam splitter including a stack of layers with a grating pitch smaller than the wavelength used and gap layers or dielectric layers.
These layers have a thickness of ¼ the wavelength of either of two substantially orthogonal polarized light components so that the polarizing beam splitter can selectively reflect one of the polarized light components.
Although the incident angle is substantially perpendicular to the polarizing beam splitter, it achieves increased polarization splitting performance.
In particular, the polarizing beam splitter, including the stacked dielectric layers having the SWS, uses the transmission of p-polarized light at Brewster's angle. Its performance therefore depends largely on changes in the effective refractive indices of the dielectric layers and the refractive indices of the media adjacent to the layers. In addition, the stacked dielectric layers having the SWS can be formed on a plane-parallel plate, as in the case of the wire-grid polarizing beam splitter, since the fine structure is produced by a semiconductor process (e.g., photolithography). Furthermore, materials that can be used in the production process can fail to provide sufficient polarization splitting performance.