Extremely small red-green-blue beam combiners for compact projection-type displays

Abstract

Laser beam combiners, which can be used for compact projection-type displays, are reviewed and a newly-proposed extremely-small laser beam combiner composed of optical waveguides and optical couplers is described. Here, the laser beams with three RGB (Red-Green-Blue) primary colors are combined into one single beam by using the laser beam combiner. Such combiners are classified into three categories, i.e., a mirror-prism type, an optical-fiber type and an optical-waveguide type. The mirror-prism-type combiners are composed of the mirrors and/or prisms for combining the laser beams. The optical-fiber-type combiners use the optical couplers made of optical fibers. On the other hand, opticalwaveguide- type combiners, which are based on the optical waveguides, are particularly useful because they enable us to construct extremely small laser combiners integrated with laser sources. Here, we proposed and analyzed a new integrated waveguide-type red-green-blue (RGB) beam combiner that can contribute to a miniaturization of the compact projection-type displays such as an eyewear projector. The essential point of this beam combiner is that it is composed of three individual waveguides for RGB light inputs, and three directional couplers. The combined output light can be obtained from the single waveguide. The waveguide supports the single mode transmission. The efficiency for output light is as high as 96%, and the size is as small as 0.06×7.8 mm. In addition, the small dependence of the output efficiency on the wavelength and the relatively large fabrication tolerance are obtained.