ABSTRACT
When light is traveling in a material such as glass or water, which has higher refractive index than its surroundings, such as air, then beyond a certain angle of incidence called critical angle, light completely reflects back. Namely, when the refracted angle in the second medium is 90 degrees, then the incident angle in the first medium is at critical angle, which is the angle beyond which light cannot escape. Light guiding can be achieved in a planar structure, where light can be trapped in planar direction. Light can be trapped in multilayers by surrounding a layer of material with high refractive index with layers of lower refractive index. A number of approaches are available to couple light between fibers, splitting light from one waveguide to multiple waveguides, and between fibers and waveguides, and from free space optics to and from waveguides.
