What is a VPH Transmission Grating?
Volume Phase Holographic (VPH) transmission gratings work much like conventional surface relief reflection gratings, except in transmission. They are periodic phase structures, whose fundamental purpose is to diffract different wavelengths of light from a common input path into different angular output paths. A VPH grating is formed in a layer of transmissive material, usually dichromated gelatin, which is sealed between two layers of clear glass or fused silica. The phase of incident light is modulated as it passes through the optically thick film that has a periodic differential hardness or refractive index. Hence the term “Volume Phase”. This is in contrast to a conventional grating, in which the depth of a surface relief pattern modulates the phase of the incident light. This key distinction is shown schematically in Figure 1. Figure 1. Conventional reflection gratings vs. VPH gratings. As with conventional reflection gratings, the spatial frequency of the periodic structure determ
