Arrayed waveguide gratings are commonly used for multiplexing and demultiplexing in WDM. High order Bessel beams can be generated by spiral diffraction gratings. Another method for manufacturing diffraction gratings uses a photosensitive gel sandwiched between two substrates. Other elements, such as filters, prisms and diffraction gratings often need large quasi-collimated beams. Moire fringes produced by diffraction gratings were the basis for the position measurement. They also painstakingly produced numerous diamond-ruled glass diffraction gratings for their spectrometers. The original high-resolution diffraction gratings were ruled. The study of polychromatics is particularly useful in the production of diffraction gratings. The primary application of fiber Bragg gratings is in optical communications systems. Recently, an alternative to prisms and gratings has been developed&colon; chirped mirrors. The interferometer consists of three parallel and equally spaced phase gratings, and an x-ray camera. Such gratings can be either transmissive or reflective. Normally a germanium-doped silica fiber is used in the manufacture of fiber Bragg gratings. It is composed of a sequence of prisms, or gratings. Raised cosine is a commonly used apodization filter for fiber Bragg gratings.