William W. Morey, Gary A. Ball, and Gerald Meltz
Many important advancements have been made in grating technology since 1978, when K.O. Hill and his colleagues first observed a photoinduced grating in a germania-doped optical fiber. Hill's gratings were made in the fiber core by a standing wave of 488 nm argon laser light. The grating exposure in this case was shown to be a two-photon process. The next major advancement in grating technology was to make a transverse exposure on the side of the fiber with an interference pattern formed with an intense UV laser source to produce the index modulation or phase grating in the fiber core. The UV exposure gave a single-photon process with an enhancement of a million times in the photosensitivity of the grating writing process compared to the use of visible laser light.
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