The historical beginnings of photosensitivity and fiber Bragg grating (FBG) technology are recounted. The basic techniques for fiber grating fabrication, their characteristics, and the fundamental properties of fiber gratings are described. The many applications of fiber grating technology are tabulated, and some selected applications are briefly described.

Bragg gratings have been produced in germanosilicate optical fibers by exposing the core, through the side of the cladding, to a coherent UV two-beam interference pattern with a wavelength selected to lie in the oxygen-vacancy defect band of germania, near 244 nm. Fractional index perturbations of approximately 3 x 10(-5) have been written in a 4.4-mm length of the core with a 5-min exposure. The Bragg filters formed by this new technique had reflectivities of 50-55% and spectral widths, at half-maximum, of 42 GHz.

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The heating and hydrodynamic expansion of the F layer caused by absorption of an incident radio wave occurs in several phases. During the first minute, energy is imparted to the electron gas through ohmic dissipation, the electron temperature is raised, field‐aligned pressure gradients are established, and the plasma begins to expand along the magnetic field. Plasma density changes are small, less than 1%, during this initial phase and the incident wave field is almost unperturbed. During the next several minutes, the plasma accelerates and the F‐region density is reduced, albeit slowly, while the electron and ion temperatures approach steady state.