Optical fibres can be designed in such a way that along the core maintain a periodic modulation of refractive index. Bragg gratings which are also called short period gratings have short periods of the order of micrometer for 1550 nm operation. A few hundred micrometer period possible in long period gratings. These gratings show superlative wavelength dependent characteristics. They are widely used in wavelength division multiplexed optical fibre networks. They are also used in the fabrication of fibre optic sensors.
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