FIBER GRATINGS AND RELATED DEVICES
The fiber grating is one of the important fiber devices and has been widely used in fiber communication systems and fiber sensor technologies. Four sections of Chapter 4 are devoted to fiber gratings: Section 4.1 for a brief introduction, Section 4.2 for its basic theory, Section 4.3 for descriptions of various fiber gratings, Section 4.4 for the sensitivities of grating and its applications in sensor technology.
4.1 INTRODUCTION TO FIBER GRATINGS
4.1.1 Basic Structure and Principle
Soon after the invention of fiber, people tried to fabricate gratings on the fiber to create desired spectral features to be used in important applications such as wavelength division multiplex technology. In earlier stages, fiber gratings were fabricated by photolithography on a lateral facet along the fiber that had been lapped and polished near its core, called D-shaped fiber grating . The performance of D-shaped gratings is generally not good enough for practical applications; and its fabrication is not suitable for mass production.
It was observed in 1978  by K. O. Hill that an index grating inside the fiber core could be generated when a 488 nm argon laser beam was injected into a section of germanium (Ge)-doped fiber for some time, and the grating period coincided with that of the laser beam standing wave in the fiber section. The index change proved to be a permanent one. This observation is considered to be the discovery of fiber photosensitivity. ...