Silicon Photonics: Fundamentals and Devices

13.8 Grating Formation Process

A large volume of work has been performed in imprinting a good grating structure in the core of the optical fibers used for telecommunication. The methods are then applied in achieving grating structures in planar waveguide circuit (PLC) waveguides. As has been described, the grating formation means the creation of a RI modulation in the core. The amplitude of the RI change depends on the photosensitivity of the material under consideration. In this subsection, we shall first consider the available methods of control of photosensitivity in the core of the guide, mainly considering silica as the core material. After that, the techniques of imprinting the RI modulation will be discussed. Related activities on other materials like SiON or SOI will be discussed at the appropriate places.

13.8.1 Photosensitivity of Glass

The photosensitivity of germanium-doped silica fiber, when using argon ion laser emitting at 488 nm, was discovered and reported in 9. In 10, it was found that a strong RI change occurs when a germanium-doped fiber is irradiated with UV light in the 240–250 nm wavelength range, around which an absorption peak due to a germania-related defect occurs. Since then, there have been considerable efforts in understanding and increasing the photosensitivity of fibers. Initially, it was thought that a core having a high concentration of germanium doping or a reduced oxidizing condition was necessary to increase the photosensitivity. Recently, other ...