An optical fiber has a core and cladding layer, where light is guided mainly in the core region. They can be fabricated from a range of glass or plastic materials. Although silica based fiber has numerous advantages, which have ensured it has become the most established and widespread. Typically for guidance, the core index must be higher than the cladding index. There are many different types of fiber with various internal structural features, offering different guidance attributes, for example highly birefringentfiber with stress inducing cladding structures.