1. Introduction

### 1.1 Optical fibers

An optical fiber is an extended cylindrical optical waveguide. In its simplest form, it consists of a core having a certain refractive index nc and is surrounded by a clad (sometimes called skin) of refractive index ncl (or ns). An optical fiber is used to guide light through its core, from one end to another, based on the principle of total internal reflection which mandates that nc must be always higher than ncl. Basically, optical fibers are made of highly pure silica glass doped with some impurities in order to increase nc or decrease ncl [1–3]. Recently, polymeric optical fibers got more attention as alternatives of some glass based optical fibers [1, 4].

Optical fibers are involved in many technological applications such as telecommunications, sensing [4, 5]; fiber lasers and fiber amplifiers [6]; fiber gratings which can act as mirrors [7, 8]; mode converters [9]; modulators; and couplers and switches [10, 11]. Optical fibers are considered ideal optical transmission media since communication cables hundreds of kilometers in length can be obtained with

low absorption and low loss due to the purity and cross-sectional uniformity of the manufactured optical fibers. Moreover, accurate tuning of the refractive indices of both core and clad guarantee extremely low scattering loss at the interfaces [1].
