Feasible predistortion loop for the linearization of Radio-over-Fiber system based on 850 nm Vertical Cavity Surface Emitting Laser and standard G.652 fiber

The main purpose of the work is to investigate a low-cost solution for systems designed for the distribution of the RF signal employing the Radio Over Fiber (RoF) technology, including the additional feature of a possible digital predistortion of the modulating signal to compensate the detrimental effects of the system nonlinearities. A Vertical Cavity Surface Emitting Laser (VCSEL) operating at 850 nm and the standard single mode fiber G-652 have been used with this purpose, since their combination is one of the cheapest possible solutions that can be implemented. The main problem present in this configuration is due to the fact that, using this kind of lasers in a fiber like G-652, that has been realized to work at wavelengths higher than 1260 nm, an undesired bimodal behavior is present in the fiber connection. We have then tested a solution based on commercial 1310nm/1550nm couplers, which are theoretically able to separate in different branches the two modes arising at 850 nm. This allows to realize an almost-Single Mode RoF link based on G-652 fiber and 850 nm VCSEL with the additional feature of making available a possible feedback signal to realize an appropriate signal pre-distortion. Aim of this thesis work is then to insert the coupler at the laser section, using the direct path to feed link based on G-652 fiber, while using the feedback path in order to digitally pre-distort the signal which modulates the laser itself. The improvements in the sysetm performance will be presented in terms of NMSE (nominal mean square error), ACPR (adjacent channel power ratio), EVM (error vector magnitude) and spectral regrowth reduction, and will allow to appreciate the beneficial effect of signal digital predistortion feature introduced.