High performance, low cost and low consumption Radio-over-Fiber systems for diversified communications applications

This dissertation aims to analyze the possibility of improving in terms of cost and consumption the future Radio-over-Fiber (RoF) systems in different telecommunications scenarios, such as current and next generation of cellular networks and in other applications such as Radio Astronomy. The RoF system studied is hence composed of a Vertical Cavity Surface Emitting Laser (VCSEL) operating at 850 nm, standard single mode fiber (SSMF) and SiGe Heterojunction PhotoTransistor (HPT), adopting the technique called Intensity Modulation-Direct Detection which is nowadays the cheapest and simplest architecture for RoF. This dissertation describes in detail the multimode propagation within the SSMF which is present at 850 nm. Through a developed mathematical model, the two-modes propagation is described and the main phenomena involved are analyzed. The model developed is able to identify the main parameters which contribute to enhance the detrimental effects produced by intermodal dispersion and modal noise both in frequency and time domain. Starting from the model, possible techniques to improve the performances are then proposed. In particular, a pre-filtering technique is realized in order to avoid the excitation of the second order mode, allowing a quasi-single-mode propagation within the SSMF. The technique is theoretically and experimentally validated both for single radio frequency sinusoidal transmission and for bandpass signal. Furthermore, the technique is validated in a LTE transmission system, making the RoF technology proposed able to transmit 256-QAM LTE signal of 20 MHz bandwidth, confirming the possibility to decrease the overall cost and consumption of the network. Further work has been done on the mathematical model. In particular the two modes propagation is exploited in order to characterize the chirp parameter of the VCSEL employed. Finally, the problem of coupling between fiber and opto-electronic devices is also discussed and investigated, in order to improve the efficiency while keeping low the costs.