Analysis of high capacity short reach optical links

Over the last few years, the global Internet traffic has grown exponentially due to the advent of the social networks, high definition streaming, online gaming, high performance computing and cloud services. The network is saturating, facing a challenge to provide enough capacity to such ever-demanding bandwidth expensive applications. Fiber optic communications is the only technology capable of dealing such high demands due to its advantages over the traditional electrical transmission technology. The short haul transmissions currently rely on direct detection due to low cost, low power and low complexity as compared to the coherent detection schemes. In order to increase the bit rate, several advance modulation formats are under investigation for short reach transmissions. Such links mostly use intensity modulation direct detection (IMDD) schemes providing a simple system when compared with the coherent receivers. In this thesis the performance of Multilevel Pulse Amplitude Modulation (MPAM) is studied using IMDD, providing good spectral efficiency as well as able to deal with the limited electronic devices bandwidth. MPAM can address the typical optical channel without the need to go with more complex and higher power modulation schemes. It provides a trade off between sensitivity and the complexity. So a simple communication system using MPAM is implemented using an external modulated laser transmitted over a distance of 2 km. In order to reduce the cost, single laser and single receiver technique is being adopted. The performance of the MPAM system in a bandwidth limited scenarios is studied with a possibility to use equalization techniques to improve the sensitivity. The utility of Forward Error Correction codes is also studied to improve the performance without increasing the latency. By increasing the number of bits per symbol, the system becomes more sensitive to the impairments. Moreover, the components and the connectors in the transmission system also introduces multipath interference (MPI) that is a key limitation to the use of advance modulation formats. Hence a detailed study is carried out to investigate the MPI effects. At the end, a novel idea based on reflective Mach-Zehnder modulator (MZM) is presented that reuses the modulated wavelength eliminating the need for a laser. As a consequent, the cost and power consumption specifically targeted for the optical interconnect environment is reduced. In a nutshell, the thesis provides an overview of the direct detection system targeted to the short optical links. It includes the studies related to the optical transmission systems and provides an insight of the available advance modulation formats and the detection schemes. Finally, the simulations and laboratory results are provided showing that adoption of MPAM is a viable solution that should be employed in high capacity short reach optical links.