|Thesis abstract: |
The present doctoral dissertation aims at finding innovative solutions for the realization of an optical backplane characterized by high capacity interconnections and low energy consumption.
Electrical copper based backplanes have been the solution adopted so far, but with growing bit rates, beyond 10Gbit/s, they can hardly fulfill the above mentioned requirements. The proposed idea is to move towards an optical backplane, where interconnections between electrical cards or servers (connected to the backplane) are realized with optical links, through a mesh of optical fibers. A preliminary comparison between standard electrical backplanes and a fiber-based optical one has been carried out in order to estimate the advantages in terms of power consumption and capacity at increasing bit-rate. A practical solution for a fiber-based optical backplane has then been designed. In order to realize a cost-effective full mesh interconnection, different topologies of fiber layout have been taken into consideration and cost-effective, energy efficient solution for the optical transmitter and receiver sides has been selected. In particular, a first prototype of the optical backplane will be based on directly modulated Vertical Cavity Surface Emitting Laser (VCSEL) and multimode optical links at 850 nm. Optical data links should also guarantee a complete transmission transparency in the interconnection with the external network, i.e. a traveling signal should be allowed to pass through the backplane network on a determined internal link without any need of opto-electrical (OE), electro-optical (EO) conversion and processing. Under this hypothesis, very high bit rate optical signals also with complex modulation formats and multiplexing (for example in wavelength or in polarization) should be able to be transferred throughout the fiber-based optical backplane. In this configuration, input/output signals will presumably travel in a single-mode fiber (SMF) coming from or going towards the external network. The employment of center-launching technique has thus been considered to guarantee single mode propagation through the multimode fiber-based optical backplane. Center-launching robustness to mechanical perturbations up to 1kHz, have been experimentally investigated in a multimode fiber link with VCSEL sources, proving error-free transmission.
In order to increase the capacity and improving the power efficiency of the whole system, we addressed the most up-to-date technologies, like VCSEL and Silicon Photonics, for the EO transceivers and their evolution for backplane-based interconnections. We will focus the attention on modulation formats as a way of increasing the link total capacity, maintaining the same number of optical interconnections. For both technologies, a performance simulative analysis of standard On-Off-Keying (OOK) and M-Pulse Amplitude Modulation (M-PAM) is reported. Furthermore, a power consumption comparison between 4-PAM and Non-Return-to-Zero (NRZ) OOK has been discussed in order to highlight the different performances and limitations for VCSEL-based and Silicon Photonics-based links.