## 2 Overview of Data Center Networks (DCNs)  
There is a growing demand for DCNs due to the increasing data traffic. Researchers have been working to improve architecture at all levels to enhance performance. Lately, Facebook, Google, and Microsoft have begun to use multiple wavelength links through coarse wavelength division multiplexing (CWDM) [28; 29; 30]. There are several structures that have been proposed for enhancing DCN performance using high bandwidth density optics, as well as optical switches [31; 32]. DCNs have developed rapidly in recent years. Large-scale data centers migrated to optical transmission technologies when they upgraded the 1 Gbps link data rate to 10 Gbps by the end of 2010. Google introduced optical links in its data centers in 2007 using 10 Gbps VCSELs and multimode fiber-based small form-factor pluggable (SFP) transceivers for distances up to 200 m. The data rates have risen from 10 to 40 Gbps, 40 to 100 Gbps, and even higher up to 400 Gbps. Lately, 100 Gbps links have been employed in production data centers. Moreover, 400 Gbps transceivers have recently been standardized for short-range (0.5-10 km) intra-data-center interconnects over standard SMF [33; 34].  
Optical transceivers are the core of DCNs and consist of a laser, modulator, multiplexer/demultiplexer (Mux/DeMux), and photodetector. Lasers and modulators can be paired, for instance, in directly modulated VCSELs. Transceivers based on VCSELs and parallel fibers are currently the most prevalent technology in DCNs. As the traffic increases, bandwidth needs are increasing dramatically. Therefore, WDM technology is essential for ultra-wide bandwidth, low-energy links. In a WDM system, several wavelength signals are combined using (de)multiplexers and transmitted over fiber. A distributed feedback laser array is usually used with Mach-Zehnder modulators (MZMs) or electro-absorption modulators (EAMs). To achieve ultra-high data rates, however, several laser sources are needed. Therefore, comb lasers emitting over several individual wavelengths are an attractive option. A multi-wavelength comb source architecture could include DeMux/Mux stages as well as broadband modulators. In this architecture, the ultra-wideband signal is spectrally divided into different sub-bands for separate processing. At the source, the ultra-wideband signal is created by integrating a large number of optical sub-bands, each modulated by an input RF signal. An ultra-wideband communication channel can be built by tightly multiplexing a very large array of sub-bands for a variety of purposes, including optical interconnections [1].  
Optical communication networks are increasingly in need of higher bandwidth due to the ever-increasing demand. Consequently, there is an urgent need to improve modulation speeds. Conventional modulation schemes for fiber optic networks typically utilize dense wavelength division multiplexing (DWDM) of separately modulated OOK signals. Nonetheless, the limited total optical spectrum and the low bit rates achievable per optical channel limit the total bandwidth available. In recent years, wideband modulation schemes using higher spectral efficiency modulation and multi-tone carriers such as optical OFDM have gained attention [35; 36; 37]. OFDM is a multicarrier high data rate transmission tech nique used in communication systems. To transmit the overall high data rate, the OFDM system employs several lower-rate orthogonal subcarriers.