Optimum Optical Frequency Combs for Telecommunications and Data Centre Networks

Optical frequency combs (OFCs) were well documented by early reports in the 60's and 70's [1] including the Nobel lecture by Hansch [2] and Hall [3]. During the last decade, there has been an immense amount of research activity focused on OFCs and their wide range of applications. These range from molecular spectroscopy [4], astronomy [5] to RF photonics [6], optical clocks [7], arbitrary waveform generation [8] and high speed optical communications [9]. An OFC can be defined, as a series of equally spaced discrete spectral lines [10]. There are various parameters that could be used to characterise an OFC, including frequency and amplitude stability, occupied bandwidth, free spectral range (repetition rate), spectral flatness, phase noise, phase correlation etc. However, the choice of optimum OFC parameters depends on the nature of the application. This work focuses on the parameter requirements for OFCs employed in next generation optical communication systems. Emerging broadband applications and bandwidth hungry services are driving the evolution of optical networks. Next generation short and long reach communication networks would be required to provide data rates of multiple Tb/s. Such high line rates are not feasible using a single wavelength channel, as the bandwidth of electronics will act as a bottleneck. However, the multi-Tb/s transmission capacity can be achieved by utilising highly parallel wavelength division multiplexing (WDM), with tens or hundreds of channels, in combination with spectrally efficient advanced modulation formats. Through such an approach, symbol rates can be maintained at levels that are compliant with the electrical bandwidth of energy-efficient CMOS driver circuitry [11]. One of the factors that has been attracting a lot of attention, with the move to higher line-rates, is maximizing the information spectral density (ISD) achieved at the transmitter. With the available spectral bandwidth becoming an extremely precious commodity, enhancing the ISD beyond what is achievable through the employment of the advanced modulation formats, becomes of paramount importance. Current optical WDM systems, using a large array of laser transmitters, require inter-channel guard bands to avoid cross channel interference/cater for the wavelength drift of the free running lasers. However, OFCs portray precise frequency spacing thereby making them an invaluable asset to densely packed communication systems. Hence, the use of OFCs for advanced multicarrier transmission techniques, like Nyquist wavelength division multiplexing (NWDM) [12], coherent optical orthogonal frequency division multiplexing (CO-OFDM) [13, 14] or time frequency packing (TFP) [15], have been investigated to realize terabit transponders. The authors will present the major benefits and shortcomings of the most commonly used techniques [16–22] that are available for the generation of OFCs. Focus will be placed on the inherent advantageous properties exhibited by the different techniques, while attention will also be paid to ways of overcoming some of the shortcomings [23–25].