Optimum Optical Frequency Combs for Telecommunications and Data Centre Networks

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Optimum Optical Frequency Combs for
Telecommunications and Data Centre Networks
P. M. Anandarajah 1 Senior Member IEEE, P. Landais 1, Senior Member IEEE, M. Deseada Gutierrez 2, L. P.
Barry1, Senior Member IEEE, and A. Kaszubowska-Anandarajah 3, Senior Member IEEE
1 School of Electronic Engineering, Dublin City University, Glasnevin, Dublin 9, Ireland
2 Pilot Photonics, Invent Centre, Dublin City University, Glasnevin, Dublin 9, Ireland
3 Connect Centre, Trinity College Dublin, Dunlop/Oriel House, Dublin, Ireland Glasnevin, Dublin 9, Ireland
Author e-mail address: prince.anandarajah@dcu.ie
ABSTRACT
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].
Keywords: Optical frequency combs, gain switching, external injection, linewidth, advanced modulation formats.
Acknowledgments: This work was supported in part by the Science Foundation Ireland (SFI) Career Development
Award 15/CDA/3640 and 13/RC/2077, the Higher Education Authority PRTLI 4 and 5 INSPIRE Programs.
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