FEATURE TERABIT NETWORKS
Coherent and Coherent-Lite As noted, coherent optical transmission is a highly efficient technology for per-wavelength data rates above 100Gb/s and distances above 80km. Coherent transmission simplifies the optical network by using chromatic dispersion (CD) and polarisation mode dispersion (PMD) compensation in the digital signal processor (DSP) chip, rather than optical dispersion compensation as has been traditional for 10Gb/s and lower rate networks. Flexibility in modulation format selection allows spectrum allocation and reach to be optimised; modulation formats from binary phase-shiſt keying (BPSK) to 64QAM can be chosen depending on the specific link requirements. Coherent also provides the best OSNR sensitivity among transmission technologies, easing network design constraints.
This technology will require an expensive 7nm DSP chip that represents significant technical risk
FlexCoherent is a coherent system with
flexible symbol rate and grid spacing that can be adjusted to maximise capacity. Tis technology enables network operators to optimise transmission performance via a range of advanced modulation formats available on a single line card. FlexCoherent technology supports an increasing variety of advanced modulation formats, and is ideal for multi-haul, simple line systems, and scales to higher data rates. Te downside of this scheme is that it uses
Figure 2: PAM-4 modulation Figure 1: DMT modulation
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a
b
c
(a) Measured signal-to-noise ratio (SNR) versus sub-carrier channel measured after DMT demodulation. 112Gb/s is transmitted with n-QAM data on 256 subcarriers spaced by 109MHz. (b) 32QAM signal, carrying five bits per symbol, transmitted on low frequency subcarrier 5; and (c) QPSK signal carrying two bits per symbol transmitted on high frequency subcarrier 250
the most complex DSP, consumes the highest power and is not the most cost-effective approach for shorter reaches. Currently under standardisation in the OIF,
‘Coherent-Lite’ is based upon standard coherent technology, but the silicon chip does not have as many features as those for today’s long-haul markets. While retaining excellent OSNR sensitivity and simplicity of network design, Coherent-Lite offers a reduced feature set comprised of single-rate/single modulation format operation, and DSP processing and forward error correction (FEC) for compensating CD and PMD up to 120km. Tis leads to lower power consumption as the chip does not need as much processing circuitry and will enable a solution for 80km. Like other coherent approaches, this scheme
works with a simple line system, offers the lowest power per bit and can achieve single- wavelength 400G. Te disadvantage is it will require an expensive 7nm DSP chip that represents significant technical risk. Advanced CMOS technology will be required to reduce power consumption and shrink the die area as
these chips need to fit in high-density packages such as Double Density QSFP (QSFP-DD) and Octal Small Form Factor Pluggable (OSFP) at 400G. State-of-the-art 7nm CMOS processing is new and this process node must achieve very aggressive power dissipation targets if it is to meet the requirements of the DCI market.
Weighing up pros and cons Considering all the pros and cons of the different schemes, coherent transmission will likely prevail for link distances that are 100km or greater coupled with its ability to scale as data rates increase from 200G to 400G and beyond. For short reach DCI applications of between 40 and 80km at 100G, the jury is still out on the preferred modulation scheme. History has shown in the fibre-optic component and transceiver market that the solution that succeeds will be the one offering the fastest time to deployment (fit for purpose), lowest power consumption and lowest cost for the total end-to-end solution. Tis enables customers to deliver efficiencies in capex and opex. As a developer of optical components and
a b (a) Simulated 56G PAM-4 optical eye diagram (b) Measured 56G PAM-4 optical eye diagram
pluggable transceivers in the coherent transmission world, Oclaro is evaluating all modulation schemes to provide solutions to help solve customers’ most complex problems. For example, it recently announced it is sampling a CFP8 module using PAM4 in an 8x50GBd configuration for 10km transmission over singlemode fibre. Tis availability of both PAM4 and coherent modulation schemes in optical products is a great step towards delivering speed and bandwidth the industry needs to handle the ongoing data explosion. If data is the world’s ‘new oil’, it will be innovative optics, based on these modulation schemes, driving the rewards.l
Adam Carter is chief commercial officer and Dan Tauber is director, systems engineering, for Oclaro
18 FIBRE SYSTEMS Issue 15 • Spring 2017
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