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InP monolithic integration Silicon photonics (no laser)

Silicon photonics (heterogeneous laser)

further capacity scaling without increasing the costs proportionally, the use of spatial-division multiplexing (SDM) over multi-core fibres is also under consideration. In addition to all these technical options, a key

1995 2005 Year Figure 2: Development of chip complexity

measured as the number of components per chip for indium phosphide based photonic integrated circuits (blue) and silicon-based integration distinguishing photonic integrated circuits with no laser (red) or with heterogeneously integrated lasers (green), [From: M. J. R. Heck et al. ‘Hybrid silicon photonic integrated circuit technology,’ IEEE J. Sel. Topics in Quantum Electronics, 2013]

signals in a single lane over a multimode fibre becomes exponentially more difficult. Parallel-lane transmission in the form of ribbon cable or wavelength division multiplexed (WDM) interfaces makes it possible to realise much higher bandwidth interconnection. Te design considerations for short-reach

transceivers used in WDM optical interconnects are very different from those used in long-haul WDM transmission links. Te choice of wavelength of operation, which could range from 850nm to 1600nm, and the channel spacing for short-reach WDM integrated transceivers will directly impact the cost, size and power consumption of the resulting transceiver module. For example, uncooled transceiver operation is preferred because eliminating the thermo-electric cooler (TEC) reduces power consumption. In addition, singlemode fibre starts to become

more attractive than multimode, not only because it supports longer transmission distances due to elimination of modal dispersion, but also singlemode fibre supports capacity upgrades more easily through WDM without the need to introduce new fibre cables. For transmission distances up to 2km with

signalling rates of less than 10Gbaud, direct modulation with on-off keying is simple, low power and cost-effective, because chromatic dispersion is not a limiting factor in this regime. However, as signalling rates move to 25Gb/s and beyond, to support links at 100G (4x25Gb/s) and 400G (16x25Gb/s, 8x50Gb/s or, eventually, 4x100Gb/s), direct modulation and on-off keying may no longer be the most effective way to support the transmission rate and reach. Novel modulation schemes such as pulse amplitude modulation (e.g. PAM4) and even digital signal processing (DSP) will be needed for certain data centre interconnect formats. Furthermore, in an effort to support

24 FIBRE SYSTEMS Issue 11 • Spring 2016 2015

design parameter for the future optical transceivers will be the material system and the fabrication technology. To achieve the low-cost targets, monolithically integrated WDM/SDM transceivers, incorporated in photonic integrated circuits (PICs), will need to be introduced. Te long-discussed silicon-photonics platform is theoretically perfectly suited for large-scale monolithic photonic-electronic integration using mature high-yield CMOS processing techniques, but optical laser sources are still technically challenging to create on silicon. Terefore, other novel alternative concepts that can combine best-in-class components from various material systems, in a heterogeneous hybrid or multi-chip integration approach, could be also considered. Figure 2 shows the amazing progress that has been achieved in the area of photonic integration, indicating that we can expect to see silicon photonics-based transceivers becoming mainstream in a few years’ time. Today, no fewer than four different combinations

of package style and technology have been proposed for 100G transmission inside the data centre, and the options are even more plentiful at 400G. Nevertheless, in due course the trade-offs between cost, power consumption and complexity will determine the most optimal transmission scheme and transceiver technology to be adopted for the higher transmission speeds.

Of mice and elephants As the next step, beyond the simple upgrade of point-to-point optical links connecting the ethernet switches within the data centre, optical

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switching of some type will be introduced gradually. As we mentioned before, current electronic switches will not be able to keep up with the required growth of switching capacity, while maintaining reasonable power consumption and costs. It is expected that by the end of this decade, new power-efficient optical circuit switches will gradually be introduced to support optical bypass of large flows, thus contributing to the reduction of core and aggregate ethernet switches. A number of new data centre network architectures – based on optical switching schemes which have been proposed in recent years can be broadly categorised as hybrid electronic/optical and all-optical.

Electronic switches will not be able to keep up with the required growth in switching capacity

In the first approach – i.e. hybrid electronic/

optical schemes – high-volume, high-capacity flows (so-called ‘elephant flows’) are separated from the smaller, low-capacity flows (‘mouse flows’) and switched using two separate, dedicated networks running in parallel. Examples of research projects that have investigated this approach include C-Trough, Helios, and OSA (see Further reading). Elephant flows are switched by an optical circuit switch – with reconfiguration times ranging from 10µs to a fraction of a second, depending on the switch technology – while the mice flows continue to be switched in the electronic domain using legacy ethernet-based electronic switches. Hybrid schemes offer the advantage of allowing an incremental upgrade of an operating data centre

Figure 3: Optical switching schemes proposed for intra-data centre interconnection include: a) a hybrid electrical/optical approach using optical circuit switching for elephant flows, and b) an all-optical approach operating at burst/packet level requiring the use of tunable wavelength converters

Electrical packet switches

Reconfigurable optical circuit switch

Control plane

Typical ethernet links

WDM optical links



ToR switches (a)

ToR switches AWGR (b) ToR switches

Number of components / PIC

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