ROB KALMAN CONNECTIVITY
with hyperscale data centre connectivity needs Keeping up
O ur world is being revolutionised by migration of computing power to the Cloud, the hyperscale data centres (HDCs) of internet giants like Google, Microsoft, Facebook, Amazon, and Apple. Enterprises are implemented outsourcing their IT infrastructures to the Cloud to harness dynamically-scalable, best-in-class computing resources while avoiding investments in their own rapidly- obsolescing data centres. Typical HDCs connect server computers via a multistage network with top-of- rack (TOR), leaf, and spine switches (Figure 1). While the short server-to-TOR switch connections are predominantly copper, the others are fibre-optic. Such connections are increasingly over single- mode (SM) fibre. Optical transceivers are the key components in these fibre-optic connections. Cost and power consumption are vital factors in HDCs so transceivers must continually improve in normalised cost ($/Gbps) and power (watts/Gbps). For a state-of-the-art 3.2Tbps switch populated with 32 x 100-Gigabit Ethernet (100GE) transceivers, the 32 transceivers currently cost approximately four times more than the rest of the switch, highlighting the importance of optical transceiver costs. Leveraging photonic integrated circuit (PIC) technologies is the key to meeting these cost and performance challenges. OPTICAL TRANSCEIVERS Current state-of-the-art data centre optical interconnects carry 100GE inverse- multiplexed over four 25Gbps NRZ optical links. As electronics advance, speed, power and cost optimisation will soon favour 50 Gbps PAM4 and 100Gbps PAM4 over 25Gbps NRZ. Each of these evolutionary technology steps comes with IC, optical component, and system integration and packaging challenges. The laser diode is made from indium phosphide (InP); for a directly-modulated laser (DML), no separate modulator is needed. The receiver’s photodetector is made from InP or may be SiGe for SiPh PICs. The highly temperature-dependent refractive index (dn/dT), small mode size, and high optical loss of InP and SiPh PICs
Today’s apparently insatiable growth of bandwidth demand
means re-thinking optical module architectures. For example, higher rate pluggables may give way to on- board optics or optics co-packaged with the switch chip ICs, argues Kaiam’s Rob Kalman.
ROB KALMAN
Mode size mismatch can also cause significant optical coupling losses. But coupling a 2μm mode to an ~8μm mode (typical of a silica waveguide or fibre) incurs > 6dB coupling loss. This mode mismatch loss can be reduced or eliminated by using a mode matching structure such as a lens or adiabatic taper to couple the different mode sizes. Kaiam’s Optical Wire Bond (OWB) technology provides a high-performance HOI solution. In OWB, one or more microlenses are mounted to micro- electromechanical systems (MEMS) platforms. Using simple equipment, the mechanical de-multiplication of the MEMS lever arm enables the microlenses to be automatically positioned in three dimensions with an accuracy of < 0.5μm while matching mode sizes. The MEMS platforms are then soldered in place by an integrated MEMS heater. THE FUTURE Moving forward, the apparently insatiable HDC bandwidth demands will require some combination of higher per-channel data rates, wavelength counts, and fibre counts. An optimal HOI solution, combined with a high-performance optical mux, demux and other optical components, will enable optical transceivers to keep up with these demands. Thus 400Gbps pluggable modules will be needed by 2018 and 1.6Tbps optical interconnects will be needed just a few years later. Pluggables may give way to on-board optics or optics co-packaged with the switch chip ICs to improve density, power consumption, and cost.
make them poor choices for implementing a wavelength mux or demux. By contrast, silica planar lightwave circuits (PLCs) do not suffer these impairments, yet leverage the cost and scale advantages of standard IC processing technologies. Unless the laser, modulator, and mux are all made from InP, a WDM transceiver requires hybrid optical integration of multiple PIC technologies.
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Transverse misalignment (µm)
Optical coupling loss for a 2μm mode diameter vs. transverse misalignment.
HYBRID OPTICAL INTEGRATION High-performance (HOI) requires the following conditions: low optical coupling losses; compactness; ruggedness; low cost; scalability to high-volume manufacturing; scalability to high parallelism; and flexible hybrid integration of different optical technologies. This long list of challenging requirements explains why HOI has been impeded for decades by lack of a suitable solution. Achieving low optical coupling losses in SM transceivers is particularly challenging. Coupling from an InP laser (typical mode diameter ~2μm) to a SM waveguide or fibre with coupling losses of < 3dB requires alignment tolerances of < 1μm. This is far beyond the capabilities of standard electronic packaging technologies.
Dr. Rob Kalman is VP of Marketing at Kaiam.
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| ISSUE 8 | Q1 2017
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