KAREN LIU MID-REACH OPTICS
better sensitivity receivers to stretch performance to longer-reach. (This simplified story neglects that the receiver end has no such benefit, as APDs instead of PIN detectors are typically needed.) THE BORDER CONFLICT HAS INTENSIFIED It may seem like the 40km products benefit from the best of both worlds, borrowing from either 10km or 80km, but industry dynamics have been pulling the two sides further apart. As the datacentre market grew to dominate market demand for 40G, 100G and now 400G optics, even shorter-reach and higher-volume products have emerged. The 10km products themselves are now binned versions of 2km products. There is not yet a standard or even a de facto standard for 80km for 100 Gbps (using 25G optics). Meanwhile, long-haul DWDM technology has become ever more capable but also ever more complex. Its technology has no trouble with the reach but struggles to match the cost expectations of the mid-reach market. The cost challenge has increased over time as the technology has become less modular. Once upon a time, 80km could “import” long-haul lasers and add link amplification only as needed. Now, long-haul products have shifted from direct detection methods to highly sophisticated and therefore expensive coherent technology reliant on digital signal processing (DSP) chips made in the latest advanced CMOS process. As the industry grapples again with the physical challenges of pushing the data rates up even further to 400G (and single channel 100G), the limitations of both short-reach link budget and long-haul cost are apparent. At this point, it appears that the ‘40km’ will continue to side with the short-reach technology – even if it can only reach 30km. It also appears that the ‘80km’ will side with coherent DWDM technology –even for applications that don’t need multiple channels. Direct-detection technology struggles to perform, yet coherent technology struggles with cost points and the first gap has emerged at 40km. This emerging crack in solution coverage threatens to widen going forward as the link budgets at even 10km become challenging. The technology gap is an opportunity for a generational turn-over to new technologies. There are a number of interesting candidates of which polymer optics is one. Passive polymers are being explored for high-density optical interconnect, particularly for nonplanar connections which can be added to electrical circuit boards. Active polymers have the potential to integrate optical components with electronics more efficiently. AN OPENING FOR POLYMER PHOTONICS
Figure 1: Limitations imposed by component speeds Source: Lightwave Logic
A TECHNOLOGY PLATFORM, NOT A MATERIAL Polymer photonics presents a new technology platform which has other potential benefits at a time when the photonics roadmap needs innovation. It has been compared to lithium niobate, the incumbent electro-optic Mach-Zehnder modulator technology. A more relevant comparison is to Indium phosphide and silicon photonics which are used to make the newer more compact modulators. But even those are not direct comparisons. Lithium niobite, indium phosphide and silicon are specific materials. Polymers are complex engineered materials. They can be designed and optimised by application. To date, most of polymer photonic devices have been Mach-Zehnder modulators at 1550nm, a class of devices widely used in long-haul transmission including coherent. They could also be engineered for 1310nm applications. Fabrication of devices from polymer uses deposition and photolithographic patterning techniques and equipment that are well established from older semiconductor fabs. They can also be added—literally on top of—devices made on InP, Si or GaAs to make hybrid photonic integrated circuits (PICs). Beyond the devices or PICs themselves, assembly and packaging is as important a determinant of cost and performance. Whether for 1310nm or 1550nm, PAM4 or coherent, polymer photonics can offer improved performance and most importantly, they open up possibilities of innovative component and package design. INNOVATION IS NEEDED BUT WILL COME The need for more data capacity everywhere in the network will continue to grow unabated. The mid-reach network promises to be a both a high-growth market and a critical technical challenge. New technology platforms such as polymer photonics can enable the optical communications industry to meet this challenge.
Components speeds, for transmitters more than receivers, is another challenge to continued speed increase. The speed of optics today is also limited by the capabilities of the electronics that drive them. As modulation speeds go up, typically the voltage required by the optical transmitter increases but the voltage available from the driver decreases. Electro-optic polymers have excellent velocity phase matching between optical and microwave propagation speed which results in unmatched speed. They also have high electro-optic response which results in lower drive voltage. For example, Lightwave Logic’s modulator roadmap starts with 50GHz modulators which is already faster than the~35 GHz used in the newest 100G and 400G Ethernet formats and coherent DWDM modulation schemes. 100 GHz modulators with 2 V drive have been demonstrated. In response to limited component speeds, the industry has been forced to use complex modulations schemes such as PAM4. PAM4 encoding cuts the usable signal to a third of its original size which stretches them out to twice the time i.e. half the speed, and exacerbates the link budget problems. At a minimum, polymer photonics’ higher speed can improve the quality of today’s PAM4 signals and lower power consumption. Alternatively, they could enable the use of simpler NRZ modulation to get the same data rates, taking back that factor of three (5 dB) in link budget. Alternatively, they could extend the roadmaps to greater speeds and open up new design possibilities. Figure 1 is a view of how both short and long- reach technologies are constrained by components speeds to the design space behind the green wall. The solid balls represent short-reach options. The textured balls are long-haul technology. The details of how product roadmaps evolved to this current state are described in a whitepaper on Lightwave Logic’s website.
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| ISSUE 16 | Q1 2019
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