Autumn 2019 - Optical Connections Magazine


and functional integration in the semiconductor industry has benefited not just from a continuous progression of denser manufacturing geometries, but also from a well-established Electronic Design Automation (EDA) ecosystem. Although still in the early phase, automation in the design tools for SiPhot platforms is starting to materialise. Design up front for volume manufacturing and test is a given in semiconductor design. Verifying the physical layout of a photonic integrated circuit (PIC) presents dierent, and arguably harder, challenges compared to a semiconductor IC, so it is key to future progress that the EDA environment develops and broadens. Adding to the challenge is the need to co-design PICs with functions that interface, and are sometimes co- packaged, but which are in dierent process technologies. The ability to have process development kits (PDKs) that support simulation environments for multiple process platforms is beginning to surface. This is paving the way for designers to optimise performance for specific applications through tightly-coupled co-design of the electronic and photonic elements. As optical network system requirements diversify and design challenges intensify, this is driving still further innovation in the use of photonic integrated circuits. Design improvements to increase the baud supported in SiPhot modulators and receivers means they can be used in high-performance systems. Ensuring that the designs are hardened – or able to support extended operating temperatures – allows for footprint- optimised designs that can be deployed in exterior cabinets which expands the use case for coherent technology into more applications. The combined use of InP for high-eciency, high- bandwidth modulators and SiPhot for integrated coherent receivers is another option for the next generation of coherent optical systems. The next generation of coherent systems is driving a new set of requirements for optical components, requiring dierent approaches to optimise either for high performance or small footprint. Photonic integration, along with co-design and co-packaging with other functional blocks, are essential elements of the design process needed to meet these requirements. For PIC design, both InP and SiPhot material platforms will have a role to play, and the ability to be able to design in, and have access to, both technologies will enable a forward path for new network architectures. BUILDING ON THE FOUNDATIONS

Fig 2: Migration from discrete electro-optical components to integrated COSA package

for high bandwidth signal propagation, reduce V- π (modulator drive voltage) and optimise optical waveguide design allows designers to extend the technology to build modulators that will support 800Gbps per wavelength with scope to extend to even higher rates. A key enabler for footprint-optimised, pluggable coherent solutions based on SiPhot is the implementation of a full coherent optical sub-assembly (COSA) into a ball-grid array (BGA) package, a favoured chip packaging option in the semiconductor domain for many years. In this case, both receive and transmit functions are integrated onto a common SiPhot die. There are several key design elements helping to achieve footprint-optimisation in this kind of highly-integrated photonic component. Area reduction on Input/ Output (I/O) ports allows higher density of I/O to get signals on and o chip in a smaller package dimension. The ability to use non-hermetic packaging in the assembly stage reduces cost and enables volume manufacturing. It also allows for direct fibre attach to the package. The trade-o for this approach, as is the goal in design approaches for footprint optimisation, is that the laser is separate, allowing better thermal and power performance. As in the examples of µICR and CDM components, co-design of multiple elements allows for higher levels of optimisation. Greater proximity and use of common BGA substrate - leveraging tried and tested flip-chip assembly processes from microelectronics manufacturing - facilitates co- packaging of dierent functional blocks like the transmit driver and receiver TIA which, in turn, reduces the component area while also improving performance. FURTHER INNOVATION IN PHOTONIC COMPONENT DESIGN As the use of silicon waveguide technology is proliferating to a wide range of dierent industries outside of optical communications, greater design eciencies and more options for design integration are becoming available. Leaps in performance

speed and performance. The ability to co-package enables lower cost, simpler packaging and assembly, leveraging the type of manufacturing flows used for microelectronics. THE RIGHT PHOTONICS TECHNOLOGY PLATFORM FOR THE RIGHT APPLICATION Having design expertise in and access to both InP and SiPhot gives maximum flexibility in system design for addressing the next-generation of divergent network requirements. InP is a mature and widely deployed technology which has the advantage of being able to support high bandwidth signals to address high-performance optical functions as well as the ability footprint-optimised applications owing to the very high degree of photonic- electronic integration and the ability for system vendors to leverage high- yield fabrication processes for volume production. Choosing the appropriate material platform for the target application means factoring in the relative importance of performance, level of integration and volume scale. With new design approaches, SiPhot performance limits can be stretched and with the ability to leverage wafer scale manufacturing, InP can also be amenable to volume applications. In some cases, the optimal choice may even be a design based on a combination of both technologies. A NEWWAVE OF OPTIMISED PHOTONIC COMPONENTS The high bandwidth characteristics of InP have already enabled system designers to design commercially viable solutions for 400G and 800G solutions. This was achieved through co-design and co-packaging of the IQ modulator and driver functions of a coherent transmitter. Various attributes of InP as a processing technology, as well as design approaches, have resulted in highly integrated and compact coherent driver modulator (CDM) components. The ability to improve insertion loss to incorporate laser sources and amplification on-chip. SiPhot has emerged as a key technology for


ISSUE 18 | Q3 2019

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