TELECOM PHOTONICS
Silicon-based Photonics Si-Nanostructures May Offer the Solution to NEXT Generation Bandwidth Demands
The phenomenal increase in the amount of traffic generated by new video-based telecommunications services such as IPTV is rapidly pushing optical transmission equipment towards higher bandwidth and a high quality of service. This trend demands a radical change in the cost model for optical telecommunications systems. It is no longer acceptable to provide high cost, low volume ”jewels“ – future development must be towards high volume, low cost commodities that can be more widely implemented throughout the network. One answer to this demand is the development of silicon-nanophotonics. Already all the fundamental building blocks for the creation of a highly integrated optical circuit have been developed, and are showing suitable performance for the proposed applications. In this paper, besides discussion on building block characteristics, the author will also show examples of implementation of filtering structures and hybrid integrated optical interfaces.
The Silicon breakthrough in optoelectronics Emerging telecommunication services are increasingly creating the demand for extremely high bandwidth and expectations for the future include extremely fast connectivity, pervasive interactive services and almost unlimited bandwidth at any time and from anywhere. In order to support this vision, communications equipment and data processing techniques must be able to increase their speed and computing power, and low-cost miniaturised optical devices must be used to handle operations now carried out by traditional equipment and servers. To reach the expected level of performance with this extraordinary degree of integration requires a considerable evolution in the current technologies. In particular, optical transmission devices which now require quite expensive fabrication processes
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need to evolve to mass production level, similar to the evolution that has happened in the past for microelectronics. This evolution can be achieved using silicon photonics, by handling optical processing in ordinary silicon chips and providing an enormous increase in performance but with very much smaller dimensions. Silicon-based photonics marks a significant break with traditional optical devices, in terms of design, technologies and fabrication processes. In comparison with the current generation of optical devices, which are designed for high-performance and relatively low-volume applications, silicon photonics represents a breakthrough which will make sophisticated functionalities available with mass-scale production and at very low cost. Silicon photonics adopts a Silicon-on-Insulator (SOI) platform, thus implementing optical processing on materials common to CMOS technologies, and combining several techniques and manufacturing approaches from both optics and microelectronics. The optical characteristics of silicon allow the miniaturising of devices, integrating a large number of functions into a single chip. This, of course, requires the development of new design and fabrication techniques due to the new challenges, but equally it opens up several application possibilities, ranging from telecommunications to sensors and biomedical applications.
THE AUTHOR EUGENIO IANNONE Mr. Iannone is responsible for business development and market validation at Pirelli Broadband Solutions. He has held this position for nearly seven years after returning from a short stint with Cisco, where he had product management responsibilities in the network dimensioning group. Mr. Iannone first joined Pirelli in 1997 as a member of the network optical design group for Pirelli Optical Systems. He has published several technical papers on network design rules and IP over WDM.
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Eugenio Iannone Pirelli Broadband Solutions S. p. A. Viale Sarca, 222 20126 Milano, Italy Tel.: +39 (02) 6442-7921 Fax: +39 (02) 6442-3455 E-mail:
[email protected] Website: www.pirellibroadband.com
FIGURE 1: Two-stage taper with an intermediate index contrast layer is one concept for fiber coupling into silicon layers.
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© 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
TELECOM PHOTONICS
Challenges in silicon-based photonics The Silicon-on-Insulator (SOI) platform is the key enabler for new-generation optical devices and its radical innovation compared to traditional optical techniques is reflected in the new paradigms that have to be developed: it is not just an evolution of silica -based PLC (Planar Lightwave Circuit) techniques and an adaptation of CMOS processes, but rather the synthesis of a new approach driven by the specific optical characteristics of Silicon. If, on the one hand, silicon shows major advantages over traditional silica-based PLC (it has, for example, a low overall insertion loss when building complex structures, it is extremely compact and has more efficient thermal tuning, amongst other things), on the other hand adopting a silicon-based platform requires the ability to master several technologies and processes in order to turn these promises into reality. Silicon has an optical refractive index that is much higher than silica and this has several implications, since it translates into dimensions that are significantly smaller: thus waveguides, resonant structures and any other elementary component can be much smaller in size. On the one hand this allows the miniaturising of devices, but on the other hand it makes fabrication tolerances and structural imperfections extremely small, thereby requiring exceptional ability to master both the design and fabrication phases in order to reduce the impact of these factors. While a silica-based waveguide has the dimension of few micrometers, silicon waveguides are a few hundred nanometers wide and even a roughness of tens of nanometers (at the limit of more advanced lithography methods for silicabased PLC) cannot be tolerated. Here the answer is to develop design methods that allow the implementation of roughnesstolerant structures and to improve frontend processes to guarantee the strict tolerances imposed by the material. Another consequence of the high silicon refractive index is the polarisation dependence of the optical structures. Polarisation dependence is a phenomenon that also affects silica-based PLCs, but in that case degradations are moderate and can be further limited with proper design. In the SOI platform the geometry of optical waveguides allows a single polarisation plane to propagate with reduced losses, while the other is disrupted, thus requiring a completely different approach, based on handling the two polarisation planes with separate parts of the device and then recombining them.
© 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
TE TM TM
TE Polarisation splitter Polarisation rotator
In order to have a complete optical platform we also need to introduce light generation and detection functions into the SOI; several advanced research activities are under way with a view to obtaining a monolithic solution for silicon-based lasers. This is however a long-term solution that has yet to achieve full technical feasibility. Currently there is no alternative to InP-based platforms for light generation, so the preferred solution for SOI devices is the hybrid integration of lasers. The same could apply for light detection, with the hybrid integration of detectors, but a technique to obtain detectors on SOIs shows a good level of maturity and promises to substitute the hybrid solution in the medium term. Mature microelectronics processes are already established for silicon, but their adoption for silicon-based photonics has to be scrutinised taking into account the different physics involved. The evolution of microelectronics processes is driving down the sizes, but when using silicon to drive light in such small structures, photons exhibit strong oscillatory-related evolution, leading to more complex design procedures and highlighting potential limits that do not affect electronics. A significant difference with both traditional PLC and microelectronic processes is in the back-end phases, namely wafer handling (dicing, polishing, etc.) and chip packaging. The major difference with traditional PLCs is that, due to the strong difference in the refractive index compared with silica, silicon structures are too small to be directly coupled with the external optical fibre (the spot size in the silicon waveguide is nearly 100 times smaller than in the optical fibre). Thus, while in silica-based PLCs the fibre coupling is a standard back-end issue, with
TE
FIGURE 2: Polarisation splitter and rotator: the light entering the device is separated in the two polarisation planes (TE and TM). The TE beam is rotated so as to coincide with the guided linear polarisation mode.
SOIs this problem needs to be resolved in the front-end design phase, implementing proper structures (tapers) which are able to convey and adapt the light in and out of the optical chip. Since one of the approaches for tapering is based on intermediate layers of different materials, the fibre coupling interacts with the optical and material frontend design. Nevertheless back-end processes need to be cheap, as in microelectronics, where packaging is a low-cost commodity. Actually many aspects of back-end can be inherited from microelectronics, such as those related to large wafer size adoption, waferlevel metrology and yield analysis methods, where SOIs can benefit from advances made in the semiconductor industry and by chipmakers.
THE COMPANY Pirelli Broadband Solutions Milano, Italy Pirelli Broadband Solutions specializes in broadband access technology and photonics. The Photonics division leverages nanotechnology and its use of tunable and pluggable solutions to deliver a complete range of components, modules and subsystems including dynamically tunable lasers, ITLAs, 10 Gbps transponders, DWDM XFP transceivers, and a CWDM/ DWDM system for metro access and metro core networks. In 2006, Pirelli Broadband Solutions posted sales of `129.4 M, with a 15.3 % growth over the previous year. For more information, visit www.pirellibroadband.com.
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TELECOM PHOTONICS
The basic building blocks As already indicated, a number of new challenges have to be faced in order to implement feature-rich optical devices on SOI-based platforms and the approaches adopted need to encompass both frontend and back-end solutions. The first, elementary building block is the optical waveguide. It has to be designed in such a way to be tolerant to roughness-related impacts, to allow waveguide crossing (that are difficult to avoid when designing complex devices made up of several interconnected blocks) and to minimise bending losses: here a trade-off among the various parameters has to be reached in order to obtain a reduced bending radius and a low loss. Although silicon shows a higher insertion loss per cm than silica, the overall losses can be kept low because the size has been reduced and the light distance travelled inside the device is really short. An efficient solution for waveguides includes wideband couplers and splitters, allowing the implementation of various light forwarding structures. What may seem a very trivial operation of allowing the light to enter a silica-based PLC device and conversely to exit it via an optical fibre becomes a much more complex task as the index contrast increases – and with SOI it becomes really challenging. The basic building block that allows the device to interface with external fibres is the taper that implements the modal adaptation between the standard fibre and the internal waveguide. Due to the very high difference in the mode diameter between the fibre and silicon waveguide (1:100) one approach is to implement a two-stage taper (fig. 1), with a layer with an intermediate index contrast. This leads to shorter and better performing tapers, even if it requires a more complex fabrication process. The nature of light propagation in silicon leads to strong polarisation dependence. Since it is necessary to implement separate structures for each polarisation plane, one of the basic building blocks needed is a polarisation splitter/combiner and rotator (fig. 2): the light entering the device is separated in the two polarisation planes (TE and TM) and the two beams are both rotated so as to coincide with the guided linear polarisation mode. In this way both signals can be processed by low-loss structures thereby minimising the polarisationdependent effects. In order to manipulate light signals it is fundamental to build resonant structures, so that filters and other complex optical
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FIGURE 3: Optical tunable filter composed of ring resonators. Such resonant structures are the basic building blocks for the manipulation of light signals. u
FIGURE 4: One concept for signal detection is the hybrid integration of standard photodiodes, coupled to waveguides with a turning mirror implemented in silicon.
processing can be implemented. The basic building block is an optical tunable filter composed of ring resonators (fig. 3). In order to build an optical resonator which combines more rings an optimum control of the fabrication process is needed to obtain a good alignment of the resonating frequencies and the proper coupling factor between the rings and the waveguide. The thermal tuning is much more effective in silicon than in silica-based devices because of the thermal conductivity characteristics of silicon. Optical detection can be achieved with a hybrid integration of standard photodiodes, coupled to waveguides with a turning mirror implemented in silicon (fig. 4). Other possible approaches are based on the exploitation of a Poly-Ge structure in a slab waveguide over a silicon layer. This is proving to be a promising solution for medium term applications. As regards generation of light, a hybrid solution has been chosen, with a flip-chip mounting of conventional laser chips: here the key is to develop a design that allows a laser passive alignment, in order to achieve wafer-level processing and inexpensive mounting.
provide significant improvements thanks to its potential for miniaturisation and for the integration of complex functions, together with cheap, mass-volume fabrication processes. Moreover, the uses of this technology are not limited to telecommunications, since many other application fields can be addressed. Silicon photonics is not only a new technique on the table, but comes with a complete rethinking and adaptation of both front-end and back-end processes, due to the special nature of the material. One way to approach this challenge is to base the development on a number of independent building blocks that address the basic functionalities needed. Research and Development into these technologies is moving quickly forward and Pirelli has already achieved the first step of designing the basic blocks. The next step is to implement the complex functionalities that can enable next generation optical access devices.
Conclusions The increasing demand for bandwidth and the widespread distribution of new services are each driving a new generation of optical communications systems, based on cheap optical devices rich in functionality. Silicon photonics, though not yet as mature as traditional silica-based PLCs, is poised to
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© 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
