© 2001 OSA/OFC 2001
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`MEMS-Based Photonic Switching in
`Communications Networks
`
`Dr. Anis Husain Chairman & CTO
`OMM, Inc 9410 Carroll Park Dr. San Diego, CA 92121
`Phone: 858-362-2806, fax: 858-362-5848 e-mail to ahusain@omminc.com
`
`Abstract: All Optical or Photonic switching is key to the next -generation intelligent
`optical networks. It enables rapid dynamic provisioning, protection and restoration of
`network connections cost-effectively, which support revenue-generating services
`including bandwidth on demand and DWDM over IP. In the past 12 months, MEMS-
`based Photonic cross connect switches have gone from the lab to commercial availability
`and are now in carrier field trials. Photonic switching is inherently capable of
`transparently switching any bit rate and data format. Optical MEMS enables switching
`any transmission wavelength, or wavelength band in a scalable, reliable, low power,
`compact size providing flexible solutions to the bandwidth demands of long haul, metro,
`and access networks. This paper will discuss OMM's MEMS based photonic switching
`subsystems and their commercial deployment
`
`A Disruptive Technology: Photonic switching ushers in the next phase of communication network
`evolution after the wide-scale deployment of DWDM.
` When DWDM and EDFAs were first
`commercialized in the mid-1990s, they were also disruptive technologies that solved the bandwidth
`depletion problem but exposed the problem of managing increased traffic density, mixed data rates, and
`new signal formats just as the surge in demand for bandwidth resulted in an explosion in the number of
`fibers and wavelength channels. Photonic switching solves the problem of managing bandwidth created the
`deployment of DWDM & EDFA technology. By being able to handle any optical bit rate and any data
`formats, telecommunications carriers can realize new, revenue-generating services with significant savings
`resulting in future proof transparent networks. Optical Switching enables carriers to converge new
`networks with legacy networks, HFC networks, and metro loops. Carriers become free from reliance on
`many ATM and SONET functions when switching and protection are performed at the optical layer.
`Furthermore, the data-centric IP layer can converge with the optical layer because of photonic switching.
`OMM is the first to commercialize MEMS-based Photonic Switching - the disruptive technology that
`realizes the full potential of DWDM.
`
`Photonic Switching Evolution: The option to switch fibers and wavelengths without electrical conversion
`is becoming a reality as even the most die-hard OEO proponents acquiesce to Hybrid-OEO switches with
`all-optical pass through capabilities. Photonic switching solutions introduce the concept of true network
`optimization. True network optimization allows the end user to customize network architectures to meet
`rapidly changing dynamics in data rates, signal formats and architectures, and the proliferation of WDM
`into the Metro and Access markets. Carrier class photonic switching solutions are just becoming available
`that allow for the transition to all-optical links where appropriate, and the addition of all-optical capability
`where requirements for electrical conversion still exist. Thus current requirements are very well suited to
`smaller photonic switch fabrics ranging from 8 to 32 ports, currently in carrier trials. As the wavelength
`density grows and the evolution to transparent networks progresses, a higher percentage of traffic will be
`switched without electrical conversion, harmonizing with the availability of larger port count switch fabrics
`beginning with 64x64 to 256x256 and eventually reaching thousands of ports.
`
`OMM’s MEMS Technology: OMM’s switching components are based on MEMS micro-machined
`mirrors fabricated on silicon wafe rs, exploiting well established, low-cost silicon VLSI CMOS foundry
`processes that have been used for years by the computer industry to provide low cost reliable processors.
`MEMS (Micro Electro Mechanical Systems) are relatively new to optical networking but have been
`deployed for over a decade in applications such as airbag sensors, accelerometers, pressure sensors,
`displays, scanners, printers and micro-fluidics. MEMS technology has emerged as the clear choice for
`building a scaleable optical switch fabric. OMM has developed both two-dimensional (2D) digital MEMS
`and three-dimensional (3D) multi-position MEMS in order to offer a range of functionality from 4x4 to
`thousands of optical cross-connect ports. The 2D product line, in full volume production, is a Telcordia-
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`tested, photonic switching subsystem, which confirms MEMS as the superior technology for photonic
`switching. Integrating MEMS, optics and electronics into the same hermetically sealed ceramic package,
`reliability is radically improved. Building the MEMS-based switch core exploits well-established and
`automated silicon foundry processes in use for more than a decade. OMM has automated all key unit
`processes including fiber align and fix, module package and fiber sealing and device testing. This is the
`key to controlling high-volume manufacturing processes and ensuring high quality parts.
`
`Electrostatic MEMS There are two primary methods of actuating a MEMS optical switch in commercial
`optical products – electrostatic and magnetic. The electrostatic method relies on the attraction of
`oppositely charged mechanical elements, and is one of the main actuation methods used for all types of
`MEMS devices. Its many advantages include repeatability; no shielding problems and well understood
`behavior. Magnetic actuation relies on attraction between magnets and typically one or more
`electromagnets. While magnetic actuation can generate larger forces, the MEMS community has generally
`not accepted its use because of the complications of integrating the magnets and difficulty of shielding
`neighboring devices from cross-talk resulting in less than optimum density on the MEMS wafer which can
`severely limit scalability. OMM’s MEMS based switches exploit the advantages of electrostatic actuation
`yielding reliable, small size, scalable switching components and subsystems.
`
`2D and 3D Architectures: MEMS-based photonic switches are based on one simple fundamental principle
`and two well-understood approaches. Photons are switched from one fiber-optic cable to another by
`routing the light through a collimating lens, reflecting it off a movable mirror, and redirecting the light back
`into one of N possible output ports. The two basic design approaches for translating this principle into
`optical switches are a two-dimensional (2D) or digital approach (N2 architecture), and a three-dimensional
`(3D) or analog approach (2N Architecture).
`
`The 2D/Digital Approach The 2D/digital approach is so named because the micro-mirrors and fibers are
`arranged in a planar fashion, and the mirrors can be either on or off at any given time. In this approach, an
`array of MEMS micro-mirrors is used to connect N input fibers to N output fibers. This is
`
`
`Fig.1 2D planar switch functional diagram, 8x8 OXC MEMS chip, 2D cross connect module,
`
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`named an N2 architecture because it uses N2 individual mirrors. For example, an 8x8 2D switch uses 64
`mirrors. Figure 1 is a functional diagram of 2D planar switch, the digital planar 2D MEMS structure, and a
`board integrated 2D Optical cross connect module. OMM integrates the associated electronic high voltage
`up-converters in the package to provide the required voltage levels at each MEMS micro-mirror. The big
`advantage of OMM’s 2D approach is that it requires only a simple TTL driver interface thereby reducing
`hundreds to thousands of pins to a simple 20 pin digital interface. Although the 2D design is inherently
`flexible, the greatest challenge in this approach lies in scaling to the types of applications that require very
`high port counts. As port count doubles, the distance light must travel through free space doubles. As the
`pitch of the micro-mirrors increases, the light propagation distance increases and the diameter of the light
`beam grows, placing tight constraints on collimator performance and mirror alignment tolerance. Such a
`tradeoff can lead to impractically large silicon devices with high costs and low yields. For these reasons,
`2D technology is best optimized in cross connect products from 4x4 to 32x32 ports. 4x4, 8x8, and 16x16
`have already been commercialized by OMM and have been shipping for over a year in applications ranging
`from Optical Add-Drop switching to fiber distribution frames, restoration as well as switching for DWDM
`over IP applications at both the core of the network as well as at the edge of the network. Several
`customers having OMM switch fabrics have reached carrier field trials and some have carried live traffic.
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`The 3D/Analog Approach – High Port Count Scalability: The 3D/Analog (or beam steering) approach is
`actually very similar to the 2D approach as it uses the same principle of moving a mirror to redirect light
`between an input and output fiber. The 3D approach is also called 2N architecture because two arrays of N
`mirrors each are used to connect N input to N output fibers. However, in this approach, as opposed to the
`2D digital switch, each mirror is required to be able to reach multiple possible positions – at least N
`positions. This approach is much less constrained by the scaling distance of light propagation as the port
`count grows. Such architectures can scale to thousands by thousands of ports with low loss and high
`uniformity. OMM’s focus in 3D technology is to bring to market a scalable robust 3D optical switch
`fabric, which is reliable, manufacturable in volume, and cost effective for the customer. To achieve this, it
`is important to emphasize ease of installation and ease of use for customers. Of primary importance is
`providing a fully connectorized switch fabric with simple TCP/IP software interface. (Shown in Figure 2.
`are a 3D dual gimbled MEMS scanner structure; a 64x64 cross connect module, the building block for a
`scalable 3D switching subsystem, and a fully integrated 3D cross connect subsystem with integrated
`electronics, control, and fiber management).
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`Fig.2 3D MEMS scanner, 64x64 cross connect module, 256x256 integrated solution
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`The central premise of OMM’s 3D approach is based on design for reliability and availability in such that
`all key parts of the switch fabric are built and assembled with redundancy and hot swap capability.
`One of the biggest challenges in moving from a digital 2D approach to a 3D analog approach is the increase
`in complexity at the controls level. There are two significant choices to make when designing a control
`system for a large 3D MEMS switch, open loop or closed loop control. While open loop control has been
`discussed and demonstrated, OMM’s experience indicates that to meet Telecordia requirements for shock,
`vibration, and long term environmental drift effects, a closed loop control scheme with continuous real-
`time optical monitoring is fundamental. Apart from the robustness of closed loop control, features such as
`a high-availability controller, dual redundant Ethernet interface to the network element, fabric database
`back-up to network element, built-in subsystem maintenance (modularity, spare channels, fault detection
`and isolation, alarm reporting), as well as NEBS Level 3, ETSI, UL, CSA and CE compliance are key to a
`successful product. OMM will begin shipments to strategic customers of 3D based switch subsystems in
`Second Quarter 2001.
`
`Conclusion: Photonic switching allows carriers to add a new dimension of flexibility and scalability to
`their DWDM networks. Different networks (Ultra Long Haul, Long Haul, Metro and Enterprise) require
`optical switching subsystems suited to their particular demands. Carriers are demanding photonic
`switching flexibility for very basic reasons, it provides: innovative new capabilities and services that
`generate new revenue in a very competitive market, and provides the potential for significant savings in
`operating and total lifetime costs by future proofing the network from rapidly evolving data rates, signal
`formats and protocols. To fill this tall order, photonic switching products must be based on a robust,
`scalable, optically transparent technology and designed for automation and volume production. Advances
`in MEMS manufacturing, optoelectronic integration and optical packaging are delivering the reliability
`required to realize the potential of the optical communications network and have established MEMS as the
`technology of choice for optical switching. OMM has shifted the paradigm in optical switching from hero
`demonstrations of elaborate concepts to commercial volume production capability of scalable switching
`solutions meeting Telcordia criteria for the entire network.
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