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Speeding Up the Internet: 

SiWave, Inc. Chose ALGOR Software for Design of MEMS Optical Switch 

Internet traffic has been growing tenfold each year as more people are getting online for communication, education and entertainment. In addition, connections are used more intensely for audio/video streams and web-hosted application service provider (ASP) utilities as well as email and web pages. Expanding fiber optic networks are currently seen as the key to fulfilling consumer demand for Internet bandwidth. At the core of optical networks are optical switches that can process multi-wavelength signals among thousands of different routes or ports. 

SiWave, Inc., an Arcadia, California supplier of optical switching components and subsystems, is developing an optical switch using Micro Electro Mechanical Systems (MEMS) technology. SiWave chose ALGOR FEA-based simulation software in the development of this product to predict the performance and durability of this tiny device. "ALGOR is a fast, intuitive simulation package that has the analysis capabilities I need, including static stress, Mechanical Event Simulation (MES), linear dynamics, heat transfer and electrostatics," said Rob Calvet, Optomechanical Engineer at SiWave.

An SiWave MEMS optical switch for fiber optic networks. 

Why Fiber Optic? 

Light-based fiber optic technology is more capable of carrying greater amounts of data than traditional electron-based telecommunication technology. This is because photons do not interact with each other, which enables them to carry higher frequency signals that encode more information per second--in other words, more bandwidth. Using Dense Wave-Division Multiplexing (DWDM) technology, many frequencies or colors of light can be channeled through a single fiber optic line, further increasing the bandwidth per line. At two terabits per second, fiber carries 1 million times as much bandwidth as a T1 line or digital subscriber line (DSL) link. With less heat generated and less power consumed, the lowest cost per unit of bandwidth-distance is delivered. 

Optical switches help make critical connections in fiber optic systems, sitting at junction points in telecommunication lines and enabling carriers to string together pathways to provide end-to-end connections. By redirecting signals between thousands of different ports, optical switches offer improvements in speed, data capacity, data management and cost over optical-electrical-optical (OEO) routers. For example, a 1000 x 1000 OEO router takes up 3-4 standard racks that are 6 feet high and uses kilowatts of power. Using optical switches, the same capacity can be handled with a system the size of a shoe box using a few watts. In addition to the lower power consumption of the system itself, the optical system reduces overhead because it takes up less space and requires less air conditioning. 

Recent estimates are that the worldwide market for optical switch systems will grow from $234 million in 2000 to $7.4 billion by 2004. Carriers look for modular, scaleable solutions, so they can increase capacity when they need it, and small to large bandwidth capacity for greater flexibility. 

MEMS for Optical Switches 

The small size of SiWave's optical switch is possible due to MEMS technology. MEMS are micromachines the size of a grain of salt or the eye of a needle that integrate mechanical elements, sensors, actuators and electronics on a common silicon substrate. In addition to optical switches within telecommunication and networking systems, MEMS applications include accelerometers in automotive airbags, inkjets in desktop printers and sensors in medical testing equipment.

A MEMS optical switch shown on a penny to demonstrate how small MEMS are. 

Although MEMS devices are typically extremely small, MEMS technology is not only about size. MEMS is a new manufacturing process that allows the creation of combined electromechanical, optical and electronic devices and systems using batch fabrication techniques. MEMS devices are fabricated using photolithographic-based semiconductor processes that selectively etch away parts of the silicon wafer or add new layers to form the various devices. Since MEMS devices are batch manufactured, new levels of capability, reliability and complexity can be combined onto a single device or an array of devices at a relatively low cost. 

The unique characteristics of MEMS devices allow the manufacture of affordable optical switching products that precisely redirect signals. 

Micro- and Macro-Scale Design Considerations 

The telecommunications industry has stringent shock specifications for all components - even if they can fit on the head of a pin. According to the Telcordia standards, all components must be able to survive a 500-g, 1-millisecond, ½-sine pulse. Equivalent to dropping a rigid object 6 feet onto concrete, this standard ensures that components will withstand shocks they might see in the field, for example, during installation or when a technician thrusts a card into an adjacent rack. 

For components such as chips, for which the specification was originally intended, the geometry is simple enough that designing to withstand shock is not a problem. For those types of components, the specification is essentially a test of workmanship. With more geometrically complex devices, such as a MEMS optical switch, the specification becomes an important design consideration. 

Shock Simulation 

Calvet began by modeling the 1-mm optical switch mirror in SolidWorks and used InCAD technology to seamlessly capture the model for use in ALGOR. After creating an automatic mesh, he worked with the geometry in Superdraw, ALGOR's precision finite element modeling tool, to refine the mesh. "Since MEMS geometry is almost two-dimensional, I sometimes use SolidWorks for portions of the geometry and then extrude the model in ALGOR's Superdraw." 

Anticipating that the optical switch might be susceptible to shock, Calvet started to plan for a built-in 100 Hz frequency isolation system within the MEMS device's packaging that would provide about 40% damping. Calvet calculated that the optical switch within the isolation system would actually experience a peak acceleration of 163 g's. It was this attenuated shock value that he input into his simulation. 

Because critical resonance frequencies are high compared to the duration of the shock, Calvet considered an inertial acceleration method a good way to approximate the behavior of the mirror under shock conditions. The inertial acceleration method required no special constraints, only a standard gravity input and a time history curve. Although the Telcordia standards call for a ½-sine pulse, the isolation system affects the shape as well as the magnitude of the shock load - resulting in a sine-shaped load - so that the optical switch will actually get shaken in two directions within just a few milliseconds. 

In addition to the shock load, Calvet input published, orthotropic material properties for the silicon and silicon compounds that comprise the optical switch. Also, he looked at which surfaces of the part were most likely to experience possible contact and defined restricted contact pairs.

ALGOR's Mechanical Event Simulation software was used to calculate stress and displacement results over time for an applied shock load on the MEMS optical switch. 

Calvet looked at both von Mises stress and displacement results. "One concern for this component is the orientation into which it displaces," said Calvet. "If the part were to contact the packaging, it might get stuck. I had to ensure the mirror's motion would not result in interference with surrounding components. In addition, I was concerned that the device might drum itself against its base so I needed to know whether contact occurred and the stresses that resulted from it." 

SiWave is proceeding with prototype testing and production for this design. The initial delivery of a prototype for testing is scheduled for the fourth quarter of 2002. "The stresses appear to be within the shock specifications," said Calvet. "If our prototype testing reveals a problem, we may improve the isolation system." 

In addition to MES, Calvet also conducted an electrostatic analysis of the optical switch design, which verified that the electrostatic potential they intended to use would result in sufficient electrostatic forces to actuate the switch.

Robert Calvet views the results of the ALGOR Mechanical Event Simulation of the optical switch.

Robert Calvet has been using ALGOR FEA for 20 years. SiWave, Inc. is a supplier of optical switching components and subsystems. SiWave, Inc. is funded by Draper Fisher Jurvetson, the leading early-stage information technology venture capital firm.

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