Sh Mems

ACKNOWLEDGEMENT

I extend my sincere gratitude to Dr.N.Premachandran, principal , Govt. Engineering College ,Thrissur , and Prof.K P Indira Devi, Head Of the Electronics and Communication Department, Govt. Engineering college Thrissur , For providing me with the necessary infrastructure for successful completion of my seminar.

I would like to convey my deep sense of gratitude to the seminar coordinator,

Smt.

C.R.

Muneera

.Asst.

Prof,

Electronics

and

communication department for her relentless support.

I am also thankful to Mr.C.D. Anilkumar, Lecturer electronics and communication department, for his suggestions.

I am thankful to all of my friends for their moral support for me.

SEMINAR REPORT’04

MEMS BASED OPTICAL CROSS CONNECTS

Abstract Over the few years an amazing amount of interest has emerged for applications of micro electro mechanical systems in telecommunications. MEMS devices are beginning to impact almost every area of science and technology. In fields as disparate as wireless communications, automotive design, entertainment, and light wave systems. Continuous growth in demand for optical network capacity has fueled the development of optical cross connects having high capacity and reliability. Micro-Electro-Mechanical-Systems devices are recognized to be the enabling technologies which provide a cost effective and reliable way to the implementation of these optical cross connects. Silicon based MEMS have proved to be the technology of choice for low cost scalable photonic applications because they allow mass manufacturing of highly accurate miniaturized parts and use materials with excellent electrical and mechanical characteristics. The use of MEMS for optical switching has tuned out to be most attractive since this application could revolutionize fiber optic telecommunications. While the promises of automatically reconfigurable networks and bit rate independent photonic switching are bright, the endeavor to develop a high port count MEMS based OXC involves overcoming challenges in MEMS design and fabrication

, optical packaging and

mirror control.

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CONTENTS 1. INTRODUCTION………………………..……………..….03 2. OPTICAL SWITCHING……………………….……..…..04 2.1. ALL OPTICAL SWITCHING..…………………….05 2.1.1. PLANAR LIGHT WAVE CIRCUITS……………..06 2.1.2. MICRO ELECTRO MECHANICAL SYSTEMS...06 2.1.3. INK JET BUBBLE SYSTEMS………….………….06 2.1.4. ELECTROHALOGRAPHY………….…………..…07

3. MEMS SWITCHES………………………………………....08 3.1. ACTUATION METHODS……………………….…....12 3.2. MEMS SWITCH ARCHITECTURES……………….12 3.2.1. 2-D ARCHITECTURE…………………………..12 3.2.2. 3-D ARCHITECTURE…………………….….…14 4. DESIGN AND FABRICATION…………………………….16 4.1. DESIGN…………………………………………………16 4.2 FABRICATION……………………………………..…..17 4.2.1. MICROMACHINING PROCESS………….……..18 4.2.2. ELECTRO STATIC MEMS MIRROR……….…..19 5. PERFORMANCE CHARACTERISTICS…………………..20 6. APPLICATIONS ….………..…………................................…21 CONCLUSION ………………….…………….....................…22 REFERENCES…….……………………..........................…...23

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1.Introduction As the modern communications and Internet becomes increasingly prevalent across the globe, fiber optics - as the defacto infrastructure that supports the information revolution - is racing to keep up. The demand for Internet services is driving the growth of data traffic worldwide. Software developers and users are constantly adopting applications that devour more and more bandwidth in order to speed delivery of information. As multiple forms of traffic place increasingly heavy burdens on fiber networks, carriers are looking for innovative ways to push more data through existing fiber. Generally, the current telecom infrastructure is a mix, with fiber optic cables in the 'core' long-haul backbone networks, some fiber and copper wire in metro or regional networks, and primarily copper wire for access networks and 'last mile' connections to customers (though other technologies -- such as cable, satellite, and fixed wireless -- are also used). The Holy Grail in telecommunications and networking today is the 'alloptical network', where every communication would remain an optical transmission from start to finish. The speed and capacity of such a network with hundreds, if not thousands, of channels per fiber strand -would be practically limitless.

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2. Optical switching Most networking equipment today is still based on electronicsignals, meaning that the optical signals have to be converted to electrical ones, to be amplified, regenerated or switched, and then reconverted to optical signals. This is generally referred to as an 'optical-to-electronic-to-optical' (OEO) conversion and is a significant bottleneck in transmission. Huge amounts of information traveling around an optical network needs to be switched through various points known as nodes. Information arriving at a node will be forwarded on towards its final destination via the best possible path, which may be determined by such factors as distance, cost, and the reliability of specific routes. The conventional way to switch the information is to detect the light from the input optical fibers, convert it to an electrical signal, and then convert that back to a laser light signal, which is then sent down the fiber you want the information to go back out on. For example, in a long-haul network, an OEO conversion may occur as often as every 600 kilometers just for amplification purposes. The basic premise of Optical Switching is that by replacing existing electronic network switches with optical ones, the need for OEO conversions is removed. The advantages of being able to avoid the OEO conversion stage are significant. First, optical switching should be cheaper, as there is no need for lots of expensive high-speed electronics. Removing this complexity should also make for physically smaller switches. Unfortunately, optical switching technology is still very much in its infancy. There have been numerous proposals as to how to implement

light

switching

between

optical

fibers,

such

as

semiconductor amplifiers, liquid crystals, holographic crystals, and tiny mirrors. In spite of recent market performance of some very important

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telecom stocks, the international telecommunications network is poised for another enormous advance by providing additional capacity and services with reduced costs. Optical cross connects will soon permit optical traffic to pass through crowded intersections with no conversion required. Optical switches of many types will facilitate pure optical switching and add/drop multiplexing in metro networks and in support of restoration, maintenance

and

testing

.

2.1. ALL OPTICAL TECHNOLOGIES Dozens of telecom systems companies and suppliers continue to offer OEO systems while keeping an eye on and supporting pure

optical-switching

technology

developments.

All

optical

technologies are those in which the electrical to optical conversion is avoided and

the

switching is

done

completely

in the

optical

domain. Most of the technologies adopted by promising candidates come from the integrated circuit (IC) industry. Planar light wave circuits (PLC), micro electro mechanical systems (MEMS), ink-jet bubble technology,

liquid-crystal

systems,

electroholography,

and

thermoelectric techniques are some of the technologies currently under development. These IC-based systems bring mass production, repeatable quality, and lower manufacturing costs than current practice. Switches and cross connects based on these technologies will perform transparent switching in which traffic stays in the optical form all the way through the network backbone and down into the metro.

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2.1.1. Planar light wave circuits Planar light wave circuits take advantage of IC practice in that layers of material are deposited and etched to create channels for either diverting or passing photons. The wall material of the channels can be reflective on command but there are no moving parts. Azanda, Kymata, Light wave Microsystems, Lynx Photonics, Nanovation, Network Photonics, OptXcon, and Optical Switch Corp. are some of the startups developing PLC technology.

2.1.2. Micro electro mechanical systems Micro electro mechanical systems(MEMS), as they apply to optical switching, are based upon IC practices that result in a movable reflective surface or mirror, the angle of which can be changed by the application of electrical power or thermal change. The optical wavelength is directed at the reflective surface, which, upon command, permits the photons to pass, or diverts them to another exit. Astarte, C-Speed, Calient, IMMI, OMM, K2 Optronics, Luxcore, Lucent technologies and Onix Microsystems are some of the MEMS-based firms.

2.1.3. Ink-jet bubble systems Ink-jet bubble systems are also IC-based, with the addition that a microscopic amount of a liquid is placed at each intersection of etched channels. With the onset of an electrical pulse the liquid is instantly heated, creating a bubble that is reflective and diverts the photons to another exit. Agilent and Alcatel are pioneering this technology. dept. of ece

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2.1.4. Liquid-crystal systems Liquid-crystal systems are also IC-based. Polymeric materials are suspended in special liquids. The materials change their alignment upon the addition of electrical power—either permitting light to pass through or diverting it. Chorum and Spectra-Switch are two of the leading liquidcrystal developers.

2.1.5.electroholography

i

Electroholography is based upon special micro crystals that can have a hologram stored in them. The hologram is of such a nature that it allows photons to pass through when it is in the 'off' position and is reflective when in the 'on' position, thereby diverting the light upon command. Trellis is currently developing this technology.

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3. MEMS SWITCHES In telecom, MEMS has become synonymous with the arrays of tiny tilting mirrors used for optical switching fabric, although the same technology is being used to make a wide range of other components as well. MEMS consist of mirrors no larger in diameter than a human hair that are arranged on special pivots so that they can be moved in three dimensions. Several hundred such mirrors can be placed together on mirror arrays no larger than a few centimeters square. Light from an input fiber is aimed at a mirror, which is directed to move the light to another mirror on a facing array. This mirror then reflects the light down towards the desired output optical fiber.

Since MEMS creates so

many mirrors on a single chip, the cost per switching element is relatively low. However, since it involves moving parts, MEMS is fairly slow to switch – requiring milliseconds to do so. This is fine for lambda provisioning or restoration but is too slow for optical burst switching or optical packet switching applications. Conventional MEMS works by reflecting the beam of light from the surface of a tiny mirror.The micro mirrors are actuated by electrostatic actuators, which are located behind the reflecting front face of the mirrors. MEMS systems have moving parts, and the speed at which the mirror moves is limited. By applying more current, the mirror can move faster, but there's a limit to how much current can be sent into the array of mirrors. If this weren't bad enough, it seems that the speed and angular displacement terms in the calculation of the required current have integer powers of around 4 or 5, and so the bottom line is that we

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have to put a lot of current into the array for a small improvement in speed. By changing the mirror design so that the angle through which light is bent is smaller, it's possible to achieve faster switching speeds. This technique is known as "fast MEMS." MEMS arrays can be built on a single-chip, single-plane approach. In other words they are 2 dimensional (2D MEMS). In a simplistic approach it’s also possible to stack a number of 2D MEMS arrays on top of each other to create a 3D MEMS array. In fact, real 3D MEMS systems are somewhat more complex than this, but the general principle holds. A huge drawback of 3D MEMS is the fact that the thousands of mirrors require complex software to coordinate their operations.

In

particular, one vendor has suggested that there are over a million lines of code in their implementation (although the reference may be to the overall switch software, and not just the MEMS subsystem). While it’s possible to test software extensively, the opportunity for bugs increases geometrically with the size of the code base. On the upside, MEMS is a very rapidly changing technology. Since it seems to have a monopoly on the high port-count optical switch market for the moment, a huge amount of investment is going into the implementations and into solving the basic problems.

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Fig. 1 MEMS Mirror Array

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Micro Electro Mechanical Systems (MEMS) are semiconductor-made micro-mechanisms, which are generally used as movable micro-mirrors that can deflect optical signals from input to output fibers. As far as medium- and large-size switching fabrics are concerned, micro-mirrors can be arranged into two-dimensional or three-dimensional arrays . In these switches, mirrors are steered in order to deflect light beams properly. Small-size switches

can be also made, as shown in the

following figure

Fig.2 MEMS Switch

In this case, the mirror slides along the 45° direction, yielding the BAR or CROSS states. MEMS switches feature good scalability. MEMS research is an outgrowth of the vast capabilities developed by the

semiconductor

industry,

including

deposition,

etching,

and

lithography, as well as an array of chemical processes such as anisotropic and highly selective etches having different etch rates for different crystallographic orientations and materials. These processes, which were originally developed to build microelectronics, are also capable of building micromechanical devices (structures capable of motion on a microscopic scale).

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MEMS are built in much the same way as a silicon integrated circuit. Various films such as polysilicon, silicon nitride, silicon dioxide, and gold are deposited and patterned to produce complicated, multilayer three-dimensional structures. However, the major difference is a release step at the end. Ina MEMS device, some of the layer materials are removed using a selective etch, leaving a device with elements that can move. The advantages

of batch-processing

techniques such as cost minimization make it economical to produce such optical cross connect switches The mirror is connected to a see-saw and either reflects the light from the optical fiber on the left to the fiber at right angles to it, or moves out of the way to allow the light to go straight into the other fiber.

Fig. 3 A two-axis micromirror for use in an all-opticalcrossconnect

Shown in the above figure is a two-axis micro mirror for use in an all-optical cross connect. The mirror is doubly-gimbaled so that light can be routed in two directions to allow complex switching functions to be performed. Such mirrors have enhanced the manufacturing of large, MEMS-based, optical cross connects. These switches have very large port counts, low losses, fast switching speed, and low costs. Clearly, the possibilities for novel optical devices and functions are endless. dept. of ece

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3.1. ACTUATION METHODS OF MEMS MIRRORS Magnetic actuation and electrostatic actuation are the two viable choices for mirror positioning. Magnetic actuation offers the benefit of large bidirectional (attractive and repulsive) linear force output but requires complex fabrication process and electromagnetic shielding . Electrostatic

actuation

is

preferred

method

for

mirror

positioning because of the relative ease of integration and fabrication.. They consist of four capacitor pads separated by two orthogonal channels parallel to the two axes of rotation of the corresponding micromirror. The mirror is grounded and the four pads are placed under a bias voltage to mechanically preload the mirror. By modulating the voltage of the four pads about the bias level it is possible to generate controlled rotations of the micro-mirrors.

3.2. MEMS Switches Architectures Switch arrays are constructed from multiple switch elements. The arrangement

usually follows one of three configurations: two-

dimensional (2-D) matrices of NxN two-position mirrors, linear arrays of NxN single-axis multiple-position mirrors [three–dimensional (3-D) 1xNarrays]

3.2.1. 2-D ARCHITECTURE Fig. 4 shows the arrangement of the first type of cross connects. The inputs are provided by a linear array of optical

N fibers. Light

emerging from the fiber array is collimated by a linear array of lenses into a set of quasi-parallel beams that propagate in free space. The outputs are taken from a similar array of fibers, equipped with a similar dept. of ece

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set of lenses, and designed to accept a similar set of beams. The axes of the input and output fibers are typically arranged at right angles.

Fig .4 principle of NxN mirror insertion free space optical cross connect

Fig .5 Illustration of NxN mirror insertion free space optical cross connect (2-D Architecture)

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The space between the input and output fiber arrays is filled with a set of small movable mirrors, capable of being inserted and removed from the intersections between the beams at an angle intermediate between their directions. A path between

i-th input fiber

and j-th output fiber is then established by the insertion of the relevant mirror M(i,j).

3.2.2. 3-D ARCHITECTURE Fig.6 shows the second type of cross connect. The inputs and outputs are again linear arrays of N fibers equipped with collimators. However, between the input and output, the beams are reflected from two linear arrays of mirrors.

Fig 6. Principle of NxN mirror rotation free space optical cross connect

Each individual mirror may be rotated through a variable angle about an axis normal to the figure. A path between input fiber and output fiber is then established by angular adjustment of mirror from the first array and mirror from the second, in a periscope configuration. NxN cross connects have been constructed using MEMS mirrors on torsion suspensions

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Fig .7.illustration freespace optical cross connect (3-D Architecture)

A similar principle is used in the third type of cross connect. The linear arrays of arrays of of

N

N square

N fibers and collimators each replaced with 2-D fibers and collimators, and the linear arrays

single-axis mirrors are replaced by 2-D arrays of

N square

dual-axis mirrors. The required mirror motion is achieved by mounting the mirror on a gimbals suspension, as in Fig. 7. This type of switch is scalable to a higher port count, and has been demonstrated using several forms of MEMS mirror

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4. Design & Fabrication Components of a large MEMS based OXC system include thousands of actuated mirrors, lenses and collimator arrays. With no doubt the MEMS mirrors, the key active element in the optical system, is the most critical technology for large OXCs

4.1. Design A two axis tilting mirror can be divided into three elements: the mirror, the springs as the mechanical support and the actuator all of which determine the important OXC system parameters such as insertion loss, settling time, and maximum port count and power dissipation.

Reflectivity of each mirror is desired to be above 97 percent. The tilt angle requirement varies from a few degrees to 10 degrees on either direction depending on the design.

Challenges in design come from the different trade-offs between desired properties of the MEMS device. As an example the stiffness of the supporting springs should meet the mirror response time and the mirror stability and immunity to shock. But the maximum stiffness is determined by the maximum tilt angle and the actuators maximum force or torque output as well as the system power budget.

A stable metal coating such as gold, along with necessary additional metal adhesion and diffusion barrier layers is often used as

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the reflective surface. These metal coatings can create an undesirable temperature dependent mirror curvature due to intrinsic stress of the metal layers and the difference in the temperature coefficients of the metal coating layers and the bulk mirror made up of a different material. This problem is severe if the coating is applied only to one side of the mirror. A thick mirror can counteract all these difficulties but then the mass of the device increases prohibitively.

4.2. Fabrication In principle the bulk mirror can be made of any material as long as reliability, reflectivity, and optical flatness requirements are met. Single crystal silicon, commonly used in MEMS, is recognized to be the most suitable technology over polysilicon or electroplated metal due to low intrinsic stress and excellent surface smoothness. The choice of the material for the mirror springs is even more important because the mirror springs will be constantly subject to twists and bends. Superior mechanical characteristics make SCS a strong and the best candidate for the mirror springs. Alternative material such as polysilicon are poor substitutes because of potential stress,hysterisis and fatigue problems. n most cases ,the same material is chosen for both the mirror and springs in order to yield a straightforward fabrication process. Besides typical lithography, deposition, and etching procedures necessary

fabrication

steps

include

deep

reactive

ion

etches

(DRIE) .silicon wafer bonding and chemical mechanical polishing (CMP). Silicon on insulator (SOI) wafer is a convenient starting material for the SCS bulk mirrors with uniform thickness

and low

intrinsic stress but they are expensive. Applying clever silicon etching and wafer bonding techniques to cost effective silicon wafers may also yield mirrors with sufficiently large ROC(radius of curvature ) and low dept. of ece

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mass. the primary differentiating factor between these MEMS mirror processes

is

device

performance

characterized

by

mirror

size,flatness,reflectivity ,maximum tilt angle and ease of mirror control. Material supply availability, length of fabrication cycles equipment bottlenecks play important roles in shortening product development cycle time to market. Ease of circuit integration mirror array size and manufacturing yield may also influence the overall switch fabric design. Arguably , a fabrication process that enables monolithic integration of electronics with MEMS may lead to MEMS mirrors with highest performance and greatest functionality.

4.2.1. Micromachining processes used to

Build MEMS devices

Fig .8. the micromachining process for making the mirror.

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4.2.2.

MEMS BASED OPTICAL CROSS CONNECTS

Illustration of an SOI based Electrostatic MEMS mirror

Fig 9(a) before release of gimbaled mirror

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Fig 9(b) after release of gimbaled mirror

Fig 10

Single MEMS mirror for OXC

5. Performance Characteristics The following are some performance paramters of optical switches. (1)

The wavelength range

it is the range of the wavelengths that can be routed without much tioattenuan .it is ussually expressed in nanometers (2)

insertion loss

it is the attenution introduced to the signal due to the insertion of the device.it is expessed in dB (3)

cross-talk attenuation

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the attenuation encountered to the undesired light waves is known as cross talk attenuation.it is measured in dB and should be as high as possible

(4)

power dissipation

it is the power consumed by the associated contrl circuitry for positioning the mirror in the proper direction and is measued in mW and (5)

switching time

it is also called the latency,and is the time between applying the control signal and the establishment of the connection.measured in ms

6. Applications optical switches can be used in a wide range of applications such as those given below.

Optical switching. Optical switches can be used as basic building blocks for network nodes to provide optical circuit or packet switching.

Switching times in the

ms range are sufficient for circuit switching. Nevertheless, to the purpose of optical packet switching, switching times in the ns range are required.

Optical add-drop multiplexing Optical add-drop multiplexers are used to add and drop specific wavelengths from multi-wavelength signals, to avoid electronic processing. For this application, wavelength selective switches are required. Switching times in the ms range are adequate.

Fiber restoration and protection switching. Small-size switches are used to restore optical paths in the event of link failure. For this application, 2x2 switches, with switching times in the ms range, are commonly used.

Signal monitoring

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For ease of network management, optical switches can be used for signal monitoring. To this purpose, wavelength-selective switches are commonly used.

CONCLUSION MEMS technology offers tantalizing possibility of advanced optical cross connects with large port count, scalability, and switching capacity that can meet the switching demands in the ever faster, denser, rapidly growing optical networks today and in the future. Of course, further research and development that considers not only device performance but also reliability and total cost, including both fabrication and maintenance, mirror control algorithm will be necessary when applying these devices to optical cross-connects and optical add/drop multiplexers. As MEMS technology continues to advance ,one thing is clear , The powerful

impact

of MEMS technology

for the

telecommunications industry will never be forgotten.

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REFERENCES 1. 'The lucent lambda router: MEMS technology of the future here today ' David J Bishop Randy Giles, Gary p Austin, Lucent technologies IEEE Communications magazine MARCH 2002, Vol.40, No. 3

2.

Lucent Technologies, “Lambda Router™ All Optical Switch,” http://www.lucent.com/products/solution

3.

MEMS Optical, “Scanning Two Axis Tilt Mirrors,” http://www.memsoptical.com/prodserv/products/twotiltmir.htm.

4.

'Micro-Mirror Array Control of Optical Tweezer Trapping Beams.' Nicholas G. Dagalakis, Thomas LeBrun, John Lippiatt. National Institute of Standards and Technology

5.

www.sercalo.com

6.

'SPIE’s International Technical Group Newsletter ' DECEMBER

2000

7.

'Silicon micro machines' David Bishop, Vladimir Aksyuk, CrisBolle, Randy Giles, dept. of ece

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and Flavio

MEMS BASED OPTICAL CROSS CONNECTS

Pardo

Micromechanics Research, Bell Laboratories Lucent Technologies, Murray Hill

8.

'Modular MEMS-Based Optical Cross-Connect With Large PortCount' N.Bonadeo, T. Chau, M. Chou, R. Doran, R. Gibson. Harel, IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 15, NO. 12, DECEMBER 2003.

9. 'A Technical Paper: Discussing Optical Phased Array Technology For

High- Speed Switching ‘ ,

Chiaro Networks whitepaper

http://www.chiaro.com/pdf/chiaroleos2002.pdf

10. ' Integrated Modeling of Optical MEMS Subsystems' Robert Stoll, Thomas Plowman, David Winick, Art Morris Coventor, 4001

Weston Parkway, Suite 200, Cary, NC 27513

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