jayaprakash(MEMS)

8/7/2019 jayaprakash(MEMS)

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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 andcommunication department, for his suggestions.

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


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SEMINAR REPORT04 MEMS BASED OPTICAL CROSS CONNECTS

AbstractOver 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 capacityhas 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 basedOXC involves overcoming challenges in MEMS design and fabrication

, optical packaging and mirror control.

dept. of ece gec thrissur 2


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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. DESIGN16

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 'all-optical 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 switchingMost networking equipment today is still based on electronic-

signals, 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.



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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 veryimportant 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 TECHNOLOGIESDozens 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, andthermoelectric techniques are some of the technologies currently

under development.



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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 opticalform all the way through the network backbone and down into the

metro.

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 movingparts.

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,



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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.

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 powereither permitting

light to pass through or diverting it.

Chorum and Spectra-Switch are two of the leading liquid-

crystal 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



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the light upon command. Trellis is currently developing this

technology.

3. MEMS SWITCHESIn 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



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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 opticalburst 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 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 its 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



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MEMS subsystem). While its 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.

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-mirrorscan 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.


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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.

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 micro-

mirror. 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 generatecontrolled 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 [threedimensional (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


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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

set of lenses, and designed to accept a similar set of beams. The axesof 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 ARCHITECTUREFig.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 fromtwo 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 N fibers and collimators each replaced with 2-D arrays

of N square fibers and collimators, and the linear arrays of N

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 & FabricationComponents 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


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yield mirrors with sufficiently large ROC(radius of curvature ) and low

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


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Build MEMS devices

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

4.2.2. Illustration of an SOI based

Electrostatic MEMS mirror


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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 muchtioattenuan .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 isknown as cross talk attenuation.it is measured in dB and shouldbe 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 inmW

and

(5) switching time

it is also called the latency,and is the time between applying thecontrol signal and the establishment of the connection.measuredin ms

6. Applicationsoptical 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 switchingcapacity 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. 'SPIEs International Technical Group Newsletter ' DECEMBER

2000

7. ' Silicon micro machines'David Bishop, Vladimir Aksyuk, CrisBolle, Randy Giles,


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and Flavio Pardo

Micromechanics Research, Bell Laboratories

Lucent Technologies, Murray Hill

8. 'Modular MEMS-Based Optical Cross-Connect With Large Port-

Count'

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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