Optical MEMS Report

Optical MEMS

ABSTRACT

Optical MEMS (Micro-Electro-Mechanical Systems) is the integration of

mechanical elements, sensors, actuators, and electronics on a common

silicon substrate through microfabrication technology. And Optical MEMS

has created a new fabrication paradigm for optical devices and systems.

Using tools developed for the integrated-circuits industry, Optical MEMS

brings unprecedented levels of miniaturization and integration to optical

communication systems. Today , the capacity of optical communication

networks is limited by the switches that are electronic in their core. Optical

MEMS is transforming the telecommunications infrastructure by providing

all-optical switches that enable high-capacity networks. Optical MEMS

devices are driving the trend toward all-optical telecommunication networks.

With the ability to directly manipulate an optical signal, MEMS have several

applications that eliminate unnecessary optical-electrical-optical (O-E-O)

conversions. In optical communication, electrical components such as

inductors and tunable capacitors can be improved significantly compared to

their integrated counterparts if they are made using optical MEMS

technology. With the integration of such components, the performance of

communication circuits will improve, while the total circuit area, power

consumption and cost will be reduced. 

In addition, the mechanical switch, it is a key component with huge potential

in various microwave circuits. Optical MEMS technology is currently used in

low- or medium-volume applications. It proves to be a boon for the modern

world.

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AKNOWLEDGEMENT

TABLE OF CONTENTS

ABSTRACT……………………………………………………………………….2

AKNOWLEDGEMENT…………………………………………………………..3

TABLE OFCONTENTS…………………………………..……………………..4

1. Introduction…………………………………….……………...................5

2. What is MEMS?..…………………….. ..….………………...................5

3. What is Optical MEMS?..………………..…...…………………............6

4. MEMS mirrors ...………………..……...……………….........................7

4.1 Types……………………………………………………………..74.2 Torsional Mirror………………………………………………….84.3 Piston mirror……………………………………………………..12

5. Applications ...…………………………....……...…………………..…..13

5.1 Display of imaging system…...………………………………...145.1.1 DMD…………………..…………………………...155.1.2 GLV………………………………………………...16

5.2 Fire Optic communication………………………………..…….185.3 Adaptive Optics………………………………………………….195.4 Data storage……………………………………………………..215.5 Use in medicine…………………………………………………22

6. Disadvantages of Optical MEMS devices..…………………………...24

7. Conclusion……………..………………………………………..............25

8. References……………………………………………………………….26

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1. Introduction

The increasing quest for transporting large amounts of data at a fast speed,

search for adaptive optics and high resolution video graphics along with

miniaturization of electronic components has evolved a new era in micro-

structure world .The sophisticated technology of Optical MEMS devices,

currently used in low- or medium-volume applications proves to be a boon

and outshines all tradiotional devices.

2. What is MEMS ?

Micro-Electro-Mechanical Systems (MEMS) is the integration of

mechanical elements, sensors, actuators, and electronics on a common

silicon substrate through microfabrication technology. While the

electronics are fabricated using integrated circuit (IC) process sequences

(e.g., CMOS, Bipolar, or BICMOS processes), the micromechanical

components are fabricated using compatible "micromachining" processes

that selectively etch away parts of the silicon wafer or add new structural

layers to form the mechanical and electromechanical devices.

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Microelectronic integrated circuits can be thought of as the "brains" of a

system and MEMS augments this decision-making capability with "eyes"

and "arms", to allow microsystems to sense and control the environment.

Sensors gather information from the environment through measuring

mechanical, thermal, biological, chemical, optical, and magnetic

phenomena. The electronics then process the information derived from

the sensors and through some decision making capability direct the

actuators to respond by moving, positioning, regulating, pumping, and

filtering, thereby controlling the environment for some desired outcome or

purpose

3. WHAT IS Optical MEMS?

The integration of micro-optics and micro-electromechanical systems has

created a new class of Microsystems , termed micro-opto-electro-

mechanical systems (MOEMS).

Optical MEMS (Micro-Electro-Mechanical Systems) is the integration of

mechanical elements, sensors, actuators, and electronics on a common

silicon substrate through microfabrication technology. And Optical MEMS

has created a new fabrication paradigm for optical devices and systems.

Using tools developed for the integrated-circuits industry, Optical MEMS

brings unprecedented levels of miniaturization and integration to optical

communication systems. Today , the capacity of optical communication

networks is limited by the switches that are electronic in their core. Optical

MEMS is transforming the telecommunications infrastructure by providing

all-optical switches that enable high-capacity networks. With the ability to

directly manipulate an optical signal, MEMS have several applications that

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eliminate unnecessary optical-electrical-optical (O-E-O) conversions. In

optical communication, electrical components such as inductors and

tunable capacitors can be improved significantly compared to their

integrated counterparts if they are made using optical MEMS technology.

With the integration of such components, the performance of

communication circuits will improve, while the total circuit area, power

consumption and cost will be reduced. In addition, the mechanical switch,

as developed by several research groups, is a key component with huge

potential in various microwave circuits. Optical MEMS technology is

currently used in low- or medium-volume applications.

4. MEMS Mirrors

• In Optical MEMS mechanical motion of microstructures is used for

manipulation of photons.

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• MEMS Mirrors are the most critical component

for optical MEMS technology.

• MEMS mirrors are useful to

– Modify the optical path on the chip

– Couple Light in and out of optical fibers

– Redirect light in image formation devices

4.1 MEMS Mirrors: Types

• Two fundamental types of mirrors based on motion

Torsional mirrors Piston mirrors

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Torsional mirrors deflect light

spatially by rotating the mirror

surface relative to the plane

Piston mirrors move

perpendicular to the plane of

the mirror and shift the phase

of the incoming light

4.2. Torsional Mirrors

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Mechanical Restoring Torque

• The restoring mechanical torque exerted on the plate by the beams for

an

angular rotation θ is given by

Mechanical Restoring Torque

TM = kθθ

where

Electrostatic Torque

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Electric Field

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Electro static torque produced by an incremental plate length dx located at

x is given by

Total electrostatic torque is obtained by integrating the incremental torque over the plate length

Stability Analysis

In the stable region, initially the electrostatic torque is greater than the

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mechanical restoring torque TE < TM

To reach a stable equilibrium point, the plate will rotate so as to

compensate the electrostatic torque that increases with square of the

voltage by a restoring mechanical torque that increases in proportion to

the angular rotation

At equilibrium TE = TM

We obtain

For equilibrium in the stable region,

A restoring torque will bring the plate back to its initial position even in the

presence of any small perturbation in the angular displacement.

4.3 Piston Mirrors

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5.Applications

A few applications of Optical MEMS include

• Display and Imaging Systems

• Fiber Optic Communication

• Optical Scanning Systems

• Adaptive Optics

• Data storage

• Medicine

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5.1 Display and Imaging Systems

MEMS technology is well suited for making sophisticated imaging

devises which includes features such as

High Resolution

High Contrast and Brightness

Small Size

Controlled in both digital mode and analog

modes

New market such as portable projector and

large screen TV Micromirror Array

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Two types of imaging devices :

– Digital Mirror Device

– Grating Light Valve

5.1.1 DMD

Digital micromirror device (DMD) technology focuses on the impact of a

fully digital video display system. It redefines the architecture of an entire

video system that can dramatically decrease costs and increase

performance from current display technologies (CRT and LCD).

The inherent nature of a digital display tends toward high-performance

applications, such as highdefinition or high-quality displays. In addition,

the digital nature of the technology matches well with today's surge in

computer graphics display. In contrast, rapid integration of digital

semiconductor technology could enable low-cost applications of the

technology, such as consumer TV or personal viewers. Future displays

will focus on the display of media from several sources, such as digital

multimedia TVs or personal ports into the "information superhighway." In

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general, the scope of the DMD display application space is very large and

seemingly ever- increasing high definition or high-quality displays. In

addition, the digital nature of the technology matches well with today's

surge in computer graphics display. In contrast, rapid integration of digital

semiconductor technology could enable low-cost applications of the

technology, such as consumer TV or personal viewers. While digital

displays have inherent high performance, this can be traded for a cost

advantage in these applications. Future displays will focus on the display

of media from several sources, such as digital multimedia TVs or personal

ports into the "information superhighway." In general, the scope of the

DMD display application space is very large and seemingly ever-

increasing.

.

5.1.2 GLV

The Grating Light Valve (GLV) technology is a micromechanical phase

grating. By providing controlled diffraction of incident light, a GLV device

will produce bright or dark pixels in a display system.With pulse width

modulation, a GLV device will produce precise gray-scale or color

variations. Built using

micro electromechanical system (MEMS) technology, and designed to be

manufactured using mainstream.IC fabrication technology, the GLV device

can be made both small and inexpensively. A variety of display systems

can be built using GLV technology each benefiting from the high contrast

ratio, fill ratio, and

brightness of this technology. In addition, GLV technology can provide

high resolution, low power consumption, and digital gray-scale and color

reproduction.

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A Grating Light Valve (GLV) device consists of parallel rows of reflective

ribbons. Alternate rows of ribbons can be pulled down approximately one-

quarter wavelength to create diffraction effects on incident light . When all

the ribbons are in the same plane, incident light is reflected from their

surfaces. By blocking light that returns along the same path as the incident

light, this state of the ribbons

produces a dark spot in a viewing system. When the (alternate) movable

ribbons are pulled down, however, diffraction produces light at an angle

that is different from that of the incident light. Unblocked, this light

produces a bright spot in a viewing system.

The Grating Light Valve uses reflection and diffraction to create dark and bright image

areas.

If an array of such GLV elements is built, and subdivided into separately

controllable picture elements, or pixels, then a white-light source can be

selectively diffracted to produce an image of monochrome bright and dark

pixels. By making the ribbons small enough, pixels can be built with

multiple ribbons producing greater image brightness. If the up and down

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ribbon switching state can be made fast enough, then modulation of the

diffraction can produce many gradations of gray and/or colors.

.

There are several means for displaying color images using GLV devices.

These include color filters with multiple light valves, field sequential color,

and sub-pixel color using "tuned" diffraction gratings.

5.2 Fibre optic communication

Optical MEMS has brought unprecedented levels of miniaturization and

integration to optical communication systems. Today , the capacity of

optical communication networks is limited by the switches that are

electronic in their core. Optical MEMS is transforming the

telecommunications infrastructure by providing all-optical switches that

enable high-capacity networks.It has features like

Switching Speed in µs/ms range

Low Crosstalk / Low Insertion Loss

Small Size / Low Cost

Scalability to Large Number of I/O Port

Optical MEMS devices are driving the trend toward all-optical

telecommunication networks. With the ability to directly manipulate an

optical signal, MEMS have several applications that eliminate

unnecessary optical-electrical-optical (O-E-O) conversions. In optical

communication, electrical components such as inductors and tunable

capacitors can be improved significantly compared to their integrated

counterparts if they are made using optical MEMS technology. With the

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integration of such components, the performance of communication

circuits will improve, while the total circuit area, power consumption and

cost will be reduced. In addition, the mechanical switch, as developed by

several research groups, is a key component with huge potential in

various processing and communication circuits.

5.3 Adaptive Optics

Optical MEMS can be used to create adaptive optics chips which can be

used for wavefront correction systems. There are a variety of application

for wavefront correction systems ranging from advanced military targeting

systems to preview systems for advanced surgeries.

Adaptive Optics systems are in use today on large Astronomical

telescopes. Adaptive optics remove the optical imperfections that result

from peering through the atmosphere. These systems are built today with

expensive macro technology. While they work very well, these systems

are very expensive. Large telescopes are not that sensitive to costs, so

they can readily afford the present significant costs in order to improve the

quality of the images that see. More widespread adoption of adaptive

optics, and wavefront correction is hampered, however, by the significant

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costs of traditional systems.

MEMS offer the promise of achieving low cost adaptive optics systems.

With lower cost, there are many applications which could benefit from

wavefront correction. Some applications under consideration for a low cost

adaptive optics system include preview systems for LASIK surgery,

ophthalmic phoropters, and fundus imaging systems. Fundus imaging

systems are used by eye doctors to image the retina, and detect

degenerative eye disease. The challenge is that image quality from

present systems is poor, and the disease can be in advanced stages

before a doctor can detect it. Improving the image quality through use of

affordable MEMS adaptive optics systems will allow surgeons to detect

degenerative eye disease at much earlier stages where it is easier to treat,

and before vision is affected.

MEMS chips have been created to meet the demanding requirements of

the vision science community. The chip will enable the creation of high

performance, cost effective adaptive optics systems. The vision science

community has significant interest in obtaining such systems for a variety

of ophthalmic applications. This chip will enable a true paradigm shift, and

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will permit the placement of high performance imaging systems into the

hands of ophthalmologists and optometrists. These chips should enable

order of magnitude improvements in the doctor's ability to image the

retina, and as such, will dramatically increase the ability to diagnose

degenerative ophthalmic conditions while they are still treatable.

Enhanced capabilities in fundus imaging will enable earlier detection of

disease, better measurement of treatment effectiveness, and the

development of improved treatment methods.

5.4 Data Storage

Optical data storage is a promising field to which the optical MEMS

technology can be applied. Tight integration of optical and mechanical

components is needed for the optical head for data storage in the next

generation. The diffraction limit in optical storage using a lens can be

overcome by near-field optical technology. An optical storage system

with a multi-probe array based on scanning near-field optical

microscopy (SNOM) is proposed. An integrated SNOM probe with a

micro aperture for generation of the optical near-field has already been

fabricated. The data is stored in the optical array of mirrors which can

be extracted easily.This gives us voluminous data storage facility at a

very low cost.

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5.5 Use in Medicine

Optical MEMS facilitates advanced surgery, ophthalmic phoropters, and

fundus imaging systems. Fundus imaging systems are used by eye

doctors to image the retina, and detect degenerative eye disease.

The challenge is that image quality from present systems is poor, and the

disease can be in advanced stages before a doctor can detect it.

Improving the image quality through use of affordable MEMS adaptive

optics systems will allow surgeons to detect degenerative eye disease at

much earlier stages where it is easier to treat, and before vision is

affected.

Optical biopsy provides high-resolution, cross-sectional imaging of tissue

noninvasively.

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There are several optical biopsy techniques including optical coherence

tomography (OCT), nonlinear optical (NLO) imaging and confocal imaging.

Conventional optical imaging systems are too bulky and have slow

imaging speed and its solution is using MEMS mirrors and lenses for

miniaturization

The following images shows Endoscopic OCT imaging and Confocal

microscopic imaging which proves to be a boon for modern medical

treatements.

1. MEMS Endoscopic OCT Imaging

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2. Confocal Microscopic Imaging ---

6.Disadvantages of Optical MEMS devices

• High driving voltage (~100V), which raises safety concern

• Small rotation angle (<30°), resulting in low imaging efficiency

• Relatively small aperture size (~0.5mm), resulting in low image

resolution

• Small linear displacement (<45μm) (small imaging depth for

confocal imaging)

• Limited degree-of-freedom (DOF) ( Multiple DOF desired.)

• Fabrication of optical MEMS components (Time consuming,

Process R&D required ,Expensive startup, Complex business

relationship )

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7.Conclusion

Optical MEMS promises to revolutionize nearly every product category by

bringing together silicon-based microelectronics with micromachining

technology, and optical components thus making possible the realization

of complete systems-on-a-chip. Optical MEMS is an enabling

technology allowing the development of smart products, augmenting the

computational ability of microelectronics with the perception ,control

`capabilities and communication of micro-sensors , micro-actuators and

micro-mirrors and expanding the space of possible designs and

applications.

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8. References:

Scientific American India, January-2008 Issue

Optical fibres and fibre optic communication systems,Subir K Sarkar,

S.Chand Publications

www.sciam.co.in

www.wikipedia.co.in

www.centralchronicle.co.in

www.freepatentsonline.com

www.omems.com

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