Lecture 19: Optical MEMS - UVic.ca › ~mech466 › MECH466-Lecture-19.pdf · - Optical MEMS can be sealed in packages with transparent housings, which allows light to pass through

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MECH 466Microelectromechanical Systems

University of VictoriaDept. of Mechanical Engineering

Lecture 19:Optical MEMS

© N. Dechev, University of Victoria

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Overview of Optical MEMS

Passive MEMS optical components

Active MEMS optical components

Overview


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Micro-optical systems presently have application in the following areas:

Optical Scanning- Medicine (minimally invasive surgery)- Small Space Inspection

Optical Alignment

Optical Communication- Switching- Optical Filtering- Spectrometry (Wavelength Separation)-Adaptive Optics and Free Space Communication

Optical MEMS Applications


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Easy to manipulate light- Optical systems involve photon re-direction, therefore, since

photons have very little momentum, micro-actuators are easily able to manipulate photons.

Simplified packaging- Optical MEMS can be sealed in packages with transparent

housings, which allows light to pass through a glass window. - This allows the optical device to be protected from dust and harsh

environmental conditions, unlike flow sensors, or micro tactile sensors, or other MEMS where the source of signal/phenomena must be in direct contact with the chip surface.

Device size- Able to pack a large number of devices into a relatively small

area/volume.

Advantages of Optical MEMS


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The DLP micro-optical projection system was developed by Texas Instruments.

It uses a matrix of micro-mirrors to selectively switch reflected light, to form a projected matrix of pixels.

Example Application:DLP (Digital Light Processing)


Individual Digital Micro Mirror [Texas Instruments] Constituent Parts of the Digital Micro Mirror [Texas Instruments]

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DLP Principle of Operation

Each mirror is 11 x 11 um in size, and a matrix may consist of 1024 x 768 mirrors, or more.

Matrix of Micro-mirrors [Texas Instruments] Individual Micro-mirror [Texas Instruments]


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By selectively switching the mirrors on and off in the presence of a light source, the mirrors will reflect the light source ‘toward’ or ‘away’ the optical path that forms the projected image.

Redirection of Light using Mirrors[Texas Instruments]



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A good source of detail information for DLP technology is: http://www.dlp.com

Duty Cycling of Mirrors to Create Shades of Grey[Texas Instruments]



DMD Matrix with Ant Leg [Texas Instr.]

DMD Matrix with Salt Crystals [Texas Instr.]

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To create the complete projection system, the DMD (Digital Micro-mirror Device) is used with a light source, optics, a color filter and a projection lens.

Complete DLP Projection System


Constituent Parts of the Complete DLP Projection System [Texas Instruments]

DMD Chip in Package with Transparent Cover [Texas Instruments]

A large area of MEMS research is in the area of optical communication.

To understand MEMS optical systems, we must become familiar with a number of sub-systems that are used within a typical optical communication system. These are:

- Optical Fibers- Lasers and laser diodes- Optical receivers (photodetector)- Focusing lenses- Diffraction lenses and gratings- Mirrors

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Optical Communication Systems


Fiber optic cables are an important part of most optical systems.

They carry digital information from one point to another, in the form of radiation (infrared to visible).

Below is a diagram of a typical fiber optic strand. Only the silica glass ‘core’ will carry the light signal, which is typically 100 um in diameter.

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Overview of Fiber Optic Cables


Fiber Optic Strand [Chang Liu]

Bulk micromachining of silicon can be used to create fiber optical alignment fixture systems.

When two fiber optic strands must be coupled together, a precise alignment is required between the two ends. This ensures low signal loss between the two fibers.

Bulk micromachining of silicon to form ‘V-groves’ in the substrate, can be used to precisely align fibers end-to-end.

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Passive Optical Alignment


Fibers positioned in V-groove[www.waveoptics.com]

SLSP profiles are micromachined into a silicon substrate.

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Fabrication by Anisotropic Etching


Fibers Positioned in V-groove[Chang Liu]

SEM Cross-Section of Bulk Micromachined Silicon.[Chang Liu]

Laser (Light Amplification by Stimulated Emission of Radiation) light is the most common light source used with fiber optic communication systems.

Edge emitting lasers diodes are formed by a junction of p-type and n-type semiconductors with a layer of Ca between the two. The beam emanates from a highly polished edge.

The ‘Divergence’ (beam spreading)of laser diodes tends to be large.

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Laser Diode

© N. Dechev, University of VictoriaIllustration of Quantum Well Laser Diode [Chang Liu]

Lenses refract light for various purposes in optical systems.

An ideal lens shape is shown in the Figure below in (a).

The ideal lens is difficult to fabricate directly using conventional micromachining, so a number of variations are shown below, (b-e):

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Micro Lenses


Binary Lens

Fresnel Lens

Equivalent lenses made in different ways [Chang Liu]

The principle of operation of a Binary lens is as follows:

Light passing through the ‘bulk material’ essentially moves in a straight line, therefore, if the ‘bulk’ is removed, the operation is essentially the same.

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Binary Lenses


Hand-Held Binary Lense [Image from Chang Liu]

Lighthouse Binary Lens

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Example of a Surface-Micromachined Fresnel Lens


Fresnel Lens fabricated using surface micromachining, and assembledby ‘folding it out of plane’ from the substrate. [D. Scharf]

An interesting ‘research example’ of a complete MEMS optical system:

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Optical Communication Systems


MEMS Optical System [D. Scharf]

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Fabrication of Micro-lenses with ‘Polymer Reflow’


Fabrication of ‘Curvey Lenses’ [Image from Chang Liu] Fabrication of ‘Curvey Lenses’ using ‘reflown’ polymers[Image from Chang Liu]

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Other Lens Technology


Variable-focus micro-lens using an ‘expandable’ polymer membrane.

As the ‘chamber’ is pressurized, it fills with fluid and expands the membrane. The different expansion volumes have different radius of curvature, and therefore different optical NA (numerical aperture).

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Fiber Optic Attenuator and Switch


Colorized SEM image of ‘on-off’ fiber optic switch, [David Bishop, Bell Labs, Lucent Technologies]

Colorized SEM image of 1x2 fiber optic switch, [David Bishop, Bell Labs, Lucent Technologies]

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Micro-Mirrors


Micro-Mirrors are a critical part of micro-optical systems, as they facilitate the re-direction of light beams.

A ‘Good’ micro-mirror has a number of important characteristics, such as:- High reflectivity (low loss of light signal)- High flatness (low loss of light signal)- High stiffness (stays flat during acceleration and deceleration)- Low mass (allows rapid acceleration and deceleration)

Micro-mirrors are used in:- DLP systems (Digital Light Projection systems)- Optical Switching (fiber optic communication systems)- Scanning (minimally invasive tools with optical scan tips)

Electrostatic Based Micro-Mirror Actuators:

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Micro-actuators for Micro-Mirror Systems


Deformable Reflective Mirror MembraneActuated using parallel plat electrostatic actuators [Image from Chang Liu]

Translating Micro-Mirror Actuated using Electrostatic Comb-Drives[Image from Chang Liu]

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Communication via Cross-Connect Switching


Consider the idea of traditional telephone communication.

For any one person to contact another person on the telephone network, an electronic link must be established.

Traditionally, this was done with an operator at a telephone ‘switchboard’.

Switchboard Operator [Image from history-grand-forks]

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Once a communication line was established, it could last for a few seconds, to a few hours, depending on the communication.

Ultimately, telephone operators were replaced with electronic switchboards in the mid-late 60’s.

This technology lasted for a while, but was again replaced by ‘fibre-optic’ technology, in the early 80’s that could carry even more information than could ‘metal electrical cables’.

This has led to a ‘fundamental’ switching problem that remains to this day, although tremendous industrial R&D is devoted to this subject.

1962 Stromberg Carlson switch with 400 lines[www.manawatelephone.com]

The basic problem is shown below:

When the light enters the ‘cross-connect’, it must be converted to an electrical signal, then electrically switched, and then converted again to an optical signal.

This process ‘takes time’ and occurs for ‘each bit’ of digital information. Therefore, it places a limit on the amount of information that can be sent/per unit time.

Transistor BasedElectronic

Cross-Connect

Light Fibre Light FibrePhoto-transistor Laser-Diode

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1XN Micro-Optical Cross-Connect Switch


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NXN Micro-Optical Cross-Connect Switch


Incomming Fibre Optic

Light Guides

Outgoing Fibre Optic

Light Guides

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Micro-Optical Cross-Connect Switches


Lucent Technologies Micro-Mirror for Cross-Connect

Two-Axis (pitch and yaw) rotatable micro-mirror for optical switching [Image from Chang Liu] An array of Two-Axis mirrors

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Optical MEMS Cross-Connect Switching


Optical mirrors in research

Dual 3D-Micromirror/motor Assemblies [M. Basha, N. Dechev]

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Assembly of Optical MEMS Cross-Connect


[M. Basha, N. Dechev]

Substrate

Motor Rotor

SupportPost

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Substrate

Motor Rotor

Mirror-Micro-Part

SupportPost

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Optical MEMS Cross-Connect


400 µm diameter Electrostatic Micromotor [M. Basha]

Free Space Optics (FSO) makes use of medium power laser beams to transmit information through space (instead of transmitting it through fibre optic cables.

The motivation to create FSO systems involves a concept commonly known as “The Last Mile” in communication networks.

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Free Space Optics for Communication


Illustration of world wide internet network[Broadband Access Network]

Local Internet Network[HowStuffWorks]

The last mile refers more generally to the last few miles of cabling required reach the home or office of an individual user on a network.

This ‘hard wired’ infrastructure represents the ‘highest cost vs. return on investment’ of the communication network.

Installing the last few miles of cable may require running overhead lines, or worse, digging up streets etc...

Imagine the costs of installing the last few miles to one building in downtown Manhattan, or downtown Los Angeles!

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Free Space Optics for Communication


The FSO systems consists of a transmitting laser (with focal system) and a receiving laser.

The typical setup and components are shown below:

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Free Space Optics System


Images from Lezer wireless [www.lasernetworks.eu]

The focal system usually consists of a MEMS-based deformable mirror element.

The deformable mirror is able to continuously re-focus the laser signal, to minimize signal loss.

The re-focusing is necessary for two reasons:

(a) to account for atmospheric disturbances due to heating

(b) to account for building sway due to wind or other disturbances.

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Free Space Optics System


Deformable Reflective Mirror MembraneActuated using parallel plat electrostatic actuators [Image from Chang Liu]

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Lecture 19: Optical MEMS - UVic.ca › ~mech466 › MECH466-Lecture-19.pdf · - Optical MEMS can be sealed in packages with transparent housings, which allows light to pass through