[PDF] - Optical Recording and Optical Recording and That audio or video is PDF Document

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Optical Recording and Communications 1

Optical Recording and Optical Recording and Communications Communications

Optical Recording and Communications 2

Observations about Optical Observations about Optical Recording and Communications Recording and Communications

Optical disks can store lots of audio or video

That audio or video is of the highest quality

Optical disks continue to play perfectly for years

Playback of optical disks involves lasers

Lasers and fibers are used in communication

Optical Recording and Communications 3

5 Questions about Optical Recording 5 Questions about Optical Recording and Communication and Communication

How is information represented digitally?

How is information recorded on an optical disk?

How is information read from an optical disk?

How can light carry information long distances?

Why does light follow an optical fiber’s bends?

Optical Recording and Communications 4

Question 1 Question 1

How is information represented digitally?

Optical Recording and Communications 5

Review of Digital Representation Review of Digital Representation

Audio or video info is a sequence of numbers

Audio or video info is a sequence of numbers

Each number can be represented digitally

Each number can be represented digitally

by putting specific symbols in a set of digits.

Digital representations often involve binary digits

Digital representations often involve binary digits,

which can each hold only two symbols:

which can each hold only two symbols: 0 0 and and 1 1. .

Each symbol is a discrete value of a physical quantity

Digital representations have

good noise

good noise-

immunity

immunity

and permit error correction (elimination of noise).

Optical Recording and Communications 6

Digital Audio Digital Audio

The air pressure in sound

The air pressure in sound is measured thousands of is measured thousands of times per second times per second

Each meas rement is

Each measurement is

Each measurement is represented digitally using represented digitally using about 16 binary digits, about 16 binary digits, each a each a 0 0 or a

r a 1

1. .

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Optical Recording and Communications 7

Question 2 Question 2

How is information recorded on an optical disk?

Optical Recording and Communications 8

Structure of CDs and DVDs Structure of CDs and DVDs

These disks have spiral tracks in reflective layers

Each track contains pits and flats

Digital symbols consist of the

Digital symbols consist of the l h f h i d fl l h f h i d fl lengths of the pits and flats lengths of the pits and flats

The track structure is made as

The track structure is made as small as can be detected by the small as can be detected by the playback system, so as to maximize playback system, so as to maximize the density of information. the density of information.

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Question 3 Question 3

How is information read from an optical disk?

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Playback Techniques Playback Techniques

Laser light is focused on the shiny layer

Reflection is weaker from a pit than from a flat

Reflected light is directed to photodiodes

Reflected light intensity

Reflected light intensity indicates pits or flats indicates pits or flats

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Playback Issues Playback Issues

Light must hit pits perfectly

Feedback optimizes the position of the light spot

Light must hit only one pit

U l li h U l li h i l i l

Use laser light

Use laser light—a single wave a single wave

Use a converging lens to focus that wave to a spot

The wave forms a “waist”

The wave forms a “waist”— —its minimum width its minimum width

Wave limits on focusing are known as diffraction

Waist can’t be much smaller than a wavelength

so pit size can’t be much smaller than a wavelength.

Optical Recording and Communications 12

Advantages of Digital Recording Advantages of Digital Recording

Freedom from noise and media damage issues

Digital representation avoids information loss

Error correction ensures clean information

S rf

nt min ti n d n’t m tt r (m h) S rf nt min ti n d n’t m tt r (m h)

Surface contamination doesn’t matter (much)

High information density

Data compression is possible

Perfect, loss

Perfect, loss-

less copies are possible

less copies are possible

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Optical Recording and Communications 13

Question 4 Question 4

How can light carry information long distances?

Optical Recording and Communications 14

Optical Communication Optical Communication

Both analog and digital representations possible

Analog representation

ften involves AM modulation of the light
ften involves AM modulation of the light

d i f d f i i d i f d f i i

and is often used for remote process monitoring.

Digital representation

ften uses pulses with discrete amplitudes as symbols
ften uses pulses with discrete amplitudes as symbols

and provides noise

and provides noise-

immunity, error correction,

immunity, error correction, compression, and channel compression, and channel-

sharing.

sharing.

Optical Recording and Communications 15

Transmission Techniques Transmission Techniques

Light in air: direct line

Light in air: direct line-

of
f-
sight

sight

Infrared remote controls

Infrared computer links

Li h i i l fib bi h Li h i i l fib bi h

Light in optical fibers: arbitrary paths

Optical cables and networks

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The Components of Optical The Components of Optical Communications Communications

Transmitters

Incandescent lamps (poor performance)

Light Emitting Diodes (adequate performance)

Laser Diodes (high performance)

Receivers

Photoresistive cells (poor performance)

Photodiodes (high performance)

Conduits

Optical Fibers (ranging from poor to high performance)

Optical Recording and Communications 17

Question 5 Question 5

Why does light follow an optical fiber’s bends?

Optical Recording and Communications 18

Total Internal Reflection Total Internal Reflection

As light enters a material with a lower index of

As light enters a material with a lower index of refraction, it bends away from the perpendicular refraction, it bends away from the perpendicular

If that bend exceeds 90

If that bend exceeds 90° °, , the light reflects instead the light reflects instead the light reflects instead the light reflects instead

That reflection is perfect:

That reflection is perfect: total internal reflection total internal reflection

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Optical Recording and Communications 19

Introductory Question Introductory Question (revisited)

(revisited)

When you submerge a digital watch in water

When you submerge a digital watch in water and tilt it just right, the watch’s face appears to and tilt it just right, the watch’s face appears to be a perfect mirror. This mirror reflection is be a perfect mirror. This mirror reflection is from from from from

A. A.

the outer (front) surface of the watch face the outer (front) surface of the watch face

B. B.

the inner (back) surface of the watch face the inner (back) surface of the watch face

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Optical Fibers Optical Fibers

An optical fiber consists of a high

An optical fiber consists of a high-

index glass

index glass core in a low core in a low-

index glass sheath

index glass sheath

When light tries to leave the core at a shallow

When light tries to leave the core at a shallow angle it experiences total internal reflection angle it experiences total internal reflection angle, it experiences total internal reflection angle, it experiences total internal reflection

Light bounces endlessly through the core and

Light bounces endlessly through the core and emerges from the end of the fiber emerges from the end of the fiber

If the glass is pure and perfect enough, the light

If the glass is pure and perfect enough, the light may travel for many kilometers through the fiber may travel for many kilometers through the fiber

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Communication Issues Communication Issues

Information is sent as a stream of digital symbols

Information is sent as a stream of digital symbols

Those symbols are usually pulses of light

If pulses spread out and overlap, information is lost

T k l f di i i T k l f di i i

To keep pulses from spreading in time

all the light must follow a single path through fiber

all frequencies of light must travel at the same speed

The fiber’s structure and materials are critical

To limit possible paths, use a “single mode” fiber

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Optical Fiber Types Optical Fiber Types

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Communication Issues Communication Issues

To limit frequency

To limit frequency-

related spreading of pulses

related spreading of pulses

minimize dispersion with monochromatic laser light

in low

in low-

dispersion glass at its optimal wavelength.

dispersion glass at its optimal wavelength.

Si li h i h h fib Si li h i h h fib

Since light attenuates as it passes through fiber

use low

use low-

loss glass

loss glass

and amplify the light periodically,

using fiber laser amplifiers.

Systems using different colors can share a fiber!

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Summary about Optical Recording Summary about Optical Recording and Communication and Communication

Optical disks store information as pits and flats

Focused laser light reads that information

Digital representations allow perfect playback

Optical fibers carry information as light