Optical CommunicationOptical CommunicationOptical CommunicationOptical Communication

From Sound to Light and BackFrom Sound to Light and BackFrom Sound to Light and BackFrom Sound to Light and Back

ApparatusApparatusApparatusApparatus

Transmitter

Receiver

Optical Fiber

Speaker

Transmitter

Receiver

Optical Fiber

Speaker

Sound WavesSound WavesSound WavesSound Waves

Longitudinal WavesLongitudinal WavesLongitudinal WavesLongitudinal Waves

٭ Longitudinal (Compression):(Compression):٭٭ Waves parallel to direction of travelWaves parallel to direction of travel

٭ Longitudinal (Compression):(Compression):٭٭ Waves parallel to direction of travelWaves parallel to direction of travel

Credit: Credit: http://www.physicsclassroom.comhttp://www.physicsclassroom.com

Wave PropertiesWave PropertiesWave PropertiesWave Properties

٭٭ AmplitudeAmplitude

٭٭ FrequencyFrequency

٭٭ WavelengthWavelength

٭٭ VelocityVelocity

٭٭ AmplitudeAmplitude

٭٭ FrequencyFrequency

٭٭ WavelengthWavelength

٭٭ VelocityVelocity

Changing Wave PropertiesChanging Wave PropertiesChanging Wave PropertiesChanging Wave Properties

What happens when you change the pitch ofWhat happens when you change the pitch of

the sound?the sound?What happens when you change the pitch ofWhat happens when you change the pitch of

the sound?the sound?

Frequency is changed. How?Frequency is changed. How?

What happens when you change the What happens when you change the volume of the sound?volume of the sound?

Amplitude is changed. Amplitude is changed. How?How?

How does sound come How does sound come from sound waves?from sound waves?

How does sound come How does sound come from sound waves?from sound waves?

٭ Air and other media have matterAir and other media have matter

٭٭ Matter oscillates when waves pass Matter oscillates when waves pass throughthrough

٭٭ Sound Pressure: Sound Pressure: ٭٭ The deviation in equilibrium pressure caused The deviation in equilibrium pressure caused

by a sound waveby a sound wave

٭ Air and other media have matterAir and other media have matter

٭٭ Matter oscillates when waves pass Matter oscillates when waves pass throughthrough

٭٭ Sound Pressure: Sound Pressure: ٭٭ The deviation in equilibrium pressure caused The deviation in equilibrium pressure caused

by a sound waveby a sound wave

How We VocalizeHow We VocalizeHow We VocalizeHow We Vocalize

• Vocal Chords are Vocal Chords are infolded membranes infolded membranes stretched across stretched across larynxlarynx

• Chords come close Chords come close together, air together, air pressure builds, pressure builds, pushes them apartpushes them apart

• ““Chopped” flow of Chopped” flow of air sustainedair sustained

• Steady oscillations Steady oscillations create soundcreate sound

• Vocal Chords are Vocal Chords are infolded membranes infolded membranes stretched across stretched across larynxlarynx

• Chords come close Chords come close together, air together, air pressure builds, pressure builds, pushes them apartpushes them apart

• ““Chopped” flow of Chopped” flow of air sustainedair sustained

• Steady oscillations Steady oscillations create soundcreate sound

How the Ear WorksHow the Ear WorksHow the Ear WorksHow the Ear Works

Credit: http://magnatone.com/HEARIN-Credit: http://magnatone.com/HEARIN-HEALTH.htmlHEALTH.html

The Light TransmitterThe Light Transmitter The Light TransmitterThe Light Transmitter

٭ Sound waves enter microphoneSound waves enter microphone

٭٭ Microphone is an electretMicrophone is an electret٭٭ Contains permanently charged plateContains permanently charged plate

٭٭ Sound waves cause plate to vibrateSound waves cause plate to vibrate

٭٭ Electrical potential differences caused by Electrical potential differences caused by vibrationsvibrations

٭٭ Voltage differences cause light in Voltage differences cause light in Light Emitting Diode (LED) to be Light Emitting Diode (LED) to be modulatedmodulated

٭ Sound waves enter microphoneSound waves enter microphone

٭٭ Microphone is an electretMicrophone is an electret٭٭ Contains permanently charged plateContains permanently charged plate

٭٭ Sound waves cause plate to vibrateSound waves cause plate to vibrate

٭٭ Electrical potential differences caused by Electrical potential differences caused by vibrationsvibrations

٭٭ Voltage differences cause light in Voltage differences cause light in Light Emitting Diode (LED) to be Light Emitting Diode (LED) to be modulatedmodulated

ModulationModulationModulationModulation

Fiber OpticsFiber OpticsFiber OpticsFiber Optics

٭ Cables made of thin glass or plastic Cables made of thin glass or plastic strandsstrands

٭٭ Not affected by electromagnetic Not affected by electromagnetic interferenceinterference

٭٭ Propagate light over long distances Propagate light over long distances with no energy loss (Total Internal with no energy loss (Total Internal Reflection)Reflection)

٭ Cables made of thin glass or plastic Cables made of thin glass or plastic strandsstrands

٭٭ Not affected by electromagnetic Not affected by electromagnetic interferenceinterference

٭٭ Propagate light over long distances Propagate light over long distances with no energy loss (Total Internal with no energy loss (Total Internal Reflection)Reflection)

Snell’s LawSnell’s LawSnell’s LawSnell’s Law٭ Describes relationship between Describes relationship between

angles of incidence and refraction angles of incidence and refraction between two different mediabetween two different media

٭٭ Media possess a Media possess a Refractive IndexRefractive Index (n) (n)٭٭ Measures how much speed of light is Measures how much speed of light is

slowed down by the mediumslowed down by the medium The more light is slowed, the higher its The more light is slowed, the higher its٭٭Refractive IndexRefractive Index

٭٭ Speed of light in a vacuum = 3 x 10Speed of light in a vacuum = 3 x 1088 m/sm/s٭٭ Refractive Index: n = 1Refractive Index: n = 1٭٭ Same as in airSame as in air

٭ Describes relationship between Describes relationship between angles of incidence and refraction angles of incidence and refraction between two different mediabetween two different media

٭٭ Media possess a Media possess a Refractive IndexRefractive Index (n) (n)٭٭ Measures how much speed of light is Measures how much speed of light is

slowed down by the mediumslowed down by the medium The more light is slowed, the higher its The more light is slowed, the higher its٭٭Refractive IndexRefractive Index

٭٭ Speed of light in a vacuum = 3 x 10Speed of light in a vacuum = 3 x 1088 m/sm/s٭٭ Refractive Index: n = 1Refractive Index: n = 1٭٭ Same as in airSame as in air

Refraction and ReflectionRefraction and ReflectionRefraction and ReflectionRefraction and Reflection

1

1= Angle of incidence

= Angle of refraction

Both angles are Both angles are taken from normaltaken from normal

2

1

2

Snell’s LawSnell’s LawSnell’s LawSnell’s Law

sin1

sin2

n2

n1

Total Internal ReflectionTotal Internal Reflection

٭ There is a special case of Snell’s There is a special case of Snell’s LawLaw

٭٭ When going from high density to When going from high density to low density, there is a point after low density, there is a point after which all of the light is reflectedwhich all of the light is reflected

٭٭ This point is the This point is the Critical AngleCritical Angle

٭ There is a special case of Snell’s There is a special case of Snell’s LawLaw

٭٭ When going from high density to When going from high density to low density, there is a point after low density, there is a point after which all of the light is reflectedwhich all of the light is reflected

٭٭ This point is the This point is the Critical AngleCritical Angle

C sin 1 n2

n1

To Normal:To Normal: To Longitudinal Axis:To Longitudinal Axis:

C ,Longitudinal 90 C

Critical AngleCritical AngleCritical AngleCritical Angle

All of the light All of the light will be reflected will be reflected when angle of when angle of incidence is incidence is greater than the greater than the critical anglecritical angle

All of the light All of the light will be reflected will be reflected when angle of when angle of incidence is incidence is greater than the greater than the critical anglecritical angle

refraction 90

sinrefraction 1

Total Internal ReflectionTotal Internal ReflectionTotal Internal ReflectionTotal Internal Reflection

٭ Allows optical fibers to carry light Allows optical fibers to carry light very long distances without any very long distances without any loss of energyloss of energy

٭ Allows optical fibers to carry light Allows optical fibers to carry light very long distances without any very long distances without any loss of energyloss of energy

What’s Wrong Here??What’s Wrong Here??What’s Wrong Here??What’s Wrong Here??

Critical angle was not Critical angle was not exceeded every timeexceeded every timeCritical angle was not Critical angle was not exceeded every timeexceeded every time

CalculationCalculationCalculationCalculation

٭٭ Find the critical angle to the Find the critical angle to the longitudinal axis in a standard longitudinal axis in a standard optical fiber.optical fiber.

n of cladding = 1.343n of cladding = 1.343

n of core = 1.557n of core = 1.557

٭٭ Find the critical angle to the Find the critical angle to the longitudinal axis in a standard longitudinal axis in a standard optical fiber.optical fiber.

n of cladding = 1.343n of cladding = 1.343

n of core = 1.557n of core = 1.557

C ,Longitudinal 90 C

C sin 1 n2

n1

SolutionSolutionSolutionSolution

C sin 1 1.343

1.557

C 59.605C ,Longitudinal 90 50.605

C ,Longitudinal 30.395

Receiver and SpeakerReceiver and SpeakerReceiver and SpeakerReceiver and Speaker

• Photodarlington on receiver Photodarlington on receiver converts light energy back into converts light energy back into electrical signalselectrical signals

• Signals are amplified through Signals are amplified through circuitrycircuitry

• Photodarlington on receiver Photodarlington on receiver converts light energy back into converts light energy back into electrical signalselectrical signals

• Signals are amplified through Signals are amplified through circuitrycircuitry

SpeakerSpeakerSpeakerSpeaker

* Speakers have both permanent * Speakers have both permanent magnets and electromagnetsmagnets and electromagnets

* Electromagnet* Electromagnet

* Speakers have both permanent * Speakers have both permanent magnets and electromagnetsmagnets and electromagnets

* Electromagnet* ElectromagnetComposed of magnetic metal Composed of magnetic metal wrapped in coil of wirewrapped in coil of wire

Current runs through wireCurrent runs through wire

Creates magnetic field around Creates magnetic field around metalmetal

SpeakersSpeakersSpeakersSpeakers

• Both types of magnets have polar Both types of magnets have polar orientationorientation

• Electromagnets can change orientationElectromagnets can change orientation• How?How?

• Both types of magnets have polar Both types of magnets have polar orientationorientation

• Electromagnets can change orientationElectromagnets can change orientation• How?How?

By changing direction of currentBy changing direction of current

Alternating Current (AC)Alternating Current (AC)

SpeakersSpeakersSpeakersSpeakers

• Amplifying circuitry switches Amplifying circuitry switches electrical signalselectrical signals

• Current constantly reversingCurrent constantly reversing• Polar orientation changes many Polar orientation changes many

times per secondtimes per second

• Amplifying circuitry switches Amplifying circuitry switches electrical signalselectrical signals

• Current constantly reversingCurrent constantly reversing• Polar orientation changes many Polar orientation changes many

times per secondtimes per second

SpeakersSpeakersSpeakersSpeakers

• Changing polar Changing polar orientation orientation changes changes interaction with interaction with permanent permanent magnetmagnet

• Electromagnet will Electromagnet will move up and down move up and down as current as current alternatesalternates

• Changing polar Changing polar orientation orientation changes changes interaction with interaction with permanent permanent magnetmagnet

• Electromagnet will Electromagnet will move up and down move up and down as current as current alternatesalternates

SpeakersSpeakersSpeakersSpeakers

Movement of coil causes speaker cone to Movement of coil causes speaker cone to move up and down, creating longitudinal move up and down, creating longitudinal sound wavessound waves