Optical Communication From Sound to Light and Back

ApparatusApparatus

Sound Waves

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

Wave Properties ٭Amplitude ٭Frequency ٭Wavelength ٭Velocity ٭Amplitude ٭Frequency ٭Wavelength ٭Velocity

Changing Wave Properties What happens when you change the pitch of the sound? What happens when you change the pitch of the sound? Frequency is changed. How? Frequency is changed. How? What happens when you change the volume of the sound? Amplitude is changed. How?

How does sound come from sound waves? Air and other media have matter ٭Air and other media have matter ٭Matter oscillates when waves pass through ٭Sound Pressure: ٭The deviation in equilibrium pressure caused by a sound wave Air and other media have matter ٭Air and other media have matter ٭Matter oscillates when waves pass through ٭Sound Pressure: ٭The deviation in equilibrium pressure caused by a sound wave

How We Vocalize Vocal Chords are infolded membranes stretched across larynx Vocal Chords are infolded membranes stretched across larynx Chords come close together, air pressure builds, pushes them apart Chords come close together, air pressure builds, pushes them apart “Chopped” flow of air sustained “Chopped” flow of air sustained Steady oscillations create sound Steady oscillations create sound Vocal Chords are infolded membranes stretched across larynx Vocal Chords are infolded membranes stretched across larynx Chords come close together, air pressure builds, pushes them apart Chords come close together, air pressure builds, pushes them apart “Chopped” flow of air sustained “Chopped” flow of air sustained Steady oscillations create sound Steady oscillations create sound

How the Ear Works Credit:

The Light Transmitter Sound waves enter microphone ٭Sound waves enter microphone ٭Microphone is an electret ٭Contains permanently charged plate ٭Sound waves cause plate to vibrate ٭Electrical potential differences caused by vibrations ٭Voltage differences cause light in Light Emitting Diode (LED) to be modulated Sound waves enter microphone ٭Sound waves enter microphone ٭Microphone is an electret ٭Contains permanently charged plate ٭Sound waves cause plate to vibrate ٭Electrical potential differences caused by vibrations ٭Voltage differences cause light in Light Emitting Diode (LED) to be modulated

ModulationModulation

Fiber Optics Cables made of thin glass or plastic strands ٭Cables made of thin glass or plastic strands ٭Not affected by electromagnetic interference ٭Propagate light over long distances with no energy loss (Total Internal Reflection) Cables made of thin glass or plastic strands ٭Cables made of thin glass or plastic strands ٭Not affected by electromagnetic interference ٭Propagate light over long distances with no energy loss (Total Internal Reflection)

Snell’s Law Describes relationship between angles of incidence and refraction between two different media ٭Describes relationship between angles of incidence and refraction between two different media ٭Media possess a Refractive Index (n) ٭Measures how much speed of light is slowed down by the medium ٭The more light is slowed, the higher its Refractive Index ٭Speed of light in a vacuum = 3 x 10 8 m/s ٭Refractive Index: n = 1 ٭Same as in air Describes relationship between angles of incidence and refraction between two different media ٭Describes relationship between angles of incidence and refraction between two different media ٭Media possess a Refractive Index (n) ٭Measures how much speed of light is slowed down by the medium ٭The more light is slowed, the higher its Refractive Index ٭Speed of light in a vacuum = 3 x 10 8 m/s ٭Refractive Index: n = 1 ٭Same as in air

Refraction and Reflection = Angle of incidence = Angle of refraction Both angles are taken from normal

Snell’s Law

Total Internal Reflection Total Internal Reflection Total Internal Reflection Total Internal Reflection There is a special case of Snell’s Law ٭There is a special case of Snell’s Law ٭When going from high density to low density, there is a point after which all of the light is reflected ٭This point is the Critical Angle There is a special case of Snell’s Law ٭There is a special case of Snell’s Law ٭When going from high density to low density, there is a point after which all of the light is reflected ٭This point is the Critical Angle To Normal: To Longitudinal Axis:

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

Total Internal Reflection Allows optical fibers to carry light very long distances without any loss of energy ٭Allows optical fibers to carry light very long distances without any loss of energy

What’s Wrong Here??

Critical angle was not exceeded every time

CalculationCalculation ٭Find the critical angle to the longitudinal axis in a standard optical fiber. n of cladding = n of core = ٭Find the critical angle to the longitudinal axis in a standard optical fiber. n of cladding = n of core = 1.557

SolutionSolution

Receiver and Speaker Photodarlington on receiver converts light energy back into electrical signals Photodarlington on receiver converts light energy back into electrical signals Signals are amplified through circuitry Signals are amplified through circuitry Photodarlington on receiver converts light energy back into electrical signals Photodarlington on receiver converts light energy back into electrical signals Signals are amplified through circuitry Signals are amplified through circuitry

SpeakerSpeaker * Speakers have both permanent magnets and electromagnets * Electromagnet * Speakers have both permanent magnets and electromagnets * Electromagnet Composed of magnetic metal wrapped in coil of wire Current runs through wire Creates magnetic field around metal

SpeakersSpeakers Both types of magnets have polar orientation Both types of magnets have polar orientation Electromagnets can change orientation Electromagnets can change orientation How? How? Both types of magnets have polar orientation Both types of magnets have polar orientation Electromagnets can change orientation Electromagnets can change orientation How? How? By changing direction of current Alternating Current (AC)

SpeakersSpeakers Amplifying circuitry switches electrical signals Amplifying circuitry switches electrical signals Current constantly reversing Current constantly reversing Polar orientation changes many times per second Polar orientation changes many times per second Amplifying circuitry switches electrical signals Amplifying circuitry switches electrical signals Current constantly reversing Current constantly reversing Polar orientation changes many times per second Polar orientation changes many times per second

SpeakersSpeakers Changing polar orientation changes interaction with permanent magnet Changing polar orientation changes interaction with permanent magnet Electromagnet will move up and down as current alternates Electromagnet will move up and down as current alternates

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