Optical Electronics

1
Optical Electronics
Adapted from Technician's guide to Fiber optics,
3rd Ed. Sterling, D. J. (2000). Delmar Publishing
2
The Basics

• Fiber optics A means to carry information from
  one point to another.
• Optical Fiber A thin strand of glass or plastic
  that serves as the transmission medium.

3
Basic Fiber-Optic System

• Transmitter Converts electrical signals to
  light.
• Drive Circuit Signal to drive current
• Source Light source such as LED or Laser.
• Medium Fiber optic cable to carry the light.
• Receiver
• Detector Accepts light and converts to
  electrical signal.
• Output Circuit Amplifies, reshapes and
  otherwise rebuilds the signal.

4
History of Fiber-Optics

• 1790's Optical Telegraph.
• Signalmen use lights in towers to relay messages.
• Transmission speed 230km in 15 minutes
• 1870 Tyndall's Experiment.
• Principle of internal reflection of light pouring
  from a spout.
• Essential principle for today's fiber-optics.

5
History of Fiber-Optics

• 1880 Bell's Photophone
• Demonstrated modulation of light for comm.
• 1950s Image carrying fiber (fiberscope).
• 1956 Glass coated glass rod Fiber Optics

6
History of Fiber-Optics

• 1960 Ruby Laser and Helium-Neon Lasers.
• 1962 Lasing from a semiconductor.
• 1970 Fiber Optic cable with lt 20dB/km losses.
  (today's are lt .2dB/km.
• 1973 Fiber replaces wire on ships and aircraft.
• 1977 2 km, 20Mbps optical link for military.

7
History of Fiber-Optics

• 1977 - ATT and GTE install Fiber Optical for
  commercial phone traffic.
• 1983 MCI installs single-mode fiber.
• Late 1980s Fiber Distributed Data Interface
  running at 100Mbps.

8
Fiber's Advantages over copper

• Wide Bandwidth
• Low Loss
• Electromagnetic Immunity
• Light Weight
• Small Size
• Safety
• Security

9
Wide Bandwidth

• Fiber optic communications can run at10 Ghz and
  have the potential to go as high as 1 Thz
  (100,000 GHz).
• A 10 Ghz capacity can transmit (per second)
• 1000 books
• 130,000 voice channels
• 16 HTDV channels or 100 compressed HDTV channels.
• Separate Voice, data and video channels are
  transmitted on a single cable.

10
Low Loss

• Loss indicates how far the data can be sent.
• Attenuation is the loss of signal strength.
• With copper, the higher the frequency, the
  greater the loss (low pass filter effects).
• In Fiber, the loss is flat until reaching very
  high frequencies.
• Severe attenuation requires repeaters to be
  placed in the path.
• Copper requires repeaters much more frequently
  then fiber.

11
Attenuation vs. Frequency
12
Electromagnetic Immunity

• Copper cables can act as an antennae picking up
  EMI from power lines, computers, machinery and
  other sources.
• Fiber is not susceptible to Electro-Magnetic
  Interference and thus no interference allowing
  error-free transmissions.

13
Light Weight

• Comparison
• Fiber 9lb per 1000 ft. (due mainly to
  packaging).
• Coax 80lb per 1000 ft.

14
Small Size

• Use where space is at a premium
• Aircraft, submarines
• Underground conduit
• High density cable areas Computer centers.

15
Safety

• No electricity thus no spark hazards so can be
  used through hazardous areas.

16
Security

• Since fiber does not carry electricity, it emits
  no EMI which could be used for eavesdropping.
• Difficult to 'tap' cable must be cut and spiced.

17
Light

• Light is an electromagnetic wave.
• Other electromagnetic waves
• Radio Waves
• Radar
• X-Rays
• Electronic Digital Pulses
• Electromagnetic energy is radiant energy that
  travels at 300,000km/s or 186,000 miles/s.

18
Electromagnetic Wave

• Consists of a oscillating electric and magnetic
  fields at right angles to each other.
• Frequency( f ) Number of cycles/second
• Wavelength ( ? ) Distance between the same 2
  points.

19
Frequency and Wavelength

• Relationship of frequency and wavelength wavelen
  gth velocity/frequency ??/f
• In free space or air velocity is the speed of
  light.
• The higher the frequency the shorter the
  wavelength.

20
Wavelength Examples

• 60 Hz power has a wavelength of 3100 miles. That
  is, the wave will have traveled 3100 miles before
  the wave begins a new cycle.
• A 55.25 MHz signal (TV Channel 2) has a
  wavelength of 17.8 feet.
• Deep red has a frequency of 430THz and wavelength
  of 700nm (billionths of a meter).

21
Electromagnetic Spectrum

• The electromagnetic spectrum is a continuous
  spectrum of energy from subsonic to RF to
  microwaves to visible light and beyond.
• Visible light has wave lengths from 380nm (deep
  violet) to 750nm (deep red).
• Ultraviolet light has a shorter wavelength and
  infrared has a longer wavelength.
• Fiber commonly uses infrared (890nm 1500nm)
  because the fiber passes it easier.

22
Electromagnetic Spectrum
23
Waves and Particles

• Light exhibits properties of waves and particles.
• Photon A particle of light.
• Quantum A bundle of energy.
• Exists in fixed discrete units (whole values).
• Energy possessed by a photon is proportional to
  its frequency. E hfwhere h is Plank's
  constant 6.63x10-34 Joule-Seconds

24
Photons

• A photon has zero mass (unlike a marble).
• If it is not in motion, it does not exist!
• The duality of light will be used in
  understanding the principles of fiber optics.
• Property of wavelength is used in describing
  characteristic of optical fiber.
• A detector absorbs the energy of the photon to be
  converted to electricity.
• An LED operates because the electrons give up
  photons at certain energy levels, which define
  the wavelength or 'color'.

25
Reflection and Refraction

• The 'Speed of Light' is simply the velocity of an
  electromagnetic wave in a vacuum.
• Light travels slower in materials.
• As light passes from one material to another, its
  direction changes.
• Refraction is the deflection of light.
• Different wavelengths of light travel at
  different speeds in the same material.

26
Index of Refraction

• The Index of Refraction is a unit representing
  the ratio of the velocity of light in a vacuum to
  the velocity of light in a material.

27
Index of Refraction

• As the index of refraction increases, the slower
  the wave will travel and the greater it will
  'bend' when entering from a material with a lower
  index.

28
Definitions for Refraction

• Normal Imaginary line perpendicular to the
  interface between 2 materials.
• Angle of incident Angle between the incident ray
  and the normal.
• Angle of Refraction Angle between the normal and
  the refracted ray.

n1 lt n2
29
Refraction for n1gtn2

• With n1 gt n2, as the incidence angle increases,
  the refractive angle increases.
• At the critical angle, the refractive angle is 90
  degrees.
• Above the critical angle, the incident ray is
  totally reflected.

n1 gt n2
n1 gt n2
30
Reflection

• With reflection, the angle of reflection is equal
  to the angle of incidence.

n1 gt n2
31
Snell's Law

• The relationship between the incident ray and
  refracted ray isn1sin?1 n2 sin?2
• For reflection to occur, angle of incidence must
  exceed the critical angle - ?c. The critical
  angle ?2 may be found by ?c arcsin(n2/n1)

32
A Practical Example

• Assuming there are 2 layers of glass with indices
  of 1.48 (n1) and 1.42 (n2) ?c
  arcsin(1.46/1.48) 80.6?

33
Fresnel Reflections

• Even when refraction occurs and light enters a
  material, a small amount is reflected back
  Fresnel Reflection (?).
• The greater the index of refraction, the greater
  the amount of losses.
• dB 10 log(1- ?)

34
Fresnel Reflections

• Fresnel losses occur when
• Light from source enters fiber
• Between connected fibers.
• Losses are the same regardless of the order of
  materials (from air to glass or from glass to
  air).

35
Total Reflection

• With the angle of incidence greater than the
  critical angle, total reflection occurs.

36
Total Internal Reflection

• With material with indices on both sides
  (cladding), the light will be continually
  reflected and follow the core.

37
Basic Fiber Optic Construction

• Two concentric layers
• Core n 1.47 typically.
• Cladding n 1.46 typically.
• Index of refraction for cladding is less than 1
  less than the index of refraction of the core.
• Jacket is a protective polymer and has no optical
  properties.

38
Total Internal ReflectionIn an Optical Fiber
39
Typical Core and Cladding Dimensions
40
Fiber Classifications

• Glass Fiber
• Glass core and glass cladding.
• Ultra-pure
• Refractive Index is controlled by adding
  impurities.
• Other impurities scatter or absorb light.

41
Fiber Classifications

• Plastic-Clad Silica (PCS)
• Glass core and plastic cladding.
• Performance not as good as glass fiber.

42
Fiber Classifications

• Plastic Fiber
• Plastic core and cladding.
• High loss and low bandwidth
• Inexpensive
• Easy to work with

43
Modes

• Mode is a mathematical and physical concept
  describing the propagation of electromagnetic
  waves through a medium.
• Allowed solution to Maxwells Equations
• Simply a path that a light wave can follow in
  traveling down the core of a fiber.

44
Dispersion

• Spreading of the light pulse which limits
  bandwidth.
• Modal Dispersion
• Material Dispersion
• Waveguide Dispersion
• Polarization Mode Dispersion

45
Step-Index Multimode (MM) or Modal Dispersion
Pulse broadening due to multi-path transmission.
Bitrate x Distance product is severely
limited! 100/140 ?m Silica Fiber 20 Mb/s
km 0.8/1.0 mm Plastic Optical Fiber 5 Mb/s
km
46
Gradient-Index (GI) Fiber

• Doping profile designed to minimize race
  conditions(outer modes travel faster due to
  lower refractive index!)
• Most common designs 62.5/125 or 50/125 ?m, NA
  0.2
• Bitrate x Distance product 1 Gb/s km

n
1.475
1.460
r
47
Single-Mode Fiber (SMF)

• Step-Index type with very small core
• Most common design 9/125 ?m or 10/125 ?m, NA
  0.1
• Bitrate x Distance product up to 1000 Gb/s
  km(limited by CD and PMD - see next slides)

48
Attenuation In Silica Fibers

• Loss of optical power as light travels through
  the fiber.
• 300dB/km for plastic
• 0.21dB/km for single-mode silica fiber.

2.5
Optical Windows
2
3
OH Absorption
2.0
1
Attenuation (dB/km)
1.5
1.0
0.5
1100
1300
900
1500
1700
Wavelength (nm)
49
Attenuation In Silica Fibers

• Sources of Attenuation
• Scattering
• Absorption
• Scattering Imperfections cause the light to
  scatter, lose direction, and be lost.
• The longer the wavelength, the less the
  scattering.

50
Attenuation In Silica Fibers

• Absorption Impurities in the glass cause the
  light energy to be absorbed.
• High OH content in silica is main loss.
• Today's high quality fiber does not suffer as
  much loss as it did just a few years ago.

51
Microbend Losses

• Small bends or imperfections in the fiber at the
  core/cladding interface will cause loss due to
  light hitting at angles that do not promote total
  internal reflection.

52
Bend Radius

• Bends increase attenuation slightly due to
  increasing the angle of incidence.
• Bends decrease tensile strength.
• Bend radius should be greater then 5x's the cable
  diameter for unstressed cable, or 10x's for
  stressed cable.

53
Numerical Aperture (NA)

• NA is the "light-gathering ability" of a fiber.
• Only light injected at greater than the critical
  angle will be propagated.
• Forms the Acceptance cone

Acceptance / Emission Cone
?
NA sin ? n2core - n2cladding
54
Numerical Aperture (NA)

• The fewer the modes, the smaller the cone.
• Plastic NA 0.50
• Graded-Index NA 0.20
• NA typically not defined for single-mode.
• As light travels in the fiber, modes are lost
  until EMD is reached.
• Light exits with a smaller cone than was
  injected.

55
Basic Transmitter Concepts

• Accepts encoded data and modulates source
• Buffers data stream (synchronization)
• Compensates for variations in current and
  temperature
• Provides output current to drive source

Basic LED Transmitter
Basic Laser Transmitter
56
Basic Receiver Concepts

• Basic receiver consists of detector,
  preamplifier, quantizer, signal detect circuit
  and output buffers.

Amplifies weak signal from detector and converts
current to voltage
Compares quantizer output to reference signal to
determine if there is sufficient S/N ratio
Converts voltage to logic states (bit stream)
Converts output to proper level (line driver)
57
Transceivers and Repeaters

• Transceiver a transmitter and receiver packaged
  together to allow both transmission and reception
  from either station.
• A regenerative repeater is a receiver driving a
  transmitter used to boost signals over long
  hauls.
• Digital regenerative repeaters not only amplify
  the signal, they reshape the pulses to their
  original form as well
• Optical amplifiers are better because no
  conversion (optical to electrical) in needed