The document summarizes key aspects of optical fiber communication including: 1) It describes the advantages of optical fiber communication over copper wire communication such as smaller size, lower transmission loss, higher bandwidth, and immunity to electromagnetic interference. 2) The basic components of an optical fiber are described including the core, cladding, buffer, and jacket. Total internal reflection is explained as the mechanism that guides light through the fiber. 3) Different types of optical fibers are discussed including plastic optical fiber, single-mode fiber, multimode step-index fiber, and multimode graded-index fiber.

1. FACULTY DEVELOPMENT
PROGRAMME
TODAYS TOPIC
OPTICAL FIBER COMMUNICATION
BY
Mr.Rajkumar D Bhure
Assoc.Prof.,ECE Dept. JBIET

2. DEPT OF ELECTRONICS & COMMUNICATION ENGINEERING,JBIET
Optical Fiber Communication
By:- Mr. Rajkumar D
Bhure
Assoc.Prof. ECE Dept.

3. Spectrum for communication

4. Attenuation Vs Wavelength

5. Fiber vs. Copper
 Optical fiber transmits light pulses
 Can be used for analog or digital transmission
 Voice, computer data, video, etc.
 Copper wires (or other metals) can carry the same
types of signals with electrical pulses

6. Advantages of
OFC over conventional Copper Wire Communication
 Small Size and light Weight
 Abundant Raw Material Availability
 Higher Band Width Low Noise
 Low transmission loss and high signal security
 Highly Reliable and easy of maintainance
 Fibers Are Electrically Isolation
 Highly transparent at particularo Wave Length
 No possiblityof ISI and echoes cross talketc.
 Immunity to natural hazardous.

7. Index of Refraction(n)
 When light enters a dense medium like glass
or water, it slows down
 The index of refraction (n) is the ratio of the
speed of light in vacuum to the speed of light
in the medium n=c/v
 Water has n = 1.3
 Light takes 30% longer to travel through it
 Fiber optic glass has n = 1.5
 Light takes 50% longer to travel through it

8. Material Used
 Basic materials are plastic and Sand(Sio2)
 The Dopents used increase or decrease RI
are:-
 GeO2 and P2O5--- To increase RI
 B2O3------ To Decrease RI

9. Elements of
Optical communication System

10. Optical Fiber
 Core
 Glass or plastic with a higher index of
refraction than the cladding
 Carries the signal
 Cladding
 Glass or plastic with a lower index of
refraction than the core
 Buffer
 Protects the fiber from damage and
moisture
 Jacket
 Holds one or more fibers in a cable

11. Plastic Optical Fiber
 Large core (1 mm) step-index multimode fiber
 Easy to cut and work with, but high attenuation (1
dB / meter) makes it useless for long distances

12. Singlemode Fiber
 Singlemode fiber has a core diameter of 8 to 9
microns, which only allows one light path or mode
 Images from arcelect.com (Link Ch 2a)
Index of
refraction

13. Multimode Step-Index Fiber
 Multimode fiber has a core diameter of 50 or 62.5
microns (sometimes even larger)
 Allows several light paths or modes
 This causes modal dispersion – some modes take
longer to pass through the fiber than others because
they travel a longer distance
 See animation at link Ch 2f
Index of
refraction

14. Step-index Multimode
 Large core size, so source power can be
efficiently coupled to the fiber
 High attenuation (4-6 dB / km)
 Low bandwidth (50 MHz-km)
 Used in short, low-speed datalinks
 Also useful in high-radiation
environments, because it can be made with pure
silica core

15. Singlemode FIber
 Best for high speeds and long distances
 Used by telephone companies and CATV

16. Multimode Graded-Index Fiber
 The index of refraction gradually changes across the
core
 Modes that travel further also move faster
 This reduces modal dispersion so the bandwidth is
greatly increased
Index of
refraction

17. Step-index and Graded-index
 Step index multimode was developed first, but
rare today because it has a low bandwidth (50
MHz-km)
 It has been replaced by graded-index multimode
with a bandwidth up to 2 GHz-km

18. Types of Optical Fibers

19. Total Internal Reflection
Snells Law :
n1 sin1  n2 sin2
Re flection Condition
1  3
When n1  n2 and as 1 increases eventually 2
goes to 90 deg rees and
n
n1 sinc  n2 or sinc  2
n1
c is called the Critical angle
For 1  c there is no propagating refracted ray

20. Light Ray confinement
nSinθ0=n1Sinθ; SinΦ=n2/n1;
NA= n1(2Δ)1/2 ; ΔT=L Δ n12/cn2

21. Acceptance Angle
The acceptance angle (i) is the
largest incident angle ray that
can be coupled into a guided ray
within the fiber
The Numerical Aperture (NA) is
the sin(i) this is defined
analogously to that for a lens
1 1 1
NA = (n12 - n2 2 2
) = 2 2
(2D n ) = n(2D ) 2
n1 - n2 n + n2
Where D º and n º 1 f# º
f
=
f
n 2 D FullAccep tan ceAngle
1
=
2 ×NA

22. Sources and Wavelengths
 Multimode fiber is used with
 LED sources at wavelengths of 850 and 1300 nm
for slower local area networks
 Lasers at 850 and 1310 nm for networks running at
gigabits per second or more

23. Sources and Wavelengths
 Singlemode fiber is used with
 Laser sources at 1300 and 1550 nm
 Bandwidth is extremely high, around 100 THz-km

24. Fiber Optic Specifications
 Attenuation
 Loss of signal, measured in dB
 Dispersion
 Blurring of a signal, affects bandwidth
 Bandwidth
 The number of bits per second that can be sent
through a data link
 Numerical Aperture
 Measures the largest angle of light that can be
accepted into the core

25. Attenuation and Dispersion

26. Measuring Bandwidth
 The bandwidth-distance product in units of
MHz×km shows how fast data can be sent
through a cable
 A common multimode fiber with bandwidth-
distance product of 500 MHz×km could
carry
 A 500 MHz signal for 1 km, or
 A 1000 MHz signal for 0.5 km
 From Wikipedia

27. Numerical Aperture
 If the core and cladding have almost the same
index of refraction, the numerical aperture will be
small
 This means that light must be shooting right
down the center of the fiber to stay in the core

28. Fiber Manufacture

29. Three Methods
 Modified Chemical Vapor Deposition (MCVD)
 Outside Vapor Deposition (OVD)
 Vapor Axial Deposition (VAD)

30. Modified Chemical Vapor Deposition
(MCVD)
 A hollow, rotating glass tube is
heated with a torch
 Chemicals inside the tube
precipitate to form soot
 Rod is collapsed to crate a
preform
 Preform is stretched in a
drawing tower to form a single
fiber up to 10 km long

31. Modified Chemical Vapor Deposition
(MCVD)

32. Outside Vapor Deposition (OVD)
 A mandrel is coated with a porous preform in a
furnace
 Then the mandrel is removed and the preform is
collapsed in a process called sintering
 Image from csrg.ch.pw.edu.pl

33. Vapor Axial Deposition (VAD)
 Preform is fabricated
continuously
 When the preform is long
enough, it goes directly to
the drawing tower
 Image from csrg.ch.pw.edu.pl

34. Drawing Apparatus
 The fiber is drawn from the preform
and then coated with a protective
coating

35. Fiber Performance

36. Attenuation
 Modern fiber material is very pure, but there is still some
attenuation
 The wavelengths used are chosen to avoid absorption
bands
 850 nm, 1300 nm, and 1550 nm
 Plastic fiber uses 660 nm LEDs
 Image from iec.org (Link Ch 2n)

37. Optical Loss in dB (decibels)
Power In Power Out
Data Link
 If the data link is perfect, and loses no power
 The loss is 0 dB
 If the data link loses 50% of the power
 The loss is 3 dB, or a change of – 3 dB
 If the data link loses 90% of the power
 The loss is 10 dB, or a change of – 10 dB
 If the data link loses 99% of the power
 The loss is 20 dB, or a change of – 20 dB
dB = 10 log (Power Out / Power In)

38. Absolute Power in dBm
 The power of a light is measured in milliwatts
 For convenience, we use the dBm
units, where
-20 dBm = 0.01 milliwatt
-10 dBm = 0.1 milliwatt
0 dBm = 1 milliwatt
10 dBm = 10 milliwatts
20 dBm = 100 milliwatts

39. Three Types of Dispersion
 Dispersion is the spreading out of a light pulse as
it travels through the fiber
 Three types:
 Modal Dispersion
 Chromatic Dispersion
 Polarization Mode Dispersion (PMD)

40. Modal Dispersion
 Modal Dispersion
 Spreading of a pulse because different modes
(paths) through the fiber take different times
 Only happens in multimode fiber
 Reduced, but not eliminated, with graded-index
fiber

41. Chromatic Dispersion
 Different wavelengths travel at different speeds
through the fiber
 This spreads a pulse in an effect named chromatic
dispersion(group Delay)
T=1/vg =1/L *dВ/dώ
 Chromatic dispersion occurs in both singlemode and
multimode fiber
 Larger effect with LEDs than with lasers
 A far smaller effect than modal dispersion

42. Modal Distribution
 In graded-index fiber, the off-axis modes go a
longer distance than the axial mode, but they
travel faster, compensating for dispersion
 But because the off-axis modes travel further, they
suffer more attenuation

43. Equilibrium Modal Distribution
 A long fiber that has lost the high-order modes is
said to have an equilibrium modal distribution
 For testing fibers, devices that can be used to
condition the modal distribution so that
measurements will be accurate

44. Mode Stripper
 An index-matching substance is put on the
outside of the fiber to remove light travelling
through the cladding

45. Mode Scrambler
 Mode scramblers mix light to excite every
possible mode of transmission within the fiber
 Used for accurate measurements of attenuation
 Figure from
fiber-optics.info
(Link Ch 2o)

46. Semiconductor Optical Sources

47. Source Characteristics
 Important Parameters
 Electrical-optical conversion efficiency
 Optical power
 Wavelength
 Wavelength distribution (called linewidth)
 Cost
 Semiconductor lasers
 Compact
 Good electrical-optical conversion efficiency
 Low voltages
 Los cost

48. Semiconductor Optoelectronics
 Two energy bands
 Conduction band (CB)
 Valence band (VB)
 Fundamental processes
 Absorbed photon creates an electron-hole pair
 Recombination of an electron and hole can emit a photon
 Types of photon emission
 Spontaneous emission
 Random recombination of an electron-hole pair
 Dominant emission for light emitting diodes (LED)
 Stimulated emission
 A photon excites another electron and hole to recombine
 Emitted photon has similar wavelength, direction, and phase
 Dominant emission for laser diodes

49. Basic Light Emission Processes
 Pumping (creating more electron-hole pairs)
 Electrically create electron-hole pairs
 Optically create electron-hole pairs
 Emission (recombination of electron-hole pairs)
 Spontaneous emission
 Simulated emission

50. Semiconductor Material
 Semiconductor crystal is required
 Type IV elements on Periodic Table
 Silicon
 Germanium
 Combination of III-V materials
 GaAs
 InP
 AlAs
 GaP
 InAs
…

51. Direct and Indirect Materials
 Relationship between energy and momentum for electrons and holes
 Depends on the material
 Electrons in the CB combine with holes in the VB
 Photons have no momentum
 Photon emission requires no momentum change
 CB minimum needs to be directly over the VB maximum
 Direct band gap transition required
 Only specific materials have a direct bandgap

52. Light Emission
 The emission wavelength depends on the energy band gap
Eg  E2  E1
ch 1.24
 
Eg E
 Semiconductor compounds ghave different
 Energy band gaps
 Atomic spacing (called lattice constants)
 Combine semiconductor compounds
 Adjust the bandgap
 Lattice constants (atomic spacing) must be matched
 Compound must be matched to a substrate
 Usually GaAs or InP

53. Common Semiconductor
Compounds
 GaAs and AlAs have the same lattice constants
 These compounds are used to grow a ternary compound that
is lattice matched to a GaAs substrate (Al1-xGaxAs)
 0.87 <  < 0.63 (mm)
 Quaternary compound GaxIn1-xAsyP1-y is lattice
matched to InP if y=2.2x
 1.0 <  < 1.65 (mm)
 Optical telecommunication laser compounds
 In0.72Ga0.28As0.62P0.38 (=1300nm)
 In0.58Ga0.42As0.9P0.1 (=1550nm)

54. Optical Sources
 Two main types of optical sources
 Light emitting diode (LED)
 Large wavelength content
 Incoherent
 Limited directionality
 Laser diode (LD)
 Small wavelength content
 Highly coherent
 Directional

55. Avoiding
losses in LED
Carrier Photon
confinement Confinement
Band-gap and refractive index engineering.
Heterostructured LED

56. Double Heterojunction LED
(important)
Fiber
Optics
Epoxy
 Double
Metal contact
heterostructure
n AlGaAs
p GaAs (active region)
 Burrus type
p Al GaAs LED
n+ GaAs
Metal contact
 Shown
bonded to a fiber
with index-
matching epoxy.

57. Double Heterostructure
 The double heterostructure is invariably used for
optical sources for communication as seen in the
figure in the pervious slide.
 Heterostucture can be used to increase:
 Efficiency by carrier confinement (band gap engineering)
 Efficiency by photon confinement (refractive index)
 The double heterostructure enables the source
radiation to be much better defined, but further, the
optical power generated per unit volume is much
greater as well. If the central layer of a double
heterostructure, the narrow band-gap region is made
no more than 1mm wide.

58. Photon confinement -
Reabsorption problem
Source of electrons
Active region (micron in thickness)
Source of holes
Active region (thin layer of GaAs) has smaller band gap, energy of photons
emitted is smaller then the band gap of the P and N-GaAlAs hence could not
be reabsorbed.

59. Carrier confinement
electrons
holes
n+-AlGaAs p-GaAs p+-AlGaAs
Simplified band diagram of the ‘sandwich’ top show carrier confinement

60. Light Emitting Diodes (LED)
 Spontaneous emission
dominates
 Random photon emission
 Spatial implications of random
emission
 Broad far field emission pattern
 Dome used to extract more of the
light
 Critical angle is between
semiconductor and plastic
 Angle between plastic and air is
near normal
 Spectral implications of random
emission
 Broad spectrum   1.452 kT
p

61. Laser Diode
 Stimulated emission dominates
 Narrower spectrum
 More directional
 Requires high optical power density in the gain region
 High photon flux attained by creating an optical cavity
 Optical Feedback: Part of the optical power is reflected back into the cavity
 End mirrors
 Lasing requires net positive gain
 Gain > Loss
 Cavity gain
 Depends on external pumping
 Applying current to a semiconductor pn junction
 Cavity loss
 Material absorption
 Scatter
 End face reflectivity

62. Laser Diode
 Stimulated emission dominates
 Narrower spectrum
 More directional
 Requires high optical power density in the gain region
 High photon flux attained by creating an optical cavity
 Optical Feedback: Part of the optical power is reflected back into the cavity
 End mirrors
 Lasing requires net positive gain
 Gain > Loss
 Cavity gain
 Depends on external pumping
 Applying current to a semiconductor pn junction
 Cavity loss
 Material absorption
 Scatter
 End face reflectivity

63. Optical Feedback
 Easiest method: cleaved end faces
 End faces must be parallel
 Uses Fresnel reflection
 n 1 
2
R 
 n 1
 For GaAs (n=3.6) R=0.32
 Lasing condition requires the net cavity gain to be one
R1 R2 expg  a  L  1
 g: distributed medium gain
 a: distributed loss
 R1 and R2 are the end facet reflectivity's

64. Phase Condition
 The waves must add in phase as given by
2 L  z  2 m
 Resulting in modes given by
2Ln

m
 Where m is an integer and n is the refractive index of the cavity

65. Longitudinal Modes

66. Longitudinal Modes
 The optical cavity excites various longitudinal
modes
 Modes with gain above the cavity loss have the
potential to lase
 Gain distribution depends on the spontaneous
emission band
 Wavelength width of the individual longitudinal

67. Mode Separation
 Wavelength of the various modes
2Ln

m
1 1 
  m  m1  2 L n   
m m  1
 The wavelength separationof the modes is
2 Ln 2
 A longer cavity   2 

m 2 Ln
 Increases the number of modes
 Decrease the threshold gain
 There is a trade-off with the length of the laser cavity