Fiber optics use total internal reflection to transmit light signals for communication. They convert electrical signals to optical signals using a transmitter, transmit the light through the optical fiber, then convert it back to an electrical signal using a receiver. Fiber optics have huge bandwidth potential and are immune to electromagnetic interference but were initially more expensive than electrical cables. Key advantages include bandwidths over 250Gbps, low signal loss, and resistance to corrosion.

1. Module 5: Digital Techniques and
Electronic Instrument Systems
5.10 Fiber Optics

2. Introduction
 Fiber optics are used to create data links.
 A fiber optic data link performs 3 functions:
 Electric signal is converted to an optical signal.
 The optimal signal is send over an optical fiber.
 The optical signal is converted again to electrical.
 The fiber optic data link is
consist of 3 parts:
 Transmitter (e.g. LED)
 Optical Fiber
 Receiver (e.g. Photodiode)

3. Advantages and Disadvantages of Fiber
Optics
 Advantages:
 Huge bandwidth:
 Ethernet cable: 1Gbps
 Fiber optics: 250Gbps
 Immunity to electrical noise
 No crosstalk
 Reduced size and weight cables
 Resistance to corrosion and temperature variations.
 Disadvantages:
 Expensive in comparison with conventional electrical
cables.
 Expensive and difficult installation.

4. Frequency and Bandwidth
 Bandwidth: Amount of information transmitted at one
time. Can be above Tbps.
 Wavelength: around 700nm.
Fiber optics
wavelength

5. 1. Properties of Light

6. Fiber Optic Light Transmission
 Light wave is a form of energy moving in wave
motion.
 When studying the propagation of light (e.g. in a fiber
optic cable), we consider it as an electromagnetic wave.
 When studying light interaction with semiconductors (e.g.
in the photoelectric effect), we consider it a sum of
particles (i.e. photons).
 When light transmitted from a source encounters a
substance can be:
 reflected
 absorbed
 refracted

7. Fiber Optic Light Transmission
 Transparent:
 Almost all light
rays pass
through.
 Translucent:
 Some light rates
can pass.
 Opaque:
 No light rates
can pass.
 Always, an amount of light rays is absorbed by
the material and the rest is reflected (which
defines its color and makes them visible) or / and
refracted, if it is not opaque.

8. Light Reflection
 Reflected rays: Rays that are not absorbed nor refracted.
(e.g. in a mirror).
 2 rays:
 The incident (falling on the relecting surface.
 The reflected ray.
 The angle of
incident and the
angle of reflection
are the equal. (Law
of reflection).

9. Refraction of light
 When the light
passes through
a medium, the
direction of rays
changes. This is
called
Refraction.

10. Refraction of Light
 View underwater.
 Blue lines: the real light
rays path from water to
the air.
 Red lines: the light path
as if there was no
water.

11. Light Diffusion
 In a non smooth material the reflected light is broken
into many light beams and scattered through many
directions.

12. The index of refraction
 Index of refraction (or refractive
index) shows how light
propagates through a medium.
 n: refractive index.
 c: speed light in vacuum (3x108
m/sec).
 v: speed of light in the material.
v
c
n
n
0 λ is the wavelength
of light in the
medium and λ0 in
the vacuum.
 v is the frequency of
light multiplied with λ. fv

13. Snell’s law
 As angle of incidence increases, angle of
refraction increases as well.
 When angle of refraction exceeds 90
degrees, then no retraction happens. All
light is reflected. (Total internal reflection).
 The angle of incidence at which the angle of
reflection is 90 degrees is called critical
angle of incidence (θc).
2211 sinsin nn
1
2
sin
n
n
c

14. 2. Structure of Fiber Optics and Light
Transmission

15. Basic structure of the Fiber Optics
 Jacket:
 Determines the mechanical
robustness of the cable.
 Usually plastic, PVC etc.
 Buffer (or coating):
 Protects the fiber optic from
physical damage. Fiber
identification etc.
 Cladding:
 Dielectric material, with index
of refraction less than the core
material. Made of glass or
plastic.
 Core:
 Light propagation medium.
Dielectric material (usually
glass). Conducts no electricity.

16. How the light propagates in the optical fiber
cable?
 2 ways to describe the propagation:
 The ray theory: to get a clear picture of the propagation
of light inside the cable.
 The mode theory: to understand the behavior of the light
inside the cable (comprehending of the properties of
absorption, attenuation and dispersion).

17. The Ray Theory
 2 types of rays propagate along the optical fiber
cable:
 Meridional rays: Rays that pass through the center axis
of the core.
 Skew rays: Rays that travel through the fiber without
passing through the center axis of the core.

18. Meridional Rays
 Can be classified as:
 Bound: Rays that propagate through the fiber by total
internal reflection.
 Unbound: Rays that are refracted from the core.
 Cladding imperfections
prevent total internal
reflection.
 Part of the bound rays
are refracted inside the
cladding. (unbound
rays).
 Unbound rays
eventually escape from
the cable.

19. How light rays enter the cable?
 For avoiding
making the rays
unbound, the rays
must enter the core
with incident angle
equal or greater
than the critical
angle.
 The range of
angles which make
the rays bounded
is the cone of
acceptance.
 All rays entering the
core with angle less
than the half-angle of
the core of
acceptance
(acceptance angle)
become unbounded
and finally lost.

20. The Numerical Aperture
 Illustrates the relation between the cone of
acceptance and the indexes of refraction.
22
claddingcore nnNA
 Shows the light-gathering ability of the optical fiber
 High NA means that the fiber optic “admits” more light.

21. Skew Rays
 The acceptance angle of
skew rates is larger than
the acceptance angle of
meridional rays.
 Most rays entering the
core are skew rays.
 The contribute to the
amount of light capacity
of the fiber optic
(especially in case of high
NA).
 However, they contribute
also the amount of light
losses of the fiber optic.
 A large portion of the number
of skew rays becomes leaky
rays.
 Leaky rays, although they
are predicted to be totally
reflected, they are partially
refracted due the curved
shape of the fiber boundary.

22. The Mode Theory
 Mode theory considers light to be electromagnetic
waves.
 Describes light propagation in the optical fiber as a
propagation of electromagnetic waves.

23. Plane Waves
 A light wave can be
represented as a
plane wave.
 Plane wave’s surfaces
are parallel plates,
normal to the direction
of the propagation.
 3 characteristics of a
plane wave:
 Direction
 Amplitude
 wavelength

24. Plane Waves
 Planes having the same
phase are called
wavefronts.
nf
c
wavelength )(
 c: speed of light in the
vacuum (3*108)
 f: frequency of light
 n: index of refraction
 Only wavefronts with angle of incident
equal or grater with the critical angle will
propagate inside the core.
 All wavefronts propagating inside the
core must have phase equal to an
integer multiplied with 2π in order for
light to be transmitted.
 If they total phase is not integer x
2π, then destructive interference
takes place. (one wavefront
eliminates the other).
 This is the reason that a finite number of
modes can be propagate along the fiber.
 A set of electromagnetic
field patterns , form a
mode.

25. Light inside the fiber optics
 The electromagnetic
wave is composed by
an electric field and a
magnetic field.
 If the magnetic field is in
the direction of the
propagation, the mode is
called TM.
 If the electric field is the
direction of the
propagation, it is called
TE.

26. The modes
 Different electromagnetic wave patterns can propagate
in a fiber optic. These patterns are called modes.
 The number of maxima of each pattern is the order of
the mode (e.g. TE0 has one maximum, TE2 has 3,
etc.).
 Each mode can be realized with different wavelength.

27. Low order and High order modes
 Low order
modes have
high incident
angle, while
high order
modes have
low incident
angle.

28. Single vs. Multi-mode Optical Fibers
 Not every mode can propagate in any fiber
optic.
 The number of modes than can propagate
depend on:
 ncore, ncladding, wavelength (λ) of the fiber optic
and core diameter (α).
222
claddingcore nn
a
V
 Normalized frequency (V)
show the number of modes
that a fiber optic can
support.
 The larger the V, the more
modes can be supported.

29. Single vs. Multi-mode Optical Fibers
 Each fiber optic has a
specific normalized
frequency, for a specific
wavelength.
 Fiber optics with V < 2.405
can support only one
mode. They are called
single mode fibers.
 Fiber optics with V > 2.405
can support more modes.
They are called multi-
mode fibers.
 Y axis shows the propagation
constant.
 The higher the propagation
constant, the less the
dispersion of the wave during
the propagation.
 The higher the mode order, the
higher the dispersion.

30. Single vs. Multi-mode Optical Fibers
 Single mode fibers:
 The core diameter is small: 8 – 10 μm.
 Only the lowest mode can be propagated (around 1300nm
wavelength).
 Normalized frequency less or equal to 2.405.
 Lower signal loss in comparison with multi-mode and higher
bandwidth. Low fiber dispersion.
 They use laser diodes to generate light.
 Multi-mode fibers:
 Can propagate up to 4 modes (TE0, TE1, TE2, TE3. Also called OM1,
OM2, OM3, OM4).
 Typical core sizes: 50 – 100 μm.
 The light enters easily the fiber and the connections are easier.
 They use LED to generate light (cheaper, less complex and last
longer).
 However, they have higher dispersion than the single mode.
 Specifically they exhibit mode dispersion: Modes arrive at the receiver at
different times.

31. More on Multi-mode Fibers
 Multi-mode fibers are basically because they are cheaper and easier to install.
 The limited bandwidth they offer in comparison with the single-mode fibers is sufficient
for more applications.
 OM1 multi-fiber optics (TE0) are 62.5/125μm fibers and offer 200/500MHz x Km.
 OM2 multi-fiber optics (TE0 & TE1) are 50/125μm fibers and offer 500MHz x Km.
 OM3 multi-fiber optics (TE0 & TE1 & TE2) are 50/125μm and offer 2GHz x Km. Are designed
for 2Gbps transmission.
 OM4 multi-fiber optics (TE0 & TE1 & TE2 & TE3) are designed for speeds from 40 to 100Gbps.
 Increasing the mode used, we increase the bandwidth. However, intermodal
dispersion limits the bandwidth.
Transmission
Standards
100 Mb
Ethernet
1 Gb Ethernet
10 Gb
Ethernet
40 Gb
Ethernet
100 Gb
Ethernet
OM1 (62.5/125) up to 2000 meters 275 meters] 33 meters Not supported Not supported
OM2 (50/125) up to 2000 meters 550 meters 82 meters] Not supported Not supported
OM3 (50/125) up to 2000 meters 550 meters 300 meters 100 meters[ 100 meters[
OM4 (50/125) up to 2000 meters 1000 meters 550 meters 150 meters[ 150 meters[

32. 3. Properties of Fiber Optics

33. Performance of Fiber Optics
 Performance is affected by:
 Signal loss
 Bandwidth
 Bandwidth in telecommunications is defined as the difference
between the highest and the lowest frequency of the signal.
 Bandwidth in data transmission is defined as bits transmitted
per second.

34. Attenuation & Dispersion
 Fiber optics properties that affect
performance are:
 Attenuation
 Dispersion
 Attenuation is a result of:
 Light absorption
 Light scattering
 Bending losses
 If the signal strength is reduced
below a specific point, the
receiver is unable to detect it.
Dispersion is the spreading of the signal. The spreading limits how fast data
can be transmitted along the fiber.
The receiver is unable to distinguish between input pulses caused by the
spreading of each pulse.

35. Attenuation
 Attenuation is the loss of power as the light travels
inside the fiber. It is caused by:
 Light absorption
 Light scattering
 Bending losses
o
i
P
P
L
nattenuatio log
10
L: Fiber’s length in km.
Pi: Signal’s power in the input.
Po: Signal’s power in the output.
attenuation units: dB/Km
example:
Signal’s input power is 400W and
the power at the receiver is 100W.
The length of the fiber is 1Km.
attenuation = 10/1 * log(400/100) =
6 dB / Km

36. Explaining Decibel scale
 dB is a logarithmic scale,
describing ratio.
1
2
log10
P
P
dB
 If P2 is twice the P1, the ratio is
10*log(2) = 3dB.
 If P2 is half the P1, then the ratio is
10*log(1/2) = -3dB
 If P1 is 1,000,000 the P1, then the
ratio is 10*log(106) = 60dB.
 log(10n) = n*log(10) = n
 log (1) = 0

37. Light Absorption
 Absorption is the conversion of optical power into
another energy form, such as heat.
 Factors contributing to light absorption:
 Imperfections in the structure of the fiber material.
 The intrinsic material properties.
 Photons are absorbed by electrons and excites them to higher
energy levels.
 The extrinsic material properties.
 Impurities in the atomic structure of the silicon (e.g. iron, nickel,
chromium) results in the absorption of photons by the electrons
of these metal and electrons excitation to higher energy levels.

38. Light Scattering
 Scattering is the result of density fluctuations in the
fiber structure.
 During the fabrication process of the fiber, there are areas
where the density of the molecular structure is higher or
lower.

39. Bending losses
 Bending losses:
 Macrobends: Too sharp bend.
 A part of the mode is converted to a higher order mode, which is
lost inside the cladding.
 Microbends: Imperfections in the fiber or external force
which deforms the cable.

40. Dispersion
 2 types of dispersion:
 Intramodal dispersion: Occurs in all kinds of optical fibers,
where more than one wavelenghts are used (e.g in WDM).
 Different wavelengths of signals of the same mode travel at
different speeds inside the fiber, so exit the fiber at different times.
 Intermodal dispersion: Occurs only in multi-mode fibers.
 Each mode travels at different speed inside the fiber, so, they do
not exit from the fiber at the same time.
 Single mode
fibers exhibit
less dispersion
than multi-
mode.

41. Wave Division Multiplexing
 Each signal is transmitted in the fiber using a different
wavelength.
 e.g. the light of LED is split to separate channels, where each
channel has a different wavelength.
 Can be used in single or multi-mode fibers.
 Can increase the bandwidth of the fiber up to 40 times.

42. 4. Optical Fiber Cables

43. Step-index and Graded-index fiber cables
 Step-index: The
refractive index of the
core is uniform.
 Graded-index: The
refractive index of the
core is varies gradually,
as a function of radial
distance from the fiber
center.
 Single mode fibers are
normally step-index.
 Graded-index multimode
fibers performance is
superior to step-index
multimode.

44. Why Graded-index fibers exhibit high
performance?
 Light travels faster in materials with low
refractive index.
 Therefore, rays that travel the longer
distance, propagate through lower
refractive indexes. So, they travel faster.
 So, the modal dispersion of the fiber
decreases.
n
c
v

45. Graded-index fibers specifications
 The relative refractive index difference shows the
difference between the core and the cladding
refractive index:
2
1
2
2
2
1
2n
nn Most common values of graded-index multimode
fibers are:
 core: 62.5μm
 cladding: 125μm
 Δ = 0.02
 They are the most common fiber optic cables
used in aircrafts.
 A basic advantage is the low sensitivity to
microbend and macrobend. This allows relatively
sharp bending during the installation procedure.

46. Fiber Buffers
 Tight buffer cables are used for fiber
installed inside buildings, aircrafts, etc.
 Loose-tube buffer cables are typically
used outdoors.

47. Types of cables

48. Zip-core cables
 1: Jacket (3mm)
 2: Tight buffer (900μm)
 3: Fiber optic
 Normally used in patch boards
 patch boards: panels in telecommunication
centers where temporary connections of circuits
are made.
 Indoor cable.

49. Distribution cable
 Tight buffer (900μm)
 Low cost cable, suitable for
short distances.
 Fibers are not individually
buffered. Difficult
termination.
 Indoor cable.

50. Breakout Cable
 Several zip-core cables
hold together.
 Each fiber is individually
buffered. Easy
termination.
 Large and more
expensive than
distribution cables.
However, the easy
installation (i.e.
termination) can
eventually make then
more economic.
 Indoor cables

51. Gel-filled loose-tube cable
 Several fibers inside
each gel-filled loose
buffer.
 Gel is used to repel
water.
 Designed to endure high
temperature and
moisture conditions.
 Can be burried into the
ground.
 Used outdoors

52. Dry loose-tube cable
 No gel.
 High durability and
easier installation than
gel-filled.
 Used outdoors.

53. Armored cables
 Steel or aluminum
armor.
 Extremely high durability
in harsh environments.
 Used outdoors.

54. Submarine communication cables
 69mm diameter
 10Kg/m
 In 2010 submarine
optical cables linked all
world’s continents.
 7: gel
 3: steel bars

55. Cables Coloring
Fiber Type Diameter (µm) Buffer Color
Multimode 50/125 Orange
Multimode 62.5/125 Slate
Multimode 85/125 Blue
Multimode 100/140 Green
Singlemode - Yellow

56. 5. Optical Splices, Connectors and Couplers

57. Fiber Optic Data Links
 Fiber optic data links perform 3 basic functions:
 Conversion of the input electrical to optical signal.
 Transmission of the optical signal through the fiber.
 Conversion of the optical signal to output electrical signal.
 Fiber optic connections permit the transfer of optical
power from one component to another. A connection
requires:
 a fiber optic splice (permanent join) or
 a connector (allows reconfiguration) or
 a coupler (combination of optical signals)
 Poor connections degrade the system performance
(coupling losses).

58. Optical Fiber Coupling Losses
 Coupling losses are due to:
 Reflection losses.
 Fiber misalignment
 Poor fiber end preparation
 Mismatches between the fiber characteristics

59. Reflection Losses
 When optical fibers are connected a part of the
source power will be reflected back, to the source
fiber.
 It is caused by a small air gap that may exists in the
interface, causing a discontinuity of the refracting index.
 Solution:
Index matching gel.

60. Fiber Misalignment
 3 main coupling errors:
 longitudinal misalignment
 Lateral misalignment
 Angular misalignment (θ
must be less than 1
degree).
 Matching gel reduces
coupling loss from fiber
separation.
 However, it does not
affect lateral
misalignment losses and
increases angular
misalignment losses.

61. Poor Fiber End Preparation
 First we remove the
buffer (usually with
chemical materials).
 Then, using a diamond
blade and constant
tension between the two
ends is used to break
the fiber.
 Finally the polishing
using and abrasive
paper and a microscope.

62. Fiber Mismatches
 Geometry mismatches
 e.g. difference in core diameter.
 NA mismatches
 different n1, n2.
 Relative refracting index difference
 different Δ.

63. Fiber Optic Splices
 A device that connects a fiber optic with the other
permanently.
 2 kinds of splices:
 Mechanical splice
 Aligning takes place mechanically.
 Fusion splice
 The end of the two fibers is melted by heat to fuse them
together.

64. Mechanical Splices
 3basic kinds:
 A tube, with a diameter
slightly larger than the fiber
optic’s diameter.
 The fiber optics are placed
inside the tube and they are
aligned.
 A groove where the two
fibers are placed.
 Rotary splices
 A transparent adhesive is
placed after the alignment,
to make the alignment
permanent and to reduce
the coupling losses.

65. Rotary splices
 The fiber is inserted into
each half of the splice.
 It’s epoxied with an
ultraviolet cure epoxy.
 Using the alignment
sleeve the two halves are
connected.
 Mechanical stability is
added due to the two
springs.
 They are used extensively
in aircraft applications due
to their durability in
environmental conditions
and easy installation.

66. Fusion Splices
 The two ends of the fiber
optics are melted to form
a connection.
 Takes longer time than
the mechanical
connection.
 Requires extensive
training by the operators
and the final result
depends greatly on their
experience.

67. Fiber Connector characteristics
 Connectors allow system
reconfiguration.
(No permanent
connections).
 Every connector is defined
by its ferrule diameter.
 Ferrules (made of ceramic)
hold the end of the fibers
and keeps them aligned.
 There are different
connectors for single mode
and multi-mode fibers.
 There are screw and push-
pull connectors.
 Most connector types are
made in two types:
 UPC.
 APC.

68. Connector installation
 Very fast installation in
comparison with splices.
 Installation procedure is
almost the same, in all
connector types:
 Buffer removal
 Boot insertion
 Plug of the fiber optic in
the connector and cramp.
 Polishing and cleaning of
the fiber end.
Boot

69. Fiber Connector evaluation
 2 parameters define the quality of the connector:
 Insertion loss: The power difference between the source
and the destination fibers (around 0.5dB).
 Return loss: The amount of reflected power (around 40 –
70dB).
 A high quality connector has low insertion loss and
high absolute return loss value.

70. UPC vs. APC Connectors
 In all fiber optic connections (using
splices or connectors) a part of the
incoming light ray is reflected back to
the source fiber.
 It is preferable the reflected light to be
lost in the cladding, than be reflected
back in the core of the source fiber.
 Angled polished connectors (APC)
are cut in 8 degrees angle to increase
the possibility of the reflected light to
be lost in the cladding.
 Depending on the application
requirements, a proper selection
between UPC and APC can be made.
 APC are usually single mode.

71. UPC vs. APC comparison
 UPC has lower insertion loss, but APC has superior
return loss, due to its diameter.

72. Types of Connectors
 Most common types:
 SC connectors (square
connector or subscriber
connector or standard
connector).
 A push-pull connector
 2.5mm ferrule diameter
 Extremely common in
datacom and telecom.
 LC connectors (lucent
connector, little
connector).
 Half the size of SC.
 1.25 mm ferrule diameter

73. Aircraft Connectors (AVIO connectors)
 Materials:
 Ferrule: Zirconium (Zr).
 External parts: Copper –
Nickel.
 Dimensions:
 Ferrule diameter: 900μm
 Boot diameter: 3mm
 Screw style.

74. AVIO Connectors specifications

75. Fiber Optics in Boeing 787

77. AFDX switches
 Boeing 787 and Airbus 380 use a new ARINC
specification: 664.
 664 is a full duplex bus, using a twisted pair of optical
cables.
 Transmitting and receiving for each LRU connected in the
network is independent.
 It is considered rather a network, than a bus. LRUs are
connected to the bus through switches.
 All connections are 10Mbps or 100Mbps.
 The network is called AFDX (Avionics Full-Duplex
Switched Ethernet).

78. Fiber Optic Couplers
 Some fiber optic links require
more than simple point-to-
point connections.
 Such connections are more
complex.
 These configurations require
devices that can distribute an
optical signal from one fiber
among two or more fibers.
These devices are called
couplers.
 Example 1x2 couplers
distributes equally the input
signal among two outputs.
 Each output signal has less
than half power of the input
signal. (losses around 3dB).

79. Types of couplers
 Optical splitters: splits
the optical power of a
signal optical fiber in two
or more fibers.
 Optical combiners:
combines the signal of
more than one optical
fibers to one fiber.

80. 6. Fiber Optic Measurement Techniques
 Laboratory Measurements:
 Attenuation
 Cut-off Wavelength (in single mode only).
 Bandwidth
 Dispersion
 Fiber geometry
 Core diameter
 Numerical Aperture
 Insertion loss
 Return loss

81. Attenuation Measurement
 Cut-back method:
 Pi and Po measurement
in a known length L.
 Cut-back to 2m and
repeat measurements
with the same Pi.
 Repeating the
measurement in a
short length fiber is
necessary to ensure
correctness.
o
i
P
P
L
nattenuatio log
10

82. Cut-off Wavelength Measurement
 The wavelength a which a mode seizes to propagate.
 It depends on the fiber length and the bending conditions.
 Attenuation is measured with
specific bending radius for
different wavelengths.
 Cut-off wavelength is the
longest λ for which attenuation
is equal to 0.1dB.

83. Bandwidth Measurement
 The fiber bandwidth is
defined as the lowest
frequency at which the
magnitude of the fiber
frequency response has
decreased to one-half
its zero frequency value.
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fP
fP
fH
i
o
 Power frequency
response:

84. Intramodal (Chromatic) Dispersion
Measurement
 Differential group delay τ(λ) is the difference
between the arrival time of different wavelengths.
 We use a reference fiber and the fiber under testing.
 The pulse delay for the reference fiber of length Lref and
the delay is tin(λ). The pulse delay for the test fiber of Ls is
tout(λ).
 The differential group delay for a specific λ per unit of
length is:
refs
outin
LL
)()(
)(

85. Fiber Geometry Measurement
 Specific fiber geometry characteristics, such as the
cladding diameter are obtained by video camera
images, analyzed by a computer.
 Core diameter is measured by the range of the
intensity of light at the end of the fiber.

86. Numerical Aperture Measurement
 It is measured with a similar way with the core
diameter.
22
claddingcore nnNA

87. Insertion and Return Loss Measurement
 Insertion loss is similar to attenuation, but is used to
indicate the attenuation in an optical fiber connector.
1
0
0
1
log10
M
M
P
P
P
P
lossinsertion
r
i
P
P
lossreturn log10
 P0 is power measured without the
interconnection.
 P1 is the power measured with the
interconnection.
 Return loss is measured using a
2x2 (or 2x1) coupler. The
returning power is measured in
one of input ports of the coupler.
 Pi is power in the source fiber.
 Pr is the power reflected.
 return loss may be defined as
10logPr/Pi. Then, it will be negative.

88. Optical Time-Domain Reflectometer
 Electronic instrument used to characterize a fiber optic.
 Injects a series of optical pulses into the fiber under test.
 It estimates attenuation, reflected losses, scattered light etc.
 It is used for field measurements (i.e. installed fiber optics, not in laboratory).

89. 7. Fiber Optics Transmitters and Receivers

90. Fiber Optic Transmitter
 Converts electrical signals to optical signals and
lunches the optical signal into the fiber.
 The part of the transmitter that converts electrical energy
(current) to optical energy (light) is the optical source.
 Optical sources must:
 Launch sufficient light power to the fiber to overcome
attenuation and coupling losses.
 Emit light at proper wavelengths.
 Optical sources are LEDs.
 Emit infrared light normally at 850, 1300 or 1550nm.
 Today, common LEDs have been replaced with VCSELs.

91. VCSEL
(Vertical Cavity Surface Emitting Laser)
 High speed, low
power
consumption.
 Can used to
design 2D
Arrays.
 Higher light
density.
 Low temperature
sensitivity.

92. Transmitter
 850nm transmitter.
 Multi-mode SC type.
 Can transmit up to 1.65 or 3.5Gbps.
Electrical InputOptical Output
 4 digital data inputs are multiplexed
with slightly different wavelengths.
 VCSELs produce light of slightly
different wavelength.
 Light is forwarded to an SC type
fiber optic cable.

93. Fiber Optic Receivers
 Photodiodes:
Semiconductor devices
that convert light into
current.

94. Fiber Optic Transceiver
 Interfaces between fiber optics and
Ethernet.
ReceiverTransmitter
Electrical Input /
Output
10 Gb Ethernet Transceiver
SR - 850 nm, for a maximum of 300 m
LR - 1310 nm, for distances up to 10 km
ER - 1550 nm, for distances up to 40 km
ZR - 1550 nm, for distances up to 80 km

95. Formulas
Formula Units Explanation
Number
A number that shows the size of the
acceptance cone.
dB/Km
Pi: input
Po: output
L: fiber optic length
Number
Power Frequency Response:
Po(f): Power at the output
Pi(F): Power at the input
nsec/Km.
Differential group delay:
tin: the group delay of the reference fiber with
length Lref
Tout: the group delay of the test fiber with
length Ls.
dB
Pi: power at the source fiber.
P0: power at the destination fiber.
dB
Pi: power at the source fiber.
Pr: power reflected.
22
claddingcore nnNA
o
i
P
P
L
nattenuatio log
10
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fP
fP
fH
i
o
refs
outin
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0
log10
P
P
lossinsertion i
r
i
P
P
lossreturn log10