NEETS 24 Fiber Optics Flashcards
the branch of optical technology concerned with the transmission of radiant power (light energy) through fibers.
Fiber Optics
to convert
an electrical input signal to an optical signal, send the
optical signal over an optical fiber, and convert the
optical signal back to an electrical signal
basic functions of a fiber optic data link
The three parts of a fiber optic data link
Transmitter
Optical Fiber
Receiver
mechanisms in the fiber waveguides weaken and distort the optical signal?
Scattering
absorption
dispersion
effect does noise have on the fiber optic signal
Noise obscures or reduces the quality of the signal.
the decrease in the amount of light reaching the
end of the fiber.
loss
In 1969, what did several scientists conclude about
optical fiber loss?
Impurities in the fiber material caused the signal loss in
optical fibers. The basic fiber material did not prevent
the light signal from reaching the end of the fiber.
How can loss be reduced during construction (or
fabrication) of optical fibers?
By removing the impurities from optical fiber.
What are the two basic types of optical Fibers?
Multimode
Single Mode Fibers
Which type of optical fiber (multimode or single mode)
tends to have lower loss and produces less signal
distortion?
Single mode fiber.
What optical fiber properties reduce connection loss
in short-distance systems?
Larger fiber core and higher fiber numerical aperture (NA).
In fiber optic systems, designers consider what
trade-offs?
Trade-offs in fiber properties, types of connections,
optical sources, and detector types in military
and subscriber-loop applications.
List seven advantages of fiber optics over electrical
systems.
*Improved system performance
*Immunity to electrical noise
*Signal security
*Electrical isolation *Reduced size and weight
*Environmental
protection
*Overall system economy.
Quantum physics successfully explained the
photoelectric effect in terms of fundamental particles of
energy called quanta. What are the fundamental particles
of energy (quanta) known as when referring to light energy?
Photons.
What type of wave motion is represented by the motion
of water?
Transverse-wave motion.
When light waves encounter any substance, what four
things can happen?
Light waves are either transmitted, refracted, reflected,
or absorbed.
A substance that transmits almost all of the light waves
falling upon it is known as what type of substance?
Transparent.
A substance that is unable to transmit any light waves
is known as what type of substance?
Opaque.
What is the law of reflection?
The law of reflection states that the angle of incidence
is equal to the angle of reflection.
When a wave is reflected from a surface, energy is
reflected. When is the reflection of energy the greatest?
When the wave is nearly parallel to the reflecting surface.
When is the reflection energy the least?
When the wave is perpendicular to the reflecting surface.
Light waves obey what law?
The law of reflection
A refracted wave occurs when a wave passes from one
medium into another medium. What determines the angle of
refraction?
Depends on the bending caused by the velocity difference
of the wave traveling through different mediums.
A light wave enters a sheet of glass at a perfect right
angle to the surface. Is the majority of the wave reflected,
refracted, transmitted, or absorbed?
Transmitted.
When light strikes a piece of white paper, the light
is reflected in all directions. What do we call this
scattering of light?
Diffusion.
Two methods describe how light propagates along an
optical fiber. These methods define two theories of light
propagation. What do we call these two theories?
The ray theory and the mode theory.
What is the basic optical-material property relevant
to optical fiber light transmission?
The index of refraction.
The index of refraction measures the speed of light in
an optical fiber. Will light travel faster in an optically
dense material or in one that is less dense?
Light will travel faster in an optical material that is less
dense.
Assume light is traveling through glass, what happens
when this light strikes the glass-air boundary?
Part of the light ray is reflected back into the glass and
part of the light ray is refracted (bent) as it enters the
air.
What condition causes a light ray to be totally
reflected back into its medium of propagation?
Total internal reflection occurs when the angle of
refraction approaches 90 degrees. This condition occurs
when the angle of incidence increases to the point where
no refraction is possible.
What name is given to the angle where total internal
reflection occurs?
Critical angle of incidence.
List the three parts of an optical fiber.
Core
cladding
coating or buffer.
Which fiber material, core or cladding, has a higher
index of refraction?
Core.
Light transmission along an optical fiber is described
by two theories. Which theory is used to approximate the
light acceptance and guiding properties of an optical
fiber?
The ray theory.
Meridional rays are classified as either bound or
unbound rays. Bound rays propagate through the fiber
according to what property?
Total internal reflection.
A light ray incident on the optical fiber core is
propagated along the fiber. Is the angle of incidence of
the light ray entering the fiber larger or smaller than the
acceptance angle (θa)
Smaller.
What fiber property does numerical aperture (NA)
measure?
NA measures the light-gathering ability of an optical
fiber.
Skew rays and meridional rays define different
acceptance angles. Which acceptance angle is larger, the
skew ray angle or the meridional ray angle?
Skew ray angle.
The mode theory uses electromagnetic wave behavior to
describe the propagation of the light along the fiber. What
is a set of guided electromagnetic waves called?
Modes of the fiber.
A light wave can be represented as a plane wave. What
three properties of light propagation
describe a plane wave?
Direction
amplitude
and wavelength of propagation.
A wavefront undergoes a phase change as it travels along
the fiber. If the wavefront transverses the fiber twice and
is reflected twice and the total phase change is equal to
1/2π, will the wavefront disappear? If yes, why?
Yes, the wavefront will disappear because the total amount
of phase collected must be an integer multiple of 2π. (If
the propagating wavefronts are out of phase, they will
disappear. The wavefronts that are in phase interfere with
the wavefronts out of phase. This type of interference
is called destructive interference.)
Modes that are bound at one wavelength may not exist
at longer wavelengths. What is the wavelength at which a
mode ceases to be bound called?
Cutoff wavelength.
What type of optical fiber operates below the cutoff
wavelength?
Multimode fiber.
Low-order and high-order modes propagate along an
optical fiber. How are modes determined to be low-order or
high-order modes?
The order of a mode is indicated by the number of field
maxima within the core of the fiber. The order of a mode
is also determined by the angle that the wavefront makes
with the axis of the fiber.
As the core and cladding modes travel along the fiber,
mode coupling occurs. What is mode coupling?
Mode coupling is the exchange of power between two modes.
The fiber’s normalized frequency (V) determines how
many modes a fiber can support. As the value of V increases,
will the number of modes supported by the fiber increase
or decrease?
Increase.
The value of the normalized frequency parameter (V)
relates the core size with mode propagation. When single
mode fibers propagate only the fundamental mode, what is
the value of V?
V ≤ 2.405.
The number of modes propagated in a multimode fiber
depends on core size and numerical aperture (NA). If the
core size and the NA decrease, will the number of modes
propagated increase or decrease?
Decrease.
Modal dispersion affects the bandwidth of multimode
systems. It is essential to adjust what three fiber
properties to maximize system bandwidth?
Core diameter
NA
index profile properties.
Attenuation is mainly a result of what three properties?
Light absorption
scattering
bending losses.
the loss of optical power as light travels
along the fiber.
attenuation
What are the main causes of absorption in optical
fiber?
Intrinsic and extrinsic material properties.
Silica (pure glass) fibers are used because of their
low intrinsic material absorption at the wavelengths of
operation. This wavelength of operation is between two
intrinsic absorption regions. What are these two regions
called? What are the wavelengths of operation for these two
regions?
Ultraviolet absorption region (below 400 nm) and infrared absorption region (above 2000 nm).
Extrinsic (OH_) absorption peaks define three regions
or windows of preferred operation. List the three windows
of operation.
The first, second, and third windows of operation are 850
nm, 1300 nm, and 1550 nm, respectively.
What is the main loss mechanism between the ultraviolet
and infrared absorption regions?
Rayleigh scattering.
Scattering losses are caused by the interaction of light
with density fluctuations within a fiber. What are the two
scattering mechanisms called when the size of the density
fluctuations is (a) greater than and (b) less than one-tenth
of the operating wavelength?
(a) Mie scattering; (b) Rayleigh scattering.
Microbend loss is caused by microscopic bends of the
fiber axis. List three sources of microbend loss.
Uneven coating applications, improper cabling procedures,
and external force.
How is fiber sensitivity to bending losses reduced?
Fiber sensitivity to bending losses can be reduced if the
refractive index of the core is increased and/or if the
overall diameter of the fiber increases.
Name the two types of intramodal, or chromatic,
dispersion.
Material dispersion and waveguide dispersion.
Which dispersion mechanism (material or waveguide) is
a function of the size of the fiber’s core relative to the
wavelength of operation?
Waveguide dispersion.
Modes of a light pulse that enter the fiber at one time
exit the fiber at different times. This condition causes
the light pulse to spread. What is this condition called?
Modal dispersion.
Refractive index profile describes the value of refractive
index as a function of radial distance at any fiber
diameter.
refractive index profile
The refractive index of a fiber core is uniform and
undergoes an abrupt change at the corecladding boundary.
Is this fiber a step-index or graded-index fiber?
Step-index.
Multimode optical fibers can have a step-index or
graded-index refractive index profile. Which fiber,
multimode step-index or multimode graded-index fiber,
usually performs better?
Multimode graded-index fiber.
List the standard core sizes for multimode step-index,
multimode graded-index, and single mode fibers.
Multimode step-index fibers: 50 μm and 100 μm. Multimode
graded-index fibers: 50 μm, 62.5
μm, 85 μm, and 100 μm. Single mode fibers: between 8 μm and
10 μm.
Multimode step-index fibers have a core and cladding of
constant refractive index n1 and n2, respectively. Which
refractive index, the core or cladding, is lower?
Cladding.
In multimode step-index fibers, the majority of light
propagates in the fiber core for what reason?
Most modes in multimode step-index fibers propagate far
from cutoff.
Multimode step-index fibers have relatively large core
diameters and large numerical apertures. These provide what
benefit?
Make it easier to couple light from a light-emitting diode
(LED) into the fiber.
The profile parameter (α) determines the shape of the
multimode graded-index core’s refractive index profile. As
the value of the α increases, how does the core’s profile
change?
From a triangular shape to step.
Light propagates in multimode graded-index fibers
according to refraction and total internal reflection. When
does total internal reflection occur?
When the angle of incidence becomes larger than the critical
angle of incidence.
What four fiber properties determine the number of modes
propagating in a multimode gradedindex fiber?
Numerical aperture (NA), relative refractive index difference (Δ), profile parameter (α), and normalized frequency (V).
Light travels faster in a material with a lower
refractive index. Therefore, light rays that travel a
longer distance in a lower refractive index travel at a
greater average velocity. What effect does this have on
multimode graded-index fiber modal dispersion and
bandwidth?
Decreases the time difference between light rays, which
reduces modal dispersion and increases fiber bandwidth.
What multimode graded-index fiber offers the best
overall performance for most applications?
62.5/125 μm multimode graded-index fiber.
What are the most distinguishing characteristics of a
multimode graded-index fiber?
A13. Source-to-fiber coupling efficiency and insensitivity
to microbending and macrobending losses.
How are source-to-fiber coupling and microbending and
macrobending losses affected by changes in core diameter
and Δ?
Coupling efficiency increases with both core diameter and
Δ, while bending losses increase directly with core
diameter and inversely with Δ.
While coupled power and bending loss favor a high Δ,
which Δ value, smaller or larger, improves fiber bandwidth?
Smaller.
What are the two basic types of single mode step-index
fibers?
Matched-clad and depressed-clad.
Which fiber cladding, matched or depressed, consists
of two regions?
Depressed.
In single mode operation, the value of the normalized
frequency (V) should remain near the 2.405 level. If the
value of V is less than 1, do single mode fibers carry a
majority of the power in the core or cladding material?
Cladding material.
What happens to the fundamental mode as the operating
wavelength becomes longer than the single mode cutoff
wavelength?
The fundamental mode becomes increasingly lossy.
Give two reasons why the value of the normalized
frequency (V) is varied in single mode stepindex fibers?
To increase performance and reduce losses caused by bending
and splicing.
Give two reasons why optical fiber manufacturers depart
from the traditional circular core and cladding, low-loss
glass fiber design?
To increase performance and reduce cost.
What five characteristics do applications using plastic clad silica (PCS) and all-plastic fibers typically have?
High NA, low bandwidth, tight bend radius, short length,
and low cost.
List the types of materials used in fabricating
low-loss, long wavelength optical fibers.
Heavy-metal fluorides, chalcogenide glasses, and
crystalline materials.
What are the two methods used by fiber manufacturers
to fabricate multimode and single mode glass fibers?
Vapor phase oxidation and direct-melt process.
Which method, vapor phase oxidation or direct-melt
process, transforms deposited material into a solid glass
preform by heating the porous material without melting?
Vapor phase oxidation.
List three benefits that properly cabled optical fibers
provide.
a. Protect optical fibers from damage or breakage during
installation and over the fiber’s lifetime.
b. Provide stable fiber transmission characteristics
compared with uncabled fibers.
c. Maintain the physical integrity of the optical fiber.
In addition to a primary coating, manufacturers add a
layer of buffer material for what reasons?
To provide additional mechanical protection and preserve
the fiber’s inherent strength.
List the three techniques used by manufacturers to
buffer optical fibers.
Tight-buffered, loose-tube, and gel-filled loose-tube.
List seven properties cable jackets should have.
Low smoke generation, low toxicity, low halogen content,
flame retardance, fluid resistance, high
abrasion resistance, and stable performance over
temperature.
List the three types of cable designs being considered
by the Navy.
Optical fiber cable component (OFCC), stranded, and ribbon
cables designs.
Describe an optical fiber cable component (OFCC).
OFCCs are tight-buffer fiber surrounded by arimid yarn and
a low-halogen outer jacket.
Two layers of arimid yarn strength members encase the
OFCC units. Why are these strength members stranded in
opposing directions?
To minimize microbending of the fibers.
Why do cable manufacturers introduce a controlled twist
to the stacked ribbons during the cabling process?
To minimize fiber stress when the cable is bent.
OFCC, stranded, and ribbon cables have different fiber
capacities. What is the approximate number of fibers that
each cable can accommodate in a 0.5-inch cable?
OFCC (12 fibers), stranded (48 fibers), ribbon (204
fibers).
Which fiber optic cable (OFCC, stranded, or ribbon) has
the worst bend performance?
Ribbon
Which fiber optic component (splice, connector, or
coupler) makes a permanent connection in a
distributed system?
splice
What are the main causes of coupling loss?
Poor fiber end preparation and poor fiber alignment.
Define the loss in optical power through a connection
loss=10log(Pi/Po)
Fiber-to-fiber coupling loss is affected by intrinsic
and extrinsic coupling losses. Can intrinsic coupling
losses be limited by limiting fiber mismatches?
Yes.
In fiber-to-fiber connections, Fresnel reflection is one
source of coupling losses. Light is reflected back into the
source fiber and is lost. What causes Fresnel reflection?
A step change in refractive index that occurs at fiber
joints, caused by fiber separation.
Reduction of Fresnel reflection is possible by reducing
the step change in the refractive index at
the fiber interface. What material reduces the step change
in refractive index at a fiber interface?
Index matching gel.
List the three basic errors that occur during fiber
alignment.
Fiber separation (longitudinal misalignment), lateral misalignment, and angular misalignment.
When the axes of two connected fibers are no longer in
parallel, the two connected fibers are in what kind of
misalignment?
Angular misalignment.
How does index matching gel affect the amount of coupling
loss caused by (a) fiber separation, (b) lateral
misalignment, and (c) angular misalignment?
(a) Reduces coupling loss, (b) does not change coupling
loss, and (c) increases coupling loss.
hich are more sensitive to alignment errors, single
mode or multimode fibers?
Single mode.
Quality fiber-end preparation is essential for proper
system operation. What properties must an optical fiber-end
face have to ensure proper fiber connection?
Be flat, smooth, and perpendicular to the fiber axis
What is the basic fiber cleaving technique for preparing
optical fibers for coupling?
Score-and-break.
Using a standard microscope to inspect a fiber-end face,
you observe that all parts of the fiberend face are in focus
at the same time. Is the fiber-end face flat, concave, or
convex?
Flat.
List six types of fiber mismatches.
Core diameter mismatch, cladding diameter mismatch, core
ellipticity, core and cladding
concentricity differences, NA mismatch, and refractive
index profile differences.
Does coupling loss from refractive index profile
difference result when the receiving fiber has a
larger profile parameter (α) than the transmitting fiber?
No.
A permanent fiber joint whose purpose is to establish an
optical connection between two
individual optical fibers
fiber optic splice
Fiber splicing is divided into two broad categories that
describe the techniques used for fiber splicing. What are
they?
Mechanical and fusion splicing.
An epoxy resin that seals mechanical splices and provides
index matching between the connected fibers.
Describe a transparent adhesive.
The Navy recommends using the rotary splice for what
two reasons?
It is a low-loss mechanical splice that provides stable
environmental and mechanical performance in the Navy
environment, and it requires only a small amount of
training.
What fiber property directly affects splice loss in
fusion splicing?
The angles and quality of the two fiber-end faces.
List two reasons why fusion splicing is one of the most
popular splicing techniques in commercial applications.
The small size of the fusion splice and the development
of automated fusion-splicing machines.
What is a short discharge of electric current that
prepares the fiber ends for fusion called?
Prefusion.
Do small core distortions formed by arc fusion’s
self-alignment mechanism have more of an affect on light
propagating through multimode or single mode fibers?
Single mode fibers.
What connection properties result in fiber optic
connector coupling loss?
Poor fiber alignment and end preparation, fiber mismatches,
and Fresnel reflection.
Which is the more critical parameter in maintaining
total insertion loss below the required level, fiber
alignment or fiber mismatch?
Fiber alignment.
Fiber optic connectors can reduce system performance
by increasing what two types of noise?
Modal and reflection.
Which type of fiber optic connector (butt-jointed or
expanded beam) brings the prepared ends of two optical
fibers into close contact?
Butt-jointed connectors.
Is coupling loss from fiber separation and lateral
misalignment more critical in expanded-beam or utt-jointed
connectors?
Butt-jointed connectors.
Is coupling loss from angular misalignment more
critical in expanded beam or butt-jointed connectors?
Expanded beam connectors.
The Navy classifies fiber optic connectors in what two
ways?
Light-duty and heavy-duty connectors.
What is the difference between passive and active fiber
optic couplers?
Passive couplers redistribute optical signals without
optical-to-electrical conversion.
Which type of optical splitter (Y-coupler or T-coupler)
splits only a small amount of power from the input fiber
to one of the output fibers?
T-coupler.
- Describe a directional coupler.
A fiber optic coupler that prevents the transfer of power
between input fibers.
List the fiber geometrical measurements performed in the
laboratory.
Cladding diameter, core diameter, numerical aperture, and
mode field diameter.
End users measure the total attenuation of a fiber at
the operating wavelength (λ). Write the equation for total
attenuation (A), between an arbitrary point X and point Y
located on an optical fiber.
A = 10 log (Px/Py) dB
Will an optical fiber’s attenuation coefficient vary
with changes in wavelength?
Yes.
What two properties of the launch condition may affect
multimode fiber attenuation measurements?
Launch spot size and angular distribution.
Does underfilling a multimode optical fiber excite
mainly high-order or low-order modes?
Low-order modes.
Multimode optical fiber launch conditions are typically
characterized as being overfilled or underfilled. Which of
these optical launch conditions exists if the launch spot
size and angular distribution are larger than that of the
fiber core?
Overfilled.
A mode filter is a device that attenuates specific modes
propagating in the core of an optical fiber. What mode
propagating along single mode fibers do mode filters
eliminate?
Second-order mode.
What are the two most common types of mode filters
Free-form Loop and Mandrel wrap
The cutoff wavelength of matched-clad and depressed-clad
single mode fibers varies according to the fiber’s radius
of curvature and length. The cutoff wavelength of which
single mode fiber type is more sensitive to length?
Depressed-clad.
Will the cutoff wavelength of uncabled fibers (λcf)
generally have a value higher or lower than the cutoff
wavelength of cabled fibers (λcc)?
Higher
Describe the -3 decibel (dB) optical power frequency
f3dB
The -3 decibel (dB) is the lowest frequency at which the
magnitude of the fiber frequency response has decreased to
one half its zero-frequency value.
Delay differences between the source wavelengths occur
as the optical signal propagates along the fiber. What is
this called?
Differential group delay τ(λ).
What determines the range of wavelengths over which
meaningful data is obtained for
calculating the chromatic dispersion?
The wavelength range of the optical source(s) used
Why do end users perform fiber geometry measurements
in the laboratory?
To reduce system attenuation and coupling loss resulting
from poor fiber fabrication.
Define cladding diameter.
The cladding diameter is the average diameter of the
cladding.
Explain the difference between multimode and single
mode core-cladding concentricity errors.
Multimode core-cladding concentricity error is the
distance between the core and cladding centers expressed
as a percentage of core diameter while the single mode
core-cladding concentricity error is just the distance
between the core and cladding centers.
Near-field power distributions describe the emitted
power per unit area in the near-field region.
Describe the near-field region.
The near-field region is the region close to the fiber-end
face
How is the core diameter defined?
The core diameter is defined as the diameter at which the
near-field intensity is 2.5 percent of the
maximum intensity
Far-field power distributions describe the emitted
power per unit area as a function of angle θ in the far-field
region. Describe the far-field region.
The far-field region is the region far from the fiber-end
face.
Will fiber coupling loss generally increase or decrease
if the mode field diameter of a single mode fiber is
decreased?
Increase.
List two effects that reflections can have on a fiber
optic data link.
Reduce the stability of the system source and increase the
signal noise present at the optical detector.
Reflectance is given as what ratio?
The ratio of reflected optical power to incident optical
power.
Does return loss include power that is transmitted,
absorbed, and/or scattered?
No.
Is it essential for end users to remeasure optical fiber
geometrical properties after installation in the field?
No
When is an OTDR recommended for conducting field
measurements on installed optical fibers or links?
When installed optical fiber cables or links are 50 meters
or more in length.
An OTDR measures the fraction of light that is reflected
back from the fiber or link under test. What causes light
to be reflected back into the OTDR?
Rayleigh scattering and Fresnel reflection.
List the types of fiber optic components considered part
of a fiber optic cable plant.
Optical fiber cables, connectors, splices, mounting
panels, jumper cables, and other passive components.
What is a temporary or permanent local deviation of the
OTDR signal in the upward or downward direction called?
A point defect.
Why is a dead-zone fiber placed between the test fiber
and OTDR when conducting attenuation measurements?
To reduce the effect of the initial reflection at the OTDR.
The amount of backscattered optical power at each point
depends on what two properties?
Forward optical power and backscatter capture coefficient.
How can test personnel eliminate the effects of
backscatter variations?
By performing the OTDR attenuation measurements in each
direction along the test fiber.
If the length of the fiber point defect changes with
pulse duration, is the OTDR signal deviation a point defect
or a region of high fiber attenuation?
A point defect.
Give the type of fault (reflective or nonreflective)
normally produced by: (a) fiber breaks, (b) fiber cracks,
and (c) fiber microbends.
(a) Reflective, (b) nonreflective, and (c) nonreflective
Explain how a point defect may exhibit an apparent gain.
A point defect may exhibit apparent gain because the
backscatter coefficient of the fiber present before the
point defect is higher than that of the fiber present after.
A point defect exhibiting an apparent gain in one
direction will exhibit what, when measured in the opposite
direction?
An exaggerated loss.
When is an optical power meter measurement recommended
for conducting field measurements on installed optical
fiber cables or cable plants?
When an installed optical fiber cable or cable plant is less
than 50 meters in length.
If an installed optical fiber cable does not have
connectors or terminations on both ends, how should the
cable be tested?
With an OTDR unless it is less than 50 meters in length.
If it is less than 50 meters in length, continuity should
be verified with a flashlight.
What are the three parts of a fiber optic transmitter?
Interface circuit, source drive circuit, and an optical
source.
Which part of a fiber optic transmitter converts the
processed electrical signal to an optical signal?
The optical source.
LEDs operating at 850 nm provide sufficient optical power
for short-distance, low-bandwidth multimode systems. List
three conditions that prevent the use of LEDs in longer
distance, higher bandwidth multimode systems.
Multimode fiber dispersion, the relatively high fiber
attenuation, and the LED’s relatively low optical output
power.
Why can multimode graded-index fiber 1300-nm systems
using LEDs operate over longer distances and at higher
bandwidths than 850-nm systems?
Longer distances and higher bandwidths are possible because
fiber material losses and dispersion are significantly
reduced at the 1300-nm region.
Semiconductor LEDs emit incoherent light. Define
incoherent light.
Light waves that lack a fixed-phase relationship
Which semiconductor sources (LD or LED) emit more focused
light and are capable of launching sufficient optical power
into both single mode and multimode fibers?
LDs.
The amount of optical power coupled into an optical fiber
depends on what four factors?
(1) The angles over which the light is emitted. (2) The size
of the source’s light-emitting area relative to the fiber
core size. (3) The alignment of the source and fiber. (4)
The coupling characteristics of the fiber (such as the NA
and the refractive index profile).
What are the two most common semiconductor materials
used in electronic and electro-optic devices?
Silicon and gallium arsenide.
What is a laser?
A laser is a device that produces optical radiation using
stimulated emission rather than spontaneous emission.
Describe stimulated emission.
A photon initially produced by spontaneous emission in the
active region interacts with the lasermaterial to produce
additional photons.
What are the three basic LED types?
Surface-emitting LEDs (SLEDs), edge-emitting LEDs (ELEDs),
and superluminescent diodes (SLDs).
Which types of LEDs are the preferred optical sources
for short-distance, low-data-rate fiber optic systems?
SLEDs and ELEDs.
What are facets?
Cut or polished surfaces at each end of the narrow active
region of an ELED.
What is lowest current at which stimulated emission
exceeds spontaneous emission in a semiconductor laser
called?
Threshold current.
Describe the output of a laser diode.
The LD’s output has a narrow spectral width and small output
beam angle.
Which type of optical source usually lacks reflective
facets and in some cases are designed to suppress
reflections back into the active region?
LED.
Which type of optical source tends to operate at higher
drive currents to produce light? Q18. Are the effects of
temperature changes on LDs more or less significant than
for LEDs?
Laser.
Specify the mechanism that SLDs lack that is required
by laser diodes to achieve lasing.
SLDs have no built-in optical feedback mechanism.
How does the source drive circuit intensity modulate
the source?
By varying the current through the source
What is a prebias?
A current applied in the laser off state just less than the
threshold current.
Is the drive circuitry generally more complex for an
LED or a laser diode? Why?
For a laser diode. The laser diode transmitter generally
contains output power control circuitry and may contain a
TE cooler and some circuitry associated with the TE cooler.
What are the two types of output interfaces for fiber
optic transmitters?
Optical connectors and optical fiber pigtails.
List five common fiber optic transmitter packages.
TO can, DIP, butterfly lead microcircuits, circuit cards,
and stand-alone optical fiber converters.
What type of source is typically used in low-data-rate
digital applications?
LED.
Why would a laser diode be used in a low-data-rate
digital application?
When extremely high transmitter output powers are required.
What type of source is generally used in high-data-rate
digital applications?
Laser diode.
Why are LEDs preferred over laser diodes for low- and
moderate-frequency analog applications?
LEDs require less complex circuitry than lasers.
What is a fiber optic receiver?
An electro-optic device that accepts optical signals from
an optical fiber and converts them into electrical signals.
Which part of the receiver amplifies the electrical
signal to a level suitable for further signal processing?
Amplifier.
Which performance parameter is the minimum amount of
optical power required to achieve a specific bit-error rate
(BER) in digital systems or a given signal-to-noise ratio
(SNR) in analog systems?
Receiver sensitivity.
Define receiver dynamic range.
The range of optical power levels over which the receiver
operates within the specified values. It usually is
described by the ratio of the maximum input power to the
sensitivity.
Describe the operation of an optical detector.
It is a transducer that converts an optical signal into an
electrical signal. It does this by generating an electrical
current proportional to the intensity of incident optical
radiation.
For efficient operation, should a detector have a high
or low responsivity at the operating wavelength?
High.
List the two principal optical detectors used in fiber
optic systems.
The semiconductor positive-intrinsic-negative (PIN)
photodiode and avalanche photodiode (APD).
What are the four most common materials used in
semiconductor detector fabrication?
Silicon, gallium arsenide, germanium, and indium
phosphide.
What is a photocurrent?
The current produced when photons are incident on the
detector active area.
Define responsivity.
The ratio of the optical detector’s output
photocurrent in amperes to the incident optical power in
watts.
How are PIN photodiodes usually biased?
Reverse-biased.
What is the dark current?
The leakage current that continues to flow through a
photodetector when there is no incident light.
Will dark current increase or decrease as the
temperature of the photodiode increases?
Increase.
Should the capacitance of the photodetector be kept
small or large to prevent the RC time constant from limiting
the response time?
Small.
Trade-offs between competing effects are necessary for
high speed response. Which competing effect (fast transit
time, low capacitance, or high quantum efficiency) requires
a thin active area?
Fast transit time.
Why is detector saturation not generally a problem in
fiber optic communications systems?
Because fiber optic communications systems operate at
low optical power levels.
Describe avalanche multiplication.
The electrons initially generated by the incident photons
accelerate as they move through the APD active region. As
these electrons collide with electrons in the semiconductor
material, they cause a fraction of them to become part of
the photocurrent.
How can the gain of an APD be increased?
By increasing the reverse-bias voltage.
Which amplifier stage (the preamplifier or the
postamplifier) is a dominant contributor of noise
and significantly influences the sensitivity of the
receiver?
The preamplifier.
List the key operational parameters used to define
receiver performance.
Receiver sensitivity, bandwidth, and dynamic range.
List the main types of receiver noise.
Thermal noise, dark current noise, and quantum noise.
What is the main factor that determines receiver
sensitivity?
Noise.
For a reduction in thermal noise, should the value of
the detector’s load resistor be increased or decreased?
For a reduction in thermal noise, should the value of
the detector’s load resistor be increased or decreased?
What are two types of noise that manifest themselves
as shot noise?
Dark current and quantum noises.
What are the two basic types of preamplifiers used in
fiber optic receivers?
The high-impedance amplifier and the transimpedance
amplifier.
Which preamplifier design (high-impedance or
transimpedance) provides improvements in bandwidth and
greater dynamic range with some degradation in sensitivity
from an increase in noise?
Transimpedance.
For what types of applications are APDs generally used?
For high-data-rate applications and for low- or
moderate-data-rate applications where receivers with
extremely low sensitivities are required.
Why is a low-pass filter generally part of a digital
fiber optic receiver?
To smooth the amplified signal to remove some of the high
frequency noise before the signal is further processed.
List four system topologies that can be constructed using
point-to-point fiber optic links.
Linear bus, ring, star, and tree topologies
Which topology (linear bus, ring, star, or tree) consists of equipments attached to one another in a closed loop?
Ring.
Which topology (bus, ring, star, or tree) has a center hub interconnecting the equipments?
Star.
Define modulation.
The process of varying one or more characteristics of an
optical signal to encode and convey information.
What is a digital signal?
A discontinuous signal that changes from one state to
another in discrete steps.
In NRZ code, does the presence of a high-light level in
the bit duration represent a binary 1 or a binary 0?
Binary 1.
How can the loss of timing occur in NRZ line coding?
If long strings of 1s or 0s are present causing a lack of
level transitions.
How is a binary 1 encoded in RZ line coding?
A half-period optical pulse present in the first half of
the bit duration.
In Manchester encoding, does a low-to-high light level
transition occurring in the middle of the bit duration
represent a binary 1 or a binary 0?
Binary 0.
What is an analog signal?
A continuous signal that varies in a direct proportion to
the instantaneous value of a physical variable.
What type of modulation do most analog fiber optic
communications systems use?
Intensity modulation.
Why has the transmission of video using analog
techniques been very popular, especially for shorter
distances?
Because cost can be minimized and complex multiplexing and
timing equipment is unnecessary.
Why is it generally only necessary to refer to
point-to-point data links when discussing the process of
fiber optic system design?
Because fiber optic systems that incorporate complex
architectures can be simplified into a collection of
point-to-point data links before beginning the design
process.
List five system design parameters considered when
system designers choose the system operational wavelength
and link components.
Launch power, connection losses, bandwidth, cost, and
reliability.
What two analyses are performed to determine if a link
design is viable?
Power budget and risetime budget.
Optical fibers or cables should never be bent at a radius
of curvature smaller than a certain value. Identify this
radius of curvature.
Minimum bend radius.
List five precautions to take when installing fiber
optic systems on board naval ships.
a. Never bend an optical fiber or cable at a radius of
curvature less than the minimum bend radius.
b. Never pull fiber optic cables tight or fasten them over
or through sharp corners or cutting edges.
c. Always clean fiber optic connectors before mating.
d. Do not kink or crush fiber optic cable during
installation of the hardware.
e. Allow only trained, authorized personnel to install or
repair fiber optic systems.
