Unit-5
Power Launching And Coupling
Subject: Fiber Optic Communication (3161005)
Semester-6th
Electronics & Communication Engineering Department
Government Engineering College, Bharuch
“He Who Can Listen To The Music
In The Midst of Noise Can Achieve
Great Things.”
Dr. Vikram Sarabhai
(1919 – 1971)
Physicist & Astronomer (India)
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Contents
▪ Introduction
▪ Source to fiber power launching
▪ Lensing schemes
▪ Fiber-to-fiber joints
▪ LED coupling to single mode fibers
▪ Fiber splicing
▪ Optical fiber connectors
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Introduction
▪ In implementing an optical fiber link, two of the major system questions are:
1. How to launch optical power into a particular fiber from some type of luminescent
source ?
2. How to couple optical power from one fiber into another.
▪ Launching optical power from a source into a fiber needs considerations such as:
✓ The numerical aperture,
✓ Core size,
✓ Refractive-index profile,
✓ Core-cladding index difference of the fiber,
✓ The size, radiance, and angular power distribution of the optical source.
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Introduction
▪ A measure of the amount of optical power emitted from a source that can be coupled
into a fiber is usually given by the coupling efficiency η defined as:
𝑃𝐹 (𝑝𝑜𝑤𝑒𝑟 𝑐𝑜𝑢𝑝𝑙𝑒𝑑 𝑖𝑛𝑡𝑜 𝑡ℎ𝑒 𝑓𝑖𝑏𝑒𝑟)
η=
𝑃𝑆 (𝑝𝑜𝑤𝑒𝑟 𝑒𝑚𝑖𝑡𝑡𝑒𝑑 𝑓𝑟𝑜𝑚 𝑡ℎ𝑒 𝑠𝑜𝑢𝑟𝑐𝑒)
Here,
✓ PF = The power coupled into the fiber
✓ PS = The power emitted from the light source.
▪ The launching or coupling efficiency depends on:
1. The type of fiber that is attached to the source
2. The coupling process (for example, whether or not lenses or other coupling
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improvement schemes are used.)
Introduction
▪ In practice, many source suppliers offer devices with a
short length of optical fiber (1 m or less) already
attached in an optimum power-coupling configuration.
▪ This section of fiber is generally referred to as a flylead
or a pigtail.
▪ Launching optical power from a fiber to fiber needs
considerations such as:
1. Fiber misalignments
2. Different core sizes
3. Numerical apertures
4. Core refractive-index profiles
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Source-to-Fiber Power Launching
▪ Optical output of a light source is
usually measured by its radiance
(or brightness) B at a given diode
current.
▪ Radiance: It is the optical power
radiated into a unit solid angle
per unit emitting surface area.
▪ It is generally specified in terms
of watts per square centimeter per
steradian.
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Source Output Pattern
▪ To
determine
the
optical
power-accepting
capability of a fiber, the spatial radiation pattern
of the source must first be known.
▪ The spatial radiation pattern of the source can be
defined as shown in diagram and characterized
by R, θ and ϕ with the polar axis.
▪ The radiance may be a function of both θ and ϕ
and can also vary from point to point on the
emitting surface.
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Radiation Pattern for Surface Emitting LED
▪ Surface-emitting LEDs are characterized by their
Lambertian output pattern, which means the source
is equally bright when viewed from any direction.
▪ The power delivered at an angle θ, measured
relative to a normal to the emitting surface, varies as
cos(θ) because the projected area of the emitting
surface varies as cos(θ) with viewing direction.
▪ The emission pattern for a Lambertian source thus
follows the relationship:
𝑩 𝜽, ∅ = 𝑩𝟎 𝑪𝒐𝒔(𝜽)
✓ Where B0 is the radiance along the normal to the
radiating surface.
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Radiation Pattern for Edge Emitting LED
▪ Edge-emitting LEDs and laser diodes have a more complex emission pattern.
▪ These devices have different radiances B(θ, 0o) and B(θ, 90o) in the planes parallel
and normal, respectively, to the emitting-junction plane of the device.
▪ These radiances can be approximated by the general form:
▪ The integers T and L are the transverse and lateral power distribution coefficients,
respectively.
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Power-Coupling Calculation
▪ Schematic diagram of a light coupled from the
source to an optical fiber is as shown in
diagram.
▪ Light outside of the acceptance angle is lost.
▪ Consider the symmetric source of brightness
B(𝐴𝑠, Ω𝑠).
✓ 𝐴𝑠 and Ω𝑠 are the area and solid emission angle
of the source, respectively.
▪ Here, the fiber end face is centered over the
emitting surface of the source and is positioned
as close to it as possible.
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Power-Coupling Calculation
▪ The coupled power can be found using the
relationship:
✓ Where, 𝐴f and Ωf
are the area and solid
acceptance angle of fiber, respectively.
▪ If source radius (rs) < fiber-core radius (a), then
the upper integration limit rm = rs.
▪ If source radius (rs) > fiber-core radius (a), then
the upper integration limit rm = a.
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Power-Coupling Calculation
▪ Case-1: Assume, a surface-emitting LED having a radius (rs) < fiber-core radius (a),
then the upper integration limit rm = rs.
▪ As we have a Lambertian emitter 𝑩 𝜽, ∅ = 𝑩𝟎 𝑪𝒐𝒔(𝜽), so
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Power-Coupling Calculation
▪ Where the numerical aperture, 𝑁𝐴 = sin(𝜃𝐴 ) .
▪ For step-index fibers, the numerical aperture is independent of the positions θs and r on
the fiber end face, so above equation becomes (for rs < a).
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Power-Coupled: LED to Step Index Fiber
▪ For step-index fibers, the numerical aperture is independent of the positions θs and r on
the fiber end face, so above equation becomes (for rs < a):
▪ Consider now the total optical power Ps that is emitted from the source of area As is
given by;
▪ Therefore, power coupled from LED (Ps) to the step index fiber can be expressed (in
terms of Ps):
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Power-Coupled: LED to Step Index Fiber
▪ Case-2: When the radius of the emitting area is larger than the radius a of the fiber-core
area. [rs > a], then the upper integration limit rm = a.
▪ Power coupled from LED to step index fiber:
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Power-Coupled: LED to Graded-Index Fiber
▪ In the case of a graded-index fiber, the numerical aperture depends on the distance r
from the fiber axis as given below:
▪ Case-1: The power coupled from a surface-emitting LED into a graded-index fiber
becomes (for rs < a):
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Power-Coupled: LED to Graded-Index Fiber
▪ Case-2: The power coupled from a surface-emitting LED into a graded-index
fiber for rs > a:
𝑃𝐿𝐸𝐷,𝑔𝑟𝑎𝑑𝑒𝑑
𝛼
= 2𝜋 𝑎 𝐵0 𝑛1 ∆
𝛼+2
2 2
2 2
2
𝑃𝐿𝐸𝐷,𝑔𝑟𝑎𝑑𝑒𝑑 = 𝜋 𝑎 𝐵0 𝑁𝐴(0)
2
𝛼
𝛼+2
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Equilibrium Numerical Aperture
▪ A light source may be supplied with a short (1- to 2-m) fiber flylead attached to it in
order to facilitate coupling the source to a system fiber.
▪ To achieve a low coupling loss, this flylead should be connected to a system fiber that
has a nominally identical NA and core diameter.
▪ A certain amount of optical power (ranging from 0.1 to 1 dB) is lost at this junction.
▪ In addition to the coupling loss, an excess power loss will occur in the first few tens of
meters of a multimode system fiber.
▪ This excess loss is a result of non-propagating modes scattering out of the fiber as the
launched modes come to an equilibrium condition.
▪ This loss is of particular importance for surface-emitting LEDs, which tend to launch
power into all modes of the fiber.
▪ Fiber-coupled lasers are less prone to this effect because they tend to excite fewer non5/15/2023
propagating fiber modes.
Equilibrium Numerical Aperture
▪ An example of the excess power
loss is shown in diagram in terms
of the fiber numerical aperture.
▪ At the input end of the fiber, the
light acceptance is described in
terms of the launch numerical
aperture NAin.
▪ When the optical power is
measured in long multimode fibers
after the launched modes have
come to equilibrium (which is often
taken to occur at 50 m), the effect
of the equilibrium numerical
aperture NAeq becomes important.
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Equilibrium Numerical Aperture
▪ At this point, the optical power in the fiber
is:
▪ Where P50 is the power expected in the fiber
at the 50-m point based on the launch NA.
▪ Since most optical fibers attain 80–90
percent of their equilibrium NA after about
50 m, it is the value of NAeq that is
important when calculating launched optical
power in multimode fibers.
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Lensing Schemes for Coupling Improvement
▪ If the source-emitting area is larger than the fiber-core area, then the resulting optical
power coupled into the fiber is the maximum that can be achieved.
▪ However, if the emitting area of the source is smaller than the core area, a miniature
lens may be placed between the source and the fiber to improve the power-coupling
efficiency.
▪ The function of the micro-lens is to magnify the emitting area of the source to match the
core area of the fiber end face exactly.
▪ Several Possible lensing schemes are:
1.
2.
3.
4.
5.
6.
Rounded end fiber
Non-imaging Microsphere (Small glass sphere in contact with both the fiber and source)
Imaging sphere (Used to image the source on the core area of the fiber end)
Cylindrical lens (Generally formed from a short section of fiber)
Spherical surfaced LED and spherical ended fiber
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Taper ended fiber.
Lensing Schemes for Coupling Improvement
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Lensing Schemes for Coupling Improvement
▪ Although lensing techniques can improve the source-to-fiber coupling efficiency, they
also create additional complexities(problems).
1. One problem is that the lens size is similar to the source and fiber core dimensions,
which introduces fabrication and handling difficulties.
2. In the case of the taper-ended fiber, the mechanical alignment must be carried out
with greater precision.
✓ Since the coupling efficiency becomes a more sharply peaked function of the spatial
alignment.
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Fiber-to-Fiber Joints
▪ Interconnecting fibers in a fiber optic system is another very important factor.
▪ These inter connection should be low-loss.
▪ These interconnects occur at:
1. Optical source
2. Photodetector
3. Within the cable where two fibers are connected
4. Intermediate point in a link where two cables are connected
▪ The connection can be:
1. Permanent bond: known as SPLICE
2. Easily demountable connection: known as CONNECTOR
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Mechanical Misalignment
▪ Mechanical alignment is the major problem when joining two fibers considering their
microscopic size.
✓ A standard multimode graded-index fiber core is 50–100 µm in diameter, which is
roughly the thickness of a human hair.
✓ A single-mode fibers have core diameters on the order of 9 µm.
▪ Radiation losses result from mechanical misalignments because the radiation cone of
the emitting fiber does not match the acceptance cone of the receiving fiber.
▪ The magnitude of the radiation loss depends on the degree of misalignment.
▪ Three different types of misalignment can occur:
1. Axial displacement or lateral displacement
2. Longitudinal Separation
3. Angular misalignment
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Mechanical Misalignment
▪ Three different types of misalignment can occur:
1. Axial displacement or lateral displacement
2. Longitudinal Separation
3. Angular misalignment
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Fiber End Face Preparation
▪ Fiber end face preparation is the first
step before splicing or connecting the
fibers through connectors.
▪ Fiber end must be:
✓ Flat
✓ Perpendicular to the fiber axis
✓ Smooth
▪ Techniques used for fiber Endpreparation are:
✓ Sawing, Grinding and polishing
✓ Controlled fracture
✓ Laser cleaving
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Fiber End Face Preparation: Grinding and Polishing
▪ This techniques can produce a very smooth surface that is
perpendicular to the fiber axis.
▪ This method is quite time-consuming and requires a fair
amount of operator skill.
▪ Although it is often implemented in a controlled
environment such as a laboratory or a factory, it is not
readily adaptable for field use.
▪ The procedure employed in the grinding and polishing
technique is to use successively finer abrasives to polish the
fiber end face.
▪ The end face is polished with each successive abrasive until
the scratches created by the previous abrasive material are
replaced by the finer scratches of the present abrasive.
▪ The number of abrasives used depends on the degree of
smoothness that is desired.
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Fiber End Face Preparation: Controlled-Fracture
▪ These techniques are based on score (small
cut) -and-break methods for cleaving fibers.
▪ The score-and-break method consists of:
1. Lightly scoring (nicking) the outer
surface of the optical fiber
2. Placing it under tension until it breaks.
▪ A heavy metal or diamond blade is used to
score the fiber.
▪ Once the scoring process is complete, fiber
tension is increased until the fiber breaks.
▪ The fiber is placed under tension either by
pulling on the fiber or by bending the fiber
over a curved surface.
▪ It requires a careful control of the curvature
and the tension.
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Fiber Splicing
▪ A fiber splice is a permanent or semipermanent joint between two fibers.
▪ These are typically used to create long optical links or in situations where frequent
connection and disconnection are not needed.
▪ Three different types of splicing can be done:
1. Fusion splicing
2. V-groove mechanical splicing
3. Elastic tube splice
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Fiber Splicing: Fusion Splicing
▪ It is the thermal bonding of two prepared fiber ends as shown in a diagram.
▪ In this method, the fiber ends are first pre-aligned and butted together using either
grooved fiber holder or under a micro scope with micromanipulators.
▪ The butt joint is then heated with an electric arc or a laser pulse so that the fiber ends are
momentarily melted and hence bonded together.
▪ This technique can produce very low splice losses.
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Fiber Splicing: V-Groove Splicing
▪ Here, the prepared fiber ends are first
butted together in a V-shaped groove, as
shown in diagram.
▪ They are then bonded together with an
adhesive or are held in place by means of a
cover plate.
▪ The V-shaped channel can be either a
grooved silicon, plastic, ceramic, or metal
substrate.
▪ The splice loss in this method depends
strongly on the fiber size (outside
dimensions and core-diameter variations)
and eccentricity (the position of the core
relative to the center of the fiber).
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Fiber Splicing: Elastic Tube Splicing
▪ It is a unique device that automatically performs lateral, longitudinal, and angular
alignment.
▪ It splices multimode fibers having losses in the same range as commercial fusion splices,
but much less equipment and skill are needed.
▪ The splice mechanism is basically a tube made of an elastic material.
▪ The central hole diameter is slightly smaller than that of the fiber to be spliced and is
tapered on each end for easy fiber insertion.
▪ When a fiber is inserted, it expands the hole diameter so that the elastic material exerts a
symmetrical force on the fiber.
▪ This symmetry feature allows an accurate and automatic alignment of the axes of the two
fibers to be joined.
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Optical Fiber Connectors
▪ A wide variety of optical fiber connectors has evolved for numerous different applications.
▪ Their uses range from simple single-channel fiber-to-fiber connectors to a multichannel
connectors used in harsh military field environments.
▪ Some of the principal requirements of a good connector design are as follows:
1. Low coupling losses:
✓ The connector assembly must maintain low losses.
✓ These low losses must not change significantly during operation or after numerous connects and
disconnects.
2. Interchangeability:
✓ Connectors of the same type must be compatible from one manufacturer to another.
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Optical Fiber Connectors
3. Ease of assembly:
✓ A service technician should be able to install the connector easily in a field environment.
✓ The connector loss should also be fairly insensitive to the assembly skill of the
technician.
4. Low environmental sensitivity:
✓ Conditions such as temperature, dust, and moisture should have a small effect on connector-
loss variations.
5. Low cost and reliable construction:
✓ The connector must have a precision suitable to the application, but its cost must not be a major
factor in the fiber system.
6. Ease of connection:
✓ Generally, one should be able to mate and demate the connector, simply, by hand.
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Optical Fiber Connectors
▪ Connectors are available in designs that screw on, twist on, or snap into place.
▪ The most commonly used connectors are the twist-on and snap-on design.
▪ The basic coupling mechanisms used in these connectors belongs to:
1. The butt-joint connector
2. The expanded-beam connector
▪ The majority of connectors uses a butt-joint coupling
mechanism.
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Optical Fiber Connectors: Butt-Joint Connector
▪ The key components are:
1. The ferrule (A long, thin stainless steel,
glass, ceramic, or plastic cylinder)
2. The precision sleeve into which the
ferrule fits
▪ The center of the ferrule has a hole that
precisely matches the size of the fiber
cladding diameter.
▪ The fiber is epoxied (bonded) into a
precision hole which has been drilled into
the ferrule.
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Optical Fiber Connectors: Expanded-Beam Connector
▪ Expanded beam connector employs lenses on
the end of the fibers.
▪ The lenses collimate the light emerging from
the transmitting fiber and focuses the beam on
the receiving fiber.
▪ The fiber to lens distance is equal to the focal
length.
▪ As the beam is collimated so even a separation
between the fibers will not make a difference.
▪ Connector is less dependent on the lateral
alignment.
▪ In addition, optical processing elements, such
as beam splitters and switches, can easily be
inserted into the expanded beam between the
fiber ends.
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Optical Fiber Connectors: Types
▪ Six widely used connector types with their main features and applications are as given
below:
Connector Type
Features
Applications
ST (Straight Tip)
Uses a ceramic ferrule and a rugged metal Designed for distribution applications
housing.
using either multimode or singlemode fibers.
SC (Square
Connector)
• Designed by NTT for snap-in connection
in tight spaces.
• Uses a ceramic ferrule in simplex or
duplex plastic housings for either
multimode or single-mode fibers.
LC (Lucent
Connector)
SFF (Small-Form-Factor) connector that Available in simplex and duplex
uses a standard RJ-45 telephone plug configurations for CATV, LAN,
housing and ceramic ferrules in simplex or MAN, and WAN applications.
duplex plastic housings.
Widely used in Gigabit Ethernet,
ATM, LAN, MAN, WAN, data
communication, Fiber Channel, and
telecommunication networks.
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Optical Fiber Connectors: Types
Connector Type
Features
Applications
MU (Miniature Unit) SFF connector based on a 1.25 Used mainly in Japan. Suitable for boardmm ceramic ferrule and a single mounted applications and for distributionfree-floating ferrule.
cable assemblies.
MT-RJ (Media
Termination—
Recommended Jack)
SFF connector with two fibers in Applications are for MANs and LANs, such as
one molded plastic ferrule and an horizontal optical cabling to the desktop.
improved RJ-45 latch mechanism.
MPO/MTP
(Multiple-Fiber,
Push-On/Pull-Off)
Can house up to twelve multimode Allows high-density connections between
or single-mode optical fibers in a network equipment in telecom rooms.
single compact ferrule.
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Optical Fiber Connectors: Types
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Optical Fiber Connectors: Types
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Optical Fiber Connectors: Types
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GTU Asked Questions
Sr. No.
Question
Marks
Year
1
List the most common type of mechanical misalignment occurring between
two joined fibers. Explain in brief anyone.
3
S2022
2
Explain following terms:
1) Power launching
2) Coupling efficiency
4
S2022
3
“The optical power launched into a fiber does not depend on the wavelength of
the source but only on it’s brightness” Justify.
4
W2021
4
Derive the equation for the power launched from LED Source in to a G.I. fiber.
7
W2021
5
List the different types of lensing schemes used in optical system.
3
W2021
6
Explain V grove fiber splicing technique.
4
W2021
7
Explain mechanical misalignment in fiber joining process.
3
W2021
8
Explain types of Mechanical Misalignment occur during fiber joining process.
3
W2022
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GTU Asked Questions
Sr. No.
Question
Marks
Year
9
Explain different fiber ‘end face’ preparation techniques.
3
W2022
10
Sketch and explain the different lensing scheme to improve coupling
efficiency.
7
W2022
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