UNIT – 3

Fiber Splicing

A Sanyasi Rao
Assoc. Prof., Dept. of ECE
Introduction
• Fiber optic splicing is an important method of joining two fiber optic
cables together.
• It is a preferred solution when an available fiber optic cable is not
sufficiently long enough for the required distance.
• Splicing is also designed to restore fiber optic cables when they are
accidentally broken.
• Typically, a splice is used outside the buildings and connectors are used to
join the cables within the buildings.
• Splices offer lower attenuation and lower back reflection than connectors
and are less expensive.

There are two main types of Splicing:

1. Fusion Splicing
2. Mechanical Splicing / V-groove
Fusion Splicing
• Fusion splicing is a permanent connection of two or more optical fibers
by welding them together using an electronic arc.
• It is the most widely used method of splicing as it provides for the lowest
loss, less reflectance, strongest and most reliable joint between two fibers.
• When adopting this method, fusion splicing machines are often used.
•Fusion Splicing involves butting two cleaned fiber end faces and heating
them until they melt together or fuse.
•Fusion Splicing is normally done with a fusion splices that controls the
alignment of the two fibers to keep losses as low as 0.05dB.
Generally, there are four basic steps in the fusion splicing process.
Step 1: strip the fiber
The splicing process begins with the preparation for both fibers ends to be
fused. So you need to strip all protective coating, jackets, tubes, strength
members and so on. And you just leave the bare fiber showing. It is noted
that the cables should be clean.
Step 2: cleave the fiber
A good fiber cleaver is crucial to a successful fusion splice. The cleaver
merely nicks the fiber and then pulls or flexes it. So to cause a clean break
rather than cut the fiber. The cleave end-face should be perfectly flat and
perpendicular to the axis for a proper splice.
Step 3: fuse the fiber
When you fuse the fiber, there are two important steps: aligning and
melting. Firstly, you need to align the ends of the fiber within the fiber
optic splicer. After proper alignment, you need to utilize an electrical arc to
melt the fibers. So you can permanently weld the two fiber ends together.
Step 4: protect the fiber
A typical fusion splice has a tensile strength between 0.5 and 1.5 lbs. And it
is not easy to break during normal handling. However, it still requires
protection from excessive bending and pulling forces.  By using heat shrink
tubing, silicone gel and mechanical crimp protectors can keep the splice
from outside elements and breakage.

https://youtu.be/ekzlonBS7d8
Mechanical Splicing
•If you want to make splices quickly and easily, the mechanical splice is a
better choice.
•A mechanical splice is a junction of two or more optical fibers.
•The fibers are aligned and held in place by a self-contained assembly.
•A typical example of this method is the use of connectors to link fibers.
•This method is most popular for fast, temporary restoration and splicing
multimode fibers.

https://youtu.be/E2jTjeNeyDA
Step 1: strip the fiber
Fiber preparation is practically the same as that of fusion splicing. Just
remove the protective coatings, jackets, tubes, and strength members. So
we can see the bare fiber. Then ensure the cleanliness of the fiber.
Step 2: cleave the fiber
The process is the same as the cleaving of fusion splicing. It is necessary to
obtain a cut on the fiber exactly at right angles to the axis.
Step 3: mechanically join the fiber
In this step, we don’t use heating as infusion splice. We simply connect the
fiber ends together inside the mechanical splice unit. The index matching gel
inside the mechanical splice apparatus is helpful. Because it can couple the
light from one fiber end to another.
Step 4: protect the fiber
Once fibers are spliced, we place them in a splice tray and then in a
splice closure. Outside plant closures don’t need to use heat. We
carefully seal shrink tubing to prevent moisture damage from the
splices.
Mechanical Splicing may have a slightly higher loss and back reflection.
These can be reduced by inserting index matching gel.

V-Groove mechanical Splicing provides a temporary joint i.e., fibers can be

disassembled if required. The fiber ends are butted together in a V-Shaped
groove.

The splice loss depends on Fiber Size and Eccentricity

Splicing Single-Mode Fibers
• In single mode fibers the lateral (axial) offset loss presents the most
serious misalignment. This loss depends on the shape of the propagating
mode.
• For Gaussian-shaped beams the loss between identical fibers is

Where
the spot size w is the mode-field radius,
d is the lateral displacement
Since the spot size is only a few micrometers in single – mode fibers, low
loss coupling requires a very high degree of mechanical precision in the
axial dimension.

https://youtu.be/8f8xtj_mcMg
Fiber Alignment & Joint Loss

Fiber splices: These are semi permanent or permanent joints which

find major use in most optical fiber telecommunication systems
(analogous to electrical soldered joints).

Demountable Fiber connectors or simple connectors: These are

removable joints which allow easy, fast, manual coupling and
uncoupling of fibers (analogous to electrical plugs and sockets).

A crucial aspect of fiber joint concerns the optical loss associated with
the connection.

A major consideration with all types of fiber-fiber connection is the

optical loss encountered at the interface.
• Fiber-to-fiber connection loss is affected by intrinsic & extrinsic coupling
losses.
• Intrinsic coupling losses are caused by inherent fiber characteristics
• Extrinsic coupling losses are caused by jointing techniques.
• Sources of loss in Fiber-to-fiber joint:
1. Fresnel Loss
2. Fiber Core Misalignment
- End separation
- Lateral misalignment
- angular misalignment
3. Fiber Parameter mismatch
-Cross section mismatch
- NA mismatch
- Core concentricity
• Intrinsic coupling losses are limited by reducing fiber mismatches between
the connected fibers
• This is done by procuring only fibers that meet stringent geometrical and
optical specifications
• Extrinsic coupling losses are limited by proper connection procedures

Losses due to Fresnel Reflection

• A small proportion of the light may be reflected back into the
transmitti ng fiber causing attenuation at the joint. This
phenomenon, known as Fresnel reflection
• Associated with the step changes in refractive index at the
jointed interface (i.e. glass–air–glass).
• The magnitude of this partial reflection of the light
transmitted through the interface is given by equation
 n1  n 
r   
n
 1  n 
Where
r – the fraction of light reflected at a single interface
n1 – refractive index of the fiber core
n – refractive index of the medium between two jointed fibers (for
air, n=1)

• In order to determine the amount of light reflected at a fiber joint,

Fresnel reflection at both fiber interfaces must be taken into account.
• The loss in decibels due to Fresnel reflection at a single interface is
given by
Loss Fres  10 log 10 1  r  ( 2)

The effect of Fresnel reflection at a fiber–fiber connection can be

reduced by the use of an index-matching fluid between the gap.
Losses due to Geometric Variations

Misalignment may occur in three dimensions:

• The separation between the fibers (longitudinal misalignment)
• The offset perpendicular to the fiber core axes (lateral
misalignment)
• Angle between the core axes (angular misalignment).
Losses due to Optical Parameter Variations

There are inherent connection problems when jointing fibers with, for
instance
- different core and/or cladding diameters
- different numerical apertures and /or relative refractive differences
- different refractive index profiles
- fiber faults (core ellipticity, core concentricity etc.

The losses caused by the above factors together with these of Fresnel
reflection are usually referred to as intrinsic joint losses.
The best results are achieved with compatible (same) fibers which are
manufactured to the lowest tolerance.
Intrinsic Losses:
Losses due to:
→ Fresnel Reflection
→ Deviation in Geometrical & Optical parameters
Minimized using fibers manufactured with lowest tolerance i.e.(same
fiber)
Intrinsic coupling losses are limited by reducing fiber mismatches between
the connected fibers.
Extrinsic Losses:
Extrinsic coupling losses are caused by jointing techniques. Fiber-to-fiber
connection loss is increased by the following sources of intrinsic and
extrinsic coupling loss:
→ Reflection losses
→ Fiber separation
→ Lateral misalignment
→ Angular misalignment
→ Core and cladding diameter mismatch
→ Numerical aperture (NA) mismatch
→ Refractive index profile difference
→ Poor fiber end preparation
→ Losses due to some imperfection in splicing
→ Caused by Misalignment
Multimode Fiber Joints
• Lateral misalignment reduces the overlap region between the
two fiber cores.
• The lateral coupling efficiency for two similar step index fibers (in a
multimode step index fiber) is given by equation

• The lateral misalignment loss in dB is given by equation

where n1 is the core refractive index, n is the refractive index of the

medium between the fibers, y is the lateral offset of the fiber core
axes, and a is the fiber core radius.
Lateral misalignment loss in multimode graded index fibers is
given by equation

Angular misalignment losses at joints in multimode step index

fibers is given by equation

where θ is the angular displacement in radians and Δ is the

relative refractive index difference for the fiber.
Losses due to Geometric Variations at a Multimode Fiber Joint

loss resulting from a mismatch of core diameters is given by equation

loss caused by a mismatch of numerical apertures is given by equation

A mismatch in refractive index profiles results in a loss given by:

The total intrinsic losses obtained at multimode fiber–fiber joints

provided by the following equation
Optical Sources
• Two most popular optical sources are LED & LASER Diodes
• LEDs are used in sort and medium distance communication systems
where power requirement is small and low bit rate.
• LASER Diodes are used for long distance and High bit rate applications.
• LEDs are associated with multimode fibers, where as LASERs are
associated with single mode fibers.
• Optical source is the major component in an optical transmitter.
• Optical transmitter converts electrical input signal into corresponding
optical signal. The optical signal is then launched into the fiber.
Characteristics of Light Source of Communication

To be useful in an optical link, a light source needs the following

characteristics:
i) It must be possible to operate the device continuously at a variety of
temperatures for many years.
ii) It must be possible to modulate the light output over a wide range of
modulating frequencies.
iii) For fiber links, the wavelength of the output should coincide with one
of transmission windows for the fiber type used.
iv) To couple large amount of power into an optical fiber, the emitting area
should be small.
v) To reduce material dispersion in an optical fiber link, the output
spectrum should be narrow.
vi) The power requirement for its operation must be low.
vii) The light source must be compatible with the modern solid state
devices.
viii) The optical output power must be directly modulated by varying the
input current to the
device.
ix) Better linearity of prevent harmonics and Intermodulation distortion.
x) High coupling efficiency.
xi) High optical output power.
xii) High reliability.
xiii) Low weight and low cost.
Classification of Sources

Gas Sources (LASER)

- High Output
-- Narrow Spectral width (0.1 – 3nm)
-- Directional Radiation
- Costly

Semiconductor Sources (LED/ILD)

- Low Output Power
-- Broad Spectrum width (30-40 nm)
- Non Directional Radiation Pattern
Basic Concepts of Absorption & Emission of Semiconductor Substances

Stimulated Absorption
Spontaneous Emission (supported by LED, results in incoherent
radiation)
Stimulated Emission (Supported by the LASER, results in to the coherent
radiation)
Light Emitting Diodes (LEDs)
p-n Junction
• Conventional p-n junction is called as homojunction as same
semiconductor material is used on both sides junction. The electron-hole
recombination occurs in relatively wide layer = 10 μm. As the carriers are
not confined to the immediate vicinity of junction, hence high current
densities can not be realized .

• The carrier confinement problem can be resolved by sandwiching a thin

layer (= 0.1 μm) between p-type and n-type layers. The middle layer may or
may not be doped. The carrier confinement occurs due to band gap
discontinuity of the junction. Such a junction is called heterojunction and
the device is called double heterostructure.

•An LED is a semiconductor forward biased P-N Junction diode is shown in

fig.
When no bias voltage present, the energy barrier prevents the
movement of the charge carriers (electrons & holes) as shown in fig. with
Energy Level diagram.
• When a forward voltage is applied, the movement of charge carriers
takes place from N to P and P to N regions. As a result some of the charge
carriers recombine in the transition region.
• The energy lost in the transition is converted to optical energy which rise
to a photon.
The wavelength of emission is calculated from the relation

c
Eg  hf  h


1 . 24
 (m ) 
E g ( eV )

hc
or  
Eg

Where
h = Plank’s Constant = 6.626 x 10-34
C= velocity of light = 3x108 m/sec
LED Structures
Homo & Hetero Junctions
• Junctions between differently doped regions of the same semiconductor
material are called a homojunction, while a junction between two different
types of materials is called a heterojunction.
• A heterojunction is an interface between two adjoining single crystal
semiconductors with different band gap.
• Heterojunction are of two types, Isotype (n-n or p-p) or Antisotype (p-n).

Draw backs of Homo Junction

• The electron hole recombination takes place in various directions over a
large area. So it requires a high current density to support the desired
level of radiated power.
• Secondly, this type of LED radiates a broad light beam. This makes the
coupling of light into an optical fiber extremely inefficient.
 Some Basic Definitions
Radiance: Radiance is a measure of the optical power radiated into a unit
solid angle per unit area of the emitting surface.
It is measured in Watts
It is also called as Brightness

Emission Response Time: This is the time delay between the application of
a current pulse and the emission of optical energy in the form of photons.
The time delay limits the Bandwidth with which the source can be
modulated directly by varying the injected current.

Quantum Efficiency: The Internal Quantum Efficiency in the active region is

the fraction of the electron-hole pairs that recombine radiatively.
The External Quantum Efficiency is defined as the ratio of the photons
emitted from the LED to the number of internally generated photons.
For high radiance and quantum efficiency, LED must have:
Optical Confinement: Achieve high level radiative recombination in the
active region of the device – yield high quantum efficiency.
Carrier Confinement: Preventing absorption of the emitted radiation by the
surrounding of the PN Junction.

• To obtain high radiance and high quantum efficiency, the charge carriers
and stimulated optical emission should be confined to Active region of P-
N junction where radiative recombination takes place. These are
respectively known as carrier confinement and optical confinement.
• To achieve better carrier & light confinement, the most common and
effectively used configuration is the “Double Heterojunction LED”
Double Heterostructure LEDs
A Double Heterostructure is formed when two semiconductor materials
are grown into a “sandwich”. One material (such as AlGaAs) is used for
the outer layers (or Cladding), and another of smaller band gap (such as
GaAs) is used for the inner layer.
The double heterostructure is a very useful structure in optoelectronic
devices and has interesting electronic properties.
When a current is applied to the ends of the pin structure, electrons and
holes are injected into the heterostructure.
The smaller energy gap material forms energy discontinues at the
boundaries, confining the electrons and holes to the smaller energy gap
semiconductor.
The electrons and holes recombine in the smaller energy gap
semiconductor emitting photons.

There are two principles:

1. Carrier Confinement
2. Optical Confinement
Carrier Confinement: Restricting the active region to a very small area.
How we achieve?
By placing a semiconductor having band gap energy in between layers of
another semiconductor having comparatively higher band gap energy.

Optical Confinement: Making the light in only one direction

How we achieve Carrier Confinement?
By placing a semiconductor having higher refractive index in between layers
of another semiconductor having comparatively lower refractive index.
LED Configurations
At present there are two main types of LED used in optical fiber links –
1. Surface emitting LED.
2. Edge emitting LED.
Both devices used a DH structure to constrain the carriers and the light to
an active layer.

Surface Emitting LEDs

In surface emitting LEDs the plane of active light emitting region is
oriented perpendicularly to the axis of the fiber. A DH diode is grown on
an N-type substrate at the top of the diode as shown in Fig. A circular well
is etched through the substrate of the device. A fiber is then connected to
accept the emitted light.
At the back of device is a gold heat sink. The current flows through the p-
type material and forms the small circular active region resulting in the
intense beam of light.
Diameter of circular active area = 50 μm
Thickness of circular active area = 2.5 μm
Current density = 2000 A/cm2 half-power
Emission pattern = Isotropic, 120o beam width.
The isotropic emission pattern from surface emitting LED is of Lambertian
pattern. In Lambertian pattern, the emitting surface is uniformly bright, but
its projected area diminishes as cosθ, where θ is the angle between the
viewing direction and the normal to the surface as shown in Fig. The beam
intensity is maximum along the normal.

The power is reduced to 50% of its peak when θ = 60o, therefore the total half
–power Beam width is 120o. The radiation pattern decides the coupling
efficiency of LED.
Edge Emitting LEDs (EELEDs)
In order to reduce the losses caused by absorption in the active layer and
to make the beam more directional, the light is collected from the edge of
the LED. Such a device is known as edge emitting LED or ELED.
It consists of an active junction region which is the source of incoherent
light and two guiding layers. The refractive index of guiding layers is lower
than active region but higher than outer surrounding material. Thus a
waveguide channel is form and optical radiation is directed into the fiber.
Fig. shows structure of ELED.
Edge emitter‘s emission pattern is more concentrated (directional)
providing improved coupling efficiency. The beam is Lambertian in the
plane parallel to the junction but diverges more slowly in the plane
perpendicular to the junction. In this plane, the beam divergence is limited.
In the parallel plane, there is no beam confinement and the radiation is
Lambertian.
To maximize the useful output power, a reflector may be placed at the end
of the diode opposite the emitting edge. Fig. shows radiation from ELED.
Features of ELED:
1. Linear relationship between optical output and current.
2. Spectral width is 25 to 400 nm for λ = 0.8 – 0.9 μm.
3. Modulation bandwidth is much large.
4. ELEDs have better coupling efficiency than surface emitter.
5. ELEDs are temperature sensitive.

Usage :
1. LEDs are suited for short range narrow and medium bandwidth links.
2. Suitable for digital systems up to 140 Mb/sec.
3. Long distance analog links.
Light Source Materials

The spontaneous emission due to carrier recombination is called electro

luminescence. To encourage electroluminescence it is necessary to select
as appropriate semiconductor material.
The semiconductors depending on energy band gap can be categorized
into,
1. Direct band gap semiconductors.
2. Indirect band gap semiconductors.
Some commonly used band gap semiconductors are shown in following
table:

Direct band gap semiconductors are most useful for this purpose. In
direct band gap semiconductors the electrons and holes on either side of
band gap have same value of crystal momentum. Hence direct
recombination is possible. The recombination occurs within 10-8 to 10-10
sec.
In indirect band gap semiconductors, the maximum and minimum
energies occur at Different values of crystal momentum. The
recombination in these semiconductors is quite slow i.e. 10-2 and 10-3 sec.

The active layer semiconductor material must have a direct band gap. In
direct band gap semiconductor, electrons and holes can recombine directly
without need of third particle to conserve momentum.

In these materials the optical radiation is sufficiently high. These materials

are compounds of group III elements (Al, Ga, In) and group V element (P,
As, Sb). Some ternary alloys Ga Al As are also used.
The fundamental quantum mechanical relationship between gap energy E
and frequency v is given as –

c
E  hv  h

hc
 where E is the energy in Joules
E

Expressing the gap energy (Eg) in electron volts and wavelength (λ) in
micro meters for this application

1 . 24
 ( m ) 
E g ( eV )
Planar LED
• The planar LED is the simplest of the structures that are available and is
fabricated by either liquid- or vapor-phase epitaxial processes over the
whole surface of a GaAs substrate.
• This involves a p-type diffusion into the n-type substrate in order to
create the junction illustrated in Figure.
• Forward current flow through the junction gives Lambertian spontaneous
emission and the device emits light from all surfaces.
• However, only a limited amount of light escapes the structure due to total
internal reflection and therefore the radiance is low.
Double Hetero – Junction LED: (Al – Ga – As)

Double –heterostructure (or heterojunction) device (LED) consists of the

two different Alloy Layers on each side of Active Region. This is the most
effectively used LED structure to achieve carrier and optical confinement.

Fig (a) shows the cross sectional drawing of a typical GaAlAs double
heterostructure LED. In this structure x>y to provide both carrier
confinement and optical guiding. Therefore regions 2 and 4 are also called
as Light guiding Layers and Regions 2,3 & 4 forms Double heterojunction
Layers (waveguide region). Region 3 is called Active region where photons
of energy hϒ are generated due to recombination process.
Fig (b) shows Energy band diagram which indicates the active region and the
electron and hole barriers which confine the charge carriers to the active
layer.
Fig (c) shows the variations in the refractive index. The lower Index of
refraction of the material regions 1 and 5 creates an optical barrier around
the waveguide region because of the higher band-gap energy of this
material. Therefore it confines the optical field to the central active region.
This dual confinement (charge carrier confinement & optical ) leads to both
high efficiency and high radiance.