Types of optical fiber

Types of optical fiber

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3 TYPES OF OPTICAL FIBER

3 TYPES OF OPTICAL FIBER
3.1 INTRODUCTION
3.2 OBJECTIVE
3.3 FIBRE TYPES – SINGLE MODE AND MULTI -MODE
3.4 CABLE CONSTRUCTION
3.5 TYPES OF FIBER OPTIC CABLE (MOST POPULAR FIBER OPTIC
CABLE TYPES)
3.6 ITU-T COMPLIANT FIBERS
3.7 SIGNAL STRENGTH AND QUALITY KPIS – DESIGN VALUES AND
MARGINS
3.13 SUMMARY
3.14 REFERENCES AND SUGGESTED FURTHER READINGS
3.15 KEY LEARNINGS
3.16 WORKSHEET

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3.1 Introduction
Fibers are classified according to the number of modes that they can propagate.
Single mode fibers can propagate only the fundamental mode. Multimode fibers can propagate
hundreds of modes. However, the classification of an optical fiber depends on more than the
number of modes that a fiber can propagate. An optical fiber's refractive index profile and core
size further distinguish single mode and multimode fibers. The refractive index profile describes
the value of refractive index as a function of radial distance at any fiber diameter.
3.2 Objective
After reading this unit, you should be able to understand:
 Multimode and single mode step-index and graded-index fibers.
 Identify the two basic types of single mode step-index fibers.
 Outdoor cable and Indoor cable applications.
3.3 FIBRE TYPES – SINGLE MODE AND MULT I-MODE
The refractive Index profile describes the relation between the indices of the core
and cladding. Two main relationships exist:
(I) Step Index
(II) Graded Index
The step index fibre has a core with uniform index throughout. The profile shows
a sharp step at the junction of the core and cladding. In contrast, the graded index has a
non-uniform core. The Index is highest at the center and gradually decreases until it
matches with that of the cladding. There is no sharp break in indices between the core and
the cladding.
By this classification there are three types of fibres :
(I) Multimode Step Index fibre (Step Index fibre)
(II) Multimode graded Index fibre (Graded Index fibre)

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(III) Single- Mode Step Index fibre (Single Mode Fibre)
3.3.1 STEP-INDEX MULTIMODE FIBE R
Step Index multimode Fiber has a large core, up to 100 microns in diameter. As a
result, some of the light rays that make up the digital pulse may travel a direct route,
whereas others zigzag as they bounce off the cladding. These alternative pathways cause
the different groupings of light rays, referred to as modes, to arrive separately at a
receiving point. The pulse, an aggregate of different modes, begins to spread out, losing
its well-defined shape. The need to leave spacing between pulses to prevent overlapping
limits bandwidth that is, the amount of information that can be sent. Consequently, this
type of fiber is best suited for transmission over short distances, in an endoscope, for
instance.

Fig : 1 STEP-INDEX MULTIMODE FIBER
3.3.2 GRADED-INDEX MULTIMODE FIBE R
It contains a core in which the refractive index diminishes gradually from the
center axis out toward the cladding. The higher refractive index at the center makes the
light rays moving down the axis advance more slowly than those near the cladding.

Fig : 2 GRADED-INDEX MULTIMODE FIBER
Also, rather than zigzagging off the cladding, light in the core curves helically
because of the graded index, reducing its travel distance. The shortened path and the
higher speed allow light at the periphery to arrive at a receiver at about the same time as

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the slow but straight rays in the core axis. The result: a digital pulse suffers less
dispersion.
3. SINGLE-MODE FIBER
It has a narrow core (nine microns or less), and the index of refraction between the
core and the cladding changes less than it does for multimode fibers. Light thus travels
parallel to the axis, creating little pulse dispersion. Telephone and cable television
networks install millions of kilometers of this fiber every year.

Fig : 3 SINGLE-MODE FIBER
3.4 CABLE CONSTRUCTION
There are two basic cable designs are:
1. Tight Buffer Tube Cable
2. Loose Buffer Tube Cable
Loose-tube cable is used in the majority of outside-plant installations and tight-
buffered cable, primarily used inside buildings.
1. TIGHT BUFFER TUBE CA BLE
With tight-buffered cable designs, the buffering material is in direct contact with
the fiber. This design is suited for "jumper cables" which connect outside plant cables to
terminal equipment, and also for linking various devices in a premises network. Single-
fiber tight-buffered cables are used as pigtails, patch cords and jumpers to terminate
loose-tube cables directly into opto-electronic transmitters, receivers and other active and
passive components. Multi-fiber tight-buffered cables also are available and are used
primarily for alternative routing and handling flexibility and ease within buildings. The
tight-buffered design provides a rugged cable structure to protect individual fibers during

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handling, routing and connectorization. Yarn strength members keep the tensile load
away from the fiber.

Fig : 4 Tight Buffer Tube Cable
The structure of a 250um coated fiber (bare fiber)
 Core (9um for standard single mode fibers, 50um or 62.5um for
multimode fibers)
 Cladding (125um)
 Coating (soft plastic, 250um is the most popular, sometimes 400um is
also used)
2. LOOSE-TUBE CABLE
The modular design of loose-tube cables typically holds 6, 12, 24, 48, 96 or even
more than 400 fibers per cable. Loose-tube cables can be all-dielectric or optionally
armored. The loose-tube design also helps in the identification and administration of
fibers in the system.
In a loose-tube cable design, color-coded plastic buffer tubes house and protect
optical fibers. A gel filling compound impedes water penetration. Excess fiber length
(relative to buffer tube length) insulates fibers from stresses of installation and
environmental loading. Buffer tubes are stranded around a dielectric or steel central

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member, which serves as an anti-buckling element.
The cable core, typically uses aramid yarn, as the primary tensile strength
member. The outer polyethylene jacket is extruded over the core. If armoring is required,
a corrugated steel tape is formed around a single jacketed cable with an additional jacket
extruded over the armor. Loose-tube cables typically are used for outside-plant
installation in aerial, duct and direct-buried applications.
Loose tube cable is designed to endure outside temperatures and high moisture
conditions. The fibers are loosely packaged in gel filled buffer tubes to repel water.
Recommended for use between buildings that are unprotected from outside elements.
Loose tube cable is restricted from inside building use.

Fig : 5 Loose Tube Cable
Elements in a loose tube fiber optic cable:
1. Multiple 250um coated bare fibers (in loose tube)
2. One or more loose tubes holding 250um bare fibers. Loose tubes
strand around the central strength member.
3. Moisture blocking gel in each loose tube for water blocking and
protection of 250um fibers
4. Central strength member (in the center of the cable and is stranded
around by loose tubes)

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5. Aramid Yarn as strength member
6. Ripcord (for easy removal of inner jacket)
7. Outer jacket (Polyethylene is most common for outdoor cables
because of its moisture resistant, abrasion resistant and stable over
wide temperature range characteristics.)
3.5 Types of Fiber Optic Cable (Most Popular Fiber Optic Cable
Types)
3.5.1 INDOOR CABLES
3.5.1.1 Simplex Fiber Cables
A single cable structure with a single fiber. Simplex cable varieties
include 1.6mm & 3mm jacket sizes.

Fig : 6 Simplex Fiber Cables
3.5.1.2 Duplex Fiber Optic Cable
Duplex-zip. This cable contains two optical fibers in a single cable
structure. Light is not coupled between the two fibers; typically one fiber is
used to transmit signals in one direction and the other receives.

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Fig : 7 Duplex Fiber Optic Cable
3.5.2 OUTDOOR LOOSE TUBE F IBER OPTIC CABLES
Tube encloses multiple coated fibers that are surrounded by a gel
compound that protects the cable from moisture in outside environments. Cable
is restricted from indoor use, typically allowing entry not to exceed 50 feet.

Fig : 8 Outdoor Loose Tube Fiber Optic Cables
3.5.3 AERIAL/SELF-SUPPORTING
Figure 8 (aerial/self-supporting) fiber cables are designed to be strung
from poles outdoors and most can also be installed in underground ducts. They
have internal stress members of steel of steel or aramid yarn that protect fibers
from stress.
Aerial cable provides ease of installation and reduces time and cost.
Figure 8 cable can easily be separated between the fiber and the messenger.

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Temperature range -55 to +85°C.

Fig : 9 Figure 8 cable
3.5.4 DIRECT-BURIED ARMORED FIBER OPTIC CABLE
Armored cables are similar to outdoor cables but include an outer armor
layer for mechanical protection and to prevent damage. They can be installed in
ducts or aerially, or directly buried underground. Armor is surrounded by a
polyethylene jacket.
Armored cable can be used for rodent protection in direct burial if
required. This cable is non-gel filled and can also be used in aerial applications.
The armor can be removed leaving the inner cable suitable for any
indoor/outdoor use. Temperature rating -40 to +85°C.

Fig : 10 Armored cable
3.5.5 SUBMARINE FIBER OPTI C CABLE (UNDERSEA FIBER OPTI C
CABLE)
Submarine cables are used in fresh or salt water. To protect them from
damage by fishing trawlers and boat anchors they have elaborately designed
structures and armors. Long distance submarine cables are especially complex
designed.

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Fig : 11 Submarine cables
3.6 ITU-T COMPLIANT FIBERS
3.6.1 ITU-T G.651 COMPLIANT MULTIMODE FIBERS AND O M1
Multimode fibers can be classified further into two as Multimode 50/125 and
Multimode 62.5/125. The classification is based on the core diameter of multimode
fibers. 50/125 have a core diameter of 50 micrometers, whereas for 62.5/125 have a
core diameter of 62.5 micrometers.
Recent classification of multimode fibers divides them as OM1, OM2, OM3 etc.
OM1 multimode fibers are 62.5/125 multimode fibers. OM2 and OM3 fibers are
compliant with ITU-T G.651 recommendations.
G.651 multimode fibers are used mainly in Local Area Networks (LAN).
Multimode fibers are not suitable for Long haul applications. Cheaper transmission
devices like lasers etc. make Multimode fibers attractive for short distance
transmission within the 300 to 500 meters reach.
For a 10GBASE-SR system demanding 2000 MHz*km, OM2 multimode fiber
can be used for a distance of up to 82 meters and OM3 fibers can be used for 300
meters. An OM2 fiber having a bandwidth of 500 MHz*km can be used for 550 meters
on a 10BASE-SX/LX networks.

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ITU-T does not have any specification for 62.5/125 multimode fibers. OM1 Fibers
also known as 62.5/125 Multimode fibers are popular in United States. OM2 and OM3
multimode fibers are also known as ITU-T G.651 fibers.
The core of MMF 50/125 has a graded index refractive index profile, which is
gradually changing from the center of the core to the cladding that enables multiple
modes with near equal velocity to travel inside the fiber.
3.6.2 ITU-T G.652 COMPLIANT SINGLE MODE FIBERS
This is the most common single mode fiber in the world. It is designed to have
minimum dispersion at around 1310nm, which is supposed to be transmission window for
single mode fibers. Conventional single mode fibers can be used at 1550nm with the use
of dispersion compensation modules.
G.652A is the first single mode fibers ITU-T classified. G.652B fibers are also
known as conventional type single mode fibers and many installers intend to use 652B
fiber by mentioning simply G.652. The major difference is in attenuation at both 1310nm
and 1550nm and polarization mode dispersion. 652B fibers have a PMD as low as 0.2
ps/sqrt.km where as for 652A fibers have a PMD of 0.5 ps/sqrt.km. Attenuation is low for
G.652B fibers.
Similarly G.652C and G.652D fibers differ in PMD value. PMD for G.652C fiber
is 0.5 ps/sqrt.km, where as for G.652D fibers have a PMD of less than or equal to 0.2
ps/sqrt.km. Both these optical fibers are known as low water peak fiber having low
attenuation at 1360nm through 1480nm, the wavelength range which is not yet used
commonly for transmission.
3.6.3 ITU-T G.653 COMPLIANT DISPERSION SHIFTED FIBER
These fibers are designed to utilize the low attenuation window of 1550nm by
minimizing the dispersion value at around 1550nm. The purpose was good, but it
generated Non-linear effects in the transmission which caused more troubles.
3.6.4 ITU-T G.654 COMPLIANT CUT-OFF SHIFTED FIBER

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This fiber is also known as low attenuation fiber. Some manufacturers have
extremely low attenuation at 1550nm for this fiber. The application area demands low
attenuation like those in Submarine optical fiber cables and terrestrial ultra long haul
optical networks. Low attenuation at 1550nm range makes this fiber suitable for
400km span without repeaters. The low attenuation ranges from 0.15 – 0.16 dB/km.
3.6.5 ITU-T G.655 COMPLIANT NO N-ZERO DISPERSION SHIF TED
FIBER
NZDSF was introduced in the mid 1990s for WDM applications. NZDSF is the
short of Non-zero dispersion shifted fiber. These are wide band transmission The non-
linear effects are successfully solved in G.655 fibers. The non-linear effects due to zero
dispersion at 1550nm in G.653 fibers are solved by G.655 fibers which are having a
non-zero value for dispersion at this wavelength range. ITU-T specifies up to G.655E
fibers (latest) from G.655A fibers which are not currently in use. G.655 fibers are most
suitable for DWDM applications.
It has a positive nonzero dispersion value over the entire C-band, which is the
spectral operating region for eribium doped optical fiber amplifiers.
Version G.655b was introduced to extend WDM application into the S-band.
Version G.655c specifies a lower PMD value of 0.2 ps√km than the 0.5 ps/√km
value of G.655a/b
3.6.6 ITU-T G.656 COMPLIANT LO W SLOPE DISPERSION NON-ZERO
DISPERSION SHIFTED FIBER
This is another type non-zero dispersion shifted fiber which has more stricter and
low dispersion slope which enables to guarantee the DWDM performance in wide
wavelength range.
It has a positive chromatic dispersion value ranging from 2 to 14 ps/(nm-km) in
the 1460 to 1625 nm wavelength band. Here dispersion slop is significantly lower than
in G.655 fibers

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It means that the chromatic dispersion changes slower with the wavelength so that
dispersion compensation is simpler or not needed. This allows the use of CWDM
without chromatic dispersion compensation.
3.6.7 G. ITU-T G.657 COMPLIANT BEND INSENSITIVE FIBER
G.657 fibers are the new comers in the market, but became a super hit in the FTTH
market. More and more installers are looking for G.657 fibers. As the name indicates, the
bend insensitive fibers are suitable for applications where multiple bends will be present.
Insensitivity to bends makes them suitable for installation at home and office
environment. G.657A is intended to compatible with G.652 D fibers. Interconnectivity
with the existing G.652 fibers are guaranteed for the G.657 A fibers. ITU-T G.657B
fibers are free from all backward compatibility requirements and do not require
complying with conventional single mode fibers. The difference between 657A and B
fibers is in the bending radius. G.657B can be bend at 7.5mm radius and less for some
manufacturers. Single mode optical fibers complying with ITU-T G.657A was developed
with the purpose of using at FTTH sites. G.657A category fibers are therefore compliant
with G.652 category fibers also. This back compatibility makes the G.657A category
fibers suitable for access networks used for FTTH. The other category, G.657.B does not
need to be compliant with G.652 fibers. Therefore G.657.B category fibers are mostly
used in indoor fiber optic cables that are installed with field installable optical connectors.
Features
 Low macro-bending loss at very low radii (≤ 15 mm)
 Compatibility with other G.652 single-mode fibre installations
 Low bending at partial bends in the mm bend radius range
 Low micro-bending loss
 Apart from its ideal use in office installations, as patch cords and/or
interconnection cables, the use of G.657 compliant fiber in Fiber-to-the-Home
networks offers significant added value to the network installers. Bend radii in

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fibre guidance ports can be reduced as well as minimum bend radii in wall and
corner mountings.
Applications
 The fibre is ideal for installation under tight bend conditions in CATV and
FTTH networks. Incorporates all the features of ITU-T G.652D optical fiber
including Low Water
 Peak (LWP) benefits, 1 Gb/s up to an indicative 5 km in data networks and
supports ATM, SONET and WDM technologies.
 All ITU-T G.657A cable constructions including FTTH tight buffered, loose
tube and ribbon.
 Supports high speed multi channel video, data and voice services in
metropolitan and access networks.
Table-1 Technical Specification

3.7 Signal strength and quality KPIs – design values and margins
Mode Material
Index of
Refraction
Profile
Operating
Windows(nm)
Core/Cladding Size
(microns)
Atten.
dB/km
Bandwidth
MHz/km
Multi-mode Glass Step 800 62.5/125 5.0 6
Multi-mode Glass Step 850 62.5/125 4.0 6
Multi-mode Glass Graded 850 62.5/125 3.3 200

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Multi-mode Glass Graded 850 50/125 2.7 600
Multi-mode Glass Graded 1300 62.5/125 0.9 800
Multi-mode Glass Graded 1300 50/125 0.7 1500
Multi-mode Glass Graded 850 85/125 2.8 200
Multi-mode Glass Graded 1300 85/125 0.7 400
Multi-mode Glass Graded 1550 85/125 0.4 500
Multi-mode Glass Graded 850 100/140 3.5 300
Multi-mode Glass Graded 1300 100/140 1.5 500
Multi-mode Glass Graded 1550 100/140 0.9 500
Multi-mode Plastic Step 650 485/500 240 5 @ 680
Multi-mode Plastic Step 650 735/750 230 5 @ 680
Multi-mode Plastic Step 650 980/1000 220 5 @ 680
Multi-mode PCS Step 790 200/350 10 20
Single-mode Glass Step 650 3.7/80 or 125 10 600
Single-mode Glass Step 850 5/80 or 125 2.3 1000
Single-mode Glass Step 1300 9.3/125 0.5 *
Single-mode Glass Step 1550 8.1/125 0.2 *
* Too high to measure accurately. Effectively infinite.
3.13 Summary
Optical fiber classification depends on more than the number of modes that a fiber
can propagate. The optical fiber's refractive index profile and core size further distinguish
different types of single mode and multimode fibers.
The first single-mode optical fibre was specified in Recommendation ITU-T
G.652, Characteristics of a single-mode optical fibre and cable, and for this reason, the
ITU-T G.652 fibres are often called, “standard single-mode fibres”. These fibres were the
first to be widely deployed in the public network and they represent a large majority of
fibres that have been installed. The agreements that led to the first publication of
Recommendation ITU-T G.652 formed a key foundation to the modern optical networks

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that are the basis of all modern telecommunications.
3.14 References and Suggested Further Readings
 ITU-T manual on OF installation
 EI of BSNL
 EI on underground OF cable laying works by BBNL
 Fiber Optics Technician's Manual
 Understanding optical communication by Dutton
 Planning Fiber Optic Networks by Bob Chomycz
 www.timbercon.com
 http://www.ofsoptics.com
 http://www.thefoa.org/
 http://www.corning.com
 http://www.fiber-optics.info
 http://www.rp-photonics.com
 http://www.occfiber.com and other websites
3.15 Key Learnings
Qu. 1 Fill in the blanks:
1. The step index fibre has a core with …………….. index throughout.
2. In …………fiber the refractive index diminishes gradually from the center axis
out toward the cladding
3. ……………. cable is used in the majority of outside-plant installations
4. ……………. cable can be used for rodent protection in direct burial if required.
5. A …………….. compliant single mode fiber is the most common single mode
fiber in the world.
6. ITU-T G.655 compliant fiber is a ……………………………………….. .fiber.
Qu.2 State True or False
1. Step Index multimode Fiber has a large core, up to 100 microns in diameter.
2. Ripcord in loose tube cable is used for for easy removal of inner jacket.
3. Tight-buffered cable, primarily used outside buildings.
4. Submarine cables are used in fresh or salt water.

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5. G.655 fibers are the new comers in the market and used in FTTH network.
6. Loose-tube cables can be all-dielectric or optionally armored.
Qu. 3 Draw a ray diagram Step index multimode fiber?









Qu.4 Write down the application area of Loose buffer tube and tight buffer tube cable?
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Qu.5 Write down the features of G.657 fibers?
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3.16 Worksheet
1. Identify the different component of loose buffer tube cable and their functions?

Sl.
No.
Components Functions
1 Outer Jacket
2 Inner Jacket
3 Primary Strength Member
4 Rip cord
5 Binders
6 Buffer Tubes
7 Central strength Member
8 Diameter of fiber with color coating
9 Cladding Diameter
10 Core diameter

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2. Write down the color coding of loose buffer tube cable?
Color Fiber No.











3. Identify the different types of multi-mode and single mode pigtails and patch cord?
Name of Cable Use
Patch cord and Pig tail
(Single mode)

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Patch cord and Pig tail
(Multi-mode)

4. Identify the components of Patch cord and Pig tail (Multi-mode & Single-mode)?
Sl.
No.
Name of Component
1 Outer Jacket
2 Aramid Yarn
3 Tight Buffer
4 Fiber


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