Fiber Optic Components

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TrainingManual

2 FIBER OPTIC COMPONENTS Version 3.0 September 1994

Fiber Optic Cable

Fiber optic cables come in many shapes and sizes. Three fiber optic cable parts are

common to all types of cable: core, cladding, and coating. The core is the path for the light

to travel. The cladding is the outer layer of the glass that constrains the light traveling through

the core and reflects it back into the core. The coating, usually silicone, provides protectionand flexibility to the fiber and can be color-coded for identification purposes.

The coating needs to be stripped back to splice the cable, but once the coating is removed

and the glass is exposed great care must be taken. After these three components are

manufactured the cable is bundled in a variety of ways.

Outside plant cable has a number of strands of glass in buffer tubes which provide

additional protection for the glass cable and are color coded for identification purposes. The

buffer tube is then surrounded by strength members which add mechanical strength to the

cable. The most common strength members are kevlar and steel or fiberglass rod. During

and after installation the strength member handles the stresses applied to the cable so that the

fiber is not damaged.

Outside Plant Fiber Optic Cable

Black

PolyurethaneOuter Jacket

OOOO

OOOOOOO

OOOOOOOO

OOOOOOOOO

OOOOOOOOOO

OOOOOOOOOO

OOOOOOOOOO

OOOOOOOOO

OOOOOOOO

OOOOOO

OOO

StrengthMembers

Optical Fiber

Cladding

Core Buffer Tube

Silicone Coating


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FIBER OPTIC COMPONENTS Version 3.0 September 1994 3

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Connectors

Connectors allow termination of the fiber into the end equipment ( multiplexors, power

meters, power sources, light terminal equipment ). Connectors provide a temporary

connection between equipment.

Standards have not been set for fiber optic connectors, so various types of connectors are

employed in fiber optic systems. Because there are so many connectors in use, it is important

to know what type a customer uses so it can be compatible with the test equipment.

D4 Connector

SMA Connector

FC Connector

ST Connector

Biconic Connector

SC Connector


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Connector Finishes

To achieve acceptable return loss measurements, connector manufacturers provide polished

physically contacting connectors (PC), angle polished connectors (APC), or high return loss

(HRL) connector types. These types of connectors will provide an acceptable return loss,ranging from 40 to 65 dB. The diagram below shows the different types of connectors and

their characteristics.

Standard Connector 14 - 20 dB ORL

Air Gap

PC Connector 40 dB ORL

Air Gap eliminated

APC Connector 65 dB ORL

Air Gap eliminated

Reflections de-

flected

Low insertion loss

HRL Connector 65 dB ORL Air Gap

Fibers are offset

More Insertion Loss

than APC

Reflections

Cladding

Core

Offset

12o

Contact on

edges


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Jumper Cables and Pigtails

Connectors can be purchased in jumper cable or pigtail form. A jumper cable is a short

piece of fiber optic cable with a connector on each end. Jumpers provide connections

between different cables, and can be easily moved from point to point.

A pigtail cable is a fiber optic cable with a connector on one end and bare fiber on the

other. Once the bare fiber end of the pigtail is spliced to the fiber transporting the

information, the pigtail allows easy manipulation of a fiber optic cable.

Fiber optic jumper cable.

Fiber optic pigtail.


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Light Distribution Frame

Light Distribution Frames (LDFs) serve two purposes; they act as patch panels and as

demarcation points.

As patch panels they allow accessibility to and maneuverability of the fiber cable. A

jumper cable is used to connect the equipment to the LDF, and a pigtail is used to connect thefiber from the span to the LDF. By bringing these two points together at the LDF, access is

gained to allow testing toward the span or equipment.

Maneuverability is available at the LDF if dark or spare fibers are employed. If a problem

occurs on an active fiber, the jumper containing the equipments signal can be moved or rolled

down to a spare fiber on each end of the system at the LDF.

The LDF also acts as a demarcation point between users to allow sectionalization of areas

of responsibility.

LDFs are equipped with bulkheads which allow proper alignment between fiber connectors.

LDF

Bulkhead

Toward Span

Toward Equipment


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Splicing

Splices are either temporary or permanent connections between two ends of a fiber.

Splices fall into two categories: mechanical and fusion. Fusion splicing is used in the long

haul, and mechanical splices are used in the local loop and LAN areas. The following is a

comparison between the two types of splices:

Mechanical Splices

Splicing Steps

1) Strip cable to the bare fiber.

2) Cleave the end of the fiber. A good cleave will result in a clean break.

3) Employ mechanical splice or fusion welder.

4) Check splice loss using a power loss test set or an OTDR.

A Good Cleave A Poor Cleave

3M Fiber LokMechanical Splice

GTE ElastomericMechanical Splice

Fusion

Some degree of expertise required

Training time required

Fusion welder (approximately $25,000)

Loss approximately .02 dB per splice

Electrodes physically weld the glasstogether

No return loss

Mechanical

Easy to use

Training time nominal

Less than $20 per splice

Loss approximately .2 dB per splice

Mechanically spliced together

Minimal Return loss


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Splice Trays and Enclosures

When two different fibers have been spliced together by means of mechanical or fusion

welding, the splice point will be stored in a splice tray. It is important to secure the splices

in the tray and ensure that there are no excessive bends to cause loss of light.

Once the cable splices have been completed and placed in a splice tray, the splice trays

are placed in an enclosure. In outside plant facilities splice enclosures are used which can

be sealed tight for protection against the elements. In inside plant facilities the splice tray can

be placed in the LDF.

Splice tray

Splice enclosure


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Transmitters

Transmitters are broken down into two categories: LASERs and LEDs. The following is a

comparison between the two types:

LEDs LASERs

Multimode or singlemode Multimode or singlemode

Used for short distances Used for long distances

-10 to -30 dBm coupled power +3 to -10 dBm coupled power

Wide linewidth Narrow linewidth

Relatively inexpensive Expensive

LASERs have a more narrow linewidth than LEDs which allows a much higher amount of power

to be coupled into the fiber optic cable.

Wavelength

Power from a laser is many timeshigher than from an LED.

Laser


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LEDs

The basic operation of an LED is shown below. A small voltage is applied across the

semiconductor, causing current to flow across the P junction which has more electrons than

the N junction. As current flows across the junction, energy is released from the LED in the

form of photons or light.

Holes

p Region

n Region

Junction

Electrons

Recombination-

+

+

+

-

-+

-+

-

Light Waves

Light emitted in other directions

is lost or blocked by package.

Light Emission

LEDs


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LASERs

LASER is the acronym for Light Amplification by the Stimulated Emission of Radiation.

Since LASERs provide stimulated emission, and LEDs provide spontaneous emission,

LASERs provide a higher power output. LASERs produce this stimulated emission by

incorporating an optical cavity for lasing. The lasing cavity is called a Fabry-Perot cavitywhich is formed by cleaving the end of the chip and giving it a reflective mirror-like finish.

The lasing action relies on a high current density. After the current passes a certain threshold

photons are emitted. Some of these photons are trapped in the cavity and reflect back and

forth. These photons combine with other photons causing amplification of light .

Oxide

StripeContact

CleavedEnd

Substrate

OpticalCavityCleaved

End

Light

Output

LASER


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Receivers

Each receiver has a detector which detects the amount of light present at the input of the

receiver. Detector sensitivity strongly depends on the wavelength of the light being received.

The appropriate wavelength operating range of 4 main types of detectors is as follows:

Detectors work best in specific wavelength regions. The following are typical

wavelength responses.

*Quantum efficiency =

InGaAs Indium Gallium Arsenide 350 nm to 1700 nm Greatest sensitivity

Ge Germanium 700 nm to 1700 nm Lower cost

Ge/Si Germanium/Silicon 400 nm to 1700 nm Widest range

SI Silicon 400 nm to 1050 Lowest cost

Wavelength Response

WavelengthMicrometers

100

80

60

40

20

0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8

Quantum

Efficiency(%)*

SiGe

InGaAs

Note different curves for different materials.

Electrons generated

Input photons


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Transceivers

Transceivers are a combination of transmitter and receiver and provide both input and

output interfaces for equipment. Transceivers are employed in FDDI, for instance. FDDI has

a standard transceiver which can be connected directly to a computer terminal or a power

meter if they have the appropriate adapters.

Transceiver

Electricaloutput

OpticalSignalout

OpticalSignalin

Electricalinput

FDDI Connector for Test Equipment

FDDI ADAPTER FIBER UNDER TEST

FIBERTECH

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Fiber Optic Components