COMPONENTS IN FIBER OPTIC COMMUNICATION SYSTEMS

7/27/2019 COMPONENTS IN FIBER OPTIC COMMUNICATION SYSTEMS

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EP 501 FIBER OPTIC COMMUNICATION SYSTEM

3.0 COMPONENTS IN FIBER OPTIC COMMUNICATION

SYSTEMS

OUTCOMES

3.1 Understand the optical devices in the fiber optics systems.

3.1.1 Explain Light Emitting Diodes (LED) with Injection Laser Diodes (ILD) as optical

sources/optical transmitters in term of the following:

a. Outage power

b. Wavelength for different colours

c. Data transmission speed

d. Light generation

e. Types

3.1.2 Explain PIN photodiode with Avalanche Photo Diodes (APD) as light detectors/optical

receivers in term of the following characteristics:

a. Responsivity

b. Dark current

c. Reaction speed

d. Spectral responses

3.1.3 State types of connector in fiber optic system: Ferrule Connector (FC), Straight Tip

(ST), Subscriber Connector (SC),

Subminiature (SMA), Lucent/Local Connector (LC).

3.1.4 Explain type of couplers/adapters used in fiber optic system:


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ST, SC, Fiber Distributed Data Interface (FDDI), FC.

3.1.5 Describe types of optical switches in fiber optic system: optical

cross-connects (OXC) and micro-electromechanical system

switching (MEMS).

3.1.6 Explain the types of repeater and amplifiers in fiber optic system: erbium-doped fiber

amplifier (EDFA), cascaded.

3.1.7 Define noise factors : Thermal Noise, Shot Noise, Dark Current Noise.

3.1.8 Calculate Signal-to-Noise Ratio related to 3.1.7.

3.2 Understand types of connection in fiber optics.

3.2.1 Explain with illustration the connection between fiber optic and connector.

3.2.2 Define the connection between fiber optic and fiber optic(splicing).

3.2.3 Explain the methods of splicing

a. Arc Fusion Splicing

b. Mechanical Splicing : Capillary type, Ribbon V-Groove Type, Elastomeric Type.

3.2.4 Differentiate the characteristics between arc fusion and mechanical splicing.

3.3 Learn multiplexing / de-multiplexing techniques in fiber optic communication.

3.3.1 Define Dense Wavelength Division Multiplexing (DWDM).

3.3.2 Describe the basic concepts of DWDM.

3.3.3 Explain the DWDM circuit components:

a. Dense wavelength-division multiplexers and de-multiplexers.

b. Dense wavelength-division add/drop multiplexer/de-multiplexer.

c. Dense wavelength-division routers.

d. Dense wavelength-division couplers.

3.3.4 Explain DWDM wavelength channel and wavelength spectrum.

3.3.5 Differentiate between DWDM and FDM.

3.3.6 List the advantages and disadvantages of DWDM.


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3.1 Optical Devices In Fiber Optics Systems

An optical communications system begins with the transmitter, which consists of a

modulator and the circuitry that generate the carrier. The carrier is a light beam that

is modulated by the digital pulses which turn it on and off. Generally, the basic

transmitter is nothing more than a light source. Whereas the receiver part of the

optical communications system is relatively simple. It consists of a detector that will

sense the light pulses and convert them into an electrical signal. This signal is then

amplified and shaped into the original serial digital data.

INPUTINPUT

Of course! It isbecause ILD canproduce a low-level forwardbias current or abrilliant lightover a muchnarrowerfrequency rangeat threshold.

Of course! It isbecause ILD canproduce a low-level forwardbias current or abrilliant lightover a muchnarrowerfrequency rangeat threshold.

Do you know that,

the most widelyused light sourcesin fiber-opticsystems is theinjection laserdiode?

Do you know that,the most widelyused light sourcesin fiber-opticsystems is theinjection laserdiode?


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3.1.1 Light Sources

Generally, a light source must meet the following requirements:

It must be able to turn on and off several tens of millions, or even billions, of

times per second.

It must be able to emit a wavelength that is transparent to the fiber.

It must be able to couple light energy into the fiber.

The optical power emitted must be sufficient enough to transmit through

optical fibers.

The performance of the fiber-optic should not be affected by the temperature

variation.

The manufacturing cost of the light source must be relatively inexpensive.

There are two types of light sources used by light wave equipment for optical fiber

transmission, light-emitting diodes ( LEDs ) andInjection laser diode(ILD).

LED is an incoherent light source that emits light in a disorderly way as compared to

ILD, which is a coherent light source that emits light in a very orderly way (see

Figure 3.1).

Incoherent radiation(a) Coherent radiation

(b)

Figure 3.1Radiation patterns for (a) LED ; (b) ILD


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LEDsare economical and are common for short distance,

low data rate applications. They are available for all three

wavelengths but are most common at 850 and 1310 nm

( 850 nm LEDs are usually the least expensive ). Light power from an LED covers a

broad spectrum, from 20 to over 80 nm . The LED is more stable and reliable than a

laser in most environments.

Injection Laser Diodes are more expensive. The advantagesof using a laser diode are in the high modulation bandwidth

( over 2 GHz ), with high optical output power and narrow

spectral width. Their application is in long distance, high data rate requirements.

Lasers are common in single mode optical fiber applications and their light power

covers a very narrow spectrum, usually less than 3 nm. This results in a low

chromatic dispersion value and hence high fiber bandwidth. Their life span is shorter

than that of an LED. Lasers are sensitive to the environment (especially to

temperature variation).

Wavelength for Different Colours

Color Wavelength (nm)

Red 780 - 622

Red 780 - 622

Orange 622 - 597

Yellow 597 - 577

Green 577 - 492

Blue 492 - 455

Violet 455 - 390

Characteristic LEDLaser

Diode


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Wavelength for Different Colours

Color Wavelength (nm)

Red 780 - 622

Cost Low High

Data rate Low HighDistance Short Long

Fiber typeMultimodefiber

Multimode and single modefiber

Lifetime High Low

Temperature sensitivity Minor Significant

PRECAU TIO N !!!!!Optical output from a laser is strong and can

easily damage the eye. Never look into laser

light or a fiber coupled to a laser. Ensure that all

Laser sources are powered off before

disconnecting the fibers.


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3.1.2 Light Detector

Optical detection occurs at the light wave receivers circuitry.

The photo detector is the device that receives the optical fiber signal and converts it

back into an electrical signal. The most important characteristics of light detectors

are :

1. Responsitivity: Responsitivity is a measure of the conversion efficiency of a

photodetector.

2. Dark current: Dark current is the leakage current that flows through a

photodiode with no light input.

3. Transit time: Transit time is the time it takes a light-induced carrier to travel

across the depletion region.

4. Spectral response: Spectral response is the range of wavelength values that

can be used for a given photodiode.

5. Light sensitivity: Light sensitivity is the minimum optical power a light

detector can receive and still produce a usable electrical output signal.

The most common types of photo detectors are the positive intrinsic negative

photodiode( PIN )and the avalanche photodiode(APD ).

PIN photodiodesare inexpensive, but they require a higher optical signal power to

generate an electrical signal. They are more common in short distance

communication applications.

The APD photodiodes are more sensitive to lower optical signal levels and can be

used in longer distance transmissions. They are more expensive than the PIN

photodiodes and are sensitive to temperature variations.

Both photodiodes can operate at similar, high-signal data rates. Some receiver photo

detector circuits operate within a narrow optical dynamic range.


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Example 3.1

Give two types of light sources and light detectors that are used in fiber-optic

systems.

Solution to Example 3.1

The light sources are: LEDs and ILD.

The light detectors are: positive intrinsic negative photodiode (PIN) and the

avalanche photodiode (APD).

With sufficientInput from theunit, is time to dosome exercises.Let me start withthe example

With sufficientInput from theunit, is time to dosome exercises.Let me start withthe example


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Activity 3A

TEST OUR UNDERSTANDING BEFORE YOU CONTINUE WITH THE

NEXT INPUT!

3.1 Light travels in a ....a Circle. b. Straight line. c. Curve. d. Random way.

3.2 Which is faster, an LED or ILD ? _______3.3 Which produces the brightest light , an LED or ILD ? ________

3.4 The most sensitive and fastest light detector is the

____________________________.

Dont forget tocompare youranswers withthe feedback onthe next page.

Dont forget tocompare youranswers withthe feedback onthe next page.


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Feedback To Activity 3A

3.1 b

3.2 ILD

3.3 ILD

3.4 Avalanche photodiode

It is too easy,isnt it?Go to the secondinput and seehow much youcan remember.

It is too easy,isnt it?Go to the secondinput and seehow much youcan remember.


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3.1.3 Types Of Connector In Fiber Optic System

i. Ferrule connector (FC)

ii. Straight Tip (ST)

iii. Subscriber connector (SC)

iv. Lucent/local Connector (LC)

3.1.4 Types of couplers/adapters used in fiber optic

ST - A slotted style bayonettype connector. This connectoris one of the most popularstyles.

SC - A push/pull typeconnector. This connector hasemerged as one of the mostpopular styles.

FC - A slotted screw-on typeconnector. This connector ispopular in single modeapplications.

SMA - A screw-on typeconnector. This connector iswaning in popularity.

FDDI - A push/pull type dualconnector. This connector isone the more popular styles.

MTRJ - A new RJ stylehousing fiber connector withtwo fiber capability.

INPUTINPUT


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LC - A small form factor opticconnector developed by Lucent

Technologies.

SC Duplex - Dual SCconnectors.

3.1.5 Types of optical switches in fiber optic system

Channel cross connecting is a key function in most communication systems. In electronic

systems, the electronic cross connecting fabric is constructed with massively integrated

circuitry and is capable of interconnecting thousand of inputs with thousands of outputs. The

same interconnection function is also required in many optical communication systems.Optical (channel) cross connection may be accomplished in two ways:

1. Convert optical data streams into electronic data, use electronic cross-connection

technology, and then convert electronic data streams into optical. This is known as

the hybrid approach.

2. Cross connect optical channels directly in the photonic domain. This is known as

all-optical switching.

The hybrid approach is currently more popular because there is existing expertise in

designing high bandwidth multichannel (NxN) non blocking electronic cross connect

fabrics. In this case, N may be in the order of thousands.

All optical switching is used in high bandwidth, few channel cross connecting fabrics (such

as router). N in this case is from 2 to perhaps 32, but photonic cross connects with N in the

range of up to 1000 are in the experimental and planning phases. An economically feasible

and reliable 1000 x 1000 all photonic, non blocking , dynamically reconfigurable switch is

currently a challenge, but the technology is promising.

Optical cross connect (OXC)


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Optical cross connect devices are modeled after the many port model: that is, N inputs ports

and N output ports, with a table that defines the connectivity between input and one or more

outputs. Mathematically, this model may be represented by a matrix relationship. Figure 3.1

illustrate the model and the matrix of a cross connect connecting device, where Ik is the

amplitude of light at input port K, oL is the amplitude of light at output port L, and (T IJ) is

the transmittance matrix. In general, the transmittance TIJ are functions of the absorption and

dispersion characteristics of the connectivity path. Ideally, the T IJ term are 1 or 0, signifying

connect or no connect, respectively, with zero connectivity loss and zero dispersion.

Figure 3.1 : Modeling an optical cress connect, mathematically and symbolically

All optical cross connect fabrics are based on at least three methods:

i. Free space optical switching

ii. Optical solid state device

iii. Electromechanical mirror based devices.

Micro electromechanical system switching (MEMS)

Micro electro mechanical systems (MEMS) is the technology of very small devices; it

merges at the nano-scale into nano electromechanical systems (NEMS) and nanotechnology.

MEMS are also referred to as micro machines (in Japan), or micro systems technology

MST(in Europe).

Micro-electro-mechanical-systems (MEMS), with its unique ability to integrate electrical,

mechanical, and optical elements on a single chip, has demonstrated high potential for

realizing optical components and systems in compact and low-cost form.

I1I2

I3


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Fig. 3.2: Free-space MEMS optical switch.

Fig 3.3: SEM of an 8 8 MEMS optical switch.

3.1.6 Types of repeater and amplifier in fiber optic system

An optical communications repeater is used in a fiber-optic communications system to

regenerate an optical signal by converting it to an electrical signal, processing that electrical

signal and then retransmitting an optical signal. Such repeaters are used to extend the reachof optical communications links by overcoming loss due to attenuation of the optical fiber

and distortion of the optical signal. Such repeaters are known as optical-electrical-optical

(OEO) due to the conversion of the signal. Repeaters are also called regenerators for the

same reason.


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Erbium-doped fiber amplifier (EDFA)

EDFA (Erbium Doped Fiber Amplifier) is a kind of fiber optic amplifier which used to re-

amplify an attenuated signal without converting the signal into electrical form. Fiber

amplifiers are developed to support dense wavelength division multiplexing (DWDM) and

to expand to the other wavelength bands supported by fiber optics. EDFA fiber optic

amplifiers function by adding erbium, rare earth ions, to the fiber core material as a do pant;

typically in levels of a few hundred parts per million Figure 3.4. The fiber is highly

transparent at the erbium lasing wavelength of two to nine microns. When pumped by a

laser diode, optical gain is created, and amplification occurs.

Figure 3.4 : Principles of spontaneous emission of erbium; only two lowest are shown

The EDFA amplifier consist of a coupling device, an erbium doped fiber and two isolator

figure 3.5. The fiber carrying the signal is connected via the isolator that suppress light

reflections into the incoming fiber. The isolator at the output of the EDFA suppresses the

reflections by the outgoing fiber figure 3.5 and 3.6. The EDFA is stimulated by a higher

optical frequency (in the UV range) laser source, known as the pump. Laser light from the

pump (980 or 1480nm) or both is also coupled in the EDFA. The pump excites the fiber

additives that directly amplify the optical signal passing through at a wavelength in the

1550nm region.


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Figure 3.5 : An EDFA amplifier consist of an erbium-doped silica fiber, an optical pump, a

coupler, and isolators at both ends.

Figure 3.6: Erbium-Doped Fiber Amplifier Design

Cascade

A configuration for SNR improvement by reducing ASE noise in EDFA repeaters for WDM

signals using cascaded optical fiber grating couplers (FGCs) is proposed. The effectiveness

of the configuration is experimentally demonstrated and discussed.

3.1.7 Noise factor

Noise corrupts the transmitted signal in a fiber optic system. This means that noise sets a

lower limit on the amount of optical power required for proper receiver operation. There are

many sources of noise in fiber optic systems. They include the following:

Noise from the light source

Noise from the interaction of light with the optical fiber

Noise from the receiver itself

Because the intent of this chapter is to discuss optical detector and receiver properties, only

noise associated with the photo detection process is discussed. Receiver noise includes

Pump 980or


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thermal noise, dark current noise, and quantum noise. Noise is the main factor that limits

receiver sensitivity.

Noise introduced by the receiver is either signal dependent or signal independent. Signal

dependent noise results from the random generation of electrons by the incident optical

power. Signal independent noise is independent of the incident optical power level.

Thermal noise is the noise resulting from the random motion of electrons in a conducting

medium. Thermal noise arises from both the photo detector and the load resistor. Amplifier

noise also contributes to thermal noise. A reduction in thermal noise is possible by

increasing the value of the load resistor. However, increasing the value of the load resistor to

reduce thermal noise reduces the receiver bandwidth. In APDs, the thermal noise is

unaffected by the internal carrier multiplication.

Shot noise is noise caused by current fluctuations because of the discrete nature of charge

carriers. Dark current and quantum noises are two types of noise that manifest themselves as

shot noise.

Dark current noise results from dark current that continues to flow in the photodiode when

there is no incident light. Dark current noise is independent of the optical signal. In addition,

the discrete nature of the photo detection process creates a signal dependent shot noise

called quantum noise.

Quantum noise results from the random generation of electrons by the incident optical

radiation. In APDs, the random nature of the avalanche process introduces an additional shot

noise called excess noise. For further information on the excess noise resulting from the

avalanche process, refer to the avalanche photodiode section.


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3.1.8 Calculate Signal to Noise Ratio.

SNR is the ratio of detected signal to uncertainty of the signal measurement. Higher is

better.

Where ;

is a PIN photo detector of responsivity

(

k is aBoltzman constant(1.38x10-23J/K)

Tis absolute temperature (K)

f is a receiver electrical bandwidth

Example 3.1

Suppose we have a system consisting of an LED emitting 10mW at 0.85m, a fiber cable

with -20 dB of loss, and a PIN photodetector of responsivity 0.5A/W. The detectors dark

current is 2 nA. the load resistance is 50; the receivers bandwidth is 10MHz, and its

temperature is 300K (27oC). the system losses, in addition to the fiber attenuation, include a

-14 db power reduction due to source coupling and a -10dB loss caused by various splices


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and connectors. Compute the received optic power, the detected signal current and power,

the shot noise and thermal noise, and the signal to noise ratio.

Solution

The total system loss is (-20) + (-10) + (-14) = -44dB. We know loss 10 log10 x = -44dB

So, transmission efficiency of 10-4.4 = 4 x 10-5.

The optic power reaching the receiver is then

PR= 4 x 10-5(10) = 4 x 10-4mW = 0.4 W

Detected signal current / photocurrent

= 0.5 (0.4) = 0.2A = 200nA

The dark current only 2nA is small compared to the signal current, so it can be ignored in

this example. The electrical signal power is

PES = (0.2 x 10-6)2 (50) = 2 x 10-12W

= 2(1.6x10-19) (0.2x10-6)(107)(50)

= 3.2 x 10-17W

Thermal Noise power

= 4 (1.38 x 10-23) (300) (107)

= 1.66 x 10-13W


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In this system, the thermal noise is nearly four orders magnitude greater than the shot noise.

The thermal noise limited result applies. We can compute the SNR from the equation

= =12

Expressed in decibels, the SNR becomes 10log1012 = 10.8dB.

Example 3.2

In Example 3.1, decrease the system losses by 6 dB. (perhaps a better fiber is used, or the

source coupling is improved). Compute the new value of SNR.

Solution;

The steps in the solution are the same as those followed in example 3.1, so we will give the

results very briefly. The 6dB improvement corresponds to an increase in received optic

power by a factor of 4. The signal photocurrent and the shot noise power increase by this

same factor, so is = 0.8A andPNS = 12.8 x 10-17W. The signal power flowing throughRL

increases 16 times toPES= 32x10-12W. The thermal noise power remains unchanged atPNT =

1.66x10-13W, still far more than the shot noise power. Then S/N=PES/PNT = 192, 16 times

that the lossier system. In decibels, we find that S/N= 22.8 dB. Comparison with the

preceding problem shows that a 6dB increase in optic power produced a 12dB improvement

in the SNR.


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3.2 Types Of Connection In Fiber Optic

Fiber Optic Connector Types and their applications

More than a dozen types of fiber optic connectors have been developed by various

manufacturers since 1980s. Although the mechanical design varies a lot among different

connector types, the most common elements in a fiber connector can be summarized in the

following picture. The example shown is a SC connector which was developed by NTT

(Nippon Telegraph and Telephone) of Japan.

A SC Connector Sample

INPUTINPUT


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SC Connector Structure

Most fiber optic connectors dont have jack and plug design. Instead a fiber mating sleeve(adapter, or coupler) sits between two connectors. At the center of the adapter there is a

cylindrical sleeve made of ceramic (zirconia) or phosphor bronze. Ferrules slide into the

sleeve and mate to each other. The adapter body provides mechanism to hold the connector

bodies such as snap-in, push-and-latch, twist-on or screwed-on. The example shown below

are FC connectors with a screwed-on mechanism.

FC Connector

ST connector simplex only, twist-on mechanism. Available in single mode and

multimode.

It is the most popular connector for multimode fiber optic LAN applications . It has a long

2.5mm diameter ferrule made of ceramic (zirconia), stainless alloy or plastic. It mates with a

interconnection adapter and is latched into place by twisting to engage a spring-loaded

bayonet socket.


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ST Connector ST Adapter (mating sleeve)

FC connector simplex only, screw-on mechanism. Available in single mode and

multimode.

FC connector also has a 2.5mm ferrule (made of ceramic (zirconia) or stainless alloy) . It is

specifically designed for telecommunication applications and provides non-optical

disconnect performance. Designed with a threaded coupling for durable connections. It has

been the most popular single mode connectors for many years. However it is now gradually

being replaced by SC and LC connectors.

FC Connector

SC connector simplex and duplex, snap-in mechanism. Available in single mode and

multimode.

SC was developed by NTT of Japan. It is widely used in single mode applications for its

excellent performance. SC connector is a non-optical disconnect connector with a 2.5mm

pre-radiused zirconia or stainless alloy ferrule. It features a snap-in (push-pull) connection

design for quick patching of cables into rack or wall mounts. Two simplex SC connectors

can be clipped together by a reusable duplex holding clip to create a duplex SC connector.


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Simplex SC Connector Duplex SC Connector

Simplex SC Adapter Duplex SC Adapter

FDDI connector Duplex only, multimode only.

FDDI connector utilizes two 2.5mm ferrules. The ferrules are sheltered from damage

because of the fix shroud that has been constructed in the FDDI connector. FDDI connector

is a duplex multimode connector designed by ANSI and is utilized in FDDI networks. FDDI

connectors are generally used to connect to the equipment from a wall outlet, but the rest of

the network will have ST or SC connectors.

FDDI Connector

Small form factor fiber optic connectors

A number of small form factor fiber optic connectors have been developed since the 90s to

fill the demand for devices that can fit into tight spaces and allow denser packing of

connections. Some are miniaturized versions of older connectors, built around a 1.25mm
http://www.fiberoptics4sale.com/page/FOFS/CTGY/Fiber_Optic_FDDI_Connectorshttp://www.fiberoptics4sale.com/page/FOFS/CTGY/Fiber_Optic_FDDI_Connectors


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ferrule rather than the 2.5mm ferrule used in ST, SC and FC connectors. Others are based on

smaller versions of MT-type ferrule for multi fiber connections, or other brand new designs.

Most have a push-and-latch design that adapts easily to duplex connectors.

LC connector simplex and duplex push and latch 1.25mm ferrule. Available in single

mode and multimode.

Externally LC connectors resemble a standard RJ45 telephone jack. Internally they resemble

a miniature version of the SC connector. LC connectors use a 1.25mm ceramic (zirconia)

ferrule instead of the 2.5mm ferrule. LC connectors are licensed by Lucent and incorporate a

push-and-latch design providing pull-proof stability in system rack mounts. Highly favored

for single mode applications.

LC Connector Simplex and Duplex

LC Simplex Adapter LC Duplex Adapter

SMA 905 and SMA 906 connector . Simplex only. Multimode only.


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SMA 905 and 906 connectors make use of threaded connections and are ideal for military

applications because of their low cost multimode coupling.

SMA 905 and SMA 906 multimode connectors are available with stainless alloy or stainless

steel ferrules. The stainless alloy ferrule may be drilled from 125um to 1550um to accept

various fiber sizes.

SMA 906 ferrule has a step, as shown in the following picture, which requires a half sleeve

to be installed when mating a SMA 906 connector with SMA 905 mating sleeves.

SMA 905 and 906 Connector

SMA 905 Adapter

3.2.2 Splicing


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We know that every line that calls for the extension of the length between both ends of the

line. Thus, the optical fiber splicing process used to connect both ends of the optical fiber.

This splicing method can reduce the rate of loss of information online as well as improving

the efficiency of the fiber system, and it's just good to do in the places online that do not

need modification.

For your information, this splicing method is divided into two types: Fusion Splicing and

Mechanical Splicing. The purpose of both methods is to optimize the splicing process in

terms of connecting the two extensions of the fibers (eg reducing loss "insertion").

Typically, the insertion loss of splicing-mode fiber is 0.1 dB Multi to 0.2 dB and the range

of this loss was very minimal compared to the connection using connectors (connectors).

a. Fusion Splicing

This method is achieved by melting the surface of the optical fiber by using high heat, for

example, sprinkle the use of electricity, where the two surfaces is melted so that it becomes

soft and so on, are connected in parallel. Since, the fiber optic core to be connected to the

external surface, such as insulation, or protective coatings can be removed. The aim is to

obtain the correct adjustments and position in both the end of the fiber.

Figure 3.6shows the position of the optical fiber in the groove of variable and Etap. Both

ends of the fiber fixed line position through the micro-variables. Once the correct position of

the fraternities and inclusion process took place through an electric arc. Meanwhile, Figure

6.1 (b) display the arcing process stages such as:

i. The beginning

Fiber is placed on the straight and parallel.

ii. Compilation stage of the fiber surface

Electrode which is opposite to the fiber will produce low-energy arcs. This is

intended to provide a flat surface at both ends and melt the cladding and insulation.

iii. Merge levels

With only the core parts of fiber, the process will be done in any merger, a high-

energy arc will be provided around the fiber. This is intended to melt the surface of

the fiber core and so on, are combined.


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iv. The final stage

Once merged it cooled for a while. At this point, the electrode will not produce the

arc. Merger process now can be seen that line is completed.

Figure 3.6 uses an electric arc fusion splicing

(a) fusion splicing equipment, (b) schematic illustration of the technique of splicing


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b. Mechanical Splicing

This method easier than fusion splicing and is an extension of techniques both ends of

optical fibers to be arranged in a straight line and, the gap between the fiber optic will be

filled with epoxy or better known as "epoxy resin. Method is to use a capillary tube in which

the ends of optical fibers will be inserted into the capillary tube and a little epoxy will be

placed into one end of the optical fiber before it is inserted into the tube. This method canalso be divided into two parts, namely:

i. Splicing Tightened Capillary Tube

Figure 3.7(a) shows the use of the capillary tube of circular and has an inner diameter of the

tube size is slightly larger than the diameter of the optical fiber. This is to facilitate the

injected epoxy type of transparent epoxy resin, between the optical fiber and the capillary

tube. This will strengthen the adhesion of epoxy between the fibers mechanically. This

technique has a low insertion loss rates up to 0.1 dB for multimod grade index optical fiber

and single mode.

ii. Splicing Loose Capillary

This method uses a rectangular capillary tube type and size of the larger diameter capillaries,

to facilitate the amalgamation of fiber optics.

In the initial stage epoxy is included in the capillaries and the next, followed by fiber optics.

Meanwhile, the other end of the fiber will be placed in the capillary and pushed in until it

meets with the end of the existing fiber. At this point, both ends of the fibers will be at the

corner of the capillaries, it can be seen in Figure 3.7(b).


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Figure 3.7 :Tubular splicing techniques, (a) Splicing tubes tighten, (b) Splicing loose tubeusing a square capillary

iii. Ribbon V- groove

splicing techniques of using V-groove in which case, both ends of the fiber is compressed.

Figure 3.8(a) shows use V-groove in the process of joining optical fibers by mechanical

means. This technique is such that it can be noted for all time by using epoxy resin.

In this splicing technique (Figure 3.8), both ends of the fiber will be placed under the V-

groove, and then, compressed by using a glass plate having a flat surface. After the

compression process is complete, then there is a long fiber. In addition to the V-groove

technique, there are many varieties for mechanical splicing techniques such as elastomeric

splicing, spring groove splicing, splicing using a glass capillary for various traces mode.,

Splicing of single mode and turns to others.


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Figure 3.8 : Ribbon V- groove splicing

iv. An elastomeric splice

An elastomeric splice contains two elastomeric (rubber like) inserts inside a glass sleeve as

shown in Figure 3.9. A V groove is molded into one insert, while the other has a flat surface.

The triangular- shaped space formed where the two insert halves mate is slightly smaller in

dimension than the diameter of the fibers being joined. When the fiber ends are pushed into

the inserts the elastomeric compresses equally on each side in contact with the fiber. As a

result, the fibers are aligned on their center axes. Even fibers with different diameters are

centered along their respective axes, maximizing the overlap of their end faces. The fibers

are usually held in place using an adhesive cured with ultraviolet(UV) light. As in the

capillary splice an index matching gel is often applied to minimize Fresnel losses. Many

manufacturers include the gel within the splice body, which reduces this assembly step


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for the technician.

Figure 3.9: Line drawing of a basic elastomeric splice.

3.2.4 Differentiate Between Fusion Splicing And Mechanical Splicing

There are two methods of fiber optic splicing, fusion splicing & mechanical splicing. If youare just beginning to splice fiber, you might want to look at your long-term goals in this field

in order to chose which technique best fits your economic and performance objectives.

Typical the reason for choosing one method over the other is economics.

Fusion Splicing:

In fiber optic fusion splicing a Fiber Optic Fusion Splicer machine is used to precisely align

the two fiber ends then the glass ends are "fused" or "welded" together using some type of

heat or electric arc. This produces a continuous connection between the fibers enabling very

low loss light transmission. (Typical loss: 0.1 dB). Fusion splicing is lower ($0.50 - $1.50

each), the initial investment is much higher ($15,000 - $50,000 depending on the accuracy

and features of the fusion splicer machine being purchased new or you can purchase a


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refurbished Fiber Optic Fusion Splicer from a reliable test equipment company for $3,000 -

$10,000 based on model and features).

Mechanical Splicing:

Mechanical splices are simply alignment devices, designed to

hold the two fiber ends in a precisely aligned position thus enabling light to pass from one

fiber into the other. (Typical loss: 0.3 dB). Mechanical splicing has a low initial investment

($1,000 - $2,000) but costs more per splice ($12-$40 each).

Performance of each splicing method, the decision is often based on what industry you are

working in. Fusion splicing produces lower loss and less back reflection than mechanical

splicing because the resulting fusion splice points are almost seamless. Fusion splices are

used primarily with single mode fiber where as Mechanical splices work with both single

and multi mode fiber.

Many Telecommunications and CATV companies invest in fusion splicing for their long

haul single mode networks, but will still use mechanical splicing for shorter, local cable

runs. Since analog video signals require minimal reflection for optimal performance, fusion

splicing is preferred for this application as well. The LAN industry has the choice of either

method, as signal loss and reflection are minor concerns for most LAN applications.

3.3 Multiplexing / Demultiplexing

Multiplexingis the process of simultaneously transmitting multiple signals over a single

communications channel (the process of combining together many separate signals to

send them over the same transmission media). This process might be a sharing frequency,

time, or space, or combination of these methods. Multiplexing has the effect of increasing

the number of communications channel so that more information can be transmitted.

Multiplexing is accomplished by an electronic circuit known as a multiplexer. The concept

of a simple multiplexer is illustrated in Fig. 3-10 below. Multiple input signals are combined

by the multiplexer into a single composite signal that is transmitted over the communication

medium. Alternately, the multiplexed signals may modulate a carrier before transmission. At


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the other end of communications link, a demultiplexer is used to sort out the signals into

their original form. In the figure, the word link refers to the physical path. The word channel

refers to the portion of a link that carries a transmission between a given pair of lines. One

link can have many (n) channels.

Fig. 3-10 concept of multiplexing

There are three basic multiplexing techniques:frequency division multiplexing(FDM), wave

division multiplexing (WDM) and time division multiplexing (TDM). The first two are

technique designed for analog signals, the third, for digital signals.

3.3.1 Dense Wavelength Division Multiplexing

In a WDM system, each of the wavelengths is launched into the fiber, and the signals are

demultiplexed at the receiving end. Like TDM, the resulting capacity is an aggregate of the

input signals, but WDM carries each input signal independently of the others. This means

that each channel has its own dedicated bandwidth and all signals arrive at the same time,

rather than being broken up and carried in time slots.

The difference between WDM and dense wavelength division multiplexing (DWDM) is one

of degree only. DWDM spaces the wavelengths more closely than WDM, and therefore

DWDM has a greater overall capacity. The full capacity is not precisely known, andprobably has not been reached.

DWDM can amplify all the wavelengths at once without first converting them to electrical

signals and can carry signals of different speeds and types simultaneously and transparently

over fiber, meaning DWDM provides protocol and bit rate independence.

MUX combines all inputs into a

single channel

DEMUX processes input signal by sorting it out

into the original individual signals

Wire or radio


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From both technical and economic perspectives, potentially unlimited transmission capacity

is the most obvious advantage of DWDM technology. Not only can the current investment

in fiber plant be preserved, but it can also be optimized by a factor of at least 32. As

demands change, more capacity can be added, either by simple equipment upgrades or by

increasing the number of lambdas on the fiber, without expensive upgrades. Capacity can be

obtained for the cost of the equipment, and the existing fiber plant investment is retained.

In addition to bandwidth, DWDM has several key advantages:

TransparencyBecause DWDM is a physical layer architecture, it can transparently

support both TDM and data formats such as asynchronous transfer mode (ATM),

Gigabit Ethernet, Enterprise System Connection (ESCON), and Fibre Channel with

open interfaces over a common physical layer.

ScalabilityDWDM can leverage the abundance of dark fiber in many metropolitan

area and enterprise networks to quickly meet demand for capacity on point-to-point

links and on spans of existing SONET/SDH rings.

Dynamic provisioningFast, simple, and dynamic provisioning of network

connections give providers the ability to provide high-bandwidth services in days

rather than months.

3.3.2 Basic Concepts Of DWDM System

At its core, DWDM involves a small number of physical-layer functions. These are depicted

in Figure 1-2, which shows a DWDM schematic for four channels. Each optical channel

occupies its own wavelength.


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Figure 1-2 DWDM Functional Schematic

A DWDM system performs the following primary functions:

Generating the signalThe source, a solid-state laser, must provide stable light

within a specific, narrow bandwidth that carries digital data modulated as an analog

signal.

Combining the signalsModern DWDM systems employ multiplexers to combine

the signals. There is some inherent loss associated with multiplexing and

demultiplexing. This loss is dependent on the number of channels but can be

mitigated with optical amplifiers, which boost all the wavelengths at once without

electrical conversion.

Transmitting the signalsThe effects of crosstalk and optical signal degradation or

loss must be considered in fiber-optic transmission. Controlling variables such as

channel spacing, wavelength tolerance, and laser power levels can minimize these

effects. The signal might need to be optically amplified over a transmission link.

Separating the received signalsAt the receiving end, the multiplexed signals must

be separated out.

Receiving the signalsThe demultiplexed signal is received by a photodetector.


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In addition to these functions, a DWDM system must also be equipped with client-

side interfaces to receive the input signal. The client-side interface function can be

performed by transponders. Interfaces on the DWDM side connect the optical fiber

to DWDM systems.

3.3.3 Main component of DWDM systems

DWDM is a core technology in an optical transport network. The essential

components of DWDM can be classified by their place in the network:

On the transmit side, lasers with precise, stable wavelengths

On the link, optical fiber that exhibits low loss and transmission performance in the

relevant wavelength spectra, in addition to flat-gain optical amplifiers to boost the

signal on longer spans

On the receive side, photo detectors and optical demultiplexers using thin film filters

or diffracting elements

Optical add/drop multiplexers and optical cross-connect components

These components and others, along with their underlying technologies, are

discussed in the following sections.

a. DWDM Multiplexers and Demultiplexers


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Because DWDM systems send signals from several sources over a single fiber, they must

include some means to combine the incoming signals. Combining the incoming signals is

achieved with a multiplexer, which takes optical wavelengths from multiple fibers and

converges them into one beam. At the receiving end, the system must be able to separate out

the components of the light so that they can be discreetly detected. Demultiplexers perform

this function by separating the received beam into its wavelength components and coupling

them to individual fibers. Demultiplexing must be done before the light is detected, because

photodetectors are inherently broadband devices that cannot selectively detect a single

wavelength.

Unidirectional and Bidirectional Communication

In a unidirectional system (see Figure 1-16), there is a multiplexer at the sending end and a

demultiplexer at the receiving end. Two systems (back-to-back terminals) with two separate

fibers are required at each end for bidirectional communication.

Figure 1-16 Multiplexing and Demultiplexing in a Unidirectional System

A bidirectional system has a multiplexer/demultiplexer at each end (see Figure 1-17) and

communication occurs over a single fiber, with different wavelengths used for each

direction.


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Figure 1-17 Multiplexing and Demultiplexing in a Bidirectional System

Multiplexers and demultiplexers can be either passive or active in design. Passive designs

are based on prisms, diffraction gratings, or filters, while active designs combine passive

devices with tunable filters. The primary challenge in these devices is to minimize crosstalk

and maximize channel separation. Crosstalk is a measure of how well the channels are

separated, and channel separation refers to the ability to distinguish each wavelength.

b. DWDM add/drop multiplexer/demultiplexer

Between multiplexing and demultiplexing points in a DWDM system, as shown in Figure 1-

17, there is an area in which multiple wavelengths exist. It is often necessary to remove or

insert one or more wavelengths at some point along this span. An optical add/drop

multiplexer (OADM) performs this removal/insertion function. Rather than combining or

separating all wavelengths, the OADM can remove some while passing others on.

OADMs are similar in many respects to SONET ADMs, except that only optical

wavelengths are added and dropped in an OADM, and no conversion of the signal from

optical to electrical takes place. Figure 1-22 is a schematic representation of the add/drop

process. This example shows both pre- and post-amplification. Some illustrated components

might or might not be present in an OADM, depending on its design.


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Figure 1-22 Selectively Adding and Removing Wavelengths

3.3.4 DWDM wavelength channel and wavelength spectrum

ITU Recommendation is G.692 "Optical interfaces for multichannel systems with optical

amplifiers". G.692 includes a number of DWDM channel plans. Channel separation set at:

50, 100 and 200 GHz

equivalent to approximate wavelength spacings of 0.4, 0.8 and 1.6 nm

Channels lie in the range 1530.3 nm to 1567.1 nm (so-called C-Band). Newer "L-Band"

exists from about 1570 nm to 1620 nm. Supervisory channel also specified at 1510 nm to

handle alarms and monitoring


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Optical Spectral Bands

Trend is toward smaller channel spacings, to incease the channel count. ITU channel

spacings are 0.4 nm, 0.8 nm and 1.6 nm (50, 100 and 200 GHz). Proposed spacings of 0.2

nm (25 GHz) and even 0.1 nm (12.5 GHz).Requires laser sources with excellent long term

wavelength stability, better than 10 pm. One target is to allow more channels in the C-band

without other upgrades.

Channel Spacing

So called ITU C-Band81 channels defined. Another band called theL-bandexists

above 1565 nm.


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ITU DWDM Channel Plan 0.4 nm Spacing (50 GHz) (All Wavelengths in nm)

ITU DWDM Channel Plan 0.8 nm Spacing (100 GHz) (All Wavelengths in nm)

3.3.5 Differentiate between DWDM and FDM


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Wavelength-division multiplexing (WDM) is conceptually same as the FDM, except that the

multiplexing and demultiplexing involves light signals transmitted through fiber-optic channels.

The idea is the same: we are combining different frequency signals. However, the difference is

that the frequencies are very high. It is designed to utilize the high data rate capability of fiber-

optic cable. Very narrow band of light signal from different source are combined to make a

wider band of light.

3.3.6 DWDM Advantages and Disadvantages

DWDM Advantages

Greater fiber capacity

Easier network expansion

No new fiber needed

Just add a new wavelength

Incremental cost for a new channel is low

No need to replace many components such as optical amplifiers

DWDM systems capable of longer span lengths

TDM approach using STM-64 is more costly and more susceptible to

chromatic and polarization mode dispersion

Can move to STM-64 when economics improve

DWDM Disadvantages

Not cost-effective for low channel numbers

Fixed cost of mux/demux, transponder, other system components

Introduces another element, the frequency domain, to network design and

management

SONET/SDH network management systems not well equipped to handle DWDM

topologies

DWDM performance monitoring and protection methodologies developing


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Activity 3B

TEST OUR UNDERSTANDING BEFORE YOU CONTINUE WITH THE

NEXT INPUT!

3.5 Name at least two shortages of fiber-optic system.

3.6 What is the meaning of coherent?

3.7 The main benefit of fiber-optic cables than electrical cable is its

______________________.

3.8 List the main types of receiver noise.

3.9 What is the main factor that determines receiver sensitivity?

3.10 For a reduction in thermal noise, should the value of the detector's load

resistor be increased or decreased?


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3.11 What are two types of noise that manifest themselves as shot noise?

Feedback To Activity 3B

3.5 Interfacing costs, strength.

3.6 Coherent refers to the emits of light from ILD that is orderly (in

phase).

3.7 Wide bandwidth

KEY FACTS

1 Laser: A coherent light source used as a transmitter in fiber-optic

systems.

2 ILD: A semiconductor diode used as a transmitter for fiber-optic3 APD: A photodiode used as a receiver for fiber-optic


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communications and having a higher responsivity compared

with the PIN photodiode.4 Repeater: Device that is used to regenerate the light signals that become

too low after they travel long distances.


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SELF-ASSESSMENT

You are approaching success. Try all the questions in this self-assessment section

and check your answers with those given in the Feedback on Self-Assessment given

on the next page. If you face any problems, discuss it with your lecturer. Good luck.

Question 3-1

a. What does LASER stand for?

b. Explain the difference between a PIN diode and an APD.

c. What is the velocity of light in free-space?

d. List 3 (THREE) primary characteristics of light detector.

Question 3-2

a. Name the 4 (FOUR) disadvantages of fiber-optic system.

b. Briefly describe the methods use to overcome the above (a) mater.


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Feedback To Self-Assessment

Have you tried the questions????? If YES, check our answers now.

Answer 3.1

a. A LASER is a coherent light source used as a transmitter in fiber-optic

systems.

b. (i) It must be able to turn on and off several tens of millions, or even billion,

of time per second.

(ii) It must be able to emit a wavelength that is transparent to the fiber.

(iii)It must be able to couple light energy into the fiber.

c. The velocity of light in free space is 3 x 108 m/s.

d. PIN photodiodes are inexpensive, but they require a higher optical signal

power to generate an electrical signal. They are more common in short

distance communication applications. As for APD, it having a higher

responsivity compared with the PIN photodiode.

Answer 3-2

a. The 4 (FOUR) disadvantages are:

i. Interfacing Costs

ii. Strength

iii. Remote Powering of Device, and

iv. Inability to Interconnect


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b. (i) Interfacing costs are referring to the costly test and repair equipment,

as well as the technology needs. Manufacturer are continuously

inventing and introducing new or improved field repair kits in order to

bust the marketing with the cheaper material.

(ii) Strength of the fiber-optic cable can be improved by steel

reinforcement.

(iii) Metallic conductors are often included in the fiber-optic cable

assembly strengthen the cable.

(iv) Microprocessors that are more efficient help the signals flow through

the optical cable reach closer to a direct electronic hardware interface.

congratulationS !!!

You have finishedthis unit

successfully.

Documents

COMPONENTS IN FIBER OPTIC COMMUNICATION SYSTEMS