COMPONENTS IN FIBER OPTIC COMMUNICATION SYSTEMS

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:

EP 501 FIBER OPTIC COMMUNICATION SYSTEM 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.

EP 501 FIBER OPTIC COMMUNICATION SYSTEM

INPUT INPUT

3.1 Optical Devices In Fiber Optics Systems

Do Do you you know know that, that, the most widely the most widely used used light light sources sources in fiber-optic in fiber-optic systems systems is is the the injection laser injection laser diode? diode?

Of Of course! course! It It is is because because ILD ILD can can produce produce aa lowlowlevel level forward forward bias bias current current or or aa brilliant brilliant light light over a much over a much narrower narrower frequency frequency range range at threshold. at threshold.

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.

EP 501 FIBER OPTIC COMMUNICATION SYSTEM

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 ) and Injection 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

EP 501 FIBER OPTIC COMMUNICATION SYSTEM

LEDs are 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 advantages of 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

LED

Laser Diode

EP 501 FIBER OPTIC COMMUNICATION SYSTEM Wavelength for Different Colours Color

Wavelength (nm)

Red Cost Data rate Distance

780 - 622 Low Low Short Multimode fiber High Minor

Fiber type Lifetime Temperature sensitivity

High High Long Multimode and single mode fiber Low Significant

P R EC A U 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

disconnecting the fibers.

powered

off

before

EP 501 FIBER OPTIC COMMUNICATION SYSTEM

3.1.2

Light Detector Optical detection occurs at the light wave receiver’s 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 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. 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.

EP 501 FIBER OPTIC COMMUNICATION SYSTEM

With With sufficient sufficient Input Input from from the the unit, is time unit, is time to to do do some exercises. some exercises. Let Let me me start start with with the the example… example…

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).

EP 501 FIBER OPTIC COMMUNICATION SYSTEM

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.

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

____________________________.

Don’t Don’t forget forget to to compare compare your your answers answers with with the the feedback feedback on on the the next next page. page.

fastest

light

d. Random way.

detector

is

the

EP 501 FIBER OPTIC COMMUNICATION SYSTEM

Feedback To Activity 3A

3.1

b

3.2

ILD

3.3

ILD

3.4

Avalanche photodiode

It It is is too too easy, easy, isn’t isn’t it? it? Go Go to to the the second second input input and and see see how much how much you you can remember. can remember.

EP 501 FIBER OPTIC COMMUNICATION SYSTEM

INPUT INPUT

3.1.3

3.1.4

Types Of Connector In Fiber Optic System i.

Ferrule connector (FC)

ii.

Straight Tip (ST)

iii.

Subscriber connector (SC)

iv.

Lucent/local Connector (LC)

Types of couplers/adapters used in fiber optic

ST - A slotted style bayonet type connector. This connector SC - A push/pull type connector. This connector has is one of the most popular emerged as one of the most styles. popular styles.

SMA - A screw-on type connector. This connector is waning in popularity.

FDDI - A push/pull type dual connector. This connector is one the more popular styles.

FC - A slotted screw-on type connector. This connector is popular in single mode applications.

MTRJ - A new RJ style housing fiber connector with two fiber capability.

EP 501 FIBER OPTIC COMMUNICATION SYSTEM

LC - A small form factor optic SC Duplex - Dual SC connector developed by Lucent connectors. Technologies.

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)

EP 501 FIBER OPTIC COMMUNICATION SYSTEM 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, o L is the amplitude of light at output port L, and (T IJ) is the transmittance matrix. In general, the transmittance T IJ 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.

I1 I2

I3

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.

EP 501 FIBER OPTIC COMMUNICATION SYSTEM

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 reach of 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.

EP 501 FIBER OPTIC COMMUNICATION SYSTEM Erbium-doped fiber amplifier (EDFA) EDFA (Erbium Doped Fiber Amplifier) is a kind of fiber optic amplifier which used to reamplify 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.

EP 501 FIBER OPTIC COMMUNICATION SYSTEM Pump 980 or

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

EP 501 FIBER OPTIC COMMUNICATION SYSTEM 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.

EP 501 FIBER OPTIC COMMUNICATION SYSTEM

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 ;

ρ is a PIN photo detector of responsivity ( k is a Boltzman constant (1.38x10-23J/K) T is absolute temperature (K) Δf is a receiver electrical bandwidth

Example 3.1 Suppose we have a system consisting of an LED emitting 10mW at 0.85µm, a fiber cable with -20 dB of loss, and a PIN photodetector of responsivity 0.5A/W. The detector’s dark current is 2 nA. the load resistance is 50Ω; the receiver’s 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

EP 501 FIBER OPTIC COMMUNICATION SYSTEM 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 µW Detected signal current / photocurrent = 0.5 (0.4) = 0.2µA = 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

EP 501 FIBER OPTIC COMMUNICATION SYSTEM

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 = 0.8µA and PNS = 12.8 x 10-17W. The signal power flowing through RL increases 16 times to PES = 32x10-12W. The thermal noise power remains unchanged at PNT = 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.

EP 501 FIBER OPTIC COMMUNICATION SYSTEM

INPUT

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

EP 501 FIBER OPTIC COMMUNICATION SYSTEM

SC Connector Structure Most fiber optic connectors don’t 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.

EP 501 FIBER OPTIC COMMUNICATION SYSTEM

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.

EP 501 FIBER OPTIC COMMUNICATION SYSTEM

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

EP 501 FIBER OPTIC COMMUNICATION SYSTEM 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.

EP 501 FIBER OPTIC COMMUNICATION SYSTEM 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

EP 501 FIBER OPTIC COMMUNICATION SYSTEM 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 3.6 shows 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 highenergy arc will be provided around the fiber. This is intended to melt the surface of the fiber core and so on, are combined.

EP 501 FIBER OPTIC COMMUNICATION SYSTEM 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

EP 501 FIBER OPTIC COMMUNICATION SYSTEM

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 can also 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).

EP 501 FIBER OPTIC COMMUNICATION SYSTEM

Figure 3.7 :Tubular splicing techniques, (a) Splicing tubes tighten, (b) Splicing loose tube using 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 Vgroove, 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.

EP 501 FIBER OPTIC COMMUNICATION SYSTEM

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

EP 501 FIBER OPTIC COMMUNICATION SYSTEM 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 you are 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

EP 501 FIBER OPTIC COMMUNICATION SYSTEM 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 Multiplexing is 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

EP 501 FIBER OPTIC COMMUNICATION SYSTEM 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.

Wire or radio

MUX combines all inputs into a single channel

DEMUX processes input signal by sorting it out into the original individual signals

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, and probably 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.

EP 501 FIBER OPTIC COMMUNICATION SYSTEM 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: •

Transparency—Because 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.

•

Scalability—DWDM 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 provisioning—Fast, 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 signal—The 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 signals—Modern 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 signals—The 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 signals—At the receiving end, the multiplexed signals must be separated out.

•

Receiving the signals—The demultiplexed signal is received by a photodetector.

EP 501 FIBER OPTIC COMMUNICATION SYSTEM •

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

EP 501 FIBER OPTIC COMMUNICATION SYSTEM 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 117, 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

EP 501 FIBER OPTIC COMMUNICATION SYSTEM 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-Band 81 channels defined. Another band called the L-band exists 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

EP 501 FIBER OPTIC COMMUNICATION SYSTEM 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 fiberoptic 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

EP 501 FIBER OPTIC COMMUNICATION SYSTEM

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?

EP 501 FIBER OPTIC COMMUNICATION SYSTEM 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 3

ILD: APD:

A semiconductor diode used as a transmitter for fiber-optic A photodiode used as a receiver for fiber-optic

EP 501 FIBER OPTIC COMMUNICATION SYSTEM communications and having a higher responsivity compared 4

Repeater:

with the PIN photodiode. Device that is used to regenerate the light signals that become too low after they travel long distances.

EP 501 FIBER OPTIC COMMUNICATION SYSTEM

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.

EP 501 FIBER OPTIC COMMUNICATION SYSTEM

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

EP 501 FIBER OPTIC COMMUNICATION SYSTEM 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.

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