CT06 Sample

Fiber Optic Communications Curriculum Manual CT06

©2007 LJ Create. This publication is copyright and no part of it may be adapted or reproduced in any material form except with the prior written permission of LJ Create.

Lesson Module: 20.72 Version 0 Issue: MT6007/A

CT06 Curriculum Manual

Fiber Optic Communications About This Learning Program

About This Learning Program

Introduction For tomorrow's engineers and technicians, training in the principles and techniques of digital communication systems will be vitally important. Every aspect of our lives is touched by digital communications, whether we are using a mobile telephone or a personal computer linked to a modem. The digital communications systems used take many forms, including satellite links, fiber optic communication systems, infra-red systems and microwave links. The CT06 ‘hands-on’ learning program builds on the understanding and practical knowledge gained from the CT02 Introduction to Digital Communications Learning Program, by covering the following aspects of fiber optic communications:

• Modern Optic Fibers • Fiber Optic Communications Systems • Characteristics and Testing of Optic Fibers What do I need to follow CT06? To follow the CT06 learning program you will need the following items: 1. 2. 3. 4. 5.

CT06 Curriculum Manual CT06 Student Workbook MODICOM 6 Fiber Optic Transmitter/Receiver Module Dual Trace Oscilloscope (Hameg HM203 or equivalent) Power Supply capable of delivering +5V at 1A, +12V at 1A, -12V at 1A. (LJ Technical Systems PS2 IC Power 60, PS4 System Power 90 or equivalent) 6. Digital multimeter (LJ Technical Systems DM1 or equivalent) 7. Set of 4mm patching leads (LJ Technical Systems order code CS1) 8. Test point connection lead (supplied with this manual)

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CT06 Curriculum Manual

The CT06 Curriculum Manual To gain a practical introduction to fiber optic communications, you will need to follow the CT06 Curriculum Manual carefully. It has been specifically designed to lead you in a step by step manner through the following topic areas:

• • • • • •

The MODICOM 6 Board An Introduction to Optic Fiber Modern Optic Fibers Fiber Optic Communication Systems The Characteristics and Testing of Optic Fibers Fault Finding Techniques

The practical exercises of the Curriculum Manual use the MODICOM 6 Fiber Optic Transmitter/Receiver training module. This is one of a range of MODICOM digital communications trainers which are available from LJ Technical Systems. As you work through the Curriculum Manual you will be guided by a series of student objectives and your progress will be monitored by questions in the theory, practical exercises and student assessments of each chapter. Your instructor has a copy of the Instructors' Guide for the Curriculum Manual. It contains solutions to all of the questions presented in the Curriculum Manual, plus background information on the practical exercises.

Your Student Workbook The Student Workbook provided is an essential part of your learning. It provides you with grids on which to draw sketches and spaces to record results, as you work through the practical exercises in the CT06 Curriculum Manual.

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Fiber Optic Communications About This Learning Program

Computerized Assessment of Student Performance If your laboratory is equipped with the DIGIAC 3000 Computer Based Training System, then the system may be used to automatically monitor your progress as you work through the chapters of the Curriculum Manual. If your instructor has asked you to use this facility, then you should key in your responses to questions using either a D3000 Data Terminal or the student console of a D3000 Base Unit. To remind you to do this, a require a keyed-in response.

symbol is printed alongside questions which

The following D3000 Lesson Module is available for use with the CT06 Curriculum Manual: D3000 Lesson Module 20.72

Further Investigation of Optic Fiber Technology A second curriculum manual is available for use with the MODICOM 6 Fiber Optic Transmitter/Receiver Module. The ST02 Fiber Optic Technology Curriculum Manual investigates the physical properties of fiber optic cables, including cable attenuation, cable end misalignment, coupling losses and polishing cable ends. In addition to the MODICOM 6 module, the ST02 Curriculum Manual requires the following items of hardware: CT6A CT6B CT6C

Fiber Optic Polishing Kit Polishing Kit Accessory Pack Physics of Fiber Optics System

Please contact LJ Technical Systems for further information.

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Fiber Optic Communications Contents

Contents

Chapter

Contents

Chapter 1

The Modicom 6 Board ........................................................... 1 - 14

Chapter 2

An Introduction to Optic Fiber ............................................ 15 - 46

Chapter 3

Modern Optic Fibers - Use of Decibels and Choice of Wavelengths used for Communications ............. 47 - 64

Chapter 4

Fiber Optic Communications Systems ................................ 65 - 94

Chapter 5

The Characteristics and Testing of Optic Fibers ............... 95 - 114

Chapter 6

Fault Finding Techniques ................................................ 115 - 132

Appendix 1

Digital Communications .................................................. 133 - 138

Appendix 2

Layout Diagram ............................................................... 139 - 140

Appendix 3

Circuit Diagrams ............................................................. 141 - 142

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Fiber Optic Communications Contents

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The Characteristics and Testing of Optic Fibers Chapter 5

Chapter 5 The Characteristics and Testing of Optic Fibers

Objectives of this Chapter Having studied this chapter you should be able to:

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■

Describe Rayleigh Scatter and its applications.

■

Outline the operation of an Optical Time Domain Reflectometer.

■

Interpret the display of information on an Optical Time Domain Reflectometer.

■

Describe the causes and effects of dispersion.

■

Outline the construction and use of typical fiber optic cables.

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5.1

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Rayleigh Scatter Generally the losses in a piece of optic fiber are due either to the light being blocked or absorbed by impurities, or scattered by an effect known as Rayleigh scattering. When the infrared light strikes a small (very very small) place where the materials in the glass are imperfectly mixed, this gives rise to localized changes in the refractive index resulting in the light being scattered in all directions. Some of the light leaves the optic fiber, some continues in the correct direction and some is even returned towards the light source. This is called back scatter. Figure 57 is not drawn to scale. In fact it is quite impossible for it to be shown in anything like the right proportions. If the discontinuity was the size shown, the optic fiber would have to be drawn about 1600 km (1000 miles) wide. Difficult to fit on the average page! core of optic fiber

backscatter incident light

scattered light

Figure 57 - Rayleigh Scatter

There are many such discontinuities throughout the optic fiber. This gives a small but continuous backscatter from all along the length of the optic fiber. This scattering effect is the cause of the reflected glare from water droplets that troubles drivers when using full driving lights in foggy conditions. Backscatter also occurs from dust particles in the atmosphere to produce the light pollution which makes astronomical observation so difficult when close to centers of population. 5.1a

Rayleigh Scatter: a

is another name for backscatter.

b

is caused by small discontinuities within the optic fibers.

c

only occurs when the optic fiber passes close to a center of population. is a form of absorption caused by impurities within the fiber.

d 96

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5.2

The Characteristics and Testing of Optic Fibers Chapter 5

The Optical Time Domain Reflectometer (OTDR) The OTDR is a measuring instrument which makes use of backscatter. It is the most versatile piece of test equipment that we have for making measurements on fiber optic systems. It provides us with two different measurements:

5.3

(i)

It can measure the magnitude of any losses that occur along the optic fiber.

(ii)

It can measure distances along the optic fiber.

The Basic Operation of the OTDR directional coupler connector

pulse generator

laser

synchronizing pulse

PIN diode or similar

optic fiber

display

Figure 58 - The OTDR

Pulse Generator The pulse generator produces an electrical pulse to switch the laser on for a very brief moment. This pulse is less than 1µs in duration.

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Laser As mentioned previously, the laser produces a flash of infrared light with a wavelength corresponding to the window which is in use in the system being tested. Directional Coupler The directional coupler is effectively a half silvered mirror system which allows the light energy to travel from left to right to enable the laser light to enter the optic fiber under test. Light arriving from the right however, is not allowed to pass straight through but is, instead, reflected downwards to the light detector. Connector There are two main methods of connecting lengths of optic fiber together. The first method, as used here, is a simple plug and socket system similar to that used for copper based coaxial cables. The alternative is called fusion splicing. This is a lower loss but permanent method. The idea is very simple. The two ends of the optic fiber are carefully prepared and brought together. An electric arc is struck between two electrodes and the glass melts and is fused together, just as in electric arc welding for metals. In the OTDR we use a connector so that it can be quickly connected to any system. In the Optic Fiber The short burst of laser light moves down the optic fiber at a velocity determined by the refractive index. As the light moves along the optic fiber it causes a continuous backscatter to occur from the Rayleigh Scatter. PIN Diode As the backscatter re-enters the OTDR, it is diverted into the PIN diode by the directive coupler. The PIN diode (or the alternative device, called an avalanche diode) converts the light into electrical energy. Display This is a cathode ray tube or in portable equipment, a liquid crystal display to make the test results available to the operator.

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Distance We obtain timing information by starting the display and the pulse generator at the same instant. This is achieved by the synchronizing pulse which switches on both the laser and the receiver at the same instant. If we know how long it takes for the backscatter light to return to the OTDR then we only have to know how fast the infrared light is traveling along the optic fiber to be able to calculate how far the light has traveled. The light takes about 5ns to travel a meter along the optic fiber. Therefore, if some light returns after say, 500ns, it follows that it has traveled a total of 100 meters. This represents 50 meters out along the optic fiber and 50 meters back. You will remember that the actual speed of propagation is determined by the refractive index of the core of the optic fiber. Speed of propagation = speed of light in free space/refractive index of the core. (The refractive index of the core of the optic fiber being tested must be punched into the OTDR otherwise all the distances will be miscalculated. The value of the refractive index is quoted by the manufacturer.) 5.3a

A typical value of the refractive index of a silica glass fiber is: a

1.5

b

2.5

c

-1.5

d

200 x 106

The synchronizing pulse simply provides a "start" signal to the pulse generator and to the display circuits to allow them to determine the travel-time of the laser light and the backscatter.

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5.4

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Losses As the light moves along the optic fiber, the light intensity is attenuated by the losses in the optic fiber and so the reflections returned to the OTDR receiver become weaker. Measurement of the amplitude of the returned signals tells us the optic fiber loss in dB/km. If a macrobend has occurred, it would show up as a drop in the signal level at a particular point. If the optic fiber has been cut as it has to be when fitting a connector, the end face of the glass causes a reflection of energy. It is also usual for this to occur at the extreme end of the optic fiber. This causes a localized increase in energy returned to the OTDR. This reflection, called a FRESNEL (the 's' is not pronounced) reflection shows up as a small spike on the display. There is always a Fresnel reflection at the start of the fiber due to the connector on the front panel of the OTDR. In Figure 59 you will see a typical fiber optic system together with its appearance on the OTDR screen. Fiber Optic System Under Test

macrobend

fusion splice

connector end of fiber

OTDR OTDR connector

OTDR Display of the Same System launch pulse

OTDR connector macrobend fusion splice connector end of fiber

power level in dB

random noise

range in km

Figure 59

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Notice that both the macrobends and fusion splices are shown as a sudden loss of power at a particular point. Indeed, it is not possible to distinguish between a macrobend and a fusion splice just by observing the OTDR display. It just shows a localized loss. The loss may show up as a vertical drop or by a sloping line depending upon the speed at which the screen is being scanned on the OTDR. The connector has a similar loss but it also has a fresnel reflection. Typical value of losses: Fusion splice: 0.05 dB Connector:

0.2 dB

Macrobend:

zero to more than 8dB depending on the severity of the macrobend.

After the end of the optic fiber, the OTDR displays random noise generated by the amplifiers in the OTDR. This noise is the same as you get from a TV when the antenna is disconnected. 5.4a

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Referring to Figure 59, the Fresnel reflection occurs: a

at a fusion splice.

b

only at the end of the fiber.

c

whenever a large loss occurs.

d

at a connector and at the end of the fiber.

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5.5

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A Real Trace Taken from an OTDR

Figure 60

Study the OTDR trace. Some parts will be unfamiliar to you and it may seem complicated at first but just try to pick out the information that you need and ignore the rest. At the time this trace was obtained the OTDR was being used to measure the loss created by a fusion splice. The numbered points 1 and 2 are measuring the power level before the splice and the points 4 and 5 are measuring the power level after the splice. The vertical line, called a marker, numbered 3 is placed on the position of the splice. The loss of power across the splice and the distance from the start of the optic fiber is shown in the box at the top right hand corner. Notice how the actual loss appears immediately after (to the right of) the cause. This also happens with fresnel reflections. The actual event is therefore physically at the start of the point where the Fresnel reflection is shown. Q

In your workbook answer the following questions to see what we can deduce from the OTDR printout:

(i)

What optic window is being used?

(ii)

What is the total length of the fiber system under test?

(iii) For how long is the laser pulsed ON? (iv) What is the refractive index of the optic fiber? (v)

What is your opinion of the quality of the splice being measured?

(vi) Draw a diagram of the system being tested showing the main features and their distances along the optic fiber. 102

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5.5a

Referring to Figure 60, the loss which is shown at an approximate range of 285 meters is: a an OTDR display error due to noise on the fiber. b

most likely caused by a connector.

c

actually at a range of only half this figure since the light has to travel along the fiber and back again. possibly due to a fusion splice or a macrobend.

d

5.6

The Characteristics and Testing of Optic Fibers Chapter 5

Two Other Applications of Backscatter (i)

Distributive Temperature Sensing (DTS)

The amount of backscatter occurring in an optic fiber is dependent upon the manufacture of the optic fiber, the optic window used, and also upon the temperature of the optic fiber. Now, when we find a characteristic of the optic fiber which depends on the temperature, it is but a small step away from using the effect to measure temperatures. This new technique is called Distributive Temperature Sensing (DTS). Basically it is an optic fiber connected to equipment operating just like an OTDR which is then passed through the areas to be measured. If it is passed through a refrigerator (minimum temperature of -190°C or -310°F), for example, the trace on the OTDR will show the backscatter level falling to a level dependent upon the temperature in the refrigerator. Similarly, a heated area (maximum temperature 460°C or 860°F) would return a higher level of backscatter. Since the OTDR is able to measure the backscatter at each point on the optic fiber, a single length of optic fiber up to 10km (6.25 miles) in length could be used to monitor many areas at the same time. It can, for example, provide a list of 40,000 temperature measurements within 1 minute.

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This would make the system ideal for locating any temperature increases in areas of high fire hazards such as oil rigs and refineries. The optic fiber could be laid in ducts with power cables. Any overheating could be detected, even rises of less than one degree and the position located to an accuracy of one meter (three feet).

5

launch pulse

4 heated zone 3

power level (temperature) 2

refrigerator

ambient temperature

1

1

2

3

4 5 range in meters

6

7

8

9

Figure 61

(ii)

Security

You will recall that one of the advantages of the fiber optic system is the high level of security offered. We saw in Chapter 2 however, that causing a macrobend would allow the light to escape and hence the data to be copied. An OTDR monitoring the line would immediately detect the power loss of the macrobend and be able to measure its distance along the optic fiber to an accuracy of approximately 0.1 meters (4 inches). The same immediate detection would occur as with the security matting shown in Figure 31 of Chapter 2. 5.6a DTS: a b c d

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is a security device for oil rigs and refineries which detects losses due to macrobends. can only measure temperatures for short periods of time, typically less than one minute. makes use of changes in backscatter level resulting from changes in temperature. is a system used to detect unauthorized copying of data.

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5.7

The Characteristics and Testing of Optic Fibers Chapter 5

Dispersion When a laser is energized by an electrical pulse, it launches a short flash of light along the optic fiber. It is an unfortunate fact that the light burst becomes longer as it moves along the fiber optic cable. The light spreads out. This effect is called "Dispersion". In Figure 62, the light pulse is shown before and after it has traveled through the cable. Can you guess what problem this will cause?

light pulse in

light pulse out

Figure 62 - Fiber Optic Cable

It is going to limit how fast we can send data - how many Bits per second we can transmit through an optic fiber optic link. In fact it is the main limit to the data transmission rate for long distance communication systems. If we send flashes of laser light down a long link in which dispersion is a problem, the flashes will merge at the far end and the ON OFF states will not be distinguished by the receiver. Over a given transmission path, there are only two remedies. Firstly, we could reduce the transmission rate so that even allowing for the spreading effect of the dispersion, the ON OFF states are still clearly separated. This is not a very exciting solution and would clash with one of the main reasons for using optic fiber. The alternative is to reduce the amount of dispersion until it is no longer a problem as shown in Figure 63.

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data signal at input

on

data signal at input

on

off

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on

on

dispersion can cause data errors

off

on on

on off reduced dispersion preserves data at output

Figure 63

To achieve this, we must investigate why the dispersion occurs.

5.8

Dispersion - the Causes and the Cures There are two important causes of dispersion in fiber optic systems. Of the two, the first, called intermodal dispersion is by far the most important, accounting for about 0.999 of the total dispersion.

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5.9

The Characteristics and Testing of Optic Fibers Chapter 5

Intermodal Dispersion - the First Cause You will recall that, to be propagated down the core of the optic fiber, the light must enter at an angle greater than the critical angle. Let us consider just two such rays of light as they travel along a section of optic fiber.

C

A B Which ray would reach C first?

Figure 64 - A Cause of Dispersion

Ray A will reach the far end before Ray B since it is traveling a shorter distance. Assuming that rays A and B are part of the same pulse of light and start at the same time, we can now see how the spreading of the pulses can occur. Each and every ray being propagated at its own angle will arrive at slightly different times at the far end. This spreading effect will occur all along the fiber so it is also important to appreciate that the longer the optic fiber, the greater the dispersion. Transmission rates that are actually possible on an optic fiber therefore depend on its length. In practice, there are only particular angles of propagation which are able to be transmitted down the optic fiber. At these angles, the electromagnetic wave, which is the light, can set up a number of complete patterns across the optic fiber. The number of complete patterns called MODES depend on the dimensions of the optic fiber core. This is very much like the way in which the width of a road determines the number of lanes of traffic it can hold. An optic fiber cannot propagate say, 6½ modes any more than a road can have 6½ traffic lanes. In both cases, they will remain at 6 until the width allows the step up to 7. Since each mode corresponds to a permissible ray angle, each mode travels at its own speed, giving rise to dispersion - hence the name intermodal dispersion (inter = between, hence dispersion between modes). Incidentally, the optic fiber used with MODICOM 6 supports over a million modes and would suffer severely from intermodal dispersion.

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The quartz glass fibers used for short distance communication have a thin core, about 50µm and this reduces the number of modes to about 1000. Much fewer than the large diameter plastic fiber but still large enough to give significant intermodal dispersion. This core diameter would sometimes be called 50 microns (1 micron is a previous name for 1µm or 1 x 10-6 meters). 5.9a

Intermodal dispersion can be reduced by: a

decreasing the diameter of the core.

b c

decreasing the transmission rate until the on/off sequences are easily detectable. increasing the number of modes.

d

using plastic fiber.

5.10 The Cure for Intermodal Dispersion A large core diameter means many modes and severe intermodal dispersion. The cure for this type of dispersion is quite simple. Reduce the core size, the number of modes decreases and the intermodal dispersion is reduced. We can do better than just reducing the intermodal dispersion, we can completely eliminate it! Simply make the core so small that only one mode is propagated. A single ray cannot possibly go at two different speeds so intermodal dispersion cannot occur. In practice the core is reduced to about 9µm. The optic fiber which now carries only a singlemode is now referred to as a 'singlemode fiber'. Singlemode fiber is used for all long distance and/or high speed communications. All our long distance telephone conversations are now carried by singlemode fiber optic systems over at least some parts of the route. The larger core optic fibers for short and medium distances carry many modes and are called 'Multimode'.

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The Characteristics and Testing of Optic Fibers Chapter 5

5.11 Cause 2 - Material Dispersion This form of dispersion occurs in both multimode and single mode optic fibers. It is only really significant in single mode usage since, being very slight, it is completely swamped by the intermodal dispersion in the multimode case. The cause is simple enough - the refractive index of a material is determined to some extent by the wavelength of the light source. Can you see how this causes dispersion? A change in refractive index will change the speed of that particular wavelength of light. Now, if your light source produces different wavelengths at the same time, we will have components of the transmitted light pulse traveling at difference speeds. The total package of light will spread out - hence the dispersion.

5.12 The Cure for Material Dispersion The cure is apparently so simple - use a light source which emits only one wavelength of light. Unfortunately, it hasn't yet been invented. Our two light sources in current use are the LED and the laser. Study Figure 65 and decide which of the two would cause the lesser amount of material dispersion.

LED Spectrum

LED spectral width 90nm (typically)

Laser Spectrum

laser spectral width 4nm (typically)

Figure 65

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The laser would cause less material dispersion because its light is more concentrated around the central wavelength. The spread of wavelength measured between the points where the power output falls to half of the peak power is called the spectral width. Some lasers have spectral widths as low as 0.1nm. The low spectral width together with its high power and fast switching makes the laser first choice for long distance communications using singlemode optic fiber. 5.12a Material dispersion is increased by: a

propagating more modes within the optic fiber.

b

using an LED as a light source instead of a laser.

c

using singlemode optic fiber.

d

increasing the core diameter of the fiber.

5.13 Fiber Optic Cables in Use We have seen that we can send light down a long length of optic fiber. The light is gradually attenuated and from time to time we have to install amplifiers called repeaters to restore the signal. Within land based and undersea systems, these are usually spaced at about 100km intervals (62.5 miles). Most repeaters convert the signal from optical to electrical, amplify it, then convert it back again. New repeaters are now being used that are able to amplify the light energy without having to convert it to electrical signals first.

5.14 Fiber Optic Cables The physical design of the cable depends upon the environment in which it is used. Indoor cable is generally thinner and lighter than outdoor cable. It is often similar in size to the cable used for your oscilloscope probes. They are usually marked "OPTIC FIBER" at intervals along the sheathing otherwise they are quite impossible to tell from copper based cables.

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The cable shown in Figure 66 is an outdoor cable for local networks, designed for direct burial underground. galvanized steel armor wires

gel-filled loose polyester tubes

optic fibers with their primary buffers

steel strength member to take the strain during installation

metallic foil moisture barriers

polyethylene sheath

polyethylene inner sheath

PVC outer sheath

Figure 66

This cable has 18 optic fibers, each with differently colored plastic sheathing to aid identification. As you will appreciate, 18 optic fibers would have enormous communication potential. The cable has a steel strength member running through the center. The purpose of this is to prevent the optic fibers being stressed during installation. It also has stranded steel armoring for protection and two layers of aluminum foil to prevent the ingress of water. Further protection against water is provided by filling the tubes carrying the fiber with a waterproof gel. Smaller indoor optic fibers use strands of KEVLAR for protection. Kevlar is an artificial yarn, yellow in color, with very fine silky threads of high strength and low stretch (also used to in the manufacture of bullet proof clothing).

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Student Assessment 5 1.

2.

3.

4.

The ingress of moisture into an optic fiber can be prevented by: a using a glass fiber rather than a plastic fiber. b

wrapping the cable with an aluminum foil barrier under the outer cover.

c

using heaters within the cable.

d

using a sheath made from PVC rather than polyethylene.

An optic fiber cable for outdoor use would: a always include a strength member. b

generally be smaller and lighter than an indoor cable.

c

always include repeaters.

d

be colored yellow if they contained kevlar.

The main function of a repeater in a fiber optic system is to: a convert amplitude modulation into frequency modulation. b

trigger two parts of a circuit at the same time.

c

restore the amplitude of the signals.

d

provide two copies of a signal to be sent to two different locations.

The Optical Time Domain Reflectometer: a is used to measure the refractive index of the buffer of an optic fiber. b

can be used to measure both losses and distances on a fiber optic system.

c

can automatically set itself to the correct refractive index for the optic fiber under test.

d

can only be used with visible light. Continued ...

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Student Assessment 5 Continued ... 5.

6.

7.

8.

Intermodal dispersion: a is caused by light entering the optic fiber at an angle which is less than the critical angle. b

can be reduced by lowering the transmission rate.

c

is the main cause of dispersion in singlemode optic fibers.

d

can be reduced by reducing the core diameter.

Light would travel through a material with a refractive index of 1.4 at a speed of: a 1.4m/s b

2.14 x 108m/s

c

4.2 x 108m/s

d

4.7 x 10-9m/s

On the display of an OTDR: a there would be no visible difference between a macrobend and a connector. b

the vertical axes is always used to measure distances.

c

a fusion splice would normally indicate a loss but would not cause a Fresnel reflection.

d

a trace which was nearly horizontal would indicate a high loss optic fiber.

Material dispersion: a would be reduced if a light source with a wider frequency spectrum were to be used. b

is the major cause of dispersion in single mode optic fibers.

c

is caused by Rayleigh scatter.

d

can be reduced by a reduction in the transmitted power. Continued ...

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Student Assessment 5 Continued ... 9.

Dispersion: a does not occur in a singlemode fiber. b

decreases if the temperature decreases and can therefore be used to detect overheating.

c

decreases steadily as the signal moves along the fiber.

d

sets an upper limit to the practical transmission rate on an optic fiber.

10. An OTDR could not operate without: a dispersion.

114

b

a laser.

c

Rayleigh scatter.

d

a cathode ray tube.

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