LAB MANUAL

Subject: Optical Communication

Paper: EC 758

Lab No. 415

Teacher In-charge:

Dr. Preeti Singh Mr. Neeraj Sharma

Lab Technician: Ms. Swaran Kaur

List of Experiments:

1. Demonstration and study of different types of Optical Fibers and connectors.

2. To establish and Study of a 650nm fiber optic analog link.

3. To establish and Study of a 650nm fiber optic digital link.

4. Study of Intensity Modulation Technique using Analog input signal. To obtain intensity

modulation of the analog signal, transmit it over a fiber optic cable and demodulate the

same at the receiver and to get back the original signal.

5. Study of Intensity Modulation Technique using digital Input signal. The objective of

this experiment is to obtain intensity modulation of digital signal, transmit it over fiber

optic cable and demodulate the same at the receiver end to get back the original signal.

6. To measure propagation or attenuation loss in optical fiber.

7. To measure propagation loss in optical fiber using optical power meter.

8. To measurement of the Numerical Aperture (NA) of the fiber.

Experiment No. 1

Objective:

Demonstration and study of different types of Optical Fibers and connectors.

Apparatus Required:

Demonstration Kit of Optical Fibers and connectors.

Theory: For much of modern telecommunication, the path over which the signals travel is optical fiber. Optical

fiber for most purposes is made of a very special kind of glass that is drawn into a very thin, long fiber.

In some ways, this is similar to the fiberglass that is used for insulation in homes. Unlike fiber glass,

however, optical fiber is made of a much different kind of glass and comes in lengths that may be

many kilometers long. Standard optical fiber is shaped like a very long thin cylinder. In the center of

the cylinder there is a core, and surrounding the core is a layer called the cladding. Both core and

cladding are glass; they are slightly different types, however. A cross section of the fiber is shown in

Figure 1.1.

Fig.1.1. Cross-sectional views of an optical fiber.

Single mode optical fiber: In fiber-optic communication, a single-mode optical fiber (SMF)

(mono mode optical fiber, single-mode optical waveguide, or uni mode fiber) is an optical

fibre designed to carry only a single ray of light (mode). These modes define the way the wave travels

through space, i.e. how the wave is distributed in space. Waves can have the same mode but have

different frequencies. This is the case in single-mode fibers, where we can have waves with different

frequencies, but of the same mode, which means that they are distributed in space in the same way, and

that gives us a single ray of light. Although the ray travels parallel to the length of the fiber, it is often

called transverse mode since its electromagnetic vibrations occur perpendicular (transverse) to the

length of the fibre. A typical single mode optical fiber has a core diameter between 8 and 10.5 µm and

a cladding diameter of 125 µm.

Multi-mode optical fiber: Multimode fiber optic cable has a large diametrical core that allows

multiple modes of light to propagate. Because of this, the number of light reflections created as the

light passes through the core increases, creating the ability for more data to pass through at a given

time. Because of the high dispersion and attenuation rate with this type of fiber, the quality of the

signal is reduced over long distances. This application is typically used for short distance, data and

audio/video applications in LANs. RF broadband signals, such as what cable companies commonly

use, cannot be transmitted over multimode fiber. Multimode fiber is usually 50/125 and 62.5/125 in

construction. This means that the core to cladding diameter ratio is 50 microns to 125 microns and

62.5 microns to 125 microns.

Multi-mode optical fiber can be built with either graded index or step index-

Step-Index Multimode Fiber: Due to its large core, some of the light rays that make up the digital pulse

may travel a direct route, whereas others zigzag as they bounce off the cladding. These alternate paths

cause the different groups of light rays, referred to as modes, to arrive separately at the receiving point.

The pulse, an aggregate of different modes, begins to spread out, losing its well-defined shape. The

need to leave spacing between pulses to prevent overlapping limits the amount of information that can

be sent. This type of fiber is best suited for transmission over short distances.

Graded-Index Multimode Fiber: Contains a core in which the refractive index diminishes gradually

from the center axis out toward the cladding. The higher refractive index at the center makes the light

rays moving down the axis advance more slowly than those near the cladding. Due to the graded index,

light in the core curves helically rather than zigzag off the cladding, reducing its travel distance. The

shortened path and the higher speed allow light at the periphery to arrive at a receiver at about the

same time as the slow but straight rays in the core axis. The result: digital pulse suffers less dispersion.

This type of fiber is best suited for local-area networks.

Fi.g.1.2 Propagation of light through different types of optical fibers.

Various optical fibers

Fig.1.3. Demonstration of different types of glass fibers and plastic fibers.

Various fiber optical connectors and adaptors

Fig.1.3. Demonstration of different types of fiber connectors and adaptors.

Result:

Various optical fibers and fiber optic connectors and adaptors have been studied through the

demonstration kit.

Experiment No. 2

Objective:

The Study of a 650nm fiber optic analog link.

Apparatus required:

1. ST2502 trainer with power supply cords

2. Optical Fiber cable.

3. Cathode ray oscilloscope with necessary connecting probe.

Theory:

Fiber optic links can be used for transmission of digital as well as analog signals. Basically a fiber

optic link contains three main elements : a transmitter, an optical fiber and a receiver. The transmitter

module takes the input signal in electrical form and then transforms it into optical energy containing

the same information.

The optical fiber is the medium which takes the energy to the receiver.

At the receiver, light is converted back into electrical form with the same pattern as fed to the

transmitter.

Transmitter:

Fiber optic transmitters are composed of a buffer, driver and optical source. The buffer provides both

an electrical connection and isolation between the transmitter and the electrical system the data. The

driver provides electrical power to the optical source. Finally, the optical source converts the electrical

current to the light energy with the same pattern. The optical source used is LED. Simple LED circuit

for analog transmission is shown below.

Fig.2.1 Transconductance drive circuit for analog transmission-common

emitter configuration

The transmitter section comprises of Function Generator which generates input signals that are going

to be used as information to transmit through optical fiber.

The Fiber Optic Link:

Emitter and Detector circuit on board form the fiber optic link. This section provides the light source

for the optic fiber and the light detector at the far end of the fiber optic links. The optic fiber plugs into

the connectors provided in this part of the board. Two separate links are provided.

The Receiver:

The AC amplifier circuit forms receiver on the board.

Block Diagram:

Fig.2.2 Analog Fiber Optic Link

Procedure:

1. Connect the power supply to the board.

2. Ensure that all switched faults are off.

3. Make the following connections.

Connect the Function generator 1KHz sine wave output to emitter‟s input.

Connect the Fiber optic cable between emitter output and detectors input.

Detector‟s output to AC amplifier input.

4. On the board, switch emitter‟s driver to analog mode.

5. Switch ON the power.

6. Observe the input to emitter with the output from AC amplifier and note that the two signals

are same.

Fig.2.3 Connections on Trainer Kit

Observations:

Input voltage = 1.2V

Detector output = 0 .1V

Amplifier output = 1.4V

Result:

The 650nm fiber optic Analog link has been established.

Experiment No. 3

Objective:

Study of a 650nm fiber optic digital link.

Apparatus required:

1. ST2502 trainer with power supply cords

2. Optical Fiber cable.

3. Cathode ray oscilloscope with necessary connecting probe.

Theory:

Fiber optic links can be used for transmission of digital as well as analog signals. Basically a fiber

optic link contains three main elements : a transmitter, an optical fiber and a receiver. The transmitter

module takes the input signal in electrical form and then transforms it into optical energy containing

the same information.The optical fiber is the medium which takes the energy to the receiver.At the

receiver, light is converted back into electrical form with the same pattern as fed to the transmitter.

Transmitter:

Fiber optic transmitters are composed of a buffer, driver and optical source. The buffer provides both

an electrical connection and isolation between the transmitter and the electrical system the data. The

driver provides electrical power to the optical source. Finally, the optical source converts the electrical

current to the light energy with the same pattern. The optical source used is LED. The simple driver

circuit is shown below.

Fig.3.1 A simple drive circuit for binary digital transmission consisting a

common emitter saturating switch.

The transmitter section comprises of Function Generator which generates input signals that are going

to be used as information to transmit through optical fiber.

The Fiber Optic Link:

Emitter and Detector circuit on board form the fiber optic link. This section provides the light source

for the optic fiber and the light detector at the far end of the fiber optic links. The optic fiber plugs into

the connectors provided in this part of the board. Two separate links are provided.

The Receiver:

The comparator circuit and AC amplifier circuit form receiver on the board.

Block Diagram:

Fig.3.2 Digital Fiber Optic Link

Procedure:

1. Connect the power supply to the board.

2.Ensure that all switched faults are off.

3. Make the following connections.

Connect the Function generator 1KHz square wave output to emitter‟s input.

Connect the Fiber optic cable between emitter output and detector‟s input.

Detector‟s output to comparator‟s input.

Comparator‟s output to AC amplifier‟s input.

4. On the board, switch emitter‟s driver to digital mode.

5. Switch ON the power.

6. Monitor both the inputs to comparator . Slowly adjust the comparator‟s bias preset, until DC

level on the input lies midway between the high and low level of the signal on the positive input.

7. Observe the input to emitter with the output from AC amplifier and note that the two signals

are same.

Fig.3.3 Connections on Trainer Kit

Observations:

Input Voltage = 0.16V

Comparator output = 0.05V

Amplifier output = 1V

Result:

The 650nm fiber optic Digital link has been established.

Experiment No. 4

Objective:

Study of Intensity Modulation Technique using Analog input signal. To obtain intensity modulation of

the analog signal, transmit it over a fiber optic cable and demodulate the same at the receiver and to get

back the original signal.

Apparatus Required:

1. ST2502 trainer with power supply cord

2. Optical Fiber cable

3. Cathode ray oscilloscope with necessary connecting probe

Theory:

Modulation:

In order to transmit information via an optical fiber communication system it is necessary to modulate

a property of light with the information signal. This property may be intensity, frequency, phase with

either analog or digital signals. The choices are indicated by the characteristics of optical fiber, the

available optical sources and detectors, and the considerations of the overall system.

Intensity Modulation:

In this system the information signal is used to control the intensity of the source. At the far end, the

variation in the amplitude of the received signal is used to recover the original information signal.

Fig. 4.1 block diagram of intensity modulated system.

The audio input signal is used to control the current through an LED which in turn controls the light

output. The light is conveyed to the detector I circuit by optic fiber. The detector is a photo transistor

which converts the incoming light to a small current which flows through a series resistor. This gives

rise to a voltage whose amplitude is controlled by the received light intensity. The voltage is now

amplified within the detector circuit and if necessary, amplified further by the amplified circuit.

The Analog Bias Voltage:

There are two problems using amplitude modulation

with an analog signal. The first is to do with the signal

itself. If you glance at the figure you will see that analog

waveform moves positive and negative of the zero line.

The second problem is that the shape of the waveform

which carries the information. Ideally the emitter

characteristics would be a straight line. Even so, we

would lose the negative going half cycles as shown.

The answer is to superimpose the sinusoidal signal on

positive voltage called the bias voltage so that both halves

of the incoming signal have an effect on the light

intensity. The combination of the linear characteristics

would be ideal but the real characteristics is not

completely straight. However, it does have a straight

section that we can use if we employ a suitable value of

bias voltage. FIG shows ideal and practical situations.

FIG 4.2. Analog

Signal

FIG 4.3. Negative half of the cycle is lost

FIG 4.4 Emitter characteristics : Ideal and

Real

Connection Diagram:

Fig. 4.5 Connection Diagram

Procedure:

1. Connect the power supply cord to the main power plug & to trainer ST2502.

2. Make the following connections as shown in FIG. 5

a. Connect the FG output marked 1 KHz sine wave to input if emitter 1.

b. Plug in a fiber optic link from output of emitter 1 LED to the photo transistor of the

detector 1.

c. Detector 1 output TP 10 to input of Amplifier TP 27.

3. In the emitter 1 block switch the mode select to analog.

4. Turn the 1 KHz preset in function generator block to fully clockwise (maximum amplitude)

position.

5. Switch on the Power Supply of the trainer and oscilloscope.

6. With the help of dual trace oscilloscope observe the input signal at emitter 1 TP 5 also; observe the

output from the detector 1. It should carry a smaller version of the original 1 KHz sine wave,

illustrating that the modulated light beam has been reconverted back into an electrical signal.

7. The output from detector 1 is further amplified by AC amplifier 1. This amplifier increases the

amplitude of the received signal, and also removes the DC component, which is present at detector

output. Monitor the output of amplifier 1 TP28 and adjust the gain adjust 1 preset until the

monitored signal has same amplitude as that applied to emitter 1 Input TP 5 .

8. While monitoring the output of Amplifier 1 TP 28 change the amplitude of modulating sine wave

by varying the 1 KHz preset in the function generator block. Note that as expected, the amplitude

of the receiver output signal changes.

Observations:

Frequency=1 KHz

S.No. Input Voltage(V) Output Voltage(V)

1 0.10 0.08

2 0.20 0.12

3 0.24 0.18

4 0.44 0.24

Result:

The Intensity Modulation and Demodulation of the Analog signal has been implemented.

FIG 4.6. Input V=0.10V FIG 4.7. Output

V=0.08V

Experiment 5

Objective:

Study of Intensity Modulation Technique using digital Input signal. The objective of this

experiment is to obtain intensity modulation of digital signal, transmit it over fiber optic cable and

demodulate the same at the receiver end to get back the original signal.

Apparatus Required:

1. ST2502 trainer with power supply cord

2. Optical Fiber cable

3. Cathode ray oscilloscope with necessary connecting probe

Connection Diagram:

Fig. 5.1 Connection Diagram

Fig. 5.2. Block Diagram

Theory:

With intensity modulation, discrete changes in light intensity are obtained(i.e. ON-OFF pulses) figure

below shows a block schematic of a typical digital optical fiber link

Initially, input digital signal from information source is suitably encoded for optical transmission. The

LED drive circuit directly modulates the intensity of the light with encoded digital signal. Hence, a

digital optical signal is launched into the optical fiber cable. The photo transistor used as detector is

followed by an amplifier to provide gain. Finally the signal obtained is decoded to give the original

digital information.

Digital Bias Voltage:

Incase of a digital signal the only information which needs to be conveyed is the ON state and OFF

state. So there is no negative part of the signal to be lost and furthermore any distortion due to non

linearity of the characteristic is of no importance –all we need to know is whether the signal is ON or

OFF. There is no need therefore to generate a bias voltage. When Amplitude Modulation is used with

digital input we employ a comparator at the receiving end of the fiber to make the waveform square

again called “cleaning it up”

Procedure:

1. Connect the power supply cord to the main power plug & to trainer ST2502.

2. Make the following connections as shown in figure 4.1.

Digital signal Generator

LED Driver Photo Transistor

Amplifier

Oscilloscope

a. Connect the 1 KHz square wave socket in function generator block to emitter 1 input.

b. Connect an optic fiber link between emitter 1 output & Detector 1 input with the help

of connector provided.

c. Detector output to comparator l's non-inverting (+ve) input

3. Switch the mode switch in emitter block to digital mode. This ensures that signal applied to

the driver's input cause the emitter LED to switch quickly between „On‟ & „Off‟ states.

4. Examine the Input to emitter 1 TP 5 on an oscilloscope this 1 KHz square wave is now

being used to amplitude modulate emitter I emitter LED.

5. Examine the output of detector 1 TP 10. This should carry a smaller version of original I

KHz square wave illustrating that the modulated light beam has been reconverted into an

electrical signal.

6. Monitor both input to comparator 1, at TP 13 & 14 and slowly adjust the "Comparator bias

1 preset until the DC Level on the negative input TP 13 lies midway between the high & low

level of the signal on the positive input TP. 14. This DC level is comparator's threshold level.

7. Examine the output of comparator 1 TP15 Note that the original digital modulating signal

has been reconstructed at the receiver.

8. Once again carefully flex the fiber optic cable we can see that there is no change in output

on bending the fiber. The output amplitude is now independent of the bend radius of the cable

and that of length of cable, provided that detector output signal is large enough to cross the

comparator threshold level. This illustrates one of the advantages of amplitude modulation of

a light beam by digital rather than analog means. Also, non-linear ties within the emitter LED

& photo transistor causing distortion of the signal at the receiver output are the disadvantages

associated with amplitude modulating a light source by analog means. Linearity is not a

problem if the light beam is switched „On‟ & „Off‟ with a digital signal, since the detector

output is simply squared up by a comparator circuit. To overcome problems associated with

amplitude modulation of a light beam by analog means, analog signals are often used to vary

or modulate some characteristic of a digital signal (e.g. frequency or pulse width.). The

digital signal being used to switch the light beam „On‟ & „Off‟.

Observations:

Digital Input Voltage 5V

Comparator Output 4V

DC bias Voltage 0.18V

output of Amplifier 5V

Results:

Intensity Modulation has been performed and its uses over analog intensity modulation have

been verified.

Experiment 6

Objective: To measure propagation or attenuation loss in optical fiber.

Apparatus Required:

1. ST2502 trainer with power supply cord.

2. Optical Fiber cable

3. Cathode ray oscilloscope with necessary connecting probe

Connection Diagram:

Fig.6.1 Connection diagrams on ST502 kit

Procedure:

1. Connect power supply cord to the main power plug & to trainer ST2502.

2. Make the following connections as shown in figure 7.1.

a. Function generator‟s 1 KHz sine wave output to Input 1 socket of emitter 1

circuit via 4 mm lead.

b. Connect 0.5 m optic fiber between emitter 1 output and detector l's input.

c. Connect detector 1 output to amplifier 1 input socket via 4mm lead.

3. Switch ON the Power Supply of the trainer and oscilloscope.

4. Set the Oscilloscope channel 1 to 0.5 V / Div and adjust 4 - 6 div amplitude by using

X 1 probe with the help of variable pot in function generator block at input 1 of

Emitter 1.

5. Observe the output signal from detector TP10 on CRO.

6. Adjust the amplitude of the received signal same as that of transmitted one with the

help of gain adjust potentiometer in AC amplifier block. Note this amplitude and

name it V1.

7. Now replace the previous FG cable with 1 m cable without disturbing any previous

setting.

8. Measure the amplitude at the receiver side again at output of amplifier 1 socket TP 28.

Note this value end name it V2.

Observations:

1. Using output of cables of different diameters:

O/P of .5m cable-.56V

O/P of 1m cable-.52V

Attenuation=20log56/52=.64dB

2. Using formula α= -1/(L1+L2)ln(V1/V2)=.429dB/m

3. Using OPM, Pn=-31.3dBm, Pf=-31.4dBm

α = .1dB

Calculations:

Calculate the propagation (attenuation) loss with the help of following formula.

V1 / V2 = e- α (L1 + L2)

Where α is loss in nepers / meter

1 neper = 8. 686 dB

L 1 = length of shorter cable (0.5 m)

L 2 = Length of longer cable (1 m)

Precautions:

a. The optical fiber must not be bent, so as to avoid bending losses.

b. The circuit must be neat and secure.

Experiment 7

Objective: To measure propagation loss in optical fiber using optical power meter.

Apparatus Required:

1. ST2502 trainer with power supply cord

2. Optical fiber cable

3. Cathode ray oscilloscope with necessary connecting probe

4. Power Meter ST2551 with power supply cord

Connection Diagram:

Fig. 7.1 Connection diagram on ST502 kit.

Procedure:

1. Connect the Power supply cord to mains supply and to the trainer ST2501.

2. Keep the mode switch in emitter 1 circuit in analog mode

3. Connect the 0.5m fiber cable in between the emitter LED & I/P of power meter.

4. Switch on the instrument fiber optic trainer & power meter (Keep the wavelength

switch in 660 nm, position). Note the reading in power meter.

5. Replace the 0.5m fiber cable with the 1m cables without disturbing any setting.

6. Again note the reading in power. This reading will be lesser then the previous one,

indicating that the propagation loss increases with increase in length.

7. Perform the same experiment with emitter 2.

Observations:

Using OPM, Pn=-31.3dBm, Pf=-31.4dBm

α = .1dB

Precautions:

a. The optical fiber must not be bent, so as to avoid bending losses.

b. The circuit must be neat and secure.

Experiment 8

Objective: To measurement of the Numerical Aperture (NA) of the fiber.

Apparatus Required:

1. ST2502 trainer with power supply cord

2. Optical Fiber cable.

3. Numerical Aperture measurement Jig/Paper & Scale.

Fig.8.1 Connection diagram on ST502

Teory:

The numerical aperture refers to maximum angle at which the incident on fiber end is totally

internally reflected and is transmitted along the fiber. The cone formed by rotation of this angle along

the axis of the fiber is the cone of acceptance of fiber. if light ray should strike the fiber end within

this cone of acceptance it will be transmitted properly else it is refracted out of fiber.

NA=

Procedure:

1. Connect the Power supply cord to mains supply and to the trainer ST2502.

2. Connect the frequency generator's 1 KHz sine wave output to input of emitter 1 circuit. Adjust its

amplitude at 5Vpp.

3. Connect one end of fiber cable to the output socket of emitter 1 circuit and the other end to the

numerical aperture measurement jig. Hold the white screen facing the fiber such that its cut face is

perpendicular to the axis of the fiber.

4. Hold fiber vertically at a suitable distance to make the red spot .

5. Record the distance of screen from the fiber end L and note the diameter W of the spot.

Compute the numerical aperture from the formula given below-

NA= (W/2) / √(W/2)^2+L^2

Result : The N.A. of fiber measured is 0.34 using trigonometric formula.

1. Dispersion is maximum at (FOR SMF) a) 1310 nm b) 1550nm c)1330nm d) none of the above 2. If zero DC bias is provided to the input of comparator circuit, (in case of digital optical fiber link). the output of the CRO will be a)'1' b) '0' c) circuit will not work properly d) output will be same as the input. 3. The full form of PMMA is ........................................................................................................................ 4. The full form of 'FC' connector stands for ................................................................................................. 5. A light ray is moving from denser to rarer medium, it will bend away from the normal. T/F