Syn Optic Mannual

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SYNERGY INSTITUTE OF TECHNOLOGY AND MANAGEMENT

DEPARTMENT OF ELECTRONICS AND COMMUNICATION

Optical Communication Lab (EC 701)

LIST OF EXPERIMENTS

1. Setting up Fiber Optic analog link

2. Setting up Fiber Optic digital link

3. Intensity Modulation System using analog input signal

4. Intensity Modulation System using digital input signal

5. Measurement of Numerical Aperture

6. Frequency Modulation System

7. Study of Propagation loss in Optical Fiber

8. Study of Bending Loss

9. Measurement of Optical Power using Optical power meter

10.Computer Communication using RS232 interface






EXPERIMENT NO. - 01

AIM: - Setting up Fiber Optic Analog Link

Study of a 650nm fiber optic analog link in this experiment you will study the relationship

between the input signal and received signal.

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

(light) energy containing the same information. The optical fiber is the medium which takes theenergy to the receiver. At the receiver light is converted back into electrical form with the same

pattern as originally fed to the transmitter.

Transmitter: -

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

provides both an electrical connection and isolation between the transmitter & the electrical

system supplying 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. Commonly

used optical sources are light emitting diodes (LED s) and Laser beam. Simple LED circuits, for

digital and analog transmissions are shown below.

Figure 1

Figure 1 shows Transconductance drive circuits for analog transmission-common emitter

configuration. The transmitter section comprises of:

1. Function Generator

2. Frequency Modulator &

3. Pulse Width Modulator block.

The Function Generator generates the input signals that are going to be used as information to

transmit through the fiber optic link. The output voltage available is 1 KHz sinusoidal signal of

adjustable amplitude, and fixed amplitude 1 KHz square wave signal. The modulator section



accepts the information signal and converts it into suitable form for transmission through the fiber

optic link.

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

The Receiver: -

The comparator circuit, low pass filter, phase locked loop, AC amplifier circuits form receiver on

the board. It is able to undo the modulation process in order to recover the original information

signal. In this experiment the trainer board is used to illustrate one way communication between

digital transmitter and receiver circuits.

PROCEDURE: -

1. Connect the Power Supply to the board.

2. Ensure that all switched faults are ‘Off’.3. Make the following connections : (as shown in figure 2)

a. Connect the 1 KHz sine wave output to emitter l's input.

b. Connect the Fiber Optics. cable between emitter output and detectors input.

c. Detector l's output to AC amplifier 1 input.

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

5. Switch on the power.

6. Observe the input to emitter 1 (TP5) with the output from AC amplifier 1 (TP28)

and note that the two signals are same.



Figure 2






EXPERIMENT NO. - 02

AIM: - Setting up Fiber Optic Digital Link

Study of an 650nm fiber optic digital link. In this experiment you will study the relationship

between the input signal and received signal.

Figure 3

Figure shows a simple drive circuit for binary digital transmission consisting a common emitter

saturating switch.

PROCEDURE: -


2. Ensure that all switched faults are ‘Off’.

3. Make the following connections (as shown in figure 4).

a. Connect the 1 KHz square wave output to emitter l's input.

b. Connect the fiber optic cable between emitter output and detectors input.

c. Detector 1's output to comparator 1’s input.

d. Comparator l's output to AC amplifier l's input.

4. On the board, switch emitter 1's driver to digital mode.

5. Switch on the power.

6. Monitor both the inputs to comparator 1 (TP13 & 14). Slowly adjust the comparators bias preset, until DC Level on the input (TP13) lies mid way between the high and low level of the

signal on the positive input (TP14).

7. Observe the input to emitter 1 (TP 5) with the output from AC amplifier 1 (TP28) and note

that the two signals are same.



Figure 4






EXPERIMENT NO. - 03

AIM: -

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

THEORY: -

Modulation: In order to transmit information via an optical fiber communication system it is

necessary to modulate a property of the light with the information signal. This property may be

intensity, frequency, phase with either digital or analog signals. The choices are indicated by thecharacteristics of optical fiber, the available optical sources and detectors, and considerations of

the over all 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.

Figure 5

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 1 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

amplifier 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.

Figure 6

If you glance at the figure you will see that analog waveform moves positive & negative of thezero line. The second problem is that it is the shape of the waveform which carries the



information. Ideally the emitter characteristic would be a straight line. Even so we would loose the

negative going half cycles as shown.

Figure 7

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 linear

characteristic would be ideal but the real characteristic is not completely straight. However, it does

have a straight section that we can use if we employ a suitable value of bias voltage. Figure 6 shows

ideal and practical situations.

PROCEDURE: -



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.



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 atdetector 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.

Figure 9






EXPERIMENT NO. - 04

AIM: - 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.

THEORY: -

Digital Modulation

With digital modulation, discrete changes in light intensity are obtained (i.e. ‘On-Off’ pulses)

figure 10 shows a block schematic of a typical digital optical fiber link.

Figure 10

Initially, input digital signal from the 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: -

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

‘Off’ state. The digital Input signal is entirely positive going as shown in figure 11

Figure 11

So, there is no negative part of the signal to be lost and further more 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 a digital input we employ a comparator at the receiving

end of the fiber to make the waveform square again called 'cleaning it up'.

PROCEDURE: -



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 changing outputon 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-linearities

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’. The next two experiments illustrate how an analog signal can be used

to modulate two specific characteristics of a digital signal.

Figure 12






EXPERIMENT NO. - 05

AIM: - Measurement of the Numerical Aperture (NA) of the fiber.

APPARATUS REQUIRED: - Numerical Aperture measurement Jig.

THEORY: -

Numerical aperture refers to the maximum angle at which the light incident on the fiber end is

very internally reflected and is transmitted properly along the fiber. The cone formed by the

rotation of this angle along the axis of the fiber is the cone of acceptance of the fiber. The light ray

should strike the fiber end within its cone of acceptance else it is refracted out of the fiber.Consideration in NA Measurement: - It is very important that the optical source should be

properly aligned with the cable and the distance from the launched point & cable be properly

selected to ensure that the maximum amount of optical power is transferred to the cable.

PROCEDURE: -

1. Connect Power Supply to the board and connect the frequency generator's 1 KHz sine

wave output to input of emitter 1 circuit. Adjust its amplitude at 5Vpp.

2. 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.

3. Hold the white screen with 4 concentric circles (10, 15, 20 & 25mm diameter) vertically at

a suitable distance to make the red spot from the fiber coincide with 10 mm circle.

Figure 134. Record the distance of screen

from the fiber end L & note the diameter W of the spot.

5. Compute the numerical aperture from the formula given above.

6. Vary the distance between in screen and fiber optic cable and make it coincide with one of

the concentric circles. Note its distance.

7. Tabulate the various distances and diameter of the circles made on the white screen and

compute the numerical aperture from the formula given.






EXPERIMENT NO. - 06

AIM: -

The Frequency Modulation System

THEORY: -

In the traditional form of FM the carrier frequency is changed or modulated by the amplitude of

the analog signal. This is not feasible in a fiber optic system since both our light sources, the LED

and the Laser, are fixed frequency devices. In fiber optic systems FM is achieved by using the

original analog input signal to vary the frequency of a train of digital pulses.

Figure 14

This is achieved by a circuit called Voltage Controlled Oscillator usually abbreviated as VCO. The

digital pulses are communicated through the optic fiber and squared up at the receiver by a

comparator in the same way as it was in amplitude modulation system. At this point we convert

the digital train back to the original analog signal by means of the Phase Locked Loop detector

(PLL). The PLL circuit performs a very simple function. It monitors an incoming signal and

produces a DC voltage output. If the input frequency increases, the DC voltage increases. If the

frequency decreases, the DC voltage decreases. In this way the original analog signal is recovered.

The output of the PLL contains many unwanted frequency components. These are removed by a

low pass filter and then finally the signal is amplified to the desired level.

PROCEDURE: -

1. Connect Power Supply to the board and

2. Ensure that all switched faults are ‘Off’.

3. Make the following connections (shown in figure 15 )

a. Connect Function generator 1 KHz sine wave signal to frequency modulator input.

b. Frequency modulator output TP2 to the emitter 1 input at TP5.Connect the optic

fiber between the emitter 1 circuit and the detector 1 circuit.

c. Detector 1 output TP10 to comparator 1 input at TP14. Comparator 1 output TP15

to the PLL detector input at TP23.

d. PLL detector output at TP26 to the low pass filter 1 input at TP19 Low Pass Filter 1

output TP20 to A C amplifier 1 input at TP27



4. Switch emitter l's driver to digital mode. This ensures that fast changing digital signal applied to

the drivers input causes the emitter LED to switch quickly between ‘On’ & Off’ states.

5. Turn the 1 KHz preset in the function generator block to fully anticlockwise (Zero amplitude)

position.

6. Switch on the Power Supply and monitor the output of the voltage controlled oscillators (VCO)

in the frequency modulator block TP2. Note that the frequency of this digital signal is at presentconstant, since the modulating 1 KHz sine wave has zero amplitude.

7. Examine the output of detector 1 (TP10 and check that the transmitted digital pulses are

successfully detected at the receiver).

8. With the help of dual trace oscilloscope monitor both inputs to comparator1. Now adjust the bias

1 preset until the bias input at TP13 is halfway between the top and bottom of the square wave

on TP14. You will remember that the function of the comparator is to clean up the square wave

after its transmission through the fiber optic link.

9. The output of comparator 1 drives the input of the PLL detector which produces a signal whose

average level is proportional to the frequency of the digital stream. This average level is then

extracted by low pass filter 1, and amplified by AC Amplifier1 to produce the original analog

signal at the amplifiers outputTP28. Examine TP28 and note that the output voltage is zero. Thisis expected since there is currently no modulating voltage in the transmitter.

10. While monitoring the input to the frequency modulator block TP1 and the output from AC

amplifier 1 TP28 turn the 1KHz preset to its fully clockwise maximum amplitude) position. Note

that the modulating 1 KHz signal now appears at the amplifiers output. If necessary, adjust the

amplifiers gain, adjust 1 preset until the two monitored signal are equal in amplitude.

11. In order to fully understand how this frequency modulation transmitter/ receiver system works,

examine the inputs and outputs of all functional blocks within the system, using an Oscilloscope.

Figure 15






EXPERIMENT NO. - 07

AIM: -

To measure propagation or attenuation loss in optical fiber

THEORY: -

Attenuation is loss of power. During transit, light pulse lose some of their photons, thus reduce

their amplitude. Attenuation for a fiber is usually specified in decibels per kilometer. For

commercially available fibers attenuation ranges from 1 dB / km for premium small-core glass

fibers to over 2000 dB / Km for a large core plastic fiber Loss is by definition negative decibels. In

common usage, discussions of loss omit the negative sign. The basic measurement for loss in afiber is done by taking the logarithmic ratio of the input power (Pi) to the output power (Po).

Where α is Loss in dB / Meter

PROCEDURE: -

1. Connect Power Supply to board.


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.


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. 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)



Figure 17






EXPERIMENT NO. - 08

AIM: - The object of this experiment into study bending loss.

THEORY: -

Whenever the condition for angle of incidence of the incident light is violated the losses are

introduced due to refraction of light. This occurs when fiber is subjected to bending. Lower the

radius of curvature more is the loss.

PROCEDURE: -

1. Connect Power Supply to board.2. Make the following connections (as shown in figure 36).

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.


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

8. Wind the FO cable on the mandrel and observe the corresponding AC amplifier output on

CRO. It will be gradually reducing showing loss due to bends.






EXPERIMENT NO. - 09

AIM: -

To measure optical power using optical power meter.

PROCEDURE: -

1. Connect the Power Supply to the board. Ensure that all switched faults are ‘Off’.

2. Connect the fiber optic cable between emitters 1's output & power meter's input.

3. On the board, switch emitter l's driver to analog mode. Keep the power meter's wavelength

selector switch in 660 nm

4. Switch on the power. Note the reading displayed in power meter.5. Switch the wavelength selector switch to 950 nm position. & note the reading displayed on

power meter.

6. Perform the same experiment with emitter 2.

Figure 19






EXPERIMENT NO. - 10

AIM: - Computer to Computer communication using RS232 interface via Fiber Optic Link

THEORY: -

There are 2 fiber optic Links provided on fiber optic trainer. We shall utilize these two links to

communicate from one PC to other & via-versa. That means it is a Duplex system of

communication using fiber optic link. The Software developed will help to transmit & receive

messages from the computers.

APPARATUS REQUIRED: - Personal computer (2 Nos. 486 or Pentium), DOS 6.0 or onwards,CD drive, RS232 cables for connecting PC's to Trainer (2 Nos.)

PROCEDURE : -

1. Keep one PC towards left and another towards right of fiber optic trainer.

2. Load the software in PC 1 & PC2, with the help of the CD supplied.

3. Keep one of the COM port free on each of PC to connect the RS232 cables. Keep

baud rate of both PC equal, say 57600.

4. Switch off both the PC's.

5. Make the following connections on the fiber optic trainer.

a. Connect Fiber Link on CHI (emitter to Detector).

b. Connect Fiber Link on CH2 (emitter to Detector).

c.Connect output of Detector 1 to comparator 1 input.

d. Connect output of Detector 2 to comparator 2 input.

e.Connect 1 KHz square wave to input of CHI (emitter).

f. Keep mode switch of both channels to digital and all Switched Faults in ‘Off’ position.

g. Switch on the Trainer. Observe input to emitter 1 and output of comparator Adjust bias

of comparator 1 for square wave output.

h. Switch 1 KHz square wave from the input of CHI to input of CH2 (emitter) and adjust

comparator 2 bias for square wave output. Switch off the Trainer.

i. Make connections as shown in diagram 17 and Switch on the Trainer.

j. Fiber optic trainer is ready for connection to PC's. Switch off the trainer.k. Connect PC1 & PC2 to D type connectors. (Any to anyone) switch on the PC's and the

Trainer & start working. Whatever you type in PC1 will be seen on the transmit

column of PC1 and will also be received in the receive column of PC2 simultaneously

& vice versa.

l. Remove any of the fiber links. To transmit & receive of that link is disconnected.

m. Change baud rate of any of the PCs & you will find that data is not transmitted. Keep

the baud rate same.

n. Reduce the baud rate of both the PCs & you will see that transmit rate is lower.

Switch off the Trainer and the PCs.



Figure 21

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