Lab4

Experiment No. 4

Study of Losses in Optical Fiber

Ammeter (0 to 100 mA)

+9V

+

↑

Anode of LED

-

OPTICAL TRANSMITTER

EXT-ANALOG

PRE-AMPLIFIER

+

D R I V E R

V

-

INTENSITY CONTROL OPTICAL FIBER LINK

Vcc

PHTODETECTOR MODULE

ANALOG OUT + V

GND

-

BLOCK DIAGRAM FOR LOSS MEASUREMENT

JUMPER SETTING DIAGRAM FOR EXPERIMENT NO. 4 1

+5V

2

SFH 756V Anode

3

+ 9V

1

Emitter of Q2 (2N2907)

2

Cathode of SFH 756V

3

Collector of Q1 (2N3904)

4

Cathode of SFH 450V

JP10

JP8

1

+

2 3

+

↑

1

-

V

2

For LED SFH 756V

3

-

4

JP10

JP8

For LED SFH 450V 1

+5V

2

SFH 450V Anode

3

+ 9V JP9

Emitter of Q2 (2N2907)

2

Cathode of SFH 756V

3

Collector of Q1 (2N3904)

4

Cathode of SFH 450V JP8

1

+

2 3

1

+

↑

1

-

V

2 3

-

4

JP9 JP8

1

2

3

Pr6 Terminal

Common for both LEDS GND Emitter of Q1 (2N3904)

JP7

1

Amplifier Output

2

3

Digital Buffer Output JP6

Base of Q1 (2N3904)

EXPERIMENT NO. 4 NAME Study of Losses in Optical Fiber.

OBJECTIVE The objective of this experiment is to measure propagation loss for two different wavelengths & bending losses in plastic fiber provided with the kit.

THEORY Optical fibers are available in different variety of materials. These materials are usually selected by taking into account their absorption characteristics for different wavelengths of light. In case of optical fiber, since the signal is transmitted in the form of light, which is completely different in nature as that of electrons, one has to consider the interaction of matter with the radiation to study the losses in fiber. Losses are introduced in fiber due to various reasons. As light propagates from one end of fiber to another end, part of it is absorbed in the material exhibiting absorption loss. Also part of the light is reflected back or in some other directions from the impurity particles present in the material contributing to the loss of the signal at the other end of the fiber. In general terms it is known as propagation loss. Plastic fibers have higher loss of the order of 180 dB/Km. 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. Another losses are due to the coupling of fiber at LED & photodetector ends.

EQUIPMENTS Experimenter Kit Photodetector Module Lab Power Supply for Photodetector Module (0 to 30V) 1 Meter and 3-Meter Fiber Cables. Jumper Wires (2 nos.) Ammeter (0 - 100 mA) - 1 no. Voltmeter (0 - 5V) - 2 nos. This experiment requires that you should perform experiment 3 - Characteristics of LED and Detector and get the data generated for optical power launched in fiber for known amount of current in LED and corresponding photodetector response. First we will find out the propagation loss in optical fiber for 660 nm wavelength and then repeat the procedure to find out the loss at 950 nm wavelength. The experimental settings are exactly same as in described in procedure of experiment no. 3.

PROCEDURE 1. Without connecting power supply, first make the jumper settings for JP6 and JP7 as shown in the jumper block diagram. 2. Now make use of jumper wires provided along with the kit to make connections of voltmeter and ammeter to the jumpers JP10 and JP8. This setting is used to measure the current flowing through the LED SFH 756V (JP10) and to measure the voltage across the same LED (JP10 and JP8).

3. Keep the optical power control potentiometer Pr6 at its maximum position (Anticlockwise rotation). 4. Select 30-cm. long optical fiber cable and insert one end of it into the cap LED SFH756V after unscrewing it first. Tighten the cap screwing it back. 5. Take a photodetector module provided along with kit. Note that it has PIN diode SFH250V mounted on it. 6. Connect the power supply wires to the variable DC voltage source in your laboratory. Note that module is provided with the external power source and not from the kit. 7. Before connecting the power supply to the module, first ensure that its output is set to DC +5V and that while connecting the supply to the module, it is in off state. Note: Photodetector module is sensitive device, so please do not try to connect it to live power supply. 8. Now insert the other end of the 30 cm. optical fiber cable into the cap of the detector SFH250V and tighten the cap screwing it back. 9. Connect the voltmeter across the test points TP1 and TP2 of photodetector module. PIN diode is reverse biased and has 1 MΩ resistance in series of it. You will be measuring voltage across this resistance. 10. Jumper JP1 on the photodetector module brings this resistance into series of the photodetector when it is shorted. It you do not short it, then you have to connect micro - ammeter across the test points TP1 and TP2 to measure photocurrent flowing through it directly. 11. Using either steps 9 or 10, you can measure photocurrent flowing through photodetector. Now switch on the power supply for the kit as well as that of photodetector module. 12. Now using optical power control potentiometer Pr6 on the kit, go on varying current through LED SFH756V and set the current corresponding to known optical output power say for 100 µW. 13. For this value of optical energy, measure the voltage across 1 MΩ resistance of photodetector module. 14. Now without disturbing any of the setting of Pr6 and without varying any supply voltage of photodetector, replace the 30-cm. fiber cable with a 1 mtr. fiber cable. Now measure the voltage across 1 MW resistance of photo detector module. 15. Find out the current flowing through the detector when fiber cable of 1 mtr. length is connected. With this value of photocurrent, find out the incident optical energy using conversion efficiency given in the data sheet of photodetector. It should be less than 100 µW due to the loss in optical fiber. The difference between 100 µW and calculated incident energy value is the loss in 1 mtr. fiber cable length for 660 nm wavelength. 16. Repeat the same procedure with 3 mtr. of a fiber cable and measure the loss for 3 mtr. of length. With the two sets of values for losses, take the average and get the loss per meter of the fiber cable at 660 nm wavelength. 17. The entire procedure from step 1 to step 16 can be repeated with LED SFH450V (950 nm) to find out the loss in optical fiber at 950 nm wavelength. While doing so, please refer the jumper wire settings for LED SFH450V. These are for JP9 and JP8 as shown in the diagram