Introduction??? Learning Objectives On completion of this chapter you will be able to: 1. Construct the block diagram for Fibre Optic Communication System. 2. Mention the sources which are used for transmission of optical signals. 3. Describe the process of converting the analog signal into optical signal. 4. Explain how the optical signals are detected at the receiving end. Material prepared by: < Physics Faculty > Topic No: < 5 > Page 1 of 6
FIBRE OPTIC COMMUNICATION SYSTEM:- It comprises of the following three main units: Transmitter - has light source and input signal modulator Optical fibre - to couple transmitter and receiver Receiver - has optical detector and signal demodulator Input (Analog) data DRIVER LIGHT SOURCE Source - to - fib re connector Transmitter Optical Fib re - to - detector connector DETECTOR OUTPUT CIRCUIT Output (Analog) data Receiver Transmitter:- It consists of analog to digital interface (driver). Here, the input signal is converted into binary form. Then, the carrier light is modulating with respect to the binary digits. The output of LED/Laser diode will be ON when the bit is 1 and OFF if the bit is 0. In this manner, the analog signal gets transformed into optical signal. Light sources for fibre optic communication: A source must meet certain following requirements: (1) Monochromatic (to avoid chromatic dispersion) (2) Intense (transmission is possible for longer distances) Material prepared by: < Physics Faculty > Topic No: < 5 > Page 2 of 6
(3) Capable of being easily modulated (for ex. Using LED and LD, modulation can be done easily unlike in gas ionization lamps and other types of lasers) 4) Small and compact (to couple fibres easily and effectively) 5) Durable and Inexpensive Light-Emitting Diode (LED) There are two types of LED. They are surface etched well emitter (S-LED) and edge emitter (E-LED). (Surface Etched well type S-LED) (Edge emitter type E-LED) Material prepared by: < Physics Faculty > Topic No: < 5 > Page 3 of 6
In the S-LED, a well is etched into the top of the planar LED structure. This enables the fibre end to be kept as close as possible to the light emitting region. In general, E-LEDs are preferred to use with small NA whereas S-LEDs are better for larger NA fibres. (Note: We have already discussed Laser (Semiconductor) diode in detail in the LASER chapter) Receiver:- The optical signal received should be first converted into digital form and then analog output. Further, the analog output has to be amplified to overcome the loss if any during its propagation through optical fibre. Photodetectors At the receiver end, conversion of optical signal into analog signal is being done with the help of photo detectors for example photodiode. The photodetectors are required to have (1) High quantum efficiency photon to electron conversion efficiency (2) Adequate frequency response speed of response (3) Low dark current low dark current low leakage current in a reverse biased diode in the absence of illumination (4) Low signal dependent noise able to respond for signal rather than noise Advantages of fibre optic communication:- Enormous band width Coaxial cable transmission 500 MHz; FOCS 10 5 Hz. Electrical isolation No need of special care for insulation, fibres are made up of insulators/dielectrics. Material prepared by: < Physics Faculty > Topic No: < 5 > Page 4 of 6
Immunity to interference and cross talk Since fibres are dielectric wave guides, they are free form em interference. There will be no interference even when large number of fibre cables bundled together. Signal security One could not draw signal from optical cable without tampering it unlike from copper wire Small size and weight Since the fibre cables are small in size, less weight and hence occupy less space, they are useful in aircrafts and satellites. Low transmission loss Transmission loss is very low compared to copper wire communication. It is reduced up to 0.2 db/km. Hence, it is used for longer distance communication with less number of repeaters. Harshness and flexibility Fibre cables are flexible, compact and extremely rough/rugged. Low cost Since fibres are made up of silica which is available in abundance, optical fibres are inexpensive now a days. Check Your Understanding (CYU) 1. What is the purpose of Driver shown in the block diagram? 2. Which type of LED is more suitable for Fibre Optic Communication? 3. Say true or false: Modulation can be done easily using LED and LD unlike in gas ionization lamps and other types of lasers 4. What are the approximate band widths for coaxial transmission and fibre optic communication systems? (Check the correct answers on next page) Material prepared by: < Physics Faculty > Topic No: < 5 > Page 5 of 6
Summary On completion of this chapter you have learned the following: (1) The Fibre Optic Communication System has three main blocks that is Transmitter - has light source and input signal modulator Optical fibre - to couple transmitter and receiver Receiver - has optical detector and signal demodulator (2) The sources used for transmitting optical signals and how they are functioning. Activity Why we are choosing the fibre optic transmission rather than coaxial cable transmission? What are the factors which are deciding the optical modulation as more efficient? Think about it. Suggested Reading 1. P.K. Palanisamy, Engineering Physics, Scitech Publications Pvt Ltd, Chennai. 2. C. Shanthi et al., Engineering Physics, Sonaversity, Salem. 3. M. Arumugam, Engineering Physics, Anuradha Agencies, Kumbakonam. (And some other open resources from internet) Answers to CYU 1. To convert analog signal into digital form 2. S-LED. 3. True. 4. 500 MHz; 10 5 Hz. Material prepared by: < Physics Faculty > Topic No: < 5 > Page 6 of 6