1. UNIT 1 OVERVIEW OF OPTICAL FIBRE COMMUNICATION

2. CONTENTS: Basics of Communication Electromagnetic Spectrum Elements of Optical Fiber Communication System Advantages of OFC.

3. Basics of Communication Communication: Transfer of information from one point to another over a distance Modulation : Long distance communication is achieved by a process called modulation. Information is superimposed on an electromagnetic wave called as CARRIER for the information signal. MODULATING Signal : Low frequency signal CARRIER SIGNAL : High frequency signal.

4. Selection of Electromagnetic carrier wave Radio Frequency spectrum (3kHz to 1 GHz) Microwave spectrum (1 GHz to 100 GHz) Optical frequency spectrum (THZ )

5. Radio Frequency Wave Electromagnetic waves ranging in frequencies between 3 KHz and 1 GHz are normally called radio waves. Radio waves are normally “ omni directional.” When an antenna transmits radio waves, they are propagated in all directions. This means that the sending and receiving antennas do not have to be aligned. The Omni directional characteristics of radio waves make them useful for multicasting, in which there is one sender but many receivers. Our AM and FM radio stations, cordless phones and televisions are examples of multicasting.

6. Disadvantages: Radio waves transmitted by one antenna are susceptible to interference by another antenna that is sending signals of the same frequency or band.

7. Microwave:

8. Continued…… Microwaves are especially suitable for this use more easily focused into narrower beams than radio waves, their comparatively higher frequencies allow broad bandwidthbandwidth high data transmission rates,data transmission rates Antenna sizes are smaller than at lower frequencies because antenna size is inversely proportional to transmitted frequency.

9. Continued….. Microwaves are used in TV, and telephone communications are transmitted long distances by microwaves between ground stations and communications satellites.communications satellites Microwaves are used radar technology. Microwaves are used radar Microwaves are used in Satellite communication Microwaves are also employed in microwave ovensmicrowave ovens

10. Limitations of Microwave: Above 40 GHz the atmosphere becomes opaque to microwaves Due to absorption from water vapor oxygen. Above 100 GHz, the absorption of electromagnetic radiation by Earth's atmosphere is so great that it is in effect opaque. The atmosphere becomes transparent again in the so-called infrared and optical window frequency ranges.

11. Transmission Lines: Depending on frequency and applications: Different cables are used: Two wire line : Flat cable Coaxial cables Microwave waveguides Optical Fibers

12. Different Transmission Lines and its Limitations: Twisted Pair Cable: Limited Frequency Spectrum (1MHz) Limited data rate 100 meter cable carry 100Mbps 5.5km cable carry 2Mbps or less Short distance between repeaters High error rate Susceptible to signal interferences such as EMI. Advantages: High availability Low Cost of Installation Low cost of Add-ons, local moves.

13. Coaxial Cables: Upper frequency limit : 4GHz Higher the frequency: higher are the losses Data rate : Coaxial cable of 185m gives a max data rate of 100Mbps.

14. Requirement of Cables: Long distance transmission needs low loss transmission line. Data rate should be high Should be compact Light weight Modern optical fiber can carry information at around 14Tb/s over 160 km fiber.

15. Optical Fiber Communication: It is a method of transmitting information from one place to another by sending light through an optical fiber.lightoptical fiber The light forms an electromagnetic carrier wave that is modulated to carry information.electromagneticcarrier wavemodulated Modulating Signal : Light signal

16. Transmission medium: Optical Fiber  It is a glass filament which carries light.  It is a cylindrical waveguide that operate at optical frequency.  It confines electromagnetic energy in the form of light.

18. Electromagnetic Spectrum: Long electrical Oscillations Radio Waves Microwaves Light Waves X-Rays

19. Electromagnetic Spectrum: Optical Communication uses infrared and visible light frequency region

20. Frequency Spectrum for Optical Fiber Communication: Light is the form of electromagnetic Wave just as radio waves, but with high frequency and shorter wavelength

21. Frequency Spectrum for Optical Fiber Communication:

22. Block diagram of OFC system: Basic Communication system: Transmitter Channel Receiver The process of communicating using fiber-optics involve the following basic steps: Creating the optical signal using a transmitter, relaying the signal along the fiber, ensuring that the signal does not become too distorted or weak. Channel and receiving the optical signal and converting it into an electrical signal. Receiver.

23. Block diagram of Optical fiber Communication System:

24. Information Source Message source Transducer Transducer: To convert non electrical signal to electrical signal Microphone converts sound waves into electrical signals Camera : Images into electrical signal. Computer: data already in electrical form

25. Optical sources The fundamental function of optical source in optical fiber communications is to convert electrical energy in the form of current into optical energy. The main optical sources currently used in optical fiber communications are lasers and light emitting diodes (LEDs).

26. Optical sources: In FOC system : Carrier signal is light signal. Information to be transmitted is superimposed on light to change the irradiance of the optical source as a function of time. Such a process of varying light level in accordance with the information signal is called : INTENSITY MODULATION

27. Light Sources: LED: LED is a non monochromatic, incoherent light source with a larger spectral width as compared to Lasers and is suitable for short distance communication. Speed of operation is slow. Lasers: Are more desirable optical source It provides highly directional, monochromatic light beam. Speed of operation is high,Lifetime is less and costly as compared to LED Used for Long distance communication

28. Information Channel: Path between the transmitter and receiver. Is a Optical Fiber made of Glass / plastic which carries information in form of light. Light propagation is modeled as rays and hence light transmission through fiber is analyzed by “RAY THEORY” Through channel different propagation effects affect the transmission rate and quality Dispersion Attenuation Loss

29. Optical detectors The information superimposed on light has to be separated in order to retrieve the original information. This process of recovering information from received light is called DEMODULATION The optical detector performs the opposite function from the source: It converts optical energy to electrical energy. The commonly used detector is the photodiode, photo transistor which produces current in response to incident light. PIN (P – Intrinsic – N diode) APD (Avalanche Photodiode)

30. Photodiodes: Photodiode is a light-sensitive device that converts the received photons into electrons. The output current of APDs are much higher than PIN as a single photon generates hundred or thousands of electrons in APD. APD are highly sensitive as compared to photodiode.

31. Block Diagram of Optical Fiber Communication:

32. Drive Circuit: To match the voltage and current according to the requirements of LED/ Laser devices. To prevent the drive current from exceeding the maximum rate and further affecting the reliability of the LED / Laser To meet expected brilliance requirements and ensure the color and brilliance homogeneity of each LED. The driver circuit should keep low power consumption so that the LED system efficiency can remain at a high level.

33. LED Driving Circuit : I b needed is low I c = βI b Transistors do have a limited amount of current handling ability Which can be improved by coupling together as a Darlington pair (often available in a single package).Darlington pair Also higher powered switches such as mosfets, can be driven for higher power requirements

34. Channel Couplers: Couplers are needed since the source emits light over large angular extent and fiber can capture light within a limited angle. Used to feed (FOCUS) the light signal into the channel. Example: In radio / TV broadcasting : coupler is antenna which couples the signal to atmosphere. In line communication: couplers are connectors. In Optical system couplers are lenses used to colliminate the light emmited from the source and direct it towards cable or photodetector.

35. Regenerators/ Repeaters: Used to boost the power level of weak signals. For weak signal repeaters/amplifiers are needed after every hundred or thousands of km to provide sufficient power to the receiver. Repeater amplify signal so that if the signal level falls below a threshold level due to losses, attenuation, distortion. Repeater detects the distorted signal, amplifies and reshapes the distorted signal and retransmits, Use of repeater improves the data rate. Repeater consists of Photodiode (APD/PIN diode) Electronic pulse generator Light source (LED/LASER)

36. Electrical receiver / Signal Processor: Amplifier : To amplify the detected signal. Filter: to filter out unwanted signal and transmit only the information signal. Destination: Information retrieved in electrical form must be converted into suitable form to be heard / seen / processed Transducer: Loudspeaker for audio message. CRT for pictures. Computer or other machines connected to OF system

37. Advantages of Optical Fiber: Greater Bandwidth Greater Information carrying capacity Immunity to cross talk Immunity to static interference Less Attenuation Greater Repeater Spacing Environmental Immunity Reliable and Durable Small size and light weight Economical

38. Advantages of Optical Fiber: Greater Bandwidth and hence greater information capacity: At high frequency of an optical fiber, data rates of 2Gbps aver 10’s of km is achieved. Twisted pair: few Mbps over 1km Coax: hundred Mbps over 1km.

39. Immunity to cross talk: OF are non metallic conductors of electric current. OF is not surrounded by magnetic field which is the prime reason to cross talk in metallic conductors which are located physically close to each other. Since there is no cross talk, it is difficult to tap the information. Information is more secure. Magnetic field produced by a current carrying wire

40. Immunity to static Interference: Since OF are non conductors of electric current. Immune to Electromagnetic Interference (EMI) that is radiations in space.

41. OF provides low Attenuation: Attenuation is markedly low in OF than compared to Twisted wire, coaxial cables. Transmission loss is less. Number of repeaters needed are less. Greater Repeater Spacing: Attenuation is less hence repeater spacing is more Cost and complexity reduced Environmental Immunity: OF are resistant to environmental extremes (whether variations). They operate over larger temperature range. Less affected by corrosive liquids, gases.

42. Reliable and Durable: OF have high tolerance to temperature, environmental conditions. OF cables last longer and are more reliable than metallic cables. Smaller Size and lighter weight: Thinner than Coaxial cable / twisted pair. Occupy less space. Economical: Raw material available in plenty. Cost is approximately same as metallic cable. OF have less loss, less no. of repeaters, low installation and system cost and improved reliability.

43. Disadvantages of Optical Fiber: A small bend on optical fiber causes loss. Alignment between fibers, connectors & core has to be precise. The cost of fusion splicing is high. The fault localization is some what difficult. Clean environment is required during splicing. A small dust particle can create high attenuation.