Ofc Ppt.pdf

2/27/2019

Introduction to OFC Unit 3

Problems of Electrical Communication systems: Why we require an alternative • Affected by EMI • Low bandwidth (4 kHz – telephone, 100-500 MHz per km – coaxial cable ) • High attenuation (20 dB/km – typically) • High system cost – due to too many repeaters for a given Bandwidth/ data rate – Eg. 32 channel (2.048 Mbps) PCM link required one repeater every 2 km

• Prone to tapping/hacking • Bulky • Requires to often replacement (due to ageing effects) 2

1

2/27/2019

Advantages of Optical Fiber Communication (Fiber Optics) • Very high bandwidth (as high as Thz) E.g. Red Led operating at 730 nm produces signal of approx 1017 Hz (f=C/λ) • Very low attenuation (lowest 0.16 dB/km) • Immune to EMI • Data security (almost impossible to tap information) • Lower system cost (fewer repeaters due to low attenuation of fibers) • Small size and low weight • Very low Bit Error Rate ( < 10-10 typically) • Effects of rain, weather, corrosion etc are irrelevant to OFC

Disadvantages of Optical Fiber Communication (Fiber Optics) • • • • • • •

Lower tensile strength Susceptible to bending losses Prone to manufacturing defects Interfacing with electronic devices Difficulty in locating faults Need of specified tools Reaction by chemicals

2

2/27/2019

Fiber Optics Applications • Telecom applications – Primarily SMFs (single mode fibers)

• Networking applications – Mostly SMFs (FTTH- Fiber direct to the home)

• Fiber optic Sensing – SMF and MMF

• Medical applications – Mostly MMF, or bundle fibers (light pipes)

• Industrial applications – Mostly MMF 5

Basics of Optical Fibre Communication An Optical Fiber Communication System consists of • Transmitter (electrical to optical) – Optical source (LED or Laser diode) + driver circuit

• Optical Fibre – Single mode fibre, or – Multimode fibre

• Receiver (optical to electrical) – Photodetector PIN or APD + receiver circuit

3

2/27/2019

Basic block diagram of OFC

Transmission of Light Inside Fiber -Optical Fiber acts as waveguide to transmit optical pulses -The transmission is based on principle of total internal reflection (TIR) *Conditions: Dense to rare medium propagation *Angle must be greater than Critical angle

-TIR is in turn based on critical angle Lets take a look

4

2/27/2019

Calculating Critical Angle

5

2/27/2019

Optical Fiber

• An optical fiber is essentially a waveguide for light • It consists of a core and cladding that surrounds the core • The index of refraction of the cladding is less than that of the core, causing rays of light leaving the core to be refracted back into the core

Physical Structure of fiber • Optical fiber is made from thin strands of either glass or plastic • It has little mechanical strength, so it must be enclosed in a protective jacket • Often, two or more fibers are enclosed in the same cable for increased bandwidth and redundancy in case one of the fibers breaks • It is also easier to build a full-duplex system using two fibers, one for transmission in each direction

6

2/27/2019

Numerical Aperture (NA)and Acceptance angle • Acceptance cone is defined as the maximum angle of incidence at which the external light rays must strike the air/glass intersection and enters the fiber core and propagate within it

Numerical Aperture • The numerical aperture is expressed as sine of maximum angle which light ray can make with the central axis of the fiber core and can travel through fiber using TIR • The numerical aperture is given by the formula: N . A.  n12  n22 • Numerical aperture is light gathering capacity of fiber

7

2/27/2019

Quick Revision

Types of fiber • Based on– refractive index profiles – Step index optical fiber – Graded index optical fiber

• Based on number of modes – Single mode OFC – Multimode OFC

16

8

2/27/2019

Step index optical fiber • It has central core having a uniform index of refraction which is surrounded by cladding that too has a uniform index of refraction but of lesser value of core. • Thus there is an abrupt change in refractive index at intersection of the fiber core and the cladding of the OFC

9

2/27/2019

Graded index optical fiber • It has a central core with non uniform index of refraction. This simply means that refractive index of the fiber core is maximum in the middle and then gradually decreases towards its outer edge up-to core cladding interface

Single and Multimode Fiber

10

2/27/2019

Wavelength Division Mux.

11

2/27/2019

LED operation

Photo detectors

12

2/27/2019

SOA: Semiconductor optical amplifier

SOA: Semiconductor optical amplifier • In semiconductor optical amplifiers (SOAs), electron-hole recombination occurs • The semiconductor optical amplifier is of small size and electrically pumped. • It is aslo potentially less expensive

13

2/27/2019

14