unit iii ocn

8/13/2019 Unit III Ocn

1/20

UNIT III

SOURCES AND DETECTORS


2/20

Optical sources:

Optical sources are active components.

Whose fundamental function is to convert the

electrical energy into optical energy. Hence

the optical sources also called as transducers.

Two types of Optical sources

1. LED2. Semi conductor laser diodes.


3/20

Advantages of LED & lasers:

Compact size

High efficiency

Good reliability

Right wavelength range

Possibility of direct modulation at relatively high

frequency.


4/20

Requirements of light sources for communication

Light o/p should be high direction.

It must emit light at required wavelength (1.3m,1.55m) ie

low losses and low dispersion.

To couple the large amount of power into an optical fiber, theemitting area should be small.

It must require very small power.

The optical o/p power is directly modulated varying the i/p

current. Optical o/p must be changes in temperature

Light source should compact size and high efficiency.

High optical o/p power & coupling efficiency.


5/20

Light emitting Diodes(LEDS)

LEDs are used in optical communication systems that

require bit rate less than approximately 100-

200Mb/s

It is mostly coupled with multimode optical fiber. The radiation from an LED is incoherent and is

emitted over a wide range of angles.

LED require less complex drive ckt.


6/20

Advantages of LED:

Simpler fabrication.

Low cost.

Reliability. Less temperature dependence.

Simpler drive circuitry.

Linearity


7/20

Principle of operation

Led can be used in fiber transmission

application are

High radiance o/p

Fast emission response time

High quantum efficiency.


8/20

Radiance: It is measure of optical power radiated into a unit

solid angle per unit area of the emitting

surface.(watt)

Emission response time:

It is a time delay b/w the application of acurrent pulse and respective optical sources.

Quantum efficiency:

It is related to the fraction of the electron-hole

pairs that recombine radiatively.


9/20

LED Structures:

For photonic communications requiring data rate

100-200 Mb/s with multimode fiber with tens of

microwatts, LEDs are usually the best choice.

LED configurations being used in photoniccommunications:

1- Surface Emitters (Front Emitters)

2- Edge Emitters


10/20

Hetro Junction Structure:


11/20

Surface emitting LED


12/20

DH Edge emitting LED


13/20

Full-Width half maximum:

The width of the spectral pattern at its half power

point is known as the full width half maximum

spectral width.

The LED FWHM is 36nm. The fundamental quantum mechanical relationship

b/w energy E and freq v

E=hv

= h C/ Where c=v

= hc/E


14/20

Internal quantum Efficiency(INT)

Internal quantum Efficiency(INT) is defined as theratio of radiative recombination to the total

recombination


15/20

INJUNCTION LASER DIODE

Laser is light amplification by stimulated

emission of radiation.

Ideal laser light has single wavelength only.

The lasing medium can be a gas, a liquid, an

insulating crystal or a semiconductor.

Semiconductor laser diodes are used as a

optical sources.


16/20

Principle of operation:

Laser action is the result of three key

processes

Photon absorption

Spontaneous emission

Stimulated emission


17/20


18/20

Spontaneous emission:

Initial state Final State

E2 E2Spontaneous emission hv

E1 E1 Spontaneous emission, an atom returns to the lower

energy state in random manner.

It gives incoherent radiation.


19/20

Stimulated emission:

Initial state Final State

E2 E2stimulated emission hv

E1 E1 hv The light associated with emitted photon is of same

frequency of incident photon and in same phase with

same polarization


20/20

Advantages & disadvantages

unit iii ocn

Documents