OPTICAL SOURCE : Light Emitting Diodes (LEDs)

INTRODUCTION

Fiber Optic Communication (1) FOC is a method of transmitting information from one place to another by sending pulses of light through an optical fiber. The physics behind the use of light as the transmitter is the amount of information that can be transmitted is directly proportional to the frequency range over which the carrier operates.

Figure 1 (a)The general communication system. Information source Transmitter (modulator) Transmission medium Receiver (demodulator) Destination b) Aa Figure 1 (a)The general communication system. (b) The optical fiber communication system. Information source Electrical transmit Optical source Optical fiber cable Optical detector Electrical receive Destination

Fiber Optic Communication (2) As the carrier frequency increases, the available transmission bandwidth will increases as well. When the bandwidth becomes larger, it will provides a larger information capacity.

Objective To explore design and operation of optical communication components : light emitting diode (LEDs).

Problem Statement The improvement of designation and operation of LEDs should be practiced in the optical communication world. This kind of alternative can bring us to the most effective optical source that will save more energy and time, but at the same time will gain more benefit.

THEORY

Requirement of Optical Source Output wavelength must coincide with the loss minima of the fiber Output power must be high, using lowest possible current and less heat High output directionality, narrow spectral width. Bandwidth should be wide enough Low distortion

Properties of LEDs (1) Light emitters converts the electrical signal into a corresponding light signal that can be injected into the fiber. The disadvantages of LEDs compared to the laser: generally lower optical power coupled into a fiber usually lower modulation bandwith harmonic distortion

Properties of LEDs (2) But, LEDs have a number of distinct advantages which have given it a prominent place in optical fiber communications. Simpler fabrication No mirror facets No striped geometry Low cost Simpler construction Reliability Less-sensitive to gradual degradation Immune to self-pulsation and modal noise problems Less temperature dependence Light output against current less affected by temperature LEDs are not threshold device Simpler drive circuitry Temperature compensation circuits unnecessary. Linearity Linear output against current characteristic

WORKING PRINCIPLE OF LEDs

Power & Efficiency of LEDs (1) In LEDs, the light emitting region consists of a p-n junction constructed of a direct band gap III-V semiconductor, which when forward biased, experiences injected minority carrier recombination, resulting in the generation of photons.

Power & Efficiency of LEDs(2) Other than radiative recombination, non-radiative recombination may be occur sometimes and generate phonons. LEDs will produce at best internal quantum efficiency of 50% for simple homojunction devices. Whereas for heterojunction devices, the internal quantum efficiency range from 60 to 80%.

ηint = (Pint / hf ) / (I/e) Power & Efficiency of LEDs (3) Internal quantum efficiency, ηint : the ratio of the radiative recombination rate to the total recombination rate. ηint = (Pint / hf ) / (I/e)

Power & Efficiency of LEDs(4) External quantum efficiency, ηext : ratio of the number of photons emitted from the device to the photons internally generated.

Power & Efficiency of LEDs(5) There are several loss mechanisms that affect the external quantum efficiency: Absorption within LEDs Fresnel losses Critical angle loss

Power & Efficiency of LEDs(6) The optical power emitted Pe into a medium : The coupling efficiency :

The Operating Principle of LEDs (1) There are homostructure, single heterostructure and double heterostructure.

The Operating Principle of LEDs (2) When a forward bias is applied, electrons from the n-type layer are injected through the p–n junction into the p-type GaAs layer where they become minority carriers. These minority carriers diffuse away from the junction, recombining with majority carriers (holes) as they do so. light is emitted from the device without re-absorption because the band gap energy in the AlGaAs layer is large in comparison with that in GaAs.

The Structure of LEDs

The Surface Emitter LEDs (1)

The Surface Emitter LEDs (2) \ The Surface Emitter LEDs (2) This kind of structure provides a low thermal impedance in the active region. The internal absorption in this device is very low due to the larger bandgap-confining layers, and the reflection coefficient at the back crystal face is high giving good forward radiance. The surface emitter LEDs can transmit the data rate less than 20 MHz. It contains the short optical link with large NA.

The Edge Emitter LEDs (1)

The Edge Emitter LEDs (2) w The Edge Emitter LEDs (2) This type of LEDs make use of the transparent guiding layers with a very thin active layer (50 to 100 μm) in order that the light produced in the active layer spreads into the transparent guiding layers. Majority of the propagating light are emitted at one end face with the light reflected back from the other end face. Its coupling efficiency is higher than the surface emitter LEDs due to the smaller NA fiber. The edge emitter LEDs radiate less power to the air compared to the surface emitter LEDs because of the reabsorption and interfacial recombination. The edge emitter LEDs can transfer higher data rate, as much as 100 MHz than the surface emitter LEDs.

CHARACTERISTICS OF LEDs

LEDs Characteristics (1) The spectral profile of LEDs is broader than the laser.

LEDs Characteristics (2) The graph of light output into the air against the drive current for LEDs.

SUMMARY

Summary (1) The double heterostructure of LEDs give the best performance due to the high radiative recombination occur. The edge emitter LEDs give rise to the high coupling coefficient with smaller NA, thus produce high radiance into the fiber.