1Prof Z Ghassemlooy Optical Fibre Communication Systems Professor Z Ghassemlooy Electronics It Division School of Engineering Sheffield Hallam University U K www shu ac uk ocr Lecture 3 Light Sources 2Prof Z Ghassemlooy Contents Properties Types of Light Source LED Laser Types of Laser Diode Comparison Modulation Modulation Bandwidth 3Prof Z Ghassemlooy Light Sources Properties In order for the light sources to function properly and find practical use the following requirements must be satisfied Output wavelength must coincide with the loss minima of the fibre Output power must be high using lowest possible current and less heat High output directionality narrow spectral width Wide bandwidth Low distortion 4Prof Z Ghassemlooy Light Sources Types Every day light sources such as tungsten filament and arc lamps are suitable but there exists two types of devices which are widely used in optical fibre communication systems OLight Emitting Diode LED O Semiconductor Laser Diode SLD or LD In both types of device the light emitting region consists of a pn 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 5Prof Z Ghassemlooy LED Structure pn junction in forward bias Injection of minority carriers across the junction gives rise to efficient radiative recombination electroluminescence of electrons in CB with holes in VB g Ehf g Ehf Electron Hole pn Homojunction LED Fermi levels 6Prof Z Ghassemlooy LED Structure Optical power produced by the Junction q hc Ihf q IP int 0 Where int Internal quantum efficiency q Electron charge 1 602 x 10 19 C Electron Hole Narrowed Depletion region p typen type Pt Fibre I P0 Photons P0 7Prof Z Ghassemlooy LED External quantum efficiency ext Loss mechanisms that affect the external quantum efficiency 1 Absorption within LED 2 Fresnel losses part of the light gets reflected back reflection coefficient R n2 n1 n2 n1 3 Critical angle loss all light gets reflected back if the incident angle is greater than the critical angle 2 2 4 x ext n Fn It considers the number of photons actually leaving the LED structure Where F Transmission factor of the device external interface n Light coupling medium refractive index nx Device material refractive index 8Prof Z Ghassemlooy LED Power Efficiency dB dBm dBm cec LPP Or the power coupled to the fibre The coupling efficiency MMSF 2 NA c GMMF 2 2 NA c The optical coupling loss relative to Pe is e c c P P L 10 log10 External power efficiency 100 P pe ep Emitted optical power 2 2 0 4 x e n FnP P 9Prof Z Ghassemlooy LED Surface Emitting LED SLED G Keiser 2000 Data rates less than 20 Mbps Short optical links with large NA fibres poor coupling Coupling lens used to increase efficiency 10Prof Z Ghassemlooy LED Edge Emitting LED ELED Higher data rate 100 Mbps Multimode and single mode fibres G Keiser 2000 11Prof Z Ghassemlooy LED Spectral Profile Intensity 0 800 900 nm 1300 1550 nm 15 4565 15 4565 Wavelength nm 12Prof Z Ghassemlooy LED Power Vs Current Characteristics Since P I then LED can be intensity modulated by modulating the I Since P I then LED can be intensity modulated by modulating the I Current I mA Power P0 mW 1 2 3 4 5 Linear region SELED ELED 50 Temperature 13Prof Z Ghassemlooy LED Characteristics Wavelength 800 850 nm 1300 nmWavelength 800 850 nm 1300 nm Spectral width nm 30 60 50 150 Output power mW 0 4 5 0 4 1 0 Coupled power mW 100 um core 0 1 2 ELED 0 3 0 4 SLED 0 04 0 08 50 um core 0 01 0 05 SLED 0 05 0 15 0 03 0 07 Single mode 0 003 0 04 Drive current mA 50 150 100 150 Modulation bandwidth 80 150 100 300 MHz 14Prof Z Ghassemlooy LED Frequenct Response 110100100010 000 Frequency MHz 0 3 Magnitude dB 800 nm LED 1300 1550 nm Multimode Single mode LD 15Prof Z Ghassemlooy Laser Characteristics The term Laser stands for Light Amplification by Stimulated Emission of Radiation Could be mono chromatic one colour It is coherent in nature I e all the wavelengths contained within the Laser light have the same phase One the main advantage of Laser over other light sources A pumping source providing power It had well defined threshold current beyond which lasing occurs At low operating current it behaves like LED Most operate in the near infrared region 16Prof Z Ghassemlooy Laser Basic Operation Three steps required to generate a laser beam are Absorption Spontaneous Emission Stimulated Emission Similar to LED but based on stimulated light emission LED mirror 1mirror 2 coherent light R 0 99R 0 90 Mirrors used to re cycle phonons 17Prof Z Ghassemlooy Absorption When a photon with certain energy is incident on an electron in a semiconductor at the ground state lower energy level E1 the electron absorbs the energy and shifts to the higher energy level E2 The energy now acquired by the electron is Ee E2 E1 Plank s law E1 E2E2 Initial state E1 Excited electron final state E1 E2 18Prof Z Ghassemlooy Spontaneous Emission E2 is unstable an