Optical Fibre Communication SystemsLecture 3: Light Sources Professor Z Ghassemlooy Electronics & It Division School of Engineering, Sheffield Hallam University U.K. www.shu.ac.uk/ocr Prof. Z Ghassemlooy 1 Contents § Properties § Types of Light Source § § § § § LED § Laser Types of Laser Diode Comparison Modulation Modulation Bandwidth Prof. Z Ghassemlooy 2 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 Prof. Z Ghassemlooy 3 Light Sources . experiences injected minority carrier recombination. but there exists two types of devices. resulting in the generation of photons. Prof. 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. Z Ghassemlooy 4 . which are widely used in optical fibre communication systems: v v Light Emitting Diode (LED) Semiconductor Laser Diode (SLD or LD).Types Every day light sources such as tungsten filament and arc lamps are suitable. Z Ghassemlooy 5 .LED .Fermi levels hf ≈ Eg Homojunction LED Prof. • Injection of minority carriers across the junction gives rise to efficient radiative recombination (electroluminescence) of electrons (in CB) with holes (in VB) p hf ≈ Eg Hole n Electron --.Structure • pn-junction in forward bias. Structure • Optical power produced by the Junction: Pt Fibre Photons P0 n-type p-type Where P0 = I ηint ηhc hf = I q qλ ηint = Internal quantum efficiency q = Electron charge 1.LED . Z Ghassemlooy 6 .602 x 10-19 C P0 Narrowed Depletion region I Electron (-) + Hole (+) Prof. LED . Z Ghassemlooy 7 . 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. Prof.External quantum efficiency ηext It considers the number of photons actually leaving the LED structure η ext = Fn 2 4n x 2 Where F = Transmission factor of the device-external interface n = Light coupling medium refractive index nx = Device material refractive index Loss mechanisms that affect the external quantum efficiency: (1) Absorption within LED (2) Fresnel losses: part of the light gets reflected back. Z Ghassemlooy .LED .Power Efficiency • Emitted optical power Pe = P0 Fn 2 4nx 2 % External power efficiency pe ηep = × 100 P • MMSF: The coupling efficiency • GMMF: ηc = NA2 NA 2 ηc = 2 The optical coupling loss relative to Pe is : Lc = − 10 log10 Pc Pe Or the power coupled to the fibre: Pc (dBm) = Pe (dBm) − Lc (dB) 8 Prof. LED.Surface Emitting LED (SLED) • Data rates less than 20 Mbps • Short optical links with large NA fibres (poor coupling) • Coupling lens used to increase efficiency G Keiser 2000 Prof. Z Ghassemlooy 9 . LED. Z Ghassemlooy 10 .Edge Emitting LED (ELED) • Higher data rate > 100 Mbps • Multimode and single mode fibres G Keiser 2000 Prof. LED . Z Ghassemlooy 11 .Spectral Profile Intensity 1300-1550 nm 800-900 nm 65 45 15 λ0 15 45 65 Wavelength (nm) Prof. Power Vs. Current Characteristics Power P0 (mW) 5 4 3 2 1 SELED Temperature Linear region ELED Current I (mA) Since P ∝ I.LED . then LED can be intensity modulated by modulating the I Prof. Z Ghassemlooy 12 50 . 50 um core .15 0.04 800-850 nm 30-60 0.LED .08 0.03-0.05 SLED 0.04-0.4-1.07 0.Single mode 0.4-5 1300 nm 50-150 0. Z Ghassemlooy 100-150 100-300 13 .100 um core .003-0.1-2 ELED 0.05-0.Characteristics Wavelength • Spectral width (nm) • Output power (mW) • Coupled power (mW) .4 SLED 0.0 • Drive current (mA) • Modulation bandwidth (MHz) 50-150 80-150 Prof.01-0.3-0. Z Ghassemlooy 14 .Frequenct Response LED Magnitude (dB) 0 -3 LD 550 0-1 130 nm 800 nm de mo gle Sin ode ltim Mu 1 10 100 1000 10.000 Frequency (MHz) Prof.LED . Z Ghassemlooy 15 .Laser . • It is coherent in nature. all the wavelengths contained within the Laser light have the same phase). (I.e. • Could be mono-chromatic (one colour).Characteristics • The term Laser stands for Light Amplification by Stimulated Emission of Radiation. 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 Prof. 90 Three steps required to generate a laser beam are: • Absorption • Spontaneous Emission • Stimulated Emission Prof. mirror 1 mirror 2 Mirrors used to “re-cycle” phonons” R = 0.Basic Operation Similar to LED. Z Ghassemlooy 16 . but based on stimulated light emission.99 “LED” coherent light R = 0.Laser . Plank's law E2 E1 E2 E1 E2 E1 Initial state Excited electron final state Prof.E1 ). Z Ghassemlooy 17 .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 . Spontaneous Emission • E2 is unstable and the excited electron(s) will return back to the lower energy level E1 • As they fall. they give up the energy acquired during absorption in the form of radiation. Z Ghassemlooy 18 . which is known as the spontaneous emission process. E2 E1 E2 Photon E1 Initial state Prof. The generated photon(s) is in phase and have the same frequency as the incident photon. it will stimulate the electron to return to the lower state level. if external stimulation (photon) is used to strike the excited atom then. • By doing so it releases its energy as a new photon. Z Ghassemlooy 19 . • The result is generation of a coherent light composed of two or more photons E2 E1 Requirement: α <0 E2 Coherent light E1 Light amplification: I(x) = I0exp(-αx) Prof.Stimulated Emission • But before the occurrence of this spontaneous emission process. τph is the photon rate. τs spontaneous recombination rate.The Rate Equations Rate of change of photon numbers = stimulated emission + spontaneous emission + loss dφ φ = Cnφ + Rsp − dt τ ph Rate of change of electron numbers = Injection + spontaneous emission + stimulated spontaneous dn J n = − − Cnφ dt qd τsp J is thecurrent density. Z Ghassemlooy 20 . C is the constant Prof. Rsp is the rate of spontaneous emission. Modes are separated by Optical confinement layers ∆λ = c δf = 2nL for i >> 1 2nL 2nL 2nL − = i i +1 i In terms of wavelength separation λ2 λ2 ∆λ = = δf 2nL c Prof. . Z Ghassemlooy 21 . 2n i = 1..Laser Diodes (LD) I L Standing wave (modes) exists at frequencies for which λi L= . 2. Spectral Profile ∆λ Intensity Modes Gaussian output profile 5 3 1 λ0 1 3 5 Wavelength (nm) Multi-mode Prof. Z Ghassemlooy 22 .LD . LD .Efficiencies Internal quantum efficiency ηint = number of photons generated in the cavity number of injected electrons ηext Pe = IE g External quantum efficiency External power efficiency Where P = IV ηep = Pe P Prof. Z Ghassemlooy 23 . Power P0 (mW) 5 4 3 2 1 LED Stimulated emission (lasing) Spontaneous emission 50 Current I (mA) Threshold current Ith Prof.Power Vs. Z Ghassemlooy 24 . Current Characteristics Temp. LD .Single Mode • Achieved by reducing the cavity length L from 250 µm to 25 µm • But difficult to fabricate • Low power • Long distance applications Types: • Fabry-Perot (FP) •Distributed Feedback (DFB) • Distributed Bragg Reflector (DBR) • Distributed Reflector (DR) Prof. Z Ghassemlooy 25 . Laser .7 to 2 nm Highly polarized Coherence length: 1 to 100 mm Small NA (→ good coupling into fiber) Agilent Technology Prof.Fabry-Perot i Strong optical feedback in the longitudinal direction i Multiple longitudinal mode spectrum Ppeak i “Classic” semiconductor laser – 1st fibre optic links (850 nm or 1300 nm) – Short & medium range links i Key characteristics – – – – – – – Wavelength: 850 or 1310 nm Total output power: a few mw Spectral width: 3 to 20 nm Mode spacing: 0. Z Ghassemlooy λ P Threshold I 250-500 um Cleaved faces 5-15 um 26 . 8 pm) Sidemode suppression ratio (SMSR): > 50 dB Coherence length: 1 to 100 m Small NA (→ good coupling into fiber) P peak SMSR λ Agilent Technology 27 Prof.Laser . Z Ghassemlooy .08 to 0. uses Bragg Reflectors for lasing iSingle longitudinal mode spectrum iHigh performance – Costly – Long-haul links & DWDM systems iKey characteristics – – – – – – Corrugated feedback Bragg Wavelength: around 1550 nm Total power output: 3 to 50 mw Spectral width: 10 to 100 MHz (0.Distributed Feedback (DFB) iNo cleaved faces. Laser .3 Laser output p-DBR active n-DBR Agilent Technology Prof.2 to 0.Vertical Cavity Surface Emitting Lasers (VCSEL) i Distributed Bragg reflector mirrors – Alternating layers of semiconductor material – 40 to 60 layers. Z Ghassemlooy 28 . each λ / 4 thick – Beam matches optical acceptance needs of fibers more closely i Key properties – – – – – Wavelength range: 780 to 980 nm (gigabit ethernet) Spectral width: <1nm Total output power: >-10 dBm Coherence length:10 cm to10 m Numerical aperture: 0. Laser diode .Single mode • External quantum efficiency • Drive current (mA) • Modulation bandwidth (MHz) Multimode 1-5 1-10 0.000 Prof. Z Ghassemlooy 29 .2 10-100 1-40 25-60 100-250 6000-40.Properties Property • Spectral width (nm) • Output power (mW) • Coupled power (µW) .1-5 1-40 50-150 2000 Single Mode < 0. Higher dispersion Less temperature dependent Simple construction Life time 107 hours Prof.Comparison LED Laser Diode i i i i i i i i i i i i i High efficiency Fast response time Higher data transmission rate Narrow output spectrum Coherent output beam Higher bit rate High launch power Less distortion Suitable for longer transmission distances Lower dispersion More temperature dependent Construction is complicated Life time 107 hours 30 i i i i i i i i i i i i Low efficiency Slow response time Lower data transmission rate Broad output spectrum In-coherent beam Low launch power Higher distortion level at the output Suitable for shorter transmission distances. Z Ghassemlooy . Modulation The process transmitting information via light carrier (or any carrier signal) is called modulation. • Direct Intensity (current) • Inexpensive (LED) • In LD it suffers from chirp up to 1 nm (wavelength variation due to variation in electron densities in the lasing area) DC RF modulating signal R I Intensity Modulated optical carrier signal • External Modulation Prof. Z Ghassemlooy 31 . Z Ghassemlooy 32 .Direct Intensity Modulation.Analogue LED LD Input signal G Keiser 2000 Prof. Direct Intensity Modulation. Z Ghassemlooy 33 .Digital LED Optical power Optical power LD i Time i Time t t Prof. 5 Gbps .External Modulation • For high frequencies 2. Z Ghassemlooy 34 .40 Gbps • AM sidebands (caused by modulation spectrum) dominate linewidth of optical signal DC MOD MOD R Modulated optical carrier signal RF (modulating signal) I Prof. Modulation Bandwidth In optical fibre communication the modulation bandwidth may be defined in terms of: • Eelectrical Bandwidth Bele .(most widely used) • Optical Bandwidth Bopt . Z Ghassemlooy 35 .Larger than Bele Optical 3 dB point G Keiser 2000 Prof.