Optical Fibres and Telecommunications Light Sources, Modulation, Transmitters and Receivers Introduction Previous section looked at Fibres. How is light generated in the first place? How is light modulated? How can useful information be sent down the fibre? How can we read back transmitted information? How can we characterise the quality of the components used? How can we characterise the quality of the information we receive? Section builds on knowledge from other areas and applies it to real-world communications. 1
Light sources Laser diodes and LED s. Review of semiconductor optical devices. Edge and surface emitting devices. Efficient fibre coupling Which source should we use? Today s lecture Simple semiconductor physics Light emitting diodes for communications. Emission characteristics of LED s Surface emitting LED s Edge emitting LED s 2
Energy/Momentum (E/k) Diagrams E Conduction Band (CB) Band Gap Energy E g Fermi Level E g Valence Band (VB) k Remember: Electron promoted to the conduction band leaves a positively charged hole in the valence band. Direct and Indirect Bandgap Materials CB Direct Indirect CB Phonon Required VB k VB Indirect materials do not emit light efficiently. Unsuitable for laser operation. k 3
Doping Semiconductors n-type k p-type k Adding impurities into the semiconductor crystal changes the energy structure. Dopants which give up electrons easily to the CB are called donors. n- type semiconductors. Dopants which take up electrons leaving holes in the VB are acceptors. p-type semiconductors. Overall effect shifts the position of the Fermi level. Degenerate doping moves the FL into the CB or VB Examples of dopants: Acceptor - Zn Donor - Te p-n Junctions. CB FL VB p n Degenerately-doped Case Holes Depletion Region Electrons FL remains constant. CB and VB bend in joining region. Holes and electrons can recombine. Light can be emitted. Holes and electrons confined to p and n regions in degenerate case. 4
p-n Junctions II Electrons Holes ev f Apply forward bias to degenerately doped junction. Both electrons and holes present in the depletion region. Recombination is possible. Spontaneous emission this is the light emitting diode (LED). hν=e g Making a Laser Now have a gain medium. Now require a cavity. Could use mirrors. More usually cleave end faces of semiconductor sample. R=((n sc -n air )/(n sc +n air )) 2 Eg. GaAs: n=3.5, r=30% No need for mirrors in most cases. 5
LED s for Commuinications For certain communications applications LED s are a good choice. Cheap Small Very long lifetimes Disadvantages with respect to laser sources. Low intensity Hard to focus Low modulation bandwidth Incoherent radiation Used in short-distance, low-bandwidth networks eg. Fibre optic local area networks (LANS) Emission Characteristics of LED s LED s are incoherent emitters. Spontaneous emission occurs in all directions in the junction. Operating wavelength often around 850nm in first telecomms window. Sources also available around 650nm for use with Plastic Optical Fibre. Broad bandwidth output (~30 to 150nm.) Often used with multimode or graded index fibre. 6
LED design Surface emitting LED - SLED +ve Electrical Contact P type Active Region N type -ve Electrical Contact Light emitted in all directions. Simplest approach is to extract light from the surface of the device. Light emission is Lambertian in profile. P θ =P 0 cos θ (41) Easy to butt-couple a fibre to the LED. LED Design Edge Emitting LED (ELED) +ve contact -ve Contact P type Active Region N type Mirror Light is confined using a waveguide. Emission from one edge of the device. Very similar in design to laser diodes. Assymetric output radiation pattern. Lambertian in plane of the junction and diffraction limited orthogonal to the junction. Higher coupling efficiencies to fiber possible. More complex coupling optics required. 7
Power output characteristics of LED s P out =(NE p η int )/t (42) Radiated Light Power Saturation Linear Region Forward Current I P out = Output Power N = Number of electrons injected E p = Energy of a photon η int = Internal quantum efficiency Now I=Ne/t (43) I= Drive Current e = electron charge. So P out =[(η int E p )/e]i (44) So P out is proportional to I. Saturation occurs when all mobile electrons are being used to produce photons. LED Output Spectrum Normalised Power λ λ p Wavelength Material E g λ p Si 1.17 1067 Ge 0.775 1610 GaAs 1.424 876 InP 1.35 924 InGaAs 0.75-1.24 1664-1006 AlGaAs 1.42-1.92 879-650 InGaAsP 0.75-1.35 1664-924 LED s have a broad, Gaussian, output bandwidth. Typical FWHM ( λ) ~30-150nm. Provides major limit on propagation distance due to chromatic dispersion. Range of output wavelength possible by materials choice. Data from Fiber-Optic Communications Technology p. 321 8
Coupling to Optical Fibre SLED Fibre Microlens Due to the poor output characteristics of the LED, normally coupled to multimode or graded index fibre. Output from LED can be improved using lenses or shaped fibre. Coupling efficiency still low ~few %. Unusual to couple more than 100 s µw into fibre. Short range link! Can approximate for a SLED: P fibre (NA) 2 P led (45) Other coupling techniques Fiber-optic communications technology p. 323 9
A real LED. Summary Introduction to this section of the course Sources Modulators Transmitters Receivers Basic semiconductor physics LED s ELED s and SLED s Coupling LED s to fibre 10