Light Sources for Optical Communications,EE 8114 Xavier Fernando RCL Lab,Requirements,Small physical dimensions to suit the fiber Narrow beam width to suit fiber NA Narrow spectral width (or line width) to reduce chromatic dispersion Fast response time (high bandwidth) to support high bit rate High output power into the fiber for long reach without repeaters,Considerations ,Ability to directly modulate by varying driving current Linearity (output light power proportional to driving current) important for analog systems Stability LED better than LASER Driving circuit issues impedance matching Reliability (life time) and cost,Solid State (Semiconductor) Light Sources,Light Emitting Diode (LED) Simple forward biased PN junction LASER Specialized LED with stimulated emission to provide : low line width, low beam width, high power and coherency,Semiconductor Physics,LEDs and laser diodes consist of a pn junction constructed of direct-bandgap III-V materials. When the pn junction is forward biased, electrons and holes are injected into the p and n regions, respectively. The injected minority carriers recombine either radiatively (a photon of energy E = h is emitted) or nonradiatively (the recombination energy is dissipated as heat).,The pn junction is known as the active or recombination region.,Energy-Bands,Pure Group. IV (intrinsic semiconductor) material has equal number of holes and electrons. Thermal excitation of an electron from the valence band to the conduction band enable it to freely move.,n-type material,Donor level in an n-type (Group V) semiconductor. The ionization of donor impurities creates an increased electron concentration distribution.,p-type material,Acceptor level in an p-type (Group III) semiconductor. The ionization of acceptor impurities creates an increased hole concentration distribution,Intrinsic & Extrinsic Materials,Extrinsic material: donor or acceptor type semiconductors. Majority carriers: electrons in n-type or holes in p-type. Minority carriers: holes in n-type or electrons in p-type. The operation of semiconductor devices is essentially based on the injection and extraction of minority carriers.,Intrinsic material: A perfect material with no impurities.,Indirect Band Gap Semiconductors,Physical Design of an LED,Double hetero structure is used to improve light output (2 p type and 2 n type materials) Each region shall also have the right refractive index to guide the light (optical property) Light is first confined in the active region (high ref. index) due to waveguide operation Then it exists via the front (surface emitting LED) or the side (edge emitting LED),Double-Heterostructure configuration,Light-Emitting Diodes,LED features: Made of GaAlAs (850 nm) or InGaAsP (S-L bands) Broad spectral output (50 to 150 nm) Optical output powers less than -13 dBm (50 W) Can be modulated only up a few hundred Mb/s Less expensive than laser diodes Edge-emitter or surface emitter structures,Edge-Emitting LED,The active region is embedded into a waveguide structure so that the light is directed an edge Larger active region More directional radiation (similar to LASER),Wavelength, Eg and the Ratio between Semiconductors,Relationship between the crystal lattice spacing, Eg, emission at room temp. The shaded area is for the quaternary alloy In1xGaxAsyP1y,Bandgap Energy,The source emission wavelength depends on the bandgap energy of the device material.,16,Bandgap Energy,For In1xGaxAsyP1y compositions that are lattice-matched to InP, the bandgap in eV varies as,17,Bandgap wavelengths from 920 to 1650 nm are covered by this material system.,Surface and Edge Emitting LED,18,Generally an LED is a broadband light source,Rate equations and Quantum Efficiency of LEDs,When there is no external carrier injection, the excess density decays exponentially due to electron-hole recombination. n is the excess carrier density, Bulk recombination rate R:,With an external supplied current density of J the rate equation for the electron -hole recombination is:,In equilibrium condition: dn/dt=0,t,n(t),Bulk recombination rate (R) = Radiative recombination rate (Rr) + Nonradiative recombination rate (Rnr),For exponential decay of excess carriers: Radiative recombination lifetime r=n/Rr Nonradiative recombination lifetime nr=n/Rnr,For high quantum efficiency, Rr Rnr r nr,Quantum Efficiency,Internal quantum efficiency is the ratio between the radiative recombination rate and the sum of radiative and nonradiative recombination rates,Where, the current injected into the LED is I, and q is the charge of an electron.,Internal Quantum Efficiency & Optical Power,Optical power generated internally in the active region in the LED is equal to the number of photons/seconds (I/q) times energy per photons (hv) times the internal quantum efficiency,4-9,External Efficiency,Not all the light internally generated exits the LED The actual light output depends on: the refractive index of the activ