ECE 228A Fall 2008 Daniel J. Blumenthal! 10.1!Lecture 10 -Transmitters!ECE 228A Fall 2008 Daniel J. Blumenthal! 10.2!Semiconductor Optical Sources! The optical sources practically used in optical communications are based on semiconductor devices! The generated optical signal is to be efficiently coupled to the output optical fiber! Other kinds of sources (non-semiconductor) and/or free space coupling is sometimes done in R&D labs, but only for advanced and prototypal research! All commercial sources comes in very compact packages, and are fiber pigtailed in the factory! Pigtailing and packaging is one of the most critical and expensive issues for these devices!Fiber Pigtail!Laser Package!ECE 228A Fall 2008 Daniel J. Blumenthal! 10.3!Semiconductor Optical Sources!➱ Compact integrated devices used to convert a modulated electrical signal to a modulated optical signal that is then efficiently coupled to the optical fiber.!Characteristic DescriptionNumber of longitudnalmodesNumber of optical frequencies laser emits. Plays a key role in bothlaser cost and how fiber dispersion will limit link bit rate.Side Mode SuppressionRatio (SMSR)A measure of how good a single mode laser is.Threshold current The minimum current required to turn on the laser. Low values arekey to decrease transmitter power dissipationLaser Noise A measure of how random the optical laser output is. Thischaracteristic can determine the ultimate performance of a link.Linewidth A measure of how noisy the laser. Plays a key role in how dispersionand crosstalk limits the transmission bit rate and capacity.Wavelength Determines the dispersion and loss operating points in the fiber andother network components.Modulation Bandwidth Determines the bit rate that can be attained by current modulation.Chirp A measure of how the optical output frequency changes with currentmodulation. Impacts transmission bit rate.Linearity Ability to reproduce and analog signal with low distortion.Fiber Output Power Power launched into fiber to achieve high signal-to-noise ratio.Wavelength Tunability The ability to tune the output wavelength over a wide range.Long Term Stability In terms of wavelength, output power and other key factors.ECE 228A Fall 2008 Daniel J. Blumenthal! 10.4!Laser Diode Issues! Direct modulation vs. External modulation! Lower cost vs. less chirp and less pattern effect! InGaAsP active region vs. InGaAlAs! No aluminum issues vs. higher temp operation! Bulk vs quantum well active region! Less complexity vs. lower threshold! Single transverse mode (vertical direction)! Essential for single mode fiber. Not an issue.! Single lateral mode (horizontal direction)! Essential for single mode fiber. Narrow waveguide required.! Single longitudinal mode! Important for long distance communication (smaller spectral width)! Current confinement?! Needed for low threshold, but don’t increase thermal resistance! Optical confinement?! Needed for single mode operation, but don’t increase thermal res.! Carrier confinement?! Needed for low threshold, but don’t affect lifetime.!ECE 228A Fall 2008 Daniel J. Blumenthal! 10.5!Semiconductor Optical Sources!Light Emitting Diode (LED)! Multimode Laser Diode! Single Mode Laser Diode!λ"λ0"λ"λ0"λ"λ0"Δλ ≈ 10 nm!Δλ ≈ 30-100 nm!Δλ ≈ .01 nm!The linewidth, Δλ, is often measured at the full width half maximum point (FWHM)!ECE 228A Fall 2008 Daniel J. Blumenthal! 10.6! Three basic optical semiconductor interactions! E2 is the bottom of the conduction band and E1 is the top of the valance band!E2!E1!Photon hν"hν"E2!E1!hν"hν"hν"E2!E1!• Incident photon Ephoton= hν= E2 - E1!• Electron - hole pair generated (EHP) !e-!h+!• Radiative recombination!• Photon spontaneously emitted with energy Ephoton= hν= E2 - E1!• Incident photon causes radiative recombination!• Two photons with same characteristics created!Semiconductor Optical Source Basics!ECE 228A Fall 2008 Daniel J. Blumenthal! 10.7!Current injection!Semiconductor p-n junction!Light generation!Semiconductor Optical Source Basics!Laser!LED!No Optical Feedback!Optical Feedback!ECE 228A Fall 2008 Daniel J. Blumenthal! 10.8! Laser = Gain medium + optical cavity! Lasing occurs with population inversion or when gain exceeds losses!Ex : Double Heterostructure laser!Ibias!Light!Contact layer!p-InP!n-InGaAsP!N-InP!Light!Cleaved facets!Semiconductor Lasers!ECE 228A Fall 2008 Daniel J. Blumenthal! 10.9!Rate Equations!RGdtdN−=N is the electron density (assumed equal to hole density)!G is the generation rate of electrons!R is the total recombination rate!ECE 228A Fall 2008 Daniel J. Blumenthal! 10.10!Rate Equations!τηNRRCNANBNRRRRRRqVIGRGdtdNststinrspi≈+++=+++==−=32N is the electron density (assumed equal to hole density)!G is the generation rate of electrons!R is the total recombination rate!ECE 228A Fall 2008 Daniel J. Blumenthal! 10.11!Absorption and Amplification! For a 2-level atomic system, we define the following transition rates assuming the induced rates from levels 1 to 2 and 2 to 1 are proportional to the photon densities (per unit frequency)! The total downward rate (2 to 1) is the sum of the induced and spontaneous rates! We also assume that the atoms are blackbody radiators in thermal equilibrium at temperature T with spectral density!W21'( )induced= B21ρ(ν)W12'( )induced= B12ρ(ν)W21'= B21ρ(ν) + A21W12'= W12'( )induced= B21ρ(ν)ρ ν( )=8πn3hν3c31ehνkBT− 1ECE 228A Fall 2008 Daniel J. Blumenthal! 10.12!Einstein Relations! In thermal equilibrium, the number of 2 to 1 and 1 to 2 transition rates are balanced! The distribution of energy states for atoms in TE for a two level system will follow the Boltzman distribution! Equating N2/N1 above, we see that the following relations (Einstein relations) must be satisfied!N2W21'= N1W12'N2B218πn3hν3c31ehνkBT− 1+ A21= N1B128πn3hν3c31ehνkBT− 1N2N1= e−hνkBTB12= B21A21B21=8πn3hν3c3ECE 228A Fall 2008 Daniel J. Blumenthal! 10.13!Energy Density! Defining the spontaneous lifetime by tspont=1/A21, we can write the transition rate per atom due to an incident optical field with a uniform spectrum and energy density ρ(ν), beam intensity I0=cU0/n (watts per square meter), U0 is the energy density of