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Optical Transmitters Peter Caputa Electronic Devices Department of Electrical Engineering Linköping University Sweden Electronic Devices 2(29) Outline Background Basic Concepts The pn-junction Emission

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Optical Transmitters Peter Caputa Electronic Devices Department of Electrical Engineering Linköping University Sweden Electronic Devices 2(29) Outline Background Basic Concepts The pn-junction Emission and Absorption rates Light-Emitting Diodes Semiconductor Lasers Source-to to-fiber coupling Summary 3(29) Background Optical transmitters convert an Electrical input signal into a corresponding Optical signal - e Optical Source photon An optical transmitter must have the following properties: -High radiance for 0.8µm 1.6µm wavelength -Emissive area no greater than core diameter -The device must match the aperture of the fiber -Easy modulation Suitable Optical sources: 1) Light-Emitting Diodes (LED:s) 2) Semiconductor lasers 4(29) Energy band model Electron energy Basic Concepts 1(5) E g - E c Conduction band (only few free electrons) + E g Bandgap energy E v Valence band (filled with electrons, only few free holes) E(k)-diagram (k-vector proportional to electron impulse) -Free electron model: E(k) 2 h k 2m 2 * + E c E(k) 2 h k 2m -Energy for electron in semiconductor: where m 2 where d E 2 dk h h 2π -E(k)-diagrams for semiconductors are usually very complicated * h 2 h6.63*10-34 Js (Plancks constant) is the effective carrier mass Basic Concepts 2(5) 5(29) Direct bandgap transitions E c E Excitation: hυe g E v k Recombination: hυe g - Large probability for excitation and recombination since E c (min) and E v (max) are at the same k-vector - Direct bandgap semiconductors are optically active (suitable for detectors, lasers, light-emitting diodes) 6(29) Basic Concepts 3(5) Indirect bandgap transitions E v E c E k k Excitation: hυe g ±E k + : Photon emission - : Photon absorption Recombination: hυe g E k ± + : Photon absorption - : Photon emission - Small probability for excitation and recombination close to E g since E c (min) and E v (max) are not at the same k-vector - Transitions must preserve both energy AND momentum Basic Concepts 4(5) Under normal conditions, all materials absorb light rather than emit it Excitation Ehυ hυ E 2 E 1 Spontaneous Emission E 2 E 1 Stimulated Emission hυ E 2 E 1 hυ hυ -E 1 Ground state energy level -E 2 Excited state energy level -Incoming photon energy Ehυ equals E g E 2 -E 1 photon is absorbed (atom ends up in excited state) -Emitted photons in random direction -No phase relationship among photons -Dominates in LED:s -Is initiated by an existing photon -Photons have matched energy, phase, direction of propagation, polarisation, k-vector -Dominates in semiconductor lasers 7(29) 8(29) Basic Concepts 5(5) Semiconductor materials and doping -Semiconductor materials: Si, Ge, GaAs, InP -N-type dopants: P, As (moves fermi level closer to conduction band) -P-type dopants: B, Ga (moves fermi level closer to valence band) Fermi level - The electron distribution over allowed energy levels at thermal equilibrium is given by the Fermi-Dirac distribution function f(e): f(e) 1+ e 1 (E EF )/ kbt where k B 1.38*10-23 J/K ; (Boltzmanns constant) E F Fermi level energy T Temperature [K] 1 f( E ) T 1 (EF + EF e )/ kb F An energy state at the Fermi level has 50% probability of being occupied by an electron 9(29) p-type pn-junction 1(2) Energy bands for doped semiconductors E c n-type E c E fc E fv E v E v pn-junction in equilibrium p-type depletion region n-type E c E f E v -The fermi level is uniform across the junction (bends E c and E v ) -Electrons (holes) accumulate on the n-side (p-side) -Built-in electric field across the junction prevents carrier diffusion 10(29) pn-junction 2(2) Forward-biased pn-homojunction (same material on both sides of the junction) p-type depletion region n-type qv 0 E c E v Homojunction problem -External voltage V 0 is applied accross the junction -Built-in electric field is reduced (depletion region becomes thinner) -Diffusion of carriers across the junction I e [ qv 1] 0 k B T / I 0 -Electrons and holes recombine (spontaneous or stimulated emission) in the depletion region - Wide depletion region (1-10µm) Carriers not confined close enough to the junction Difficult to obtain high carrier densities Double Heterostructure 11(29) -Insert a thin (~0.1um) active layer having a reduced band gap between the n-layer and p-layer -Bandgap discontinuity carriers confined to the active layer -Refractive index difference dielectric waveguide -Active layer thickness controls which optical modes are generated 12(29) Emission and Absorption Rates 1(4) Rate definitions -Rate of spontaneous emission: R spon AN 2 -Rate of stimulated emission: R stim BN 2 ρ em -Rate of absorption R abs B'N 1 ρ em where: N 1 Atomic density in the ground state N 2 Atomic density in the excited state ρ em Spectral density of the electromagnetic energy At thermal equilibrium, the atomic densities are distributed according to Boltzmann statistics: N 1 / N ( Eg / kb T) (hυ/kb T) 2 (1) e e 13(29) Emission and Absorption Rates 2(4) In thermal equilibrium, the upward and downward rates must be equal: AN 2 + BN 2 ρ em B'N 1 ρ em Using the rate definitions and Eq.(1), the spectral density then becomes: ' (B /B)e A/B ρ em (hυ/kb T) 1 (2) In thermal equilibrium, Eq.(2) corresponds to spectral density of blackbody radiation: π h υ / c ρ em (hυ/kb T) e 1 (3) Identify Einstein constants from Eq.(2) and Eq.(3) ( 3 8 π h υ / c )B B ' B A 3 Emission and Absorption Rates 3(4) Electron and hole Fermi-Dirac distributions -Recombination probability is proportional to electron concentration at E c and hole concentration at E v f f c v (E (E c v ) ) 1+ e 1+ e 1 (Ec Efc )/ kbt 1 (Ev Efv )/ kbt Occupation probability for electrons in conduction band Occupation probability for holes in valence band Emission and absorption rates -Spontaneous emission rate: (sum over all possible transitions such that E c -E v hυphoton energy) R spon (ω) A(E Ec v,e c )f -Stimulated emission rate: R stim (ω) B(E Ec -Absorption rate: R abs (ω) B(E Ec v v,e, E c c )f )f c c v (E (E (E c c v [ f v (Ev)] ρ E ) 1 cv [ f v (Ev)] ρ ρ E ) 1 cv em [ f c (Ec)] ρ ρ E ) 1 cv em c c c 14(29) 15(29) ρ cv Emission and Absorption Rates 4(4) where m r 3/2 (2m r) ( hω 2 3 E 2 π h mc mv (mc + m v) g ) 1/2 -Joint density of states (number of states per unit volume per unit energy range) -m r reduced mass m c effective electron mass in conduction band m v effective hole mass in valence band Results from emission and absorption rates 1) R spon can exceed R stim and R abs if k B T hυ 2) hυ 1eV for radiation in visible or near infra-red region k B T 25 mev at room temperature R stim /R spon 1/(e (hυ /k -1) 1 Thus, R spon R stim at room temperature B T) All lasers must operate away from thermal equilibrium (pump lasers with external energy) Light-Emitting Diodes (LED:s) 16(29) LED properties: - LED:s are forward biased pn-homojunction or pn-double heterostructures - Spontaneous recombination dominates - Incoherent (no phase relationship) light is emitted Internal Optical Power where: I I/q h ω η int I/ η int P int η int ( hω / q)i forward bias current carrier injection rate photon energy internal quantum efficiency (fraction of electron-hole pairs recombining through spontaneous emission) q rate of photon generation 17(29) Light-Emitting Diodes (LED:s) Emitted Power where: η ext η ext η P η T f (θ) Fresnell transmittivity η Pe ext int ext int ( hω / q)i external quantum efficiency (fraction of photons escaping from the device) 1 θc T (θ)(2π sinθ)dθ 4π 0 f θ incident angle θ c critical angle p-type n-type θ c Example: θ 0 n 3.5 ; refractive index T f (0) 4n/(n+1) 2 η ext n -1 (n+1) %!!! Light-Emitting Diodes (LED:s) Total Quantum Efficiency η tot where: emitted power applied electrical power V 0 voltage drop over the device typically: hω qv 0 η η tot ext η int Responsivity R LED P typically: e /I η ext R LED 10 mw/a η int ( hω / q) η η ( hω / qv0) ext (typically 1%) int non-linear due to active region temperature increase Power vs. Current for 1.3um LED 18(29) 19(29) LED Spectrum The spectrum affects performance through fiber dispersion Spectral width ( λ( λ) ) is given by: hc hc λ Ephoton where λ 2 Ephoton Ephoton γ λ λ E E photon photon 2 k E B T photon Theoretical spectral width: (Room temperature 2k B T0.052eV) Measured InGaAsP LEDs with three different active layer compositions λ γ λ 0.85µm nm 1.3µm nm 1.55µm nm Semiconductor Lasers 20(29) Semiconductor laser properties -Emit high-power coherent light (~100mW) through stimulated emission -Narrow angular spread -High coupling efficiency (~50%) -Narrow spectral width permits high bit rates (~10Gb/s) Requirements for laser action: 1) Population inversion and optical gain 2) Positive feedback to obtain a laser oscillator Population Inversion Population inversion is obtained when an external energy source raises the atomic population from the ground state to the excited state -Fermi-level separation exceeds the bandgap under forward biasing (heavy doping of p-type and n-type layers) -Exponential optical gain in active layer when injected carrier density exceeds transparancy value N T -Peak gain coefficient: N gp (N) g0 1 + ln N0 N Injected current density [cm -3 ] N N T N T Transparency value (1-1.5*10 18 cm -3 for InGaAsP) g p g 0 when N N 0 g p 0 when N N T -When N N T : Stimulated emission rate Absorption rate Gain for 1.3um InGaAsP laser for various injected carrier densities 21(29) 22(29) Positive Feedback for Lasers Two mirrors form a Fabry-Perot cavity -No external mirrors, cleaved facets are enough -Typically 30% mirror reflectivity -Self-oscillation when gain internal losses: current injection 1 1 g α + ln 2L R1R where 2 n-1 R m Mirror reflectivity n+ 1 int αint + αmirr 2 α int internal loss α mirr mirror loss α cav cavity loss n refractive index of gain medium α cav Active region L cleaved facets R 1 R 2 gain medium mirrors z0 zl Laser Operation 1(2) -Forward biased pn-junction -At high enough injection, stimulated emission starts to dominate -When photons are generated, only a small fraction leave the cavity photon density builds-up -Resonant modes are produced: mirrors z0 zl -Photon wavelength must satisfy: aλ L a 1, 2, Mode spacing π k L 23(29) 24(29) Gain Spectrum Resonant modes Laser Operation 2(2) Light Emission a) Below threshold -Gain is less than cavity loss -Light emission is broad as in LED b) At threshold -A few modes start dominating the emission spectrum Dominant mode c) Above threshold -The gain spectrum is unchanged -Dominant mode in light emission due to stimulated emission 25(29) Laser Structures 1(2) Broad area laser Gain guided laser Oxide stripe laser Junction stripe laser -Current injected over a broad area covering whole laser -Current injected over a narrow stripe (~5µm) confines light in the stripe -Active layer thickness ~0.1µm -Light emission region(1*5 µm 2 ) -Light in elliptic spot (1*100 µm 2 ) DRAWBACKS: -High threshold current -Beam is not stable as laser power is increased -Typical threshold mA DRAWBACKS: -Mode stability problems -Beam is not stable as laser power is increased 26(29) Index guided laser Laser Structures 2(2) Ridge-waveguide structure (weak index guiding) Buried heterostructure (strong index guiding) -Introduce an index step n L in vertical direction to form a wave guide -Weak index guided laser has n L ~0.01 (left figure) -Strong index guided laser has n L ~0.1 (right figure) -Light in elliptic spot (2*1 µm 2 ) -Single-mode light is emitted by controlling width and thickness of active layer -Stable spatial distribution of light as laser power increases Laser Spectral Characteristics (a) Typical laser spectra: a) gain-guided laser b) index guided laser Gain-guided laser spectra -many excited modes causes broad spectral line width Index-guided laser spectra -many excited modes around lasing threshold -when driving current increases, the mode with smallest cavity losses reaches threshold first and starts to dominate -reduced cavity length (L 100µm) increases mode spacing -single mode spectral width λ10-4 nm are achievable -mode-suppression ratio 30dB for a good single-mode laser (b) 27(29) Coupling efficiency n c : nc (1- R f)(na) R f Fiber front-end reflectivity NA Fiber Numerical Aperture NA 2 (n 2 core n 2 cladd) Butt coupling Source-Fiber Coupling 2 -Bring fiber close to source and hold in place by index-matching epoxy n c 1% (surface-emitting LED) n c 10% (edge-emitting LED) n c 10% (laser into single-mode fiber) Lens coupling -Rounded fiber n c 40% -Spherical lens n c 70% Coupling issues -Alignment -Optical feedback 0.1% feedback is enough to destabilize laser linewidth broadening, mode hopping Prevent by antireflective coating, cut fiber at slight angle Butt coupling Lens coupling 28(29) 29(29) LED:s are characterized by: Summary -Spontaneous emission -Incoherent light -Large angular spread -Low total quantum efficiency ( 1%) -Large spectral width ( λ30nm-100nm) LED:s are suitable for low cost applications Mb/s, 100Mb/s, transmitting up to a few kilometers Semiconductor lasers require: 1) Population inversion and optical gain 2) Positive feedback to obtain a laser oscillator Semiconductor lasers are characterized by: -High-power coherent light (~100mW) through stimulated emission -Narrow angular spread -High coupling efficiency (~50%) -Narrow spectral width permits high bit rates (~10Gb/s)

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