Description

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

Information

Category:
## Science & Technology

Publish on:

Views: 61 | Pages: 29

Extension: PDF | Download: 0

Share

Transcript

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)

Related Search

Similar documents

We Need Your Support

Thank you for visiting our website and your interest in our free products and services. We are nonprofit website to share and download documents. To the running of this website, we need your help to support us.

Thanks to everyone for your continued support.

No, Thanks

SAVE OUR EARTH

We need your sign to support Project to invent "SMART AND CONTROLLABLE REFLECTIVE BALLOONS" to cover the Sun and Save Our Earth.

More details...Sign Now!

We are very appreciated for your Prompt Action!

x