Semiconductor Optical Sources

Source Characteristics Important Parameters Electrical-optical conversion efficiency Optical power Wavelength Wavelength distribution (called linewidth) Cost Semiconductor lasers Compact Good electrical-optical conversion efficiency Low voltages Los cost

Semiconductor Optoelectronics Two energy bands Conduction band (CB) Valence band (VB) Fundamental processes Absorbed photon creates an electron-hole pair Recombination of an electron and hole can emit a photon Types of photon emission Spontaneous emission Random recombination of an electron-hole pair Dominant emission for light emitting diodes (LED) Stimulated emission A photon excites another electron and hole to recombine Emitted photon has similar wavelength, direction, and phase Dominant emission for laser diodes

Basic Light Emission Processes Pumping (creating more electron-hole pairs) Electrically create electron-hole pairs Optically create electron-hole pairs Emission (recombination of electron-hole pairs) Spontaneous emission Simulated emission

Semiconductor Material Semiconductor crystal is required Type IV elements on Periodic Table Silicon Germanium Combination of III-V materials GaAs InP AlAs GaP InAs … Periodic Table of Elements

Direct and Indirect Materials Relationship between energy and momentum for electrons and holes Depends on the material Electrons in the CB combine with holes in the VB Photons have no momentum Photon emission requires no momentum change CB minimum needs to be directly over the VB maximum Direct bandgap transition required Only specific materials have a direct bandgap

Light Emission The emission wavelength depends on the energy band gap Semiconductor compounds have different Energy band gaps Atomic spacing (called lattice constants) Combine semiconductor compounds Adjust the bandgap Lattice constants (atomic spacing) must be matched Compound must be matched to a substrate Usually GaAs or InP

Direct and Indirect Materials Only specific materials have a direct bandgap Material determines the bandgap Material Element Group Bandgap Energy Eg (eV) Bandgap wavelength lg (mm) Type Ge IV 0.66 1.88 I Si 1.11 1.15 AlP III-V 2.45 0.52 AlAs 2.16 0.57 AlSb 1.58 0.75 GaP 2.26 0.55 GaAs 1.42 0.87 D GaSb 0.73 1.70 InP 1.35 0.92 InAs 0.36 3.5 AnSb 0.17 7.3

Common Semiconductor Compounds GaAs and AlAs have the same lattice constants These compounds are used to grow a ternary compound that is lattice matched to a GaAs substrate (Al1-xGaxAs) 0.87 < l < 0.63 (mm) Quaternary compound GaxIn1-xAsyP1-y is lattice matched to InP if y=2.2x 1.0 < l < 1.65 (mm) Optical telecommunication laser compounds In0.72Ga0.28As0.62P0.38 (l=1300nm) In0.58Ga0.42As0.9P0.1 (l=1550nm)

Optical Sources Two main types of optical sources Light emitting diode (LED) Large wavelength content Incoherent Limited directionality Laser diode (LD) Small wavelength content Highly coherent Directional

Light Emitting Diodes (LED) Spontaneous emission dominates Random photon emission Implications of random emission Broad spectrum (Dl~30nm) Broad far field emission pattern Dome used to extract more of the light Critical angle is between semiconductor and plastic Angle between plastic and air is near normal Normal reflection is reduced Dome makes LED more directional

Laser Diode Stimulated emission dominates Narrower spectrum More directional Requires high optical power density in the gain region High photon flux attained by creating an optical cavity Optical Feedback: Part of the optical power is reflected back into the cavity End mirrors Lasing requires net positive gain Gain > Loss Cavity gain Depends on external pumping Applying current to a semiconductor pn junction Cavity loss Material absorption Scatter End face reflectivity

Lasing Gain > Loss Gain Gain increases with supplied current Threshold condition: when gain exceeds loss Loss Light that leaves the cavity Amount of optical feedback Scattering loss Confinement loss Amount of power actually guided in the gain region

Optical Feedback Easiest method: cleaved end faces End faces must be parallel Uses Fresnel reflection For GaAs (n=3.6) R=0.32 Lasing condition requires the net cavity gain to be one g: distributed medium gain a: distributed loss R1 and R2 are the end facet reflectivities

Cleaved Cavity Laser The cavity can be produced by cleaving the end faces of the semiconductor heterojunction This laser is called a Fabry-Perot laser diode (FP-LD) Semiconductor-air interface produces a reflection coefficient at normal incidence of For GaAs this reflection coefficient is Threshold condition is where the gain equals the internal and external loss Longer length laser has a lower gain threshold

Phase Condition The waves must add in phase as given by Resulting in modes given by Where m is an integer and n is the refractive index of the cavity

Longitudinal Modes

Longitudinal Modes The optical cavity excites various longitudinal modes Modes with gain above the cavity loss have the potential to lase Gain distribution depends on the spontaneous emission band Wavelength width of the individual longitudinal modes depends on the reflectivity of the end faces Wavelength separation of the modes Dl depends on the length of the cavity

Mode Separation Wavelength of the various modes The wavelength separation of the modes is A longer cavity Increases the number of modes Decrease the threshold gain There is a trade-off with the length of the laser cavity

Cleaved Cavity Laser Example A laser has a length of L=500mm and has a gain of Solving this for wavelength gives (1550-5.65) nm < l < (1550+5.65) nm The supported modes are calculated based on the constructed interference condition The minimum and maximum orders are mmin=2249 mmax=2267 The number of modes is 18 With a wavelength separation of Dl=0.69nm

Single Longitudinal Mode Lasers Multimode laser have a large wavelength content A large wavelength content decrease the performance of the optical link Methods used to produce single longitudinal mode lasers Cleaved-coupled-cavity (C3) laser Distributed feedback laser (DFB) laser

Cleaved Coupled Cavity (C3) Laser Longitudinal modes are required to satisfy the phase condition for both cavities

Periodic Reflector Lasers Periodic structure (grating) couples between forward and backward propagating waves For l=1550 nm, L=220 nm Distributed feedback (DFB) laser Grating distributed over entire active region Distributed Bragg reflector (DBR) laser Grating replaces mirror at end face

Laser Wavelength Linewidth

Summary of Source Characteristics Laser type FP laser: Less expensive, larger linewidth DFB: More expensive, smaller linewidth Optical characteristics Optical wavelength Optical linewidth Optical power Electrical characteristics Electrical power consumption Required voltage Required current

Example Laser Specifications Let look at an example specification sheet Phasebridge “Wideband Integrated Laser Transmitter Module” Laser + External Modulator Specifications Wavelength: 1548 nm < l < 1562 nm Average power: 5 < Pt < 9 mW Threshold current Ith=40mA TEC cooler Line width: 10 MHz We need to convert from Df to Dl Dl=0.008 nm

Semiconductor Optical Detectors

Semiconductor Optical Detectors Inverse device with semiconductor lasers Source: convert electric current to optical power Detector: convert optical power to electrical current Use pin structures similar to lasers Electrical power is proportional to i2 Electrical power is proportional to optical power squared Called square law device Important characteristics Modulation bandwidth (response speed) Optical conversion efficiency Noise Area

pin Photodiode p-n junction has a space charge region at the interface of the two material types This region is depleted of most carriers A photon generates an electron-hole pair in this region that moves rapidly at the drift velocity by the electric field Intrinsic layer is introduced Increase the space charge region

I-V Characteristic of Reversed Biased pin Photocurrent increases with incident optical power Dark current, Id: current with no incident optical power

Light Absorption Dominant interaction Photon absorbed Electron is excited to CB Hole left in the VB Depends on the energy band gap (similar to lasers) Absorption (a) requires the photon energy to be smaller than the material band gap

Quantum Efficiency Probability that photon generates an electron-hole pair Absorption requires Photon gets into the depletion region Be absorbed Reflection off of the surface Photon absorbed before it gets to the depletion region Photon gets absorbed in the depletion region Fraction of incident photons that are absorbed

Detector Responsivity Each absorbed photon generates an electron hole pair Iph = (Number of absorbed photons) * (charge of electron) Rate of incident photons depends on Incident optical power Pinc Energy of the photon Ephoton= hf Generated current Detector responsivity Current generated per unit optical power l in units of mm

Responsivity Depends on quantum efficiency h, and photon energy

Avalanche Photodiode (APD)

Minimum Detectable Power Important detector Specifications Responsivity Noise Equivalent noise power in or noise equivalent power NEP Often grouped into minimum detectable power Pmin at a specific data rate Pmin scales with data rate Common InGaAs pin photodetector Pmin=-22 dBm @B=2.5 Gbps, BER=10-10 Common InGaAs APD Pmin=-32 dBm @B=2.5 Gbps, BER=10-10 Limited to around B=2.5 Gbps