Presentation on theme: "IV. Laser Diode (LD) or Semiconductor Laser"— Presentation transcript:
1 IV. Laser Diode (LD) or Semiconductor Laser Operation MechanismCharacteristics of LDLD Design (1): control of electronic propertiesLD Design (2): control of optical propertiesAdvanced LD StructuresApplications of LD
2 Introduction to the Semiconductor Laser LASER — Light Amplification by Stimulated Emission of RadiationThe Laser is a source of highly directional, monochromatic, coherent light.The Laser operates under a “stimulated emission” process.The semiconductor laser differs from other lasers (solid, gas, and liquid lasers):small size (typical on the order of 0.1 × 0.1 × 0.3 mm3)high efficiencythe laser output is easily modulated at high frequency by controlling the junction currentlow or medium power (as compared with ruby or CO2 laser, but is comparable to the He-Ne laser)particularly suitable for fiber optic communicationImportant applications of the semiconductor lasers:optical-fiber communication, video recording, optical reading, high-speed laser printing. high-resolution gas spectroscopy, atmospheric pollution monitoring.
3 From LED to LD: Improvement by an Optical Cavity
4 Comparison between an LD and LED Laser DiodeStimulated radiationnarrow linewidthcoherenthigher output powera threshold devicestrong temperature dependencehigher coupling efficiency to a fiberLEDSpontaneous radiationbroad spectralincoherentlower output powerno threshold currentweak temperature dependencelower coupling efficiency
5 Stimulated Emission Stimulation emission The two basic requirements for a stimulated emission process to occur:(1) providing an optical resonant cavity to build up a large enough photon fielda very large photon field energy density (12) will enhance the stimulated emission over spontaneous emission(2) obtaining population inversion conditionunder the population inversion condition (n2 > n1) the stimulated emission is to dominate over absorption of photons from the radiation field
6 Optical Resonant Cavity parallel reflecting mirrors to reflect the photons back and forth, allowing the photon energy density to build up.The Fabry-Perot faces (cavity)The reflecting ends of the laser cavityThe gain in photons per pass between the Fabry-Perot faces must larger than the losses (such as the transmission at the ends, scattering from impurities absorption, and others)In the semiconductor laser, optical resonant cavity is made by cleaving.Cleave the oriented sample (GaAs) along a crystal plane (110), letting the crystal structure itself provide the parallel faces.
7 Resonant modes of a laser cavity Longitudinal modesdetermine the output-light wavelengthLateral modesleading to subpeaks on the sides of the fundamental modes, and resulting in “kinks” in the output-current curve.suppressed by the “stripe-geometry” structureTransverse modesgenerating “hot spots”suppressed by “thin active layer “ designSuppressing lateral and transverse mode is necessary to improve the performance of lasers.Single-mode laser: the laser operates in the fundamental transverse and lateral modes but with several longitudinal modes.Single-frequency laser: the laser operates in only one longitudinal mode.
8 Longitudinal modes of a laser cavity For stimulated emission, the length L of the cavity must satisfy the condition (for resonant): m [ 0 / 2n ] = L or m 0 = 2 n Lm is an integral number and is the refraction index in the semiconductor corresponding to the wavelength 0 (n is generally a function of 0)The separation 0 between the allowed modes in the longitudinal direction isSince dn/d0 is very small, 0 02 / 2Ln (for m = 1)For typical GaAs laser of 0 = 0.94 nm, n = 3.6 and L = 300 m, 0 = 4 Å.
9 Population Inversion (1) Forward biasing a p-n junction formed between degenerate semiconductors under high-injection condition. Population inversion appears about the transition regionThe condition necessary for population inversion is (EFC - EFV) > Eg where EFC, and EFV are the quasi-Fermi levelsIn the figure shows then energy diagrams of a degenerate p-n junction(a) at thermal equilibrium(b) under forward bias(c) under high-injection condition
10 Population Inversion (2) (a) incoherent (spontaneous) emission EFC - EFV > h > Eg(b) laser modes at threshold There modes correspond to successive numbers of integral half-wavelengths fitted within the cavity(c) dominant laser mode above threshold h = Eg
11 Carrier and Optical Confinement Carrier and Optical Confinement can be obtained by using the heterostructure design in the LDCarrier Confinementreduce the threshold current densitylaser can operate continuously at room temperatureOptical Confinementconfinement factor : the ratio of the light intensity within the active layer to the sum of light intensity both and outside the active layer = 1 - exp ( - C n d )n : the difference in the reflective indexd : the thickness of the active layerthe larger the n and d are, the higher the will beOptical confinement provides effective wave-guide for optical communication
12 Homojunction and Heterojunction Laser Homojunction Laserpulse mode outputlarge threshold current densityoperated at low temperaturebroad spectral width of output lightImprovement Heterojunction LaserHeterojunction Laser(1) Single-Heterojunction Laser (SH Laser)(2) Double-Heterojunction Laser (DH Laser)(3) Stripe-geometry DH Laser(4) Single quantum well (SQW) Laser(5) Multiple quantum well (MQW) Laser(6) Strained layer superlattice (SLS) structure
14 Threshold Current Density Gain (g)the incremental optical energy flux per unit lengthThreshold Gainthe gain satifies the condition that a light wave makes a complete traveral of the cavity without attenuation is the confinement factor, is the loss per unit length, L is the length of the cavity, R is the reflectance of the ends of the cavityThreshold Current Density (Jth)the minimum current density required for lasing to occurTo reduce Jth, we can increase , , L, R and reduce d,
15 Characteristics of the DH laser Threshold current density vs. active layer thicknessThe threshold current density decreases with decreasing d, reaches a minimum, and then increases. The increase of Jth at very narrow active thickness is caused by poor optical confinement.Output power vs. diode currentThe light-current characteristics is quite linear above threshold.Temperature dependenceThe threshold current increases exponentially with temperature Jth ~ exp [ T/T0 ]
16 Emission Spectra of the typical DH laser Emission spectra of a perfect laserabove the threshold, the laser may approach near-perfect monochromatic emission with a spectra width in the order of 1 to 10 Å.High-resolution emission spectra (of a typical stripe-geometry DH laser)Sub-peaks, which are evenly spaced with a separation of = 7.5 Å, appear in the spectra. belong to the longitudinal modes.Because of these longitudinal modes, the stripe geometry laser is not a spectrally pure light source for optical communication.
17 Design considerations for laser diode performance Low threshold currentlow threshold can be generated by electronic devices which can be modulated at high speed to provide a high speed modulation in the output(1) reducing the active layer thickness (d)↣ Quantum-Well (~ Å), Strain Quantum-Well(2) N-doped active region(3) Stripe geometryLateral confinementto avoid the “kink” effect, which produces noise in the optical transmitterreduce the lateral dimension of the Fabry-Perot cavity(1) Stripe geometry (Gain-guided cavity)(2) Buried heterostructuresSelective Optical Cavityto reduce the laser linewidth(1) Distributed Feedback (DFB) structures
18 Stripe Geometry LaserUsing the “gain-guided cavity” to carry out the lateral confinementAdvantages of a stripe geometry structureRemoving side peaks from the main modes by suppression of the lateral mode.Reducing the threshold currentless stringent demands on fabrication (because of the smaller active volume and the greater protection offered by isolating the active region from an open surface along two sides)Fundamental mode operation is valid for all stripe widths below µm.Different types of stripe-geometry structure:oxide stripeimplantationselective diffusionMesa stripeburied heterostructuresridge structures
19 Single Frequency Laser Single frequency lasers is desirable in the optical fiber communication system to increase the bandwidth of an optical signal.This is because light pulses of different frequencies travel through optical fiber at different speeds thus causing pulse spread.Dispersion mechanisms for a step-index fiber: (1) intermodal dispersion (2) waveguide dispersion (3) material dispersionDispersion effects can be minimized by using long wavelength sources of narrow spectral width (a single frequency laser) in conjunction with single mode fibers.Methods to achieve the single frequency lasers:(1) Frequency Selective FeedbackExternal Grating, Distributed-Feedback (DFB), Distributed Bragg Reflector (DBR)(2) Coupled CavityCleaved Coupled Cavity (C3) laser
20 Distributed Feedback (DFB) Laser In periodic structures, special effects occur when the wavelength of the wave approaches the wavelength of the periodic structure. In semiconductor crystals, this leads to bandgaps and Bragg reflections.The wavelength selective periodic grating with a corrugated structure, made by E-beam lithography and RIE, is incorporated into to the laser.The period of the grating is d = 2qB /2n where B is the Bragg wavelength give bywhere 0 is the oscillating wavelengthDFB lasers have been made with sawed end facets or with antireflection coating to suppress the Fabry-Perot modes.The DFB laser’ main advantage is its very small temperature dependence.
21 Distributed Bragg Reflector (DBR) Laser In the DBR laser, the period reflecting mirror stack is placed outside the active lasing region.The advantages of the DBR lasers:high coupling efficiency between the active lasing region and the passive waveduide structures.the wavelength of the output light is tunable.The reflective index of the stack is alterable by current injection.The wavelengths that get the highest feedback must satisfy B = 2 q (nr1 d1 + nr2 d2) where is a positive integerThe values of nr1 d1 and nr2 d2 can be altered electronically, therefore can have a certain degree of wavelength tunability
22 Cleaved-Coupled-Cavity (C3) Laser The C3 laser consists of two standard Fabry-Perot cavity laser diodes which are self-aligned and very closely coupled to form a two-cavity resonator.Because the laser light has to travel through an additional cavity (modulator), the only radiation that is reinforced is at a wavelength that resonates both in the laser’s cavity and also in the modulator.The two cavities can have their currents controlled independently and this is the main advantage of the C3 laser.
23 Quantum Well LaserIf the thickness of the active region Ly is made small enough (Ly ~ the “de Broglie wavelength” = h/p < 500Å, depending on the materials for GaAs, Ly ~ 20 nm), the carriers are confined in a finite potential well in which the energy band splitting into a “staircase” of discrete levels (the quantization effect)E-h recombination can only occur with “n = 0 transition” in the quantum well.In a quantum well (QW), a large number electrons all of the same energy can recombine with a similar block of holes.Hence, a QW laser should gives a much narrower output wavelength, unlike the other lasers with the bulk effect, where recombining carriers are distributed in energy over a parabolically varying density of states
24 Multiple Quantum Well (MQW) Laser Several single quantum wells are coupled into a “multiple quantum well (MQW)” structure.The significantly reduced temperature sensitivity of MQW lasers has been related to the staircase density of states distribution and the distributed electron and photon distributions of the active region.This optical confinement helps to contain the otherwise large losses from a narrow active region, leading to low threshold currents.An MQW is the active region of a laser that can emit a single frequency at several different wavelengths, known as a multiple array grating integrated cavity (MAGIC) laser.
25 Graded Index Separate Confinement Heterostructure (GRINSCH) Laser A narrower carrier confinement region (d) of high recombination is separated from a wider optical waveguide regionOptical confinement can be optimized without affecting the carrier confinementGRINSCH-SQW and GRINSCH-MQWThe threshold current for a GRINSCH is much lower than that of a DH laserFor a standard DH laser, both mirror and absorption losses increase rapid for thin active region, leading to very high threshold current.
27 Vertical Cavity Surface Emitting Laser (VCSEL) The structure of an VCSEL is very much like a standard heterojunction LED.Advantages of the VCSEL:the possibility of single frequency operation due to the short cavitythe removal of the fragile cleavage process that creates the end mirrors in a standard laser.The success of the VCSEL depends on incorporating high reflectivity mirrors in the structuresThe incorporations of DBR and MQW structures highly improve the performance of the VCSEL.Various DBRs in the VCSEL:crystalline BRDamorphous DBR stacksMgF/ZnSe DBR