8. Semiconductor lasers8Semiconductor lasersypA typical semiconductor laser is formed froma semiconductor diode and a pair of plane-parallel mirrors.In operation, the diode is forward biasedIn operation the diode is forward biasedThe populations are so large that f e+ f h> 1 for some photon energy (above the bandgap energy), thereby giving gain in the semiconductor material.If the gain per pass exceeds the mirror transmission loss and any other losses experienced by the beam (e.g, diffraction, absorption loss in nominally transparent parts of the structure, loss from scattering off material, or structure imperfections)the structure will lase.Semiconductor laser structuresThere are two basic configurations edge-emittingedge emittingsurface emitting.Edge emitting lasers(1)Edge-emitting lasers(1) The edge-emitting laser usually is based on a The edge-emitting laser usually is based on a waveguide structure.Edge-emitting lasers(2)Ed itti l(2)A “slab” waveguide is formed from the p and n AlGaAs layers to give waveguiding in one direction, surrounding the GaAs layer. The AlGaAs is essentially transparent at the laser operating wavelength has a relatively lower refractive index than the GaAs, both confining the optical mode and electrons and holes injected b th fi i th ti l d d l t d h l i j t d Improve the effectiveness of the stimulated emission gain.The mirrors in a laser are usually formed from the natural reflectivity of the semiconductor-air interfacereflectivity of the semiconductor-air interfaceThese plane-parallel mirrors form a Fabry-Perot cavity, and such lasers are known as Fabry-Perot lasers.Edge-emitting lasers(3)Edge emitting lasers(3)To obtain enough gain per pass to overcome this relatively large mirror loss the laser ca it needs to be t picall 100s of microns mirror loss, the laser cavity needs to be typically 100s of microns long. Heat dissipation is always a problem in semiconductor laser structures since relatively high current densities (e.g., 100s of A/)i d t t ffi i t i A/cm 2or more) are required to generate sufficient carrier densities in the diode. In most edgeemitting laser diodes, therefore, it is desirable to g g ,,confine the current injection and the optical mode in a relatively narrow stripe to minimize the total dissipation, and to allow for some heat spreading. Also, confining in a narrow stripe gives a p g ,g p g mode shape that is more nearly the same size in both directions, as is desirable if we want to couple into. Hence, a long narrow contact stripe is often used.contact stripe is often used.The injection of current into this narrow stripe can itself cause a weak guiding effect in the lateral direction, giving rise to a “gain-guided”laserguided laser.Edge-emitting lasers(4)Edge-emitting lasers(4)In modern lasers, this gain guiding is usually supplemented by In modern lasers this“gain guiding”is usually supplemented by refractive index guiding to give “index-guided” lasers. A commong g q g p g y index guiding technique is to etch a ridge in the top cladding layer of the slab guide, which tends to give a higher effective index for the laser mode in the region just below the ridge, hence givingidisome waveguiding.Edge-emitting lasers(5)Edge-emitting lasers(5)In a more sophisticated index-guided structure the index is larger in the center partly because there is only the low bandgap active material (InGaAsP) present.In this structure, a deep mesa ridge is formed in the originalIn this structure,a deep mesa ridge is formed in the original layered material,down to just below the active region; then the additional InP layers are “regrown”on the sides, burying the activeon the sides“burying”the activeheterostructure (hence the name“buried heterostructure”).)This kind of structure is particularlyefficient at injecting carriers only intothe active region in the middle of thelaser mode.There are many variants of the buriedThere are many variants of the buriedheterostructure concept.g g()Edge-emitting lasers(6)It is difficult to get the laser beam in an edge-emitting laser to be the same dimensions in both directions.h di i i b h di iThe beam as it leaves the laser is small in the “vertical” direction, and relatively larger in the “horizontal” direction.d l ti l l i th“h i t l”di tiAs it propagates into thefar field, the situationf fi ld th it tireverses because ofdiffraction, with a relativelydiff i i h l i llarge beam in the verticaldirection and a smallerbeam in the horizontal direction.Edge-emitting lasers(7) Edge-emitting lasers(7)Edge-emitting lasers(8)Edge-emitting lasers(8) p pA separate confinement heterostructure is a more sophisticated heterostructure in which a greater number of different layers of material are added,with some of the layers being primarily present to guide(or confine)the optical mode,and some being theremodeprimarily to position the electron and hole populations optimally for gain.In the GRINSCH,the material is graded approximately quadratically around about the thin active region.The approximately parabolic grading of index gives good control over the waveguide mode profile,and the thin active layer with deep potential wells for electrons and holes results in good overlap of the excited electron and hole populations for strong gain. The active region is also in the middle of the optical mode where the amplitude is highest,and hence the effective gain is also highest.highest highestOutput spectrum of a laser at a current just Output spectrum of a laser at a current justabove thresholdDistributed feedback(DFB)laser Distributed feedback (DFB) laserpp g y For applications where the laser wavelength must be more closely controlled (as in telecommunications), it is common to use either distributed Bragg reflector (DBR) or distributed feedback (DFB) laser structures.Both of these rely on the use of periodic grating structures, usually formed by corrugating an interface in the laser structure. The period of the corrugations is a (small) integer number of half-Th i d f th ti i(ll)i t b f h lf wavelengths, which isalso the basic structureof the simplest DFBlaser.A distributed Bragg reflector laser structure A distributed Bragg reflector laser structureThis high reflectivity arises because all of the reflections off of different periods in the grating add up in phase.S h i f d bSuch a mirror formed by anoptical structure with a periodf h lf l h i ll dof half a wavelength is calleda Bragg mirror or a distributedBragg reflector(DBR)Vertical-cavity surface-emitting lasers(1) Vertical-cavity surface-emitting lasers(1)e ve c c v y su ce e g se(VCS)s eThe vertical-cavity surface-emitting laser (VCSEL) is like a DBR laser, but made in the vertical direction.The motivations for making the VCSEL are not so much to obtaingnarrow-linewidth, single-frequency operation, but more to make lasers that can have intrinsically circular beam profiles, therefore making them easier to interface to fibers, and to allow the construction of arrays of lasers.VCSELs are also very small compared to edge emitters, because CS l ll d d i bthey avoid the long waveguide region.VCSELs have been enabled partly by the development of low-loss VCSEL h b bl d tl b th d l t f l l mirrors made integral to the semiconductor structure. These mirrors are formed from alternating quarter wave layers of low and high are formed from alternating quarter-wave layers of low and high index transparent semiconductors.Vertical cavity surface emitting lasers(2) Vertical-cavity surface-emitting lasers(2)(a)Emitting through anetched hole in theh d h l i hsubstrate.(b)Emitting through the topof the structure.(c)Emitting through atransparentt tsubstrate.Vertical cavity surface emitting lasers(3) Vertical-cavity surface-emitting lasers(3) Various other advantages come from the quantum confinement Various other advantages come from the quantum confinement effects seen in such thin, quantum-well layers.In particular, the density of states in quantum wells has a muchIn particular,the density of states in quantum wells has a much more favorable form for laser gain, being more abrupt.y g yThis better form of the density of states leads to significantly improved differential gain in the laser, which can be particularly important in high-speed modulation.In addition, the quantum confinement effects also give another degree of freedom in designing structures, since the quantum confinement can change the laser wavelength without changing fi t h th l l th ith t h i the composition.Almost all modern high-performance laser structures now use Almost all modern high performance laser structures now use quantum-well active layers.Laser gain dynamics(1)Laser gain dynamics(1)We can understand some of the basic phenomena that occur aswe try to modulate a laser at progressively higher speeds basedon a relatively simple rate equation model.s ode,we eed o co s de wo coup ed spec s.In this model, we need to consider two coupled aspects.One aspect is how the carrier density is affected by the number ofphotons in the cavity, and the other is how the number of photons h t i th it d th th i h th b f h tin the cavity is affect by the carrier density.We therefore consider two simple “rate equations” –first orderq pdifferential equations that are coupled to one another.Laser gain dynamics(2)g y()Consider first the rate of change of the number of carriers per unit ,,gvolume, N, in the laser gain medium.We are passing current, I, into the laser diode. Some fraction of the carriers in this current add to the carrier density in the active (gain) region of the device (usually most of them in a(i)i f h d i(ll f h iwelldesigned laser diode).If the volume of the gain region is then in the gain regionIf the volume of the gain region is V gain, then, in the gain region, The number of carriers added per unit volume per unit time I/I / eV gainWe expect there will be some recombination of the carriers that does not add photons to the cavity mode of interest.we presume this undesired recombination is characterized by a simple lifetime, , so we have the number of undesired carrier recombinations per unit volume per unit time = N /recombinations per unit volume per unit time=NLaser gain dynamics(3)g y()The number of photons added to the light beam in the laser mode p g y p y gper unit length inside the laser cavity is simply the gaincoefficient, g, times the number of photons in the laser mode. if the number of photons per unit volume in the laser mode is Np , the number of photons added to the beam per unit volume per unit length inside the cavity is gNp .The photons are traveling at a velocity vg inside the cavity, where vg is the group velocity.Hence the number of photons added to the beam per unit volume H h b f h dd d h b i lper unit time is v g N g p.Because a carrier is removed from N for each photon added by this Beca se a carrier is remo ed from N for each photon added b this stimulated recombination process, we have the number ofstimulated carrier recombinations per unit volume per unit time stimulated carrier recombinations per unit volume per unit timev g N g pLaser gain dynamics(4)Adding the creation and recombination rates calculated above with the appropriate signs, we have a net rate equation for the carrier densitycarrier densityLaser gain dynamics(5)g y()For the photons in the laser mode of interest in the cavity, We can lump all of these photon loss mechanisms into a photon lifetime, , for this cavity mode, to obtain the number of photons lost from the cavity per unit cavity volume per unit time = N p/ Photons are being added to the laser mode by the process of stimulated emission. We know that, per unit volume of the gain material, v g N g p photons are being added per unit time.As a result of both of these effects, to calculate the number of photons being added to the cavity mode per unit cavity volume per unit time, we need to introduce a correction factor called the “mode confinement factor”, G, so that the number of photons added perN g punit cavity volume per unit time = G vgLaser gain dynamics(6)we obtain a rate equation for the number of photons per unit volume in the cavity modevolume in the cavity modesteady state situationsteady state situationSuppose for the moment that we were running the laser in aS f th t th t i th l isteady state manner, at some fixed current, I o. In this condition, the gain would be and the carrier and photon densities would the gain would be g o, and the carrier and photon densities would be N o and N po, respectively. In this steady state situation, the p(carrier and photon densities would also be stable (i.e., dN / dt= 0 and dN p/ dt= 0)The gain dynamics of the laser in a simple The gain dynamics of the laser in a simple “small-signal” model(1)The gain dynamics of the laser in a simplesmall signal model (2)“small-signal”model(2)This equation is that of a simple damped harmonic oscillator, This equation is that of a simple damped harmonic oscillator driven by the term on the right hand side.We can usefully define a resonance (angular) frequencyThe gain dynamics of the laser in a simpleThe gain dynamics of the laser in a simple “small-signal” model (3)output power at different modulation frequencies output power at different modulation frequenciesLaser diode markets(1)()Laser diode markets(2) Laser diode markets(2)Laser diode markets(3)Laser diode markets(4) Laser diode markets(4)Laser diode markets(5)。