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Quantum Electronics, IEEE Journal of

Issue 6 • Date June 1970

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Displaying Results 1 - 25 of 33
  • [Front cover and table of contents]

    Page(s): 0
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  • Semiconductor lasers - Two years later

    Page(s): 275 - 277
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    First Page of the Article
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  • Optical losses in "close-confinement" epitaxial p-n junction lasers - Theory and experiment

    Page(s): 290 - 291
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    The optical losses of "close-confinement" (single heterojunction) epitaxial injection lasers fabricated by liquid-phase epitaxy (LPE) have been described in terms of a simple waveguide model. Experimental and theoretical comparisons were made of LPE lasers with identical impurity profiles except for the addition of the AlxGa1-xAs p+-p heterojunction (DeltaE_{g}sim 0.1eV) within1-2 muof the p-n junction. This reduces the room-temperature absorption loss and hence the laser threshold-current density. The values of the internal laser loss α at 300°K range from 20 to 40 cm-1, while in the homojunction LPE lasers α is typically close to 100 cm-1. The laser gain coefficient β in the close-confinement lasers ranges from 3 to6 times 10^{-3}cm/A; values ofsim3 times 10^{-3}cm/A. are estimated in similarly doped homojunction LPE lasers. However, β in diffused lasers is known to be substantially lower,sim leq 10^{-3}cm/A. We believe that substantial electron confinement exists in the homo-junction LPE laser because of the doping discontinuity at the p+p interface, which is absent in the diffused lasers. Thus, the β values are correspondingly lower in the diffused lasers. The addition of the (AlGa)As heterojunction does not, apparently, increase the electron confinement greatly as compared to homojunction LPE lasers when the active region width is about 2 μ (the optimum value for overall laser performance) for a given doping level. Values of the exterior differential quantum efficiency of 43 percent are obtained at 300°K in close-confinement lasers, which means that most of the light internally generated is emitted. Because of the reduced internal loss, the spontaneous exterior efficiency is also greatly increased by the addition of the heterojunction (a factor of 2-3). State-of-the-art values of the threshold current densit- of the optimum structures are 10 000 A/cm2for a cavity length of 20 mils, with values of 8000 A/cm2in exceptional units. A power conversion efficiency at 300°K of 10 percent has been measured, which agrees with the theoretically predicted value. It is noteworthy that the diode series resistance is unaffected by the addition of the heterojunction. The laser characteristics have been studied as a function of varying acceptor concentration, donor concentration, and bandgap energy discontinuity at the p+-p interface. Semiquantitative agreement for the laser loss between theory and experiment at 300°K and 77°K is obtained for an active region width ofsim 2 mu, using reasonable values of the index of refraction differences between the various laser regions and free carrier losses. The major difference between simple waveguide theory and experiment is in the dependence on the width of the active region. Instead of a decrease in α with increasing width, the opposite occurs in the range investigated because the theory is not applicable when the width of the active region substanfially exceeds the minority carrier diffusion length. Furthermore, higher order mode propagation has been neglected. Such modes have been experimentally observed whend cong 5 mu. The addition of the heterojunction greatly reduces the laser loss due to light leakage from the active region into the p+ region. The contribution to the laser loss resulting from leakage into the n-type side of the junction is small. It is unaffected, of course, by the addition of the heterojunction. This loss could be further reduced by use of a double heterojunction structure. Based on the theoretical results and comparison with experiment in which the Zn concentration is changed,it is concluded that the dominant loss is due to free carrier absorption (holes mainly) in the active region of the close-confinement lasers. Thus, it is not expected that t View full abstract»

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  • Characteristics of bistable CW GaAs junction lasers operating above the delay-transition temperature

    Page(s): 300 - 304
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    The conditions for bistable operation of CW GaAs junction lasers are developed in terms of the previously published double-acceptor trap theory. The experimental CW operation of such devices is shown to agree well with the theoretical results. In addition the fabrication of these bistable lasers is described and several pulsed experiments are reported that indicate a significant increase in the number of trapping centers in the vicinity of the junction. View full abstract»

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  • Densities of states and properties of spontaneous and stimulated emission in semiconductor lasers

    Page(s): 310
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    The spontaneous and stimulated absorption spectral functions are calculated using a band model consisting of a tail part given by Halperin and Lax, a parabolic part above the tail, and an optical model with an energy-dependent matrix element and no selection rule for the radiative transitions. The use of a parabolic band above the tail is justified since a second-order perturbation calculation shows negligible distortion of the band in this region. The parameters associated with the densities of states are determined self-consistently, involving no adjustable constants. Contrary to the generally accepted assumption and the results of Stern's calculation using Kane's density of states of a long and reasonably large-state density in the conduction-band tail, it is shown that the tail is negligibly small compared to the valence-band tail. Therefore, it is concluded that, for a typical laser, the electron quasi-Fermi level at threshold above 77°K should be in the parabolic portion of the conduction band instead of in the tail as is usually assumed without justification. The energy dependence of the matrix element is that for the average parabolic conduction band to an acceptor-level transition and should be a suitable one due to small conduction-band tail and the population of most of the holes in the vicinity of the acceptor ionization energy. General properties such as the gain current relationship and the temperature dependence of current and carrier quasi-Fermi levels are compared with those calculated without band tails and with band tails given by Kane's model. Considerable difference in each case is found and discussed. The calculations of the temperature dependence of the threshold current and the current dependence of the super radiance spectra are then applied to GaAs diffused diodes with substrate doping of3 times 10^{18}cm-3, taking into account the temperature dependence of the cavity loss and the nonuniform accepter distribu- - tion in the p layer. Detailed comparison with experimental data is made and good quantitative agreement is obtained in either case, showing strong support to the conclusion on the band-tail structure. The approximations of using the linear screening and Gaussian statistics that were employed by Halperin and Lax, and Kane to obtain analytical expressions for the densities of states used in Stern's calculation are discussed. View full abstract»

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  • Carrier lifetime and threshold of Pb1-xSnxTe lasers in a magnetic field

    Page(s): 313
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    It is well known that a magnetic field modifies not only the photon energy, but also the threshold current required by semiconductor lasers. However, the extent to which a given field modifies this threshold current depends very strongly upon temperature and on the lifetimes of the carriers in the valence and conduction bands. Theoretical calculations that predict values of threshold current as a function of magnetic field in mixed-crystal Pb1-xSnxTe lasers are presented. The carrier lifetimes in these materials have recently been studied in some detail and turn out to be related to the composition of the material and to its preparation history. Assuming that the carrier Lifetimes are largely determined by the acoustical phonon density or by random distribution of short-range scattering centers, the Kubo formalism can be used to express the individual Landau-level densities of states. From these, the quasi-Fermi levels at any pumping rate can be directly calculated. Fork-conserving transitions, the optical density of states is then determined by a convolution integral method recently reported elsewhere, and the threshold current is computed numerically by the method of Lasher and Stern. Using some of the measured values of carrier mobilities given by Calawa et al., for compositions withx = 0.07andx = 0.20, and using the measured values of Butler and Harman for zero magnetic-field threshold to adjust the gain requirement parameter, threshold currents at 77°K are computed for magnetic field strengths up to 250 kG. Curves are obtained that predict a threshold current for the higher mobility material that decreases by a factor of about 3 with increasing magnetic field out to the vicinity of 10 kG, flattens out, and then slowly increases. For the lower mobility material, an overall higher input current is required; however the curve continues to decrease to 25 kG before flattening out and then beginning to rise- - . View full abstract»

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  • Index of refraction changes in semiconductors with population inversion

    Page(s): 316
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    The effect of population inversion on the index of refraction in semiconductors is calculated. A simple model of the absorption edge is modified to include the effects of population inversion. By correlating the density of states in the conduction band with the absoption edge, a modification of the absorption edge occurs because of the filling of states in the conduction band. The effective density of hole states in the valence band is neglected because the density of states in the valence band is much larger than the density of states in the conduction band. When the exponential tail in the conduction band is filled to an energy Ef, a propagating electromagnetic wave does not attenuate for energies below Efdefined as the quasi-Fermi level for electrons in the inverted region. No attenuation occurs because electrons in the valence band can not be excited to the filled states of the conduction band. For a wave with an energy above Ef, band-to-band transistions are allowed because of vacant states in the conduction band. Therefore, for waves with energiesE = homega > E_{f}, the absorption process is the same as the absorption process for a material without population inversion. Free carrier effects are neglected. The change in the index of refraction is calculated for various slopes of the band tails and for various values of Fermi levels below the nominal-band edge. The results show that at low temperatures there is a positive contribution to the index for energies slightly below the Fermi level. At energies much less than the Fermi level, the contribution to the index of refraction is negative. At absolute zero, the crossover between a negative contribution and a positive contribution to the index isE_{t} = E_{f} - 0.3725E_{0}where E0is the slope of the conduction-band tail. As the temperature increases, the value of Etincreases. The magnitude of the index of refraction change increase- - s as the slope E0increases. View full abstract»

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  • Resonant modes of GaAs junction lasers - III: Propagation characteristics of laser beams with rectangular symmetry

    Page(s): 317 - 324
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    In Parts I and II of this work [1], [2], the resonant modes of GaAs lasers, as predicted from a proposed model, are found to be in very good agreement with experimental results that emphasize the frequency domain. This paper, Part III, supplements the previous work by investigating the spatial field distributions of junction-laser beams. For the first time, the propagation characteristics of the beams are described mathematically in a manner that is consistent with detailed beamwidth measurements in the Fresnel and Fraunhofer regions of the diffraction field. This correlation between theory and experiment is made possible by the use of a lens-slit combination that ensures the generation of reproducible beams with rectangular symmetry. The astigmatic beam solutions describing the propagation characteristics beyond the slit include beam-broadening effects produced both by the lens and by phase variations at the laser mirror, along the junction plane. These latter variations have not been previously reported. Their existence is deduced from our measurements beyond the slit, which show a greater beam expansion along the junction plane than can be theoretically accounted for by the lens system alone. Near threshold, at a distance of one beamwidth from the center of the mirror illumination, the phase lag is found to have a current-dependent magnitude of a few degrees. View full abstract»

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  • Observation of intrinsic quantum fluctuations in semiconductor lasers

    Page(s): 325
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    The first experimental observation of the excitation of the natural resonance of a semiconductor laser by the quantum fluctuations intrinsic to the laser is reported. Such excitation and the resulting resonant-like peaks in the microwave noise spectrum of the laser intensity were initially predicted by McCumber and later calculated in detail for semiconductor lasers by Haug. In addition, we present experimental and theoretical results that show that high-level excitation of the resonance by internal or forced modulation of the population inversion lowers the resonant frequency due to the nonlinearity present in the rate equations. In the experiments to be described, intensity noise spectra of continuously operating GaAs injection lasers were observed at microwave frequencies with a high-speed photodiode and a microwave spectrum analyzer. Because of the low level of the photocurrent produced by these fluctuations, it was necessary to reduce the intrinsic noise level of the system by using phase-sensitive detection techniques. In this way, the following experimental observations have been made. First, a sharp peak in the intensity noise spectrum has been observed at currentsIfrom 1.5 to 100 percent above threshold (Ith). Second, at constant heat-sink temperature, the frequency of the peak varies, with current as(I/I_{th} - 1)^{1/2}. Third, while the intensity fluctuations relative to the square of the laser intensity continuously decrease (by three orders of magnitude) with increasing current, the absolute value of the noise peak increases (by more than two orders of magnitude) to a maximum value that is maintained with further increases in current. Finally, the frequency of the noise peak at constant current level above threshold shows no variation with heat-sink temperatures between 80°K and 150°K. The above observations were made on lasers in which the resonance was not strongly excited by combination tones present in th- - e active medium and consequently for which there were no deep intensity pulsations. For lasers in which the intensity spontaneously pulses, the resonance peak has also been observed at currents very near threshold. However, in these diodes, the frequency at the peak increases with current as predicted by the theory only over a small range near threshold. Beyond this range, the resonance is excited by the combination tones, causing the frequency to decrease to a minimum value before increasing farther as the current is increased. Such behavior can be qualitatively understood in terms of the decrease in the average inversion (and consequently a reduction of the resonant frequency), which accompanies the self-induced pulsing of the laser intensity. Computer calculations based on nonlinear rate equations have confirmed this behavior. View full abstract»

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  • Theory of multimode effects including noise in semiconductor lasers

    Page(s): 325
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    In recent years a number of powerful methods have been developed that treat quantum fluctuations in ordinary lasers. Among these are quantum mechanical Langevin equations and ac-number Fokker-Planck equation associated with quantum systems. First, the quantum mechanical Langevin equations for the semiconductor laser are shown. Then the extension of the Fokker-Planck method to multimode action in semiconductor lasers is given, and then the exact stationary solution, which predicts the relative probability of mode configurations. In particular the Paoli-Ripper model that treats in a combined way the intrinsic resonance of a single-mode semiconductor laser and the effect of nonlinear polarization is discussed. View full abstract»

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  • GaAs lasers utilizing light propagation along curved junctions

    Page(s): 367 - 371
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    A junction laser in which the light propagation within the device is along curved-junction paths is reported here. Path curvatures with radii ranging from 0.25 to 1.00 mm were obtained by the intentional introduction of curved diffused junctions. It was found that for radii of curvature smaller than a critical value of about 0.4 mm, the threshold-current density increases very rapidly. This value is in good agreement with the theoretical value, which is obtained by assuming that the gradient of the refractive index across the GaAs junction determines the minimum bending radius for low-loss propagation. The design of these curved-junction devices is critically dependent on the suppression of lasing along undesired junction paths perpendicular to the desired direction by bending these paths through a smaller than critical radius. View full abstract»

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  • Excitons and phonons in processes of laser oscillation from II-VI semiconductors

    Page(s): 388
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    The determination of the nature of optical laser transitions is one of the basic problems of semiconductor laser physics. It was shown earlier in certain mixed CdSx-CdSe1-xcrystals that in the case of intense dipole-allowed exciton absorption, the indirect exciton transition provides the principal channel for laser action using free excitons with the assistance of optical phonons at relatively high temperatures and using bound excitons at relatively low temperatures. In the present work new evidence is reported on the above CdSx-CdSe1-xcrystals as well as on other II-VI compounds (ZnSx-CdS1-xand ZnSe-CdSe), enabling conclusions about the characteristics of the oscillations produced by the complex phonon spectra of mixed crystals. Lasing of the above crystals was produced by optical two-photon pumping using a ruby laser. Experimental results showed that with two-photon pumping involving excitons in crystals of CdS, CdS-CdSe, ZnS-CdS, and ZnSe-CdSe, lasing can be obtained at temperatures no higher than120-140degK. When the temperature is not too low (T geq 60degK), the laser action always preceeds by means of free excitons interacting with LO phonons. At lower temperatures than 50°K most of the crystals lase on bound excitons, with or without the involvement of optical phonons, depending on the spectral distribution of losses. In two cases (CdS and ZnS0.09-CdS0.91) atT < 50degK, the lasing occurred on transitions of the Auger recombination type involving excitions. The mechanism of laser action is a strong function of temperature. An important feature of mixed crystals is the dependence of the phonons involved on the composition of the samples. This dependence was studied in the CdS-CdSe crystals. These crystals have a complex phonon spectrum; they are characterized by two types of longitudinal optical phonons, LO1and LO2whose intensity ratio is dependent on the crystal composition. For crystals in which one of the components is dominant at 77°K, it is shown that the oscillation occurs via free excitons with either the LO1or LO2longitudinal optical phonon more intense. For crystals with comparable concentrations of their components, both LO1and LO2phonons take part simultaneously in the lasing process (two-phonon transitions). View full abstract»

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  • [Back cover]

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  • Influence of reflectivity on the external quantum efficiency of GaAs injection lasers

    Page(s): 314 - 316
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    The differential external quantum efficiency ηextof Fabry-Perot-type GaAs injection lasers has been measured as a function of the mean reflectivityR. By evaporation of antireflective films,Rhas been varied between 0.25 and 0.06. The observed increase of ηextwith decreasingRis interpreted in terms of a simple model. Values for the optical losses α, the gain factor β, and the internal quantum efficiency at 77°K are obtained for individual diodes. View full abstract»

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  • Second-harmonic generation with the GaAs laser

    Page(s): 356 - 360
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    The second harmonic of pulsed GaAs injection lasers was generated with the nonlinear crystal α-iodic acid (HIO3) phase matched by angular tuning. The injection laser was operated with an external cavity, and the HIO3crystal was placed inside the cavity to increase the conversion efficiency. The maximum harmonic power observed was 15 μW with a fundamental power of 4.5 watts inside the cavity. Since the lasing bandwidth was typically 30 Å and the calculated bandwidth for collinear harmonic generation was only 6.3 Å, a grating was used to narrow the lasing bandwidth to 3 Å. The expected large increase in harmonic power did not occur. It is shown that this is due to sum-frequency generation by laser modes lying outside the harmonic bandwidth. It is also found that the spectral width of the harmonic becomes bigger than the collinear bandwidth when a divergent laser beam is used. View full abstract»

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  • Frequency stabilization and narrowing of optical pulses from CW GaAs injection lasers

    Page(s): 335 - 339
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    Self-induced intensity pulsations of continuously operating GaAs injection lasers have been frequency stabilized and narrowed by applying to the laser microwave feedback signals derived from the electrical and optical outputs of the laser itself. The width of the optical pulses has been reduced to less than 180 ps at a pulse rate whose spectral width was simultaneously reduced to less than 30 kHz. Significant differences between electrical and optical methods of feedback are demonstrated and discussed. View full abstract»

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  • Microwave self-modulation of a diode laser coupled to an external cavity

    Page(s): 328 - 334
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    An external cavity coupled to a conventional Fabry-Perot GaAs diode laser operating continuously has been found to cause modulation of the light output at a frequency within the range 0.5 to several GHz. The modulation depth is close to 100 percent and the linewidth can be made as narrow as 180 kHz. The modulation is thought to be stimulated by the intensity noise fluctuations, which peak at the well-known spiking frequency fr. The oscillations are strongly enhanced by a frequency locking action of the external cavity, being efficient when the external cavity round-trip time2L/c, or a multiple thereof, corresponds to the inverse of the spiking frequency. Since the latter is dependent on both pump current and temperature, the system can simply be tuned by adjusting the pump current. For a fixed resonator length, the narrow-band oscillations occur in a small current range, in which an increase in frequency with increasing current at a rate of 400 kHz/mA is observed. A small-signal analysis based on simple rate equations shows the influence of the external cavity on the intrinsic resonance frequency fr. It demonstrates that self-modulation can only occur for small values of the coupling coefficient ε between the laser diode and the external cavity. View full abstract»

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  • Comments on the reliability of GaAs Stripe-Geometry juntion lasers

    Page(s): 372 - 373
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    Experiments with stripe-geometry lasers show that Byer's results for degradation of pulsed diodes can not be generalized to CW conditions. Diodes subjected to CW injection-current density of8 times 10^{3}A/cm2show very little degradation after close to 1000 hours of operation at room temperature. View full abstract»

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  • High-peak-power room-temperature GaAs laser arrays

    Page(s): 377 - 382
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    The current procedures that are employed in the design and fabrication of high-peak-power room-temperature GaAs laser arrays are presented. The methods of material selection and heat-sink preparation are illustrated. Specific mention is made of the laser-chip packing densities that produce effective laser-array brightness in the 100 kW/cm2region. Actual arrays have been constructed that produce 1 kW peak power with 400 amperes peak of modulator current at 1 kHz, or 600 watts peak at 300 amperes peak at 5 kHz. For modulator-current pulses less than 35 ns, these arrays can be expected to operate for 1010pulses. For field-lens apertures only a few inches in diameter, beam divergences from these arrays of only a few milliradians can be realized. Data relating the power output as a function of pulsewidth, repetition rate, and modulator current for several array configurations are presented. Differential quantum efficiency and overall power-efficiency data and curves are illustrated for typical diode and array configurations. The schemes employed to select and interface various optical components necessary to produce the desired beam divergences from reasonably sized apertures are discussed. Brief mention is made of the parameters of the laser-junction radiation densities necessary to maximize the array operating lifetime. View full abstract»

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  • Superlinear dependence of gain on current density in GaAs injection lasers

    Page(s): 311 - 312
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    Measurements of threshold-current density and incremental efficiency on homostructure and heterostructure GaAs lasers show an increasingly superlinear dependence of gaingon current densityJwith increasing temperature. The indexnin the relationg propto J^{n}satisfies the relationn = [1 + (kT/E_{t})^{2}]^{1/2}where Etis a measure of the band-tail depth. View full abstract»

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  • Band tailing and the spontaneous spectra of compensated epitaxial GaAs laser junctions

    Page(s): 305 - 310
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    The spectra of the spontaneous emission from GaAs laser diodes fabricated by vapor-phase epitaxy and containing a high degree of compensation in the p-type side of the junction are, in many respects, similar to those previously reported for the cathodoluminescence of homogeneous p-type material. The temperature dependence of the spectra of these diodes supports a model where the emission is attributed to radiative transitions between a narrow band of states near the conduction-band edge and an exponential distribution of states extending the valence band into the forbidden gap. An increase in temperature then results in an increase or a decrease in the radiative power output depending on the position of the quasi-Fermi level for electrons relative to the above narrow band of states and a distribution of nonradiative levels below it. View full abstract»

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  • Optically excited bulk semiconductor lasers

    Page(s): 392 - 400
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    Optical pump experiments on homogeneous samples provide a method of obtaining a clearer understanding of the details of laser action in semiconductors. For this reason, the quantum-mechanical rate equations for a bulk semiconductor laser that explicitly include a monochromatic light field as a pump source are developed. The theory treats general optical transitions and is valid for both pure and doped homogeneous semiconductors. The developed theory gives the intensity of the laser output in the entire region from below to above threshold as a function of the intensity and frequency of the pump field, the temperature, doping level, and material constants. Also the variation of the laser light frequency at threshold upon these parameters is given by the theory. Information about the dependence of the necessary pump intensity and frequency for threshold is gained. The theoretical calculations are compared with experimental measurements and found to be in general agreement. View full abstract»

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  • Electrical and optical characteristics of InAs junction lasers

    Page(s): 383 - 388
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    Electrical and light emission characteristics of pulsed InAs p-n junction lasers, prepared by diffusing Zn into Sn-doped n-type crystals with a substrate concentration of 1018cm-3, were studied at 4.2, 20.4, and 77°K. Data of variation of capacitance with bias showed these to be abrupt junctions. At injection currents below threshold, the temperature independence of the slopes of the I-V characteristics and the observed variation of the integrated light intensity with current suggest the injection mechanism be photon-assisted tunneling. The peak of the spontaneous radiation shifts with the applied voltage in the expected manner. The time-resolved laser spectra are seen to shift to longer wavelengths during pulsed operation of the laser. The line shift results from joule heating of the junction, and follows the variation of bandgap with temperature. From a plot of threshold current density as a function of reciprocal length gain and loss factors have been calculated. An independent value of the gain factor was obtained from the observed variation with current of the superradiantly narrowed linewidth from a nonlasing structure. Mode-confinement studies indicate that the mode confinement is due to the optical gain in the junction region. View full abstract»

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  • Internal Q switching in GaAs- GaxAl1-xAs heterostructure lasers

    Page(s): 297 - 299
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    Efficient internalQswitching and delay effects are observed in a range of temperatures from less than 100 to 400°K in solution grown GaAs-GaxAl1-xAs heterostructure injection laser diodes. The characteristic transition temperature Ttabove which long stimulated emission delays are found, decreases with the laser cavity length (i. e., higher threshold current density). Ttalso decreases with the thickness of the active region. The onset of hole injection seems to occur simultaneously with the appearance of the pronounced delay andQ-switching effects. View full abstract»

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  • Repetitively Q -switched light pulses from GaAs injection lasers with tandem double-section stripe geometry

    Page(s): 339 - 352
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    Self-induced sustained pulsations have been observed in the light output from GaAs injection lasers with tandem double-section stripe geometry. The pulsing behavior of the laser is analyzed using a repetitivelyQ-switched model similar to that of Basov. Exact numerical solutions to the nonlinear rate equations have been obtained, which are to be compared with our experimental results and with the results in the earlier work of Basov et al. An approximate approach using a phase-plane analysis is described, which yields a simple way for the determination of the regions of excitation currents that leads to the sustained pulsing solution. View full abstract»

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Aims & Scope

The IEEE Journal of Quantum Electronics is dedicated to the publication of manuscripts reporting novel experimental or theoretical results in the broad field of the science and technology of quantum electronics..

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Meet Our Editors

Editor-in-Chief
Aaron R. Hawkins
Brigham Young University