Influence of electrostatic confinement on optical gain in GaInNAs quantum-well lasers

There remains controversy surrounding the cause of the magnitude and temperature sensitivity of the threshold current density of 1.3-/spl mu/m GaInNAs quantum-well (QW) lasers, with several authors attributing the strong temperature sensitivity to hole leakage, due to the relatively low valence band offset in GaInNAs/ GaAs QW structures. We use a Poisson solver along with a ten-band k.p Hamiltonian to calculate self-consistently the influence of electrostatic confinement on the optical gain in such lasers. We find that the inclusion of such effects significantly reduces the hole leakage effect, with the electrostatic attraction of the electrons significantly increasing the binding of heavy holes in the QW region. We conclude by comparison with previous theoretical and experimental studies that the room temperature threshold current is generally dominated by monomolecular recombination, while the temperature sensitivity can be explained as predominantly due to Auger recombination.     

Ultrafast operation of optically pumped resonant periodic gain GaAssurface emitting lasers

Generation and characterization of picosecond optical pulses from vertical-cavity resonant-periodic-gain GaAs-AlGaAs surface-emitting lasers optically pumped by picosecond dye-laser pulses is reported. The output pulseshape was obtained from the cross correlation of pump and signal pulses. Dependence of signal pulsewidth and pulse delay on pump power were investigated. The results are in good agreement with a single rate equation model of the pulse formation. A cavity lifetime of 8.3 ps, compared with a gain medium transit time of ~0.1 ps, is determined for this very high-Q structure     

High-efficiency CW operation of MOCVD-grown GaAs/AlGaAs vertical-cavity lasers with resonant periodic gain

A GaAs/AlGaAs vertical-cavity surface-emitting laser with resonant periodic gain has been grown by metal-organic chemical vapour deposition. The as-grown structure exhibits an optically pumped CW threshold below 15 mW at 300 K and a single-ended power efficiency up to 45%. Fundamental Gaussian and higher-order modes are observed with spectral widths (FWHM) as low as 0.27 AA.<>     

Femtosecond periodic gain vertical-cavity lasers

A periodic gain structure is used as the gain medium in a mode-locked vertical-cavity surface-emitting laser to decrease the effect of carrier diffusion. A laser pulse width of under 190 fs is directly generated from the laser cavity     

Modal gain in coupled-stripe lasers

Using an effective index model, we calculate the eigenvalue equation for a large array of evanescent-wave-coupled multistripe lasers. The special cases of symmetric and antisymmetric modes for both the index- and gain-guided lasers are treated in detail. The main findings are 1) weak index-guided coupling, with small differences in optical confinement factors, and 2) stronger gain-guided coupling, with parameter-dependent selection of the mode symmetry having higher gain.     

Influence of facet coating on the dual wavelength operation of asymmetric InGaAs-GaAs quantum-well lasers

A low-reflectivity SiN/sub y/O/sub z/ film was deposited on one facet of asymmetric In /sub x/Ga/sub 1-x/As-GaAs quantum-well (QW) laser diodes containing three different QW compositions with the indium fractions x=0.25,0.21, and 0.15, located, respectively, from the n-doped to p-doped sides of the device. Lasing is only observed on the two higher In-content QWs. The reduction of the reflectivity of one facet, with the other as-cleaved, leads to an increase in the transition cavity length, below which the diode initially lases on the shorter wavelength QW (x=0.21) at threshold and above which the diode lases on the longer wavelength QW (x=0.25). No lasing occurs on the shortest wavelength QW (x=0.15). With sufficient current injection simultaneous lasing on the two QWs is observed. When dual-wavelength emission occurs the facet reflectivity determines whether the short or long wavelength emitting QW lases first as the pump current is increased. A separation of up to 31 nm between the two wavelengths is observed under the dual wavelength emission conditions.     

Effect of inhomogeneous line broadening on gain and differential gain of quantum dot lasers

Detailed theoretical analysis of the size fluctuation in InAs-GaAs quantum dot (QD) lasers is presented. Analytical expressions for the inhomogeneous line broadening and the optical gain are derived for a Gaussian size fluctuation distribution. The effect of size fluctuations on the QD carrier density, modal gain, and differential gain is studied. Red shifts in the gain peak is observed when size fluctuations increases. The energy detuning between the gain peak and the differential gain peak for a pyramidal quantum dot system having an average base length of 130 /spl Aring/ and standard deviation of 7 /spl Aring/ is about 12 meV.     

紫外二极管泵浦固体激光器(DPSS)在工业应用中大展拳脚

窄脉宽、短波长紫外二极管泵浦固体激光器(DPSS)的最新进展促进了工业生产系统的发展。过去。DPSS激光器比较适用于科研而不适于工业生产。随着DPSS激光器的进展，现已开辟出很多可能的应用，包括红外、脉冲连续波以及9开关产生具有多脉冲宽度的脉冲光波。与其他种类的激光器相比，DPSS激光器在调控脉冲形状、重复频率和光束质量等方面具有较大的灵活性，其生成的谐波允许用户获得适于多种材料加工的较短波长的光束。     

Polarization-dependent nonlinear gain in semiconductor lasers

We have numerically studied the nonlinear gain coefficients in terms of spectral hole burning for the optical fields in parallel and orthogonal polarizations in semiconductor lasers by solving the equation of motion for the density matrix in perturbation series. The electronic band structures and the transition matrix elements used in the calculations are obtained by diagonalizing Luttinger's Hamiltonian. In the present analysis for InGaAsP lasers, the cross-saturation coefficient for the parallel polarizations is twice as large as the self-saturation. Also, the cross-saturation coefficient for the orthogonal polarizations, which affects the polarization switching and polarization bistable operations of the laser, rests between the two. The relative magnitude of self-saturation coefficients and cross-saturation coefficients for orthogonal polarizations satisfies the condition for polarization bistable operations. We also discuss the effect of carrier heating on gain saturation coefficients     

Polarization-dependent gain saturations in quantum-well lasers

Theoretical analyses of polarization-dependent optical gain saturation are given for semiconductor quantum-well (QW) lasers to investigate the conditions of polarization switching and bistable operations. Nonlinear susceptibilities, which give saturation coefficients, are obtained in the perturbative analyses of density matrices, where the relevant electronic states in the QW are calculated by diagonalizing Luttinger's Hamiltonian, thus including valence band mixing. The present formulation is applied to InGaAsP QW lasers with edge-emitting and vertical-cavity surface-emitting laser (VCSEL) structures, and the self- and cross-saturation coefficients with parallel and orthogonal optical polarizations are numerically calculated, which are compared with those of bulk lasers. For the edge-emitting case, the saturation coefficients are strongly dependent on the photon energies, and the bistable operation condition is not satisfied in the gain peak, different from a bulk laser which showed only a slight energy dependence. In a VCSEL, the saturation coefficients are also dependent on the photon energies but the bistable operation condition is always satisfied     

Effective nonlinear gain in semiconductor lasers

An effective nonlinear gain is introduced for semiconductor lasers by taking into account the effect of laser structure and the associated distribution of the mode intensity along the cavity length. It should be used in the analysis of laser dynamics and noise in place of the material nonlinear gain parameter. A general expression for the effective nonlinear gain is given by using the Green's function method. The results obtained for Fabry-Perot and distributed feedback lasers show that the effective nonlinear gain could be considerably enhanced. The exact value of the enhancement factor depends on cavity parameters. Affected by the laser structure, the nonlinear gain has a different power dependence than expected from material considerations alone     

Optical gain and gain suppression of quantum-well lasers withvalence band mixing

The effects of valence band mixing on the nonlinear gains of quantum-well lasers are studied theoretically for the first time. The analysis is based on the multiband effective-mass theory and the density matrix formalism with intraband relaxation taken into account. The gain and the gain-suppression coefficient of a quantum-well laser are calculated from the complex optical susceptibility obtained by the density matrix formulation with the theoretical dipole moments obtained from the multiband effective-mass theory. The calculated gain spectrum shows that there are differences (both in peak amplitude and spectral shape) between this model with valence band mixing and the conventional parabolic band model. The shape of the gain spectrum calculated by the new model becomes more symmetric due to intraband relaxation together with nonparabolic energy dispersions. Optical intensity in the GaAs active region is estimated by solving rate equations for the stationary states with nonlinear gain suppression     

Influence of nonuniform carrier distribution on the polarizationdependence of modal gain in multiquantum-well lasers and semiconductoroptical amplifiers

We investigate the modal gain seen by transverse-electric (TE) and transverse-magnetic (TM) modes of bulk and multiquantum-well (MQW) lasers given a nonuniform distribution of active region carriers. We find that the dependence of modal gain on the nonuniformity of carrier profile differs for TE and TM modes. This experimentally observable phenomenon is proposed as a measure of carrier density nonuniformity. We discuss the importance of the confinement picture for TE and TM modes, in the generalized presence of some asymmetry, in assuring injection-level independent polarization insensitivity in semiconductor lasers and optical amplifiers     

Theoretical gain spectrum of far-infrared Raman lasers

The four-level density matrix theory of the Raman FIR laser is extended to include arbitrary pump and emission field strengths and arbitrary pump frequency offsets from resonance. The results accommodate the full range of spectral variation that is observed when CH₃F Raman FIR lasers are operated at widely differing values of the rotational quantum number J. The extended theory is also useful for optimizing the temperature and pressure of these broadly tunable lasers to achieve maximum spectral coverage     

Thermal effects on the linear gain in free-electron lasers

The authors develop a self-consistent formulation of the linear gain in both helical and planar wiggler configurations in the presence of an axial energy spread derived from a beam pitch-angle spread. Such a beam may be thought of as monoenergetic but with a non-vanishing emittance. The analysis includes collective Raman effects for both the helical and planar wiggler systems, and describes the gain at harmonics in the case of a planar wiggler. General dispersion equations are derived, and solved numerically, for each wiggler configuration which includes a general thermal function which describes the effect of the pitch-angle spread     

Effect of nonlinear gain on the bandwidth of semiconductor lasers

The effect of nonlinear gain on the frequency response of semiconductor lasers at high bias powers is analysed using the multimode rate equations. In spite of the strong damping of the relaxation oscillations due to nonlinear gain, bandwidths of over 40 GHz appear to be attainable in semiconductor lasers.     

Effect of nonlinear gain on single-frequency behaviour of semiconductor lasers

The effect of nonlinear gain on the single-frequency behaviour of semiconductor lasers is analysed using a two-mode rate-equation model. We find that the asymmetric nature of the nonlinear gain is responsible for bringing the sidemode above threshold when the power in the main mode exceeds a critical level. We obtain an analytic expression for the critical current density at which the sidemode reaches threshold and apply it to discuss the single-frequency range of distributed-feedback semiconductor lasers.     

Q-switched operation of quasi-three-level lasers

A theoretical model for longitudinally pumped quasi-three-level lasers under Q-switched, repetitively Q-switched, and cw operating conditions is developed. The model is applied using 8% Ho:YAG as the gain medium, and it is found that for typical conditions the effective energy storage time can be much less than the upper state lifetime, in agreement with experiment but not predicted by earlier theories. This effect is attributed to details of laser design and pumping conditions, as opposed to being an intrinsic property of the gain material     

Impact of gain dispersion on the spatio-temporal dynamics ofmultisection lasers

We present a refined model for multi-section lasers, introducing an additional equation for material polarization in the well-known traveling wave model. We investigate the polarization-induced changes in the spectral properties of the optical waveguide. Finally, we show the relevance of this model for a more realistic simulation of the complicated dynamical behavior of multi-section distributed feedback (DFB) lasers, such as fast self-pulsations, multi-stability, and hysteresis effects due to mode competition     

Mode confinement and gain in junction lasers

For a three layer dielectric sandwich, the center dielectric region must have a dielectric constant higher than the outer regions and some finite thickness for a wave to be "bound" to the center region. We have calculated the transverse propagation constants for even TE modes when the dielectric sandwich is unsymmetrical. The resultant field distributions are used to derive a formula for the gain of a junction laser of the formalpha = frac{alpha_{1} + Palpha_{3}/nR + falpha_{2}}{g}, where the αn's are the attenuation (or gain) constants associated with the various regions of the junction structure. Curves ofP/nR,f, andgas a function of active region thickness are given from which the condition for laser oscillation or the net gain or loss per unit length of a given device may be found.