Thermal impedance of VCSELs with AlOx-GaAs DBRs

The authors have measured the thermal impedance of vertical-cavity surface-emitting lasers (VCSELs) with oxide/GaAs DBRs and shown that it is comparable to that of VCSELs with all-semiconductor DBRs. A VCSEL with an 8-μm oxide aperture shows a thermal impedance of 2.8°C/mW. By varying the aperture size, the thermal conductance of the material below the active area is 0.255 W/cm°C. These results demonstrate that the oxide is not a major barrier to heat transport out of the active region     

High-Temperature Continuous-Wave Single-Mode Operation of 1.3-$mu$m p-Doped InAs–GaAs Quantum-Dot VCSELs

In this letter, we have demonstrated continuous-wave single-mode operation of 1.3-$mu{hbox {m}}$ InAs–GaAs quantum-dot (QD) vertical-cavity surface-emitting lasers (VCSELs) with p-type modulation-doped QD active region from 20 $^{circ}hbox{C}$ to 60 $^{circ}hbox{C}$ . The highest output power of 0.435 mW and lowest threshold current of 1.2 mA under single-mode operation are achieved. The temperature-dependent output characteristics of QD-VCSELs are investigated. Single-mode operation with a sidemode suppression ratio of 34 dB is observed at room temperature. The critical size of oxide aperture for single-mode operation is discussed.      

VCSELs based on arrays of sub-monolayer InGaAs quantum dots

Vertical-cavity surface-emitting lasers (VCSELs) with an active region based on sub-monolayer InGaAs quantum dots and doped AlGaAs/GaAs distributed Bragg reflectors were grown by MBE. VCSELs with current aperture of 3 μm in diameter demonstrate single-mode lasing in 980-nm range with the threshold current of 0.6 mA, maximum output power up to 4 mW, and external differential efficiency of 68%. Multimode VCSELs with a (10–12)-μm aperture demonstrate ultralow internal optical loss of 0.09% per pass, which compares favorably with the best results obtained in similar lasers with undoped distributed Bragg reflectors. Original Russian Text ? S.A. Blokhin, N.A. Maleev, A.G. Kuz’menkov, Yu. M. Shernyakov, I.I. Novikov, N.Yu. Gordeev, V.V. Dyudelev, G.S. Sokolovskiĭ, V.I. Kuchinskiĭi, M.M. Kulagina, M.V. Maximov, V.M. Ustinov, A.R. Kovsh, S.S. Mikhrin, N.N. Ledentsov, 2006, published in Fizika i Tekhnika Poluprovodnikov, 2006, Vol. 40, No. 5, pp. 633–638.     

Polarization- and Transverse-Mode Dynamics in Optically Injected and Gain-Switched Vertical-Cavity Surface-Emitting Lasers

In this paper, we summarize our recent results on nonlinear polarization- and transverse-mode dynamics of vertical-cavity surface-emitting lasers (VCSELs) induced by optical injection (OI) or current modulation. Due to the surface emission and cylindrical symmetry, VCSELs lack strong polarization anisotropy and may undergo polarization switching (PS). Furthermore, VCSELs may emit light in multiple transverse modes. This provides new features to the rich nonlinear dynamics induced in VCSELs by an external perturbation. We demonstrate for the case of orthogonal OI that new Hopf bifurcation on a two-polarization-mode solution delimits the injection locking (IL) region and that PS and IL of first-order transverse mode lead to a new resonance tongue for large positive detunings. Similarly, the underlying polarization-mode competition leads to chaotic-like behavior in case of gain switching and the presence of two transverse modes additionally reduces the possibility of regular dynamics.      

Polarization selection in VCSELs due to current carrier heating

The problem of polarization switching of light from a vertical cavity surface emitting laser (VCSEL) is discussed. It is shown that heating of holes in the p-type distributed Bragg reflector and of electrons and holes in the quantum wells of the active region lead to switching of the polarization of the VCSEL. The model developed here makes it possible to explain the results of experiments on switching of the polarization of VCSELs at fixed active region temperatures. Fiz. Tekh. Poluprovodn. 33, 887–893 (July 1999)     

Design and technology of vertical-cavity surface-emitting lasers with nonconducting epitaxial mirrors

Design and technology problems in the fabrication of vertical-cavity surface-emitting lasers (VCSELs) equipped with nonconducting distributed Bragg reflectors (DBRs) and fabricated using molecular-beam epitaxy are considered. VCSELs with an active region were formed on the basis of InGaAs quantum wells and incorporated an AlGaAs/GaAs bottom DBR and an oxidized AlGaO/GaAs top DBR; the diameter of the oxidized aperture was equal to 7–12 μm. The devices exhibit a continuous-wave lasing at room temperature with threshold currents of 0.5–1.5 mA, a differential efficiency as high as 0.5 mW/mA, and a highest output power of 3 mW. __________ Translated from Fizika i Tekhnika Poluprovodnikov, Vol. 37, No. 10, 2003, pp. 1265–1269. Original Russian Text Copyright ? 2003 by Maleev, Kovsh, Zhukov, Vasil’ev, Mikhrin, Kuz’menkov, Bedarev, Zadiranov, Kulagina, Shernyakov, Shulenkov, Bykovskii, Solov’ev, M?ller, Ledentsov, Ustinov.     

1.3-μm GaInAsN Vertical-Cavity Surface-Emitting Lasers by Oxide-Planarized and Surface-Relief Processes for Single-Mode Operation

In this letter, we investigate and characterize the 1.3-mum single-mode vertical-cavity surface-emitting lasers (VCSELs) with two GaInAsN strained multiple quantum wells as the active region. Surface relief technique and a thick silicon oxide were used for the spatial mode filtering and the planarization processing, respectively. The VCSELs with a 5-mum-diameter surface-relief aperture and a 12-mum-diameter oxide-confined aperture at room temperature exhibit a threshold current of 3 mA, a slope efficiency of 0.14 mW/mA, a maximum operation temperature of 90 degC, and a single-mode behavior. These VCSELs show a maximum light output power of 1 mW for the single fundamental mode with a transverse-mode suppression of more than 30 dB and also show a clear eye-opening feature operated at 2.488 Gb/s and 12.6 mA     

Transverse-mode control of vertical-cavity surface-emitting lasers

Transverse-mode control of vertical-cavity surface-emitting lasers (VCSELs) has been investigated. A theoretical model takes into account the distributions of carriers, optical field, and temperature. Using a method of finding self-consistent solutions for the carrier diffusion, optical field, and thermal conduction equations, we have studied the influence of current spreading, injected current density, gain-guided aperture, and window diameter on the transverse modes. The calculated results agree well with those of experiments and show that the transverse-mode evolution of VCSELs depends on the changes of gain and refractive index induced by carriers and heating; decreasing temperature rise and profile width, current spreading, and gain-guided aperture dimension, increasing homogeneity of the injected carriers at the lasing region, and decreasing window diameter are effective methods to suppress high-order transverse modes.     

Engineering the polarization-dependent saturation in quantum-wellsurface-emitting semiconductor lasers

We discuss how the polarization dependence of the saturation in vertical-cavity surface emitting lasers (VCSELs) can be influenced by the design of the quantum-well (QW) gain medium. As an important concept in our discussion, we use carrier reservoirs, i.e., we separate the carrier population into a number of subpopulations. Specifically, we treat VCSELs in which the carriers are separated on the basis of their spin, their momentum, or on the basis of their spatial position. By numerically analyzing the rate equations for one specific case, we show how a properly chosen polarization dependence of the saturation leads to polarization self-modulation     

AlGaAs vertical-cavity surface-emitting laser responses to 4.5-MeV proton irradiation

We have irradiated single- and multimode AlGaAs vertical-cavity surface-emitting laser (VCSEL) arrays operating at a nominal wavelength of 780 nm with 4.5-MeV protons and doses ranging from 10 to 30 Mrad in the active region. We observed a peak power reduction of about 2% per Mrad in the 14-/spl mu/m aperture, multimode VCSELs. Single-mode VCSELs having an aperture of 6 /spl mu/m exhibited a smaller peak power reduction of 0.4%-1% per Mrad. A slight shift in the current threshold was observed only for the multimode VCSELs at dose levels above 10 Mrad. First results indicate a reduced VCSEL peak laser power output that is dominated by a temperature shift caused by the radiation induced increase in resistive heating. In contrast, the power reduction in edge-emitting lasers is dominated by the enhanced radiation induced nonradiative recombination rate. The VCSEL irradiation was performed with a focused ion micro beam that was rastered over the device surface, ensuring a very uniform exposure of a single device in the array.     

Influence of interface growth steps on the anisotropy of excitonic emission in ZnCdSe/ZnSe quantum wells

Spectra of excitonic luminescence of single-quantum-well ZnCdSe/ZnSe heterostructures at T = 8 K are investigated. Two types of linear-polarization anisotropy of the excitonic emission are revealed. The first type of anisotropy corresponds to the luminescence polarization along the [011] axis and is related to the emission from the lower level of the doublet state of heavy-hole excitons localized in the islands elongated in the $[0\overline 1 1]$ direction. The second type of anisotropy is related to the generation of free excitons with wave vectors exceeding that of light. Such a process is made possible by the elastic scattering of light at anisotropic interface-roughness structure originating from the growth steps. In this case, the emission is polarized along the [0-11] axis and the anisotropy is caused by deformation effects that lead to the perturbation of the hole states in the vicinity of the growth steps. The deformation appears due to a mismatch between the lattice constants of the barrier and the well materials.     

Numerical investigation of self-heating effects of oxide-confined vertical-cavity surface-emitting lasers

We present a comprehensive numerical model to simulate self-heating effects of oxide-confined vertical-cavity surface-emitting lasers (VCSELs) under continuous-wave operation. The model self-consistently accounts for the close interaction between optical, electrical, and thermal processes in VCSELs. In particular, hot carriers and nonequilibrium optical phonons in the quantum wells are modeled by solving a carrier energy balance equation and an optical phonon rate equation. Our numerical simulations reveal that they are responsible for aggravated thermal rollovers in VCSELs' L-I characteristics. Detailed comparisons are made and good agreement is obtained between simulations and experiments for the L-I-V and lasing wavelength characteristics of VCSELs with varying oxide aperture size. Various mechanisms that result in the L-I thermal rollover behavior are also investigated with the aid of simulations.     

A comprehensive model for the modal dynamics of vertical-cavitysurface-emitting lasers

We present a quasi-3-D dynamic model of vertical-cavity surface-emitting lasers (VCSELs). The interdependent processes of carrier transport, heat generation and dissipation, and optical fields are solved self-consistently for each point in time and space. An effective index model is adopted for the evaluation of the optical fields in the complex layer structure. The inclusion of a temperature- and carrier-density-dependent refractive index, and its time dependence, allows us to study the evolution of the transverse optical field distributions under dynamic conditions. The model is applied to a typical index-guided structure with a 7-μm oxide aperture. A direct comparison is made using "cold" cavity modes, which is a normal technique when modeling the dynamics of VCSELs. Significant discrepancies are demonstrated both at smalland large-signal modulation, which indicates the need of a more sophisticated model for accurately predicting and understanding the geometry-dependent modal evolution     

Influence of anisotropies on transverse modes in oxide-confinedVCSELs

We present a comprehensive fully vectorial model for the cavity eigenmodes of oxide-confined vertical-cavity surface-emitting lasers (VCSELs) with the details of their complex structure. It includes device-inherent symmetry-breaking mechanisms like noncircular geometries and material anisotropies related to the elasto-optic and electro-optic effect. The latter is accounted for in the model starting from the material and doping profiles. We compare these theoretical results with experimental findings of spectrally and polarization-resolved transverse mode nearfields of oxide-confined VCSELs with two different aperture diameters. Within a parametric study of the influence of aperture anisotropies, we are able to calculate frequencies and gains of all transverse mode families, their polarization dependence and their spatial mode profiles which are in good agreement with the experimental findings     

Design of Stable Single-Mode Chaotic Light Source Using Antiresonant Reflecting Optical Waveguide Vertical-Cavity Surface-Emitting Lasers

The influence of external optical feedback (EOFB) on the lasing characteristics of 1.55-mum antiresonant reflecting optical waveguide (ARROW) vertical-cavity surface-emitting lasers (VCSELs) with various core radii are analyzed theoretically. It is found that ARROW VCSELs with large core radii (i.e., > 4 mum) respond more sensitively to EOFB than that with small core radii (i.e., les 4 mum). Furthermore, strong EOFB-driven ARROW VCSELs with large radii show more difficult to sustain single-mode chaotic oscillation than those with small radii. This is because radiation loss margin of ARROW reduces with the increase of core radii so that the presence of carrier spatial-hole burning deteriorates the stable single-mode operation of VCSELs. However, if the dimensions of ARROW as well as the radius of injection current aperture can be selected appropriately, stable single-mode chaotic light sources can be obtained from EOFB-driven ARROW VCSELs with large core radius.     

Picosecond polarization dynamics and noise in pulsed vertical-cavity surface-emitting lasers

We have measured on picosecond time scales the polarization behavior of vertical-cavity surface-emitting lasers (VCSELs) pumped with short optical pulses. Our data describe the temporal properties of the intensity and intensity noise for orthogonally polarized modes as well as noise correlations between the modes in both the circular and the linear detection bases. A variety of behavior is observed that depends on the polarization of the pump, the strength of the pump, the VCSEL operating temperature which determines the carrier spin-flip rates, and the degree of birefringence and dichroism in the laser. We develop an extension of the "split density" model which includes carrier relaxation processes that are important on picosecond time scales. Our numerical simulations illustrate how spin-flip processes lead to anti-correlated fluctuations in the circularly polarized modes and how anti-correlated noise can arise between the linearly polarized modes without switching between two "preferred" modes, as is commonly seen on longer time scales.     

Engineered polarization control of GaAs/AlGaAs surface-emittinglasers by anisotropic stress from elliptical etched substrate hole

The engineered polarization control of surface emitting (SE) lasers, based on the observed evidence of the polarization determination, is reported. Thermally stressed epitaxial layers including an active region are made anisotropic by an elliptically etched substrate structure. This stress causes an anisotropy in optical gain of the active region. The polarization control has been demonstrated with about 80% reproducibility in etched-well-type surface emitting (SE) lasers by employing a thick gold film or a polyimide as a stress-enhancing material