High-speed modulation of vertical-cavity surface-emitting lasers

The IM (intensity modulation) and FM (frequency modulation) characteristics of vertical-cavity surface-emitting lasers are studied. The laser has high FM efficiency and broad IM bandwidth (near 8 GHz) at a very low bias (4.5 mA). Five Gb/s pseudorandom direct intensity modulation of this laser with open eyes is demonstrated.<>     

Selection and modulation of high-order transverse modes invertical-cavity surface-emitting lasers

A theoretical proposal to double the pulse repetition frequency by alternating current modulation of two orthogonal high-order transverse modes of a vertical-cavity surface-emitting laser (VCSEL) is presented. Static and dynamic characteristics of weakly index-guided VCSELs in a multitransverse mode regime are analyzed. An efficient model that takes into account all the transverse modes supported by the waveguide is developed. Mode partition noise in a current modulated VCSEL is studied, taking into account the azimuthal degree of freedom. Different transverse modes can be excited with a probability that is numerically calculated. For high injection currents, modes that are not favored in the steady state can be excited with higher probability. The excitation probability is similar for azimuthally orthogonal modes. This symmetry can be broken by selecting a particular high-order transverse mode by using azimuthal-dependent current profiling. This selection can be achieved over current ranges as wide as ten times the threshold current. Current modulation of this transverse mode is then analyzed. Alternate current modulation of two orthogonal high-order transverse modes is also studied. Alternating modulation of these modes can double the pulse repetition frequency obtained by modulating just one high-order transverse mode, without increasing injected current density levels. This current-induced spatial switching leads to high-frequency beam steering in the laser azimuthal direction     

Theoretical analysis of modulation response and second-orderharmonic distortion in vertical-cavity surface-emitting lasers

A rate-equation model is developed, with the consideration of size effects, to analyze the steady state and dynamic behavior of index-guided vertical-cavity surface-emitting lasers. The size dependence of spatial hole burning, cavity loss, as well as thermal resistance of device cavity are taken into account. Using this model, the influence of size effects on the amplitude modulation response and second-order harmonic distortion are studied. It is found that a laser with a small core radius exhibits better modulation response and less harmonic distortion than that of a large waveguide device, however, there is a tradeoff between the output power and modulation efficiency of the lasers     

Circuit model for multiple transverse mode vertical-cavity surface-emitting lasers

The modeling of multimode operation in vertical-cavity surface-emitting laser diodes is difficult because spatial interaction between modes makes the problem very complex. In this paper, a novel circuit model is presented for such lasers, which is based on spatially dependent rate equations, and thus spatial hole burning and carrier diffusion are taken into account. These effects are studied in direct-modulated microwave vertical-cavity surface-emitting lasers by showing linear and nonlinear simulation examples in both time and frequency domains.     

Effects of built-in polarization on InGaN-GaN vertical-cavity surface-emitting lasers

We investigate the effect of built-in spontaneous and piezoelectric polarization on the internal device physics of current-injected GaN-based vertical-cavity surface-emitting lasers (VCSELs) with strained InGaN quantum wells. Advanced device simulation is applied to a previously manufactured device design featuring dielectric mirrors and an indium-tin-oxide current injection layer. Contrary to common perception, we show: 1) that only a small fraction of the built-in quantum-well polarization is screened at typical injection current densities and 2) that the polarization of the AlGaN electron stopper layer has a strong effect on the VCSEL threshold current which can be partly compensated for by higher p-doping.     

High-power single-mode antiresonant reflecting optical waveguide-type vertical-cavity surface-emitting lasers

Antiresonant reflecting optical waveguide (ARROW) techniques are employed in vertical cavity surface emitting lasers (VCSELs) to achieve high-power single-mode emission. Using the effective-index method and fiber mode approximation, the cold-cavity lateral modal behavior for the circular shaped ARROW VCSEL demonstrates significant reduction of radiation loss from that of a single antiguide, while maintaining strong discrimination against high-order modes. The circular-waveguide is created by selective chemical etching and two-step metal-organic chemical vapor deposition growth, with proton implantation used to confine the current injection to the low-index core region. A single-mode CW power of 7.1 mW has been achieved from an 8 /spl mu/m diameter ARROW device (index step /spl Delta/n = 0.05, emission at /spl lambda//sub 0/ = 980 nm) with a far-field FWHM of 10/spl deg/. Larger aperture (12 /spl mu/m) devices exhibit multimode operation at lower drive currents with a maximum single-mode continuous-wave output power of 4.3 mW.     

Influence of optical feedback on the polarization switching of vertical-cavity surface-emitting lasers

We study theoretically the effect of isotropic and polarization-selective optical feedback on the polarization switching of vertical-cavity surface-emitting lasers. We use the framework of the spin-flip model that takes into account the spin degree of freedom of carriers in the semiconductor quantum well. We analyze the polarization switching dynamics for different values of the spin relaxation rate and find a good agreement with recent experimental results . We also study the influence of the external cavity length and find different dynamical behaviors for short and long external cavities.     

Influence of optical feedback time-delay on power-drops in vertical-cavity surface-emitting lasers

The influence of optical feedback time delay on the average period of low-frequency fluctuations (LFF) in external cavity vertical-cavity surface-emitting lasers (VCSELs) is studied experimentally. The regularity of power dropouts is shown to increase with increased external cavity length. Coexistence of LFF and stable operation is observed when the external cavity length is close to the VCSEL coherence length.     

Transient response of vertical-cavity surface-emitting lasers ofdifferent active-region diameters

The multimode dynamics of vertical-cavity surface-emitting lasers with different active-region diameters was measured under subnanosecond electrical excitation (800-ps pulse duration, 100-ps risetime). The dynamics is characterized by the delayed onset of higher order modes which have a turn-on delay that is dependent on the active-region diameter and the excitation parameters. A simple model that can be used to estimate this turn-on delay for large-area devices is presented. Polarization-resolved measurements show that, under this fast excitation condition, both orthogonal polarization states are isomorphic. The influence of the observed dynamics on the relative intensity noise of these devices is also discussed     

Frequency response of polarization switching in vertical-cavity surface-emitting lasers

We present an experimental study of the current-driven polarization modulation properties of VCSELs. In some VCSELs, abrupt polarization switching (PS) between two polarization modes is observed at a particular value of the pump current. We investigate the dynamics and the associated dominating time scales of PS as these features are strongly linked with the underlying physical mechanism causing the PS. To this end, we measure both for gain- and index-guided VCSELs the critical modulation amplitude necessary to steadily force PS back and forth across the PS point as a function of the modulation frequency. This yields the current-driven polarization modulation frequency response, which we compare with the thermal frequency response of the studied devices. The dynamic behavior turns out to be strikingly different for the different VCSEL types. Thermal effects only play a minor role in the PS in our index-guided VCSELs, while they really seem to lie at the origin of PS in the gain-guided VCSELs. By implementing this in a rate-equation based theoretical model, we are able to explain the peculiarities of the measured response curves and to reproduce the experimental findings.     

Numerical study of optical injection dynamics of vertical-cavity surface-emitting lasers

We study numerically the transverse-mode dynamics of a vertical-cavity surface-emitting laser with external optical injection. We consider the case in which two transverse modes with parallel polarizations are exited. Varying the strength and frequency of the injected field, we find a rich variety of complex behaviors. We show that by increasing the optical injection strength the laser does not turn into a single-mode laser, as would be expected if the transverse modes have perpendicular polarizations.     

Spectral characteristics of vertical-cavity surface-emitting lasers with external optical feedback

The measurement of the effects of external optical feedback on the spectra of VCSELs (vertical-cavity surface-emitting lasers) is reported. It is surprising that VCSELs have a sensitivity to optical feedback comparable to that of conventional edge-emitting lasers such as DFBs despite their significantly different structures. This is because the extremely short cavity length of VCSELs negates the effects of their highly reflective output mirrors. As in edge-emitting lasers, VCSELs exhibit well-defined regimes of feedback effects in their spectra. Since optical isolators cannot be easily applied to VCSELs due to their array structure, these lasers may be most useful in applications which are not sensitive to the spectral qualities of the light source.<>     

外部光反馈作用下垂直腔面发射激光器的电光特性

研究了外部光反馈对垂直腔面发射激光器（VCSELs）电光特性的影响。通过计算光反馈参数对比了VCSELs与边发射激光器（EELs）的光反馈灵敏性。基于复合腔理论计算了存在外部光反馈时垂直腔面发射激光器的阈值电流及斜率效率等电光特性参数。实验结果表明,分布布拉格反射镜（DBR）反射率不同的VCSEL,在反馈率为10%的外部光反馈作用下,其阈值电流及斜率效率发生了不同程度的变化。实验结果有效验证了理论计算的结论。     

Gigabit-per-second cryogenic optical link using optimizedlow-temperature AlGaAs-GaAs vertical-cavity surface-emitting lasers

We describe the design of GaAs-AlGaAs vertical-cavity surface-emitting lasers (VCSELs) that are optimized for operation at very low temperatures and the experimental demonstration of a free-space optical interconnect for cryogenic electronic systems using a VCSEL. We demonstrate high-speed modulation of the optical link at a data rate of up to 2 Gb/s at 77 K, with a very low bit-error rate of <10^-13 , and thermally stable operation is achieved over a wide range of cryogenic temperatures without laser bias current compensation. Cryogenic VCSELs with excellent lasing characteristics have been achieved over the entire temperature range from 150 K to 6 K, including high output power (22 mW), high power-conversion efficiency (32%), high slope efficiency (~100%), low threshold voltage (1.75 V) and current (1.7 mA), as well as a high-modulation bandwidth (12 GHz) for a 16 μm diameter device at 80 K     

Light-current characteristics of vertical-cavity surface-emitting lasers with external optical feedback

The influence of external optical feedback (OFB) on the light-current characteristics of the vertical-cavity surface-emitting lasers (VCSELs) was investigated theoretically and experimentally. By calculating the OFB sensitivity parameter, the OFB sensibility of the VCSELs was compared with the edge emitting lasers. Based on the compound cavity theory, the light-current characteristic parameters of the VCSELs with external OFB, such as the threshold current and the slope efficiency, were calculated. The experimental results indicated that the threshold current of the VCSELs with different DBR reflectivities decreased to different degrees, accompanied with a decrease of slope efficiency when under 10% feedback ratio of the external OFB, which is in good agreement with the theoretical calculation.     

High-speed modulation of 850-nm intracavity contacted shallowimplant-apertured vertical-cavity surface-emitting lasers

GaAs-AlGaAs quantum-well (850 nn) vertical-cavity surface-emitting lasers, with lateral current injection and shallow implanted apertures, show small signal modulation bandwidths of at least 11 GHz and large signal data rates of at least 10 Gb/s. The devices achieved a maximum output power of 2.1 mW, with a threshold current and voltage of 1 mA and 1.71 V, respectively. The shallow implantation step provides photolithographically precise aperture formation (using O⁺ ions), for efficient lateral current injection into the quantum-well active region of the laser, from intracavity contacts. The device aperture was 7 μm in diameter, and the opening in the annular top contact was 13 μm in diameter. The optical spectrum showed several transverse modes     

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.     

Dynamic behavior of vertical-cavity surface-emitting lasers

A quasi-three-dimensional dynamic model of index-guided vertical-cavity surface-emitting lasers is developed. Detailed structure of Bragg reflectors and lateral optical confinement are considered into the model. A three-dimensional waveguide problem is reduced to one dimension by using the effective-index method. The dynamic response of optical field is solved by the time-domain algorithm. In addition, the lateral variation of carrier concentration, refractive index, and spontaneous-emission profile are also determined in a self consistent manner. Using this model, the influence of carrier transport and hot carriers on the dynamic behavior of vertical-cavity surface emitting lasers is studied. It is found that these nonlinearities have significant influence on the relaxation-oscillation frequency and modulation bandwidth of the devices     

Nonlinear dynamics of vertical-cavity surface-emitting lasers

The dependence of the transient response of a vertical-cavity surface-emitting laser (VCSEL) on its aperture size is investigated subject to direct current modulation and external optical feedback. It is shown that lasers with small aperture size suppress higher order bifurcations and chaos even under large-signal modulation and external optical feedback. Furthermore, the noise characteristics of VCSELs under the influence of external optical feedback are studied via the calculation of relative intensity noise. It is found that the level of external optical feedback for the onset of coherence collapse is high for devices with small aperture size. On the other hand, the small-signal response of lasers is also analyzed through the calculation of third-order harmonic distortion. It is shown that harmonic distortion is minimized in small devices. Therefore, VCSELs with small aperture size have better immunity to irregular response under direct current modulation and external optical feedback     

ESD-induced degradation of vertical-cavity surface-emitting lasers

The sensitivity of proton implanted, vertical-cavity surface emitting lasers (VCSELs) to electrostatic discharge (ESD) pulses is investigated under human body model test conditions. Rather low degradation threshold pulse amplitudes were observed in forward bias (+1500 V) as well as reverse bias (-800 V) step stress tests. Monitoring both the electrical and optical parameters of the VCSELs during ESD stress, it was found that in forward bias ESD stress tests the optical degradation precedes the electrical degradation