T. Knodl

Cavity solitons as pixels in semiconductor microcavities

Cavity solitons are localized intensity peaks that can form in a homogeneous background of radiation. They are generated by shining laser pulses into optical cavities that contain a nonlinear medium driven by a coherent field (holding beam). The ability to switch cavity solitons on and off and to control their location and motion by applying laser pulses makes them interesting as potential ‘pixels’ for reconfigurable arrays or all-optical processing units. Theoretical work on cavity solitons has stimulated a variety of experiments in macroscopic cavities and in systems with optical feedback. But for practical devices, it is desirable to generate cavity solitons in semiconductor structures, which would allow fast response and miniaturization. The existence of cavity solitons in semiconductor microcavities has been predicted theoretically, and precursors of cavity solitons have been observed, but clear experimental realization has been hindered by boundary-dependence of the resulting optical patterns—cavity solitons should be self-confined. Here we demonstrate the generation of cavity solitons in vertical cavity semiconductor microresonators that are electrically pumped above transparency but slightly below lasing threshold. We show that the generated optical spots can be written, erased and manipulated as objects independent of each other and of the boundary. Numerical simulations allow for a clearer interpretation of experimental results.

Multistage bipolar cascade vertical-cavity surface-emitting lasers: theory and experiment

We present an overview over our research on bipolar cascade vertical-cavity surface-emitting lasers (VCSELs) emitting at 980 nm wavelength, including the scaling properties and the influence of design variations on laser performance as well as strategies for GaAs Esaki junction optimization. We experimentally demonstrate high-performance two- and three-stage devices, the latter of which with 130% differential quantum efficiency. The derived analytical expressions for the scaling behavior are confirmed by measurement data and show a significant improvement in the steady-state as well as the dynamic performance with respect to active region stacking. From the investigated design variations, the influence of oxide apertures and active region spacing on laser performance is clearly extracted and reveals that current spreading is present in the cavity and can lead to severe limitations in optical performance. The GaAs tunnel diodes have been optimized with respect to the donor concentration and the additional incorporation of an In/sub 0.1/Ga/sub 0.9/As layer on the n/sup ++/-side, leading to a zero-bias specific resistance of about 1/spl middot/10/sup -4/ /spl Omega/cm/sup 2/.

Current-spreading-induced bistability in bipolar cascade vertical-cavity surface-emitting lasers

We present experimental evidence that current spreading in the cavity of two-stage bipolar cascade vertical-cavity surface-emitting lasers is responsible for the formation of bistability loops in the light versus current characteristics. The bistable behavior strongly varies with the detuning between cavity resonance and gain maximum and is attributed to the wavelength and carrier density dependent absorption coefficient. In order to explain the measured dependencies, an analytical model is presented that very well matches the light output characteristics.

Scaling behavior of bipolar cascade VCSELs

We present theoretical and experimental results on the scaling properties of bipolar cascade vertical-cavity surface-emitting lasers with respect to active region stacking and out-coupling mirror reflectivity. The measurement results at around 980-nm wavelength confirm the theoretically expected behavior of an almost linear increase in slope efficiency and a reduction in threshold current density with the number of stages. A three-stage laser shows a differential quantum efficiency of 128% in continuous-wave operation at room temperature. For the threshold current density, we obtain a decrease from 3.75 kA/cm/sup 2/ for the conventional VCSEL to about 1.2 kA/cm/sup 2/ for a three-stage device. Such multi-diode cascade VCSELs are highly interesting for optical links, analog systems, and high-power applications.

Investigation of the capacitance of integrated DFB-EAMs with shared active layer for 40 GHz bandwidth

Electro absorption modulators (EAM )integrated with distributed feedback (DFB) lasers are attractive light sources for modulation speeds up to 40 Gbps. A model was developed that considers intrinsic carrier phenomena and extrinsic parasitics of a DFB-EAM. With this model, the small signal measurements could be verified and design rules were identified. Simulation results are in excellent agreement with measured data and allow for detailed insight in carrier pile-up and field spreading related effects. By design optimization, the 3 dB bandwidth of the devise was boosted from 18 GHz to 41 GHz.

40 GHz monolithic integrated 1.3 /spl mu/m InGaAlAs-InP laser-modulator with double-stack MQW layer structure

Monolithic integrated DFB laser-modulator devices (EML) based on the double-type MQW stack are studied. First experimental results on optimization strategies for high-speed double stack EMLs are presented. The small-signal 3 dB bandwidth is improved, with respect to that previous device design, by more than a factor of two, up to the 40 GHz regime. This is done by decreasing the additional pin-junction capacitance in the EAM section by mesa width shrinkage and replacing the n-type InP substrate by a si-type one. Thus, the novel double-stack EML approach, fabricated in the InGaAlAs-InP material system, has the potential to meet the requirements of future 40 Gbps optoelectronic transceivers.

Influence of design variations on bipolar cascade VCSEL performance

The basic idea of bipolar cascade vertical-cavity surface-emitting lasers (VCSELs) is the monolithic stacking of active regions that are electrically coupled using Esaki tunnel junctions in between. Thus, carriers can be recycled for additional radiative recombination resulting in a higher roundtrip gain for cascade VCSELs compared to the conventional structure. This offers an additional design freedom to improve optical performance of VCSELs. Firstly, it is possible to scale down the threshold current density and increase the differential quantum efficiency simultaneously. Secondly, cascade VCSELs have the potential to boost optical output power. Such devices might be an interesting solution for high-power VCSEL applications, optical links with gain and analog systems. The scalability of device performance with respect to active layer stacking has been successfully demonstrated recently, even with a slope efficiency well exceeding unity in continuous wave operation at room temperature. However, to improve cascade laser performance further, we have investigated various designs to achieve a better understanding of device behavior. In this paper, we present the comparison of electrical and optical properties of three different two-stage diode cascade VCSEL designs. It is observed that current spreading in the cavity and device degradation are important issues that have to be considered in bipolar cascade VCSELs.

Improvement of diode cascade VCSEL performance

Summary form only. The performance of conventional VCSELs is generally limited by the extremely low roundtrip gain in the cavity. Thus, VCSEL devices require high mirror reflectivity but also are characterized by an increase in the threshold current density compared to edge emitting lasers. To overcome this distinct disadvantage, diode cascade VCSELs are proposed as excellent candidates to improve the performance of VCSEL structures. Such a VCSEL could also exhibit high differential quantum efficiency well exceeding unity that may be interesting for low noise applications. Previously, we could demonstrate the first continuous-wave, room temperature operating diode cascade VCSEL at 980 nm wavelength. In this letter, we present the improvement of the diode cascade VCSEL output characteristics by reducing the current spreading in the cavity region.