F. Hopfer

20 Gb/s 85

980 nm vertical-cavity surface-emitting lasers based on submonolayer growth of quantum dots show clearly open eyes and operate error free with bit error rates better than 10 at 25 and 85degC for 20 Gb/s without current adjustment. The peak differential efficiency only reduces from 0.71 to 0.61 W/A between 25 and 85degC; the maximum output power at 25degC is above 10 mW.

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We have studied the modulation properties of a vertical cavity surface-emitting laser (VCSEL) coupled to an electrooptical modulator. It is shown that, if the modulator is placed in a resonant cavity, the modulation of the light output power is governed predominantly by electrooptic, or electrorefraction effect rather than by electroabsorption. A novel concept of electrooptically modulated (EOM) VCSEL based on the stopband edge-tunable distributed Bragg reflector (DBR) is proposed which allows overcoming the limitations of the first-generation EOM VCSEL based on resonantly coupled cavities. A new class of electrooptic (EO) media is proposed based on type-II heterostructures, in which the exciton oscillator strength increases from a zero or a small value at zero bias to a large value at an applied bias. A EOM VCSEL based on a stopband-edge tunable DBR including a type-II EO medium is to show the most temperature-robust operation. Modeling of a high-frequency response of a VCSEL light output against large signal modulation of the mirror transmittance has demonstrated the feasibility to reach 40 Gb/s operation at low bit error rate. EOM VCSEL showing 60 GHz electrical and ~35 GHz optical (limited by the photodetector response) bandwidths is realized.

C Error-Free Operation of VCSELs Based on Submonolayer Deposition of Quantum Dots

We report on recent progress in high-speed quantum-dot (QD) vertical-cavity surface-emitting lasers (VCSELs). Advanced types of QD media allow an ultrahigh modal gain, avoid temperature depletion, and gain saturation effects. Temperature robustness up to 100degC for 0.96-1.25 mum range devices is realized in the continuous wave (cw) regime. An open eye 20 Gb/s operation with bit error rates better than 10-12 has been achieved in a temperature range 25degC - 85degC without current adjustment. A different approach for ultrahigh-speed operation is based on a combination of the VCSEL section, operating in the CW mode with an additional section of the device, which is electrooptically modulated under a reverse bias. The tuning of a resonance wavelength of the second section, caused by the electrooptic effect, affects the transmission of the system. The approach enables ultrahigh-speed signal modulation. 60 GHz electrical and ~35 GHz optical (limited by the photodetector response) bandwidths are realized.

Ultra high-speed electro-optically modulated VCSELs: modeling and experimental results

Easy to handle light sources with non-classical emission features are strongly demanded in the growing field of quantum communication. We report on single-photon emission from an electrically pumped quantum dot with unmatched spectral purity, making spatial or spectral filtering dispensable.

High-Speed Quantum-Dot Vertical-Cavity Surface-Emitting Lasers

We show experimentally that polarization mode hopping in quantum dot vertical cavity surface emitting lasers (VCSELs) takes place between nonorthogonal elliptically polarized modes. In contrast to quantum well VCSELs the average dwell time decreases with injection current. This decrease is by 8 orders of magnitude: from seconds to nanoseconds and is achieved without any modifications of the VCSEL internal anisotropies. The observed scaling happens in a range of currents as wide as 8 times the threshold value.

Non-classical light emission from a single electrically driven quantum dot

Advanced types of QD media allow an ultrahigh modal gain, avoid temperature depletion and gain saturation effects, when used in high-speed quantum dot (QD) vertical-cavity surface-emitting lasers (VCSELs). An anti-guiding VCSEL design reduces gain depletion and radiative leakage, caused by parasitic whispering gallery VCSEL modes. Temperature robustness up to 100°C for 0.96 - 1.25 &mgr;m range devices is realized in the continuous wave (cw) regime. An open eye 20 Gb/s operation with bit error rates better than 10-12 has been achieved in a temperature range 25-85°C without current adjustment. A different approach for ultrahigh-speed operation is based on a combination of the VCSEL section, operating in the CW mode with an additional section of the device, which is electrooptically modulated under a reverse bias. The tuning of a resonance wavelength of the second section, caused by the electrooptic effect, affects the transmission of the system. The second cavity mode, resonant to the VCSEL mode, or the stopband edge of the second Bragg reflector can be used for intensity modulation. The approach enables ultrahigh speed signal modulation. 60GHz electrical and ~35GHz optical (limited by the photodetector response) bandwidths are realized.

Polarization switching and polarization mode hopping in quantum dot vertical-cavity surface-emitting lasers

980-nm VCSELs based on submonolayer growth show for 20-Gbit/s large-signal modulation clearly open eyes without adjustment of the driving conditions between 25degC and 120degC. To access the limiting mechanism for the modulation bandwidth, a temperature-dependent small-signal analysis is carried out on these devices. Single-mode devices are limited by damping, whereas multimode devices are limited by thermal effects, preventing higher photon densities in the cavity.

Novel concepts for ultrahigh-speed quantum-dot VCSELs and edge-emitters

Quantum dot (QDs) heterostructures structurally represent tiny 3D insertions of a narrow bandgap material, coherently embedded in a wide-bandgap single-crystalline matrix. The QDs are produced by conventional epitaxial techniques applying self-organized growth and behave electronically as artificial atoms. Strain-induced attraction of QDs in different rows enables vertically-coupled structures for polarization, lifetime and wavelength control. Overgrowth with ternary or quaternary alloy materials allows controllable increase in the QD volume via the island-activated alloy phase separation. Repulsive forces during overgrowth of QDs by a matrix material enable selective capping of coherent QDs, keeping the defect regions uncapped for their subsequent selective evaporation. Low-threshold injection lasing is achieved up to 1350 nm wavelength at 300K using InAs-GaAs QDs. 8 mW VCSELs at 1.3 μm with doped DBRs are realized. Edge-emitters demonstrate 10 GHz bandwidth up to 70°C without current adjustment. VCSELs show ~4 GHz relaxation oscillation frequency. QD lasers demonstrate above 3000 h of CW operation at 1.5 W at 45°C heat sink temperature without degradation. The defect reduction technique (DRT) applied to thick layers enables realization of defect-free structures on top of dislocated templates. Using of DRT metamorphic buffer layers allowed 7W GaAs-based QD lasers at 1500 nm.

Temperature-Dependent Small-Signal Analysis of High-Speed High-Temperature Stable 980-nm VCSELs

We demonstrate experimental evidence of polarization switching accompanied by polarization-mode hopping in quantum-dot vertical-cavity surface-emitting lasers. In our case, the polarization switching is associated with a change of linearly polarized light to elliptically polarized one, hence switching takes place between elliptically polarized states. Current-modulation measurements show that the polarization switching is of thermal origin.

QD lasers: physics and applications

We report on the modeling, epitaxial growth, fabrication, and characterization of 830-845 nm vertical cavity surface emitting lasers (VCSELs) that employ InAs-GaAs quantum dot (QD) gain elements. The GaAs-based VCSELs are essentially conventional in design, grown by solid-source molecular beam epitaxy, and include top and bottom gradedheterointerface AlGaAs distributed Bragg reflectors, a single selectively-oxidized AlAs waveguiding/current funneling aperture layer, and a quasi-antiwaveguiding microcavity. The active region consists of three sheets of InAs-GaAs submonolayer insertions separated by AlGaAs matrix layers. Compared to QWs the InAs-GaAs insertions are expected to offer higher exciton-dominated modal gain and improved carrier capture and retention, thus resulting in superior temperature stability and resilience to degradation caused by operating at the larger switching currents commonly employed to increase the data rates of modern optical communication systems. We investigate the robustness and temperature performance of our QD VCSEL design by fabricating prototype devices in a high-frequency ground-sourceground contact pad configuration suitable for on-wafer probing. Arrays of VCSELs are produced with precise variations in top mesa diameter from 24 to 36 μm and oxide aperture diameter from 1 to 12 μm resulting in VCSELs that operate in full single-mode, single-mode to multi-mode, and full multi-mode regimes. The single-mode QD VCSELs have room temperature threshold currents below 0.5 mA and peak output powers near 1 mW, whereas the corresponding values for full multi-mode devices range from about 0.5 to 1.5 mA and 2.5 to 5 mW. At 20°C we observe optical transmission at 20 Gb/s through 150 m of OM3 fiber with a bit error ratio better than 10-12, thus demonstrating the great potential of our QD VCSELs for applications in next-generation short-distance optical data communications and interconnect systems.