David Gready

High Speed 1.55 μm InAs/InGaAlAs/InP Quantum Dot Lasers

We report static and dynamic characteristics of InAs/InP quantum dot (QD) lasers emitting near 1.55 μm. The gain section was optimized for a high speed operation using a unique spatially resolved model. The measured modulation capability dependence on structural parameters (barrier width and the number of QD layers) is consistent with the model predictions. Short cavity lasers with a modal gain of more than 10 cm-1 per dot layer exhibit a small signal modulation bandwidth above 9 GHz and large signal modulation at up to 22 Gb/s with an on/off ratio of 3 dB.

High-Speed Low-Noise InAs/InAlGaAs/InP 1.55-

We present the static and dynamic properties of InAs quantum-dot (QD) lasers emitting near 1.55 μm. The used laser material comprises four QD layers and exhibits a high modal gain of about 40 cm-1. The 340-μ.m-long lasers show a room temperature threshold current of 38 mA and a maximum output power of 16 mW. The small signal modulation response is highly damped and carrier transport limited with a moderate 3-dB bandwidth of 5 GHz. This is accompanied by a flat relative intensity noise spectrum at a low level of -150 dBc/Hz. Neverthe- less, the laser exhibits record large signal modulation capabilities for a 1.5-μ.m QD laser: 15 Gb/s with a 4-dB on/off ratio.

\mu{\rm m}

The optical and structural properties of a new kind of InAs/InGaAlAs/InP quantum dot (QD)-like objects grown by molecular beam epitaxy have been investigated. These nanostructures were found to have significantly more symmetrical shapes compared to the commonly obtained dash-like geometries typical of this material system. The enhanced symmetry has been achieved due to the use of an As2 source and the consequent shorter migration length of the indium atoms. Structural studies based on a combination of scanning transmission electron microscopy (STEM) and atom probe tomography (APT) provided detailed information on both the structure and composition distribution within an individual nanostructure. However, it was not possible to determine the lateral aspect ratio from STEM or APT. To verify the in-plane geometry, electronic structure calculations, including the energy levels and transition oscillator strength for the QDs have been performed using an eight-band k·p model and realistic system parameters. The ...

Quantum-Dot Lasers

The small signal modulation response of semiconductor lasers is commonly used to predict large signal modulation capabilities. Recent experiments suggest that this prediction may fail in some quantum dot (QD) lasers. We present a model supported by experiments, which shows that when the small signal modulation response is limited by gain compression and the gain is large, the laser can be modulated at very high bit rates. This effect is inherent to dynamics governing all semiconductor lasers but the conditions needed for high bit rate modulation in the presence of narrow small signal bandwidths are only obtainable in QD lasers.

Electronic structure, morphology and emission polarization of enhanced symmetry InAs quantum-dot-like structures grown on InP substrates by molecular beam epitaxy

We describe a detailed model for carrier dynamics in tunneling injection quantum dot lasers. The model includes the spatial dependence of the mobile carriers and the coupling between mobile carriers and confined carriers. The Poisson and Schrodinger equations are solved together, which enables consideration of band structure modification due to an electron-hole interaction. The model is solved both in steady state and for temporal perturbations. Steady-state carrier distributions and the band structure are presented, as well as large and small signal modulation responses.

On the relationship between small and large signal modulation capabilities in highly nonlinear quantum dot lasers

We present a comprehensive model of a tunneling injection quantum dot laser. The spatially resolved model incorporates homogeneous and inhomogeneous gain broadenings, both of which are found to be important. The tunneling injection improves dynamical properties as a result of an efficient carrier injection into the laser state and also because of an effective lowering of the carrier temperature. The lower temperature yields lower carrier densities, which reduces scattering and hence narrows the homogeneous linewidth, improving the modulation response. The inhomogeneous broadening is naturally found to deteriorate the modulation response.

Carrier Dynamics in Tunneling Injection Quantum Dot Lasers

The conventional distributed feedback and distributed Bragg reflector edge-emitting lasers employ buried gratings, which require two or more epitaxial growth steps. By using lateral corrugations of the ridge-waveguide as surface gratings the epitaxial overgrowth is avoided, reducing the fabrication complexity, increasing the yield and reducing the fabrication cost. The surface gratings are applicable to different materials, including Al-containing ones and can be easily integrated in complex device structures and photonic circuits. Single-contact and multiple contact edge-emitting lasers with laterally-corrugated ridge waveguide gratings have been developed both on GaAs and InP substrates with the aim to exploit the photon-photon resonance in order to extend their direct modulation bandwidth. The paper reports on the characteristics of such surface-grating-based lasers emitting both at 1.3 and 1.55 μm and presents the photon-photon resonance extended small-signal modulation bandwidth (> 20 GHz) achieved with a 1.6 mm long single-contact device under direct modulation. Similarly structured devices, with shorter lengths are expected to exceed 40 GHz small-signal modulation bandwidth under direct modulation.

Effects of Homogeneous and Inhomogeneous Broadening on the Dynamics of Tunneling Injection Quantum Dot Lasers

InP-based two-section distributed Bragg reflector lasers employing surface defined lateral gratings, which are compatible with low-cost nanoimprint technology, were fabricated. A newly developed inductively coupled plasma reactive ion etching process was utilized for obtaining large aspect ratios and good device performance. The lasers operate in a single mode up to 50°C under continuous-wave conditions. A modulation bandwidth of about 10 GHz was obtained for 0.9-mm-long devices.

Development of high-speed directly modulated DFB and DBR lasers with surface gratings

With high modal gain InAs/AlGaInAs/InP quantum dot laser material short cavity ridge waveguide lasers were fabricated with cavity lengths down to 275 μm. These devices show new record values in direct digital signal modulation at 22 GBit/s. In addition also strong improvement are expected from this laser material in the static properties, in particular on the linewidth due to the suppression of the linewidth enhancement factor. First distributed feedback lasers on similar quantum dot laser material were processed and preliminary linewidth measurements indicate a significant linewidth reduction. Quantitative investigations are under way and will be presented at the conference.

1.3-

We report on the static and dynamic characteristics of InAs/InGaAlAs/InP quantum dot lasers. The lasers exhibit a record model gain of >; 70 cm-1 with 6 QD layers and reached with an optimized active region design a record digital modulation rate of 20 GBit/s at 1.55 μm wavelength range.