Göran Adolfsson

Effects of Lateral Diffusion on the Temperature Sensitivity of the Threshold Current for 1.3-

We present an experimental and theoretical investigation of the temperature dependence of the threshold current for double quantum well GaInNAs-GaAs lasers in the temperature range 10 degC-110 degC. Pulsed measurements of the threshold current have been performed on broad and narrow ridge wave guide (RWG) lasers. The narrow RWG lasers exhibit high characteristic temperatures (T0) of 200 K up to a critical temperature (Tc), above which T0 is reduced by approximately a factor of 2. The T0-values for broad RWG lasers are significantly lower than those for the narrow RWG lasers, with characteristic temperatures on the order of 100 (60) K below (above) Tc. Numerical simulations, using a model that accounts for lateral diffusion effects, show good agreement with experimental data and reveal that a weakly temperature dependent lateral diffusion current dominates the threshold current for narrow RWG lasers.

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A regenerator for differential phase-shift keying data, consisting of two Mach-Zehnder interferometers and two amplitude regenerators, is examined theoretically. The result, which shows that the phase error is suppressed if phase-preserving amplitude regenerators are used, provides a theoretical description of an already experimentally implemented regenerator.

Double Quantum-Well GaInNAs–GaAs Lasers

By inserting index perturbations at certain positions along a semiconductor Fabry–Perot laser cavity the threshold gain for one or several of the longitudinal cavity modes can be selectively lowered to facilitate, e.g., single-mode or two-color operation. Previous design methods were limited to a fairly small number of perturbations, leading to only weakly perturbed cavities and thus a limited freedom in tailoring the spectral properties of the laser. In our approach we fully account for all multiple-reflection events and use a search space that permits any distribution of the locations and lengths of the perturbations. We are therefore able to design cavities with almost arbitrary spectral properties with very low threshold gain values for, e.g., the lasing modes of a two-color cavity. Constraining the design by reducing the geometrical freedom, which can be used to increase the smallest feature size to simplify fabrication, we seamlessly approach the weakly perturbed regime while maintaining much of the freedom for spectral engineering.

Suppression of phase error in differential phase-shift keying data by amplitude regeneration

We present epitaxial growth of GaInNAs on GaAs by molecular beam epitaxy (MBE) using analog, digital and N irradiation methods. It is possible to realize GaInNAs quantum wells (QWs) with a maximum substitutional N concentration up to 6% and a strong light emission up to 1.71μm at 300K. High quality 1.3μm GaInNAs multiple QW edge emitting laser diodes have been demonstrated. The threshold current density (for a cavity of 100x1000μm2) is 300, 300, 400 and 940A/cm2 for single, double, triple and quadruple QW lasers, respectively. The maximum 3dB bandwidth reaches 17GHz and high-speed transmission at 10Gb/s up to 110°C under a constant voltage has been demonstrated.

Spectral engineering of semiconductor Fabry-Perot laser cavities in the weakly and strongly perturbed regimes

We present results from measurements of the subthreshold lateral spontaneous emission profile in 1.3-mu m wavelength ridge waveguide InGaNAs quantum-well lasers using a scanning near-field optical microscopy technique. The measurements reveal the presence of significant lateral carrier diffusion which has a profound effect on the temperature dependence of the threshold current. This effect is frequently omitted when the characteristic temperature of the threshold current is considered.

Dilute nitrides and 1.3µm GaInNAs quantum well lasers on GaAs

Dilute-nitride lasers with record performances have been used to build uncooled transceivers and failure free 10 Gb/s optical transmission was achieved over 815 m of multimode Corning InfiniCor fiber under the LRM standard.

Direct Observation of Lateral Carrier Diffusion in Ridge Waveguide InGaNAs Lasers

In this work, we optimise the structure of an uncooled directly modulated 1.3 mum GaInNAs ridge waveguide laser for high temperature operation. The static and dynamic performance of the optimised design is analyzed using an accurate in-house 2D electro-opto-thermal laser simulator. The optimised structure is shown to have a lower threshold current, higher efficiency, higher modulation bandwidth and lower vertical beam divergence compared to a reference structure with a conventional design. Large-signal 10 Gbit/s digital modulation simulations were performed and demonstrate the improved performance of the optimised structure especially under high temperature operation.

10 Gb/s uncooled dilute-nitride optical transmitters operating at 1.3 µm

We present state-of-the-art performance of 1.3 μm GaInNAs lasers on GaAs grown by molecular beam epitaxy. The lowest achieved threshold current density is 297, 150 and 133 A/cm2 per quantum well (QW) for single, double and triple QW broad area lasers with a cavity length of 1 mm. The characteristic temperature is 93-133 K in the ambient temperature range of 10-80 °C for broad area lasers depending on the cavity length, and increases to 163-208 K for ridge waveguide lasers as a result of temperature insensitive lateral carrier diffusion in QWs. The maximum 3 dB bandwidth of 17 GHz is achieved in a double QW laser. Uncooled 10 Gb/s operation up to 110 °C has been demonstrated.

Static and dynamic performance optimisation of a 1.3 μm GaInNAs ridge waveguide laser

Spectrally engineered semiconductor Fabry-Perot laser resonators are designed to enhance the optical feedback for selected longitudinal modes, which thereby require less gain for lasing. This is achieved by introducing refractive index perturbations along the length of the resonator. However, the physical realization of these resonators is a challenge because of very narrow tolerances; in particular the need for precise positioning of the end facets of the resonator in relation to the perturbations, and the excess propagation loss associated with the perturbations, has been a major concern. We report on a method to achieve high-quality end facet mirrors enabling precise positioning relative to the perturbations, the latter which are realized as lateral corrugations of the waveguide. Measurements show that the mirror quality is comparable to that of cleaved mirrors and that the additional loss introduced by the perturbations adds < 10 cm-1 to the overall propagation loss, provided that the perturbations are densely enough spaced along the resonator. This implies that the number of perturbations should be large, which is beneficial for the realization of strongly perturbed resonators enabling the most flexible engineering of the spectral properties of the laser.

High performance 1.3 μm GaInNAs quantum well lasers on GaAs

In this work we have measured the lateral spontaneous emission profile for the lasers, using scanning near-field optical microscopy (SNOM). Since the near-field of the spontaneous emission maps the lateral carrier distribution in the active region, this measurement provides a way to directly measure the lateral diffusion. The obtained profile therefore represents the optical mode which is well confined beneath the ridge. Carriers that diffuse outside the ridge can hence not contribute to optical gain and are therefore associated with a leakage current.