Efficient 1.3 mm GaInNAs Quantum Well Lasers for Uncooled, High Speed Operation

Doktorsavhandling, 2006

The rapid expansion of tele and data transmission systems requires an ever increasing capacity in modern optical fibre communication networks. With the implementation of short distance, high density networks, such as access networks, there is a need for cost-effective optical transmitters compatible with single mode fibres. Directly modulated, long wavelength (1.3 and 1.55 µm) semiconductor lasers for uncooled, high speed operation are therefore under development. With the traditional InP based lasers being relatively temperature sensitive there is a need for new laser technologies with improved high temperature performance. GaAs based long wavelength lasers offer such an opportunity where new gain materials such as (In)GaAs quantum dots and GaInNAs quantum wells (QWs) are used to enable emission at wavelengths longer than what can be achieved with conventional GaAs based materials. The work presented in this thesis deals with GaInNAs materials and lasers, aiming for efficient 1.3 µm QW lasers for high speed operation over a wide temperature range. By replacing a small amount of As by N, the strong N-induced lattice perturbation results in unique physical properties of this material, such as a large reduction of the bandgap energy and an increased conduction band offset. The epitaxial material is grown by molecular beam epitaxy (MBE). Using optimized growth conditions and suitable material compositions, 1.3 µm GaInNAs single QW lasers with GaNAs barriers, exhibiting a record low threshold current density of 320 A/cm2 for MBE-grown lasers, are demonstrated. Subsequent optical gain measurements show that N incorporation in the barriers results in population of excited states, in agreement with k×p calculations, which is disadvantageous for the thermal stability and the high speed modulation properties. This led to the fabrication of GaInNAs double QW lasers with GaAs barriers, emitting at 1.28 µm and exhibiting a threshold current density of only 300 A/cm2 (150 A/cm2/QW), the lowest reported for any GaInNAs laser at this wavelength. From this material, ridge waveguide lasers with a record modulation bandwidth of 17 GHz at room temperature were fabricated. The temperature dependence of the modulation bandwidth, bandwidth limiting effects and basic parameters of importance for the laser dynamics were also studied. Finally, as a results of the excellent temperature stability, uncooled operation over the temperature range 25-110°C was demonstrated at both 2.5 and 10 Gb/s.