Optimized active region design for high speed 850 nm VCSELs

Paper i proceeding, 2009

Short wavelength (850 nm) VCSELs operating at speeds of 25 Gb/s and above are needed for future high capacity, short reach data communication links. The modulation bandwidth is intrinsically limited by the differential gain of the QWs used in the active region of the VCSEL. In this work we explore the use of strained InGaAs/AlGaAs QWs and benchmark the performance against conventional GaAs/AlGaAs QWs. An 8-band k⋅p model [1] was used to calculate the energy band dispersions, using band offsets from model solid theory [2]. In all cases, the QW and barrier compositions and QW thickness were chosen for a gain peak at 845 nm, enabling emission at 850 nm with a small detuning between the gain peak and the cavity resonance. With increasing In-concentration the QW thickness is reduced and the Al-concentration in the barrier is increased to maintain the gain peak at 845 nm and the number of QWs is increased to maintain optical confinement and enable operation at a low carrier density for high differential gain. It was found that the incorporation of up to 10% In leads to a significant reduction in threshold carrier density and increase in differential gain. This is due to an increased separation and reduced mixing between the highest heavy-hole and light-hole valence bands (Fig.1). A further increase of In concentration leads to a less marked improvement. With an optimum active region design (5 x 4 nm In0.10Ga0.90As/Al0.37Ga0.63As QWs) a differential gain twice as high as that of a conventional design with 3 x 8 nm GaAs/Al0.30Ga0.70As QWs was predicted (Table 1). The improvement of differential gain was experimentally confirmed by extracting the resonance frequency and its dependence on current from the modulation response of VCSELs with optimized InGaAs/AlGaAs QW and conventional GaAs/AlGaAs QW active regions. The differential gain was calculated from the corresponding D-factors (Fig.2) [3]. Excellent agreement was obtained between theory and experiments (Table 1). VCSELs with an optimized InGaAs/AlGaAs QW active region have a modulation bandwidth of 20 GHz at 25° and 15 GHz at 85°C [4] and have enabled error-free transmission over 50 (100) m multimode fiber up to 32 (25) Gb/s at a bias current density as low as 11 kA/cm2 under direct current modulation.