An overview is given about the recent improvement in 1.55 μm QD lasers for direct modulation. Based on improved QD epitaxy, which reduces the inhomogeneous size distribution, record values in small signal modulation bandwidth of more than 15 GHz and in digital modulation of up to 35 GBit/s were obtained. Due to the high modal gain and robust ground state transition, the temperature dependence of the laser performance could be very much improved with characteristic temperatures of T0 = 125 K and T1 near to 400 K. This allow a temperature stable modulation bandwidth between 15-60 °C of (14 +/- 1) GHz sufficient for 25 GBit/s digital modulation.
An overview is given about the recent improvement in 1.5 μm QD lasers for direct modulation. Based on improved QD epitaxy with a reduced inhomogeneous size distribution, record values in small signal modulation bandwidth of more than 15 GHz and in digital modulation of up to 36 GBit/s were obtained. Due to the high modal gain and robust ground state transition, the temperature dependence of the laser performance could be very much improved with characteristic temperatures of T0 = 125 K and T1 near to 400 K. Also the impact of the temperature on the digital modulation speed will be discussed.
Self-organized InAs quantum dot (QD) lasers based on InP substrate were grown by means of solid source molecular beam epitaxy (SSMBE). Six InAs QD layers with high dot density and highly uniform dot sizes were used as active medium. Broad area (BA) and ridge waveguide (RWG) lasers with different cavity lengths were processed and characterized. Also the influence of a post-growth rapid thermal annealing (RTA) process on the laser characteristics was investigated. The lasers showed a high modal gain of 12 - 14.5 cm-1 per dot layer and a threshold current density for infinite cavity length of 120 A/cm2 per dot layer. In pulsed operation, as-cleaved BA lasers with a cavity length of 292 μm can be operated up to 120 °C. High characteristic temperature values were obtained with T0 = 125 K (20 °C to 45 °C) and T0 = 100 K up to 120 °C. The slope efficiency of about 0.28 W/A can be kept constant over a wide operating temperature range of up to 100 °C. Mounted RWG lasers with 388 μm cavity length and operated in pulsed mode showed a maximum output power of 120 mW a slope efficiency of 0.42 W/A at 15 °C. The lasers can be operated at 150 °C with 25 mW output power. These results demonstrate very well the temperature insensitive lasing performance expected in nearly ideal QD lasers due to the high density of states localized at the transition energy, which allow a very robust ground state lasing.
Due to the discrete density of states distribution and spatial localization of carriers in quantum dot (QD) material, the
dynamics should be strongly enhanced in comparison to quantum well material. Based on improved 1.5 μm
InAs/InGaAlAs/InP QD gain material short cavity ridge waveguide lasers were fabricated. Devices with cavity, lengths
of 230 to 338 μm with high reflection coatings on the backside exhibit record value for any QD laser in small and large
signal modulation performance with up to 15 GHz and 36 GBit/s, respectively, obtained at 14 °C. Due to the high
temperature stability of threshold current and external differential efficiency, the lasers exhibit also nearly constant
modulation bandwidth between 14-60 °C.
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