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InP-based, vertical-cavity surface-emitting lasers (VCSELs) utilizing a buried tunnel junction (BTJ) emitting at 1.55 μm with improved active region and reduced parasitics are demonstrated. A superior modulation bandwidth > 10 GHz is achieved up to 85 °C. The VCSEL device is investigated in detail, analyzing all bandwidth-limiting elements. With their improved temperature range, these VCSELs are especially qualified for uncooled operation in passive optical networks. These lasers show open eyes and error-free transmission up to 25-Gb/s modulation speed at room temperature. Uncooled error-free operation over a wide temperature range under constant bias conditions for ultra-low-cost systems is demonstrated at a 12.5-Gb/s data rate up to 85 °C. Due to a well-tailored mode-gain detuning, the laser characteristics are practically invariant with temperature at fixed bias conditions. Further possible design optimizations targeting at data rates well above 25 Gb/s are discussed.