We demonstrate a lattice relaxation control by in situ curvature measurement for a metamorphic buffer. Using this relaxation control, we investigated a thin (240 nm) In 0 .15 Ga 0 .85 As metamorphic buffer for fabricating an unstrained In 0 .10 Ga 0 .90 As quasi-substrate on a GaAs substrate and succeeded in fabricating a 1.3-μm metamorphic InGaAs multiplequantum well laser diode (LD) on the metamorphic buffer. We confirmed that the LD was directly modulated at 25 Gb/s with a high-characteristic temperature (T 0 = 187 K).Index Terms-Diode lasers, metamorphic, quantum well lasers, semiconductor growth.