The separate-confinement heterostructure (SCH) of an electroabsorption modulator integrated with a distributed feedback laser diode (EAM-LD) was optimized to obtain a clear optical waveform (eye opening) without penalty of chirp characteristics. The electric field applied to the multiple-quantum-well (MQW) structure was controlled by employing the proper thickness of undoped SCH layers to attain gentle absorption characteristics of the EAM at the optical mark level. As a result, the eye opening of the modulated emission at 10 Gb/s was improved and the intersymbol interference was reduced. No severe drawback on chirp characteristics was found in an /spl alpha/-parameter measurement. EAM-LD's with the optimized SCH structure exhibited good transmission characteristics of power penalties under 1.5 dB at 10 Gb/s, and the reliability of 10 Gb/s transmission characteristics was also confirmed. We have also investigated how the optical waveform after transmission was affected by the dependence of the /spl alpha/-parameter on bias voltage. Measurement and numerical calculation of the optical waveform after transmission showed that the optical waveform was severely affected by the /spl alpha/-parameters at deep reverse voltages.