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Future optical transmission systems and signal processing circuits will require optical pulse sources capable of producing subpicosecond (sub-ps) pulses with low timing jitter at repetition rates of tens of gigahertz. In this paper, we present the theory, design, and measurements of a novel InP-based hybrid mode-locked laser diode (MLLD) structure with an ultrafast monolithically integrated, reverse-biased, uni-traveling-carrier (UTC) absorber. The necessity of an ultrafast absorber to obtain sub-ps pulses is analyzed and explained with our advanced time-domain rate-equation model. The realized MLLD demonstrated clean sub-ps pulses of 900 fs at 42-GHz repetition rate and the potential in an optimized device to reach values around 600 fs.