With a modulator-based $10\mathrm{GHz}$ optical-frequency-comb generator at $1.55\mu \mathrm{m}$ , we report a $20\mathrm{GHz}$ repetitive train of optical pulses as short as $450\mathrm{fs}$ . The timing stability of the $20\mathrm{GHz}$ pulses, in addition to the phase for optical-comb modes, shows a strong dependence on the relative frequency detuning between the comb generator’s cavity and the seed cw laser. With a new and simple scheme, the comb generator’s cavity resonance was locked to a narrow-linewidth seed laser within an estimated optical-frequency range $⩽6\mathrm{MHz}$ , enabling high-fidelity $20\mathrm{GHz}$ subpicosecond pulses and stable optical-frequency-comb generation for indefinite periods.