Linewidth narrowing of the radio frequency beat-tones and the optical-modes is experimentally investigated in a $\sim 40\mathrm{GHz}$ quantum-dash mode-locked laser diode subject to optical injection of $10\mathrm{GHz}$ pulses. In comparison to the $75\mathrm{kHz}$ linewidth exhibited by the beat-tones in passive mode-locking conditions, a remarkable reduction to less than $8\mathrm{Hz}$ is achieved when the laser is under optical injection. From this beat-tone signal, an integrated root-mean-square timing jitter of $64\mathrm{fs}$ is calculated. In addition, a quadratic profile of the optical linewidth with the wavelength is observed in active locking, reaching a minimum of $1.7\mathrm{MHz}$ for the longitudinal modes around $1530\mathrm{nm}$ and progressively increasing to $37.4\mathrm{MHz}$ for modes at $1525\mathrm{nm}$ .