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We report tunable harmonic mode-locking in continuous-wave (CW) optically injected InAs/InP quantum-dash lasers emitting at 1.56 μm. The frequency of generated microwave pulses is twice the laser's self-mode-locked frequency. Autocorrelation traces show Gaussian pulses with low time-bandwidth product ( ≈0.6). Mapping of the dynamics shows that this coherent microwave oscillation (MWO) region has maximum locking range ≈600 MHz (≈5 pm). Measurement of the optical linewidth of each individual mode at this regime shows strong narrowing relative to free running values by two orders of magnitude. The RF linewidth of the generated signal is ≈160 kHz confirming the coherence among the optical modes and low timing jitter.