In this paper, we describe generation and application of wide narrow linewidth optical frequency combs using dual-mode injection-locking of InP quantum-dash mode-locked lasers. First, the dependence of the RF locking-range on the device's absorber voltage is experimentally investigated. Under optimized absorber voltage, a continuous wide RF locking-range of is achievable for lasers with 21 GHz repetition rate. The total RF locking-range of is possible considering locking-range for positive and negative absorber voltages. This wide tuning of the repetition rate, a record for a monolithic mode-locked laser, is reported from a two-section device without any additional passive section or extended-cavity for repetition rate tuning. It is shown that the effective RF locking-range in dual-mode injection corresponds to the optical locking-range and repetition rate tuning under CW injection, which is wider when the free-running mode-locking operation is “less stable.” The widest comb consists of 35 narrow lines within 10 dB of the peak, spanning and generating 3.7 ps pulses. Second, we show the first demonstration of multi pump phase-synchronization of two 10 Gb/s DPSK channels in a phase-sensitive amplifier using dual-mode injection-locking technique. The phase-sensitive amplifier based on the “black box” scheme shows more than 7 dB phase-sensitive gain and error free performance for both input channels with 1 dB penalty.