“…7 that the frequency varies within a range of ±2.15 MHz, which is a relatively normal level for P1-based microwave generator. This long-term stability is expected to be significantly reduced from several MHz to KHz by further introducing external optical feedback [21,23,24,28,29,31], optical injection [33], filtered feedback [34], or phase-locked loop [47]. This is the aim of our next work.…”