Abstract
Photonic microwave generation techniques have been widely studied in recent years. It has characteristic of low propagation loss and no electromagnetic interference in fiber optics communication. The period-one (P1) nonlinear dynamics of semiconductor lasers is one of the way to generate photonic microwave. In this study, we compare the photonic microwave signals generated by semiconductor lasers subject to optical injection (OI) system and optical feedback (OF) system. In addition, we investigate the minimum linewidth and maximum linewidth reduction ratio under different operation conditions. Narrower linewidth and better stability increase the resolution of velocimeter and improve radio-over-fiber communication. In the OI system, the lowest P1 linewidth operation point is around the detuning-insensitivity point. We further analyzed the linewidth of the master laser (ML) and slave laser (SL) constituting the P1. In the OF system, the P1 microwave linewidth is found to decrease when the feedback strength and the external cavity length increase. Here we analyzed the linewidth of the master frequency (MF) and secondary frequency (SF) constituting the P1. Compared with the OI system, the P1 linewidth in the OF system can be as low as 209 kHz in stead of 4.685 MHz and the linewidth reduction ratio can be as high as 98% in stead of 82.3%. In sum, with the OF system, it has narrower linewidth of photonic microwave, higher linewidth reduction ratio, and broader operating region. Finally, to quantitatively compare the P1 microwave linewidth in the OI and the OF systems, we let the linewidths of the ML and the SL in the OI system to be the same while the linewidths of the MF and SF in the OF system to be the same. In the OF system, the ratio of the P1 microwave linewidth to the sum of the two optical signals constituting the P1 oscillation is 0.02, which is lower than the OI system which has a value of 0.1.