Spectral purity analysis on tunable, V- and W-band microwaves generated through two mutually injected semiconductor lasers at period-one nonlinear dynamics

This study analyzes the spectral purity of microwaves generated using two mutually injected semiconductor lasers with highly different injection powers. While the strong optical injection from the first laser excites the period-one dynamics in the second laser for tunable microwave generation over the V and W bands, the weak optical injection from the second laser regenerates the period-one dynamics in the first laser to form an external cavity for the establishment of high spectral purity. The extent of spectral purity is determined by the two lasers and the weak optical injection, and is independent of the generated microwave frequency. The microwave linewidth generally reduces with the injection strength and delay time of the weak injection, where a linewidth narrower or even much narrower than 61 Hz is highly likely achieved. The phase noise generally decreases with the injection strength and delay time of the weak injection, where the lowest phase noise is achieved at the offset frequency of 10 and 100 kHz, with values of -78 and -98 dBc/Hz, respectively. The phase noise variance initially reduces and later increases with the injection strength and delay time of the weak injection when the contribution of phase noise from beating between the external cavity modes becomes significant. The side-peak suppression ratio generally reduces with the injection strength and delay time of the weak injection, where a ratio of more than 30 dB is achieved over a large range. The quantitative agreement between the numerical prediction and the experimental observation suggests that it is practically feasible to further improve the spectral purity experimentally achieved in our prior study to a level where, for example, a microwave linewidth down to the order of 100 Hz or smaller is achieved.