Acquiring Narrow Linewidth Microwave Signals Based On An Optically Injected Semiconductor Laser

Recently, a new method about Radio-over-Fibe (RoF) has been studied extensively, which combined fiber with microwave communication perfectly by using fiber cable links for transmitting radio signals. RoF has been become the research focus of communication technology because it has many advantages such as strong anti-interference, strong confidentiality, wide coverage, low cost, easy installation etc. The key point of RoF is the generation of optical microwave. Lately, a technique based on the injected semiconductor laser operated at the period-one (PI) oscillation has been paid more attentions. This method can not only generate microwave with SSB structure, but also the microwave has high frequency and can be adjusted continuously. But the microwave signals generated by this injected semiconductor laser are unstable enough and need extra methods for frequency locking.Firstly, we investigated the frequency locking of microwave signals generated by optically injected semiconductor laser with fiber Bragg grating (FBG) feedback by Lang-Kobayashi (L-K) mode theoretically. Results show that, microwave can be obtained and adjusted continuously at appropriate injecting strength. Linewidth is narrowed from MHz to kHz by introducing moderate FBG feedback and adjusting feedback strength elaborately. Furthermore, we analyzed the influence of feedback strength and feedback time on linewidth. In addition, the performances of microwave signals, generated by an optically injected semiconductor laser operated at the PI oscillation under 1/2 subharmonic microwave modulation, have been investigated experimentally. The experimental results show that, under suitable injection condition, the microwave signals output from an optically injected semiconductor operated at PI oscillation may have SSB optical spectra structure, and the linewidth of the microwave signals are relative wide (about order of MHz). After adopting 1/2 subharmonic locking technique, the linewidth of the obtained microwave signal can be reduced from tens of MHz to tens of kHz. Furthermore, we have analyzed the influences of the power and frequency of the subharmonic microwave on the phase noise of the generated microwave signals, and further mapped the subharmonic microwave locking region in the parameter space of the power and frequency of the subharmonic microwave.