Selection of the Oscillation Mode in Multi-Mode Oscillators by Initializing the Cavity Signal

We demonstrate a new method to select the oscillation mode in multi-mode oscillators with an instantaneous gain saturation. The method is based on initializing the cavity signal by feeding into the cavity an external signal for a duration that can be as short as the cavity delay. After the external signal is turned off, the steady state oscillation mode is exactly determined by the frequency of the external signal. By changing the external signal frequency, any specific mode can be selected over a broad continuous operating frequency region, which can be wider than the full width at half maximum (FWHM) bandwidth of the cavity filter. The method is not based on injection-locking effect and therefore the frequency of the external signal can be arbitrary chosen in a broad operating bandwidth, and it is not limited by the narrow locking range of the oscillator. When the frequency difference between the external signal and the central frequency of the filter is small, in comparison with the filter FWHM bandwidth, the selected mode is the mode with the nearest frequency to that of the external signal. However, as this frequency difference is increased, the selected mode is slightly shifted toward the filter central frequency. These results were experimentally demonstrated in an optoelectronic oscillator (OEO) and were compared to a comprehensive numerical simulation. A good quantitative agreement between theory and experiments were obtained. A fundamental limit on the minimum duration, which is required to initialize the cavity signal is given.