|M.Sc Student||Avelin Tzvi|
|Subject||Selection of the Oscillation Mode in Multi-Mode|
Oscillators by Initializing the Cavity Signal
|Department||Department of Electrical and Computers Engineering||Supervisor||PROF. Moshe Horowitz|
|Full Thesis text|
Ultra-low phase noise Radio-frequency signals are essential in various applications such as Radars and Analog to Digital converters (ADC). According to the Leeson model, the phase noise of an oscillator is proportional to the quality factor (Q) of its cavity. The Q-factor may be increased by increasing the cavity length without significantly increasing the cavity loss. Since the loss in RF cables is very high, Yao and Maleki have suggested in 1996 to increase the cavity length by propagating the RF signal in a long fiber. In such device, called Optoelectronic Oscillator (OEO) the RF signal is converted into changes in light, which propagates in a very long fiber and is then converted back into an RF signal by using a photo-detector. In a long cavity oscillator, the mode spacing is very low such that the cavity filter cannot be used to choose a specific mode. Therefore, in an oscillator with a fast saturable amplifier, the oscillator can potentially oscillate in one of a large number of modes. When the oscillator is turned on the oscillator operates in one of few modes that are randomly chosen according to noise. Therefore, it is important to deterministically choose a specific mode over a broad frequency region. It is also important to choose the mode in a minimum time duration.
In this thesis 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 operating frequency region, which can be wider than the full width at half maximum (FWHM) bandwidth of the cavity filter. The method is demonstrated experimentally in an OEO and is studied numerically by using a comprehensive numerical simulation. A very good agreement between theory and experiments is obtained. We also study the minimum injection duration, which is required to choose a specific mode. We show that this duration is slightly lower than the cavity round trip time. When the injection duration becomes shorted, the oscillating mode may slightly change each time the OEO is turned on. However, the injection frequency and the injection duration deterministically affect the average selected mode.