Ph.D Thesis
Ph.D Student Fleyer Michael Injection Locking and Noise in Delay-Line Oscillators Department of Electrical Engineering Professor Moshe Horowitz

Abstract

The synchronization of oscillators, which was discovered by Huygens as early as in the 17th century, has been studied in a variety of systems and has found applications in different fields of science and engineering. Delay-line oscillators such as fiber lasers or opto-electronic oscillators (OEOs), have a cavity length that is significantly longer the oscillation wavelength.  Injection locking of such oscillators may lead to interesting effects such as oscillation death, irregular, or even chaotic oscillation.

In this thesis, we study injection locking and noise in delay-line oscillators and its applications to obtain a tunable ultra-low noise RF signals and a novel acoustic sensor.

The cavity mode spacing in delay-line oscillators is often much smaller than the bandwidth of its amplifier. Therefore, the oscillator can potentially oscillate in many cavity modes. However, in oscillators with an homogeneously broadened amplifier, oscillation is obtained only in cavity modes with the highest small-signal gain. These modes cause the saturation of the whole gain spectrum, thus preventing oscillation from other cavity modes. We show in our work that a stable oscillation can be obtained in homogeneously broadened oscillators even if the small-signal gain of the oscillating mode is significantly smaller than of other modes. The oscillation frequency is chosen by a short-time quasi-injection locking of the oscillator to an external source. After the injected signal is turned off, the oscillator continues generating the selected mode for an unlimited duration due to wave mixing in the amplifier. We study the effect theoretically and demonstrate it experimentally in an OEO. The effect can be used for "frequency memory" of ultra-low phase noise signals and to accurately determine the oscillation frequency.

We also study the noise in delay-line oscillators that are injection-locked to a periodic external signal. We apply the results to demonstrate a novel tunable ultra-low phase noise OEO and a novel acoustic fiber sensor. The sensor is based on the high sensitivity of the injection-locked oscillator phase to changes in its resonance frequency. The injection locking of the oscillator enables to enhance the output signal, to control the sensor bandwidth, and to accurately detect signals at very low frequencies.

In long-cavity oscillators that are based on fibers, noise is added due to the Rayleigh scattering. We have developed a comprehensive model that enables, for the first time, to obtain an excellent quantitative agreement between theory and experiments for the noise spectra induced by this effect.