Study of Dual Injection-Locked Optoelectronic Oscillators

Low phase noise oscillators are critical components in several radio frequency (RF) applications. Low phase noise RF oscillators are used as frequency standards or clocks in: Doppler radar, free space and fiber-based telecommunications systems, global positioning systems and secure communications protocols. In all such applications, the oscillators must provide electromagnetic signals at a specified frequency. Such oscillators must have low phase noise as random deviations from the nominal phase rotation reduce their effectiveness as frequency standards or clocks. Furthermore, because these oscillators are used as frequency standards, they must not generate electromagnetic signals at any frequencies other than the nominal oscillating frequency. Signals at unwanted frequencies are referred to as spurious modes. Optoelectronic oscillators (OEOs) have proven to be promising sources of low phase noise radio frequency signals. The OEO is a ring oscillator with both optical and electrical paths. The OEO uses an optical fiber spool as an in-loop cavity to generate a time delay in an RF signal superimposed on a continuous wave optical beam. The low loss per unit length of optical fiber means that it can be used to generate very long delays. These long delays result in spectrally-narrow oscillating modes with low phase noise. However, long delays also lead to oscillator modes that are too narrowly spaced to be filtered by conventional RF filters. The dual injection-locked optoelectronic oscillator (DIL-OEO) has been proposed as a means to suppress spurious OEO modes. The DIL-OEO works by injection-locking a high-Q multi-mode master OEO to a low-Q single-mode slave OEO. In so doing, it is possible to generate an RF signal with both low phase noise and low spurious modes. In this work, we present a study of the injection-locking process in DIL-OEOs. We study the effects of injection-locking on both the phase noise and spurious modes of the DIL-OEO. We constructed a modular DIL-OEO which allowed us to independentally vary the injection levels and loop lengths in the DIL-OEO. Using our modular DIL-OEO, we conducted a systematic study of the effects of injection-locking parameters on the OEO's phase noise and spurious mode levels. The results of our systematic study allowed us to construct the first theoretical models describing the power spectrum of the DIL-OEO's steady-state signal. We also present theoretical models describing the dynamics of injection-locking in DIL-OEOs. Our dynamical models allow us to predict the locking bandwidth and stability of the DIL-OEO. In order to perform our experimental study, we required a measurement system capable of measuring the very low phase noise levels of our OEOs. We contructed a cross-correlation delay-line measurement system in order to measure the phase noise of our OEOs. In this work, we present a detailded description of our measurement system. We include a theoretical description of the delay-line measurement system. We also describe the experimental techniques used to calibrate, validate and optimize our measurement system. Because the DIL-OEO consists of two single-loop OEOs injection locked to each other, it was necessary to optimize each of the single-loop OEOs. In this work, we present experimental data showing the results of techniques we employed to minimize the phase noise of single-loop OEOs. Finally, guided by our experimental data and theoretical models, we constructed an optimized DIL-OEO with both low phase noise and low spurious mode levels. In this work, we present experimental data demonstrating a 60 dB reduction in the nearest neighbour spurious mode of a 4 km OEO. This spurious mode reduction was achieved without increasing the phase noise of the OEO at offset frequencies within 1 kHz of the DIL-OEO's 10 GHz central oscillating tone. The phase noise of the DIL-OEO at 10 kHz is only 8 dB higher than that of a 4 km single-loop OEO. In addition to presenting our optimized DIL-OEO, we present strategies for optimizing any DIL-OEO in order to achieve spurious mode suppression with a minimal increase in phase noise