High-performance Optoelectronic Oscillator

High-spectral-purity, high-RF-stability and low-phase-noise optoelectronic os-cillator (OEO) as a key device has drawn great interests in its potential applications of microwave or millimeter-wave generation and signal processing in RF and optical systems. However, difficulties of mode competition, frequency stability and phase noise must be overcome. In this dissertation, we utilize an optical domain combined dual-loop structure to improve the signal side-mode-suppression ratio (SMSR) and spectral purity. Unpumped erbium-doped fiber (EDF) is inserted to improve long-term frequency stability. We also use polarization modulation to generate fundamental or frequency-doubled microwave signal. Finally, the self-regeneratively mode-locked fiber laser loop further decreases the phase noise of the system.We firstly theoretically and experimentally study the coupled frequency-doubling optoelectronic oscillator based on polarization modulation and polarization multi-plexing. The polarization modulation makes a breakthrough at the limitation of the modulation bandwidth of electro-optic modulator, not only generating both of funda-mental and frequency-doubling microwave signals, but also improving the stability of the system without the use of biased voltage. A polarization beam splitter (PBS) is served as a polarization-modulation-to-intensity-modulation converter, and simulta-neously as a power splitter, which divides the signal into two optical paths with or-thogonal polarization states to form an optical domain combined dual-loop structure, realizing the polarization multiplexing. The proposed approach only a single photo- diode (PD) is used, which reduces the noise of the generated signal and the cost of the scheme. In addition, the polarization multiplexing would not introduce the3-dB com-bining loss, which lowers the gain requirement of an electrical amplifier in the OEO loop. A fundamental microwave signal at9.95GHz or a frequency-doubled micro-wave signal at19.9GHz are generated with a wavelength-independent SMSR as high as78dB obtained. The SMSR is almost independent on laser wavelength. The phase noise of the generated19.9-GHz signal was-103.45dBc/Hz at10-kHz frequency off-set. Experiment also measured the repeated phase noise for about2minutes. The phase noise at10-kHz frequency offset fluctuates within±0.4dBc/Hz, showing a good short-term frequency stability.A coupled optoelectronic oscillator (COEO) by an unpumped EDF is proposed and experimentally demonstrated. Because the reflections in the connectors and de-vices in the cavity of the fiber laser would form standing waves in the EDF, spatial hole burning (SHB) effect would be generated, resulting in smaller absorption for the higher-power mode. Therefore, the undesirable competitive modes are eliminated and the SMSR is improved. The self-regeneratively mode-locked structure sharing inten-sity modulator with OEO and forming a fiber laser, further lowers the phase noise of the oscillation signal. A10.664-GHz microwave signal is generated with a phase noise of-120.58dBc/Hz at10-kHz requency offset and maintains mode-locking for more than20minutes. The incorporation of the unpumped EDF increases significantly the stability and spectral purity of the COEO. In addition, the experiment also proves that the longer the EDF is, the better the suppression of the undesirable modes is. But the length of the unpumped EDF could not be too long since the large absorption in the unpumped EDF would reduce the loop gain in the COEO until the COEO can not stably oscillate.