Frequency Stabilization Techniques Of Optoelectronic Oscillators

Microwave oscillators are the source of electrical systems such as communication,measure-ment,spaceflight and radar systems.Benefited from the development of the microwave photonics,the optoelectronic oscillator(OEO)is proposed in 1994,in which the oscillation is formed by a cycle of a radio over fiber(RoF)link and a microwave feedback.The OEO is capable of high spec-tral purity microwave signal generation even at frequencies up to 100 GHz,which is an attractive feature.Whereas,most free-running OEOs suffer from inferior frequency stability due to the high environmental sensitivity of their optical fiber and the microwave bandpass filter in the loop.In this thesis,the frequency stabilization techniques of the OEO are studied.Firstly,the research status and commonly used techniques are introduced.Then the theoretical model of the OEO is established,which is presented by its oscillation threshold,oscillation power and frequency spectrum.A supplementary study of the influence of flicker noise and the non-linearity of the microwave amplifiers on the OEO system is carried out,and several modifications of the phase noise and the oscillation power to the classical Yao-Maleki model are proposed accordingly.On the basis of aforementioned analysis,frequency stability optimization of an X-band op-toelectronic oscillator(OEO)using the technique of phase-locked loop(PLL)and dual loop self-injection-locking(DSIL)is proposed and demonstrated.The employment of DSIL suppresses the phase noise and reduces the frequency drifts of the OEO while the side modes originated from the self-injection-locking(SIL)loops are mutually suppressed.Then the DSIL OEO is phase locked to an oven-controlled crystal oscillator(OCXO)through a self-designed phase-locked loop(PLL)circuit.The relationship between the loop transfer characteristics of a PLL and the phase noise of the oscillation signal is analyzed.The close-in phase noise of the OEO are optimized by properly choosing the bandwidth of the loop filter of the PLL.The phase noise of the OEO is suppressed by 42 dB at 100 Hz offset and 21 dB at 10 kHz offset with PLL and DSIL,achieving-55 dBc/Hz@10 Hz and-125 dBc/Hz@10 kHz.The frequency overlapping ADEV of the obtained output achieved 7.03 × 10-12 at average time of 100 s,which is over 3 orders of magnitude better than that of the DSIL OEO and the free-running OEO.Then a coupled OEO(COEO)combined with injection-locking(IL)and phase locked loop(PLL)with improved frequency stability is demonstrated.The IL operation suppresses the side modes and conducts frequency pulling of the COEO frequency to the injection frequency,while the PLL stabilizes the OEO loop and thereof helps the injection-locked state to remain.Experiment results reveal that the IL-PLL COEO possesses reduced close-in phase noise of-72 dBc/Hz@10 Hz,-103 dBc/Hz@1 kHz,-121 dBc/Hz@10 kHz and a better long term stability with a frequency overlapping ADEV of 6.41 × 10-12 at average time of 100 s.