Research On The Phase Noise Of Opto-Electronic Oscillators

Microwave and millimeter-wave oscillators are the core components of communication,radar and electronic warfare systems. Phase noise is the one of the key characteristics of oscillators; however, phase noise performances of the traditional electronics oscillators have been developed to the theoretical limit and are difficult to be further improved. Opto-electronic oscillator (OEO) is a new type of oscillator combining microwave and photonics technologies. OEO uses optoelectronic resonator instead of electronic resonator, thanks to the electro-optic modulation technology and the low loss characteristics of optical fiber,the optoelectronic resonator's quality factor is usually very high and doesn't decrease with the increasing of oscillation frequency. It is expected that OEOs have the ability to generate extremely low phase noise signals even in the microwave and millimeter-wave bands, and has broad application prospects. Up to now, significant progresses on the phase noise study of OEO have been made; even so, further studies on the phase noise of OEO are still necessary because the current theories are insufficient yet. This dissertation focuses on the phase noise of OEO's oscillation signal and the residual phase noises (RPNs) of the components in OEO, a series of research work has been done and listed as follows:(1) The research status on the phase noise of OEO is summarized. Typical structures of OEO are listed and phase noise characteristics of them are compared, in addition, the present theoretical phase noise models of OEO are also studied and inadequacies of these models are pointed out. In particular, the classical quasi-linear model of OEO is deduced in detail and the relationship between the phase noise and noise sources in OEO is clarified.(2) Theoretical and experimental investigations on the RPNs of the main devices in OEO are carried out. In the experiments, the microwave amplifiers' RPNs are measured by using the traditional RPN measurement method, besides, a dual-laser cross-correlation method is proposed to measure the RPNs of optoelectronic devices.The experimental results show the relationship between the RPNs and the operating points of the Mach-zehnder modulator (MZM) and the photodetector (PD). In addition,the phase noise induced by laser's relative intensity noise (RIN) in the PD is also measured by a newly proposed method, the experimental results clearly show the relationship between the RIN-induced phase noise and the incident optical power levels of the PD.(3) The relationship between the microwave phase noise and the group velocity dispersion (GVD) in the microwave optical link is studied; a theoretical model of the output microwave phase noise relative to the GVD parameter is derived. A two-tone cross-correlation method is proposed to measure the RPN of the microwave optical link, this method can overcome the shortcoming of traditional method which can not measure the RPN of large-delay devices, and the noise floor of traditional measurement system can be reduced by up to 20 dB by this method. The RPNs of the microwave optical links are measured by the method, the measurement results show that when the fiber length increases from 1 m to 6 km, the RPN at the offset frequency 10 kHz of the microwave optical link deteriorates about 10 dB due to GVD, which can verify the validity of the theoretical model established in this paper.(4) A theoretical phase noise model of the single-loop free-running OEO is established in phase domain. This model indicates that the phase noise of the OEO's oscillation signal is related to the RPN of the components inside the OEO loop, and a quantitative relationship is given. Compared with the classical quasi-linear phase noise model, the phase noise model in phase domain can describe the phase noise of the OEO more accurately at the close-in offset frequencies. In addition, a two-channel phase noise measurement system based on delayed homodyne method is developed,the developed system can achieve a noise floor of -130 dBc/Hz@lkHz and -170 dBc/Hz@10kHz. The single-loop OEO's phase noise is measured by the developed system and the effect of opto-electronic hybrid link's RPN on the phase noise of OEO is invesitgated, the results are consistent with the theoretical phase noise model.(5) An injection-phase locked OEO is proposed. Utilizing the injection locking to reduce the close-in phase noise and increase the side-mode suppression ratio(SMSR). Using the phase-locked loop (PLL) to improve the frequency stability of the OEO by detecting the phase difference between the output signal of the OEO and the external injection signal, also to further improve the close-in phase noise performance.OEO based on this structure is able to avhieve single mode oscillation, low phase noise and high frequency stability at the same time. A theoretical phase noise model in phase domain of the injection-phase locked OEO is established, the model indicates that the phase noise of the injection-phase locked OEO is related to the injection phase noise, injection power, PLL bandwidth and components' RPN in the OEO loop.(6) Prototype of the injection-phase locked OEO is developed and studied experimentally, which includes the temperature control and drive circuit of the laser,bias control circuit of the modulator, PLL control circuit, temperature control box of the fiber and so on. The phase noises of injection-phase locked OEO, injection locked OEO and free-running OEO are compared, results show the superiority of the injection-phase locked structure. Results also show that the far-off phase noise of the injection-phase locked OEO will be closer to that of the injection source when the injection power becomes larger. Under the situations of the electrical filter's center frequency and 3dB bandwidth are 9.6 GHz and 20 MHz, single mode oscillation at 9.6 GHz is realized by a 6 km fiber injection-phase locked OEO. When the injection locking bandwidth is 1.98 kHz, the phase noises of the OEO's output signal are about-125 dBc/Hz@1kHz and -147 dBc/Hz@10kHz, the SMSR is higher than 80 dBc, the Allan deviation is about 1.37×10-11@ls and the long term frequency stability approaches to 0.1 ppm.