Research In Key Technologies And Applications Of Optoelectronic Oscillators

High quality microwave reference is one of the most important elements in modern electronic systems, especially for measurement,communication, sensor, and radar systems. With the rapid development of novel wireless communication systems and high frequency electronic systems, microwave oscillators with higher frequency, lower phase noise,and better stability are expected.For microwave generation, resonate cavities have crucial roles in energy storage and mode selection, which could directly decide the phase noise performance of a resonator. However, traditional resonate cavities including quartz crystal resonator and dielectric resonator face great limitations in output frequency and phase noise performance. Considering the many advantages of optical resonate cavities such as high quality factor, high operation frequency, and immunity to electromagnetic interference, optoelectronic oscillators can generate microwave signals with wide frequency range and high spectrum purity, indicating promising applications in electronic systems.In this thesis, several key technologies and important applications of optoelectronic oscillators are investigated as following:Firstly, the noise sources and theoretical model of optoelectronic oscillator are studied to evaluate the energy storage ability and phase noise performance. By improving the quality factor of resonate cavity while suppressing noises including RIN, Rayleigh Scattering, shot noise,and noises from electrical amplifiers, the phase noise performance of optoelectronic oscillators could be efficiently improved.Secondly, to improve the long term frequency stability of optoelectronic oscillators under wide operating temperature range, we proposed a novel broadband and wide range feedback tuning scheme for PLL stabilization of tunable optoelectronic oscillators. The proposed scheme was achieved in a simple and feasible way based on a dual-parallel Mach-Zehnder modulator. Owing to the full RF phase tuning range, wide temperature range operation of the PLL-locked OEO was achieved without any thermal control. Due to the wide operation bandwidth of the proposed tuning scheme, the PLL-locked OEO could stay locked while changing the oscillating frequency in a wide band. The long-term frequency stability of PLL-locked OEO oscillating at 8 GHz and 10 GHz was measured respectively, The calculated overlapping Allan Deviation was four orders of magnitude better than the free-running OEO and reached lower than 10-10 after 1000-s averaging time. The phase noise performance was also improved at low offset frequencies and remained the same at high offset frequencies.Thirdly, a self-oscillating optical frequency comb generator based on an optoelectronic oscillator employing cascaded modulators was proposed and experimentally demonstrated. By incorporating the optoelectronic oscillator loop into the optical frequency comb generator,11 comb lines would be generated with the frequency spacing equals to the oscillation frequency of the OEO. 10 and 12 GHz optical frequency combs are demonstrated with the spectral power variation below 0.82 and 0.93 dB. The corresponding spectral pure microwave source are also generated and evaluated. Thanks to the frequency independent characteristics, the single-sideband phase noise of the microwave sources are as low as -122 dBc/Hz and -115 dBc/Hz at 10 kHz frequency offset.Fourthly, we proposed and experimentally demonstrated a novel self-oscillating triangular pulse generator based on a 90o photonic-assisted phase shifter. The fundamental harmonic is produced by the optoelectronic oscillation loop for generating triangular pulses.Meanwhile, the even-order harmonic suppression and the phase matching between odd-order harmonics are simultaneously realized by carrier phase-shifted double sideband modulation using a dual-parallel Mach-Zehnder modulator. Furthermore, the 9 times amplitude ratio between the fundamental and third-order harmonics can be achieved by properly tuning the gain of the open oscillation loop. In the proof-of-concept experiment, self-oscillating triangular pulses with repetition rates of 4 and 5.7 GHz are successfully generated with the phase noises of about -114 dBc/Hz and -122 dBc/Hz at 10 kHz offset frequency. The corresponding root-mean-square errors between the generated and the ideal triangular waveforms are 5.9777e-4 and 6.0002e-4 respectively.