Basic Research On Tunable Optoelectronic Oscillator And Its Application

Optoelectronic oscillator（OEO）is a ring resonator composed of an optoelectronic hybrid feedback link,which can simultaneously generate microwave signals and optically-carried microwave signals.The signals from the OEOs are featured with ultra-low phase noise and broadband tunability.Hence,OEO is recognized as a promising candidate to break through the electronic bottleneck,which has the potential to improve the performance of radar,wireless communication and aerospace systems.This dissertation focuses on tunable OEO and its application.The main research work includes OEO-based microwave signal generation,novel OEO architecture,and OEO-related application on temperature sensing,microwave frequency division,and multiformat microwave waveforms generation.The main contents of this dissertation are listed as follows.（1）An OEO scheme based on electrically-controlled stimulated Brillouin scattering is proposed for generating frequency-definable single-tone microwave signals.By employing a single laser source and an electro-optic frequency shifter,the frequency of the Brillouin pump light can be finely tuned.On this condition,frequency-definable microwave signals can be generated by the proposed OEO.Hence,the problems of poor tuning resolution and frequency instability in the traditional OEO based on stimulated Brillouin scattering are solved.In the experiment,precisely tunable single-tone microwave signals with a tuning resolution of 10 MHz are generated.In addition,a two-tone Brillouin pump light is generated by using electro-optic modulation technique.In such a case,two-tone microwave signals with freely-definable frequency in the range of2.4 GHz to 13 GHz are generated by using the OEO based on stimulated Brillouin scattering.（2）A Fourier domain mode locking OEO is proposed based on electrically-controlled stimulated Brillouin scattering.The prominent advantage of this scheme lies in that it solves the problem of the poor linearity in the conventional Fourier domain mode locking OEO.In the proposed scheme,two electro-optic frequency shifters are employed to generate a frequency-tunable Brillouin pump light and a fast frequency-scanning probe light via a single laser source.By using phase-modulation-to-intensity-modulation conversion based on stimulated Brillouin scattering,a fast-scanning microwave photonic bandpass filter is realized,which is employed in the OEO cavity to achieve mode selection.Through setting the OEO loop delay to be equal to an integer multiple of the scanning period,linearly chirped microwave waveforms with a high linearity can be generated,whose frequency range can be accurately defined.In the experiment,linearly chirped microwave waveforms with bandwidths from 1 GHz to 4 GHz,and center frequencies from 15.644 GHz to 17.144 GHz are generated.The maximum time-bandwidth product and the chirp rate are measured to be 82000 and 0.195 GHz/μs,respectively.Additionally,the linearity of the generated LCMW is smaller than 1.7%,and the phase noise is measured to be-113.98 d Bc/Hz@10 k Hz.The proposed Fourier domain mode locking OEO can be applied in the high-precision imaging radar.（3）An active mode locking OEO is proposed for generating microwave pulse trains with low close-to-carrier phase noise.This scheme can address the issue that it is difficult to achieve stable multi-mode oscillation in a conventional OEO due to the mode competition effect.In the proposed scheme,an electric amplitude modulator is used in the OEO cavity to achieve active mode locking.On this condition,coherent multi-mode oscillation can be realized in the OEO to generate microwave pulse trains.Through tuning the frequency of the signal applied to the electric amplitude modulator,the repetition rate of the generated microwave pulse can be varied.The feasibility of the proposed OEO scheme is numerically evaluated and experimentally demonstrated.In the experiment,microwave pulse trains centered at 4.01 GHz and with tunable repetition rates are generated.The experimental results indicate that the close-to-carrier phase noise of the active mode locking OEO is 40 d B lower than that of the conventional OEO.In addition,a dual-loop active mode locking OEO is proposed for suppressing the supermode noise.Benefited from the“Vernier”effect introduced by the dual-loop architecture,the supermode noise in the microwave pulse train under harmonic mode locking status is largely suppressed.In the experiment,the supermode noise suppression ratios are measured to be 60 d B and 55 d B under 2^nd-and 5^th-order mode locking status,respectively.Furthermore,an active mode locking OEO with tunable center frequency is realized by using a microwave photonic filter.In the experiment,the carrier frequencies of the generated microwave pulse signals can be tuned in the range of 2 GHz to 8 GHz.Active mode locking OEO is expected to be applied in the pulse Doppler radar.（4）Research on the OEO-related application has also been carried out.Firstly,the temperature sensor is proposed by employing an OEO based on stimulated Brillouin scattering.In this scheme,two types of optical fiber with different Brillouin frequency shifts are employed as the sensing fiber outside the OEO cavity and the gain fiber in the OEO cavity,respectively.On this condition,a single-tone oscillation signal at a temperature-dependent low frequency is generated from the OEO,which facilitates high-speed and high-sensitive temperature interrogation.In the experiment,temperature interrogation around 1.2 GHz is realized in the proposed OEO-based sensor,where the sensing sensitivity is measured to be 1.364 MHz/℃.Then,a broadband reconfigurable microwave frequency divider is proposed by using an OEO.Through switching the modulation mode of the electro-optic modulator in the OEO cavity,the frequency-division OEO can achieve two types of frequency division factors.In the experiment,2and 3 frequency division of the input microwave signals in the range of 3.18 GHz to 8.18GHz is realized.Finally,multiformat microwave signal generation is achieved by varying the bias voltage of the electro-optic modulator in the OEO cavity.Through varying the waveform of the low-frequency electrical signal applied to the bias port of the electro-optic modulator,the proposed OEO can work at different oscillation modes.In the experiment,sinusoidal,pulse and rectangular waveforms at high frequencies are directly generated,which can find application in multifunctional microwave systems.