The Study On Frequency Tunability Of Optoelectronic Oscillator Based On Optical Waveguide Ring Resonator

High quality microwave signal source is the basis of microwave system application.The quality of microwave signal and phase noise directly determine the working performance of microwave system.The traditional electric microwave signal source has the disadvantages of large volume,high cost,poor signal amplitude-frequency characteristic and high phase noise.As a new type of microwave source,optoelectronic oscillator can produce microwave signals with frequency from hundreds of k Hz to hundreds of GHz,low phase noise and high quality factor.It can also output optical and electrical signals at the same time.It has high application value in the fields of satellite navigation and positioning,radar and electronic countermeasure,microwave measurement and communication.Although the optoelectronic oscillator can produce high-frequency and high-quality microwave signals,the tunability of microwave frequency is also required in these application fields.For example,the tunability of optoelectronic oscillator frequency plays a key role in measuring the Doppler frequency shift of some signals.Therefore,people pay more and more attention to the research on the tunability of the output frequency of the optoelectronic oscillator.However,most researchers pay attention to the broadband tunability of the output frequency of the optoelectronic oscillator and ignore the research on the fine tuning of the frequency.In the application of radar system,the high-finesse tunable frequency can improve the detection performance of radar.At the same time,the high-frequency resolution of signal source is very helpful to improve the accuracy of satellite navigation and positioning and the measurement integrity of microwave measuring instruments.Therefore,this paper mainly studies the high-finesse tunability of the output frequency of the optoelectronic oscillator.The main research contents are as follows:(1)The basic structure of optoelectronic oscillator is briefly introduced;The threshold condition and phase condition of the optoelectronic oscillator are analyzed in detail,and the spectrum characteristics and phase noise characteristics of the oscillation signal are analyzed.It is concluded that the phase noise is independent of the oscillation frequency and only related to the frequency offset.(2)The filtering effect of optical waveguide ring resonator is applied to optoelectronic oscillator system instead of long optical fiber delay line to realize the single-mode output of optoelectronic oscillator.Firstly,the optoelectronic oscillator Simulink simulation model based on the waveguide resonator is established to verify the feasibility of the designed system.The optoelectronic oscillator based on the waveguide resonator introduces the waveguide resonator on the basis of the minimum loop optoelectronic oscillator.The filtering effect of the waveguide resonator in the optoelectronic oscillator system is verified by comparing the oscillation spectrum of the optoelectronic oscillator based on the minimum loop and based the waveguide resonator.The mode interval of the optoelectronic oscillator based on the minimum loop is40.32 MHz.The mode interval of the optoelectronic oscillator based on the optical waveguide ring resonator is 2.12 GHz,and its oscillation spectrum ranges from 2.12 GHz to 19.233 GHz(the span of the spectrum analyzer is set to 20 GHz).In addition,the free spectral range of the optical waveguide ring resonator is 2.169 GHz.It can be seen that the waveguide resonator plays a filtering effect in the optoelectronic oscillator system.Then the side mode suppression ratio and phase noise of the optoelectronic oscillator are analyzed based on the minimum loop and based on the waveguide resonator.The side mode suppression ratio of the oscillation spectrum of the optoelectronic oscillator based on the minimum loop is 39.472 d B,and the phase noise at the frequency offset of 10 k Hz is-87.9 d Bc/Hz;The side mode suppression ratio of the optoelectronic oscillator based on the optical waveguide ring resonator is 58.881 d B,and the phase noise at the frequency offset of 10 k Hz is-100.54 d Bc/Hz.Through comparison,it is found that the optoelectronic oscillator based on the optical waveguide ring resonator is easier to obtain a purer microwave signal.The phase noise at different oscillation frequencies is tested and analyzed.It is verified experimentally that the phase noise has nothing to do with the oscillation frequency,but only with the frequency offset.(3)When the laser frequency is locked at the resonant frequency of the waveguide resonator,the optoelectronic oscillator can realize stable oscillation.Therefore,the laser frequency is controlled by locking the laser frequency in different positions of the resonant spectrum of the waveguide resonator,so that the output signal of the optoelectronic oscillator can achieve high-finesse tunable output with a tuning step of 20 k Hz.Then,the phase noise of the oscillation signal at 2 GHz at different frequency locking positions is analyzed.It is obtained that the phase noise will not change with the change of frequency locking parameters,and the oscillation frequency will shift with the change of frequency locking parameters.The innovation of this paper:(1)The OEO loop is constructed by using the filter effect of optical waveguide ring resonator,which can realize single mode output of OEO and reduce the requirement of filter.In addition,the microwave filter is constructed by the optical waveguide ring resonator in our loop,and no additional electrical filter is introduced,which reduces the influence of device noise on OEO output signal.(2)When the laser frequency is locked at different positions of the resonant spectrum of the optical waveguide ring resonator,the amplitude of the demodulation curve will change,so as to feedback and control the change of the laser frequency,and finally realize the high-finesse tuning of the output of the optoelectronic oscillator based on the optical waveguide ring resonator.At the same time,no additional electro-optic and microwave devices are added,and the stability and phase noise performance of the optical oscillator will not be affected.(3)The use of optical waveguide ring resonator instead of long optical fiber in OEO not only increases the light delay time,but also makes OEO truly miniaturized and practical due to the small size of optical waveguide ring resonator.