Research On Tunable Frequency-multiplying Optoelectronic Oscillator

Optoelectronic oscillator(OEO)with the advantages of high spectral purity,high Q value and low phase noise,is an important research direction in microwave photonics.Since the bandwidth of the modulator and the adjustable range of the electrical filter in traditional OEO implementation are limited,the frequency and tunable range of the oscillating signal will be limited.The tunable microwave signal source with low phase noise and high frequency has been widely used in electronic radar.,wireless communication and other electronic technology fields.Therefore,it is significant to study the tunable frequency-multiplying OEO.In this paper,starting from the tunable OEO based on phase shift fiber Bragg grating(PS-FBG)and phase modulator(PM),and combining with the principle of dual parallel Mach-Zehnder modulator(DP-MZM)to achieve frequency-doubled and quadruple-frequency signals,a new tunable frequencymultiplying OEO structure is proposed.The main contents are shown as follows:(1)The schemes of the frequency-multiplying OEO in literatures are introduced,and their merits and drawbacks are compared.The principles of basic single-loop OEO and the related major devices are illustrated.The measurement of phase noise and the tunability of different OEO structures are discussed.(2)To analyze the PS-FBG in transfer matrix method,it can be found that,when the phase shift point is located at the center of the grating and the amount of phase shift is ?,the notch appears at the center of the reflection spectrum and reaches to the deepest.As the length of the grating and the depth of the refractive index modulation increase,the shape of the notch becomes narrower and smaller.According to the simulation results,a PS-FBG with a notch bandwidth of 0.00133 nm is produced.The produced PS-FBG has a good filtering filter performance.The tunable microwave photonic bandpass filter composed of PM and PS-FBG is studied,and the OEO fundamental frequency signal is simulated and analyzed.A microwave signal with a central frequency of 20 GHz and a side mode rejection ratio of 20 d B is generated.By tuning the laser wavelength,a microwave signal with a frequency range from 10 GHz to 50 GHz is obtained,which verified the good tunability of OEO.The process to achieve frequency-multiplying signal using DP-MZM is analyzed in simulation.The radio frequency spurious suppression ratio is 20 d B,which has a simple structure and great frequency-multiplying result.(3)A tunable frequency-multiplying OEO scheme realized by the combination of a DP-MZM and a PS-FBG is proposed.The experimental results show that when the fundamental frequency is 5.8 GHz,the side-mode rejection ratio of the signal is 45 d B and the free spectral range(FSR)is 312 k Hz.By adjusting the optical wavelength of the tunable laser,the frequency-doubled microwave signal with a tunable frequency range from 11 to 20 GHz and the frequency-quadrupled microwave signal with a tunable frequency range from 22.5 to 26 GHz are generated.The phase noise of the fundamental,frequency-doubled and frequency-quadrupled signal at 10 k Hz offset frequency are-105.9d Bc/Hz,-103.3d Bc/Hz and-86.2d Bc/Hz,respectively.Compared with the existing structure,the structure proposed in this paper can achieve the microwave signal only by adjusting the wavelength of the incident light,and there is no optical filter in the system while generating the doubled and quadruple frequency signal.There is no optical notch filter,electrical filter or SBS structure in system,which can significantly reduce system cost and complexity.