Linearized Microwave Photonic Links Based On Phase Modulation

With the rapid development of microwave communication, fiber technology and optoelectronic devices, a new discipline microwave photonic appeared. Photonic technology has many advantages, such as low loss, light weight, high bandwidth, immunity against electromagnetic interference, etc., so have been widely used in communications, radar system, electronic warfare. As the core of optical processing, microwave photonic links are being used to conveying RF signals.In this paper, we analyzed the key technologies of the microwave photonic links, derived and built a simulation model of the performance parameters, studied a linear microwave photonic link based on phase modulation and achieved all-optical broadband microwave signal down-conversion based on phase modulation.First of all, we analyzed two key technologies including photoelectric and electro-optic based on intensity modulator and phase modulator. In order to identify the mathematical expression and relationship among the key parameters such as gain, noise figure, nonlinear distortion and dynamic range, the modeling and simulation of the microwave photonic link has been established.Then, we analyzed two methods of phase modulation to intensity modulation based on dispersive medium and filter by using either the single-mode fiber or optical filter. A method to improve the linearity of the analog photonic link is proposed and experimentally demonstrated, which consists of a phase modulator and an optical tunable bandpass filter. The ratio of the second-order optical sideband and the optical carrier band could be adjusted by changing the bandwidth and center wavelength of the filter, which suppress the third-order intermodulation distortion by32dB than that of single sideband phase-modulated signal transmission. Subsequently the spurious-free dynamic range of the link is113.1dB-Hz2/3which is improved by14dB.Finally, we analyzed the technology of optical down-conversion based on two modulators. A method to improve the linearity of the down-converted analog photonic link is proposed and experimentally demonstrated, which consists of two phase modulators and a polarizer. Down-conversion of a10-GHz microwave signal to100-MHz intermediate frequency is successfully achieved. Using anisotropic electro-optic coefficient of lithium niobate phase modulator, by carefully optimizing the angles between the transverse-electric and transverse-magnetic modes, the third-order inter-modulation distortion (IMD3) is suppressed. Subsequently the spurious-free dynamic range of the link is93dB-Hz2/3which is improved by13dB than other method.