Linearized Microwave Photonic Link Based On Phase Modulation

With the rapid development of microwave communication technology,optical communication technology and photoelectric device,microwave photonics is gradually formed.Microwave photon link is the key technology.It has the advantages of wide bandwidth,low loss,light weight and insensitivity to electromagnetic interference.Microwave photonic link has obvious advantages in microwave generation and microwave signal processing using photonic technology,and it is widely used in satellite communications,cable TV and other fields.In microwave photonic link,a microwave signal is often modulated on an optical carrier by an external modulator.Owing to the intrinsic nonlinear in external modulator,harmonic distortions and inter-modulation distortions are generated,which affect the transmission characteristics of microwave photonic link.Because the third-order inter-modulation distortion is near the fundamental frequency and it is often difficult to be filtered through the existing filter,it is necessary to eliminate the third-order inter-modulation distortion and improve the performance of the link by proper designing the link structure.Firstly,some key parameters of the microwave photon link are introduced,and the concept of dynamic range is derived from the theoretical analysis.Then,the paper introduces the electrical-to-optical signal conversion technology in the microwave photonic link,which provides the theoretical basis for this paper.Secondly,several schemes for improving the dynamic range of microwave photonic link are analyzed.Each has its advantages and disadvantages.The link based on a dual-parallel Mach-Zehnder modulator has a problem of bias drift.The structure based on digital signal processing technology is complex because the analog-to-digital conversion is needed.The microwave photonic link based on phase modulation is widely investigated since it is simple and can overcome the limitation of bias drift.A microwave photonic link based on two phase modulators is proposed.The simulated results show that the scheme can suppress the third-order inter-modulation distortion.The signal power at the fundamental frequency is also partly suppressed.In addition,it cannot eliminate the second-order inter-modulation distortion and can be improved further.Finally,a simple and optimized microwave photonic link structure is proposed,the dual-sideband signal is converted to single-sideband signal using a wavelength division multiplexer,the simulation results show that the proposed scheme improves the dynamic range and enhances theperformance.The key concept is that the phase modulation signal is converted to the intensity modulation signal using a two-channel wavelength division multiplexer.The third-order inter-modulation distortion can be suppressed by properly suppressing the light carrier power.Since the two modulated signals are out of phase by ?,the second order nonlinear distortion is also be eliminated using the balanced detection after the combination of the two signals.Furthermore,the power of the baseband signal is doubled,thus improving the dynamic range.In addition,the chromatic dispersion is also avoided due to the single-sideband modulation.