Signal Processing Technologies Research On Broadband Multi-carrier RF Photonic Links With High Performance

As the civil communication demands on the development of data communication and multimedia services, and the requirements of high-speed,broadband, large capacity and the integration of comprehensive information processing needs in military applications, microwave photonic which combines the flexible, elaborate microwave and broadband, low loss photonic conquers the band-limited bottleneck of traditional microwave link, and will provide technical supports for the future demands. The technical challenges of linearized ultra-broadband, multi-carrier radio frequency photonic links are poor uniformity, narrow dynamic range and low link performance. From the linear transformantion mechanism between microwave and photonic, the relevant researchs were performed on the limited linearization bandwidth,multi-source nonlinear compensation, dynamic range improvement and link performance optimization.An adjustment-free linearization approach where high dynamic range could be kept during the carrier change was researched and designed, by extracting the distortion information from baseband signal and then modulating the distorted RF signal. The extracted "distortion information"was then used to modulate the distorted optically-carried RF signal. The new intermodulation will have the same amplitude and opposite phase, as the original one, resulting in the third-order inter-modulation distortions (IMD3)compensation. In experiment, the IMD3 nonlinear are suppressed 60 dB uniformly under typical input RF power, while the carrier is tuned from 4 GHz to 12 GHz. The spurious-free dynamic range (SFDR) is kept 125 dB within 1-Hz bandwidth without attendant optimization of link parameters.A novel scheme to effectively suppress the cross-modulation distortion,(XMD) and IMD3 in the wide-band, multi-carrier RF photonic link system was researched and designed. Rather than construct a critical nonlinear path in hardware, we directly extract the nonlinear sidebands from the received signal to linearize the distorted signal in the DSP unit, avoiding the less XMD suppression caused by the non-synchronization between compensate and distorted signals. The compensation factors depend on the bias angle of the MZM and OIP3, the scheme is simple and easy to implement and generality.Large suppression of XMD and IMD3 were demonstrated by 33 dB and 25 dB, respectively, resulting in an improved SFDR by more than 22 dB.A novel scheme to effectively eliminate the XMD by cascading two standard Mach-Zehnder modulators (MZMs) was proposed and demonstrated.When the bias angle of the cascaded modulator is specifically designed, the XMDs, both from the photonic link itself and from the nonlinear electrical amplifiers, are well suppressed. Experimentally, a suppression ratio of 33 dB is achieved by the cascading scheme. Through using the high performance pre- and post-amplifiers, the measured link gain and noise figure (NF) are 23.5 dB and 16 dB, respectively, which highly improved the performance of RF photonic link and effectively eliminated the distortion.Furthermore, a novel digital linearization scheme to suppress the frequency-folded multi-source nonlinear distortions in ultra-broadband,multi-carrier RF link with photonic bandpass sampling was researched and designed. Benefitting from the photonic bandpass sampling link is equivalent to a baseband digital nonlinear link, all the frequency-folded 3rd nonlinearities are suppressed 33 dB around by drawing from the distortion regenerating and subtracting algorithm and SFDR is improved from 109 to 117 dB@1 Hz.Furthermore, we explore the harmonic phase change in photonic bandpass sampling RF link with saturated PD. Different from the amplitude dependent nonlinearity, the AM-PM nonlinearity results in ?/2 phase difference.