Generation And Processing Of Microwave Photonic Signals Based On Spectral Shaping

As the explosive development of microwave technology in the application areas of radar system, wireless transmission and radio astronomy, recently the generation, processing and manipulation techniques of high-frequency microwave signals have been major concerns in academia and industry. Thanks to the inherent characteristics of light, such as high speed, large bandwidth and low loss, photonic techniques are usually used to solve the microwave problems. Under this condition, microwave photonics emerges as an emerging interdiscipline, which is aimed at breaking the bottleneck of traditional microwave technology on bandwidth and processing rate. Aim at the hotspots in microwave photonics, this thesis studies on two aspects including the generation and the all-fiber processing of microwave photonic signals. On one aspect, novel spectral shaping methods have been explored to improve the flexibility of arbitrary waveform generation; on the other aspect, we aim at optimizing the system performance in radio over fiber transmission by suppressing the system impairments using all-fiber techniques.In the area of microwave photonic generation, firstly this thesis proposes a filter structure with sinusoidal response based on polarization interference, and established the theoretical model. Based on this model, we present two methods to design an optical filter with triangular-shaped response function. Subsequently, the mechanism of frequency-to-time mapping has been analyzed, and an experimental setup is established based on the mapping principle to generate triangular-shaped signals. Then, we extend the cascaded spectral shaper to the area of arbitrary waveform generation and verify the effectiveness by theoretical and experimental results. Finally, two dimensional tunable microwave signal generation is demonstrated by structure modification of the spectral shaper; for the all-fiber processing of optical microwave signals, the downlink transmission system is experimentally established. A pre-distortion scheme based on spectral shaping is proposed to solve the dispersion-induced power fading effects, which obviously improves the system receiving sensitivity. The main contributions of this dissertation are listed as follows:(1) Generation of triangular-shaped microwave signals with parallel filter structure is proposed. A comb filter based on polarization interference (PI) is designed by combining a polarization maintaining fiber and a polarizer. The formula which can illustrate the concept is derived in theory and the variation trend of the filter response as the fiber length is analyzed. Subsequently, based on Fourier expansion theory, we analyze the harmonic components of a triangular waveform and study the effects of the high-order harmonic to the overall envelop. In absence of the high-order harmonic, two sinusoidal-shaped spectra corresponding to the fundamental harmonic and third-order harmonic are generated by parallel PI filters. A polarization beam combiner is employed subsequently to superpose the two spectral components without interference to obtain triangular-shaped spectrum. Finally, theoretical principle of the frequency to time mapping technique is discussed and used to achieve triangular waveform generation. Experimental results verify that the frequency and pulse width of the generated pulses can be tuned with the PMF and single mode fiber, which meets well with the theoretical model.(2) Arbitrary waveform generator based on cascaded PI filters is designed. Firstly, we analyze the influences of the input polarization to the response function of the PI filter, and discuss the extinction ratio range of the output sinusoidal spectrum. A cascaded system structure based on PI filters is established and the theoretical expression of its output spectrum is derived, which shows that the system can generate combination of multi-components of the Fourier expansion. Moreover, arbitrary waveform can be obtained by adjust the output amplitude of each spectral component. The simulation results show that the accuracy of the output spectrum is relative to the filter amounts. However, the algorithm flexibility increases with the filter amounts. Consequently, we utilize genetic algorithm to solve the flexible calculation of the polarization relation between each PI filter. By just using three cascaded filters, we achieve the sinusoidal, triangle and rectangle shaped spectrum generation. After that triangular-shaped waveform is obtained by poured the generated spectrum into a frequency-to-time mapping module composed of a dispersive fiber and a photodetector.(3) Two-dimensional tunable microwave signal generation is achieved by optimizing the filter structure based on polarization interference. A tunable DGD element replaces the PMF to control the free spectrum range of the output spectrum, and a bandwidth-tunable filter is cascaded subsequently to adjust the spectral bandwidth. The function relationship of the signal frequency and the DGD value is derived. Experimental results verify the effectiveness of this proposed scheme. Microwave signals with frequency range of4GHz-30GHz and pulse width of0.7nm-3nm is generated.(4) A photonic scheme to generate phase-coded microwave signals using all-fiber components is proposed. A filter module that is composed of two polarization controllers, a piece of polarization maintaining fiber and a polarizer is employed to generate sinusoidal optical spectrum. The phase modulator is cascaded ahead of the filter module to bring in small variation of the differential group delay in the delay interference process, which can result in frequency shifting of the optical spectrum. Through the frequency to time conversion in a dispersive medium, the frequency shift of the optical spectrum is mapped into the time domain. Consequently, the phase of generated RF signals can be continuously shifted by tuning the applied voltage onto the phase modulator. In out experiment, phase-coded RF signals with symbol periods of0.1-ns and carrier frequency of25-GHz are obtained and the continuous tunability of the generated signal phase is demonstrated.(5) An all-fiber predistortion method is presented to improve the performance of optical microwave signals in radio over fiber transmission. The predistortion process includes two approaches based on optical spectral shaping technique. First, we theoretically show that optimal CSR for DSB signals is3dB and this optimal CSR can be achieved via filtering process. Secondly, a pre-distortion operation is used by adding beforehand phase shift between the two sidebands of the transmitted signals to cancel the dispersion effects for RoF systems. The experimental results show that by employing this pre-distortion method the DSB signals with an optimal CSR of3dB achieve1.2dB sensitivity improvements over SSB signals with an optimal CSR of0dB.