The Study Of High Performance Single Passband Microwave Hotonic Filter Based On Stimulated Brillouin Scattering

Filters are applied to select the wanted signals and suppress the noises and theinterference signals. Microwave photonic filters that carry on equivalent tasks as theirelectrical contourpart are capable of bypassing the limitation of electronic bottleneck andbringing complementary advantages such as high operational frequency range, largeinstantaneous bandwidth, low frequency-dependent losses, immunity to electromagneticinterference and other capabilities that are difficult or even impossible for traditionaltechnologies in terms of tunability and reconfigurability. As an important functional device,prominent filter contributes to keep the communication systems to work well. Therefore itis of crucial importance to design and build microwave photonic filters with highperformance, and they play vital roles in commercial, military and defense applications.Thus in this thesis, a high performance single passband microwave photonic filter based onthe combination of phase modulation and stimulated Brillouin scattering (SBS) is designedwith high stability, large tuning range, high selectivity and reconfigurability attributing tothe properties of SBS effect including high energy transfer efficiency, narrow effectiveband and high flexibility.To identify the limitation in the SBS based filter system, a basic filtering system is builtusing two individually tunable lasers. By measuring the filter response, a wide tuning rangefrom0.5GHz to18.4GHz is obtained. During measurement, this system shows poorstability in center frequency, and its usable tuning range is restricted to twice of theBrillouin frequency shift.Taking other filter’s performace into consideration, modulation techniques areemployed to produce stable pumps relative to carrier. On this basis, Brillouin gain and lossspectra compensation is firstly proposed to cancel the unwanted passband by properlyarranging the frequency space between two adjacent pumps. Numeric simulation showsthat the maximum available tuning range is proportional to the number of pumps. In theproof of concept trial, a filter system using two tunable light sources is built and the tunablerange is from0.5GHz to30GHz. In theory, this range could be further expanded intomillimeter wave region in the presence of more pumps. In order to enhance filter’s selectivity, subtracting the gain spectrum with two Brillouinloss spectra is proposed by locating the two losses in the wings of gain spectrum. Thesubtraction enables linewidth reduction of the gain spectrum as well as filter bandwidthreduction via SBS processing. Further more, the effective gain coefficients are increased tomitigate the degradation of system signal to noise ratio caused by spectra subtractiong. Inthe numeric analysis, filter bandwidth of this proposal decreases from10.98MHz to4MHzvia optimizing system parameter which implies a doubled selectivey improvement.Finally, filter bandwidth broadening using BPSK modulation is proposed. In thisscheme, the BPSK modulation performs power spectrum density enlargement ofmonochromatic pump wave such that the linewidth of Brillouin gain spectrum is broadened,whcih results in the enlargement of filter bandwidth through spectrum mapping. The use ofthis approach is flexible in the dynamic control of filter bandwidth due to that the pulse rateof BPSK modulation can be timely altered. Consequently, the filter bandwidth can bearbitrarily reconfigurated over several tens of megahertz up to several gigahertz, which issignificantly preferable in modern communication system.