The Study Of Reconfigurable And Tunable Microwave Photonic Filter

Microwave photonics is a multidisciplinary field involving optics,microwaves and electrical engineering.As an important application of microwave photonics,microwave photonic filters have attracted extensive attention and research in academia and industry.Compared with the traditional microwave filter,it has the advantages of low transmission loss,small size,light weight,large available bandwidth,wide application to the various environments,immune to electromagnetic interference and so on.They also have the advantages of tunable and reconfigurable.With the increase of the communication transmission rate and the high precision requirement of information processing,when implementing the RF signal multiplexing,the channel bandwidth needs to be reasonably configured according to the channel requirements.So the resources can be located reasonable and avoid unnecessary bandwidth redundancy and energy loss.Therefore,it is meaningful to research and design of microwave photon filters with flexible bandwidth and continuous tuning of the center frequency.In order to overcome the periodicity of the frequency response of traditional microwave photonic filters,researchers have applied the stimulated Brillouin scattering effect to process optical signals in a variety of filter structures in the past decade.The filter principle based on the stimulated Brillouin scattering effect selectively amplifies or attenuates the signal,but does not amplify or attenuate the out-of-band noise.Therefore,the stimulated Brillouin scattering spectrum can be regarded as a high filter in optical domain.The stimulated Brillouin scattering spectrum has a narrow linewidth,which is about tens of megahertz in a common single-mode fiber.Stimulated Brillouin scattering is a non-linear effect with a low threshold.The requirement for the length of the fiber in the microwave photonics subsystem is not very strict,and the integration of the microwave photonic filter can be realized by using the waveguide as a transmission medium.The system volume and energy loss promote the commercial development of microwave photonic filters.The design idea of this paper is based on stimulated Brillouin scattering effect,combined with optical frequency comb technology and digital modulation technology,to achieve a flexible reconstruction of the filter passband bandwidth.Due to the high controllability of the frequency interval and the baseband pattern rate of the optical frequency comb in the electrical domain,the filter bandwidth reconstruction designed in this paper has high controllability and precision.Similarly,the filter center frequency designed in this paper can be continuously tuned by using the frequency shift characteristics of the stimulated Brillouin scattering effect.Specific design ideas,research work and innovations are as follows:1.The basis of the filter design in this paper is the combination of stimulated Brillouin scattering and phase modulation.Two independent tunable laser sources are used as the pump light and the probe light respectively,and the Brimouun scattering spectrum is used to amplify the sideband of the phase modulation signal.After demodulation in the photodetector,the corresponding frequency is achieved.The tuning range is 2 times of Brillouin frequency shift.2.A two passbands microwave photonic filter suitable for WLAN is designed and implemented.According to the requirements of IEEE802.11b/g/n for the WLAN bandwidth,the bandwidth of the stimulated Brillouin scattering filter based on a single light source cannot meet the requirement of the bandwidth because of the narrow Brillouin spectrum line width.In this paper,the optical frequency comb is produced by the cascade of the MZM modulator and the DPMZM modulator.When the frequency interval of the optical frequency comb is smaller than the intrinsic line width of the Brillouin gain spectrum,the multiple gain spectrums are overlap.By controlling the flatness of the frequency comb,the flatness of the effective gain spectrum can be controlled.By combining mathematical model calculation and simulation,a set of values of each input parameter under better flatness can be obtained.It provides theoretical support for the selection of experimental data.3.An optical frequency comb based on multi-tone signal modulation is proposed,and a bandwidth-reconfigurable microwave photonic filter with arbitrary passband is designed and implemented.The key to generating a flat optical frequency comb based on multi-tone signal modulation is to control the relative amplitude between the multi-tone signals in the electrical domain.Due to the non-linear response of the signal generator,adapter and RF cable when transmitting multi-tone signals,RF signals of different frequencies will have different losses during transmission.In this experiment,the pre-distortion compensation was performed before the multi-tone signal was connected to the filter system,and a relatively flat optical frequency comb was obtained.The frequency interval between multi-tone signals is arbitrarily set,which breaks the fixed frequency limit of the optical frequency comb generated by the single RF signal modulation method.Through the flexible controlling of the frequency of the pump light,the multiple passbands with selectable number and tunable center frequency,two passbands with reconfigurable bandwidth and broadened single passband filter are realized.4.By using BPSK modulation technique to broaden the bandwith of the filter is analyzed.Through theoretical research and experimental tests,it is found that when the input baseband code is fixed,the passband bandwidth of the filter is broadened as the code rate increases,but the passband selectivity gradually decreases;When the input pattern rate is constant,the passband bandwidth of the filter gradually smaller as the duty ratio of the pattern increases,and the passband selectivity is increased.A method of combining digital modulation and analog modulation techniques is proposed to broaden the bandwidth of the filter.By using BPSK modulation technology,the bandwidth of the filter is reconstructed by changing the pattern rate.The characteristics of the discrete spectrum of the comb-like pump light are used to compensate the attenuation when the bandwidth is enlarged.When the same bandwidth is achieved,the selectivity based on mixed cascade modulation is superior to individual digital modulation techniques.5.An ultra-wideband microwave photonic filter with triple notch structure based on photonic processing is proposed and implemented.The tunable optical filter is used to filter out part of the sidebands of the phase modulation signal to realize the conversion from phase modulation to intensity modulation.Through adjusting the bandwidth and center frequency of the optical filter,a single passband response that satisfies the bandwidth requirement of the ultra-wideband filter is generated.Multiple Brillouin attenuation spectra are generated based on multiple pump lights.Another laser light source is coupled with the multi-tone signal modulation.The phase modulation is balanced by separately tuning the output power of the coupling laser.The relative amplitude between the upper sideband of the signal and remaining lower sideband due to the non-steep edge of the transmission curve of the tunable optical filter is controlled.Then an ultra-wideband microwave photonic filter with three notches having high suppression ratio is achieved.The bandwidth,center frequency,and notch position can be individually tuned in this structure.The filter overcomes the disadvantages of poor tuning of the electric UWB filter and its vulnerability to electromagnetic interference.Therefore,it is also applicable to filters with other frequency and bandwidth requirements.