Research On High Performace Microwave Signal Generation And Filtering Based On SBS Effect In Optical Fiber

In recent years,photons with the advantages of low transmission loss,wide bandwidth,and anti-electromagnetic interference have been used for the generation and processing of microwave signals,and microwave photonics technology has emerged.Microwave photonics can be applied to communications,radar,sensing,imaging,and satellite communications.Therefore,it has received extensive research attention.Microwave photonics is to modulate the microwave signal into the optical domain,and then convert it into electrical signal after optical sampling,frequency conversion,or other processing.Microwave photonics technology can generate microwave signals with high frequency,low phase noise,and sweeping signals with large time-bandwidth product.Moreover,longdistance transmission of signals can be realized which can solve problems of electric signals such as large loss,short transmission distance,limited frequency and bandwidth,and electronic interference.At the same time,it is possible to realize fast processing of highfrequency microwave signals with a relatively simple system and reduce the cost of highfrequency electrical signal processing.Among them,the generation of single-frequency and multi-frequency microwave signals is particularly important for the local oscillator signals for the transmission,up-conversion,and sensing.But the performances such as frequency adjustment flexibility,spectral purity,and phase noise of the microwave signals generated by the existing schemes still need to be improved.Microwave photonics filter(MPF)is the key component of high-frequency broadband signal processing.Its roll-off curve and suppression ratio directly determine the signal-to-noise ratio of the filtered signal.Therefore,optimizing the performance of microwave signal generator and MPF has important research value for perfecting microwave photonics subsystems and improving high-frequency signal generation and processing technology.Stimulated Brillouin Scattering(SBS)effect has the characteristics of low threshold and high gain.The Brillouin gain spectrum has a narrow bandwidth,and the center frequency can be flexibly tuned which is a potential candidate for the optoelectronic oscillator(OEO)as a mode selector.The frequency tuning resolution of the generated microwave signal can be greatly improved.The SBS gain spectrum can be broadened by means of sweep pumping,which can be used to design a MPF with programmable bandwidth and shape.It has broad application prospects in the field of microwave photonics.In this paper,using SBS,a series of theoretical analysis and system experiment verification are carried out for the performance improvement of microwave signals generation and MPF.The specific contents include:1.Brillouin optoelectronic oscillator design with high frequency tuning resolution and low phase noise.Microwave signals with high frequency,high spectral purity,and low phase noise can be widely used in communication and radar systems.The OEO has the advantage of high Q value and can generate microwave signals with ultra-low phase noise at different frequencies,which fully meet the requirements of the above applications.In this paper,the SBS gain is used as the mode selection filter in the OEO loop.By controlling the pump wavelength of SBS,flexible frequency adjustment of the generated microwave signal can be realized.The output of a highly stable laser is used as the carrier of the phase modulation(PM)for the pump frequency shifting and the OEO oscillating loop carrier at the same time.One of the high-order sidebands of PM is selected by a narrowband optical filter as the SBS pump.By changing the frequency of the electrical drive signal of the PM and the sideband order,electrical signals with a wide frequency range and high resolution can be realized.The microwave signals with a frequency tuning resolution of 10 MHz and a tuning range of up to 40 GHz are obtained.This is minimum frequency tuning resolution realized by SBSbased OEO systems.At different frequencies,the generated microwave signals maintain the characteristics of the single mode.At 100 k Hz offset frequency,the single sideband(SSB)phase noise is lower than-120 d Bc/Hz,and the main factor that introduce noise into the system is the active devices.This scheme verifies the feasibility of SBS-based OEO to generate microwave signals with ultra-low phase noise and provides guidance for analyzing and reducing system noise.2.High-stability Brillouin OEO scheme combined with optical injection locking and optical phase-locking loop.In order to solve the problem of electronic bottlenecks in the development of traditional microwave signal to high frequency,an SBS-based OEO scheme is adopted.The optical frequency comb technology,optical injection locking(OIL)technology and optical phase locking technology are innovatively combined to realize microwave signals with arbitrary high frequency and ultra-low phase noise.In this paper,a cascaded intensity and phase modulation is used to generate an optical frequency comb(OFC),and the output of the slave laser(SL)is injected into one of the high-order sidebands of the OFC.It can provide sufficient pump power while ensuring high sideband suppression ratio.In the experiment,by adjusting the frequency of the drive signal and selecting different sidebands of the OFC for injection locking,the microwave signal generation with the frequency tuning range of40 GHz and the tuning resolution of 8 MHz is verified.A dual-loop architecture is involved to guarantee the single-mode oscillation.The side-mode suppression ratio(SMSR)is higher than 60 d B,and ultra-low phase noise at different frequencies are maintained.Since the pump signal and the loop carrier are coherent,the extra frequency noise introduced by the laser difference is avoided to some extent.By using devices with wider response bandwidths,the generated signal maintains high performance while the signal frequency can be extended to millimeter-wave band.Based on the above research,this paper proposes a stability-improved OEO system combined with an optical phase-locked loop(OPLL)to compensate for the temperature drift induced by ambient temperature and loop jitters.Taking a 5.8 GHz microwave signal as an example,at the 0.1 Hz offset frequency,the SSB phase noise of the generated signal is reduced from 40 d Bc/Hz to-10 d Bc/Hz compared with the free-running condition.After the phase locking,the microwave signal generated by OEO has a frequency fluctuation of less than 5 Hz in 1 h.In the case of free running,the corresponding frequency fluctuation is8 k Hz,and the frequency stability is improved by 1600 times.Based on this method,the phase noise of OEO under low offset frequency can be significantly suppressed,which greatly improves the long-term stability of the signal and lays a foundation for the practical application of Brillouin OEO.3.Dual-frequency Brillouin OEO with independent tunability.The applications of dual-frequency microwave signals in frequency conversion,sensing,and communication put higher requirements on the frequency tunability and stability of the signals.In order to solve the mode competition problem of dual-frequency OEO,we adopts OIL and polarization multiplexing technology.Benefiting from the flexible tuning of SBS,it generates highly stable dual-frequency microwave signals with independent frequency tunability.In this paper,cascaded intensity and phase modulation is used to generate the OFC.Two SLs are respectively injection-locked into different sidebands of the OFC.The output signals are used as the SBS pumps to generate two gains with orthogonal polarization states.The orthogonal polarization-multiplexing modulation is introduced to realize two modes oscillation in the same OEO loop.This scheme breaks the mode competition and produces dual-frequency microwave signals with a frequency range of 40 GHz.The frequency of the generated signal can be independently adjusted with10 MHz resolution,and high spectral purity and low phase noise are guaranteed simultaneously.4.Brillouin MPF based on frequency-sweeping pumpConventional electrical filters usually have limited frequency regulation,and roll-off curves are not ideal.In this paper,by extending the SBS gain spectrum,a MPF with programmable shape and bandwidth,low in-band jitters,and ideal roll-off curve is designed which can meet the signal shaping and filtering requirements in the communication systems.A sweeping-frequency signal is modulated into optical domain and used as SBS pump after the carrier-suppressed single-sideband modulation.The gain spectrum of SBS is extended to realize MPFs with tunable wavelength and bandwidth.By introducing digital feedback to control the electrical signal,the in-band fluctuation of the MPF is suppressed to within2 d B.The digital control of the SBS-based MPF is realized by controlling the center wavelength and bandwidth of the pump.Furthermore,this study analyzes the effects of the sweeping pump time on the filter performance and transmission quality of digital signals.The results provides theoretical configuration for the SBS-based MPF scheme using sweeping pumps.In order to improve the pump power efficiency of SBS,the phase modulation scheme is used to detect the intensity and phase information of SBS at the same time.The MPF with improved rejection ratio is realized,and a dispersion compensation module is introduced to avoid the noise from dispersion.The polarization state and dispersion control are adopted to realize simple control of the filter waveform.This scheme improves the pump power efficiency of SBS and provides a new method for waveform control of SBS MPF.The SEO-based OEO proposed solves the problem of frequency adjustment flexibility and greatly improves the long-term stability of the signal frequency.Based on the SBSbased MPF,the center wavelength and bandwidth are flexibly configured,and the pump sweeping time is analyzed in detail.Further,this research also proposes the practical research direction of OEO and MPF.It can be seen that the SBS has potential applications in the field of microwave photonics,and it is worthy of further study to meet the needs of modern development requirements.