Mechanism Of Narrow Linewidth Tunable Microwave Photonic Generation Based On Forward Brillouin Fiber Laser

The Brillouin scattering in fiber has been widely used in fiber communication system,fiber laser,fast and slow light technology,fiber sensing and microwave photonic due to its low threshold and narrow gain bandwidth characteristics.Especially in the field of microwave photonic,the generation technology of microwave photonic with narrow linewidth and tunable frequency based on stimulatedBrillouin scattering has become a research hotspot of scholars.In previous studies,the methods of generating tunable microwave photonic with narrow linewidth based on stimulatedBrillouin scattering can be divided into three categories:（1）Fiber laser based on backward stimulatedBrillouin scattering.The researchers generated two different backwardBrillouin Stokes lasers by using two pump sources of different wavelengths,which generated microwave photonic by beating frequency.Based on the narrow linewidth characteristics of the backwardBrillouin fiber laser,the narrow linewidth microwave photonic on the order of k Hz is realized.The microwave photonic tunability is realized by fixing one pump source wavelength and adjusting the other pump source wavelength.However,the linewidth of microwave photonic generated by Brillouin fiber lasers is limited to the order of k Hz due to the 30 MHz gain bandwidth of the backwardBrillouin scattering.（2）Opto-electric oscillator based on forward stimulatedBrillouin scattering.The forward stimulatedBrillouin scattering signal in the fiber is converted from phase modulation to intensity modulation by the Sagnac loop structure and the signal feedback is realized by electro-optic modulator and photoelectric converter.Using the gain bandwidth of forward stimulatedBrillouin scattering less than 6 MHz,and combined with the ring resonator structure,the microwave photonic generation of narrow linewidth and single longitudinal mode is realized.However,the Sagnac loop,electro-optic modulator and photoelectric converter in the oscillator will increases the loss in the cavity,which limits the optimization of microwave photonic linewidth.（3）Passive mode-locked lasers based on forward stimulatedBrillouin scattering.Using the photoacoustic interaction of short photonic crystal fiber or single mode fiber,and combined with the acoustic frequency characteristics of forward stimulatedBrillouin scattering,based on the nonlinear polarization rotation effect of passive mode-locking and saturable absorption effect,the microwave photonic signal with a soliton pulse repetition rate corresponding to the frequency of the forward stimulatedBrillouin acoustic mode is generated.However,the low Q value in the cavity of the mode-locked laser restricts the optimization of the linewidth of the microwave photonic because of the short length fiber of meter magnitude.We propose a narrow linewidth tunable microwave photonic generation scheme based on forwardBrillouin fiber laser.Based on the existing research on narrow linewidth microwave photonic,an all-fiber double-ring forwardBrillouin fiber laser is constructed.By using the forward stimulatedBrillouin scattering with gain bandwidth less than 6 MHz,and combining the Vernier effect of double-ring cavity and the principle of resonant cavity linewidth compression,stable narrow-linewidth single-longitudinal mode microwave photonic are generated.In existing frequency tunable microwave photonic research foundation,we propose the incorporation of a Sagnac loop with an unpumped erbium-doped fiber in a short-length ring cavity,using the annular cavity to narrow the microwave photonic linewidth,by adjusting the polarization controller inside the ring,matching acoustic mode frequencies of microwave photonic of different orders to longitudinal modes in a ring cavity,the microwave photonic of this order acoustic mode frequency is output,so as to realize the tunable frequency of microwave photonic.The main contents include:（1）Briefly describes the concept of fiber Brillouin scattering.The characteristics and application fields of forwardBrillouin scattering in optical fibers are introduced.The development of forwardBrillouin scattering and microwave photonic technology based on forward stimulatedBrillouin scattering are analyzed.Finally,the main research content of this paper is introduced.（2）Theoretical description of the forward and backwardBrillouin scattering,including the principle of the forward and backwardBrillouin scattering,dispersion curve analysis and mathematical equation study of the acoustic and optical field.Then the forward stimulatedBrillouin scattering in single mode fiber is described theoretically,detected experimentally,and simulated.（3）A double-ring forwardBrillouin fiber laser is proposed to generate single longitudinal mode microwave photonic with narrow linewidth.The forward stimulatedBrillouin scattering gain is provided by 5 km single mode fiber and the main ring cavity is compressed to narrow the linewidth of microwave photonic.The 300 m single mode fiber is used as a secondary ring,and forms a double-ring cavity structure with the main ring.Based on the vernier effect,the single longitudinal mode output of microwave photonic is realized.The self-seededBrillouin pump formed by erbium-doped fiber and forward stimulatedBrillouin scattering R₀₇order Stokes light beat frequency at 300 m W and 980 nm pump threshold power,generating R₀₇order microwave photonic with frequency of 319.79 MHz and linewidth less than 1 Hz.The acoustic mode suppression ratio and side mode suppression ratio are 22 dB and 36 dB respectively,realizing the single longitudinal mode output of R₀₇order microwave photonic.The power stability and frequency stability fluctuated only±1 dB and±0.5 MHz during the stability measurement experiment within 20 minutes.（4）A forwardBrillouin fiber laser based on unpumped Erbium-doped fiber Sagnac loop is proposed to generate frequency tunable narrow linewidth microwave photonic.A 100 m single mode fiber is used to provide forward stimulatedBrillouin scattering gain,and a ring cavity is constructed to narrow the linewidth of microwave photonic.A Sagnac loop with an unpumped erbium-doped fiber is introduced,by adjusting the polarization controller in the Sagnac loop,the acoustic mode frequencies of different order of microwave photonic are matched with the longitudinal mode in the ring cavity,and the microwave photonic of this order acoustic mode frequency are output,so as to realize the frequency tunability of microwave photonic.At the pump threshold power of 980 nm at 200 m W,narrow linewidth microwave photonic with tunable range of 128～271 MHz with frequency intervals of about50 MHz are generated,corresponding to Stokes acoustic mode frequencies of R₀₃～R₀₆order forward stimulatedBrillouin scattering.The linewidths of tunable R₀₃～R₀₆order microwave photonic are 11.9 Hz,11.1 Hz,10.3 Hz and 10.8 Hz,respectively,and the longitudinal mode suppression ratios are 38 dB,28 dB,20 dB and 30 dB,respectively.