Optimization Of Key Performance Parameters And Application Investigations Of Brillouin Random Fiber Laser

Random fiber lasers originated from the conventional three-dimensional laser with replaced reflector by stray particles in disordered medium.And the one-dimensional waveguide properties of optical fibers were utilized to constrain the resonant light in random lasers after the appearance of low-loss optical fibers.Thus the performance of random lasers in terms of gain,laser efficiency,directionality,and stability were greatly improved.Among a variety of random fiber lasers with different gain mechanisms and structures,the Brillouin random fiber laser stands out by its narrow linewidth,low noise,low threshold and high optical signal-tonoise ratio.However,the Rayleigh scattering in fibers provided the randomly distributed feedback.And the long gain fiber required for provide enough Brillouin gain caused the dense distribution of random modes within the Brillouin gain bandwidth.These make Brillouin random fiber lasers still have urgent problems to be solved,such as poor stability and low lasing-efficiency compared with conventional fiber lasers with fixed-cavity.In this paper,we focus on the investigation of the characteristics and optimization of key performance parameters of the Brillouin random fiber laser.And we aim for achieving a Brillouin random fiber laser with low noise,narrow linewidth,high stability,and single-longitudinal-mode laser output.The research content of this paper and the main work are as follows:1.Exploring the physical mechanism of optical effects including stimulated Brillouin scattering,Rayleigh scattering and so on in Brillouin random fiber lasers.And utilizing coupled mode equations establish the physical model of Brillouin random fiber lasers.Predict the behavior and phenomena of laser resonance and laser output.And eventually explain and analyze the currently existing laser output instability.2.A dual-coupler fiber ring resonator(DCFRR)is designed based on Vernier principle and composite-cavity theory as an all-fiber filter.It is used to partially filter the densely distributed random modes in the Brillouin random fiber laser caused by the long resonant cavity.And eventually achieving a stable random laser output.3.An unpumped er-doped-fiber sagnac loop(UESL)is built based on the saturable absorption property of erbium-doped fibers.Therefore,the optical signal passing into the UESL forms a standing wave to induce a dynamic grating in the erbium-doped fiber to provide filtering function.Due to the weak refractive index modulation of the grating and the construction method,the sagnac ring has a narrow filtering bandwidth and the ability to autotracking the center wavelength of the transmitted light.This effectively filters out random modes in the cavity of Brillouin random fiber laser and improves the stability of the laser output.4.The high Brillouin gain coefficient of the highly nonlinear fiber is used to shorten the resonant cavity length of the Brillouin random fiber laser.Thus the number of random modes in the resonant cavity is reduced.And then a small number of random modes are filtered using the narrow filter bandwidth and the ability of center-wavelength auto-tracking of the UESL to achieve a single-longitudinal-mode and narrow-linewidth random laser output.5.Benefit from the high signal-to-noise ratio,high fidelity of the Brillouin random fiber laser and the high sensitivity of the transmissivity of the DCFRR to vibration signals.And the DCFRR-based Brillouin random fiber laser is applied to fiber-optic acoustic sensing to achieve wide-frequency-range acoustic and even ultrasonic measurement with high sensitivity in the short and long distance.