Research On Random Fiber Laser Based On Random Fiber Grating

Fiber lasers have the advantages of good directionality,high anti-interference ability,high optical efficiency,easy thermal management,and compact structure,so they have been widely used in communication,sensing,material processing,life science,national defence and military,aerospace and other fields.As a new type of fiber laser,random fiber lasers have attracted extensive attention of researchers in the last decade due to their unique cavity structure,optical properties,simple fabrication,low cost,and great potential for applications in optical imaging and fiber sensing.Random fiber lasers using random fiber grating to provide random feedback have become one of the important routes to achieve highperformance random fiber lasers due to their better performance in terms of pump threshold,spectral bandwidth,and optical signal-to-noise ratio.However,the random fiber lasers based on random fiber grating still have some problems so far,including the lack of lowthreshold lasing with controllable mode,the need for additional filters and low power output.Therefore,this thesis conducts systematic research on random fiber lasers based on random fiber grating,including the design,optimization and experimental verification of the random fiber grating itself and the structure of random fiber lasers,and proposes and implements four different kinds of high-performance random fiber lasers based on random fiber grating.The main research results are as follows:(1)An erbium-doped random fiber laser with low threshold and controllable lasing mode is proposed and demonstrated.A high-reflection random fiber grating with a reflectivity of up to 93.5% written by femtosecond laser is used to provide random feedback.The properties of the random fiber grating are theoretically studied by the transfer matrix method,and it is proved that the internal light intensity distribution exhibits exponential attenuation due to Anderson localization.The discrete lasing mode,the threshold of only5.7 m W,and the narrow-band spectrum of only about 0.4 pm near the threshold in our random fiber laser reveal the characteristics of the laser and its performance enhancement brought by the Anderson localization effect.By tuning the center wavelength of the gratingbased filter we achieved the control of the lasing mode of the random fiber laser,and the laser realized long-term stable operation for the selected 3 lasing modes.(2)An erbium-doped random fiber laser that can achieve stable single-wavelength lasing without additional filter is proposed and demonstrated.A partial-refletion random fiber grating that provides random feedback is designed so that the highest reflection peak is significantly higher than the other reflection peaks,thereby ensuring that the laser is only lasing at the highest reflection peak of the random fiber grating(due to the lowest lasing threshold there).The calculation based on the transmission matrix method shows that the reflectivity of the partial-refletion random fiber grating should be designed in the range of30%-90%.The threshold of the realized laser is only 6.4 m W.Thanks to the good high temperature resistance of the random fiber grating written by femtosecond laser,we further demonstrate the application of this random fiber laser in high temperature sensing.Experiments show that the sensor can work stably at a high temperature of 500 °C for a long time,and has an optical signal-to-noise ratio of up to 70 d B.(3)A novel random distributed Bragg reflector fiber laser is proposed and demonstrated.The random fiber grating fabricated by femtosecond laser is designed so that its highest reflection peak is significantly higher than other peaks in the corresponding high-reflection grating reflection band,which enables the proposed erbium-doped random fiber laser to obtain stable single-wavelength lasing under the spectral filtering of the high-reflection grating.The random fiber grating is 76 mm long,and the selected lasing zone reflection peak bandwidth is only 13 pm.Through the selection of the longitudinal mode of the random fiber grating,the distributed Bragg reflector fiber laser achieves a single-frequency lasing with a linewidth of 3.9 k Hz.The effective length of the random fiber grating is theoretically studied,and then the effective cavity length of the laser is calculated to be 104 mm,which is consistent with the experimentally measured effective cavity length of 109 mm.(4)A novel high-power random Raman fiber laser with a Fabry-Perot cavity structure is proposed and demonstrated.A low-reflection random fiber grating with a length of only1.86 mm is written using a femtosecond laser to provide localized random feedback.Due to the large wavelength spacing between adjacent reflection peaks of the ultrashort random fiber grating,it is easy to use the high-reflection grating at the other end of the laser cavity for spectral filtering,so that it is relatively easy to realize a high-power random Raman fiber laser with single-wavelength lasing.The laser has a threshold of only 2.16 W,a slope efficiency of up to 91.56%,an optical signal-to-noise ratio of 55 d B,and an output power of 6.44 W.Theoretical calculations also show that the four-wave mixing effect between a large number of longitudinal modes in the random Raman fiber laser with a cavity length of2 km makes the broadening of the lasing spectrum conform to the turbulent-like square root broadening law.