Research On Narrow Linewidth Random Fiber Laser Based On Random Fiber Grating

As a new type of fiber laser without a fixed cavity,random fiber laser enjoys the advantages of high efficiency,compact structure,and low cost,which has become a potential light source in the fields of communication,sensing,biology,etc.The narrow line width characteristic is one of the attractive developing direction.The existing narrow-linewidth random fiber lasers are mostly demonstrated by Rayleigh scattering or artificial random scattering enhancement point schemes,which have disadvantages such as longer feedback fiber,weak feedback intensity,and serious mode competition resulting in higher laser pump threshold,low slope efficiency and higher relative intensity noise level.To solve the above problems,this paper demonstrates a narrow linewidth random fiber laser based on random fiber grating arrays.The random fiber gratings are designed and experimentally prepared based on the simulation analysis.with the gratings,a ring-cavity narrow linewidth random fiber laser has been built.Moreover,a distributed sensing system has been demonstrated based on the random fiber laser.It is believed that the laser has great application potential in the fields of sensing and communication.The main research contents of the paper include:(1)Based on the photon localization theory and the transfer matrix method,a simulation analysis of various parameters in random fiber gratings is carried out.The simulation results show that the number of cascaded gratings,the modulation depth of a single grating,and the longer the length of a single grating will both influence the overall feedback intensity of RFBG;the number of cascaded gratings,the length of a single grating,and the spacing between gratings are related to the density of reflection peaks in the RFBG reflection spectrum;the number of cascaded gratings,the length of a single grating,the modulation depth of a single grating and the spacing between gratings all have an impact on the full width at half maximum of the reflection peak in the RFBG reflection spectrum.Two sets of random fiber gratings are designed for different performance aims of lasers.The first group of parameters of grating is that the length of a single grating is 2mm,the reflectivity is about5%,the number of grating is set about 15,and the spacing is random in 2-7mm.The second group of grating parameters is that the length of a single grating is 1mm,the reflectivity of a single grating is about 2%,the number of the gratings are 15/30/50/75,and the spacing is between 1-2mm.The two sets of gratings were prepared by the phase mask method.The feedback intensity of the strongest feedback peak in the first set of gratings reaches 16.7d B,and the intensity difference with the other feedback peaks is over 4d B,which is beneficial in stable single longitudinal mode output.The spectrum of the second group of gratings is consistent with the simulation results.(2)The random fiber laser with a ring cavity is designed for the purpose of stable lasing output.Various measurement methods are used to show the performance of the laser.Experiments show that the center wavelength of the laser is around 1549.2nm,the optical signal-to-noise ratio is 72d B,the threshold is about 20.7m W,and the slope efficiency is14.5%.The maximum out power of the laser is around 53.12m W.Within 6 minutes,the fluctuation of the output power is lower than 2%and the fluctuation of center wavelength is lower than 0.02nm.The linewidth of the laser is less than 774Hz.Relative intensity noise above 0.4MHz is lower than-140d B/Hz.The phase noise above 1k Hz is lower than-12d B(rad~2/Hz).Moreover,this paper has investigated the influence of the number of cascaded gratings on the laser output.Judging from the change law of the overall laser performance,the increase in the number of cascaded gratings(15-75)will raise the slope efficiency and relative intensity noise level of the laser,while the linewidth,the pump threshold and the phase noise are correspondingly reduced.This experiment provides reference value for the design of narrow linewidth random fiber lasers in the future.(3)A distributed sensing system is built based on the narrow-linewidth random fiber laser.The laser realizes a nearly distortion-free restoration of the 5k Hz vibration signal without frequency stabilization,packaging,and amplification.The noise ratio reached 28d B,and the other sensing channels remained unresponsive.It shows the application potential of narrow linewidth random fiber laser in the field of distributed sensing.