Research On Mode Modulation Of Random Fiber Laser And Its Applications

Random fiber laser(RFL),a new type of fiber laser,has attracted extensive attentions and researches in the past few decades.As a continuation of the development of traditional random laser(RL),multiple scattering effects in one-dimensional fiber structure are used as effective optical feedback mechanism for RFLs,combing with nonlinear gain providing optical amplification.Therefore,the unique random lasing is realized without any conventional resonant cavity structures.From the very first random fiber laser based on photonic crystal fiber filled with laser dyes and nano scattering particles,to the coherent feedback random fiber laser composed of randomly distributed fiber grating structure,and to the random distributed feedback fiber laser based on purely distributed Rayleigh scattering along commercial standard single mode fiber,the development of random fiber lasers have gone through coherent feedbacks to incoherent feedbacks,from strongly disordered scattering structures to weakly disordered structures.Finally,the development of RFLs have been flourished in incoherent feedback RFLs,leading to plentiful progresses in high power/high efficiency,narrow line width,broadband tunable RFLs and successfully applied in applications such as optical sensing,ultra long-distance optical fiber communication.The development of RFL has also expanded from the initial study of spectral and power characteristics to the study of polarization,new wavelength generation,time-domain characteristic,non-linear mechanism and multi mode characteristic.Throughout the history of the development of RFLs,the majority of the researches mainly focus on continuous wave operation and single transverse mode excitation.This is limited by the open structure formed by random scatterings in RFL,blocking the generation of pulsed random lasing based on traditional resonant cavity mechanism.On the other hand,although the use of single mode fiber greatly reduces the lasing threshold and is widely used as lasing medium,this also hinders the analysis and research of multi mode characteristics of RFL.Therefore,the research on more complex behaviors of RFL,such as pulse generation and multi-transverse mode excitation are still in the infancy.Plentiful of fundamental problems are still remaining and urgently needed to be solved and developed.Following research frontier of RFL,we focus on the study of mode modulation of RFL and its applications.Starting from the characteristics of longitudinal mode,time-domain pulse and spectral bandwidth,to the low spatial coherence of transverse mode,new mechanisms of mode control are studied and more efforts are focused on the application of multi transverse modes RFL.The main research contents of this dissertation are summarized as follows:(1)The mode control characteristics of active fiber RFL under coherent feedback mechanism are studied.The output characteristics regulation of RFL is analized from the stabilization and selection of lasing wavelength to the time domain pulse generation.Active fiber RFL with coherent feedback mechanism is based on random resonant feedbacks provided by random distributed fiber gratting array.Therefore,it has advantages such as short fiber length,low threshold,compact structure and abundant random resonances.As a result,the output wavelength of this type RFL also suffers from strong fluctuations over time due to the intense mode competition effect.Thus,the mode control of the coherent feedback RFL has important scientific significance.We propose for the first time that through introducing local gain perturbation by external control light irradiated on the random distributed fiber grating array,stable single peak random lasing output can be obtained at some specific control light injected positions.The lasing wavelength can also be effectively adjusted by changing the position of the injection light,which provides a new method to stabilize and regulate the operation behavior of coherent feedback RFL.Besides,by introducing graphene based saturable absorber,the Q-switched mode-locking pulse generation of coherent feedback RFL are realized for the first time taking advantage of the abundant resonant characteristics.By analyzing the resonant frequencies,the operation mechanism of localized mode and global mode resonance is revealed,which provides a new idea for the generation of pulsed RFL and the regulation of time domain mode characteristics.(2)Supercontinuum(SC)generation seeded by RFL is proposed and studied.RFL based on Rayleigh scattering feedback mechanism has a unique opened structure,which is independent of wavelength selector and supports ultra-wide spectrum feedback.In order to fully explore the bandwidth characteristics of the lasing process of this opened structure,nonlinear optical fiber is introduced here and combined with nonlinear effects such as modulation instability,SC excited by RFL is proposed.The output bandwidth can be effectively modulated by changing the pump power.As a bridge between the pump laser and the SC excitation,the RFL can convert the pump laser with wavelength located in the normal dispersion region into the SC spectrum.Full band and flat SC spectrum can be obtained by fully utilizing the advantages of completely-opened structure.Furthermore,by enhancing the distributed Rayleigh scattering effect and effectively reducing the lasing threshold,backward SC is obtained for the first time.Through the analysis of time domain dynamic characteristics,it is found that the backward SC spectrum has lower relative intensity fluctuation in time domain,and can be used to realize broadband light source with lower noise.(3)Expand the research of RFL to multi transverse modes regime.The speckle effect of multimode RFL and its application in speckle free imaging illumination are mainly focused here.The use of traditional single mode or few modes fiber limits the study of multi transverse modes characteristics of RFL.In addition,the development of multimode RFL is also hindered by the high lasing threshold characteristics of large core diameter multimode fiber laser.Here,combination of single mode fiber based random lasing structure and extra-large mode area step-index multimode fiber(MMF)is applied to realize multimode RFL with extremly low spatial coherence.The results show that multimode RFL not only possesses low spatial coherence that satisfies speckle free imaging illumination,but also is superior to the traditional multimode incoherent light source due to high spectral density originated from laisng process.In order to satisfy the requirement of broadband spectrum light source for imaging systems such as optical coherence tomography(OCT),decoherence characteristics of supercontinuum is also studied in MMF.The contribution of spectral bandwidth,core diameter and fiber length to the decoherence process is revealed,which provides guidance for realization of highly efficient decoherence.The main advantage of traditional RFL is simpler structure and easy to realize high power/efficiency.Therefore,speckle characteristics of high power multimode RFL are also studied.A 56 W high power multimode RFL is obtained based on a master oscillator power-amplifier(MOPA)configuration.The phenomenon that the effective transverse mode can be excited by increasing laser power is revealed,which helps to further reduce the spatial coherence.(4)Based on the above speckle free imaging using multimode RFL,in order to explain the dependence of speckle contrast on the change of output light field of MMF and the decrease of spatial coherence caused by the increase of laser power,the effects of mode number and modal power distribution on speckle formation in MMF are thoroughly investigated through theoretical analysis of modes in MMF.The electric field distribution of the transverse mode supported by the large core diameter step index MMF is calculated theoretically.According to the principle of mode decomposition and composition of optical fiber,specific modal weight coefficients are imposed to each transverse mode.In this way,the overall mode composition can be modulated,and the influence of mode number and different order transverse modes on the final mode field formation can be studied flexibly.The process of speckle formation is simulated by introducing random phase modulation.By analyzing speckle contrast of the speckle patterns,the influence of different mode composition on spatial coherence of multimode fiber output is studied.The results show that the contribution of different order modes to speckle formation is unequal,and the speckle contrast caused by lower order transverse modes is significantly smaller than that of higher order transverse modes.Therefore,the lowest speckle contrast value of the whole mode composition appears in the regime that the power of the low order transverse mode is dominant while the power of the whole modes is relatively uniform.This work provides theoretical guidance for designing,optimizing and regulating the spatial coherence of multimode fiber output.