| Fiber lasers are widely used in industrial processing and manufacturing,communication transmission,medical diagnosis and scientific research because of good stability,compact structure,high power and high efficiency.In order to achieve mode selection and narrow linewidth of fiber lasers,external cavity feedback element is usually introduced.Whispering gallery mode(WGM)microcavity,as a kind of external cavity element,is popular in researchers because of ultrahigh quality factor(Q)and the narrow resonance linewidth.However,the lasing mechanisms of a laser with feedback of a WGM microcavity is still not disclosed.Therefore,we take WGM microsphere as the external feedback element and build a single-mode and multimode laser rate equation model respectively,Combining with the waveguide coupling theory.The lasing mechanisms are deeply and completely analyzed.Secondly,the laser rate equation model of active WGM microcavity is constructed to explore the pumping and lasing mechanism of active microcavity,which paves the way for optimizing laser performance.The specific research work is as followed:1.The coupling model of microcavity waveguide is constructedIn order to explore the influence of coupling conditions on transmission and reflection of the microcavity waveguide coupling system,we constructed the micro-cavity waveguide coupling model and obtained the following conclusions:(1)The smaller the eigen-loss of the micro-cavity is,that is,the larger the Q value is,the greater the reflection corresponding to the resonant center frequency of the micro-cavity is.(2)For a WGM microcavity with ultra-high Q,when precisely coupled,the reflection is close to 1 and the transmittance is close to 0.On the contrary,for a WGM microcavity with an ultra-low Q value,the reflection is close to 0 and the transmittance is close to 1.(3)When the transmission has the minimum value,the reflection has the maximum value,which increases with the increase of the inner coupling frequency.(4)The strong coupling between the clockwise and counterclockwise WGM in the microcavity can obtain a large reflection.For the micro-cavity waveguide coupling system,we also get the effect of the coupling conditions on the gain threshold.The threshold at the critical-coupling point can achieve a minimum when almost all the light energy enters the microcavity,and the threshold value decreases with the increase of the microcavity size.2.Investigation on microcavity-feedback fiber-loop lasers by rate equation modelsMicrocavity-feedback fiber-loop laser gives a promise for single-mode and tunable lasing.A WGM microcavity-feedback fiber-loop laser is constructed and dynamic lasing mechanisms are investigated by rate equation models.For the single-mode lasing,the results show that the lasing efficiency can be maximized by tuning the coupling of waveguide and WGM microcavity,and two critical coupling rates exist for a fixed internal coupling coefficient of clockwise and counter-clockwise WGMs.By simply increasing the total length of fiber-loop laser,multi-mode lasing originated from one WGM feedback of a silica microsphere is explored,the lasing behaviors are well predicted by the rate equations and the WGM resonance linewidth can be effectively evaluated from the beat notes bandwidth of multiple lasings.We wish the results demonstrated here can be used for optimization on the lasing output of a fiber-loop laser with an external feedback mirror.3.Investigation on active microcavity lasers by rate equation modelsTo explore lasing mechanisms of active microcavity lasers,we built the active microcavity laser rate equations model based on coupled mode theory.The lowest threshold power and highest pump efficiency can be obtained at the critical-coupling point.For the slop efficiency,its maximum point is in the over-coupling region and near the critical-coupling point.This provides a good theoretical basis for optimizing input and output characteristics of active microcavity and improving the laser efficiency of active microcavity in the future. |
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