Theoretical Research And Suppression Of Transverse Mode Instability In High Power Fiber Laser

As the age of photon in twenty-first Century,high power fiber laser as a new generation laser,compared with gas,chemical,solid and other lasers,has the advantages of high photoelec-tric conversion efficiency,good heat dissipation,excellent beam quality,compact and so on.It is widely used in industrial processing,national defense,scientific research and other fields.Transverse mode instability?TMI?becomes the main nonlinear effect that restricts the power scaling of fiber laser in signle mode regime,and restricts the demanding field applications for high brightness lasers.As the main limitation of high power fiber lasers,TMI is of great signif-icance for theoretical research and suppression methods.In this thesis,the results of theoretical research and suppression methods for TMI in high power fiber lasers are obtained:Based on the rate equation,heat conduction equation and wave equation,a coupled mode model,depicting TMI effect,of high dimensional and multi physical fields is established.The reliability of the model is further verified,and the influence factors of the coupling coefficient are also discussed.In order to apply to a kW fiber laser system,when the gain saturation effect is added to the TMI model,the saturation of the pump light and signal light are both considered.In the theoretical model,the effects of stimulated Raman scattering and photon-darkening are introduced respectively.Both of the two are accompanied by the heat effect,which aggravate the effect of transverse mode instability.The theoretical results show that the threshold of transverse mode instability decreases significantly.The new criterion of the transverse mode instability pump threshold,which is the new criterion to judge the transverse mode instability threshold,can be used to effectively judge the suppression ability of the TMI of different types of large mode field fibers.From the three aspects of laser parameters,nonlinear effect and optical fiber design,we explore the suppression methods of TMI.By reducing the quantum loss effect,increasing the gain saturation effect and increasing the high order mode loss,TMI threshold of can be improved.In terms of nonlinear effects,we can suppress the stimulated Raman scattering effect and photon-darkening effect,then effectively suppress the TMI effect.In the aspect of optical fiber design,four types of fiber are designed:partially-doped fiber,variable core-cladding ratio fiber,low numerical aperture fiber and low thermal optical coefficient fiber.It is theoretically proved that these four kinds of optical fibers have the inhibition effect on the TMI effect.Two characteristics of transverse mode instability are observed experimentally:threshold characteristics and mode degradation.By changing the oscillator power,pump wavelength and pump configuration and bending mode selection,the TMI is suppressed.The experimental re-sults show that the suppression effect of the counter-pumped scheme is better,increasing the threshold greater than 50%,both the Raman suppression ratio and the narrow line width laser output can be significantly improved.It is an effective method to suppress the TMI effect in en-gineering.The effect of stimulated Raman scattering on transverse mode instability is observed in the experiment.By controlling the nonlinear growth of stimulated Raman scattering effect,the thermal effect caused by Raman conversion can be effectively reduced,and the TMI will be mitigated.It is further verified that the laser with transverse mode instability is signal light rather than Raman light.Based on the iterative design method of microwave waveguide,the iterative design method of multimode optical waveguide based on coupled mode theory is constructed.This iterative design method has the characteristics of reliability,universality,and so on.In the tapered waveg-uide,the LP_0mmode converter can be realized in multi mode,multi band,high efficiency,large bandwidth and reversible conversion.Based on the iterative design method of coupled mode the-ory,a multimode optical waveguide iterative design method based on beam propagation method is constructed.The LP₁₁-LP₀₁mode converter is realized in the axisal bending waveguide struc-ture,the maximum conversion efficiency can be achieved at the highest level of 75.54%,and the maximum device optimization can be improved by 17.57%using this method.These mode converters are promising for inverse mode conversion in fiber lasers emerging TMI.