| Thermal-lens effect,as one type of the thermal effect,it will change the intrinsic refractive index(RI)distribution of the gain fiber in high-power operation.Any of the gain fiber,no matter of the traditional larger-mode-area(LMA)fiber or the specially-designed novel very-larger-mode-area(VLMA)fiber,will experience the thermal-lens effect due to the quantum effect along with the pump absorption.Summarized from the previous studies on the thermal-lens effect,the research contents mainly focus on the decrease of the nonlinear threshold caused by the thermal-lens induced mode shrinking.As is well known,each supported mode in the gain fiber can be potentially propagated and amplified.The thermal-lens induced RI variation will not only influence the mode competition caused by the mode field variation,but also will form an irregular thermally-induced waveguide in the gain fiber and then induce the mode coupling.These two aspects will necessarily affect the mode propagation properties in the fiber laser system.However,pertinent studies on these aspects are nearly vacant.Therefore,in this thesis,we will systematically investigate the effect of thermal lens on the mode propagation properties in the fiber laser system,the main contents are illustrated as follows:1.The development history of fiber laser and the physical limitations on the power scaling are introduced.The investigation state of thermal lens,which is one of the most important limitation issues,is systematically analyzed and the research significance is proposed based on these backgrounds.2.The thermal-lens induced mode field variation is quantitatively analyzed.By the deduction of the heat conduction equation,the RI variation in the active fiber is given.Based on this,the thermal-lens induced mode field variations in the straight and bent fiber are respectively discussed.It is found that the mode shrinking is more severe in the fiber with smaller core NA or with larger mode field.Simultaneously,it is found that both of bending and thermal-lens effect will bring reduction of the mode field.Thus,a novel parameter,bend radius limitation is proposed to delimitate the mode field shrinking.Moreover,the limitations of bend radius of various fibers are given for guide the practical application.3.The effect of thermal-lens on the mode coupling is investigated.The mode coupling coefficient is deduced based on the couple-mode theory.It is firstly demonstrated and verified that the mode coupling can be induced by the thermal-lens effect by calculating the mode coupling coefficient.The variation of coupling coefficient with thermal load and fiber parameters are discussed in straight and bent fiber,respectively.It is shown that the mode coupling coefficient is generally enhanced by the thermal load and the mode field variation.In the straight fiber,when thermal load is small,the coupling coefficient can be reduced by the increment of fiber core,while it is enhanced when thermal load is large enough.In spite of this,in the bent fiber,increasing the fiber core,reducing the core NA,or decreasing the bending diameter will all enhance the mode coupling,especially when the NA is quite low,the mode coupling coefficient can be enhanced most severely even with low thermal load.The pertinent study on the mode coupling coefficient provides foundation on the investigation of mode coupling effect in the high power fiber laser system.4.The thermal-lens induced mode coupling effect in the high power fiber amplifier is investigated.The numerical model for describing such mode coupling effect is established firstly,with both consideration of longitudinal thermal load distribution and relative effect on the mode field distribution and gain effect.This model is applied to two typical fiber amplifiers,single-frequency short-cavity fiber amplifier and the traditional fiber amplifier to investigate the inner mode behavior.It is found that,even with pure fundamental mode(FM)seed input,the higher-order mode(HOM)can also be induced.The coupling mode content is mainly decided by the absolute value of mode coupling coefficient until the input HOM content is higher than a certain level.The thermal-lens induced mode coupling is most severe with co-pumped scheme while weakest with counter-pump scheme.With a given total pump absorption,the mode coupling in traditional fiber amplifier,which need longer gain fiber,the HOM content is much smaller compared with the high-absorbed case.In spite of this,the triggered HOM content is much higher than the level of noise that triggering the transverse mode instabilities(TMI).5.The design and application of the thermally-guiding very-large-mode-area(TG VLMA)fiber.The design of TG VLMA fiber is proposed to use the thermal-lens effect rather than suppression.By summarizing the research status of the novel VLMA fibers,the advantage and significance on the investigation of TG VLMA fiber is proposed.The mode field evolution and beam quality evolution are studied,which shows that the mode behavior with thermal load can be concluded into three stages and corresponds to mode is“cladding-confined”,“quasi-core confined”,and“core-confined”states.The beam quality can be optimized by controlling the thermal load since it is not changing monotonously with the thermal load.Moreover,we make an investigation on the adiabatic criterion of such fiber,which shows as long as the pump absorption coefficient is small,the adiabatic criterion can be met.According to this,the propagation characteristic of a single-mode TG VLMA fiber amplifier is numerically studied.It is shown that the seed power means a lot to the amplifier behavior,which should be large enough to certificate high output efficiency.Moreover,it is revealed that the initial RI difference of TG VLMA should be larger than 10-4to maintain a linear increment of output power with pump power.It is also shown that the co-pumped scheme is most beneficial to the nonlinear effect suppression.The pertinent study provides significant guidance on the application of TG VLMA fiber amplifier. |
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