Research On Surface Treatment Of Special Laser Glass

High-power fiber lasers are widely used in industrial manufacturing,national defense and military fields due to their high conversion efficiency,high reliability,high beam quality,compact structure,and ability to handle harsh environments.In recent years,the output power of high-power fiber lasers has reached the order of 10,000 watts or even hundreds of thousands of watts.However,during long-term high-power operation,fiber lasers may experience reduced output power,increased pump thresholds,and unstable performance,that is,the photodarkening effect,which greatly reduces the stability and service life of the device and becomes the bottleneck of its further development and application.From a material point of view,under high-power operation,impurities in the fiber end face or core/clad interface,as well as defects and cracks generated during the preparation process,will form strong absorption points,resulting in irreversible and destructive damage to the fiber.Therefore,it is necessary to reduce the existence of these defects in laser glass and optical fiber as much as possible,which is especially important for non-silica-based special laser glass that uses the rod-in-tube method to prepare optical fiber preforms.In order to study and explore the material,physics,and optical basic sciences and application issues involved in the process of microstructure evolution and darkening dynamics of glass fiber in high-power fiber lasers under extreme conditions such as intense lasers,our laboratory cooperates with the China Academy of Engineering Physics carry out the research work of"Research on the process of microstructure evolution and darkening dynamics of optical fiber under extreme conditions（U1830203）".Taking this project as an opportunity,this paper systematically carries out research on the surface treatment of three types of commonly used special laser glasses（phosphate,germanate and tellurite glasses,referred to as PAB,GGB and TTZ/TGZ glasses,respectively）,in order to master the surface treatment methods of special laser glasses and clarify the glass surface/subsurface defect removal mechanism under different process conditions,and to provide a theoretical and experimental basis for reducing the loss of special glass fiber core/cladding interface and improving its damage resistance under extreme conditions.This paper is divided into three chapters.Chapter 1 is the introduction.It first introduces the high-power fiber lasers and the generation mechanism and suppression methods of photodarkening effect.And then the research progress of surface treatment of silica and special laser glass at home and abroad is reviewed.Finally,it summarizes the structure and properties of three types of commonly used special laser glass,and the research purpose and research idea of this paper are represented.In chapter 2,the reaction probabilities of each compositional oxide of special laser glasses in acid from the perspective of thermodynamics are analyzed firstly and the results show that the etching reaction of these glasses in acid can proceed spontaneously at room temperature,and the reaction is exothermic.Then the etching rate of PAB glass in different acids and the morphology after etching are further analyzed,and 40%hydrofluoric acid is determined as the etchant for PAB glass.In GGB glass,the surface morphology and deposits after hydrochloric acid etching are analyzed,and it is concluded that the main component of the deposits is Ge O₂ crystal,which is produced by the hydrolysis of Ge⁴⁺ions in the solution after etching.In TTZ and TGZ glasses,the effects of hydrochloric acid concentration,temperature,and etching time on its etching rate and surface morphology are systematically studied.The results show that hydrochloric acid is easy to etch tellurite glass.Based on this,the effect of chemical etching on the surface quality and transmittance of three types of special laser glass is further studied.The results show that the etching will expose defects and cracks in the glass subsurface,and increase the scattering of light at the same time,resulting in an increased roughness and a decreased transmittance of glass.The subsequent heat treatment promotes the surface of the glass to become flat again under the action of surface tension,the transmittance is restored,and the roughness is reduced to a lower level,which is beneficial to improve the laser damage resistance of the laser glass.In chapter 3,the removal of surface and subsurface defects of special laser glass with different polishing agents（Ce O₂,Al₂O₃,Fe₂O₃ and Zr O₂）,and the impurity elements and their depth distribution on the surface of glass after grinding and polishing are studied.The results show that Zr O₂ is the optimal polishing agent for PAB and TTZ glasses,while the optimal polishing agent for GGB glass is Ce O₂.After polishing with the optimal polishing agent,the root-mean-square roughness of the surface of PAB,GGB and TTZ glass can reach 2.64,7.04and 3.97 nm,respectively.In addition,after grinding and polishing,a large amount of Fe,Si and a small amount of metal elements in the polishing powder will remain on the subsurface of the glass,and they are mainly distributed in the depth range of 50 nm below the glass surface.The surface roughness of glass before and after light etching with hydrochloric acid is measured quasi-in-situ using the method of femtosecond laser marking.The results show that by etching with concentrated hydrochloric acid（12 mol/L）,the subsurface defects of the glass can be exposed in a very short time（5 s）,thereby increasing the surface roughness.Therefore,when using concentrated hydrochloric acid to etch special laser glass,the etching should be controlled in a short period of time so that the subsurface defects of the glass can be just completely removed as the best.