Study On Tm³⁺-Doped Silica Fiber Fabricated By Sol-Gel Method

Due to narrow line width,long coherence length,low intensity noise and good monochromaticity,single frequency lasers have broad application prospects in fields such as coherent detection and nonlinearity.Nowadays,distributed Bragg feedback（DBR）structures are mostly used for high performance single frequency lasers to carry out corresponding research on line width narrowing,power enhancement,intensity noise suppression and wavelength expansion,owing to the advantages of short cavity（centimeter level）single frequency lasers such as simple system structure,good integration,difficulty in mode hopping and power stability.In terms of the power enhancement of 2.0μm single frequency lasers,the main problem limiting the output power of single frequency lasers is the short active fiber in the short cavity structure.The preparation of Tm³⁺highly doped active fibers is the key to achieving high fiber gain.At present,although multi-component soft glass fibers have achieved high concentration doping of Tm³⁺,there are still difficulties in fusing them with commercial optical fibers,which seriously limits the practical application of high-performance,high-power single frequency lasers.Therefore,this paper innovatively proposes to use sol-coating method to prepare Tm³⁺highly-doped silica fiber with high gain.The following is a summary of all work.In the first part,the micro mechanism of the sol-gel method was firstly studied and the preparation process of Tm³⁺highly-doped glass film was emphatically explored.The effects of H₂O,ethanol and reaction temperature on the Tm³⁺highly-doped sol film were summarized.It is concluded that transparent sol with Tm³⁺concentration up to 30 mol%can be prepared at room temperature when the ratio of silica to water and alcohol is 1:3.5:7.5.After spin-coating,the wet film undergoes drying at 90℃,water removal at 1050℃and rapid sintering.Finally,we got a transparent and defect-free glass film with the thickness of about 1μm.This part of work laid the foundation for coating on the inner wall of silica capillary.And finally,we successfully prepared Tm³⁺highly-doped silica fiber with the size of 4/125μm through secondary melting taper.This sol-coating method makes it possible to realize a new preparation of Tm³⁺highly-doped silica fiber.The second part is about the laser performance test of the Tm³⁺highly-doped silica fiber.Based on the easily fusion between the optical fiber and the commercial passive optical fiber,an all-optical fiber linear cavity structure is built with 2.3 mol%Tm₂O₃ silica fiber.The laser output is realized at1947 nm.The slope efficiency of 2.3,4.6 and 6.5 cm optical fiber is tested and the maximum efficiency is 14.1%.The signal-to-noise ratio is 70 d B.At the same time,the small signal gain coefficient of the optical fiber is 0.48 d B/cm,and the loss coefficient is 1.22 d B/cm@1310 nm,absorption coefficient is 2.56 d B/cm@808 nm.The laser power jitter is controlled at～0.1 m W within3.5 h.In order to improve the laser performance of Tm³⁺highly-doped silica fiber and optimize the composition of the core material,the Tm³⁺highly-doped silica glass is prepared by the sol-gel method combined with the high-temperature sintering process.We explored the glass forming performance and spectral performance of silica glass with different La/Al ratios under the high concentration of Tm³⁺.When the La/Al ratio decreases from 0.54 to 0.16,the high doping concentration of thulium ion is 7.36 wt.%and can still be achieved without crystallization in silica glass.The infrared transmittance of the glass reaches 90%and the measured absorption cross section at 789 nm is 0.92×10²²cm^-2.The spectral properties of the glass with the composition of 1.8Tm₂O₃-1.7La₂O₃-10.3Al₂O₃-86.2Si O₂（mol%）were tested at 26～180℃.The experimental data showed that the three main absorption cross sections at 789,1210 and 1650 nm decreased respectively.The absorption intensity at 808 and 1900 nm increased with temperature rise,the absorption intensity at 1560 nm was less temperature-dependent.The fluorescence properties of the glass were tested in 42～200℃.The results showed that the fluorescence intensity at 1820 nm decreased with the increase of temperature,while slightly increased in 1520～1718 nm,and the fluorescence spectrum broadened towards the short wavelength by about 26 nm.