摘要
窄线宽单频(或单纵模)光纤激光器由于优良的光束质量、较窄的线宽以及较低的热效应等,而在激光雷达、激光测距、光传感和超远距离探测等领域有着强烈需求。目前利用掺稀土多组分磷酸盐光纤采用短腔结构已实现了单频激光输出。然而,即使采用高纯度原料,并精细熔炼,多组分磷酸盐有源光纤的传输损耗也远高于磷酸盐玻璃的本征损耗。研究表明,这种损耗主要来自光纤纤芯和包层间的界面缺陷。当用管棒法制备光纤预制棒时,不可避免地在光纤表面、亚表面留下划痕、坑洞等缺陷,以及在玻璃预制棒表面吸附各种杂质。这些缺陷和杂质残留于拉制的磷酸盐玻璃光纤的纤芯/包层界面处,引起光纤对光的散射和吸收,增加了光纤损耗。针对上述问题,本课题着重研究对光纤预制棒进行化学处理以改善磷酸盐光纤纤芯/包层界面质量,为拉制低损耗掺稀土多组分磷酸盐光纤,研制窄线宽单频光纤激光器奠定材料基础。
实验选择了一种光学光谱性质优良、物化性质稳定的多组分磷酸盐玻璃60.9P2O5-15BaO-10Al2O3-1.5Er2O3-12.6(K2O-Nb2O5-Sb2O3--Yb2O3)为研究对象,探讨了掺稀土磷酸盐激光玻璃在酸性溶液中的化学溶解机制。分析了酸性溶液中酸浓度、溶解温度、溶解时间和超声波作用对多组分磷酸盐玻璃化学浸蚀的影响。依据热力学和动力学理论,讨论了酸性溶液对磷酸盐玻璃浸蚀反应的方向、限度,制约反应速度的各种因素,对玻璃的质量损失曲线、溶解速率曲线和腐蚀液中的离子浓度曲线进行了拟合,建立了溶解动力学模型。据此对浸蚀反应的多种控制参数进行了优化。实验表明,酸性溶液能浸蚀磷酸盐玻璃,消除玻璃表面划痕等缺陷。
实验发现,随着反应时间延长,磷酸盐玻璃表面产生不溶于盐酸溶液的晶化颗粒状覆盖层。此覆盖层严重阻碍了盐酸溶液对玻璃的进一步溶解,并且这些晶化颗粒降低了玻璃透过率。通过在光纤拉制温度下对玻璃进行热处理,由于微小颗粒具有更低的熔点,玻璃表面微米尺度晶化颗粒可重新玻璃化,并提高玻璃的光透过率。
研究表明,采用化学浸蚀和高温热处理相结合的方法能有效改善磷酸盐玻璃光纤预制棒表面平整性,去除玻璃表面杂质,改善由此预制棒拉制的磷酸盐光纤纤芯/包层界面匹配性,降低光纤的损耗。
Narrow linewidth single frequency (or single-longitudinal-mode) fiber laser has a great potential application in laser radar, laser ranging, optical sensor and long distance exploration due to its excellent beam quality, ultra-narrow linewidth and low thermal effects. At present, narrow linewidth single-frequency laser have been obtained in phosphate fiber with a short cavity configuration. However, even though high-purity raw materials and fine melting are adopted, the transmission loss of phosphate fiber is also much higher than the intrinsic loss of phosphate glass. The loss mainly comes form the interface defects and impurities between fiber core and cladding. The defeats and impurities, such as the scratches and holes, mainly come from the cold processing of phosphate fiber preform. They would cause the light scattered and absorpted, leading to a high loss. Therefore, the aim of this dissertation is to improve the fiber core-cladding interface quality and lay a foundation for material process used in narrow linewidth single frequency fiber laser.
Phosphate glass with the molar composition 60.9P2O5-15BaO-10Al2O3-1.5Er2O3 -12.6(K2O-Nb2O5-Sb2O3--Yb2O3) was chose in this dissertation. The dissolution mechanism of phosphate glass in acid medium was investigated. The etching effect factors such as solution concentration, temperature, reaction time and ultrasonic stirring etc were also investigated. Based on thermodynamic and kinetic theory, the reaction direction and limit of phosphate glass in acid etching solution was discussed. A dynamic model of dissolution was established to fit the mass loss curves, the dissolution rate curve and the ion concentration curve. Accordingly, a variety of reaction parameters were optimized. Experiments show that the acid solution could remove the glass surface contaminations and defects.
A white opaque layer was formed on the glass surface by prolonging the immersion time in hydrochloric acid. This opaque layer limits the diffusion and exchange process of ions, reducing the glass transmittance. The micron-scale crystallines have a lower melting point, so they are easy to re-vitrificated with certain heat treatment and the light transmittance of glass is improved.
The results show that, chemical etching followed by high-temperature heat treatment is an effective method to improve the quality of phosphate glass surface, which make the fiber core/cladding interface better matching in rod in tube technology.
引文
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