近年来,光子晶体光纤由于自身灵活多变的结构而具有很多传统光纤无法比拟的奇异特性,其在各领域的应用已经引起广泛关注。本文首先对光子晶体光纤的概念、分类和原理作了简要介绍,回顾了国内外光子晶体光纤的发展,介绍了光子晶体光纤的理论分析方法、制备工艺和相关参数及特性,讨论了几种不同材料及结构制成的光子晶体光纤的特点,对光子晶体光纤在激光、传感和通信等领域的应用以及我们的研发成果进行了介绍。最后,对光子晶体光纤进一步的发展和研究进行了展望。 Photonic crystal fibers (PCFs) have attracted increasing attention in recent years due to the unique flexible structures that enable unprecedented advantages and superior performance compared with traditional optical fibers. In this paper, the concept, classification and principle of PCFs are introduced first. Then the development of PCFs in both academic and industrial fields is reviewed, and the theoretical analysis method, preparation process and related parameters and properties of PCFs are illustrated. Next, the PCFs made of different materials and diverse structures are characterized. Furthermore, the applications of PCFs in fiber lasers, fiber-optic sensing and communications, as well as our works in the fields are discussed. Finally, comprehensive insights into the overall situation, challenges and prospectives of PCFs are provided.
PCF的有效模场面积(Aeff)是由空气孔的直径d和孔间距Λ决定的,与光纤的尺寸无关,所以可以通过调节d和 Λ 以及 d / Λ 来调整Aeff。一般,包层的填充率的增大会有效的减小纤芯面积和纤芯–包层折射率差,使模场面积减小,但也不尽然。Vittoria Finazzi等人对三角形4环单实芯石英PCF的有效模场面积进行了详细分析,发现当纤芯直径( d co = 2 Λ − d )小于波长时,由于芯径太小而不能很好地限制光波,模场会迅速变大,有效模场面积随芯径的变化有最小值 [46] 。
在高功率激光器中,由于功率大,模场面积小,能量密度大,会导致显著的非线性效应,而且一些单模光纤中因耦合效率等因素,也需要大模场面积。所以,大模场单模PCF可以很好地解决这个问题。较小的包层空气填充率就可保证大模场面积,可是也会造成较大的限制损耗,从 [46] 可知限制损耗随有效模场面积与 Λ 和 d / Λ 都有关,我们选定 d / Λ = m ,以 d = m Λ ,代入
d co = 2 Λ − d (10)
得到
d co = ( 2 − m ) Λ (11)
从(11)式可知,令 m = 0.6 , Λ = 1.0 μ m 时, d co < λ ,Aeff随 Λ 增大而减小, Λ = 1.2 μ m 时, d co > λ ( 1.55 μ m ) ,Aeff随 Λ 增大而增大,即把Aeff与芯径的关系转换成了Aeff与孔间距的关系,如果 Λ < 1.2 μ m 时,限制损耗随Aeff的减小而减小,当 Λ > 1.2 μ m 时,限制损耗随Aeff的增大而减小,限制损耗并不是随有效截面积单调递增或递减。所以在设计PCF时,如果时,我们在追求大模面积的同时,不能忽略随之增大的限制损耗,一定要综合考虑。
将HE11x和HE11y模式的有效折射率 n eff x = β x / k 0 和 n eff y = β y / k 0 代入(13)式可得:
B = | n eff x − n eff y | (14)
B即是两个正交偏振模的有效折射率之差,差值越大保偏性能越好。PCF中获得高双折射一般通过提高几何尺寸的非对称程度或改变某一方向的应力等方式。提高非对称性的方法有:改变纤芯附近的空气孔的形状、尺寸或排列,改变包层空气孔的形状、尺寸或排列。提高某一偏振方向上的应力的方法有:在空气孔中填充各向异性材料。传统光纤的双折射一般在10−4的量级上,而光子晶体光纤已经到了10−2的量级 [32] ,具有优越的保偏性能。这与PCF特殊的纤芯和包层的结构参数有关,如椭圆形单实纤芯三角形PCF,纤芯椭圆率 η = d y / d x = 1 (dx和dy为椭圆的短轴和长轴)时,即为圆形,双折射B = 0,当η偏离1时,双折射不断增大,并且随着纤芯面积S的减小,双折射B越大 [48] 。当纤芯的形状和大小不变,改变包层孔径d和孔间距 Λ 的大小,双折射也会相应的变化,同一波长在不同的孔间距具有不同的双折射,且每一模式下双折射曲线都有一个峰值 [48] 。
张炳涛,陈月娥,赵兹罡,王 勇. 光子晶体光纤的发展和应用 Development and Applications of Photonic Crystal Fibers[J]. 应用物理, 2019, 09(01): 30-50. https://doi.org/10.12677/APP.2019.91005
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