气凝胶光纤光子器件实用化技术研究

Optical microfibers and nanofibers(MNFs) are important topics for fiber photonics and nanophotonics. The advantages of the MNF include relatively mature and effective fabrication method, low optical transmission loss, strong evanescent field, tight optical confinement, modifiable waveguide dispersion, as well as built-in transitions to traditional fibers and fiber devices seamlessly. However, the optical quality of the MNFs quickly degrades after fabrication when their diameters are close to or smaller than the optical wavelength of the guided light. The optical transmission loss increases rapidly within a few hours or even a day, which is due to surface light scattering from dust and from cracks induced by water vapour in air. This quality degradation will limit their practical applications. We can solve this problem by integrating MNFs into hydrophobic aerogels and surface chemistry modification. More importantly, the integration will not weaken the advantages of MNFs. We will investigate the atmospheric drying method to prepare hydrophobic aerogel thin films on the surface of MNFs and their optical devices in this project. Compared to integrated MNFs into bulk aerogels by a supercritical drying method, this new integration approach and technology will greatly reduce the production cost and cycle time, and also achieve more functional fiber devices. The successful implementation of this project will be able to solve the basic stability problem of MNFs devices in practical applications. By combining the unique advantages of the aerogel film and the MNFs, we will fabricate the innovative aerogel-MNF photonic devices including ultrawide band MNF couplers, microfiber knot resonators, MNF supercontinuum generation, the dissolved gas sensing element, and the stable frequency comb using aerogel films to control the distance between MNFs and microspheres.

微纳光纤是光纤光子学和微纳光子学的研究热点方向。微纳光纤的优势包括相对成熟有效的制备方法、光学传输损耗低、倏逝场强、光纤模式束缚性好、色散特性可灵活调控、以及与传统光纤器件可实现无缝连接等。但研究表明，当微纳光纤的直径接近或低于光学波长时，其光学传输损耗在一天内甚至几个小时内会迅速增加。这种因尘埃、水气以及表面微裂造成的质量退化，限制其只能短期实验所用。通过气凝胶和微纳光纤的集成和表面化学疏水处理，将能解决微纳光纤及其器件的稳定性问题且保持其优良特性。本项目将研究常压干燥法来制备气凝胶薄膜微纳光纤器件。相比超临界态干燥块状气凝胶微纳光纤集成，这种新的集成方法和技术将极大的降低制备成本和周期，还将带来新的应用。结合气凝胶薄膜的独特优势，研究的创新气凝胶光纤光子器件主要包括气凝胶微纳光纤超宽谱耦合器、谐振器、超连续谱光源、溶水气体传感器件、和微纳光纤在气凝胶薄膜微间隔控制下的稳定微球频率梳等。

常压干燥法制备气凝胶与微纳光纤结合的优势包括制备流程安全，降低成本和时间等。该方法可避免通常的高温高压超临界态干燥法，常压干燥法制备中难点是控制气凝胶微纳孔的收缩和塌陷。我们成功将常压干燥法制备的气凝胶与光纤器件集成。这种集成的显著特征是保护光纤表面，溶胶凝胶制备，极低的包层折射率，高纤芯包层折射率差，和疏水的光纤表面。我们通过对凝胶表面疏水处理后再干燥的流程制备气凝胶薄膜，能对光纤表面有效保护。我们研究了微纳光纤在低折射率气凝胶包层作用下的模场特性，发现在一定条件下，模场的有效折射率可以非常接近包层折射率，进而研究混合材料的微纳芯光纤的模场和耦合特征，可将这种光纤与传统通信光纤低损耗集成。利用气凝胶的极低折射率，可作为微流体的包层材料来形成新的波导结构，数值孔径大，对收集荧光信号的增强有很好的效果，但实验中我们也发现疏水表面不利于流体进入通道。我们发现反谐振光纤的空气孔也有利于作为低折射率包层，而且未封闭的包层也有利于气体或微流体的快速交换。在此启发下，我们设计实现了低包层折射率，响应速度快的光纤，在高灵敏度气体传感和微流体传感方面很有应用潜力。我们在实验中尝试疏水气凝胶薄膜与微纳光纤或微结构光纤结合，发现只要把气凝胶的表面疏水基加到光纤表面，就能实现光纤表面的疏水特性。利于这一原理与技术，我们可以更进一步对空芯光子晶体光纤的包层孔选择性疏水，这样可以制备功能型选择性疏水光纤，在微流体的取样和检测方面有较大优势。在溶胶凝胶法制备气凝胶的过程中，我们尝试了很多种制备方法，例如有机溶剂法，无机溶剂法，也尝试用光聚合的方法对溶胶进行微纳尺度的固化，在光纤表面制备微纳尖锥，取得了一定的进展。通过优化参数，这种全光纤的微锥结构可以产生高质量的贝塞尔光束，在光学成像、粒子微操控、激光微加工等方面有应用前景。