Liquid-core/liquid-cladding Optical Waveguides Using Thermal Gradients Formation and Their Application to Chemical Analysis

提出了一种基于温度梯度的液芯/液包层光波导(TG-LLW)，将不锈钢毛细管中的水层流置于热源中，在层流中形成温度梯度。水在不锈钢管(内径0.8 mm；外径1.6 mm，长15 cm)中用高效液相色谱仪泵形成层流，流量为0~9 cm^3min~(-1)(平均线速度为0~30 cm S~(-1))。不锈钢管被放置在一个热源，即循环热水中，其温度从室温变化到70度。实验中，热源温度越高，流量越大，得到的导光强度越高。CFD模拟表明，热源温度的升高导致了流动中温度梯度的增加，即流动中折射率梯度的增加。此外，CFD计算还阐明了流量的影响。导光强度随距离呈周期性变化，且周期随热源温度的升高而缩短。用渐变折射率光纤理论解释了这一现象。也就是说，折射率梯度越小，导光周期越长。TG-LLW的这一性质将有助于在未来的光流控系统中操纵光，即聚焦和分散引导光。

Proposed was a liquid-core/liquid-cladding optical waveguide using thermal gradients (TG-LLW), where laminar flow of water in a stainless capillary tube was placed in a heat source, and thermal gradients across the laminar flow were formed. Laminar flow of water was formed in a stainless steel tube (i.d., 0.8 mm; o.d., 1.6 mm, 15 cm long) using an HPLC pump, whose flow rate was 0 to 9 cm^3 min^<-1> (the average linear velocity of the flow was 0 to 30 cm s^<-1>). The stainless tube was placed in a heat source, i.e., circulated hot water, whose temperature was changed from room temperature to 70 degree. In the experiments, the higher the temperature of the heat source was and the greater the flow rate, the higher intensity of the guided light was obtained. CFD simulation showed that the increase in the temperature of the heat source resulted in the increase in thermal gradients in the flow, i.e., the increase in refractive index gradients in the flow. Moreover, the CFD calculation also elucidated the effect of the flow rate. The guided light intensity changed periodically with the distance, and the period became shorter with the increase in the temperature of the heat source. Thisphenomenon is explained by the theory of graded index optical fiber. That is, the smaller refractive index gradient results in the longer period of the guided light. This nature of TG-LLW should be useful to manipulate the light, i.e., focusing and dispersing guided light in optofluidic systems in future.