Analysis of liquid-to-solid coupling and other performance parameters for microfluidically reconfigurable photonic systems

In this paper, we analytically investigate the coupling of light from liquid-core waveguides to conventional solid-core waveguides and a series of other optical properties of liquid waveguides in order to gauge the practicality of such a system for use in microfluidically reconfigurable photonic systems. A finite element model of the system was constructed and relevant properties such as mode field diameter, attenuation, bending loss, and efficiency of evanescent and end-fire coupling were investigated as a function of the liquid waveguide Peclet number and the relative difference in refractive index. For pure liquid systems we show that the mode field diameter decreases monotonically with increasing Peclet number and that bending losses could be significantly reduced by increasing the Peclet number. More critically, we observed irreversible evanescent coupling, in which the light coupled in the solid waveguide is entrapped within the solid rather than coupled back into the liquid waveguide. This effect was caused by the lengthwise variation in the propagation constant of the liquid core due to downstream diffusion. We demonstrate that coupling efficiencies as high as 84% can be obtained for fluid based end-fire coupling by taking advantage of the tunable mode field diameter. By developing techniques for coupling light between liquid and solid states we hope to be able to overcome the drawbacks of solid waveguide systems (e.g. unchangeable structure and properties) and liquid waveguide systems (e.g. diversion and attenuation) yielding a new paradigm for reconfigurable photonics.