Abstract
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.
Highlights
Since its inception, Optofluidics [1,2], or the fusion of microfluidics with optics, has had significant impact in a number of areas including: optical biosensors, lensing and imaging [3,4,5], nanomanipulation [6,7] and on-chip waveguiding and lasing [8,9]
A popular approach to the latter of these is through the use of liquid core/fluid cladding waveguides [10,11,12] which represent a new type of optical element whereby a high refractive index liquid is cladded by a lower refractive index fluid are used to guide light around on a microfluidic chip
Compared to conventional solid waveguides, liquid waveguides have a number of significant advantages including: physical adaptability, chemical adaptability and thermal stabilization
Summary
Optofluidics [1,2], or the fusion of microfluidics with optics, has had significant impact in a number of areas including: optical biosensors, lensing and imaging [3,4,5], nanomanipulation [6,7] and on-chip waveguiding and lasing [8,9]. Compared to conventional solid waveguides, liquid waveguides have a number of significant advantages including: physical adaptability (i.e. the light path or mode profile can be reconfigured on command by adjusting the local flow conditions), chemical adaptability (i.e. varying levels of gain media or non-linear solutions can be introduced to or removed from the waveguide) and thermal stabilization. Despite these advantages, liquid state optical components are not likely to ever completely replace solid state optical components due to limitations in switching speed, general robustness and lack of advanced functionality such as active filtering or electro-optic modulation [13,14]. In addition to characterizing the coupling efficiency and length as a function of waveguide Pectlet number, uniquely fluid-optical effects are demonstrated including irreversible evanescent coupling and dynamic mode field matching
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