Combining whispering gallery mode lasers and microstructured optical fibers: limitations, applications and perspectives for in-vivo biosensing

Abstract

Whispering gallery modes (WGMs) have been widely studied over the past 20 years for various applications, including biological sensing. While the WGM-based sensing approaches reported in the literature have shown tremendous performance down to single molecule detection, at present such sensing technologies are not yet mature and still have significant practical constraints that limit their use in real-world applications. Our work has focused on developing a practical, yet effective, WGM-based sensing platform capable of being used as a dip sensor for in-vivo biosensing by combining WGM fluorescent microresonators with silica Microstructured Optical Fibers (MOFs). We recently demonstrated that a suspended core MOF with a dye-doped polymer microresonator supporting WGMs positioned onto the tip of the fiber, can be used as a dip sensor. In this architecture the resonator is anchored to one of the MOF air holes, in contact with the fiber core, enabling a significant portion of the evanescent field from the fiber to overlap with the sphere and hence excite the fluorescent WGMs. This architecture allows for remote excitation and collection of the WGMs. The fiber also permits easy manipulation of the microresonator for dip sensing applications, and hence alleviates the need for a complex microfluidic interface. More importantly, it allows for an increase in both the excitation and collection efficiency compared to free space coupling, and also improves the Q factor. In this paper we present our recent results on microstructured fiber tip WGM-based sensors and show that this sensing platform can be used in clinical diagnostics, for detecting various clinically relevant biomarkers in complex clinical samples.