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Using optical fiber sidewall to capture light from an adjustable depth

It is commonly accepted that light captured by the fiber core sidewall is from a depth of less than 1 μm limited by the evanescent-wave (EW) field. Yet we demonstrate in this paper that, with some simple modifications, the fiber core sidewall is also able to capture light from an adjustable depth beyond the EW field. The key elements are an air-clad fiber core cylinder; a liquid sample droplet surrounding a short segment of the core; a narrow, collimated and position-tunable excitation beam perpendicular to the fiber axis to excite the sample. A theoretical investigation of the operation of the mechanism is given, which is verified by a preliminary experiment.