Sensitive fluorescence detection with microstructured optical fibers

Abstract

Optical fibers are ideal for environmental sensing applications because of their ability to transmit optical signals to and from the sensing region without the use of free-space optics. By accessing the evanescent field, the fiber itself can act as the sensing element and long interaction lengths can be achieved. Microstructured optical fibers are particularly suited to such applications as the species of interest can occupy the air spaces within the fiber. Here we use a suspended nanowire design that provides the high evanescent overlap of a nanowire with the long interaction length and robustness of a conventional fiber. The fluorescence-detection approach proposed in this paper is attractive because of its simplicity. When one end of the fiber is dipped into the sample, capillary forces draw the liquid into the voids within the fiber. The evanescent field of the pump light excites the fluorescent labels and a portion of the fluorescence is captured by the fiber core and propagates to the fiber tips. The aim of this work is to improve the sensing architecture to increase the sensitivity of the sensor, and to examine the primary factors such as the background glass fluorescence currently restricting the detection limit in this type of sensor. This work is focused on biological detection in liquid samples using Quantum dots, but through careful selection of the fluorescent dye this can be extended to a large number of alternative applications. Using this system we are able to detect quantum dot concentrations as low as 10 pM. © 2011 SPIE.