Experimental Investigation of Fused Biconical Fiber Couplers for Measuring Refractive Index Changes in Aqueous Solutions

A detailed experimental study of a simple and compact fiber optic sensor based on a fused biconical fiber coupler is presented, in which the sensitivity is improved by operating the coupler beyond the first coupling cycle. The sensor is demonstrated to perform high sensitivity measurements of refractive index changes by means of variation of sugar concentration in water. The device is operated to achieve a linear transmission response, allowing a linear relation between the sugar concentration and the output signal. The initial sensitivity was measured as 0.03 units of normalized transmission per unit of sugar concentration (g/100 mL), with a noise detection limit of a sugar concentration of 0.06 wt% of sugar concentration. Improvements in sensitivity were studied by operating the coupler beyond their first coupling cycles; achieving an improved sensitivity of 0.15 units of normalized transmission per unit of sugar concentration, and a minimum detection limit of 0.012 wt% of sugar concentration. From this result, the minimum detectable refractive index change is estimated as 2 × 10
^-5
refractive index unit.