Comparative Study of Microfiber and Side-Polished Optical Fiber Sensors for Refractometry in Microfluidics

A simple theoretical analysis is undertaken to compare the change of guided mode index of microfiber and side-polished single mode fiber sensors to the refractive index of surrounding fluids. This is followed by an experimental investigation using a broadband Mach-Zehnder interferometer to compare the sensitivity of the two types of sensing fibers by measuring the interferometer response to fluid refractive indices ranging from 1.330 to 1.451. The experiments show that a microfiber sensor of diameter close to 5 $\mu{\rm m}$ and an average length of around 500 $\mu{\rm m}$ generates a response similar to that generated from a side-polished fiber sensor of length 7 mm. A compact microfluidic cell is fabricated to enable a microfiber based refractive index sensor to be used in a microfluidics environment. Experimental refractometric results obtained using this arrangement are reported.