Femtosecond Laser Microfabricated Optofluidic Mach–Zehnder Interferometer for Refractive Index Sensing

An approach for refractive index (RI) sensing is proposed using a waveguide-based optofluidic Mach-Zehnder interferometer (MZI). In this optofluidic MZI, the sensing arm is an epoxy-based negative photoresist (SU-8) core fabricated with femtosecond laser-induced two-photon polymerization technique, while the reference arm is the glass substrate. By integrating the SU-8 core with a microchannel, a RI sensor is realized with which the RI of sodium chloride (NaCl) solution has been measured. Effects of the chip length, microchannel length, and core size on the sensitivity of RI measurement have been revealed, indicating a shorter chip length, longer microchannel, and smaller core size can achieve a higher sensitivity, which is advantageous for sensing applications.