Whispering Gallery Mode Dip Sensor for Aqueous Sensing

We report fabrication of a 4 mm thick, preassembled whispering gallery mode (WGM) sensor that can be repeatedly dipped into aqueous solutions and lifted. We built the viable photonic sensor assembly by bending an optical fiber by 90° and molding its tip into a sphere, thus, a long stem holding a submillimeter sensor at the end of a short arm of fiber, and positioning a pair of parallel cone-tipped tapers along the long stem so that the tips touch the sensor. Our sensor head is an optical fiber device just a few millimeters thick and yet has a sensitivity of the resonance wavelength shift comparable to the one obtained with conventional WGM sensors in a planar arrangement. Since dipping and lifting from the solution changes the temperature of the sensor, affecting the resonance wavelength, we enclosed a thermistor within the sensor head to monitor the temperature. We demonstrate that the resonance shift in repeated transfer of the sensor head between water and a solution of sucrose, after correction by the temperature change, is reproducible and agrees with a theoretical estimate of the shift for different concentrations.