Zamboni, Riccardo (2016) Optofluidic application of a Mach-Zehnder interferometer integrated in lithium niobate for droplet sensing. [Magistrali biennali]
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The aim of this thesis' work is to realize and characterize an integrated detector for the measurements of speed and volume of droplets. The measure is simultaneously made on a flux of droplets moving in microchannel, detecting single droplet one by one. The device consists in a novel configuration chip made of lithium niobate, on which a microfluidic circuit and an optical waveguide Mach-Zehnder interferometer (MZI) are integrated on the same platform. The system is composed of two main stages: Optical and Microfluidic. The first is a waveguide that divides in two y-branches and two parallel arms to configure MZI. The arms of interferometer are crossed orthogonally by a microfluidic channel, in which droplets are generated by Microfluidic Stage. This one consists in a cross junction, where droplets are produced by cross-flow of immiscible phases. During the flowing of droplets in the channel, arms of MZI illuminate alternately dispersed and continuous phases. In this way, an intensity signal is collected at the end of MZI, where all information of the droplets can be read thanks to the difference in diffraction index between two liquids. This novel configuration has several advantages. It can allow for different functionalities such as: measure of droplet speed: it can be detected the time of passing droplet in front of guide; measure of droplet length: it can be easily provided from the time of the flowing of single menisci, both ahead and back; measure of droplet refraction index: with interferometric signal of the different liquids in front of arms. In this work the geometry of the two main stages are deep investigated in order to optimize the performance of this device. Both cross-junction and MZI in waveguide were already studied in literature, but a low-loss MZI design for optofluidic droplet application has not been mentioned yet. In this work all elements of MZI in waveguide are accurately described with care for low-loss and compatibility with droplet size: from Y-branches to tapered section. Both theoretical and fabrication process studies have been realized on all of key section of MZI configuration. All of them were fabricated in single mode channel waveguide at a wavelength of 632.8 nm by titanium in-diffusion. Subsequently a wide analysis was made in order to guarantee reproducibility: they were optically characterized with Near Field technique. Results were reported and reached performance are highlighted. After the best configuration was obtained, the optical signal of the flowing droplets have been studied. To reach this goal, single waveguide has been used as optical stage, properly coupled with microfluidic one. These samples have helped to understand optofluidics interaction between droplet and guided beam. Promising results have been achieved in this configuration and presented in thesis. First results and tests are reported in this work for MZI configuration with air droplet, four working regimes are founded and studied. The device has been tested with random sequence of droplets in the same running section and MZI signal demonstrated that it can recognize all these droplet in length and velocities reconstructing the sequence.
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