In this thesis, we aim to characterize optical properties of thin dielectric films coated on curved surfaces. Indeed, optical thin films attract a great deal of attention especially the ones coated on silica based optical waveguides used as sensor system. Therefore, the step index optical fiber is used in the thesis as a substrate due to the fact that the sensor technology tends towards to fiber optic based platforms. In the thesis, a step index optical fiber is coated with polyvinyl alcohol (PVA), then its thickness is mathematically estimated exploiting Fresnel scalar diffraction method. Phase front of the laser light wave comes across with a phase object (fiber optic), transmits through of it and diffracts. Whole process is modeled by using numerical analysis methods and compared to experimental results to obtain desired parameters in MATLAB. The conventional least-squares method is used for comparison purpose. Although the emphasis is on optical thin film characterization, we demonstrate the application area of diffraction from fiber optic as sensor. It is used to detect adulteration of olive oil that is big concern for the food industry. The refractive index of various mixture of olive oil and sunflower oil is measured with intend to detect adulteration. This feature makes it a good candidate for fiber optic based refractive index sensor and it may bring practicability and precision to the sensing process. This dissertation gives detailed information about diffraction from fiber optic both theoretically and experimentally. The experiments were realized by using 632.8 nm continuous wave laser. Both of the experimental results demonstrate that phase diffraction method is a powerful technique to characterize optical thin films and to sense refractive index of the surrounding medium.