In this thesis, we developed an in-fiber all optical switching device exploiting a transient grating as a control mechanism that was formed by interference of laser light beams via the Kerr effect. The switching device is designed by partial removal of the fiber cladding and replacing the polished cladding with a nonlinear polymer film (slab waveguide) that exhibits higher third order (Kerr) optical nonlinearity. The proposed device structure is analyzed considering Four Wave Mixing (FWM) of Gaussian beams of the grating and propagating modes of the optical fiber in the evanescent region where the nonlinear material is placed. The fields of modes and the grating forming beams interact in the nonlinear medium according to the matching conditions and evoke power transfer among the fiber modes. Thus, the coupling between the modes is directed by means of the transient grating. In the experimental part, polymeric thin films are prepared and linear refractive indices are measured using Fresnel diffraction method (developed in the laboratory) by matching the model with the experimental output. Then, z-scan method is employed to characterize the third order nonlinear optical properties of the thin films and pump-probe experiments are exploited to ensure existence of the transient grating and its diffraction capability. Finally, the side polished fiber is coated with the Methyl Red doped PVA composite polymer and by generating transient gratings on the polymer film, the switching capability of the device is introduced. The switching can be achieved either by a bulk refractive index change (no grating) or a transient grating between the modes of the optical fiber.