"In this thesis various techniques to enhance the interaction of the evanescent waves in an optical fiber with the surrounding media are investigated. In most cases the coupling and overlapping between core cladding modes were exploited. A detailed description of several fiber-based devices, their applications, existing problems, and possible solutions, are presented placing emphasis on the mechanism to achieve strong evanescent field to enhance the sensor sensitivity. The thesis also presents a thorough literature investigation to describe the working principle of each device. It focuses on the fundamentals for the excitation of core-cladding modes in conventional optical fibers and photonic crystal fibers. The thesis begins with a theoretical investigation of different mechanisms for evanescent interaction of a guided wave with the external media, and in the particular case of optical fibers, the strategies that have been explored to enhance the evanescent fields. The basic concepts to understand the design and manufacturing of optical fiber devices are described. Fundamental aspects of couplers, wavelength filters, fiber tapers and long-period gratings are presented. It then focuses on two experimental demonstrations based on long-period fiber gratings in combination with fiber tapers. A novel scheme for tuning the resonance wavelength in a fiber grating and a fiber interferometer for refractive index sensing are presented. More specifically, long-period fiber gratings made by mechanical pressure over conventional single-mode fibers are reported. First, the characteristics of a mechanically induced long-period fiber grating made by pressing a pair of grooved plates over single-mode fiber tapers are analyzed. Robust fiber tapers with diameters ranging from 90 to 125 μm were used in order to shift the resonance wavelengths toward shorter wavelengths. It was found that this technique is particularly suitable for tuning the resonance wavelengths to shorter wavelengths below the limit imposed by the grooved plate period."