We report the fabrication of a multimode
interferometer (MMI), based on a multimode polymer optical fiber (POF), sandwiched between two single-mode silica fibers. The POF used in this work is a step-index fiber with core and cladding composed of a cycloolefin polymer (COP) and PMMA respectively. The low moisture absorption of the COP material and the benefit of having an exposed core for refractive index measurements in MMI devices lead us to perform an etching of the PMMA layer. Additionally, we report the inscription of a Bragg grating in the COP core material of an etched single- mode-multimode-single-mode (SMS) structure. The two fiber structures were simultaneously characterized to strain, temperature, humidity and refractive index. The ability to detect refractive index by using a POF based SMS structure is reported for the first time in this paper. Additionally the capability for dual parameter measurement was discussed using both FBG and SMS structures.