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 indexmeasurements in MMI devices lead us to perform an etching of the PMMAlayer. 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.35139FCT-Fundacao para a Ciencia e Tecnologia through Portuguese national funds [UID/EEA/50008/2013]hiPOF Ph.D. Scholarship [PTDC/EEI-TEL/7134/2014, SFRH/BD/88472/2012, IF/01664/2014]INITIATEFINEP [0112039300]