Multimode interference occurring in a singlemode-multimode-singlemode fiber structure is numerically investigated with a wide-angle beam propagation method using the cylindrical coordinate. Two application examples in optical sensing are presented. 1) The wavelength sensitivity of the singlemode-multimode-singlemode structure is utilized and with an appropriate length of the multimode fiber section, the device is developed as an edge filter for a ratiometric wavelength measurement system. 2) When the multimode fiber cladding is removed, it becomes the singlemode-multimode core-singlemode structure and can be developed as an all-fiber refractometer sensor with an optimal length of multimode core section. The designed refractometer can offer an estimated resolution of about 5.4x10-5 for refractive index from 1.33 to 1.45, which is competitive as compared to existing waveguide based optical refractometer sensors while offering a much simpler configuration.© (2006) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.