A reflection type fiber-optic biosensor utilizing multimode interference(MMI) in coreless silica fibers has been investigated and applied to measure refractive indexes of liquids and to monitor antigen-antibody immunoreactions.It has been demostrated by the experiments that the wavelength corresponding to the interference trough in the output spectra increases linearly with the increase of refractive index of surrounding luquid when the refractive index is lower than 1.4.The sensitivity of refra...