Experimental analysis of optical fiber multimode interference structure and its application to refractive index measurement

We investigated a fiber-based multimode interference phenomenon in the wavelength domain by using a white light source and an optical spectrum analyzer. This phenomenon was produced by a larger-core optical fiber joined at both ends with smaller-core optical fibers. We examined the variation of interference wavelength with changes in the length of the larger-core fiber. The interference wavelengths were blueshifted and the interference signals were sharpened with an increase in the length of the larger-core fiber. The calculated results agreed well with the measured results. Next, we investigated how the input and output fibers with a small core influence the interference signal characteristics. By comparing the amplitude differences of the interference signal we find the conditions of input and output (I/O) fibers for higher sensitivity. In addition, an interference-signal shift was observed by changing the medium surrounding an multimode interference (MMI) structure. The amount of shift increased at a longer wavelength. This leads to the sensitive detection of the refractive index. Finally, a demonstration of the optical fiber refractometer with a multimode interference structure was given by refractive-index measurements of ethanol/water solutions.