Multimode interference devices with single-mode-multimode-multimode fiber structure.

We present a single-mode-multimode-multimode (SMm) fiber structure with a few interesting device applications. Unlike the single-mode-multimode-single-mode (SMS) structure, SMm has the unique feature of more than one mode in the output fiber. A detailed physical understanding of the transmission properties, and the differences from the SMS structure, is presented. The device can be used to excite selected modes in the output multimode fiber (MMF). This can be used to reduce the number of modes in high-speed MMF applications using an all-fiber structure instead of bulk optics. In yet another possible application, we show a way of designing sensitive refractive index sensors for measurement in different RI ranges.