Wavelength tuning of multimode interference bandpass filters by mechanical bending: experiment and theory in comparison

We present the tuning of multimode interference bandpass filters made of standard fibers by mechanical bending. Our setup allows continuous adjustment of the bending radius from infinity down to about 5?cm. The impact of bending on the transmission spectrum and on polarization is investigated experimentally, and a filter with a continuous tuning range of 13.6?nm and 86?% peak transmission was realized. By use of numerical simulations employing a semi-analytical mode expansion approach, we obtain quantitative understanding of the underlying physics. Further breakdown of the governing equations enables us to identify the fiber parameters that are relevant for the design of customized filters.