Discrete self-imaging in small-core optical fiber interferometers

Multiple cladding modes can exist in a small-core optical fiber unaccompanied by core modes, yet this fact has not been sufficiently explored in literature to date. In this article, we study the self-imaging of cladding modes in small core optical fiber interferometers. Our analytical and numerical simulations and experiments show that unlike the self-imaging of core modes, self-imaging of cladding modes only appears at a set of discrete positions along the interferometer axis with an equal spacing corresponding to some discrete values of fiber core radius. This is the first observation of the discrete self-imaging effect in multimode waveguides. More strikingly, the self-imaging period of cladding modes grows exponentially with fiber core radius, unlike the quadratic relationship in the case of core modes. The findings bring new insights to the mode propagation in an optical fiber with a core at micro/nanoscale, which may open new avenues for exploring multimode fiber technologies in both linear and nonlinear optics.