An ultra-compact multimode interference coupler as an optimum all-optical switch based on nonlinear modal propagation analysis

This paper proposes modal propagation analysis (MPA) as an advantageous approach to studying an all-optical switch based on a small-dimension multimode interference (MMI) coupler at the threshold of the nonlinear regime. The finite-difference method is applied as a rigorous numerical method of solving the nonlinear modal equations and measuring the modal propagation constant. The characterizations of two initial modes demonstrate the changes that are induced by nonlinear effects, such as the conversion of a sinusoidal profile to a Gaussian profile in the direction of propagation, the high oscillation of the induced phase, and a variable shift in wavelength. Furthermore, all of the abovementioned changes manifest differently for each mode. The Gaussian profiles of guided modes accompanied by other observed phenomena lead to more efficient interferences among the modes to demonstrate switching applications at a small MMI length scale. The procedure for designing an optimum switch requires the implementation of a series of several studies of the switching operation based on related parameters, such as the contrast ratio between the ON and OFF outputs, which is called the switching performance gain (SPG); this parameter is used to optimize the switch via the output width, and the insertion loss is used in the same manner. The results indicate the efficiency of the approach at an MMI length of a few micrometers and indicate that SPG is sensitive to both output width and input intensity. To our knowledge, this is the first study to demonstrate power switching using nonlinear modal propagation analysis in the low-intensity nonlinear regime in a configuration created by guided-mode interferences.