This paper, propose a novel approach to access all optical switching performance usable in telecommunications and
photonic signal processing circuits by one continuous MMI coupler under a nonlinear regime to achieve the switching at smallest length of Multimode interference waveguide. In past methods, there have been MMI switch with length about a few millimeters, but in this paper an MMI is presented for switching purposes with a length less than 100μm. In this approach, the wave propagation is studied base on MPA method. The nonlinear wave equation in presence of Kerr nonlinear effect is solved as a set of multiple coupled nonlinear equations, so each equations are related to one of guided modes in Multimode waveguide. With ability of calculating the electric field at the outputs channels, we are able to optimize the Multimode waveguide in terms of length. switching is performable with a variation of input intensity or electric field, thus the output fields have been measured with 105 different amounts of input electric fields for each lengths which are switching candidates, then switching operation is appeared in special length with its dependence input electric fields that each electric fields relate to switch the light to bar or cross outputs, this switch is the outstanding proposal for new all optical digital signal processing due to the compact size.