Following base calculations about waveguide and working mode(TE/TM), Mach-Zehnder  interferometer can be designed and simulated in Lumerical Interconnect via using neff and ng values. 
Basic Mach-Zehnder interferometer setup is given below. According to the phase properties of sample, constructive or destructive interference can be observed in outputs. In bulk optics, each beam splitter apply 90o phase shift to reflected light.