Propagation constant can be changed by adjusting waveguide geometry(height is fixed, only width can be adjusted) or manipulating material properties via thermal or electrical disturbances. This configuration is called as "balanced" and "active" MZI. 
As a second option, path length difference can be created during fabrication via adjusting waveguide lengths. This configuration is called as "imbalanced" and "passive" MZI. Output intensity formula can be organized as :
\(I_{output}=\frac{I_{input}}{2}[1+cos(\beta\delta_L)]\)
\(\delta_L\) is path length difference between two arms of interferometer. Output intensity change sinusoidally dependent on propagation constant, hence on wavelength of light source. 
\(I_{output}=\frac{I_{input}}{2}[1+cos(\frac{2{\pi}n_{eff}}{\lambda}\delta_L)]\)
Theoretical calculations can be validated in Lumerical Interconnect, which simulates designed concept in system level. Part of the configuration from GDS file(standardized CAD file format), which includes optical grating fiber in TE mode to be able to transfer light effectively from optical fiber into waveguide, Y-Branch and 2 waveguides is given below.