Suppose that a light source with input intensity \(I_i\) and electric field  \(E_i\) is split, resulting in intensities \(I_{i1}\) and \(I_{i2}\) and electric fields \(E_{i1}\) and \(E_{i2}\).  Using a beam-combiner, we will call the output of combining  input intensities \(I_{i1}\) and \(I_{i2}\) , \(I_o\)  while we will call the output of combining electric fields \(E_{i1}\) and \(E_{i2}\), \(E_o\).

The MZI pictured below contains an input light source, a beam-splitter, a beam-combiner, and an output light-beam. Each component is connected by waveguides and in the case of the branches denoted by electric fields \(E_{i1}\) and \(E_{i2}\) the waveguides connecting the beam-splitter to the beam-combiner are identical in length and material. This what is known as a balanced MZI: the lengths from the beam splitter to the beam-combiner are equal on both branches. Under this geometry, any wavelength of light given at the input will be split and constructively combined at the output.