We derive in concrete the electric field through the usage of the Fourier-Bessel series that appear from the complex exponential in the most fundamental representation of an electric field emitted by an optical source and resulting in the explicit apparition of modes that encompasses the field dynamics in a vertical cavity. This artificial mathematical methodology has as an advantage the explicit exhibition of harmonics that is perceived as an inherent effect of the chirp phenomenon. Power spectrum encompasses to some extent the morphology of the one measured in lab.