In this work, we investigate the transformation of a squeezed by photon-number quantum state of a single-mode optical signal during phase modulation process. Within the framework of the semiclassical model of the phase  modulator, we obtained estimations for the statistical properties of individual modes and mode sub-ensembles of the signal spectrum. We shown that for the case of modulation of squeezed vacuum, the entire spectrum of the  modulated signal evaluates to the entangled state of a number of frequency modes. The results obtained in this work enable the use of squeezed modulated light states in quantum key distribution systems and various interferometric applications.