Quantum model of electromagnetic (EM) signals of lasers was suggested and the properties of this model were established. Transfer from quantum model of EM signal to its classical approximation was fulfilled. Strict classical and quantum descriptions of spectral apparatus operation were given and a link between them was established for classical spectrum analyzer (SA). It was shown that this SA transforms time statistics of photons into statistics of their space distribution onto SA output plane, i.e. the link between quantum object (laser signals) and interacting with it SA was established.