Change in the electromagnetic field frequency because of stimulated radiation in a medium

The spectrum of a two-photon electromagnetic field in a transparent dispersing medium consists of three branches. Excitation of the secondary branches of the spectrum implies a change in the electromagnetic field frequency during passage through a medium of finite size if excited impurity atoms are present.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 5, pp. 65–69, May, 1982.I am grateful to the participants of the seminar supervised by V. L. Bonch-Bruevich for favorable remarks.