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The purpose of the paper is to consider the transformation of electromagnetic wave in a medium with permittivity and conductivity changing in time as the finite sequence of rectangular periodic pulses.

Parametric phenomena in active media have been attracted attention for a long time in connection with the possibility of electromagnetic wave generation and amplification both by the interaction of charges with a spatially periodic medium [1], [2] and by the time modulation of the medium parameters [37]. In the systems with distributed parameters, nonstationarity of the medium caused by the moving of sinusoidal wave of the permittivity disturbance has mainly been considered. In this case, the solution to the problem is obtained approximately as an expansion in powers of a small parameter.

The investigation of transient electromagnetic phenomena is of importance for the problems of electro-magnetic signal controlling by the temporal adjustment of medium parameters, i.e. in optoelectronic systems [8]. One of the possible ways of such an adjustment is the changing of medium parameters by a finite sequence of pulses.

The problem of the electromagnetic wave propagation in a time-varying medium is formulated as a Volterra integral equation of the second kind [9]. The temporal variation of medium parameters is considered as a finite sequence of the periodic rectangular pulses. The solution to the problem is obtained by the Direct Numerical Calculation Method [10]. On the basis of the proposed method an original software for numerical modeling of the wave propagation in a time-varying medium has been created. The structure of the program is created allowing for the further developing and extending its functionality. In general, the software enables us to consider an initial field with an arbitrary time-spatial dependence. Here we consider two initial fields, a plane wave and a Gaussian beam.