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It is shown that inhomogeneous broadening of the spectral line of activeimpurities may sustain simultaneously parity (P) and time (T) symmetries of amedium, in a finite range of field frequencies, what is forbidden by thecausality principle in media without broadening. If a spectral width of apropagating pulse is less than the inhomogeneous broadening, the medium forsuch a pulse becomes quasi-PT symmetric. The effect of the broadband quasi-PTsymmetry in a finite frequency domain, is illustrated on examples ofunidirectional diffraction of pulses in Bragg and Laue geometries, propagatingin photonic crystals.

Broadband quasi-PT Symmetry Sustained by Inhomogeneous Broadening of the Spectral Line