We present an energy-efficient method for conversion of relatively long (nanosecond) optical pulses into an extraordinary light structure-a packet of ordered picosecond pulses which differs from the known types of ordered ultrashort pulse patterns (like soliton molecules). The method relies on revealed peculiarities of nonlinear evolution of an ultrashort dark pulse implanted in a nanosecond bright pulse when they propagate in an optical fiber. Under certain conditions, energy of the background nanosecond pulse, which initially contains a single ultrashort dark pulse, can be mostly converted into a structured burst of ultrashort bright pulses. This burst can feature relatively high (manifold of the initial value) peak power and ultrahigh (sub-THz) intraburst pulse repetition. This dark pulse evolution develops at prorogation distances of a few tens of nonlinear lengths in telecom fibers under conditions of anomalous dispersion. Thus, it can be considered on the one hand as an important limitation for fiber transmission of particular optical waveforms, and on the other hand as a promising method for ultrashort pulse bursts generation.