One way to excite a dark envelope soliton is to use a dark input pulse - a narrow dip in the amplitude of a continuous wave background. This approach has been used previously to excite dark solitons for surface spin waves in yttrium iron garnet (YIG) thin film strips¹ and for lasers in photorefractive crystals.² Theoretically, however, an initial dark signal should not evolve into a single dark soliton, but a pair of dark solitons that have opposite phase jumps and an overall phase change of zero.^3,4,5,6 Experimentally, such dark soliton pairs have been observed for solitons in various systems,^7,8,9,10 but their phase signature has never been demonstrated so far.