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Widely stretchable soliton crystals in a passively mode-locked fiber laser.

We present the first direct demonstration of a new type of stable and extremely elastic soliton crystals, the bond length and bond strength of which can be individually controlled in a wide range. The stretching and compressing can be repeated many times, conserving the overall structure by incorporating a highly asymmetric tunable Mach-Zehnder interferometer into a specially designed passively mode-locked fiber laser. The temporal structure and dynamics of the generated soliton crystals were measured using an asynchronous optical sampling system with picosecond resolution. We demonstrated that a stable and robust soliton crystal can be formed by two types of primitive structures: single dissipative solitons and (or) pairs each consisting of a dissipative soliton and a pulse with a lower amplitude. Continuous stretching and compression of the soliton crystal by an extraordinarily high factor of more than 30 has been demonstrated, the smallest recorded separation between the pulses being as low as 5 ps, corresponding to an effective repetition frequency of 200 GHz. Collective pulse dynamics, including soliton crystal cracking and transformation of crystals comprising high/low-amplitude pulse pairs to the crystals of similar pulses, has been observed experimentally.