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[Published Article] Xueming Liu's team published a paper in PRL to reveal the dynamics of the generation of soliton molecules
Source : Administrator   Time : 2018-07-10

       For a long time, real-time measurement of complex optical ultrafast process has been an important topic in ultrafast laser field. Recently, our group observed the complete formation of soliton molecules (SMs) in mode-locked fiber lasers for the first time. This work is published in the latest issue of Physical Review Letters [Physical Review Letters 121, 023905 (2018)], which is titled as “Real-time observation of the buildup of soliton molecules”.[pdf]

       As a type of nonlinear local wave, solitons exist widely in various subjects of physics, such as hydrodynamics, condensed matter physics, acoustics, and optics. Optical soliton has particle characteristics. Multiple solitons can form a bound state through interaction, similar to chemical molecules, which is known as soliton molecules. For a long time, the electronic instruments were hard to obtain non-repetitive transient information. Because their detection speeds are limited by mechanics or electronics. How to achieve real-time measurement of transient physical phenomena has been a great challenge for researchers. The emergence of time-stretch dispersive Fourier transform (TS-DFT) technology overcomes the speed limitation of traditional spectrometer, and can realize fast and real-time spectral measurement. In recent years, based on TS-DFT technique, real-time detection of ultrafast transient evolution of optical pulses has been realized, and new understanding of the interaction between optical solitons has been obtained. However, the complete dynamics of SMs in ultrafast optical systems has not been reported yet.

       Fig. 1. (a) The first stage of the birth dynamics of SMs: the raised relaxation oscillation (RO); (b) The peak evolution of pulse at time of 34.5 ns corresponds to data in the section with blue line box in (a); (c) The intensity distribution of the pulses propagating in the cavity in roundtrip of -570 corresponds to data in the section with red line box in (a).

       Based on TS-DFT technology, Professor Liu and his team studied the complete buildup of SMs in passively mode-locked fiber lasers. They found that the birth dynamics of a stable SM experiences five different stages, i.e., the raised relaxation oscillation (RO) stage, beating dynamics stage, transient single pulse stage, transient bound state, and finally the stable bound state, as shown in Fig. 2. In the raised RO stage, the pulse evolution follows a law that only the strongest one can ultimately survive and, meanwhile, the pulses periodically appear at the same temporal positions for all lasing spikes during the same RO stage (named as memory ability), as shown in Fig. 1(b). Moreover, they have found that the buildup dynamics of SMs is quite sensitive to both the polarization state of intracavity light and the fluctuation of pump power. These results provide new perspectives into the ultrafast transient process in mode-locked lasers and bring useful insights into laser designs and applications.

       Fig. 2. Experimental real-time interferograms during the formation and evolution of SM. They include: beating dynamics (A and B regions), transient single pulse, transient bound state, and the stable bound state.

       The above research is supported by National Natural Science Foundation of China and Key Scientific and Technological Innovation Team Project in Shaanxi Province.

       The original link:http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.121.023905