Recent advancement in technology has increased the need to enhance the capacity of optical fiber communications. As such, the propagation of optical pulses in multimode fiber laser have attracted significant interest among researchers. For instance, the use of space division multiplexing method which involves spatial modes propagating in multimode fibers have been developed for improving the efficiency of optical fibers communications.
In a recently published literature, a spatiotemporal mode-locked multimode fiber laser is proposed and demonstrated experimentally. This has highly benefited the field of telecommunications and high-power fiber laser sources and most importantly, it has provided the much-needed foundation for investigating the nonlinear dynamics in optical fibers. Consequently, soliton molecules have been widely associated with nonlinear fiber optics. In analogy to molecules in chemistry, soliton molecules (also called boundary soliton state) consist of several individual solitons, balancing by repulsive and attraction forces. Owing to its unique properties, it has potential applications in improving ultrafast characterization as well as in optical fiber transmission. Currently, they have been observed in single-mode fiber lasers. However, little emphasis have been given to investigating nonlinear dynamics in spatiotemporal mode-locked multimode fiber.
Recently, a group of researchers at Tsinghua University: Mr. Huaqiang Qin, Professor Xiaosheng Xiao, Mr. Pan Wang, and Professor Changxi Yang experimentally investigated nonlinear dynamics in spatiotemporal mode-locked multimode fibers lasers. They observed the soliton molecules which are increasingly important in optical fiber applications. Their research work is currently published in the research journal, Optics Letters.
Briefly, the authors used the nonlinear polarization rotation method for spatiotemporal mode locking. Meanwhile, spatial and spectral filters were used for stabilization. They further observed soliton molecules as well as spatiotemporal mode-locking state through adjustments on the waveplates cavity. Eventually, they measured the beam profiles and spectra of the observed molecules and also compared the corresponding spatial of various beam parts and the autocorrelation traces in the same bund state.
From the conducted experiment, the authors observed soliton molecules with various self-organizations. This included soliton pairs, soliton quartets, soliton triplets with each having pulse separations different from the others. Additionally, they presented the transition states of different operations with pump power. The beam profiles indicated the distribution of the beam energy which corresponded to the distribution of the transverse modes. Different transverse modes have different spectral regions and different resonant frequency. Furthermore, for the continuous wave state near the spatiotemporal mode locking, the cavity energy is sufficient to support more pulses. This is attributed to the fact that the soliton molecules emerge directly from continuous wave, unlike in the case for the single-mode fiber lasers.
The study by Professor Xiaosheng Xiao and his colleagues is the first to experimentally observe the soliton molecules in the spatiotemporal mode-locked multimode fiber laser. The obtained experimental result is a significant contribution towards understating of the complex nonlinear dynamics of the propagation of pulses in the multimode fibers. It will also enhance its various applications in fiber lasers sources and telecommunications.
Qin, H., Xiao, X., Wang, P., & Yang, C. (2018). Observation of soliton molecules in a spatiotemporal mode-locked multimode fiber laser. Optics Letters, 43(9), 1982.Go To Optics Letters