Observation of dark-bright vector solitons in fiber lasers

Solitons are localized nonlinear waves that maintain their shape while propagating in dispersive media. They are as a result of the cancellation of the nonlinear and dispersive effects in the medium that provides solutions to partial differential equations used to describe various physical systems. In particular, soliton formation and dynamics in single-mode fibers have attracted significant research attention. This is a main area of interest in nonlinear optics studies.

Presently, the nonlinear Schrödinger equation is mostly used to describe light propagation in single-mode fibers. This can be attributed to the role of the natural balance between the fiber dispersion and self-phase modulation in the formation of solitons. This applies to both bright and dark solitons. And the former is mostly formed in the anomalous dispersion fibers while the latter are formed in normal dispersion fibers.

Previously, slight birefringence has been observed in single-mode fibers due to various factors including imperfections, effects of fiber bending and fiber strains. On the other hand, the coupled nonlinear Schrödinger equations have been used to describe light propagation in weakly birefringent fibers about self-phase modulation and cross-polarization phase modulation. Alternatively, theoretical studies have shown the ability to form vector solitons with mutually coupled polarization components through cross-polarization coupling. However, experimental observations of coherently coupled dark-bright vector solitons in single-mode fibers have not been reported so far.

To this note, researchers at Jiangsu Normal University: Professor J. Ma, Professor Luming Zhao, and Professor Deyuan Shen together with Dr. Guodong Shao, Dr. Yufeng Song, Dr. Y. J. Xiang and Professor M. Richardson, and led by Professor Dingyuan Tang at Nanyang Technological University, investigated the coherently coupled dark-bright vector solitons in a single-mode fiber laser using either anomalous or normal cavity dispersion. They also investigated the properties of the vector solitons and compared them with those provided in the literature based on the coupled nonlinear Schrödinger equations. The work is currently published in the journal, Optics Letter.

Briefly, the research team cross-examined the features of dark-bright vector solitons provided in the literature. To overcome the challenges involved in the experimental observation of dark-bright vectors solitons, an innovative approach was adopted. The light was circulated in an active fiber ring cavity instead of traversing over long fiber length. The approach enables fine-tuning of the effective cavity dispersion and birefringence.

The authors confirmed the existence of new forms of optical solitons in coupled nonlinear systems. For instance, dark-bright vector solitons were observed in single-mode fiber lasers with either normal or anomalous dispersion cavity. These solitons exhibited properties that are exactly similar to those found in theoretical predictions based on the coherently coupled nonlinear Schrödinger equations.

In summary, the study by Professor Dingyuan Tang and his colleagues is the first to experimentally confirm the existence of dark-bright vector solitons in single-mode fiber lasers. In general, the coupled nonlinear Schrödinger equations can be used to describe the properties of a wide range of physical systems. Also, the dark-bright vector soliton formation comprises of new nonlinear wave dynamic phenomenon. Therefore, the study provides vital information that will be used in future studies for multi-compound soliton formation in other physical systems.