A team at Northwestern Polytechnical University (China) has demonstrated near-chirp-free solitons in normal-dispersion, hybrid-structure fiber lasers that contain a few meters of polarization-maintaining fiber. The solitons typically have a bandwidth and duration of 0.74 nm and 1.95 ps, respectively. Although conventional and dissipative solitons can be created by managing the dispersion and nonlinearity of fiber lasers, the generation of chirp-free solitons in all-normal-dispersion fiber lasers remains a challenge for ultrafast optics. The chirp-free pulse is called a birefringence-managed soliton because its formation relies on birefringence-related phase-matching theory. To meet the periodic boundary condition of the fiber laser, the pulse is based on a phase-matching principle that incorporates the birefringence, normal-dispersion, and nonlinear effects. “Such pulse is termed as birefringence-managed soliton because its two orthogonal-polarized components propagate in an unsymmetrical ‘X’ inside the polarization-maintaining fiber, partially compensating the group delay difference induced by the chromatic dispersion and resulting in the self-consistent evolution,” the researchers said. A schematic diagram of normal-dispersion fiber lasers. Simulated spectra (b) and temporal profiles (c) of a birefringence-managed soliton and its two orthogonal-polarized components. The diagram depicts a research development that could support both laser physics and soliton mathematics. Courtesy of D. Mao, Z. He, Y. Zhang, Y. Du, C. Zeng, L. Yun, Z. Luo, T. Li, Z. Sun, and J. Zhao. Professors Dong Mao and Jianlin Zhao led the work, and when they and their team simulated the evolution of the pulse, they found that the phase-matching effect confined the spectrum broadened by self-phase modulation, and that the saturable absorption effect reduced the pulse stretched by normal dispersion. This mechanism ultimately enabled the team to generate the near-chirp-free soliton. The birefringence-managed solitons exist universally in the normal-dispersion regime, independent of the cavity length or mode-locking elements in the single-mode fiber/polarization-maintaining fiber lasers. The researchers achieved similar birefringence-managed solitons in four different normal-dispersion fiber lasers with cavity lengths spanning 7.8 to 25.6 m. Analysis supporting the experimental observations confirmed that the birefringence could be adapted to shape solitons at the normal-dispersion regime. The researchers configured the laser cavity to include a section of gain fiber, a saturable absorber, an output coupler, a polarization-maintaining fiber, and standard low-birefringent, single-mode-fibers. A polarization-insensitive isolator in the cavity ensured the unidirectional propagation of the pulse. “When the pulse circulates in the cavity, the coupling behavior from single-mode fiber to polarization-maintaining fiber must be considered,” the researchers said. “The laser operation depends on the angle between y-polarized component and fast axis of polarization-maintaining-fiber. For example, near-chirp-free birefringence-managed solitons can be formed when the angle ranges from 0.1 π to 0.4 π, and giant-chirp dissipative solitons are achieved for the angle of 0 or 0.5 π.” The proposed approach could be used to directly generate near-chirp-free solitons in normal-dispersion fiber lasers without external compression — which could be advantageous for wavelength bands below 1.3 μm, where the silica fiber dispersion is typically normal. The technique could be extended to propagate near-chirp-free solitons over long distance in normal-dispersion regimes with periodically spaced single-mode and polarization-maintaining fibers. From its fiber laser, which delivered switchable, near-chirp-free birefringence-managed solitons and giant-chirp dissipative solitons, the team believes its device and method could serve as a flexible, multifunctional pulse source. The pulse energy of birefringence-managed solitons is confined to a limited level, so high-order harmonic mode-locking could be achieved in long-cavity normal-dispersion fiber lasers, using the researchers’ approach. The properties of the birefringence-managed soliton, and the way that it is formed, are fundamentally different from other types of pulses in mode-locked fiber lasers. As such, the singular qualities of the birefringence-managed soliton could open new avenues of research in laser physics, soliton mathematics, and related applications. The research was published in Light: Science & Applications (www.doi.org/10.1038/s41377-022-00713-y).