Over the past decades, researchers have demonstrated conventional solitons, stretched pulses, self-similar pulses, and dissipative solitons by managing the dispersion and nonlinearity of fiber lasers.
However, new types of robust pulses were less discovered in mode-locked fiber lasers since dissipative solitons were reported in ~2000s. On the other hand, the self-similar pulses and dissipative solitons formed in normal-dispersion regime are highly chirped, and direct generation of chirp-free solitons without external compression in all-normal-dispersion fiber lasers remains a long-term challenge in ultrafast optics.
In a new paper published in Light Science & Application, a team of scientists, led by Professor Dong Mao and Jianlin Zhao from Key Laboratory of Light Field Manipulation and Information Acquisition, Ministry of Industry and Information Technology, Northwestern Polytechnical University, and co-workers have reported a new class of chirp-free pulse in normal-dispersion fiber lasers containing a section of polarization-maintaining fiber, which is termed as birefringence-managed soliton since its formation relies on the birefringence-related phase-matching theory. Simulation results fully reproduce the experimental observations, and show that two orthogonal-polarized components of pulse propagate in a "X" manner in the polarization-maintaining fiber, partially compensating the group delay dispersion to achieve unique vector soliton. The property and formation mechanism of birefringence-managed soliton fundamentally differ from other types of pulses in mode-locked fiber lasers, which will open new research branches in laser physics, soliton mathematics, and their related applications.
The configuration of the laser cavity is shown in Fig. 1(a), which includes a section of gain fiber, a saturable absorber, an output coupler, and a few meters of polarization-maintaining fiber. The other fibers and pigtails of fiber components are the standard low-birefringent single-mode-fibers. A polarization-insensitive isolator ensures the unidirectional propagation of pulse. These scientists summarize the operational principle of their fiber laser:
"When the pulse circulates in the cavity, the coupling behavior from single-mode-fiber to polarization-maintaining-fiber must be considered. 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 (Fig. 1(b) and 1(c)) 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 bandwidth and duration of the typical mode-locked birefringence-managed solitons are 0.74 nm and 1.95 ps respectively, giving the time-bandwidth product of 0.41 and confirming the near-chirp-free property."
"Numerical results and theoretical analyses fully reproduce and interpret the experimental observations, and show that, the fiber birefringence, normal-dispersion, and nonlinear effect follow a phase matching principle, enabling the formation of the near-chirp-free soliton. Specifically, the phase matching effect confines the spectrum broadened by self-phase modulation and the saturable absorption effect slims the pulse stretched by normal dispersion." they added.
"Such pulse is termed as birefringence-managed soliton because its two orthogonal-polarized components propagate in an unsymmetrical "X" manner inside the polarization-maintaining fiber, partially compensating the group delay difference induced by the chromatic dispersion and resulting in the self-consistent evolution." the scientists explain.
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