Induced solitons formed by cross polarization coupling in a birefringent cavity fiber laser

We report on the experimental observation of induced solitons in a passively mode-locked fiber ring laser with birefringence cavity. Due to the cross coupling between the two orthogonal polarization components of the laser, it was found that if a soliton was formed along one cavity polarization axis, a weak soliton was also induced along the orthogonal polarization axis, and depending on the net cavity birefringence, the induced soliton could either have the same or different center wavelengths to that of the inducing soliton. Moreover, the induced soliton always had the same group velocity as that of the inducing soliton. They form a vector soliton in the cavity. Numerical simulations confirmed the experimental observations.

💡 Research Summary

This paper reports the first experimental observation of induced solitons in a passively mode‑locked fiber ring laser with a birefringent cavity. By incorporating a polarization‑maintaining fiber and a polarization controller into the cavity, the authors were able to adjust the net cavity birefringence and thus control the relative phase and amplitude of the two orthogonal polarization components. When a strong soliton forms on one polarization axis (e.g., the horizontal axis), a much weaker soliton is simultaneously generated on the orthogonal axis (vertical) through cross‑phase modulation (XPM). The induced soliton always travels with the same group velocity as the primary soliton, resulting in a temporally locked pair that constitutes a vector soliton.

Two distinct regimes were identified depending on the net birefringence. In the low‑birefringence case, the induced soliton shares the same central wavelength as the primary soliton, indicating perfect spectral synchronization. When the birefringence is increased, the induced soliton’s central wavelength shifts by about 0.2 nm relative to the primary soliton, while still maintaining group‑velocity locking. This wavelength offset arises from the balance between XPM‑induced phase shifts and the differential group‑delay introduced by birefringence.

The experimental setup consists of a 10 m erbium‑doped fiber (EDF) gain section, ~1.5 km of standard single‑mode fiber (SMF), and a polarization‑maintaining segment that defines the cavity’s birefringence. Passive mode‑locking is achieved via nonlinear polarization rotation (NPR), and the output is monitored with an optical spectrum analyzer, high‑speed photodetector, and a polarization analyzer. By varying the polarization controller, the authors could reproducibly switch between the two regimes and verify that the induced soliton’s power is typically 10 dB lower than that of the primary soliton.

To corroborate the observations, the authors performed numerical simulations based on the coupled nonlinear Schrödinger equations (CNLSE) for the two polarization components. The model includes measured values for the nonlinear coefficient, dispersion, gain, loss, and birefringence. Simulations starting from a strong pulse in one polarization and noise in the orthogonal polarization reproduce the spontaneous growth of a weak, group‑velocity‑locked soliton. The simulated spectra and wavelength shifts match the experimental data, confirming that XPM is the dominant mechanism for soliton induction.

The findings have several implications. First, they demonstrate that a soliton in one polarization can “drag” a soliton in the orthogonal polarization without external seeding, offering a new route to generate vector solitons in fiber lasers. Second, the ability to control whether the two solitons share the same wavelength or are spectrally offset provides a tool for polarization‑division multiplexing schemes where inter‑polarization coupling can be exploited for signal processing or stabilization. Third, the group‑velocity locking ensures that the two components remain temporally overlapped, which could improve pulse energy scaling and spectral broadening in ultrafast fiber sources.

In conclusion, the paper establishes that cross‑polarization coupling in a birefringent cavity can induce a weak soliton that co‑propagates with a strong soliton, forming a stable vector soliton pair. The experimental results, supported by rigorous numerical modeling, deepen our understanding of nonlinear polarization dynamics in fiber lasers and open avenues for advanced polarization‑controlled ultrafast photonic devices. Future work may explore active control of the induced soliton’s amplitude, interactions among multiple vector solitons, and integration of this mechanism into practical communication and sensing systems.