Vector soliton
In physical optics or wave optics, a vector soliton is a solitary wave with multiple components coupled together that maintains its shape during propagation. Ordinary solitons maintain their shape but have effectively only one polarization component, while vector solitons have two distinct polarization components. Among all the types of solitons, optical vector solitons draw the most attention due to their wide range of applications, particularly in generating ultrafast pulses and light control technology. Optical vector solitons can be classified into temporal vector solitons and spatial vector solitons. During the propagation of both temporal solitons and spatial solitons, despite being in a medium with birefringence, the orthogonal polarizations can copropagate as one unit without splitting due to the strong cross-phase modulation and coherent energy exchange between the two polarizations of the vector soliton which may induce intensity differences between these two polarizations. Thus vector solitons are no longer linearly polarized but rather elliptically polarized.
Definition
C.R. Menyuk first derived the nonlinear pulse propagation equation in a single-mode optical fiber under weak birefringence. Then, Menyuk described vector solitons as two solitons with orthogonal polarizations which co-propagate together without dispersing their energy and while retaining their shapes. Because of nonlinear interaction among these two polarizations, despite the existence of birefringence between these two polarization modes, they could still adjust their group velocity and be trapped together.Vector solitons can be spatial or temporal, and are formed by two orthogonally polarized components of a single optical field or two fields of different frequencies but the same polarization.
History
In 1987 Menyuk first derived the nonlinear pulse propagation equation in SMF under weak birefringence. This seminal equation opened up the new field of "scalar" solitons to researchers. His equation concerns the nonlinear interaction between the two orthogonal polarization components of the vector soliton. Researchers have obtained both analytical and numerical solutions of this equation under weak, moderate and even strong birefringence.In 1988 Christodoulides and Joseph first theoretically predicted a novel form of phase-locked vector soliton in birefringent dispersive media, which is now known as a high-order phase-locked vector soliton in SMFs. It has two orthogonal polarization components with comparable intensity. Despite the existence of birefringence, these two polarizations could propagate with the same group velocity as they shift their central frequencies.
In 2000, Cundiff and Akhmediev found that these two polarizations could form not only a so-called group-velocity-locked vector soliton but also a polarization-locked vector soliton. They reported that the intensity ratio of these two polarizations can be about 0.25-1.00.
However, recently, another type of vector soliton, "induced vector soliton" has been observed. Such a vector soliton is novel in that the intensity difference between the two orthogonal polarizations is extremely large. It seems that weak polarizations are ordinarily unable to form a component of a vector soliton. However, due to the cross-polarization modulation between strong and weak polarization components, a "weak soliton" could also be formed. It thus demonstrates that the soliton obtained is not a "scalar" soliton with a linear polarization mode, but rather a vector soliton with a large ellipticity. This expands the scope of the vector soliton so that the intensity ratio between the strong and weak components of the vector soliton is not limited to 0.25-1.0 but can now extend to 20 dB.
Based on the classic work by Christodoulides and Joseph, which concerns a high-order phase-locked vector soliton in SMFs, a stable high-order phase-locked vector soliton has recently been created in a fiber laser. It has the characteristic that not only are the two orthogonally polarized soliton components phase-locked, but also one of the components has a double-humped intensity profile.
The following pictures show that, when the fiber birefringence is taken into consideration, a single nonlinear Schrödinger equation fails to describe the soliton dynamics but instead two coupled NLSEs are required. Then, solitons with two polarization modes can be numerically obtained.
Image:Vectorsoliton origin in 2009.jpg|500px|Why vector solitons are generated?
FWM spectral sideband in vector soliton
A new pattern of spectral sidebands was first experimentally observed on the polarization-resolved soliton spectra of the polarization-locked vector solitons of fiber lasers. The new spectral sidebands are characterized by the fact that their positions on the soliton's spectrum vary with the strength of the linear cavity birefringence, and while one polarization component's sideband has a spectral peak, the orthogonal polarization component has a spectral dip, indicating the energy exchange between the two orthogonal polarization components of the vector solitons. Numeric simulations also confirmed that the formation of the new type of spectral sidebands was caused by the FWM between the two polarization components.Bound vector soliton
Two adjacent vector solitons could form a bound state. Compared with scalar bound solitons, the polarization state of this soliton is more complex. Because of the cross interactions, the bound vector solitons could have much stronger interaction forces than can exist between scalar solitons.Vector dark soliton
Dark solitons are characterized by being formed from a localized reduction of intensity compared to a more intense continuous wave background. Scalar dark solitons can be formed in all normal dispersion fiber lasers mode-locked by the nonlinear polarization rotation method and can be rather stable. Vector dark solitons are much less stable due to the cross-interaction between the two polarization components. Therefore, it is interesting to investigate how the polarization state of these two polarization components evolves.In 2009, the first dark soliton fiber laser has been successfully achieved in an all-normal dispersion erbium-doped fiber laser with a polarizer in cavity. Experimentally finding that apart from the bright pulse emission, under appropriate conditions the fiber laser could also emit single or multiple dark pulses. Based on numerical simulations we interpret the dark pulse formation in the laser as a result of dark soliton shaping.