Coupling

Coupling defines the power transferring between electromagnetic(EM) fields of different frequencies, different polarization, or different spatial modes. Typically, the wave coupling mechanism can be known as the perturbation where the polarization of the beam is slightly altered(change in polarization |∆P|<<| D|); hence the permittivity, refractive index, and the impermeability of the medium are altered accordingly [1]

The coupled-wave theorem is responsible for formulating the coupling of waves with different frequencies. It describes the entire field as a linear combination of the waves with different frequencies, each with spatially varying amplitude. The beams may comprise the dependency toward the longitudinal direction after applying the appropriate perturbation. Slowly varying amplitude approximation deduces the coupling wave equation into a first-order differential equation by addressing the practical interest, and [1] discusses the theorem behind this phenomenon.

In optical fiber communication, the coupling effect among different polarization is considered an adverse effect to the coherent detection process (detector is sensitive to the polarization and the field phase). The imperfection of the fiber geometry and the composition may behave as a perturbation, leading to the transfer of power among different polarization. Modal birefringence effect ensures the maintenance of polarization over a coherent distance by eradicating the effect of perturbation as much as possible [2]. The coherent length may be a considerable distance concerning the beat length, which experiences an exact 2π amount of linear retardation. Our conviction is that this type of polarization coupling may occur even in the radiation EM beam in the telecommunication industry.

Our objective (in the project "Spatial mode coupling for Laguerre Gaussian beam (LG)") was the investigation of the coupling among spatial modes with different spatial distributions. Literally, [1] has described the coupling among different spatial modes of plane waves under the coupling mode theory. Besides, the coupled-mode theory has been applied in both periodic and directional wave-guides for codirectional and contradirectional coupling scenarios. Under the project, we extended the coupling mode theory for the case of LG beams.

Jia-Ming Liu. Photonic devices. Cambridge University Press, 2009.

John M Senior and M Yousif Jamro. Optical fiber communications: principles and practice. Pearson Education, 2009.