M.A. Abelmas1, O.V. Ivanov2

1 Ulyanovsk State Technical University (Ulyanovsk, Russia)

2 Ulyanovsk Branch of the V.A. Kotelnikov Institute of Radio Engineering and Electronics of the RAS (Ulyanovsk, Russia)

1 abelmax1998@mail.ru; 2 olegivvit@yandex.ru

In the calculation of modern fiber-optic sensors operating with the interaction of modes of contacting optical fibers, the approximation of linearly polarized modes is usually used. It is considered that the polarization direction at the cladding boundary coincides with the polarization direction inside the fiber and the longitudinal component of the field is not taken into account. However, in structures with interaction through the evanescent field at the boundary of the fiber cladding, the field can be non-uniform near the surface, and the longitudinal component can make a contribution comparable to the radial and azimuthal components. At the same time, the exact calculation of the spatial distribution of the field and the polarization direction over the entire cross-section of the fiber and at its surface has not been previously carried out.

The aim of this paper is to do an exact calculation of the hybrid modes of a coreless optical fiber and the spatial distribution of electric fields for different mode numbers. It also compares the vector distribution of the field for the LP21 mode inside the fiber and at the cladding surface.

The dependences of the mode propagation constants on the cladding radius are presented. A comparison of the longitudinal and transverse components of the electric fields at the fiber cladding boundary proves the significance of the longitudinal component in the formation of the evanescent electric field. The contribution of different components to the formation of the electric field is demonstrated depending on the radial mode number. Polarization characteristics of the electric fields at the cladding boundary are determined. The dependence of the field polarization direction on the azimuthal angle is presented. A comparison of the polarization vector inside and at the cladding surface is carried out, and a significant deviation of the vector under the fiber surface is revealed.

The practical significance of the presented results is important for finding the mode coupling coefficients, which are calculated when creating fiber-optic sensors based on contacting fibers.

Abelmas M.A., Ivanov O.V. Evanescent electromagnetic fields of LP2m modes of coreless optical fibers. Radiotekhnika. 2025. V. 89.
№ 6. P. 114−125. DOI: https://doi.org/10.18127/j00338486-202506-11 (In Russian)

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