A study published in Photonics offers a new perspective on the localization of light in the core of solid-core micro-structured optical fibers, the effect of orbital and spin parts of the Poynting vector (energy flow), and singularities arising in it. This approach can help understand the energy leakage’s core modes and the solid-core micro-structured optical fibers production with a simple design and low losses.
Light Localization in Solid-Core Micro-Structured Optical Fibers
Fiber-optics communication and singular optics have been researched extensively in solid-core micro-structured optical fibers.
Fiber-optics communication deals with data transmission through optical fibers and singular optics investigates light propagation with various phase singularities.
These fields deal with light beams with orbital angular momentum across optical fibers. These beams can be employed in optical communications to compress a signal conveyed via a fiber due to their independent propagation with unique topological charges.
Various fibers, including ring-shaped, multicore, ring-core photonic crystal, negative curvature ring-core, inverse-parabolic graded-index, and hollow-core photonic bandgap fibers, have been used to explain the propagation of orbital angular momentum light beams.
This enabled researchers to focus on transverse energy flows for free-space propagating beams to reorient the energy flow in the optical vortex’s cross-section.
Previously, it was established that the total energy flow (Poynting vector) is the sum of the spin and orbital energy flow. Therefore, this approach was introduced to paraxial fields. Afterward, this approach was modified to work with non-paraxial vector fields. This transformation was possible due to spin-orbit interaction.
In holey fibers, losses decrease monotonically with wavelength. Interpreting the resonant loss reduction in all solid bandgap fibers (ASBGFs) requires understanding the departure of the Poynting vector’s orbital part from the radial direction.
In resonant loss reduction, the Poynting vector’s orbital component exhibits singularities in the fiber cross-section and vortices in the cladding rods. Vortex movements of the transverse Poynting vector’s orbital portion are not detected in ASBGF-like holey fibers.
Depending on all solid bandgap fibers’ cladding element configurations’ symmetry and the number of cladding rod layers, narrow transmission bands with low losses can be detected.
Investigating the Effect of Poynting Vector Vortex Motion on the Fiber Losses
Researchers investigated the behavior of the spin and orbital components of the Poynting vector of the fundamental leaky core mode in all solid bandgap and holey fibers, demonstrating their impact on the degree of light localization in these fibers.
The fiber’s waveguide losses were slightly compensated by the spin component of the transverse Poynting vector. The waveguide losses in the all-solid dielectric pipe were determined by the radial projection of the Poynting vector’s orbital part.
Significant Findings of the Study
The fundamental features of the leaky waveguide core modes are singular spots in the transverse component of the Poynting vector. The transverse part of the Poynting vector for the ASBGF fundamental modes can be represented as a set of singular points.
In all solid bandgap and holey fibers, the fundamental core mode’s spin part of the Poynting vector primarily contributes to the core mode energy flow’s axial component, and the small part governs the core mode energy movement in the transverse direction.
All of the Poynting vector’s components are affected by the orbital component. Still, the transverse energy flow of core modes and the resulting all-solid bandgap fibers and holey fiber waveguide loss are most sensitive to this factor.
The spin component has a vortex configuration, while orbital parts function identically for polarization and influence mode energy outflow between cladding holes. This phenomenon can explain a monotonic drop in waveguide losses in HFs when increasing the cladding hole radius decreases the orbital part of the energy flow between the holes.
Increasing the number of holey fibers cladding holes reduces waveguide losses, but the core mode energy leakage pattern does not change. In all solid bandgap fibers, new core mode vortex movements emerge in the cladding rods, driving energy to the fiber center.
A further investigation of the singular properties of the Poynting vector’s transverse component can be a valuable tool for determining the relationship between the location of singular points and the cross-section structures of solid-core micro-structured optical fibers at which minimal losses for the leaky mode in a given spectral range are obtained.
Alagashev, G., Stafeev, S., Kotlyar, V., & Pryamikov, A. (2022) The Effect of the Spin and Orbital Parts of the Poynting Vector on Light Localization in Solid-Core Micro-Structured Optical Fibers. Photonics. https://www.mdpi.com/2304-6732/9/10/775