Demonstration of true-eigenmode propagation in few-mode fibers by selective LP mode excitation and near-field observation

Abstract

We observed the near-field patterns of light output from fewmode fibers (FMFs) in which an LP mode was selectively excited. It was confirmed from the variation of the intensity profile with the wavelength that the true eigenmodes of a circular core fiber are guided in single-core stepindex and graded-index FMFs. On the other hand, we discovered a new phenomenon that LP 11 modes propagate as eigenmodes oriented along a specific axis in 4-LP mode 12-core FMF. In addition, we confirmed that this phenomenon is not due to the elliptical deformation of the core by observing the NFP and calculating the eigenmodes in an elliptical core using elliptical cylindrical coordinates and the Mathieu function.

Predictable and unpredictable phenomena in optical fibers for space-division/mode-division multiplexing transmission: statistical analysis of coupling and mysterious behavior of modes

“…was not inserted, the mode pattern of LP 11 appeared when the phase plate was rotated by a certain angle, as shown in the first row of Fig. 12(a) [49]. The mode pattern did not change even when the half-wave plate was rotated while the azimuth angle of the phase plate was fixed.…”

“…It consists of a tunable laser, a selective LP mode exciter (rotatable halfwave and rotatable phase plates), and an NFP observation system (rotatable polarizer and infrared camera). The angle of the polarizer was measured clockwise from the negative x-axis [15,49]. Fig.…”

“…When the x-polarized LP even 11 mode was incident on the SC-FMF at a certain angle, the observed NFP that passed through the output polarizer rotated by a certain angle was quenched at a certain wavelength and it turned out to be linearly polarized light, as shown in Fig. 11 [49]. However, when the wavelength of the light source was slightly changed, an extinction angle did not appear, and a donutshaped intensity distribution corresponding to the locally different elliptical polarization states was observed.…”

“…When the output polarizer It can be seen from (a) that the principal axis of the mode parity was located in the horizontal direction in this fiber core. In (b), mixing between the TMH and TEH mode groups occurred [49].…”

“…On the other hand, when the incident LP even 11−x mode was rotated by 45 degrees in the azimuth direction, both the mode pattern and the polarization direction were rotated and varied with the wavelength as shown in Fig. 12(b) [49].…”