Performance evaluation of selective mode conversion based on phase plates for a 10-mode fiber

Abstract

We numerically and experimentally evaluate the performance of higher-order mode conversion based on phase plates for 10-mode fibers (10MFs). The phase plates have the phase jump of π between multiple planes, which match the phase patterns of linearly polarized (LP) modes of 10MF. First, we numerically investigate the effects of the fabrication errors such as the phase-difference error and the slope in the phase jump of the phase plate. The simulation results for the mode conversion to LP11 indicate that such errors make the spatial pattern of the converted beam asymmetric. In order to maintain the symmetric pattern, the phase-difference error is required to be less than ± 2%, and the ratio of the slope width to the input beam waist should be suppressed to be less than 0.05. Next, we calculate the coupling power efficiencies of the excitation of LP modes in 10MF when the converted beams after the phase plate are launched into 10MF using a lens. As the calculation results, highly accurate adjustment of the input beam waist is required to suppress the crosstalk due to coupling of undesirable LP modes by less than -20 dB. For mode excitation of LP11 or LP12, crosstalk of more than -20 dB is not avoidable even if the input beam waist is carefully adjusted. In contrast, the crosstalk for the mode excitation of LP21 or LP31 is easily suppressed to be less than -20 dB without careful adjustment of the input beam waist. These results suggest that phase plates are not applicable to mode conversion to LP11 and LP12 in 10MF while they are suitable for conversion to LP02, LP21 and LP31. Finally, we experimentally demonstrate conversion from LP01 to LP21 and LP31 modes in 10MF using phase plates. We obtain nearly ideal LP21 and LP31 modes with the small crosstalk due to the coupling of the other undesirable LP modes.

Holographically encoded volume phase masks

“…This issue has been addressed in detail for conventional narrow band phase plate mode converters. 38 Similar overall efficiency results are expected for broadband HPM-based fiber mode converters, but are beyond the scope of this paper.…”

“…However, since the phase shift is only π at the design wavelength, this is the only wavelength with effective mode, as has been demonstrated by recent results for fiber modes. 38 To demonstrate the capabilities of HPMs, we encoded such a four sector phase mask in a transmission volume Bragg grating, thus creating a mode converting HPM. Here, we report a series of experiments to test the theoretical predictions of the HPM multiwavelength operation presented above.…”

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

“…To overcome this problem, we previously proposed a method of realizing MDM transmission using true eigenmodes as transmission channels by constructing a multi/demultiplexer of true eigenmodes utilizing that of LP modes [10]. As long as the transmission distance is less than several kilometers so that the mode conversion or mode coupling is negligibly small [12], the MDM transmission using true eigenmode will be effective for a MIMO-free transmission. However, since perturbations such as macro-and microbending and the elliptical deformation of the core may occur in actual optical fibers, it is necessary to experimentally analyze how the eigenmodes propagate in an actual fiber and to study the effect of these perturbations.…”

Chromatic characterization of ion-exchanged glass binary phase plates for mode-division multiplexing

“…As a result, a number of technologies have been proposed and implemented to encode information in the amplitude, phase, frequency, and polarization of the optical wave. One promising candidate to further increase fiber capacity is mode-division multiplexing (MDM) in few-mode fibers (FMFs), which uses transverse space modes as a new information-bearing dimension [1][2][3][4]. A fundamental element for MDM is a modal transformer, which converts the fundamental mode LP 01 of a single-mode fiber (SMF) into the modes LP 11 , LP 21 … of a FMF.…”