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We propose a technique for reducing the differential modal gain (DMG) that occurs in a two-mode erbium-doped fiber (2M-EDF) by inscribing voids in the core center of a 2M-EDF with a femtosecond laser. We show that an empty void inscribed at the core center can attenuate the linearly polarized () mode selectively while suppressing excess loss for the mode. We also reveal that DMG can be controlled by means of void diameter. The longitudinal position dependence of the void in a 2M-EDF was also investigated considering its influence on gain and noise figure (NF) characteristics. Finally, we realize a sufficiently low DMG of less than 0.5 dB in the full C-band as well as a sufficient gain and NF by using the proposed technique.

We discuss the extensibility of the S bend model to five mode ring core fiber (RCF). We propose a MUX/DEMUX for the five mode RCF and clarify design conditions from the viewpoint of connectivity. We evaluate the in-bobbin state with different barrel sizes and confirm the extensibility of the S bend model to five mode transmission. Furthermore, a transmission experiment demonstrates the reduction in MDL possible by promoting binding in five-mode RCF.

We demonstrate a ring-resonator-based gain-clamped 4-LP-mode erbium-doped fiber amplifier (EDFA). In this study, we show that a gain-clamped EDFA can reduce temporal modal gain variation simultaneously in all modes. We experimentally evaluate the transient response of a 4-LP-mode EDFA and succeed in reducing the modal gain variation for all modes at 1530 and 1550 nm from 4.0 to 1.1 dB and from 2.8 to 0.7 dB, respectively, by using a gain-clamping technique.

A simple and efficient method for estimating modal amplitude and phase of multimode near field patterns (NFPs) based on artificial-neural-network (ANN) with the help of the optimization method is proposed. The inferred amplitude and phase of measured NFPs based on ANN are refined by using a grey-wolf optimizer (GWO). By using the proposed method, the image correlation between reproduced and measured NFPs is improved without re-training of ANN, which is the most time-consuming part of ANN-based numerical modal decomposition technique. Numerical examples of three and six mode cases are presented for the estimation using simple ANN. For six-mode case, the correlation is greatly improved by using the optimizer. Finally, the estimation of the measured NFPs from three-mode exchanger and six-mode mode conversion grating is implemented, and 5% improvement in the correlation value is observed for six-mode case. The proposed method offers alternative way to improve the correlation without using elaborated ANN.

A multicore fiber design with more than 10 coupled cores is investigated numerically and experimentally. It is revealed that a randomly coupled 12-core structure with a square lattice core arrangement can be realized within a 125 μm cladding by employing fiber twisting of more than a few π rad/m. A 125 μm-clad twisting-rate-controlled 12-core fiber is then fabricated and a relative core density of 12 compared with conventional single-mode fiber is achieved while maintaining a low spatial mode dispersion characteristic of 8.4 ps/√km at 1550 nm. Finally, we experimentally evaluated the performance of 12 spatial channels by conducting a 24 × 24 MIMO transmission experiment.

We report the design, fabrication, and evaluation of a cladding pumped two-LP-mode six-core moderately coupled erbium-doped fiber (EDF) amplifier. In this study, we show that moderately coupled multi-core EDF can realize a high pump power density in a cladding by allowing core-to-core power coupling. We also experimentally evaluate the amplification characteristics and confirm an average mode-core gain of 18 dB, a differential mode-core gain of 4 dB, and an average noise figure of 7 dB in the C-band. Moreover, we confirm an average mode-core gain of 15 dB, a differential mode-core gain of 5 dB, and an average noise figure of 7.5 dB in the L-band.