High-Flat-Gain and $\mathrm{C}+\mathrm{L}$ Band Distributed Raman Amplifier Applied in 3 Mode-Group-Division Multiplexing and WDM Long-Haul MIMO-Free Transmission over 104-km Ring-Core Fiber

We demonstrate 3 orbital angular momentum mode-group-division multiplexing assisted with wavelength-division multiplexing for MIMO-free transmission over 104-km ring-core fiber. The distributed Raman amplifier in system shows high flat gain
$(> \mathbf{10dB})$
over the
$\mathbf{C}+\mathbf{L}$
band.