Six-mode SDM transmission over 960  km with a reach extension of five times enabled by a 6M-EDFA

For the first time, to our knowledge, we demonstrate a six-mode transmission over a 960-km fiber link using in-line integrated amplification provided by a six-mode erbium-doped fiber amplifier (6M-EDFA) for a 28-GBaud dual-polarization QPSK signal. This transmission distance is five times longer than that of previously reported works. The integrated 6M-EDFA enabling this long-haul transmission exhibits modal gains of >17.6 dB , while the gains increase to 25 dB with an input power of −25 dBm . Importantly, a Gaussian-like erbium doping profile has been proposed to optimize the differential modal gain to 1.15 dB, ensuring a more uniform signal-to-noise ratio between spatial modes after long-haul transmission.