Since the 1980s, optical fiber communication technology has gone through the development process of time division multiplexing (TDM), wavelength division multiplexing (WDM), polarization division multiplexing (PDM), and quadrature amplitude modulation (QAM). The transmission rate and capacity of the optical communication network are constantly improving, and the system capacity is gradually approaching the Shannon limit. In recent years, for the sake of effectively breaking through this capacity limitation, space division multiplexing (SDM) technology has attracted great attention. For example, the mode division multiplexing (MDM) technology makes it possible to simultaneously propagate several spatial modes in a few-mode fiber (FMF), which thereby greatly improves the fiber capacity. Few-mode erbium-doped fiber amplifiers (FM-EDFAs) can amplify multiple spatial modes at the same time for extending the transmission distance of MDM signals and help to greatly reduce the cost of MDM systems. The combination of the MDM technology and WDM-based optical transport network (OTN) can greatly alleviate the increasing bandwidth pressure. At the same time, dual-polarization quadrature phase shift keying (DP-QPSK) or QAM formats have been widely used in coherent communication systems. The amplification and transmission of dual-polarization signals in free-space FM-EDFAs have been reported in some references. Up to now, few papers have reported the amplification and transmission results of high-speed dual-polarization signals in all-fiber FM-EDFAs. Therefore, it is also worthwhile and practical to investigate the amplification and transmission performance of the dual-polarization signals in all-fiber FM-EDFAs.

This paper aims to experimentally study the amplification and transmission performance of high-speed DP-QPSK MDM signals in an all-fiber FM-EDFA. For this purpose, we build up a 100 Gbit/s DP-QPSK MDM system, including an MDM transmitter unit, the FM-EDFA, and an MDM receiver unit (Fig. 1). The MDM transmitter unit is composed of several commercial OTN optical transmitters (TXs), a serial of variable optical attenuators (VOAs), a mode-selective photonic lantern (MSPL), and a few-mode polarization controller (FMPC). The all-fiber FM-EDFA is developed from two homemade few-mode isolated wavelength division multiplexers (FM-IWDMs) and a section of few-mode erbium-doped fiber (FM-EDF) (Fig. 2). Two co-propagating LP_11a and LP_11b modes as pump lasers are excited at 1480 nm by another MSPL. The MDM signals are amplified by the FM-EDFA and then are input to the MDM receiver unit for mode demultiplexing and coherent reception. The MDM receiver unit is composed of an FMPC, an MSPL, a serial of wavelength-selective switches (WSSs), and multiple OTN optical receivers (RXs). To measure the amplification of the DP-QPSK MDM signals, this study employs the wavelength mapping method to calculate the modal gain with an optical spectrum analyzer (OSA).

Firstly, the modal gain and noise figure of the FM-EDFA are tested (Fig. 3). When the pump power of each mode is 24.5 dBm, the minimum differential modal gain (DMG) of 1.27 dB is obtained. With the pump power of each mode increasing to 29.2 dBm, the average modal gain and the DMG are up to 21 dB and 1.97 dB, respectively. Secondly, we test the receiver sensitivity curves of each channel with and without the FM-EDFA (Fig. 4). Compared with the MDM system without the FM-EDFA, the one with the FM-EDFA shows that the receiver sensitivities of LP₀₁, LP_11a, and LP_11b channels are degraded by 0.55 dB, 1.47 dB, and 0.99 dB, respectively. The polarization-dependent loss (PDL) of each channel is also measured. In the MDM system with the FM-EDFA, the PDL of each channel is also raised to some degree. Finally, the influence of DMG on the sensitivity equalization is studied in the amplification experiment of two modes (LP₀₁ and LP_11b), in which the DMG is changed by adjusting the pump power (Fig. 6). It is found that the channel sensitivity equalization is independent of the DMG, and the channel sensitivity degradation is related to the amplified spontaneous emission (ASE) noise and the PDL from the FM-EDFA.

In this paper, an amplification and transmission system is built up for 100 Gbit/s DP-QPSK MDM signals, which mainly includes the transceiver units of MDM signals and the all-fiber FM-EDFA with FM-IWDMs. According to the amplification and transmission experiment for three modes of LP₀₁, LP_11a, and LP_11b, it is shown that the receiver sensitivity of each channel at the bit error rate of 10^-2 is, respectively, degraded by 0.55 dB, 1.47 dB, and 0.99 dB due to the introduction of the FM-EDFA. The influence of DMG on sensitivity equalization is also studied in the amplification experiment of two modes (LP₀₁ and LP_11b), and there is no direct correlation between them. However, the DMG will affect the optical power margin of each channel. The conclusions can provide a reference for MDM amplification and transmission of dual-polarization signals.