高速DP-QPSK模分复用信号在少模掺铒光纤放大器中的传输实验

Transmission Experiment of High-Speed DP-QPSK Mode Division Multiplexing Signals with Few-Mode Erbium-Doped Fiber Amplifier

模分复用(MDM)与高速光传送网(OTN)相结合,能缓解日益增长的带宽需求压力和降低已有相干通信设备的使用成本。搭建了100 Gbit/s双偏振正交相移键控(DP-QPSK)MDM信号放大传输系统,主要包括MDM信号收发单元和少模掺铒光纤放大器(FM-EDFA),其中FM-EDFA采用少模隔离型波分复用器(FM-IWDM)构建。三模(LP_(01)、LP_(11a)和LP_(11b))放大传输实验表明,相对于无FM-EDFA的MDM系统,各信道的接收机灵敏度(以10-2误码率为参考)分别劣化0.55 dB、1.47 dB和0.99 dB。研究了两模(LP_(01)和LP_(11b))放大情形下模式增益差(DMG)对信道灵敏度均衡性的影响,结果显示两者无明显的依赖关系。所得结论可为双偏振(DP)信号的MDM放大传输研究提供参考。

Objective 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.Methods 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).Results and Discussions 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 LP_(01),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 (LP_(01) 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.Conclusions 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 LP_(01), 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 (LP_(01) 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.