With the rapid growth of internet traffic, the demand for large transmission capacities from all walks of life has grown dramatically. The current transmission capacity of a single-mode fiber (SMF) in a fiber optic transmission system is rapidly approaching the Shannon limit. Solving the transmission capacity problem has become a top priority. One method to solve this problem is to apply mode-division multiplexing (MDM). In MDM, multi-mode fibers have severe intermodal dispersion and large nonlinear impairment; this is less effective for long-haul transmission. Few-mode fibers have less intermodal dispersion and more potential for long-haul fiber optic communications. Currently, China is catching up in the field of MDM and mainly adopts the intensity modulation direct detection (IMDD) method for experiments that is not suitable for long-distance transmission. The number of modes used in the studied MDM system is small; this has a limited effect on improving the capacity of the communication system. This study adopts polarization multiplexing and advanced digital signal processing technologies to construct a single-channel mode division multiplexing optical fiber transmission system based on an in-phase quadrature (IQ) modulation heterodyne coherent detection system. We successfully realize a 1000 km transmission of 32 Gbaud 16 quadrature amplitude modulation (QAM) signals in two degenerate modes, LP11a and LP11b. After equalization using the time-domain and frequency-domain multiple-input multiple-output least mean square (MIMO-LMS) algorithms, the bit error rate (BER) is lower than the soft-decision forward error correction (SD-FEC) threshold (5.2×10^-2).

At the transmitter side, external cavity lasers (ECL) generate light wave. The generated continuous light wave is modulated by a 16QAM signal through an IQ modulator. The 16QAM signal loaded into an arbitrary waveform generator (AWG) is generated offline using MATLAB. The modulated signal is divided into two paths by a polarization beam splitter (PBS) and transmitted in the polarization-maintaining fiber. One path passes through the delay line and is then combined with the other path by a polarization beam combiner (PBC) to complete polarization multiplexing. The polarization-multiplexed signal is amplified in the erbium-doped fiber amplifier (EDFA) and divided into two paths together through a 1×2 coupler equal. One path is decorrelated through a delay line with a length of 3 m and delay time of 15 ns and then injected into the fiber for transmission. The fiber optic link adopts a loop structure in which the loop switch is controlled by two acousto-optic modulators (AOM). Long-distance transmission is achieved by setting the AOM to control the number of transmission turns of the multiplexed signal in the loop. The signals are modulated into the LP11a and LP11b modes by the mode-multiplexing module, and the signals under the two modes are jointly transmitted in the few-mode fiber (FMF). The signals enter the mode-demultiplexing module through a few-mode fiber and are boosted using an EDFA. Owing to the insertion loss of the AOM switch and coupler, the signals must be amplified by the EDFA after entering the optical fiber loop. We solve the problem of the uneven gain of EDFAs by adopting a wavelength selective switch (WSS). The output of the WSS is sent back to the mode multiplexer to conduct MDM and 50 km FMF transmissions again until the total transmission distance can meet our requirement. On the receiver side, a coherent optical receiver conducts heterodyne detection on the output signal and performs digital signal processing (DSP). In offline DSP, the received electrical signal is first processed by frequency-domain dispersion compensation, and the compensated signal is then downsampled. Quadruple signal rate is preserved for clock recovery during downsampling. After clock recovery, the signal is downsampled again and the original signal is recovered by the MIMO-time domain (TD) LMS, MIMO- frequency domain (FD) LMS, carrier phase recovery, detection-directed LMS (DDLMS) algorithms. Finally, the BER calculation is performed for the signal.

Figure 2 shows the BERs of the two modes measured under different OSNR conditions compared with the additive white Gaussian noise (AWGN) channel simulation results. In the case of a low signal to noise ratio (SNR), the BER is close to the theoretical channel result, whereas in the case of a high SNR (about 20.5 dB), the BER is 1×10^-2 that is 2.5 dB away from the theoretical value. Figure 3 shows the BERs of the two modes after 1000 km transmission under different input fiber powers. In the case of different input fiber powers, after 1000 km transmission, the BERs of both the LP11a and LP11b modes can meet the SD-FEC threshold (5.2×10^-2). Because the indices of refraction of LP11a and LP11b are close, the BERs of the different modes show little difference. As shown in Fig. 4, the two modes exhibit similar performance at all transmission distances and both can meet the SD-FEC threshold (5.2×10^-2).

In this study, we experimentally build a dual-mode polarization-multiplexed 16QAM signal 1000 km few-mode fiber transmission system based on heterodyne coherent detection. At the receiving end, the MIMO-TDLMS and MIMO-FDLMS algorithms are used for channel equalization, and a single-channel 512 Gbit/s transmission rate is achieved. The BER can meet the SD-FEC threshold (5.2×10^-2), and the corresponding net data rate is 400 Gbit/s. Although achieving longer transmission distances requires improved mode-dependent losses in the links, the results confirm the potential of few-mode fibers for future high-capacity long-distance transmission systems.