In recent years, the explosive growth of data volume has challenged the backbone transmission network whose core technology is optical fiber transmission. In the past, single-mode fiber transmission has long been the first choice for large-capacity and long-distance transmission due to its low loss and high bandwidth. Till now, single-mode fiber still occupies most of the optical transmission networks. However, the rate of single-mode fiber transmission which combines polarization division multiplexing (PDM) and wavelength division multiplexing (WDM) technologies is limited to 100 Tbit/s. It becomes weaker and weaker in the face of the expected increase of several orders of magnitude in the demand for transmission rate. With the emergence of more mature mode division multiplexing (MDM) and demultiplexing technologies, low-dispersion, low-loss few-mode fibers (FMFs), and more advanced digital signal processing (DSP) algorithms, it becomes possible to use few-mode fibers to achieve greater capacity and longer distance transmission.
Our few-mode transmission experiment uses a self-developed graded few-mode fiber that can transmit six modes. In the experiment, we choose two modes of LP11a and LP11b for transmission. Compared with other modes, the LP11 mode has a lower loss, and this kind of few-mode transmission can perform power control and dispersion control more easily than the few-mode transmission of different linear polarization modes. The transmission distance of each loop is 50 km, and 1000 km transmission is achieved by transmitting 20 loops. In terms of the experimental setup of the long-distance few-mode fiber loop experiment, at the transmitting end, 80 laser sources with a frequency interval of 50 GHz output a total of 80 carriers through the arrayed waveguide grating control. The two IQ signals output by the arbitrary waveform generator modulate the WDM signal of 79 channels and another test signal in the IQ modulator respectively, and then a section of decorrelation signal is generated through the delay line and is used to perform polarization division multiplexing. After being amplified by the erbium-doped fiber amplifier (EDFA), it is divided into two independent signals through delay and de-correlation again, and then multiplexed and transmitted by the mode multiplexer in the loop.
After entering the loop, the two independent signals are mode multiplexed and modulated in two modes of LP11a and LP11b in the mode multiplexer and output. The ring includes 50 km of few-mode fiber, mode multiplexer/demultiplexer, EDFA, wavelength selective switch (WSS), and acoustic-optic modulator (AOM). EDFA balances the optical power of each mode signal, and WSS controls the flatness between channels of each mode signal after the EDFA power balancing. The dispersion of the FMF link in all fiber modes is about 21.01 ps/(nm·km), and the effective area of the used fiber is 121 μm2 when transmitting the LP11 mode. After 20 FMF loop transmissions for a total of 1000 km, the measurement channel signal is selected using a wavelength division multiplexer, and the coherent optical receiver detects the signal. The detected signal is captured by an oscilloscope with a sampling rate of 80 GSa/s and processed by DSP. In order to reduce the number of oscilloscope input ports, we use heterodyne coherent detection, so we only need to use a 4-channel oscilloscope to achieve coherent detection of two-mode signals. The frequency difference between the local oscillator signal and the detected transmission signal is about 18 GHz.
In offline DSP, the signal passes through frequency domain dispersion compensation, down-sampling (retaining twice the symbol rate), clock recovery, multiple-input multiple-output (MIMO) frequency domain least mean square (FDLMS), MIMO time domain least mean square (FTLMS), carrier phase recovery, and direct decision least mean square (DDLMS) in sequence and quadrature amplitude modulation (QAM) demapping and bit error rate (BER) calculation.
We experimentally tested the transmission performance of the two modes (LP11a and LP11b) under different optical signal-to-noise ratios (OSNRs) and compared them with additive white Gaussian noise (AWGN) channel simulation tests. In the interval of the OSNR of each channel in the experiment, the BER is close to the theoretical channel result under the condition of low signal-to-noise ratio (SNR). Since the crosstalk between modes and polarizations is dominant in the noise when the SNR is relatively high and cannot be completely eliminated, it may lead to a large difference between the BER performance and the theoretical value when the SNR is high.
We tested the C30 channel BER performance of the two modes under back-to-back (BTB) case and transmission distances of 250 km, 500 km and 1000 km, respectively. After adding the frequency- and time-domain joint algorithm called MIMO-FTDLMS, even with the huge inter-channel symbol interference caused by the other three-way crosstalk and the channel state changes caused by the inevitable disturbance superposition of each channel, every channel can be effectively recovered. This greatly shows the effectiveness of the algorithm in multimode transmission. Likewise, both modes exhibit similar performance in transmission, and the BER is less than the low-density parity check (LDPC) soft decision threshold of 28% redundancy at all transmission distances. In the experiment, the BERs of 80 channels, two modes and two polarization multiplexing signals transmitted over 1000 km are all below the soft decision threshold, thus the total net transmission rate is 32 Tbit/s.
The excellent performance of the system benefits from the two-stage cascaded MIMO equalization algorithm and self-made low-loss, low-dispersion few-mode fiber. This few-mode long-distance transmission system provides a new solution for the next generation optical backbone network transmission. At the cost of algorithm complexity at the receiving end, the quaternary phase shift keying (QPSK) format used in traditional long-distance transmission is replaced by 16QAM with higher spectral efficiency. In addition, less costly few-mode fibers are used at the same rate for spatial multiplexing.
The main limitation of the current system is the complexity of the algorithm. The least mean square (LMS) algorithm in the cascaded time-frequency domain will bring a large delay to the system, and the algorithm needs to be optimized in terms of feedback structure and parallelism.
Our experimental design verified the transmission of the WDM-MDM-PDM-16QAM system over a 1000 km few-mode fiber. By adjusting channel flatness through WSS, and using MIMO-TDLMS and MIMO-FDLMS two-stage MIMO algorithms for channel equalization at the receiving end, we finally achieve a transmission rate of 32 Tbit/s with 80 channels of two-mode and dual-polarization signals. After the transmission system is combined with multi-core optical fiber, it is expected to achieve a transmission rate increase of 1?2 orders of magnitude.
