基于循环频移器(RFS)的多载波产生原理，得到了间隔为25 GHz，载噪比为25 dB的20个子载波的多载波输出，将波特率为16 Gbaud(即64 Gbit/s)的归零码十六进制正交幅度调制（RZ-16QAM）信号加载到这些子载波上，并且进行偏分复用（PDM）生成PDM-RZ-16QAM信号，实验实现了2.56 Tbit/s PDM-RZ-16QAM信号792 km标准单模光纤(SSMF)的传输和相干接收。实验测得的背靠背相干光波分复用（CO-WDM）系统在误码率为1×10-3的情况下，光信噪比需求比理论值(FEC)增加了4.1 dB。传输后，最佳入射光功率处的误码率为4.5×10-3，小于前向纠错的软判决门限（2.4×10-2），此时对应的CO-WDM PDM-RZ-16QAM中心信道信号恢复后x和y偏振方向的星座图清晰规则，说明传输性能很好，并且系统的频谱效率达到了5.1 bit/(s·Hz)。

20-tone, 25 GHz spacing multi-carrier with the tone-to-noise ratio (TNR) larger than 25 dB is generated based on recirculating frequency shifter (RFS). Each subcarrier is modulated with a return-zero 16-quadrature amplitude modulation (RZ-16QAM) signal at 16 Gbaud and then polarization division multiplexing (PDM) is employed to further increase the capacity. At last, 2.56 Tbit/s, PDM-RZ-16QAM, coherent optical wavelength division multiplexing (CO-WDM) transmission over 792 km standard single-mode fiber (SSMF) with 25 GHz channel spacing using coherent detection is experimentally demonstrated. Compared to the theoretical limits, the measured optical signal to noise ratio (OSNR) sensitivity curve in back-to-back system is shifted by approximately 4.1 dB at a bit-error rate (BER) of 1×10-3. After transmission, the BER value of 4.5×10-3 at the optimal launchpower is under the threshold value of soft-decision forward error correction (FEC) of 2.4×10-2, and the corresponding constellations diagrams of x and y polarizations for recovered CO-WDM PDM-RZ-16QAM signals of the central channel show the excellent performance. The spectrum efficiency of our transmission system is 5.1 bit/(s·Hz).