Maximizing the transmission capacity of individual wavelength channels is necessary to meet the increasing capacity and distance requirements of metro optical networks. Orthogonal frequency division multiplexing (OFDM) technology can tolerate certain chromatic dispersion when signals are loaded onto each subcarrier, thus maximizing the transmission capacity within limited bandwidths during optical fiber transmission. In addition, intensity modulation-direct detection (IM-DD) is currently the most widely used method in metro optical network access layers. However, it is severely affected by fiber chromatic dispersion and cannot meet the needs of long-distance transmission in other layers of metro optical networks. Therefore, the IM-DD OFDM system combining the two technologies has received increasing attention. However, as the capacity and distance requirements of next-generation metro optical networks increase further, the dispersion problem will exceed the tolerable limit of OFDM, and the impact of nonlinear effects will become more obvious, causing a serious decline in system performance. Digital back-propagation (DBP) and optical phase conjugation (OPC) technologies are commonly used to compensate for chromatic dispersion and nonlinear effects simultaneously. However, DBP requires solving the inverse non-linear Schrodinger equation of the fiber channel, which has a high computational cost. When using OPC technology, when two sections of fiber have the same length, the even-order chromatic dispersion and pulse broadening caused by nonlinear effects accumulated in the first section of fiber will be completely recovered in the second section of fiber theoretically. However, traditional OPC schemes based on single-pump degenerate four-wave mixing (DFWM) have signal wavelength shifts at the phase conjugator, which changes the group velocity dispersion parameters in the second section of the fiber link. As a result, the OPC needs to be slightly shift from the midpoint of the fiber link to achieve complete signal impairment compensation. There is also a polarization sensitivity problem that reduces the efficiency of four-wave mixing (FWM), thus affecting the compensation performance of OPC waves in the system.
We propose a wavelength-shift-free OPC compensation scheme based on orthogonal polarization pumping non-degenerate four-wave mixing (NFWM) for IM-DD OFDM optical communication systems. It simultaneously compensates for chromatic dispersion and suppresses the impact of the nonlinear effects. First, we theoretically derive the principle of generating an OPC using orthogonal polarization pumping NFWM in a highly nonlinear fiber (HNLF). Based on the above principle, we design a wavelength-shift-free OPC implementation method to obtain an OPC wave with the same wavelength as the original signal in the orthogonal polarization state. Then, the factors that affect the power of the generated OPC wave are specifically analyzed. Finally, according to the optimized parameter settings, a simulation verification is performed.
The pump optical power, the nonlinear coefficient, and the length of HNLF play a key role in the performance of IM-DD OFDM systems based on orthogonal polarization pumping NFWM for generating OPC. First, the impact of pump optical power is analyzed. Fig. 3 shows that the bit error rate (BER) varies with the change in the signal optical power injected into the OPC at different pump optical power values. It can be seen that a larger pump power will cause a sudden increase in the BER as the optical signal power continues to increase. The main reason for this is that the increase in pump power will lead to a large amplified spontaneous emission noise within the bandwidth of the generated OPC wave. The noise cannot be filtered out by an optical filter and will affect its compensation effectiveness. Next, the impact of the nonlinear coefficient and length of the HNLF on the system performance is analyzed. As shown in Fig. 4, with an increase in the nonlinear parameters, the BER is lower when the HNLF is shorter. However, its performance degrades as the length of HNLF increases. Finally, we compare the performance of the traditional OPC scheme without calculating the shift value at the midpoint, the traditional OPC scheme with midpoint-shift-value calculation, and our wavelength-shift-free OPC scheme. The BER curves varied with the received optical power (ROP), as shown in Fig. 5. It can be seen that our proposed system can achieve a 7% HD-FEC threshold at a rate of 114.375 Gbit/s through a standard single-mode fiber link with a length of 240 km, and the constellation points are relatively clear with few noise points.
We theoretically analyze and verify the feasibility and effectiveness of wavelength-shift-free OPC compensation for IM-DD OFDM optical communication systems based on orthogonal polarization pump NFWM. To achieve better performance for the system, we study various parameters that affect system performance. The performance comparison between the proposed scheme and the traditional OPC scheme is conducted, and it is found that the system based on wavelength-shift-free OPC transmission achieves a BER of 7% for HD-FEC threshold at an ROP of -10 dBm, while the system based on traditional OPC scheme cannot achieve the decision threshold even after midpoint-shift-value calculation under this transmission condition. Our scheme can provide a theoretical basis for the design of high-speed long-distance IM-DD OFDM optical communication systems.
