基于注入锁定载波恢复的同源相干光传输技术

Coherent Optical⁃Transmission Technology Based on Optical⁃Injection⁃Locking Carrier Recovery

为了解决传统异源相干探测方案成本较高、数字信号处理算法复杂的问题,同源自零差相干探测方案被视为较有潜力的解决方案之一。同源自零差相干探测系统在信号传输的同时也传输一个同源的直流导频光,作为本振(LO)。但是该方案中本振光在链路中传输存在功率损耗的问题,降低了相干接收灵敏度,限制了传输距离。利用光注入锁定(OIL)则能够在保持频率与相位锁定的同时,有效提高接收端LO光功率,从而解决功率衰落问题。然而链路中偏振态(SOP)的随机变化可能导致OIL长时间处于0功率注入并影响其锁定状态稳定性。在OIL之前引入主动偏振加扰,能够避免潜在的长时间注入光功率过低的情况发生,因此抵抗了偏振衰落。通过理论与仿真研究,揭示了主动偏振加扰对OIL系统锁定/失锁特性的影响,拟合了主动偏振加扰频率与OIL最小失锁阈值功率之间的关系,并开展了大容量的同源自零差相干探测系统传输实验验证。结果表明,OIL能为LO在窄带宽内提供高增益,显著提高信噪比,且外加主动偏振加扰能使OIL系统更容易锁定,在同源自零差相干探测系统中能保持较高灵敏度的相干探测效果。这种方案能有效抵抗光传输过程中的各种衰落,对于偏振衰落问题提供了一种可以解决的方案。

Objective In the big-data era, owing to the promotion of explosive digital applications such as artificial intelligence(AI), cloud computing, virtual reality/augmented reality(VR/AR), and data centers, network traffic has increased continuously, thus placing higher requirements on optical-fiber communication systems. In this context, because of concerns such as bandwidth and cost, the conventional coherent optical communication technology is no longer advantageous for supporting industrialized short-distance transmission. Additionally, owing to frequency-selective fading caused by low sensitivity and dispersion, the conventional intensity modulation direct detection(IMDD) technology commonly used for short distances cannot continue to cooperate with the further upgrading and evolution of next-generation communication. Simultaneously, the increase in data-center traffic has become the main component of the global Internet Protocol(IP) traffic. For short-distance coherent optical communication, the self-homodyne coherent detection(SHCD) system significantly reduces the system complexity, cost, and power consumption compared with conventional coherent optical communication systems. It is considered the most promising solution in data centers but still presents the issues of power and polarization fading. In this study, we use optical injection locking(OIL) to overcome power fading and introduce active polarization scrambling(APS) prior to OIL, which can avoid the potential occurrence of long-term, low-optical-power injection and thus prevent polarization fading. This approach facilitates OIL and maintains a highly sensitive coherent detection effect in the SHCD system.Methods First, a numerical model was established for theoretical and simulation analyses. During the simulation, we simulated the frequency change of the injected light amplitude prior to OIL to replace the external APS and then analyzed the effect of APS on the OIL. We discuss the locking characteristics under different injected light-amplitude variation frequencies, as well as the effect of APS on the locking/unlocking characteristics(locking range and minimum unlocking threshold power) of OIL. We fitted the relationship between the APS frequency and the minimum unlocking threshold power for OIL. Subsequently, we conducted transmission experiments to verify the high-capacity SHCD system.Results and Discussions By utilizing OIL, we effectively increased the local oscillator(LO) power at the receiving end while maintaining the frequency and phase locking, thereby overcoming the issue of power fading. Introducing APS prior to OIL can avoid potential situations where the injected light power is extremely low for a long time, thus preventing polarization fading. When the frequency of the injected light-amplitude change is relatively low, the OIL system experiences locking loss. However, as the frequency increases gradually, the output phase difference changes irregularly, thus resulting in nonperiodic output-phase-difference oscillations. When the frequency reaches a certain threshold, the output phase difference stops oscillating and remains relatively stable, thus indicating a locking state(Figs. 3 and 4). As the frequency of the APS increases, the locking range increases accordingly(Fig. 6). Additionally, the minimum unlocking threshold power for the OIL differs depending on the APS frequency. When the APS frequency is low, the minimum threshold power for locking loss is minimally affected by the change in the APS frequency. When the APS frequency is high, the minimum threshold power for locking loss decreases significantly as the APS frequency increases(Fig. 8).The APS deteriorates the system performance slightly, and the severity of the deterioration is proportional to the APS frequency. The higher the frequency, the more significant is the performance degradation. However, the phase diagram shows that when the APS frequency is high, the probability of locking loss is reduced significantly(Fig. 11). Therefore, an appropriate APS frequency must be selected for practical use to achieve high-sensitivity SHCD.Conclusions OIL can provide high gains for LO within a narrow bandwidth, thus significantly improving the signal-to-noise ratio.When the APS frequency is high, the minimum threshold power for locking loss decreases significantly with the increase in the APS frequency, thus indicating that the OIL system with external APS exhibits a more stable locking state and stronger anti-locking-loss ability. By selecting an appropriate APS frequency at the transmitting end and utilizing OIL at the receiving end, the SHCD system can be matched well, thus resulting in a stable and high-sensitivity self-homodyne coherent transmission. This provides a favorable solution for addressing signal fading in optical-fiber transmission and is a promising method for achieving high-performance, low-cost,and simplified high-speed optical digital signal processing(DSP) transmission in short-range scenarios in the big-data era.