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.

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.

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.

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.