Currently, the information capacity of communication systems based on a single-mode fiber (SMF) is approaching its physical limits. To solve this problem, spatial division multiplexing based on orbital angular momentum (OAM) has been intensively investigated. Theoretically, owing to their orthogonality characteristics, OAM modes can help realize infinite multiplexed channels. Hence, the capacity and spectrum efficiency of existing optical fiber communication can be enhanced by using the orthogonality and infinity properties of the OAM modes. The vortex mode amplifier is one of the key devices in optical communication systems and is essential for improving the performance of future vortex mode multiplexing systems applicable in long-distance and high-capacity optical fiber communication. Currently, the vortex mode amplifier heavily relies on the ring-core erbium-doped fiber (RC-EDF), which is not only conducive to the stable transmission of the vortex beam but also improves the conversion efficiency of the pump beam. The modal gain is an important index of ring-core erbium-doped vortex fiber amplifiers. Therefore, the design and fabrication of an RC-EDF with a high modal gain are essential to satisfy the growing demand for data traffic.
The modal gain mainly depends on the intensity distribution of the pump and signal modes as well as the doping distribution of erbium ions. The ring core structure is used to optimize the field distribution of the pump modes, thereby improving the conversion efficiency of the pump beam. Owing to the limitations of the doping technology, the doping distribution of erbium ions in actual fabricated fibers differs considerably from that of the theoretical design. Therefore, in this study, we simplify the Er doping factor in the theoretical design to ensure that the doping distribution of the erbium ions is uniform. First, the effects of the doping region and width of the ring core on the gains of the first- and second-order vortex modes are analyzed, and subsequently, these effects are optimized to realize a high-gain ring-core erbium-doped vortex fiber amplifier. According to the relative refractive index profile of the RC-EDF, the modal intensity profiles of the vortex modes (|l|=1-2) and the pump fundamental mode (PFM) are numerically simulated using the finite-element method. The effects of the RC-EDF length and signal wavelength on the amplifier gain characteristics are studied using numerical simulation based on the rate and light propagation equations, which guides the optimized fiber parameters in RC-EDF fabrication. Furthermore, an experimental setup is built to characterize the amplification performance of the proposed RC-EDF.
The proposed RC-EDF has cladding and inner-ring-core radii of 62.5 μm and 1.6 μm, respectively. We ensure a large refractive-index difference of 0.012 between the ring core and cladding, which is sufficient to achieve mode splitting. The proposed RC-EDF supports high-order vortex modes (|l|=1-2). Modal gains (|l|=1-2) are optimal when the thickness of the ring core and the width of the doping region are 5 μm and 6 μm, respectively. The simulation results show that the gains of the first- and second-order vortex modes can reach 35.4 dB in the C band [Fig. 2(b)]. The fabricated RC-EDF has a cladding radius of 62.5 μm, an inner ring-core radius of 1.7 μm, and a ring core thickness of 5 μm. Moreover. the refractive-index difference between the ring core and cladding is 0.0127. An experimental setup is built to characterize the amplification performance of the first- and second-order vortex modes in the proposed RC-EDF. Furthermore, an infrared camera is used to detect the output beam profile of the RC-EDF. The results show that the clean vortex modes (|l|=1-2) are stably excited and transmitted in the RC-EDF. The maximum gain (|l|=1-2) is 32.6 dB at 1530 nm [Fig. 7(b)]. The maximum output power values of the first- and second-order vortex modes are 14 dBm and 15 dBm, respectively (Fig. 8). The fabricated RC-EDF has a small overlap area between the pump and signal modes, and the doping distribution of the erbium is uneven, which causes the gains to be different in the theoretical calculations and experimental measurements.
In this study, a ring-core erbium-doped fiber that supports the first- and second-order vortex modes is designed, and modal gains (|l|=1-2) are improved by optimizing the thickness of the ring core and the width of the erbium-doping region. Based on these optimized parameters, we perform a theoretical simulation and find that the vortex mode gains are higher than 35.4 dB in the entire C band. The performance of the fabricated RC-EDF is experimentally characterized. The gains of the vortex modes (|l|=1-2) are higher than 32.6 dB at 1530 nm wavelength. The proposed high-gain vortex mode amplifier based on the ring-core erbium-doped fiber is expected to be widely used in long-distance and large-capacity spatial division multiplexing fiber communications.
