There is burst growth in the demand for network capacity, with the rapid development of new technologies, such as big data and cloud computing, etc. As the cornerstone of the global network, the traditional communication system based on the single-mode fiber has been rapidly developed for nearly 50 years. Its capacity has approached the nonlinear Shannon limit, which cannot meet the demand of network bandwidth surge. Mode-division multiplexing (MDM) technology based on multiple orthogonal modes in few-mode fibers (FMFs) is expected to overcome this capacity crunch.
In recent years, aided by FMFs with low group delay and loss, multiplexers and demultiplexers with low insertion loss and high isolation, and digital signal processing technology with low complexity, a series of significant works are demonstrated in long-haul MDM transmission systems, which greatly accelerates the commercialization of MDM technology.
Currently, parallel single-mode erbium-doped fiber amplifiers (SM-EDFAs) are utilized to amplify the transmitted signals in most of the reported MDM long-haul transmission systems. Compared with SM-EDFAs, few-mode erbium-doped fiber amplifiers (FM-EDFAs) are more low-cost and power-efficient, as they can simultaneously amplify multiple modes of FMFs. However, previously demonstrated FM-EDFAs are constructed with many discrete bulky free-space components, such as isolators and power combiners, which not only lack stability and flexibility, but also are expensive and suffer from a high coupling loss. The span length and transmission distance of long-haul MDM transmission systems with above FM-EDFAs are restricted, which makes FM-EDFAs less practical.
To address the problems, a research group led by Prof. Lin Zhang, associate Prof. Zhiqun Yang and Dr. Yaping Liu at Tianjin University and Peng Cheng National Laboratory, in collaboration with Dr. Cheng Du's Team at FiberHome Telecommunication Technologies Co., Ltd, has experimentally demonstrated a high-gain integrated in-line few-mode amplifier enabling long-haul FMF transmission.
The FM-EDFA exhibits superior optical properties, such as the averaged modal gain of 25 dB, output power of ~21 dBm, differential modal gain (DMG) of <1.1 dB and noise figures (NFs) of <6.5 dB over the C band. Combined with a new discrete coupled transmission architecture, the group has demonstrated a 28-Gbaud quadrature phase-shift keying (QPSK) transmission with the in-line few-mode amplification based on the integrated 3M-EDFA over a 3840-km FMF link, which represents the longest transmission distance among all the existing MDM transmission systems with in-line FM-EDFAs. The relevant research results were published in Photonics Research, Volume 10, No. 12, 2022 (Tao Xu, Tianyu Gao, Yanze Wang, Wenhao Li, Wei Li, Cheng Du, Zhiqun Yang, Yaping Liu, Lin Zhang. High-gain integrated in-line few-mode amplifier enabling 3840-km long-haul transmission[J]. Photonics Research, 2022, 10(12): 2794).
The integrated FM-EDFA is a dual-stage amplifier and constructed by integrated passive isolators and power combiners. In order to achieve high output power and low NFs, forward and backward pumping configurations are applied to the first and the second stages of the amplifier, respectively (Fig. 1a). The results show that the averaged modal gain is >25 dB and the DMG is <1.1 dB (Fig. 1b). Besides, the NFs are <6.5 dB (Fig. 1c).
Fig. 1 (a) Schematic of the dual-staged integrated FM-EDFA. (b) the modal gain and DMG, (c) NF of the amplifier
Then, the group construct a recirculating three-mode loop system, to verify the practicality of the 3M-EDFA, which can effectively compensate the 64-km fiber link loss at very low cost (Fig. 2a). A frequency-domain equalization is used to recover the data. Moreover, a cyclic mode permutation scheme is applied to suppressed DMD-induced pulse spreading, which effectively reduces the algorithm complexity. A 28-Gbaud QPSK transmission over 3840-km FMF are demonstrated. After 3840-km transmission, the average BER is below the forward error correction limit (Fig 2b).
Fig. 2 (a) Experimental setup for the MDM transmission over 3840-km FMF with the proposed 3M-EDFA. (b) BER versus the transmission distance.
Compared to previous FM-EDFAs with bulk free-space components, the dual-stage integrated FM-EDFA exhibits superior properties, such as low DMG, high modal gain, stability and flexibility, representing a necessary step toward practical MDM transmission systems. In addition, from the viewpoint of MDM's commercialization, integrated FM-EDFAs are economical.
