Explicit Alignment Learning for Neural Machine Translation

Abstract

Even though neural machine translation (NMT) has become the state-of-the-art solution for end-to-end translation, it still suffers from a lack of translation interpretability, which may be conveniently enhanced by explicit alignment learning (EAL), as performed in traditional statistical machine translation (SMT). To provide the benefits of both NMT and SMT, this paper presents a novel model design that enhances NMT with an additional training process for EAL, in addition to the end-to-end translation training. Thus, we propose two approaches an explicit alignment learning approach, in which we further remove the need for the additional alignment model, and perform embedding mixup with the alignment based on encoder--decoder attention weights in the NMT model. We conducted experiments on both small-scale (IWSLT14 De->En and IWSLT13 Fr->En) and large-scale (WMT14 En->De, En->Fr, WMT17 Zh->En) benchmarks. Evaluation results show that our EAL methods significantly outperformed strong baseline methods, which shows the effectiveness of EAL. Further explorations show that the translation improvements are due to a better spatial alignment of the source and target language embeddings. Our method improves translation performance without the need to increase model parameters and training data, which verifies that the idea of incorporating techniques of SMT into NMT is worthwhile.