Word alignment plays a crucial role in various natural language processing tasks, such as serving as cross-lingual signals for sentence embedding, reducing hallucination and omission in machine translation, and facilitating the construction of training data for simultaneous speech translation.Current state-of-the-art approaches usually rely on: (1) supervised data and large-scale weakly supervised data constructed from Wikipedia and (2) multilingual Transformer encoder-based models.However, we find that the current state-of-the-art encoder-based method, BinaryAlign, suffers from the issue of insufficient labeled data, and we further improve it with self-training with a small amount of parallel data. In addition, considering the impressive performance of multilingual large language models on many natural language processing tasks, we also explore the possibility of using these decoder-based large language models as word aligners. We observe that although fine-tuning large language models with labeled data produces acceptable results, augmenting the training with pseudo-labeled data further enhances model performance. Based on the findings, we propose a semi-supervised framework to improve the large language model-based word aligners. Experimental results demonstrate that the proposed method with a small amount of parallel data outperforms the current state-of-the-art method on various word alignment datasets.

Recently, many works have been attempting to adapt Large Language Models (LLMs) for sentence embedding, with most of them fine-tuning LLMs towards the contrastive objective and enabling bi-directional attention for better performance, using LoRA to address the large model scale.In this work, we suggest that this adaptation can also be simply and effectively achieved using causal attention and with even fewer trainable parameters through soft prompt tuning, as an alternative to fine-tuning with LoRA and other methods with extra post-training tasks.Our method only optimizes a few learnable tokens while keeping the rest of the model frozen.Through experiments on a diverse set of evaluation tasks, we find that simply tuning only a few tokens can achieve a competitive performance with that of fine-tuning with LoRA. The percentage of trainable parameters can be reduced to less than 0.001%. Moreover, we also demonstrate that turning causal attention to bi-directional attention with or without extra post-training tasks does not provide additional benefit when soft prompt tuning is applied, suggesting that causal attention can be naturally used in decoder-only LLMs for sentence embedding adaptation.

The problem of hallucination and omission, a long-standing problem in machine translation (MT), is more pronounced when a large language model (LLM) is used in MT because an LLM itself is susceptible to these phenomena. In this work, we mitigate the problem in an LLM-based MT model by guiding it to better word alignment. We first study the correlation between word alignment and the phenomena of hallucination and omission in MT. Then we propose to utilize word alignment as preference to optimize the LLM-based MT model. The preference data are constructed by selecting chosen and rejected translations from multiple MT tools. Subsequently, direct preference optimization is used to optimize the LLM-based model towards the preference signal. Given the absence of evaluators specifically designed for hallucination and omission in MT, we further propose selecting hard instances and utilizing GPT-4 to directly evaluate the performance of the models in mitigating these issues. We verify the rationality of these designed evaluation methods by experiments, followed by extensive results demonstrating the effectiveness of word alignment-based preference optimization to mitigate hallucination and omission. On the other hand, although it shows promise in mitigating hallucination and omission, the overall performance of MT in different language directions remains mixed, with slight increases in BLEU and decreases in COMET.

The field of cross-lingual sentence embeddings has recently experienced significant advancements, but research concerning low-resource languages has lagged due to the scarcity of parallel corpora. This paper shows that cross-lingual word representation in low-resource languages is notably under-aligned with that in high-resource languages in current models. To address this, we introduce a novel framework that explicitly aligns words between English and eight low-resource languages, utilizing off-the-shelf word alignment models. This framework incorporates three primary training objectives: aligned word prediction and word translation ranking, along with the widely used translation ranking. We evaluate our approach through experiments on the bitext retrieval task, which demonstrate substantial improvements on sentence embeddings in low-resource languages. In addition, the competitive performance of the proposed model across a broader range of tasks in high-resource languages underscores its practicality.