Diffractive neural networks have recently emerged as a promising framework for all-optical computing. However, these networks are typically trained for a single task, limiting their potential adoption in systems requiring multiple functionalities. Existing approaches to achieving multi-task functionality either modify the mechanical configuration of the network per task or use a different illumination wavelength or polarization state for each task. In this work, we propose a new control mechanism, which is based on the illumination's angular spectrum. Specifically, we shape the illumination using an amplitude mask that selectively controls its angular spectrum. We employ different illumination masks for achieving different network functionalities, so that the mask serves as a unique task encoder. Interestingly, we show that effective control can be achieved over a very narrow angular range, within the paraxial regime. We numerically illustrate the proposed approach by training a single diffractive network to perform multiple image-to-image translation tasks. In particular, we demonstrate translating handwritten digits into typeset digits of different values, and translating handwritten English letters into typeset numbers and typeset Greek letters, where the type of the output is determined by the illumination's angular components. As we show, the proposed framework can work under different coherence conditions, and can be combined with existing control strategies, such as different wavelengths. Our results establish the illumination angular spectrum as a powerful degree of freedom for controlling diffractive networks, enabling a scalable and versatile framework for multi-task all-optical computing.

翻译：衍射神经网络作为一种全光学计算框架，近年来展现出广阔的应用前景。然而，此类网络通常针对单一任务进行训练，限制了其在需要多功能性的系统中的潜在应用。现有实现多任务功能的方法，要么针对每个任务改变网络的机械结构，要么为每个任务使用不同的照明波长或偏振态。本研究提出一种基于照明角谱的新型调控机制。具体而言，我们通过振幅掩模对入射光场进行整形，从而选择性调控其角谱分布。通过采用不同的照明掩模实现不同的网络功能，使得掩模成为独特的任务编码器。值得注意的是，我们证明在近轴区域内，即使角谱范围非常狭窄，仍能实现有效的功能调控。我们通过数值模拟验证了所提方法：训练单个衍射网络执行多种图像到图像的转换任务。特别展示了将手写数字转换为不同数值的印刷体数字，以及将手写英文字母转换为印刷体数字和希腊字母的案例，其中输出类型由照明角谱分量决定。研究表明，该框架可在不同相干条件下工作，并能与现有调控策略（如不同波长）相结合。我们的研究结果确立了照明角谱作为调控衍射网络的重要自由度，为构建可扩展、多功能的全光学计算框架提供了新途径。