Opt Express. 2022 Nov 21;30(24):43439-43452. doi: 10.1364/OE.473705.
Mode filters are fundamental elements in a mode-division multiplexing (MDM) system for reducing modal cross-talk or realizing modal routing. However, the previously reported silicon mode filters can only filter one specific mode at a time and multiple modes filtering usually needs a cascade of several filters, which is adverse to highly integrated MDM systems. Here, we propose a unique concept to realize compact, scalable and flexible mode filters based on backward mode conversion gratings elaborately embedded in a multimode waveguide. Our proposed method is highly scalable for realizing a higher-order-mode-pass or band-mode-pass filter of any order and capable of flexibly filtering one or multiple modes simultaneously. We have demonstrated the concept through the design of four filters for different order of mode(s) and one mode demultiplexer based on such a filter, and the measurement of two fabricated 11μm length filters (TE1-pass/TE2-pass) show that an excellent performance of insertion loss <1.0dB/1.5dB and extinction ratio >29dB/28.5dB is achieved over a bandwidth of 51.2nm/48.3nm, which are competitive with the state-of-the-art.
模式滤波器是模式分复用（MDM）系统中的基本元件，用于减少模式串扰或实现模式路由。然而，以前报道的硅基模式滤波器一次只能滤除一个特定的模式，而多模式滤波通常需要几个滤波器级联，这不利于高度集成的 MDM 系统。在这里，我们提出了一种独特的概念，基于精心嵌入在多模波导中的反向模式转换光栅来实现紧凑、可扩展和灵活的模式滤波器。我们的方法具有高度的可扩展性，可实现任意阶的高阶模式通过滤波器或带模式通过滤波器，并能够灵活地同时滤除一个或多个模式。我们通过设计四个用于不同阶模式的滤波器和一个基于这种滤波器的模式解复用器来验证了这一概念，并且对两个制造的 11μm 长度滤波器（TE1 通过/TE2 通过）的测量表明，在 51.2nm/48.3nm 的带宽范围内，插入损耗<1.0dB/1.5dB，消光比>29dB/28.5dB 的优异性能是可以实现的，这与现有技术相当。
