Optical filters are essential parts of advanced optical communication and sensing systems. Among them, the ones with an ultrawide free spectral range (FSR) are especially critical. They are promising to provide access to numerous wavelength channels highly desired for large-capacity optical transmission and multipoint multiparameter sensing. Present schemes for wide-FSR filters either suffer from limited cavity length or poor fabrication tolerance or impose an additional active-tuning control requirement. We theoretically and experimentally demonstrate a filter that features FSR-free operation capability, subnanometer optical bandwidth, and acceptable fabrication tolerance. Only one single deep dip within a record-large waveband ($\mathrm{S}+\mathrm{C}+\mathrm{L}$ band) is observed by appropriately designing a side-coupled Bragg-grating-assisted Fabry–Perot filter, which has been applied as the basic sensing unit for both the refractive index and temperature measurement. Five such basic units are also cascaded in series to demonstrate a multichannel filter. This work provides a new insight to design FSR-free filters and opens up a possibility of flexible large-capacity integration using more wavelength channels, which will greatly advance integrated photonics in optical communication and sensing.

中文翻译：

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在 S + C + L 波段具有超宽可调性的无自由光谱范围的滤波器

滤光器是先进光通信和传感系统的重要组成部分。其中，具有超宽自由光谱范围 (FSR) 的那些尤为关键。它们有望为大容量光传输和多点多参数传感提供对大量波长通道的访问。宽 FSR 滤波器的现有方案要么受到有限的腔长度或较差的制造公差，要么强加了额外的有源调谐控制要求。我们从理论上和实验上证明了一种具有无 FSR 操作能力、亚纳米光学带宽和可接受的制造公差的滤波器。在创纪录的大波段（$\mathrm{秒}+\mathrm{C}+\mathrm{升}$波段）通过适当设计侧耦合布拉格光栅辅助法布里-珀罗滤波器来观察，该滤波器已被用作折射率和温度测量的基本传感单元。五个这样的基本单元也串联起来以演示多通道滤波器。这项工作为设计无 FSR 滤波器提供了新的见解，并开辟了使用更多波长通道进行灵活大容量集成的可能性，这将极大地推进光通信和传感中的集成光子学。