As the scientific community seeks efficient optical neural interfaces with sub-cortical structures of the mouse brain, a wide set of technologies and methods is being developed to monitor cellular events through fluorescence signals generated by genetically encoded molecules. Among these technologies, tapered optical fibers (TFs) take advantage of the modal properties of narrowing waveguides to enable both depth-resolved and wide-volume light collection from scattering tissue, with minimized invasiveness with respect to standard flat fiber stubs (FFs). However, light guided in patch cords as well as in FFs and TFs can result in autofluorescence (AF) signal, which can act as a source of time-variable noise and limit their application to probe fluorescence lifetime in vivo. In this work, we compare the AF signal of FFs and TFs, highlighting the influence of the cladding composition on AF generation. We show that the autofluorescence signal generated in TFs has a peculiar coupling pattern with guided modes, and that far-field detection can be exploited to separate functional fluorescence from AF. On these bases, we provide evidence that TFs can be employed to implement depth-resolved fluorescence lifetime photometry, potentially enabling the extraction of a new set of information from deep brain regions, as time-correlating single photon counting starts to be applied in freely-moving animals to monitor the intracellular biochemical state of neurons.

中文翻译：

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扁平光纤和锥形光纤自发荧光在脑组织深度分辨荧光寿命测光中的比较研究

随着科学界寻求与小鼠大脑皮层下结构的有效光学神经接口，正在开发一系列广泛的技术和方法，以通过基因编码分子产生的荧光信号监测细胞事件。在这些技术中，锥形光纤 (TF) 利用变窄波导的模态特性来实现从散射组织中进行深度分辨和大体积光收集，并且相对于标准扁平光纤短截线 (FF) 的侵入性最小。然而，在跳线以及 FFs 和 TFs 中引导的光会导致自发荧光 (AF) 信号，它可以作为时变噪声的来源并限制它们在体内探测荧光寿命的应用. 在这项工作中，我们比较了 FF 和 TF 的 AF 信号，突出了包层成分对 AF 生成的影响。我们表明在 TF 中产生的自发荧光信号与引导模式具有特殊的耦合模式，并且可以利用远场检测将功能性荧光与 AF 分开。在这些基础上，我们提供了证据表明 TFs 可用于实现深度分辨荧光寿命光度测量，随着时间相关单光子计数开始自由应用，有可能从大脑深部区域提取一组新信息——移动动物以监测神经元的细胞内生化状态。