On-the-fly Correction of Multi-Mode Fibre Bending with Fibre Bragg Gratings for Two-Photon Fluorescence Imaging of Living Neurons

该项目旨在通过多模光纤（MMF）解决活体神经元体积双光子荧光成像的局限性。MMFs对弯曲的高灵敏度极大地限制了其在体内成像中的应用。通过光纤布拉格光栅（fbg）的结合，我们的目标是表征光纤弯曲和纠正弯曲引起的光畸变。荧光成像早已成为生物医学成像的有力工具。由于炎症的危险，在体内神经元成像中，以最小的侵入性将可见性扩展到表面以外以研究深层组织的挑战尤为重要。双光子荧光成像已经成为一种优越的方法，因为它增加了组织穿透，同时减少了长波光、光漂白和背景荧光的散射。它需要一个体积激励焦点，辐射通过内窥镜探头传递和收集。梯度折射率透镜传统上用于此。然而，由于mmf的尺寸小得多，它提供了一种侵入性小得多的替代方案。MMFs在远端产生复杂的斑点图案，这是通过光纤的确定性传播的产物。入射波形可以用空间光调制器（SLM）来形成一个轴向拉长的激发焦点，这是双光子荧光成像所需要的。在以前的工作中，mmf在很大程度上是刚性的，或者用很少的纤维弯曲处理。然而，mmf在成像中对弯曲非常敏感，这会导致明显的光畸变。通过弯曲MMF传播的光将在产生的散斑模式中经历变化。这意味着SLM需要重新编程以重塑入射波前并重新聚焦光。在活体双光子荧光成像中，纤维弯曲是一个需要解决的重要挑战，因为动物的任何运动都会破坏图像质量。因此，研究如何校正由MMF弯曲引起的光畸变是非常重要的。对MMF弯曲的实时校正将是一项革命性的技术，它可以在清醒的、有行为的动物的大脑区域中成像，这些区域迄今为止完全无法用这种分辨率进行研究。其中一些区域，如伏隔核，在成瘾等现象中起着重要作用，因此代表了非常重要的研究领域。为了解决成像系统中光纤弯曲的处理问题，fbg将用于动态校正由于弯曲引起的光畸变。fbg是由周期性折射率变化组成的光纤传感器件，可以用飞秒激光器写入MMF中。它们选择性地反射光纤芯折射率调制中与布拉格条件共振的波长。fbg以前用于监测岩土工程应用中的弯曲应变。通过对fbg能否用于获取弯曲信息的原理证明，开辟了许多研究的可能性。一个简单的模型可以从MMF核心的中心向外轴向看到不同的fbg，从而获得弯曲方向的信息。更复杂的反射器可能包括研究波分复用，其中fbg在同一光纤中调谐到不同的波长。然后可以同时测量多个FBG结构。这将为进一步实现MMF成像工程可能性提供重要途径。由此产生的技术不仅可以应用于生物医学领域，还可以应用于临床成像和工业检查。该项目属于EPSRC光通信研究领域，与光学设备和子系统以及医学成像重叠。

The project aims to tackle limitations in volumetric two-photon fluorescence imaging of live neurons through a multi-mode fibre (MMF). The high sensitivity of MMFs to bending significantly restricts utility in in vivo imaging. Through the incorporation of fibre Bragg gratings (FBGs), we aim to characterise fibre bending and correct light distortion caused by bending on-the-fly.Fluorescence imaging has long been established as a powerful tool in biomedical imaging. The challenge to extend visibility beyond the surface to investigate deeper-lying tissue with minimal invasiveness is especially crucial in in vivo neuron imaging due to the danger of inflammation. Two-photon fluorescence imaging has emerged as a superior method due to increased tissue penetration while reducing the scattering of longer wavelength light, photobleaching, and background fluorescence. It requires a volumetric excitation focus to which radiation is delivered and collected from through an endoscopic probe. Gradient index lenses have traditionally been used for this. However, MMFs offer a much less invasive alternative owing to their significantly smaller size. MMFs produce complicated speckle patterns at the distal end which are the product of deterministic propagation through the fibre. The incident waveform can be shaped with a spatial light modulator (SLM) to produce an axially elongated excitation focus as required for two-photon fluorescence imaging. In previous work, MMFs have largely been held rigid or treated with little fibre bending. MMFs in imaging are, however, extremely sensitive to bending which results in significant light distortion. Light propagating through a bent MMF will experience changes in the produced speckle pattern. This means that the SLM needs to be reprogrammed to reshape the incident wavefront and refocus the light. In in vivo two-photon fluorescence imaging, fibre bending is an important challenge to address since any motion of the animal will otherwise destroy image quality. It is therefore hugely important to study how the light distortion caused by MMF bending can be corrected for. An on-the-fly correction for MMF bending would be a revolutionary technology to image neurons in awake, behaving animals in brain regions that have hitherto been completely inaccessible for study with this resolution. Some of these regions, such as the nucleus accumbens, have a major role in phenomena such as addiction, and as such represent hugely important areas for study. To tackle the treatment of fibre bending in imaging systems, FBGs will be employed for an on-the-fly correction of light distortion due to bending. FBGs are fibre optic sensing devices consisting of periodic refractive index variations which can be written into the MMF with a femtosecond laser. They selectively reflect wavelengths which are on resonance with the Bragg condition in the fibre core index modulation. FBGs have previously been used to monitor bending strain in geotechnical applications. Following a proof of principle of whether FBGs can be used to obtain information on bending opens up many research possibilities. A simple model could see distinct FBGs axially outwards from the centre of the MMF core to attain information on the bending orientation. More complex reflectors may include investigating wavelength division multiplexing, wherein FBGs are tuned to different wavelengths in the same fibre. Multiple FBG structures can then be measured simultaneously. This will provide an important pathway into further MMF engineering possibilities for imaging. The resulting technology will have applications not only in biomedical settings but also in clinical imaging and industrial inspection. This project falls within the EPSRC Optical Communications research area, overlapping with Optical Devices and Subsystems and Medical Imaging.