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

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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.