Generating Airy beams through multimode fibres.

Focussing light through a multimode fibre (MMF) is the basis of holographic endoscopes, which currently enable detailed imaging of deep tissue. Achieving high fidelity and purity diffraction-limited foci has been shown to be possible, when fully controlling the amplitude, phase, and two orthogonal polarisation states of the input field. Yet, generating more complex field distributions with similar performance remains to be assessed. Here, we demonstrate the generation of Airy beams through an MMF containing in excess of 90 % of the optical power delivered by the fibre. We discuss two distinct methods for generating optical landscapes: the direct field and the Fourier domain synthesis. Moreover, we showcase the flexibility of the Fourier domain synthesis to modify the generated beam.

Advancing the path to in-vivo imaging in freely moving mice via multimode-multicore fiber based holographic endoscopy.

Significance: Hair-thin multimode optical fiber-based holographic endoscopes have gained considerable interest in modern neuroscience for their ability to achieve cellular and even subcellular resolution during in-vivo deep brain imaging. However, the application of multimode fibers in freely moving animals presents a persistent challenge as it is difficult to maintain optimal imaging performance while the fiber undergoes deformations. Aim: We propose a fiber solution for challenging in-vivo applications with the capability of deep brain high spatial resolution imaging and neuronal activity monitoring in anesthetized as well as awake behaving mice. Approach: We used our previously developed M3CF multimode-multicore fiber to record fluorescently labeled neurons in anesthetized mice. Our M3CF exhibits a cascaded refractive index structure, enabling two distinct regimes of light transport that imitate either a multimode or a multicore fiber. The M3CF has been specifically designed for use in the initial phase of an in-vivo experiment, allowing for the navigation of the endoscope's distal end toward the targeted brain structure. The multicore regime enables the transfer of light to and from each individual neuron within the field of view. For chronic experiments in awake behaving mice, it is crucial to allow for disconnecting the fiber and the animal between experiments. Therefore, we provide here an effective solution and establish a protocol for reconnection of two segments of M3CF with hexagonally arranged corelets. Results: We successfully utilized the M3CF to image neurons in anaesthetized transgenic mice expressing enhanced green fluorescent protein. Additionally, we compared imaging results obtained with the M3CF with larger numerical aperture (NA) fibers in fixed whole-brain tissue. Conclusions: This study focuses on addressing challenges and providing insights into the use of multimode-multicore fibers as imaging solutions for in-vivo applications. We suggest that the upcoming version of the M3CF increases the overall NA between the two cladding layers to allow for access to high resolution spatial imaging. As the NA increases in the multimode regime, the fiber diameter and ring structure must be reduced to minimize the computational burden and invasiveness.

Near perfect focusing through multimode fibres.

Holographic, multimode fibre (MMF) based endoscopes envision high-quality in-vivo imaging inside previously inaccessible structures of living organisms, amongst other perspective applications. Within these instruments, a digital micro-mirror device (DMD) is deployed in order to holographically synthesise light fields which, after traversing the multimode fibre, form foci at desired positions behind the distal fibre facet. When applied in various imaging modalities, the purity and sharpness of the achieved foci are determinant for the imaging performance. Here we present diffraction-limited foci, which contain in excess of 96% of optical power delivered by the fibre which, to the best of our knowledge, represents the highest value reported to date. Further, we quantitatively study the impact of various conditions of the experimental procedure including input polarisation settings, influence of ghost diffraction orders, light modulation regimes, bias of the calibration camera and the influence of noise.

Giant refractometric sensitivity by combining extreme optical Vernier effect and modal interference.

The optical Vernier effect consists of overlapping responses of a sensing and a reference interferometer with slightly shifted interferometric frequencies. The beating modulation thus generated presents high magnified sensitivity and resolution compared to the sensing interferometer, if the two interferometers are slightly out of tune with each other. However, the outcome of such a condition is a large beating modulation, immeasurable by conventional detection systems due to practical limitations of the usable spectral range. We propose a method to surpass this limitation by using a few-mode sensing interferometer instead of a single-mode one. The overlap response of the different modes produces a measurable envelope, whilst preserving an extremely high magnification factor, an order of magnification higher than current state-of-the-art performances. Furthermore, we demonstrate the application of that method in the development of a giant sensitivity fibre refractometer with a sensitivity of around 500 µm/RIU (refractive index unit) and with a magnification factor over 850.

Optical Harmonic Vernier Effect: A New Tool for High Performance Interferometric Fibre Sensors.

The optical Vernier effect magnifies the sensing capabilities of an interferometer, allowing for unprecedented sensitivities and resolutions to be achieved. Just like a caliper uses two different scales to achieve higher resolution measurements, the optical Vernier effect is based on the overlap in the responses of two interferometers with slightly detuned interference signals. Here, we present a novel approach in detail, which introduces optical harmonics to the Vernier effect through Fabry-Perot interferometers, where the two interferometers can have very different frequencies in the interferometric pattern. We demonstrate not only a considerable enhancement compared to current methods, but also better control of the sensitivity magnification factor, which scales up with the order of the harmonics, allowing us to surpass the limits of the conventional Vernier effect as used today. In addition, this novel concept opens also new ways of dimensioning the sensing structures, together with improved fabrication tolerances.

Multimode Fabry⁻Perot Interferometer Probe Based on Vernier Effect for Enhanced Temperature Sensing.

New miniaturized sensors for biological and medical applications must be adapted to the measuring environments and they should provide a high measurement resolution to sense small changes. The Vernier effect is an effective way of magnifying the sensitivity of a device, allowing for higher resolution sensing. We applied this concept to the development of a small-size optical fiber Fabry⁻Perot interferometer probe that presents more than 60-fold higher sensitivity to temperature than the normal Fabry⁻Perot interferometer without the Vernier effect. This enables the sensor to reach higher temperature resolutions. The silica Fabry⁻Perot interferometer is created by focused ion beam milling of the end of a tapered multimode fiber. Multiple Fabry⁻Perot interferometers with shifted frequencies are generated in the cavity due to the presence of multiple modes. The reflection spectrum shows two main components in the Fast Fourier transform that give rise to the Vernier effect. The superposition of these components presents an enhancement of sensitivity to temperature. The same effect is also obtained by monitoring the reflection spectrum node without any filtering. A temperature sensitivity of -654 pm/°C was obtained between 30 °C and 120 °C, with an experimental resolution of 0.14 °C. Stability measurements are also reported.

Neuromuscular electrical stimulation alters gene expression and delays quadriceps muscle atrophy of rats after anterior cruciate ligament transection.

INTRODUCTION: Neuromuscular electrical stimulation (NMES) is used to improve quadriceps mass after anterior cruciate ligament (ACL) injury. We studied the effect of NMES on mRNA levels of atrophy genes in the quadriceps muscle of rats after ACL transection. METHODS: mRNA levels of atrogin-1, MuRF-1, and myostatin were assessed by quantitative PCR and the polyubiquitinated proteins by Western blot at 1, 2, 3, 7, and 15 days postinjury. RESULTS: NMES minimized the accumulation of atrogenes and myostatin according to time period. NMES also prevented reduction in muscle mass in all muscles of the ACLES group at 3 days. CONCLUSIONS: Use of NMES decreased the accumulation of atrogenes and myostatin mRNA in the quadriceps muscles, inhibiting early atrophy at 3 days, although it did not prevent atrophy at 7 and 15 days after ACL transection. This study highlights the importance of therapeutic NMES interventions in the acute phase after ACL transection.

Palmitate modulates the early steps of insulin signalling pathway in pancreatic islets.

Insulin stimulates its own secretion and synthesis by pancreatic beta-cells. Although the exact molecular mechanism involved is unknown, changes in beta-cell insulin signalling have been recognized as a potential link between insulin resistance and its impaired release, as observed in non-insulin-dependent diabetes. However, insulin resistance is also associated with elevated plasma levels of free fatty acids (FFA) that are well known modulators of insulin secretion by pancreatic islets. This information led us to investigate the effect of FFA on insulin receptor signalling in pancreatic islets. Exposure of pancreatic islets to palmitate caused up-regulation of several insulin-induced activities including tyrosine phosphorylation of insulin receptor and pp185. This is the first evidence that short exposure of these cells to 100 microM palmitate activates the early steps of insulin receptor signalling. 2-Bromopalmitate, a carnitine palmitoyl-CoA transferase-1 inhibitor, did not affect the effect of the fatty acid. Cerulenin, an acylation inhibitor, abolished the palmitate effect on protein levels and phosphorylation of insulin receptor. This result supports the proposition that protein acylation may be an important mechanism by which palmitate exerts its modulating effect on the intracellular insulin signalling pathway in rat pancreatic islets.