We report a new route to mitigate focal-ratio degradation (FRD) in multimode optical fiber links. Our approach uses a custom multicore fiber (MCF) with dissimilar cores, which is then tapered at both ends to form multimode cores and create mode-selective photonic lantern (PL) transitions. The cores of the MCF are single-moded at 1550 nm and sufficiently spaced such that there is no observable core-to-core cross talk after a 7 m fiber length. The mode-selective PLs at each end of the MCF, in combinations with the low core-to-core cross talk of the MCF at 1550 nm, ensure that the relative amount of power in each spatial mode is preserved throughout the fibre link. As such, a PL-MCF-PL link using this approach has the potential to be resistant to FRD by inhibiting the coupling of light from lower-order to higher-order modes. We show that the full PL-MCF-PL link exhibits lower FRD than a custom multimode fiber that guides the same number of modes. A study of how the FRD behavior of the PL-MCF-PL link varies as a function of wavelength indicates some bandwidth limitations due to the wavelength-dependent properties of the PLs and the cross coupling in the MCF.
Large telescopes use adaptive optics to correct aberrations in wavefronts over large areas. Such aberrations can be measured by a Shack-Hartmann wavefront sensor, an array of individual sensor elements each comprising a micro-lens with a detector at its focal plane. Any phase variation across the wavefront incident upon a sensor element causes the focal spot to move on the detector. The location of the spot therefore provides information about the magnitude and direction of the local tilt in the wavefront at that element.
We demonstrate a novel wavefront sensor based on coupling in a multi-core optical fibre. The fibre contains three identical single-mode cores symmetrically positioned at the corners of an equilateral triangle. The fibre is designed to have negligible coupling between the cores, so that their power distribution is faithfully transmitted along the fibre despite any time-varying perturbations. However, the input end of the fibre is locally narrowed down so that the cores couple over 1/4 of a coupling beatlength. This section acts like the lens in a Shack-Hartmann element, in that it converts phase variations at the input (due to wavefront tilt) into intensity variations at the output. Thus sensing of wavefront tilt can be physically remote from detection, in a compact lens-less all-fibre structure.
We fabricated a three-core fibre with cores of numerical aperture 0.11, diameter 10 μm and core-to-core centre separation of 40 μm. It was designed so that, for 1550 nm light, less than 0.1% of the light coupled from one core to the others over a length of 150 m. The input section was narrowed over about 40 mm by tapering (ie, heating and stretching) the fibre with an oxy-butane flame. To obtain the required 1/4 beatlength of coupling, we launched 1550 nm light into one core and monitored the outputs in all three as the fibre was elongated. Tapering was stopped when coupled power was maximised, corresponding to 1/2 beatlength of coupling . The narrowed section was then cleaved in the middle, yielding 1/4 beatlength of coupling.
To investigate wavefront tilt, the narrowed end of the fibre was mounted in a reflective puck on a mirror mount 7 cm from the end of a single-mode fibre carrying 1550 nm light. This projects a plane wave onto the sensor. The angle and direction of tilt were varied by adjusting the mirror mount, mimicking a perturbation of the plane wavefront. The position of a 635 nm laser beam reflected by the puck onto a screen recorded the angle of tilt. In our preliminary experiments, a range of tilt angles of 2.5 degrees could be measured, along with the direction of the tilt.
In due course, we envisage a full wavefront sensor array constructed from a multi-core fibre containing many such groups of three cores, tapered at one end and connected to a remotely-placed sensor array at the other.
 R. J., Black et al., Electron. Lett. 22, 1311-1313 (1986)
We present a multifunctional endoscope capable of imaging, fluid delivery and fluid sampling in the alveolar space. The endoscope consists of an imaging fibre bundle fabricated from cost effective OM1 PCVD graded index preforms made for the telecommunications market. These low-cost fibres could potentially make our endoscope disposable after a single use. The performance of our low-cost imaging fibre bundle is shown to be comparable to the current commercial state-of-the-art. The imaging fibre bundle is packaged alongside two channels for the delivery and extraction of fluids. The fluid delivery channels can be used to deliver fluorescent smart probes for the detection of pathogens and to perform a targeted alveolar lavage without the removal of the imaging fibre as is currently standard procedure. Our endoscope is fully biocompatible and with an overall outer diameter of 1.4 mm allowing it to fit into the standard working channel of a bronchoscope. We demonstrate the use of our endoscope in ex-vivo human lungs. We show alveolar tissue and bacterial imaging over two wavelength bands 520 nm – 600 nm and 650 nm – 750 nm both commonly used for bacterial smart probe detection.
We present a toolkit for a multiplexed pH and oxygen sensing probe in the distal lung using multicore fibres. Measuring physiological relevant parameters like pH and oxygen is of significant importance in understanding changes associated with disease pathology. We present here, a single multicore fibre based pH and oxygen sensing probe which can be used with a standard bronchoscope to perform in vivo measurements in the distal lung.
The multiplexed probe consists of fluorescent pH sensors (fluorescein based) and oxygen sensors (Palladium porphyrin complex based) covalently bonded to silica microspheres (10 µm) loaded on the distal facet of a 19 core (10 µm core diameter) multicore fibre (total diameter of ~150 µm excluding coating). Pits are formed by selectively etching the cores using hydrofluoric acid, multiplexing is achieved through the self-location of individual probes on differing cores. This architecture can be expanded to include probes for further parameters. Robust measurements are demonstrated of self-referencing fluorophores, not limited by photobleaching, with short (100ms) measurement times at low (~10µW) illumination powers.
We have performed on bench calibration and tests of in vitro tissue models and in an ovine whole lung model to validate our sensors. The pH sensor is demonstrated in the physiologically relevant range of pH 5 to pH 8.5 and with an accuracy of ± 0.05 pH units. The oxygen sensor is demonstrated in gas mixtures downwards from 20% oxygen and in liquid saturated with 20% oxygen mixtures (~8mg/L) down to full depletion (0mg/L) with ~0.5mg/L accuracy.