Optical Waveguide Theory

Although biological cells are mostly transparent, they are phase objects that differ in shape and refractive index. Any image that is projected through layers of randomly oriented cells will normally be distorted by refraction, reflection, and scattering. Counterintuitively, the retina of the vertebrate eye is inverted with respect to its optical function and light must pass through several tissue layers before reaching the light-detecting photoreceptor cells. Here we report on the specific optical properties of glial cells present in the retina, which might contribute to optimize this apparently unfavorable situation. We investigated intact retinal tissue and individual Muller cells, which are radial glial cells spanning the entire retinal thickness. Muller cells have an extended funnel shape, a higher refractive index than their surrounding tissue, and are oriented along the direction of light propagation. Transmission and reflection confocal microscopy of retinal tissue in vitro and in vivo showed that these cells provide a low-scattering passage for light from the retinal surface to the photoreceptor cells. Using a modified dual-beam laser trap we could also demonstrate that individual Muller cells act as optical fibers. Furthermore, their parallel array in the retina is reminiscent of fiberoptic plates used for low-distortion image transfer. Thus, Muller cells seem to mediate the image transfer through the vertebrate retina with minimal distortion and low loss. This finding elucidates a fundamental feature of the inverted retina as an optical system and ascribes a new function to glial cells.

https://www.pnas.org/content/pnas/104/20/8287.full.pdf

Müller cells are living optical fibers in the vertebrate retina

Nowadays, smart composite materials embed miniaturized sensors for structural health monitoring (SHM) in order to mitigate the risk of failure due to an overload or to unwanted inhomogeneity resulting from the fabrication process. Optical fiber sensors, and more particularly fiber Bragg grating (FBG) sensors, outperform traditional sensor technologies, as they are lightweight, small in size and offer convenient multiplexing capabilities with remote operation. They have thus been extensively associated to composite materials to study their behavior for further SHM purposes. This paper reviews the main challenges arising from the use of FBGs in composite materials. The focus will be made on issues related to temperature-strain discrimination, demodulation of the amplitude spectrum during and after the curing process as well as connection between the embedded optical fibers and the surroundings. The main strategies developed in each of these three topics will be summarized and compared, demonstrating the large progress that has been made in this field in the past few years.

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Fiber Bragg grating sensors toward structural health monitoring in composite materials: challenges and solutions.

A topical review of numerical and experimental studies of supercontinuum generation in photonic crystal fiber is presented over the full range of experimentally reported parameters, from the femtosecond to the continuous-wave regime. Results from numerical simulations are used to discuss the temporal and spectral characteristics of the supercontinuum, and to interpret the physics of the underlying spectral broadening processes. Particular attention is given to the case of supercontinuum generation seeded by femtosecond pulses in the anomalous group velocity dispersion regime of photonic crystal fiber, where the processes of soliton fission, stimulated Raman scattering, and dispersive wave generation are reviewed in detail. The corresponding intensity and phase stability properties of the supercontinuum spectra generated under different conditions are also discussed.

Supercontinuum generation, photonic crystal fiber

Fiber grating sensors hold immense potential for biomedical applications due to their inherent properties like small size, biocompatibility, non-toxicity, chemical inertness and electromagnetically inert nature. Grating based sensors are well known technology for structural health monitoring (SHM) in the arena of civil and aerospace; however investigations for their use in the field of medicine are fairly recent and they have not been yet commercialized for the same. As these sensors detect strain, temperature, pressure, vibration, curvature and refractive index of the surrounding material even in high magnetic and electric field environments, they can serve diagnostic purposes in diverse areas of healthcare e.g. biomechanics, cardiology, gynecology, very low temperature monitoring and immunosensing to name a few. Most importantly, they can be used efficiently for thermal and pressure mapping even during MRI procedure where conventional sensors may fail. The paper is a review of various application areas of fiber grating based devices, the status of precedent and ongoing research around the world, and discussion on the issues hampering their rapid growth in the field.

Fiber grating sensors in medicine: Current and emerging applications

We report the formulation of an ABCD matrix for reflection and refraction of Gaussian light beams at the surfaces of the hyperboloid of revolution that separate media of different refractive indices. The analysis includes an arbitrary angle of incidence and is based on matching the optical phase at the interface. Finally, we deduce expressions for spot sizes and wave-front radii and use them to obtain the ABCD matrix. Based on the formulated ABCD matrix for refraction under paraxial approximation, we also report a simple theoretical investigation of the coupling efficiency of a laser diode to a single-mode fiber with a hyperbolic lens formed on its tip.

ABCD matrix for reflection and refraction of Gaussian light beams at surfaces of hyperboloid of revolution and efficiency computation for laser diode to single-mode fiber coupling by way of a hyperbolic lens on the fiber tip.

Laser diode to single-mode fibre excitation via hyperbolic lens on the fibre tip: Formulation of ABCD matrix and efficiency computation

Effect of optical Kerr effect nonlinearity on LP11 mode cutoff frequency of single-mode dispersion-shifted and dispersion-flattened fibers

We introduce and analyze the upside-down taper lens end drawn from step-index fibers. Also, we model the refractive-index distribution and present the ABCD transformation matrix of this fiber end under paraxial approximation. The analysis can be useful for designing micro-optic image systems and laser diodes to single-mode fiber coupling optics.

Analysis of an upside-down taper lens end from a single-mode step-index fiber

A simple numerical method based on Chebysev technique is proposed to calculate the propagation characteristics of single mode fibers having arbitrary index profiles. The method also formulates a linear relation of K 1 (W)/K 0 (W) with 1/W. The method predicts the normalised fiber parameters and field functictions in the core and cladding as well for step and parabolic index fibers excellently.

Somenath Sarkar

Papers

ABCD matrix for reflection and refraction of Gaussian light beams at surfaces of hyperboloid of revolution and efficiency computation for laser diode to single-mode fiber coupling by way of a hyperbolic lens on the fiber tip.

Laser diode to single-mode fibre excitation via hyperbolic lens on the fibre tip: Formulation of ABCD matrix and efficiency computation

Effect of optical Kerr effect nonlinearity on LP11 mode cutoff frequency of single-mode dispersion-shifted and dispersion-flattened fibers

Analysis of an upside-down taper lens end from a single-mode step-index fiber

Novel method for studying single-mode fibers involving Chebysev technique