Tomasz Nasilowski

Transversal Load Sensing With Fiber Bragg Gratings in Microstructured Optical Fibers

We present fiber Bragg grating based transversal load sensing with a highly birefringent microstructured optical fiber. For the bare fiber, the change of the Bragg peak separation under a transverse line load was simulated with a finite-element model and experimentally verified. We also show that microstructured optical fibers with fiber Bragg gratings can be successfully embedded in a carbon fiber reinforced composite material. The linear dependence of the Bragg peak separation to a transversal stress in the composite sample was measured to be 15.3 pm/MPa.

Fiber Bragg Gratings in Germanium-Doped Highly Birefringent Microstructured Optical Fibers

We present a dedicated fiber Bragg grating (FBG) inscription experiment to investigate the compatibility of a microstructured optical fiber (MOF) with conventional FBG inscription setups. For the studied MOF, the angular orientation of the fiber in the interferometric excimer laser setup was found to have no significant influence on the final reflection of the inscribed FBGs. We also show that an array of multiplexed FBGs can be inscribed in a single MOF with a repeatability and quality that match fiber sensing requirements.

Response of FBGs in Microstructured and Bow Tie Fibers Embedded in Laminated Composite

Fiber Bragg gratings in bow tie fiber and highly birefringent microstructured optical fiber are embedded in a carbon fiber reinforced epoxy. The Bragg peak wavelength shifts of the embedded gratings are measured under controlled bending, transversal loading, and thermal cycling of the composite sample. We obtain similar axial and transversal strain sensitivities for the two embedded fiber types. We also highlight the low temperature dependence of the Bragg peak separation of the microstructured fibers, which is an important advantage for this application. The results show the feasibility of using microstructured fibers in structural integrity monitoring.

Large-mode-area photonic crystal fiber with double lattice constant structure and low bending loss

We report on a bendable photonic crystal fiber for short pulse high power fiber laser applications. This fiber uses a double lattice structure and enables single mode operation with a very large mode area that reaches 1454 µm² when the fiber is kept straight and 655 µm² in the fiber bent around a 10 cm radius. Single mode operation is enforced by the very large bending loss in excess of 50 dB/m experienced by the higher order modes, whilst bending loss for the fundamental mode is smaller than 0.01 dB/m. We outline the principles of our fiber design and we explore the guiding properties of the fiber.

Experimental study of dispersion characteristics for a series of microstructured fibers for customized supercontinuum generation

We demonstrate an experimental study of the chromatic dispersion properties for a series of microstructured fibers (MSFs) dedicated for a supercontinuum generation. With white-light interferometry application we analyze experimentally how the small variations of structural parameters, i.e. an air-hole diameter and a lattice constant, influence dispersion characteristics in different groups of MSFs. Our study provides useful information on how to design the fiber which is less sensitive to the fabrication imperfections. Moreover those investigations are the initial step to the development of the customized or tunable supercontinuum light sources based on MSFs with slightly changed structural parameters which can generate light with a different spectrum range, adapted to a proper application.

Bragg Grating Inscription in GeO

We present KrF excimer laser-induced dynamics of Bragg grating growths in GeO2 doped microstructured optical fibers. The studied fibers all have 6 rings of airholes in a hexagonal lattice and a GeO2 doped region in the center of the microstructure. We compare the growth rates of fiber Bragg gratings in the different microstructured fibers with UV grating inscription. The influence of the doping level, the airhole filling factor, the airhole pitch distance and the fiber orientation are investigated. We expand the range of microstructured optical fibers in which Bragg gratings can be inscribed, achieving reflection strengths that are useable for FBG-based sensing applications, even for doped regions with GeO2 concentrations as low as 1.36 mol% and 0.45 mol%.

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We report on a photonic crystal fiber with a large mode area designed for compact high power fiber lasers and amplifiers. The fiber suppresses higher order modes when bent around a 10-cm radius and enables single mode operation in small footprint laser and amplifier architectures. We experimentally confirm the peculiar bending properties of this fiber in its passive version, by reporting on the measurement results of fundamental mode loss in bent and straight fibers, and of the influence of the bending plane orientation on this fiber loss.

-Doped Microstructured Optical Fibers

Microstructured optical fibers can be precisely tailored for many different applications, out of which sensing has been found to be particularly interesting. However, placing silica optical fiber sensors in harsh environments results in their quick destruction as a result of the hydrolysis process. In this paper, the degradation mechanism of bare and metal-coated optical fibers at high temperatures under longitudinal strain has been determined by detailed analysis of the thermal behavior of silica and metals, like copper and nickel. We furthermore propose a novel method of enhancing the lifetime of optical fibers by the deposition of electroless nickel-phosphorous alloy in a low-temperature chemical process. The best results were obtained for a coating comprising an inner layer of copper and outer layer of low phosphorous nickel. Lifetime values obtained during the annealing experiments were extrapolated to other temperatures by a dedicated model elaborated by the authors. The estimated copper-coated optical fiber lifetime under cycled longitudinal strain reached 31 h at 450 °C.

Photonic Crystal Fiber With Large Mode Area and Characteristic Bending Properties

We review exceptional properties of the photonic crystal fibres enabling sensing applications of this new class of fibres. First, the sensing capabilities of highly birefringent index guided fibres are discussed. This includes dispersion characteristics of phase and group modal birefringence in different fibre structures, and sensitivity of these parameters to hydrostatic pressures and temperature. We demonstrate that index guided and photonic bandgap holey fibres of specific construction can be used as wide-band fibre-optic polarizer. We also show that combining of geometrical and stress effects makes it possible to design the holey fibres with either zero phase or group modal birefringence at virtually any given wavelength. Finally, different designs and performance of PCFs suitable for gas sensing are overviewed.

New Methods of Enhancing the Thermal Durability of Silica Optical Fibers

We analyze the mechanisms contributing to the overall sensitivity of optical sensors based on holey fibers. For this purpose we investigate the shift of the Bragg-matched wavelength in a sensor with a Bragg grating as well as the change of birefringence in a polarimetric sensor. For the sake of simplicity we concentrate on the response of the sensor to temperature. We show that the change of refractive index of silica is not always the dominating source of the sensors response.