Arrays of Regenerated Fiber Bragg Gratings in Non-Hydrogen-Loaded Photosensitive Fibers for High-Temperature Sensor Networks

We report about the possibility of using regenerated fiber Bragg gratings generated in photosensitive fibers without applying hydrogen loading for high temperature sensor networks. We use a thermally induced regenerative process which leads to a secondary increase in grating reflectivity. This refractive index modification has shown to become more stable after the regeneration up to temperatures of 600 °C. With the use of an interferometric writing technique, it is possible also to generate arrays of regenerated fiber Bragg gratings for sensor networks

Reliable Fiber Sensor System with Star-Ring-Bus Architecture

This work presents a novel star-ring-bus sensor system and demonstrates its effectiveness. The main trunk of the proposed sensor system is a star topology and the sensing branches comprise a series of bus subnets. Any weakness in the reliability of the sensor system is overcome by adding remote nodes and switches to the ring and bus subnets. To construct the proposed star-ring-bus sensor system, a fiber ring laser scheme is used to improve the signal-to-noise ratio of the sensor system. The proposed system increases the reliability and capacity of fiber sensor systems

Arrays of regenerated fiber bragg gratings in non-hydrogen-loaded photosensitive fibers for high-temperature sensor networks

We report about the possibility of using regenerated fiber Bragg gratings generated in photosensitive fibers without applying hydrogen loading for high temperature sensor networks. We use a thermally induced regenerative process which leads to a secondary increase in grating reflectivity. This refractive index modification has shown to become more stable after the regeneration up to temperatures of 600 °C. With the use of an interferometric writing technique, it is possible also to generate arrays of regenerated fiber Bragg gratings for sensor networks. © 2009 by the authors

Fiber Optic Sensors for Extreme Environments

Optical fiber based sensors offer several important advantages over electronic sensors, including low manufacturing cost, miniature and flexible structures, immunities to electromagnetic fields, and the capability of distributive and multi-parameter sensing on a single fiber. Extreme harsh environments such as temperature >800°C or as low as a few Kelvin, present unique challenges and opportunities to fiber optic sensors. For example, hydrogen gas leak detection in cryogenic environment is critically important in the production and use of liquid hydrogen fuels. But the sensitivity of conventional Palladium (Pd) coated hydrogen sensors degrade rapidly when temperature decreases. Another example is the quick diminishing of conventional type-I gratings with temperature range beyond 500°C, which prevent the FBG implementation in numerous high temperature applications. The objective of this thesis is to explore new fiber sensing technologies that have significant performance enhancements, or were previously not possible in extreme environment applications. Optically heated fiber sensors were developed for cryogenic Hydrogen gas and liquid level sensing in environments as well as room temperature gas flow sensing. Regenerated gratings were developed for high temperature pressure sensing. Novel in-fiber sensing techniques such as Rayleigh and Raman scattering were also exploited for fully distributed sensing operations. These technologies and devices offer reliable and flexible sensing solutions extreme environments in energy, transportation and telecom industry

Herstellung funktioneller Dünnschichtelemente auf den Stirnflächen von Lichtleitfasern mittels hochauflösender lithografischer Strukturierungsverfahren

Die Entwicklung von Technologien für lithografische Prozesse zur Herstellung hochfrequenter diffraktiver Dünnschichtbauelemente direkt auf den Stirnflächen von Lichtleitfasern erfordert die Modifizierung der für die Prozessdurchführung auf Wafern optimierten lithografischen Verfahren. Dies umfasst die Schichtherstellung, einzustellende Prozessparameter und die Prozessführung. Durch die Strukturierung metallischer Schichten werden faserbasierte Lithografiemasken für die Faser-zu-Faser-Belichtung zur effizienten und reproduzierbaren Fotolithografie auf Faserstirnflächen erzeugt. Beschrieben wird zudem die hochfrequente DUV-Interferenzlithografie an einem für die Elektronenstrahllithografie spezifizierten Resist auf unterschiedlichen Substratmaterialien. Die direkte Oberflächenstrukturierung von Hochfrequenzgittern in auf die Stirnflächen von Quarzglaslichtleitfasern aufgebrachten hochbrechenden Schichten aus As35S65 erzeugt effiziente fasergekoppelte Gitter mit hohem Brechungsindexkontrast.In this thesis the development of technologies for lithographic processes for the fabrication of high-frequency diffractive thin film elements directly on the end-faces of optical fibres is described. For this purpose lithographic procedures, optimised for wafer processing lithographic procedures, have to be modified in a convenient way. This concerns the fabrication of the layers as well as the processing and the required process parameters. In dependence on used materials and structural dimensions different functionalities of the structured layers can be implemented.Two different systems are applied for exposure. Direct laser writing at 442 nm exposure wavelength is used for the fabrication of structures with grating periods larger than 3 µm. For the realisation of high-frequency gratings with periods smaller than 1 µm laser interference lithography utilizing an excimerlaser emitting at a wavelength of 248 nm is applied. At this specific wavelength is investigated the feasibility to structure a resist which is actually designed for electron-beam-lithography also by interference lithography. Due to its unique properties like small sensitivity to humidity, good long-term stability, good adhesion and simplified process management this resist is particularly well-suited for the development of technologies for the structuring of materials on fibre-end-faces. Suitable parameters for the realisation of high-frequency grating structures regarding exposure and general processing parameters for different substrate layers are determined.Structuring of metallic layers on fibre-end-faces results in fibre based lithographic masks which can be used for efficient and reproducible photolithography on fibre-end-faces by fibre-to-fibre exposure.Efficient fibre-coupled high-index-contrast gratings can be realised by direct excimerlaser interference lithography surface-structuring of high-frequency gratings in high-index layers of As35S65 at the end-face of fused-silica-fibres. Wavelength and polarising filters based on such gratings are designed, appropriate exposure conditions are determined and the properties of realised structures are compared to results of simulations