Fiber Bragg Grating Sensors and Sensor Arrays

Fiber Bragg gratings have found widespread application in sensor systems, e. g. for temperature, strain or refractive index measurements. The concept of fiber Bragg gratings allows also in a simple way the realisation of arrays of such sensors. The development of such optical fiber sensor systems often requires special fibers and grating structures which may go beyond more conventional Bragg grating structures in typical communication fibers. Concerning fibers there is, for example., a need of achieving fiber gratings in small diameter fibers and fiber tapers as well as in microstructured fibers. Special fiber grating structures are of interest e.g. in the visible wavelength range, which requires smaller spatial structures compared to more conventional gratings in the near infrared wavelength region. Examples for such modern developments in fiber Bragg grating technology for sensor applications will be presented and discussed. Introduction Fiber Bragg gratings (FBGs) have become standard elements in the development of new types of fiber optical sensors and sensor systems [1-2]. Standard techniques for making such fiber Bragg gratings are phase mask recording technique and the interferometric recording technique. Such gratings have been developed also for use in harsh environments like high temperatures [3]. Multiple gratings are of specific interest in sensor systems in order to allow quasi distributed sensing and for achieving sensor multiplexing. So called draw tower gratings, which are recorded by single pulses during the fiber drawing process, have been developed especially for this purpose [4-7]. These examples show the need for specific developments adapted to the needs of fiber sensor applications. Further special developments should therefore extend our capabilities to apply fiber Bragg gratings for additional sensing applications. We will discuss in the following such developments for improved and extended sensor applications such as: FBGs in small diameter fibers FBGs in fiber tapers FBGs for short wavelengths FBGs in microstructured fibers Fiber Bragg gratings in small diameter fibers Due to their small diameter, optical fibers are attractive sensor elements for embedding in structural materials. Examples are fiber reinforced thermoplastic composites. By embedding sensors, such as fiber Bragg gratings, into the material it becomes possible to monitor the structural behaviour during production and/or operation. For such applications it should be avoided that the composite structure is disturbed by the sensor element. Therefore smaller diameters of optical fibers than typically used in communication application are desirable. Special fibers for this purpose have been fabricated with fiber diameters of 80nm for grating inscription during the fiber drawing process. It has been shown that also for such small diameter fibers efficient gratings can be achieved, which Advances in Science and Technology Online: 2008-09-02 ISSN: 1662-0356, Vol. 55, pp 138-144 doi:10.4028/www.scientific.net/AST.55.138 © 2008 Trans Tech Publications, Switzerland This is an open access article under the CC-BY 4.0 license (https://creativecommons.org/licenses/by/4.0/) show similar properties to gratings in conventional sized fibers with a diameter of 125μm (fig. 1) (see also paper C-4: L16 [8]). . Fig. 1 Comparison of experimentally achieved fiber Bragg gratings in a conventional fiber with diameter of 125 μm (left) and a fiber with diameter of 80μm (right) Fiber Bragg gratings in tapers The interaction of the guided light in a fiber with a FBG with the surrounding material can be used for a refractive sensor. In a more complex setup with metallized fibers also the effect of generating plasmonic waves can modify in a sensitive way the reflection properties for FBGs. Such effects require, however, an efficient overlapping of the guided light. For this purpose fibers with a small core diameter are needed with a reasonable extension of the evanescent wave in the medium to be measured. One possible approach are side polished fibers. In this case only in one orientation good overlapping of the guided light is obtained, which is a limiting factor for the achievable sensitivity. Optimized sensitivity can be therefore expected for tapered fiber structures with extremely small fiber cores. Typical techniques for making fiber tapers are drawing or etching technologies. The concept for such a tapered sensor probe is shown in fig. 2. In the short tapered fiber end a fiber Bragg grating structure is inscribed. Additional layers of metal and high refractive index tantalpentoxide are applied to achieve increased field amplitudes at the core boundary and to shift the surface plasmon resonance to smaller refractive index values of the surrounding medium (adapted to water refractive index). This FBG is combined with a second FBG in the conventional part of the fiber for temperature measurements and temperature compensation. This FBG sensor element can now be used as a refractive sensor, where the refractive index value can be modified by specific biological binding reactions. Specific senor elements were made by an etching process achieving sensor tip diameters of a few micrometers and down to submicrometer dimensions. A result from a sensor probe with a diameter of 3μm is shown in figs. 3 and 4. Besides the good field overlapping such FBGs in tapered fibers also give the opportunity of small scale sensor nano probes. In a similar way it is possible to use tapers made by a taper drawing process. 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