Investigation of fiber optic sensors

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Supervisor:
Dr. Gerhátné Udvary Eszter
Department of Broadband Infocommunications and Electromagnetic Theory

In my BA thesis work I try to describe fiber optic sensors and to introduce FBG, laboratory measurements with FBG and their evaluation.

Fiber can be used as a sensor in two ways. Firstly, when the fiber carries the information between the sensing element and the processing element, and secondly, when the fiber itself is the sensor. With fiber optic sensors we can measure position, shift, strain, inflection, temperature, and electromagnetic fields. There is also the opportunity of distributed sensing, offering the possibility of a large number of sensors being supported by a single fiber optic line. In line with environmental effects, the intensity, phase, polarization, wavelength, spectral content of the light running through the fiber change. Therefore there are intensity-based, spectral-based, polarimetric and interferometric sensors and their combinations. Measurement is usually defined as intensity-based measurement. There are many inventive investigating methods, and many new ones emerge today.

My thesis mainly deals with FBG sensors - FBG is the shortened form of Fiber Bragg Grating. Bragg grates reflect particular wavelength of light and transmit all others. Sensing principle lies in the fact that due to environmental effects (inflection, strain, temperature change) grating length and their grating structure changes, as a result the reflected wavelength, too. The wavelength shift information can be related to the measurand.

In the OMT laboratory of BME we tested many measurement techniques for the examination of inflection and temperature relations. We measured the transmitted and reflected spectra of FBG and its power. Thanks to a good idea, the spectral overlap of FBG fiber and sensor fiber with optical power meter can be traced accurately, offering the possibility of a cheap, simple FBG sensor measuring.

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