A four-core optical fiber is introduced as a strain based temperature sensor to investigate the phase shift based on the temperature variations. An interferometric fringe pattern is obtained by the coherent waveguides from the four cores. A small piece of a four-core fiber is winded around a solid stainless steel cylinder to form a tight circular loop, which is exposed to a temperature change from 50 °C to 92 °C. Shear strain due to the expansion of the steel rod at this temperature interval causes an optical path length difference between the inner and outer core pairs, resulting a total phase shift of 20.4±0.29 rad, which is monitored with a CMOS camera. Using the phase changes, two dimensional shear strain is determined.
In this paper, an interferometric fiber optic vibration sensor based on a four-core optical fiber is described. When the light is coupled into the four cores, each core acts as a mutually coherent waveguide with the other ones, which allows obtaining an interference fringe pattern at the far field. Vibrating a section of the four-core optical fiber causes a path difference between the light beams guiding in the separate cores, which results in a shift in the fringe pattern. Such a mechanism allows one to relate the fringe shift to the vibration amplitude and frequency. In this study, a source, which is capable to generate 100 Hz frequency sound waves is attached to the optical fiber to maintain vibration of the section of the fiber. A single slit and a photodetector are used to detect the shifting of the fringe pattern that causes a change in the phase of the guiding light. When a He-Ne laser beam is coupled into the optical fiber, the structured fringe pattern is projected onto the slit behind the photodetector, then a small part of the fringe pattern is analysed. Thus, an interferometric fiber optic vibration sensor based on a four-core optical fiber, which has a simple structure and high sensitivity, is accomplished.
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