The guidance of the fundamental mode of microstructured fibers with a Ge-doped core can be cut off by filling the
holes with a material, liquid in our case, that increases the refractive index in the air-holes up to a value between the
refractive index values of the Ge-doped region and the silica. A section of such a liquid-filled fiber defines a short-pass
filter widely tunable by adjusting slightly the refractive index values. Thus, a small change of temperature or a small
strain of the fiber can be used to adjust the transmittance of the filter. Alternatively, temperature changes and strain could
be determined by measuring the cutoff wavelength. A simple measurement of the power transmitted through a short
section of liquid-filled fiber can be used to monitor temperature and strain, enabling the measurement of fast transients
such as mechanical vibrations. Several Y-shaped fibers with a Ge-doped core were fabricated. This microstructure with
only three big holes surrounding the core makes straightforward the filling of the fiber with liquids. The experimental
characterization includes the measurement of the cutoff wavelength of a number of devices with different lengths,
different microstructure geometries and different refractive index liquids. The cutoff wavelength and the transmission
were measured as a function of temperature and strain. A simple theoretical simulation permits to explain the
experimental results.
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Salvador Torres-Peiró, Antonio Díez, José L. Cruz, Miguel V. Andrés, "Cutoff properties of liquid-filled Ge-doped microstructured fibers," Proc. SPIE 7839, 2nd Workshop on Specialty Optical Fibers and Their Applications (WSOF-2), 783918 (14 October 2010);