Strain sensing based on modal interference in multimode fibers (MMFs) has been extensively studied, but no experimental or theoretical reports have been given as to how the system works when strain is applied not to the whole MMF but only to part of the MMF. Here, using a perfluorinated graded-index polymer optical fiber as the MMF, we investigate the strain sensing characteristics of this type of sensor when strain is partially applied to fiber sections with different lengths. The strain sensitivity dependence on the length of the strained section reveals that this strain sensor actually behaves as a displacement sensor.

We investigate, for the first time, the temperature and strain dependencies of the spectral dips of the multimodal interference patterns in hetero-core-fiber structures at telecom wavelength. With increasing temperature and strain, the spectral dips shifted to shorter wavelengths with dependence coefficients of -3.5 nm/°C and -0.62 pm/με, respectively. Their absolute values were ~60 times larger and ~30 times smaller than the temperature and strain sensitivities of a standard single-mode-multimode-single-mode sensor, respectively. This result implies that the hetero-core-fiber structures can be potentially used to develop highly sensitive temperature sensors with reduced strain sensitivity. Among a variety of optical fiber sensors that have been developed so far for structural health monitoring, [1][2][3][4][5][6] we focus on what we call a hetero-core fiber structure, in which a

Structural properties and spatial ordering in multilayered ZnMgTe/ZnSe type-II quantum dot structures J. Appl. Phys. 111, 033516 (2012) Piezoelectric superlattices as multi-field internally resonating metamaterials AIP Advances 1, 041504 (2011) Effect of built-in electric field on the temperature dependence of transition energy for InP/GaAs type-II superlattices J. Appl. Phys. 110, 123523 (2011) Polarization property of deep-ultraviolet light emission from C-plane AlN/GaN short-period superlattices Appl. Phys. Lett. 99, 251112 (2011) Tunable superlattice in-plane thermal conductivity based on asperity sharpness at interfaces: Beyond Ziman's model of specularity J. Appl. Phys. 110, 113529 (2011) Additional information on J. Appl. Phys. The effect of periods on the accumulation and release of stress in GaAs/AlInAs superlattices structure is reported here. It is observed that in GaAs/AlInAs superlattices, when the Indium ͑In͒ content is greater than 10%, stress accumulates monotonically as the number of period increases. In GaAs/AlInAs superlattices with an In content of 5%, the accumulated stress is larger when the number of periods is less than 10. However when the number of periods exceeds 10, it was observed that suddenly there is a significant increase in defects and stress release. However, with any further increase in period number, there is once again an accumulation of stress.

We observe multimodal interference in single-mode-multimodesingle-mode sensors comprising short polymer optical fibers (POFs) with lengths from 100 mm down to 7 mm. Characteristic spectral peaks/dips are observed not in the conventionally used telecom band but around 1000 nm. We find that the dip wavelength depends on temperature but that the sensitivity is much lower than those obtained in the longer wavelength range when longer POFs are used. We discuss the possibility that, even with the reduced sensitivity, our success in observing multimodal interference in millimeter-long optical fibers will be a basis toward the combined use of frequency and intensity information in conventional fiber-optic single-point sensors for discriminative measurement of multiple physical parameters.

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