Fiber-optic vector accelerometer using orthogonal bragg grating inscription over innermost cladding of a multi-clad fiber

A fiber-optic vector accelerometer is demonstrated based on output power detection of orthogonal fiber Bragg gratings (FBGs). The compact sensing probe consists of a short section of multi-clad fiber (MCF) containing two concatenated, orthogonally positioned FBGs inscription over the core and innermost cladding. We use femtosecond laser side-illumination to ensure the two grating inscriptions are precisely positioned and strong refractive index modulation. Once the configuration was achieved, two well-defined groups of resonances appear on the reflection spectrum separately, corresponding to two 'cladding' FBG reflections and two 'core' FBG reflections. The two 'cladding' resonances are polarized by the asymmetrical grating distribution over the fiber cross-section, thus perceive the vector fiber bend. The key of this configuration is that the orthogonal 'cladding' FBGs are highly sensitive to acceleration together with getting the orientation of unknown source in two-dimensional plane. Moreover, the power fluctuations can be referenced out by measuring power of the core mode resonances that are insensitive to fiber bending.