A Highly Sensitive Two-Dimensional Inclinometer Based on Two Etched Chirped-Fiber-Grating Arrays

High Accurate Mathematical Tools to Estimate the Gravity Direction Using Two Non-Orthogonal Inclinometers

This study provides two mathematical tools to best estimate the gravity direction when using a pair of non-orthogonal inclinometers whose measurements are affected by zero-mean Gaussian errors. These tools consist of: (1) the analytical derivation of the gravity direction expectation and its covariance matrix, and (2) a continuous description of the geoid model correction as a linear combination of a set of orthogonal surfaces. The accuracy of the statistical quantities is validated by extensive Monte Carlo tests and the application in an Extended Kalman Filter (EKF) has been included. The continuous geoid description is needed as the geoid represents the true gravity direction. These tools can be implemented in any problem requiring high-precision estimates of the local gravity direction.

Partially etched chirped fiber Bragg grating (pECFBG) for joint temperature, thermal profile, and refractive index detection

In this work, a partially etched chirped fiber Bragg grating (pECFBG) is introduced, as a compact sensor for multi-parametric measurement of temperature, thermal gradients over the active length, and refractive index. The sensor is fabricated by wet-etching a portion of a 14-mm linearly chirped FBG with linear chirp profile. The resulting device has two active areas: the unetched part of the grating (2 mm) can be used either as a uniform temperature sensor, or to detect thermal gradients experienced through the grating length by means of a spectral reconstruction technique; the etched part (12 mm), besides having a similar thermal sensitivity, is exposed to refractive index sensing through the introduction of a sensitivity to external refractive index. Overall, the pECFBG structure behaves as a compact sensor with multi-parameter capability, that can both measure temperature and refractive index on the same grating, but also spatially resolve temperature detection through the grating section. The results have been validated through both a model and experimental setup, showing that the mutual correlation algorithm applied to different spectral parts of the grating is able to discriminate between uniform and gradient-shaped temperature profiles, and refractive index changes.

Review of Chirped Fiber Bragg Grating (CFBG) Fiber-Optic Sensors and Their Applications

Fiber Bragg Gratings (FBGs) are one of the most popular technology within fiber-optic sensors, and they allow the measurement of mechanical, thermal, and physical parameters. In recent years, a strong emphasis has been placed on the fabrication and application of chirped FBGs (CFBGs), which are characterized by a non-uniform modulation of the refractive index within the core of an optical fiber. A CFBG behaves as a cascade of FBGs, each one reflecting a narrow spectrum that depends on temperature and/or strain. The key characteristic of CFBGs is that their reflection spectrum depends on the strain/temperature observed in each section of the grating; thus, they enable a short-length distributed sensing, whereas it is possible to detect spatially resolved variations of temperature or strain with resolution on the order of a millimeter over the grating length. Based on this premise, CFBGs have found important applications in healthcare, mechanical engineering, and shock waves analysis, among others. This work reviews the present and emerging trends in CFBG sensors, focusing on all aspects of the sensing element and outlining the application case scenarios for which CFBG sensors have been demonstrated.