We demonstrate a fiber-optic 3D vector displacement sensor based on the monitoring of Bragg reflection from an eccentric grating inscribed in a depressed-cladding fiber using the femtosecond laser side-illumination and phase-mask technique. The compact sensing probe consists of a short section of depressed cladding fiber (DCF) containing eccentrically positioned fiber Bragg gratings. The eccentric grating breaks the cylindrical symmetry of the fiber cross-section and further has bending orientation-dependence. The generated fundamental resonance is strongly sensitive to bending of the fiber, and the direction of the bending plane can be determined from its responses. When integrated with axis strain monitoring, the sensor achieves a 3D vector displacement measurement via simple geometric analysis.

We present a compact simultaneous strain and high-temperature measurement scheme using dual fiber Bragg gratings (FBGs) written in single mode fiber (SMF) and thin-core fiber (TCF). The different effective refractive indexes between the cores of SMF and TCF are the key point because such FBGs support two separate resonances in the reflection spectrum for sensing measurement. These two resonances have different optical parameters, elastic-optic coefficient, and thermal-optic coefficient, they shift with different sensitivities under axial strain and temperature variations. The etching process is utilized on the grating of TCF to increase the sensitivity differences of the dual FBGs. In the experiment, simultaneous measurement of strain and temperature with the high strain sensitivity of 3.25 pm/με and high temperature up to 800 °C is realized respectively. This sensor scheme suits the practical application for strain and high-temperature measurement simultaneously.

The magnetic fluid is used to integrate with a special fiber Bragg grating (FBG) inscribed over the core and innermost cladding of a multi-clad fiber, which acts as a bridge between magnetic field measurement and FBG. The FBG is etched by hydrofluoric acid and packaged with a magnetic fluid inside a capillary. The reflection of the sensor provides two resonance modes of which the cladding mode intensity shows a great response to both the magnetic field intensity and the direction. Meanwhile, the fundamental core mode intensity is quiet stable, and thus the proposed sensor clearly demonstrates the potential to simultaneous measurement of magnetic field and temperature.

A fiber-optic Mach-Zehnder interferometer is proposed and demonstrated experimentally. The device consists of a short piece of thin-core fiber (TCF) sandwiched between two standard single mode fibers. A microscope direct-writing technique with femtosecond laser exposure is utilized to induce a refractive index change near the interface between the core and cladding of the TCF. This added refractive index profile is helpful for the coupling of the core-to-cladding modes, resulting in a well-defined interference spectrum. The ultra-high temperature property of the proposed interferometer is demonstrated up to 900 °C with a sensitivity of 94.3 pm/°C over repeated measurements. And the repeated operations of thermal annealing are employed to improve the thermal stability and measurement performance of the interferometer.

Herein we propose and experimentally demonstrate a hyperelastic polymer (polydimethylsiloxane, PDMS) fiber Fabry-Perot interferometer (FPI) for nanoforce measurement. The femtosecond laser pulse illumination and slit beam shaping are utilized to form refractive index modulation as the reflection mirrors of the FPI in the fiber. The PDMS fiber FPI, which exhibits an ultra-low Young's modulus, can be stretched by applied microforce and further result in the spectral variation of the FPI. In the measurement, another matching FPI is employed to introduce the optical Vernier effect. Due to the hyperelastic performance of the PDMS fiber and the magnification of the Vernier effect, the proposed sensor has force sensitivity as high as 45.721 nm μN ⁻¹ within linearly elastic range from 0 to 1400 μN. The proposed sensor is easily fabricated, assembled, and very stable, and it has therefore the potential to be a beneficial sensing technique for biochemical analysis and materials science.

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.