We propose two types of temperature-compensated all-fiber sensors based on a novel laser machined Fabry-Perot interferometer (FPI) whose sensitivity is adjusted by controlling the FPI gauge length and fiber material. The sensors are demonstrated to achieve precise multiparameter measurements under high temperatures. One sensor is an enhanced sensitivity FPI cascaded with a femtosecond laser (FSL) inscribed fiber Bragg grating (FBG). We propose a modified calculation formula considering the temperature dependence of strain/pressure sensitivities to ensure measurement accuracies. The experiment shows that the cascaded sensor exhibits excellent sensing performance with the maximum strain and pressure sensitivities of $\rm 41.6 pm/\mu \varepsilon and -222 pm/MPa$ , respectively, and in a stable environment, its strain-temperature and pressure-temperature crosstalk are $\rm 0.139 \mu \varepsilon/^{\circ} C$ and 0.027 MPa/°C, respectively. The other sensor is the FPI itself with a length-matched gutter. It achieves an ultralow average thermal drift of ∼0.1 pm/°C over 16-400°C. Experimental results show that it exhibits low temperature-pressure and temperature-strain cross sensitivities of $\rm \sim -0.0017 MPa/^{\circ}C and \sim 0.015 \mu \varepsilon/^{\circ} C$ , respectively.

Accurate measurement of strain is one of the most important issues for high temperature environments. We present a highly integrated all-fiber sensor to achieve precise measurements of strain/high-pressure, which consists of a fiber Bragg grating (FBG) inscribed by an 800 nm femtosecond laser cascaded with a micro extrinsic Fabry–Perot (FP) cavity fabricated by the 157 nm laser micromachining technique. FBG is sensitive to temperature, but insensitive to strain/pressure, whereas the FP is sensitive to strain/pressure, but has a small dependence on temperature. Therefore, such a cascaded sensor could be used for dual-parameter measurement and can work well at high temperatures. Experimental results indicate that this device exhibits a good strain characteristic at high temperatures and excellent high-pressure performance at room temperature. Due to its highly sensitive wavelength response, the proposed sensor will have remarkable potential applications in dual parameter sensing in harsh environments.

We propose a design method of a weak fiber Bragg grating (FBG) array, in which the reflectivity of the FBG increases along the fiber, and this increase is limited by the dynamic range of the distributed optical fiber sensing (DOFS) system. For a DOFS system, this design method can not only well compensate for the impact of fiber loss, within a certain range and caused by radiation, on the deterioration of the system performance, but also can reduce the requirements for the dynamic range of the system in nuclear environments. Based on this design method, we propose a short-distance acoustic detection system based on broadband ultraweak FBGs assisted phase-sensitive optical time domain reflectometry ( $\Phi $ -OTDR), and assess the system’s ability to detect pipeline leakages. This work not only provides guidance for the design of FBG sensing systems but also provides a new promising scheme for the application of DOFS system in nuclear environments.

An orientation-dependent displacement sensor based on grating inscription over a fiber core and inner cladding has been demonstrated. The device comprises a short piece of multi-cladding fiber sandwiched between two standard single-mode fibers (SMFs). The grating structure is fabricated by a femtosecond laser side-illumination technique. Two well-defined resonances are achieved by the downstream both core and cladding fiber Bragg gratings (FBGs). The cladding resonance presents fiber bending dependence, together with a strong orientation dependence because of asymmetrical distribution of the “cladding” FBG along the fiber cross-section.