A high-spatial-resolution strain sensor in OFDR was experimentally demonstrated by use of combining distance compensation (DC) and image wavelet denoising (IWD) method. Compared with using one method alone, the measured distribution strain could be varied from 200 to 2000 μϵ at a spatial resolution of 2.56 mm. Moreover, four times enhancement of the accuracy for the strain senor was achieved without hardware modification. This method is a significant step toward a high performance OFDR system, especially for evaluating the rapid change of object's structural condition.

A method, i.e., vector projections, using two outer cores of multicore fiber without calibration and OFDR was proposed to realize three-dimensional (3D) shape sensing. A complement for apparent curvature vector method using three outer cores of multicore fiber was also proposed to determine the viable fiber cores arrangement for shape sensing. Compared with apparent curvature vector method, the vector projections method based on two outer cores of multicore fiber decreased the number of fiber cores, which eliminating the requirement for fiber cores arrangement. Based on these methods, a reconstructed shape with a higher accuracy was obtained based on integrating all fiber shape from all cores combinations. The maximum root-mean-square errors (RMSEs) of end position for the vector projections and apparent curvature vector method were 13.1 and 12.4 mm, while a higher accuracy, i.e., maximum RMSEs of 11.9 and 12.1 mm, was obtained based on the integrated fiber shapes.

A high-spatial-resolution, i.e., 2.0 mm, strain sensor based on a Rayleigh-scattering-enhanced SMF, i.e., exposed SMF (E-SMF) with direct UV laser, was experimentally demonstrated. The enhancement of Rayleigh-scattering (RS) intensity could be tuned by adjusting the exposure parameters, i.e., distance between SMF and Phase mask, energy of UV laser and velocity of the SMF. A 1.0 m long E-SMF with a RS enhancement of 37.3 dB was obtained, where the exposure time was only 100 s. Compared with the un-exposed SMF (UE-SMF), the strain profiles of the E-SMF could be clearly demodulated without fluctuation at a spatial resolution of 2.0 mm using traditional cross-correlation algorithm when the applied strain was from 200 to 2600 μϵ.

High-spatial-resolution distributed sensing at high temperature is crucial in many industrial areas, such as aero-engines, nuclear power, furnaces, and fuel cells. Here, we propose and demonstrate a large-scale multiplexed high-density weak in-fiber micro-cavity (MC) array for distributed high-temperature sensing with millimeter spatial resolution. The proposed in-fiber MC, featured by an intrinsic Fabry-Perot interferometer (IFPI) with a short cavity length of 100 μm and a low peak reflectivity of ∼−55 dB, was formed by two weak reflectors created in a conventional single-mode fiber (SMF) by using femtosecond laser point-by-point inscription. Several high-density MC arrays, consisting of identical weak IFPIs over 1000, were fabricated by using different femtosecond laser pulse energy to investigate the transmission loss (TL) of MC arrays. The experimental result shows that the TL induced by a single MC could be low as 0.0009 dB. Moreover, the high-temperature performance of the MCs was studied via cyclic heating and cooling between room temperature and 1000 °C, showing a temperature sensitivity of −2.29 GHz/°C (i.e., 18.4 pm/°C). Furthermore, distributed high-temperature sensing was demonstrated by employing the fabricated in-fiber MC array with the demodulation of optical frequency domain reflectometry, and a high spatial resolution of 1 mm was achieved at a high temperature of 1000 °C. As such, the proposed high-density weak in-fiber MC arrays are suitable for distributed high-temperature sensing in harsh environment, and hence have wide prospection of application in the fields of aerospace, nuclear power, metallurgy, and electrochemical industry.

A nondestructive measurement method based on an Optical frequency domain reflectometry (OFDR) was demonstrated to achieve Young’s modulus of an optical fiber. Such a method can be used to measure, not only the averaged Young’s modulus within the measured fiber length, but also Young’s modulus distribution along the optical fiber axis. Moreover, the standard deviation of the measured Young’s modulus is calculated to analyze the measurement error. Young’s modulus distribution of the coated and uncoated single mode fiber (SMF) samples was successfully measured along the optical fiber axis. The average Young’s modulus of the coated and uncoated SMF samples was 13.75 ± 0.14, and 71.63 ± 0.43 Gpa, respectively, within the measured fiber length of 500 mm. The measured Young’s modulus distribution along the optical fiber axis could be used to analyze the damage degree of the fiber, which is very useful to nondestructively estimate the service life of optical fiber sensors immersed into smart engineer structures.

Two types of series-integrated fiber Bragg gratings (SI-FBGs), i.e., strong and weak SI-FBGs, were inscribed in a standard single-mode fiber (SMF) using the femtosecond laser point-by-point technology. In the SI-FBGs inscribing system, the grating pitch of each FBG and the distance between the two adjacent FBGs in the SI-FBGs can be flexibly controlled by adjusting the inscription parameters. The strong SI-FBGs with different grating pitches and the weak SI-FBGs with an identical grating pitch were employed to successfully measure the temperature distribution in a tube furnace and the strain distribution on a cantilever beam, respectively. A high spatial resolution of less than 1 mm was achieved during the distributed temperature sensing experiment. Moreover, the spatial resolution could be improved by decreasing the distance between the two adjacent FBGs, i.e., decreasing the FBG length and the space between the two adjacent FBGs. Hence, the inscribed high-quality SI-FBGs have great potential to be developed as various quasi-distributed sensors with a high spatial resolution.