In this paper, we present a highly sensitive inline fiber curvature sensor based on an eccentric core fiber (ECF) and off-axis air cavity Fabry-Pirot interferometer (OACFPI). This ECF-based bending sensor is fabricated by fusion-splicing a section of ECF between a single-mode fiber and off-axis air cavity fiber. Due to the location of the core in the ECF, a bend applied to the ECF-based OACFPI leads to elongation or shortening of the optical path length in the air cavity, which makes the system suitable for bending measurement. To analyze bending characteristics, three ECFs with different eccentric core locations are investigated separately. Additionally, a swept source is employed for measurement. The experimental results demonstrate that, the bending sensitivities of the sensors at the two opposite most-sensitive directions are 0.241 nm/m-1 (sensor 1), 0.131 nm/m-1 (sensor 2), and 0.0807 nm/m-1 (sensor 3), and -0.245 nm/m-1 (sensor 1), -0.156 nm/m-1 (sensor 2), and -0.077 nm/m-1 (sensor 3). The temperature sensitivities of the sensors are 0.84 pm/0C, 0.92 pm/°C, and 0.94 pm/0C, respectively. The proposed sensors are compact, versatile, and are expected to have potential applications in biomedical sensing.

Effective quench detection technologies are extremely important for protecting high temperature superconducting (HTS) applications. In recent years, an optical fiber encapsulated HTS tape (OF-tape) is proposed for quench detection by using distributed temperature sensing (DTS) technology. In this paper, an HTS coil (OF-coil) wound with a 39 m long OF-tape was prepared and energized by an AC overcurrent of 700 A to verify the feasibility of temperature observation during quench events. Furthermore, anti-stokes intensity was also measured to understand dependence of temporal resolutions on response times of coil quenches. The main results show that temperature variations of the OF-coil can be correctly measured after quench and the maximum temperature is shown at the middle turn of the OF-coil. Particularly, for the method of detecting anti-stokes intensity, quench response time of temporal resolution (TR.) with 30 ms is about 500 ms faster than that with 300 ms. However, when the TR. is 30 ms, more noise signal is observed. Accordingly, a trade-off should be considered between response time and noise signal for efficient quench detection.

A high-overtone bulk acoustic resonator (HBAR) consisting of a piezoelectric film with two electrodes on a substrate exhibits a high quality factor (Q) and multi-mode resonance spectrum. By analyzing the influences of each layer’s material and structure (thickness) parameters on the effective electromechanical coupling coefficient (K_eff²), the resonance spectrum characteristics of K_eff² have been investigated systematically, and the optimal design of HBAR has been provided. Besides, a device, corresponding to one of the theoretical cases studied, is fabricated and evaluated. The experimental results are basically consistent with the theoretical results. Finally, the effects of K_eff² on the function of the crystal oscillators constructed with HBARs are proposed. The crystal oscillators can operate in more modes and have a larger frequency hopping bandwidth by using the HBARs with a larger K_eff²·Q.

A modified reverse time migration (RTM) method for dispersed flexural waves is proposed to detect and image the crack in a thin plate. The key point of this method is to take the time reversal (TR) of the expanded flexural wave at a receiver as the backward wave in the cross-correlation imaging condition of RTM, thus the expanded backward wave will be re-compressed also due to dispersion of the flexural wave. The backward wave is time reversed again when arriving at a certain point on the plate, and then correlated to the forward wave derived from the original source. The image of the crack is obtained by superimposing the cross-correlation results of many source-receiver pairs in transmitting and receiving arrays. The experimental studies on imaging a crack in a thin plate are conducted, and the transmitted and received waveforms are recorded with high fidelity by a laser vibrometer, thus, ensuring that an accurate and clear image of the crack is obtained by this modified RTM process.