Wavelet-Based Demodulation of Multimode Etched Fiber Bragg Grating Refractive Index Sensor

In-Line Mach Zehnder Interferometer Based on Ytterbium Doped Fiber with Up-Taper Structure in Fiber Ring Laser and Its Application in Sensing

We report an ytterbium (Yb) doped fiber Mach Zehnder interferometer (MZI) based on the up-taper fiber structure in a fiber ring laser (FRL) cavity. Different from the traditional FRL sensing system, in which additional filters are required, the designed structure simultaneously acts as a filter, sensor and gain medium. Furthermore, thanks to the high thermal-optical coefficient of Yb doped fiber, the temperature sensitivity of 0.261 nm/°C can be achieved in the range of 10-50 °C. In addition, benefiting from the unique characteristics of the laser system itself, the designed structure has a narrower linewidth (-0.2 nm) and a higher signal-to-noise ratio (SNR) (-40 dB) than the sensor system based on a broadband light source (BBS). Meanwhile, the refractive index (RI) response and stability of the system are measured. The RI sensitivity is up to 151 nm/RIU, and the wavelength fluctuation range within two hours is less than 0.2 nm. Therefore, the designed structure is expected to play a significant role in human life safety monitoring, aircraft engine temperature monitoring, etc.

Soft joint shape measurement device based on FBG with a simple demodulating system

Soft joint shape measurement is challenging because, in most cases, it relies solely on internal sensors. Existing shape estimation methods commonly take measurements at discrete points and utilize curve-fitting schemes, which are inefficient for complex joint shapes that require continuous measurements. Therefore joint shape measurement sensors rely on the fiber Bragg grating (FBG) due to its sensitivity, immunity to electromagnetic interference, and flexibility. Nevertheless, FBG demodulation is still an open research case. Hence, we propose a shape measurement device appropriate for FBG-based continuous measurements that employs a sensor with only three FBGs thrusting inside the soft joint to measure its 3D shape. Moreover, we develop a simple demodulating system exploiting the FBG's filter overlapping properties and design a calibrating process for FBG signals. Soft joint shape measurement experiments highlight our method's effectiveness, providing a relative error within 0.7%. Further tests involving continuum robot measurement reveal that the achieved precision is of the same level as a motion-capturing system.

Detecting of the Crack and Leakage in the Joint of Precast Concrete Segmental Bridge Using Piezoceramic Based Smart Aggregate

Precast concrete segmental bridges (PCSBs) have been widely used in bridge engineering due to their numerous competitive advantages. The structural behavior and health status of PCSBs largely depend on the performance of the joint between the assembled segments. However, due to construction errors and dynamic loading conditions, some cracks and leakages have been found at the epoxy joints of PCSBs during the construction or operation stage. These defects will affect the joint quality, negatively impacting the safety and durability of the bridge. A structural health monitoring (SHM) method using active sensing with a piezoceramic-based smart aggregate (SA) to detect the crack and leakage in the epoxy joint of PCSBs was proposed and the feasibility was studied by experiment in the present work. Two concrete prisms were prefabricated with installed SAs and assembled with epoxy joint. An initial defect was simulated by leaving a 3-cm crack at the center of the joint without epoxy. With a total of 13 test cases and the different lengths of cracks without water and filled with water were simulated and tested. Time-domain analysis, frequency-domain analysis and wavelet-packet-based energy index (WPEI) analysis were conducted to evaluate the health condition of the structure. By comparing the collected voltage signals, Power Spectrum Density (PSD) energy and WPEIs under different healthy states, it is shown that the test results are closely related to the length of the crack and the leakage in the epoxy joint. It is demonstrated that the devised approach has certain application value in detecting the crack and leakage in the joint of PCSBs.