Microfiber Bragg Grating Bonded Using Tapered Cantilever for High-Sensitivity Ultrasonic Detection

Cantilevered Microfiber Bragg Grating Sensor With High Ultrasonic Sensitivity and Designable Resonant Frequency

“…(1) where a is a strain sensitivity of a conventional FBG with a value of 1.2 pm/με, and A is the amplification factor representing the ratio of input and output strains of the cantilever structure. As it is well known that the voltage of an ultrasonic signal is proportional to the strain [14], A can be written as,…”

“…The cantilever beam of the CMFBG includes a part of the pigtail (with a length of 2 mm), a taper region (with a length of 2.25 mm), and a waist region (with a length of 8.25 mm). Unlike the common cantilevered measurement method, in which the length of the cantilever beam is literally infinite [14], the cantilever beam of the CMFBG considered is short as, in this experiment, the length was only 12.5 mm. In the work described in the following, CMFBGs with diameters of 35 μm are referred to as a '35 μm CMFBG', the diameter of the CMFBG is described by D, and the length of the taper region and waist region are termed Lt and Lw, respectively.…”

“…Fig. 6(a) shows the results of the simulation of the CMFBG and the CFBG, extracted from the neural axis of the waist region (as our previous study demonstrated that the axial strain of the elements on the neural axis of the optical fiber model represents the key response of the sensor [14]). The waveforms are not closely similar to those seen from the experimental results because of the shortened simulated propagation distance, the frequency characteristics of the actuator, and the adhesive conditions, where it is difficult for these to be completely consistent with the experiment.…”

“…A similar trade-off between the dynamic range and the sensitivity is also found in superstructured gratings [13]. To tackle this limitation in the system optical design, a microfiber Bragg grating (MFBG) has been proposed, taking advantage of the mechanics of the device [14] where the MFBG amplifies the ultrasonic signal by gathering the ultrasonic energy using the small diameter of the fiber. This provides a simple way to improve the sensitivity, while retaining the high dynamic range of a normal FBG.…”

Multiparameter measurement depending on the cavity-length differentiation characteristics of a microfiber Fabry–Perot interferometer

“…Usually, FBG sensors use only wavelength as the means of detection for a measured parameter, which limits their ability to directly distinguish between different parameters and can easily lead to cross-interference [21,22]. To overcome this problem, researchers have been exploring new approaches and structures, such as the use of multi-grating structures [23][24][25], special fiber structures [26][27][28][29] fiber Fabry-Perot interference [30][31][32][33] structures, polymer coatings [34][35][36][37], and multi-sensor networks [38][39][40] to improve the multiparameter measurement capability and cross-interference resistance of FBG sensors. For example, Fu et al [25] proposed a fiber sensor based on a cascaded long-period fiber grating with a double-clad fiber composite tapered structure.…”