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Strain, Displacement, and Force Sensors

In addition to the sensing parameter of temperature, strain is another universally used parameter that needs to be tested in a lot of applications. Strain is defined as the relative deformation of the element under test, that is, AL/L, often with a unit of microstrain (|i?). FBG strain sensors have been used more often than FFPI sensors, thanks to their high capacity of multiplexing. However, FBGs often have a length of around 10 mm, while FFPI strain sensors can be fabricated with a length of several tens of micrometers. Conventional FBGs cannot withstand temperature higher than 300°C. Therefore, FFPI strain sensors are excellent for some specific applications, e specially for those requiring very compact size and operating in high-temperature environments.

Rao and Ran [14] developed FFPI strain sensors for high- temperature application by using laser micromachining technologies, including fs-laser micromachining [15], 157-nm laser micromachining [16], and also PCFs [17]. Strain can be measured with good linearity at a temperature of 800°C, as shown in Figure 4.3.

There were also numerous publications focused on how to enhance the strain sensitivity of FFPI sensors. By splicing a section of hollow- core ring PCF in between two SMFs, an FFPI sensor with strain sensitivity of 15.4 pm/|l? was achieved [18]. Several other groups, including Yiping Wang [19,20], Benli Yu [21], and their coworkers, developed sensitive FFPI strain sensors based on microbubbles whose dimension and shape were precisely controlled by optimizing the fusion splicing process. The strain sensitivity of FFPI sensors was recently enhanced up to 43 pm/|l?.

For displacement measurement, a simple scheme was developed by Wang et al. [22] by using SMF as both the input and output fiber. A reflective film, attached to the object under test, together with the end face of the SMF formed an FFPI. The displacement of the object corresponded to the FFPI cavity length and was measured

High-temperature strain sensing by PCF—FFPI fabricated by 157-nm laser micromachining technology

Figure 4.3 High-temperature strain sensing by PCF—FFPI fabricated by 157-nm laser micromachining technology.

by the interferometer interrogating method, with accuracies of 0.05 nm over the dynamic range of 0.005-79.1 nm and 0.5 nm over the dynamic range of 79.1-3200 nm. Similar methods were further investigated by Yu [23], Huang [24], and their coworkers. FFPI sensors can also be used for angle sensing after certain kinds of packaging and mounting [25]. Vibration can be considered as dynamic strain or displacement, which can be measured by integrated cantilever or microstructured beam [26].

Force sensors are of great importance for industrial measurement and also for some bio-applications like micro-surgery. Liu et al. [27] integrated a micro FFPI at the surgery tool tip and the FFPI was interrogated by an optical coherence tomography (OCT) system. The outer diameter of the force sensing tool was less than 1 mm. The calibration characteristics were studied and the dynamic range of force sensing was at the scale of several millinewtons. Recently, Gong et al. [28] developed a sensitive force sensor based on an FFPI with a short section of optical microfiber in the cavity. A short FBG was inscribed and then drawn into optical microfiber with a diameter of several micrometers by the heating-and-drawing method. The far end of the fiber was cleaved so that the broadband Fresnel reflection and the narrowband reflection from the FBG interfere. The use of the optical microfiber enabled an ultrasensitive FFPI force sensor, with a sensitivity of 0.221 pm/|J,N. Good repeatability of force sensing was demonstrated, and the sensitivity can be controlled by the diameter of the optical microfiber, as shown in Figure 4.4.

Sensitive force sensing based on FFPI with a section of optical microfiber

Figure 4.4 Sensitive force sensing based on FFPI with a section of optical microfiber. (a) The sensitivity is much higher than that using a section of single mode fiber instead of the microfiber. (b) Tuning the sensitivity by controlling the diameter of the microfiber.

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