Self-Enclosed FFPI Strain, Pressure, Refractive Index, and Temperature Sensors
The second type of MFFPI sensor is based on a self-enclosed cavity structure, enabling fabrication of sensors that can measure high temperatures, strain, pressure, and refractive index. To form the self- enclosed MFFPI cavity, a circular micro-hole with a typical depth and diameter of tens of micrometers is first made at the tip of a cleaved optical fiber via laser micromachining (see Figure 3.16). Next, an in-line MFFPI cavity is formed and enclosed by simply splicing the engraved fiber to another cleaved fiber, so the cavity is automatically enclosed by the cladding of the fiber. This micro FP etalon can be operated at temperatures of up to 800°C due to its robust, stable, and reliable structure. This device is also insensitive to temperature change due to the hollow-core structure of the etalon and the ultralow thermal expansion coefficient of the silica.
The self-enclosed FP sensor, shown in Figure 3.17a, could be directly used as a high-temperature strain sensor. As shown in Figure 3.18, such a strain sensor can also be operated under very high temperature with good linearity.
To add pressure, temperature, and refractive-index measurement capability to this sensor, the fiber near the air-filled FP cavity is cleaved precisely to form a diaphragm, as shown in Figure 3.17b. For pressure measurement, the diaphragm is about several micrometers thick. When pressure is applied to the sensor, the diaphragm is pushed, which shortens the cavity length of the air cavity. Because the diaphragm is flat and of almost uniform thickness, its center deflection AL, under applied pressure AP, can be calculated by AL = 3(1 — p,2)R4AP/16Eh3, where L, R, and h are the cavity length, the radius, and the thickness of the diaphragm, respectively, E is Young’s modulus, and |1 is Poisson’s ratio. An example is shown in Figure 3.19 where the sensor is used to measure high pressure of up to 60 MPa, and the pressure response shows good linearity (Figure 3.19b).
If the fiber is cleaved near the air cavity with a much longer distance (more than 100 |!m) to form an IFPI, as shown in Figure 3.17c, refractive index and temperature monitoring can be obtained simultaneously. There are two cavities: an air cavity and a much longer all-fiber IFPI. Based on this sensor head, refractive index and temperature can be measured by testing the fringe contrast and optical path difference (OPD) of the IFPI, respectively, as shown in Figure 3.16. RI sensitivity of ~44.9 dB/RIU and temperature OPD coefficient of ~2.67 nm/°C are achieved as shown in Figure 3.20.
Finally, the cleaved fiber can be machined once again on its end face by using a designed mask pattern to form an acceleration sensor
Figure 3.16 Fabrication process of the self-enclosed FFP in-line etalom (a) creating a circular micro-hole at the fiber tip using a 157-nm laser; (b) splicing the fiber with the micro-hole to another cleaved fiber; (c) completing the FP etalon (strain sensor) after an arc-fusion splicing operation; (d) cleaving the fiber near air FP cavity to form pressure, temperature, and refractive sensors; and (e) fabricating accelerometer sensor. (Rao, Y. J. and Ran, Z. L. 2011. Laser Focus World, 47(11), 71; Rao, Y. J. et al. 2007. Optics Express, 15(22), 14936-14941; Ran, Z. L. et al. 2008. Optics Express, 16(3), 2252-2263; Ran, Z. et al. 2013. IEEE Sensors Journal, 13(5), 1988-1991; Ran, Z. et al. 2009. Journal of Lightwave Fechnology, 27(15), 3143-3149; Ran, Z. et al. 2011. IEEE Sensors Journal, 11(5), 1103-1106; Ran, Z. et al. 2011. In 2lst International Conference on Optical Fiber Sensors (OFS-21), Ottawa, ON, Canada, Papers 7753-109,7753-113, and 7753-114; Ran, Z. et al. 2011. 2lst International Conference on Optical Fibre Sensors (OFS-21) (pp. 775317-775317). International Society for Optics and Photonics.)
Figure 3.17 (a) Photograph of the self-enclosed FP cavity (strain sensor). (b) Photograph of the pressure sensor. (c) Photograph of the refractive and temperature sensor. (d) Top view of the accelerometer. (Rao, Y. J. and Ran, Z. L. 2011. Laser Focus World, 47(11), 71; Rao, Y. J. et al. 2007. Optics Express, 15(22), 14936-14941; Ran, Z. L. et al. 2008. Optics Express, 16(3), 2252-2263; Ran, Z. et al. 2013. IEEE Sensors Journal, 13(5), 1988-1991; Ran, Z. et al. 2009. Journal of Lightwave Technology, 27(15), 3143-3149; Ran, Z. et al. 2011. IEEE Sensors Journal IEEE, 11(5), 1103-1106; Ran, Z. et al. 2011. 21st International Conference on Optical Fiber Sensors (OFS-21), Ottawa, ON, Canada, Papers 7753-109, 7753-113, and 7753-114; Ran, Z. et al. 2011. 21st International Conference on Optical Fibre Sensors (OFS-21) (pp. 775317-775317). International Society for Optics and Photonics.)
(see Figure 3.17d). This sensor is sensitive to the acceleration which direction is perpendicular to the fiber end face due to its double- clamped acceleration sensing structure. The sensor spectrum and response to acceleration are shown in Figure 3.21. Its sensitivity is ~1.8 rad/g.