Sapphire FFPI Sensor
Sapphire fiber (Al2O3) is a single-crystal material, whose melting point is 2040°C . It is an ideal material to make a fiber-optic high- temperature sensor, but is difficult to be machined due to its extreme hardness. The sapphire fiber sensor structure, shown in Figure 3.22a, comprises a micro extrinsic air cavity and an intrinsic FP cavity. For comparison, a silica fiber-optic sensor with the same structure is also fabricated, as in Figure 3.22b. As the air cavity is not sensitive to temperature, the intrinsic FP cavity with a short section of fiber acts as a temperature-sensitive element. The sensor fabrication process consists of three basic steps: First, a series of 157-nm laser pulses are used to produce a circular hole on a sapphire fiber tip using a photo mask. Second, by splicing the sapphire fiber with a cleaved silica SMF end to enclose the hole, an air FP cavity was formed. Third, by cleaving the sapphire fiber near the air FP cavity using 157-nm laser, an FP temperature sensor is formed. The photo of the sapphire fiber sensor is shown in Figure 3.23.
Figure 3.18 (a) Reflective spectrum of the etalon. (b) Static strain responses of the FPI. (From Ran, Z. L. et al. 2009. Journal of Lightwave Technology., 27(15), 3143-3149.)
Figure 3.19 (a) Reflective spectrum of the pressure sensor. (b) Pressure responses of the sensor. (From Ran, Z. et al. 2011. IEEE Sensors Journal, 11(5), 1103-1106.)
The temperature characteristics of the sapphire fiber and silica fiber temperature sensors were both measured from 100°C to 1100°C. The spectra of the two sensors at different temperatures are shown in Figure 3.24a and b, respectively. It can be seen that the fringe contrast of the sapphire FP sensor is more stable than that of the silica FP sensor at high-temperature regions.
The temperature responses of the two types of sensors are shown in Figure 3.25. For the sapphire FP sensor, the pit wavelength shift at 1550 nm wavelength range is ~50.12 nm, corresponding to a sensitivity of ~50 pm/°C. It has good linearity of ~99.9%, while for the silica FP sensor, the wavelength shift is only ~8.86 nm, with a linearity of ~99.3%.