Temperature-Insensitive or Temperature-Compensated Sensing
Temperature variation exists everywhere for the field trial of optical fiber sensors. Therefore, it is very important to develop temperature-insensitive or temperature-compensated sensors for detecting all kinds of other parameters. Great contributions have been made to solve this problem. There are mainly three strategies. The first is by using extrinsic FFPI sensors, which have low-temperature sensitivity. The second is by designing the packaging or structure of the FFPI so that the temperature effect can be compensated. The third is by measuring the temperature-insensitive readout parameter, like the fringe contrast in dB.
It is known that extrinsic FFPI has low-temperature sensitivity, compared with their intrinsic counterparts. It comprises a hollow cavity with two reflective mirrors, often formed simply by cleaved optical fiber with the Fresnel reflection. The reasons for the low- temperature sensitivity include two aspects. One is the thermally induced changes on the refractive index of air is smaller than that of silica. The other is the inherent compensation effect, that is, as temperature increases, the thermo-expansion of the mirrors of the FP cavity tends to reduce the cavity length, while the expansion of the side wall of the cavity tends to increase the cavity length . By using extrinsic FFPI sensors, the influence of temperature can be neglected when the requirement for measurement accuracy is not very high.
The second strategy is by compensating the temperature effect via designing the sensor structure [35,38,98]. One temperature compensation method is by packaging the FFPI sensor with a specially designed structure. A metal wire was used and its end face served as one of the reflective surfaces for the FFPI . The expansion of the outer tube for alignment tends to make the cavity length longer, while the expansions of the optical fiber and the metal wire tend to make the cavity length shorter. Thus, the temperature effect was compensated. Temperature compensation of FFPI sensors can also be achieved by using hollow-core fiber and different kinds of fibers.
Another design was often used for pressure sensors, as shown in Figure 4.14 (lower picture) . The two ends of the FFPI were attached to the inner tube and middle tube, respectively. As temperature increased, the expansions of the two tubes introduced similar displacement in the same direction onto each end of the FFPI. Therefore, the thermal-induced strain on the FFPI can be compensated.
Figure 4.14 Packaging of the FFPI for temperature compensation.
Figure 4.15 Dual-parameter (temperature and refractive index) sensing by FFPI via simultaneous detection of fringe contrast and wavelength shift.
The third strategy is by measuring the fringe contrast of the reflective spectra of the FFPI, which is not influenced by the temperature variations [75,78,99-101]. Thus, the FFPI can perform temperature-insensitive measurement. This method is often used for refractive index sensing. By using the FFPI based on three-beam interference, the refractive index and temperature can be measured by the fringe contrast changes and the wavelength shift of the reflective spectra, respectively, as shown by the results in Figure 4.15 . The cross talk between the two measured parameters is very low.
Besides the refractive index, FFPI sensors can be designed and used for simultaneous measurements of temperature and another parameter like relative humidity, pressure, or strain [102-104]. The main idea is that the dependences, that is, the sensitivity coefficient, of wavelength shift and the fringe contrast on the two parameters are different. Thus, a 2 X 2 coefficient matrix can be used for data processing .