Physical and Biochemical Sensors Based on FFPIs
Fiber-optic sensors have been widely used in a number of fields, ranging from distributed sensing of strain at kilometer scale and biomolecule detection at micrometer or even nanometer scale. For FFP sensors, they have mainly been used with miniature size for sensing of both physical and biochemical parameters.
Physical parameters, including temperature, strain, displacement, force, pressure, acoustic or ultrasonic waves, and electric and magnetic fields, were detected for smart structure and infrastructure health monitoring, oil/gas and electric power industry, etc.
Temperature detection is required in a lot of industrial applications, in order to save energy, or to monitor the production process during which temperature may influence much. Conventional temperature detection methods include the thermocouple, heat resistance, and infrared radiation detection. They may be influenced by ambient electromagnetic interference or may not be available for applications in harsh environments. Fiber-optic temperature sensors can withstand temperatures up to 1000°C and have the advantages of immune to electromagnetic interference, and environmental vibrations.
Fiber-optic FPIs were first embedded in composite materials for temperature sensing, demonstrated by H. Taylor and coworkers [1,2]. The measurement range was 20-200°C when the FFPI was embedded in a graphite-epoxy composite material. They also demonstrated that the temperature sensitivity can be enhanced by 2.9 times when the FFPI was embedded into stainless-steel tubes.
One of the most important applications of FFPI sensors is for high- temperature measurement, thanks to the inherent high melting point of silica. FFPI sensors based on the common Ge-doped optical fibers can be used under a temperature of about 600°C . By using special optical fibers such as PCFs or sapphire fibers, the measurement range can be extended up to 1200°C or even above 1500°C . There were numerous papers related to fiber-optic high-temperature sensing, and another bunch of papers for strain or pressure sensing under high- temperature environment.
Figure 4.1 shows the high-temperature sensing performance of an FFPI sensor based on fusion splicing a section of solid-core PCF to SMF A temperature range of —20°C to 1200°C was measured with good linearity and good repeatability .
The most recent report on FFPI temperature sensors described a sourceless high-temperature sensor , in which the thermal radiation, with a broad band, of the measurement environment was employed as light source for the detection of FP interference and no external light source was required. This is a smart idea that significantly simplifies the experimental setup and also reduces the cost. By employing the interference from a sapphire wafer, the fringe contrast of the reflective spectrum was good and the temperature can be measured up to 1593°C, as shown in Figure 4.2.
Figure 4.1 High-temperature sensing performance of PCF-FFPI.
Figure 4.2 Sourceless high-temperature FFPI sensor based on sapphire wafer.
As described in Chapter 2, FFPI can be formed with different kinds of optical fibers or fabricated by various micromachining technologies. Theoretically speaking, all of these FFPIs can be used for temperature sensing, as long as there is no cross talk between temperature and other parameters like strain. For example, Lee et al.  performed high-temperature sensing by using an FFPI based on dielectric films coated on the end face of optical fiber layer-bylayer. The coating was annealed under 800°C and was still smooth. Temperature sensing between 250°C and 750°C was demonstrated by wavelength shift detection.
Li et al.  developed an ultrahigh-sensitive FFPI temperature sensor based on a graphene diaphragm. The graphene film was grown by chemical vapor deposition (CVD) method and transferred onto a ferrule into which the fiber was inserted. The fiber end and the graphene film formed an FFPI, and the wavelength shift of the interference fringes was measured. A high sensitivity of 1.87 nm/°C was achieved, about two orders of magnitude higher than that of the conventional intrinsic FFPIs. The FFPI can be used for high-temperature sensing of up to 1000°C. There were also FFPIs based on other materials like silicon, which has the advantage of temperature sensing at high frequency of up to 2 kHz and is useful for some special applications .
The above methods for temperature sensing were based on the thermal expansion of silica optical fiber. There was another kind of method, which determined temperature by measuring the thermal- induced changes of materials inside the FP cavity.
By changing the gas pressure within the FP cavity, ambient temperature can be determined by measuring the slope of the spectral shift versus pressure. This method, in principle, is not sensitive to the changes of the cavity length. Thus, strain-insensitive temperature sensing of up to 1000°C was performed with strain of up to 3600 |i? . Similarly, Liu et al.  developed a fiber-optic temperature sensor based on differential pressure detection by using a silicon diaphragm. The temperature-related pressure changes of the sealed air introduced the deformation of the silicon diaphragm, which can be measured by the wavelength shift of the FFPI sensor.
Liquid, instead of air, can also be filled into the FFPI cavity, and its thermo-optical characteristics can be used for temperature sensing. Zhao et al.  developed an FFPI temperature sensor by filling anhydrous ethanol in the FP cavity. The refractive index of ethanol, thus the phase shift of the reflected beam from the FFPI, varied with temperature changes. Good linearity of temperature sensing was demonstrated. Recently, Yang et al.  developed an FFPI formed in a mercury-filled silica tube. The mercury-filled tube was fusion spliced with an SMF. The mercury surface and the end face of SMF formed an FFPI. The cavity length decreased when temperature increased and the volume of mercury was expanded. An ultrahigh temperature sensitivity of up to -41.9 nm/°C was achieved.