Pressure sensors have been extensively investigated in the last decades, especially for oil/gas and space applications. Kao and Taylor  developed a simple intrinsic FFPI pressure sensor by fixing the FFPI in a cylindrical housing by epoxy, as shown in Figure 4.5. The FFPI was always under tension during the test. The deformation of the diaphragm induced by external air pressure changes was transferred into the strain on the FFPI and was further measured by detecting the phase changes.
Liquid level sensor, which measures pressure at several kPa, is useful for oil storage and many other applications. Lu and Yang  developed a liquid level sensor by packaging FFPI in a specially designed mechanical structure, with a resolution of 2 mm over the measurement range of 2 m. The resolution was further enhanced to 0.4 mm . The size of the sensor was large. By using a fused silica ferrule and CO2 laser heating bonding technology, a simple extrinsic FFPI was formed by aligning the optical fiber with a diaphragm attached to the end of the ferrule . In this case, there was no strain on the FFPI sensor and the deformation of the diaphragm directly corresponded to the phase changes of the FFPI. Similar to other extrinsic FFPIs, it has the advantage of low-temperature cross sensitivity (0.013 nm/°C) with a pressure sensitivity of 5.3 nm/kPa. Due to its high sensitivity, it can be used for liquid level sensing, with a resolution of 0.7 mm over 5-m measurement range . Moreover, the size of the sensor was reduced to 0.5 mm, which can be integrated to other systems easily. Some other structures of the FFPI pressure sensors were designed for special use; for example, 45° angled fiber and cross-axial configuration were used for better surface mounting , as shown in Figure 4.6. A packaging design of temperature-insensitive
Figure 4.5 Packaged FFPI pressure sensor.
Figure 4.6 FFPI pressure sensor based on (a) coaxial and (b) cross-axial configuration.
FFPI pressure sensor was developed for downhole applications . Pressure of up to 50-60 MPa was measured with a low-temperature sensitivity.
The size of the FFPI pressure sensor can be further reduced by micromachining technologies. Thanks to the compact size, the sensing performance including stability of the sensor can also be enhanced. Different kinds of micromachining technologies for fabricating the FFPIs were introduced in Chapter 2. Here, we discuss the sensing performance for FFPI by different fabrication methods.
Chemical etching has the advantages of low cost and capability for mass production. It is based on the different etching rate of the fiber core and cladding due to their different material composition. An FFPI can be formed by fusion splicing the etched fiber with another cleaved fiber . By cleaving one end of the FFPI and further polishing the fiber end into a thin diaphragm, the FFPI pressure sensor was formed. The pressure sensitivity can be adjusted by the thickness of the diaphragm. Usually, the roughness of the etched bottom of the air cavity is not good enough for high-performance FFPI and needs to be improved by optimizing the charge intensity and duration during the splicing.
By using laser micromachining technologies, the FFPI can be fabricated through similar steps as described above by using the etching method. By using fs-laser micromachining, the fabricated FFPI sensor can work under high temperatures of up to 700°C , which is one of the advantages of the micromachined FFPI sensor. Figures 4.7 shows the pressure and temperature sensing performance of it. During the fabrication process, the laser spot needs to be precisely aligned with the optical fiber end, which makes this technology
Figure 4.7 (a) Pressure and (b) temperature sensing performance of the laser micromachined FFPI sensor.
complex and relatively high cost. Therefore, it is often used for harsh environment sensing with high-performance requirements.
There were also other kinds of FFPI pressure-sensing structures fabricated by simply cleaving and splicing. One example is by splicing a section of silica rod between two SMFs . The silica rod acted only as a support for forming the FFPI structure. However, this structure has relatively high cross talk between pressure and temperature because both parameters had a strong effect on the readout signal, that is, the spectral shift of the FFPI. Another hybrid optical fiber FFPI structure was fabricated, and a passive temperature compensation scheme was used to reduce 85% of the temperature dependence . By detecting the length of the air gap, a pressure sensitivity of 0.316 nm/psi was obtained. The temperature cross sensitivity was about 0.026 psi/°C.
There are two schemes of FFPI for transferring the pressure variation to the air cavity length changes of the FFPI. One is by using a thin diaphragm at the end of a hollow or etched optical fiber, introduced as above. The pressure is added on the diaphragm along the axis of the fiber, and the deformation of the thin diaphragm corresponds to the cavity length changes. The other is by adding pressure on the lateral direction of the sensing structure. Early trials were based on aligning two optical fiber ends in a silica capillary or ferrule. Pressure was exerted on the lateral direction of the capillary and induced axial strain of the FFPI so that it changed the cavity length of the FFPI.
In order to make the sensing structure compact and reliable, PCFs or other kinds of microstructured fibers were used instead of hollow silica ferrules. PCFs can be fusion spliced in between two SMFs and form an FFPI . Pressure on the lateral direction can be measured by detecting the wavelength shift induced by the axial strain. Frazao , Guan , and their coworkers demonstrated FFPI pressure sensors using different kinds of PCFs, and sensitivities of 0.82 nm/ MPa and 0.023 nm/MPa were obtained, respectively.
Different kinds of materials were used for fabricating the FFPI pressure sensors. Polymers have advantages of being easy-to-fabricate, and are also easily deformed so that FFPI sensors based on polymer materials may have high sensitivity of pressure. Eom et al.  developed an FFPI pressure sensor by packaging a lensed fiber and a hybrid polymer thin diaphragm in a glass ferrule. The cavity length changes were detected to determine the external pressure. Highly deformable material, PDMS, was used to improve the sensitivity. A high sensitivity of 1.41 p,m/kPa and a detection resolution of 0.03 kPa were obtained.
Wang et al.  developed an FFPI pressure sensor for underwater blast wave pressure detection, by coating at the end face of an optical fiber with two reflective films sandwiched with a polymer spacer with a thickness of 50 |lm. The repeatability is 1.82% within the full range of 0-55 MPa. The response time is 0.767 |ls. Bae and Yu  developed a UV-molding method for fabricating the FFPI pressure sensor. The fabrication process is simple and the calibration curve was achieved with excellent linearity.
Besides the polymer materials, silicon has also been used frequently to achieve unique properties of sensing. Liu and Han  developed an FFPI gas pressure sensor by attaching a silicon pillar to the optical fiber tip. The pressure was determined by measuring the temperature of the silicon, which was heated by a visible laser beam. In this experiment, the unique absorption property of silica was used, that is, high absorption at visible wavelength for heating and low absorption at near infrared band for generating two-beam interference.
Other kinds of special FFPI pressure sensors were developed for shock pressure testing with fast response time. More than 10 years ago, Gander et al.  fabricated an FFPI pressure sensor by forming the cavity with a cleaved SMF and a 2-p.m-thickness copper diaphragm and sealing the structure by epoxy. The frequency response of the sensor on the dynamic pressure was measured. Shock wave was investigated with an FFPI pressure sensor with response time at the millisecond level, which used a silicon dioxide diaphragm as the transducer. Further, by sealing a laser micromachined cavity with aluminized polycarbonate diaphragm, an FFPI pressure sensor with a fast response time of 3 ^s was developed .