Microstructured FFPI Sensors
Photonic Crystal Fiber FFPIs
PCFs including index-guided PCF and photonic bandgap fiber as two main types, have been extensively investigated since they were invented in the 1990s [79,80]. As the technologies for the fabrication and characterization of PCFs, as well as the fusion splicing of PCF with SMFs, were developed, PCFs were used for enhancing the performance of FFPIs. PCF-FFPIs have been used for the measurement of various parameters, including high temperature, strain, pressure, and refractive index.
Fiber-optic sensors have the advantage of being capable of surviving in harsh environments such as those with high temperatures. FFPIs are even better for high-temperature sensing than the conventional FBGs. One example of the PCF-FFPIs was fabricated by fusion
Figure 2.16 (a) FFPI fabricated by fusion splicing a short section of PCF with SMF and (b) the cross section of the PCF. (Reprinted from Zhu, T., Ke, T., Rao, Y., and Chiang, K. S. 2010. Fabry— Perot optical fiber tip sensor for high temperature measurement. Optics Communications, 283(19), 3683-3685, Copyright 2010, with permission from Elsevier.)
splicing a short section of solid-core PCF with a SMF , as shown in Figure 2.16a. The PCF has a solid core with a diameter of 5.1 |lm, surrounded by six large holes with a diameter of 10.5 p,m, as shown in Figure 2.16b. The interference occurred between the reflections from the SMF-PCF interface and the end face of the PFC core. The fringe visibility was about 12 dB, which was high enough for harsh environment sensing. The PCF-FFPI can measure temperature as high as 1200°C due to its pure silica structure. The repeatability of sensing was good. Another similar FFPI structure was fabricated by using a section of polarization-maintaining PCF and high-temperature sensing was also demonstrated .
A section of hollow-core PCF was fusion spliced in between two SMFs to form an EFFPI , as shown in Figure 2.1 . The cavity length can be as long as several centimeters with good fringe visibility due to the low transmission loss of PCF compared with hollow-core fibers (or capillaries), which is important to enhance spatial frequency
Figure 2.17 EFFPI based on fusion splicing a section of PCF in between two SMFs.
multiplexing capability. The fringe contrast can be greatly enhanced by coating the end face of SMFs and then fusion splicing with PCF. Based on this simple PCF-EFFPI structure, temperature-insensitive strain sensing was performed.
Other kinds of PCFs were also used to form similar PCF-EFFPI structure, with different sensing performance [84,85]. Similar structures were employed for pressure sensing based on the lateral deformation of the PCF .
PCFs can also be used as a hollow-core fiber for fabricating the intrinsic-extrinsic hybrid EFFPI, similar as a hollow-core capillary. In 2008, Rao et al.  developed a PCF-FFPI structure for refractive index sensing. A hollow-core PCF was fusion spliced with a SMF and then cleaved with a PCF length of 2.3 mm. The fusion splicing procedures and parameters for PCFs were different from the conventional ones between two SMFs. Then another SMF was fusion spliced to the PCF and then cleaved by a fs laser with a SMF length of 20 |lm. The PCF part was an EFFPI, while the short SMF cap was an IFFPI. As calibration, the fringe visibility was measured as a function of the refractive index. Experimental results indicated the fringe visibility of the PCF hybrid FFPI was insensitive to temperature. Another similar intrinsic-extrinsic hybrid PCF-FFPI structure was fabricated by changing the type of PCF, the length of PCF, and SMF . The fringe visibility was enhanced up to about 30 dB when the hybrid PCF-FFPI was put in air, and was about 8 dB when putting the device in water.
Due to the multi-hole structure of PCFs, the fusion splicing parameters should be specifically set up for fabricating the PCF-FFPI structure. There was another way to fabricate a PCF-FFPI. As described in Reference 89, a film was formed by fully collapsing the air hole of a hollow-core PCF, after fusion splicing the PCF with a SMF. Further, they fabricated a PCF-FFPI by forming two films through collapsing the air holes at each end of the PCF .
The most promising aspect of PCF for fiber-optic sensing is the multi-hole structure, which is excellent for biochemical sampling and also for efficient light-matter interaction. The label-free biochemical detection is usually performed by refractive index measurement. An ultrahigh sensitivity, of 38,000 nm/RIU, for fiber-optic refractive index sensing  was obtained by detecting the wavelength shift
Figure 2.18 PCF-FFPI with liquid sampling channels.
from the mode coupling in PCF and by filling a single hole of PCF. The detection limit of refractive index was on the order of 10-7.
Deng et al.  fabricated a PCF-FFPI by fusion splicing a short section of hollow-core fiber and PCF in sequence with a SMF. The structure is shown in Figure 2.18. The solid core of the PCF served as a reflection surface, and the air holes were sampling channels for external gas under test to enter into the hollow fiber chamber. A low fusion power and a short fusion time were used during the fusion splicing of PCF, in order to avoid the collapse of the air holes. The sensitivity of wavelength shift versus refractive index was 1639 nm/RIU, which was higher than many other kinds of fiber-optic refractometers.