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Microstructured Optical Fiber FFPIs

Besides the PCFs, there are other kinds ofMOF including suspended- core fiber (SCF) and polarization-maintaining fiber (PMF). FFPIs based on SCF and PMF were also developed [93-95].

SCF-FFPIs were fabricated by fusion splicing a short section of SCF with SMF and used for strain, temperature, and refractive index sensing. One SCF had three holes with a diameter of 20 |!m and a core diameter of 3.2 p,m. Another SCF had four holes with a diameter of 43 |lm and a core diameter of 5 |lm. The lengths of the two SCFs were 1 and 0.84 mm, respectively, for the three- and four-hole SCFs. In order to exert strain on the FFPI, the cleaved end of the SCFs was spliced with a hollow-core PCF. The strain and temperature sensing performance of the SCF-FFPIs had no big difference from FFPIs

PMF—FFPI structure and its reflective spectrum

Figure 2.19 PMF—FFPI structure and its reflective spectrum.

using other types of optical fibers. However, it was good for refractive index sensing, thanks to the large holes around the suspended core.

Polarization maintain fibers (PMF) were also used for fabricating FFPIs based on inscription of a pair of FBGs [96]. The relatively high reflectance of the FBGs enabled a fine spectra with FSR of 40 pm and a linewidth of each fringe of 1 pm, which was helpful for high- resolution sensing.

PMF, as another main kind of microstructured fibers, can be employed for the microfluidic applications by using the two symmetric air holes in the fiber cladding [97,98]. The microfluidic PMF-FFPI structure is shown in Figure 2.19. The solid-core diameter was 6 |lm and the diameter of the air holes is 26 |lm with a center-to-center distance of 60 p,m. By fusion splicing the PMF, a fiber tube, PMF in sequence with a SMF, FFPI was formed based on the reflections from the two ends of the fiber tube, which had an inner diameter of 75 p,m for liquid sampling [97]. An inlet was introduced by etching the joint of the SMF and the PMF and the microfluidic flow was activated by an external pressure/vacuum pump. The far end of the PMF was cleaved at an angle to avoid reflections. The RI sensitivity of 1051 nm/ RIU was achieved, with good repeatability and low cross sensitivity of temperature.

(a) Specialty optical fiber designed for fabricating FFPI sensors and (b) SEM and (c) microscopic images of the after etching

Figure 2.20 (a) Specialty optical fiber designed for fabricating FFPI sensors and (b) SEM and (c) microscopic images of the after etching.

There were FFPIs based on purpose-designed MOFs. One example is shown in Figure 2.20, with the central core region doped with TiO2, a pure silica barrel ring, a P2O5-doped ring, and an outer cladding [99]. The P2O5-doped ring can be etched at a high rate, and the TiO2-doped core has a low etching rate. Therefore, a flat core end face, as well as a thin cladding, can be fabricated after chemical etching, as shown in Figure 2.20b and c. After fusion splicing with a lead-in SMF, the MOF-FFPI with a short cavity length was formed (Figure 2.21). However, thanks to the thin, but relatively long

FFPI formed by the etched microstructured fiber

Figure 2.21 FFPI formed by the etched microstructured fiber.

(360 Дш), etched cladding, a strain resolution of 0.5 Д? and low cross sensitivity of 0.055 Д?/°С were obtained.

 
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