IFFPI Structures Based on Fusion Splicing of Different Kinds of Fibers
The main point in the fabrication of IFFPI sensors is to form reflective mirrors in the optical fiber, in order to provide at least two reflective beams for generating the interferometric signal. Previously, FBG pairs, reflective films, and air gaps were introduced as reflective mirrors. For all three structures, it is necessary to fabricate the reflective elements first and then to form the FP cavity based on the two reflective mirrors. Indeed, there is another simpler method to fabricate IFFPI sensors, that is, by directly fusion splicing two different kinds of optical fibers. As long as there is a refractive index difference between the two fibers, there is a reflective signal according to the Fresnel equation. By considering the light with an incident angle of 90°, the Fresnel equation can be simplified and the reflection can be expressed as
Here n1 and n2 are the refractive indices of the two optical fibers.
The first example is the work by Tsai and Lin published in 2001 . They used Corning SMF (SMF-28) and the 3M SMF (FS-SN- 3224), respectively. The core diameters were 8.4 and 4 |lm, respectively. The refractive indices of the fiber cores were 1.4488 and 1.456 at 1550 nm. Due to the core diameter difference, the reflectance at the spliced joint of the two fibers depended on the core refractive index difference within the radial range of 0-2 p,m, as well as the refractive index difference of the core of SMF-28 and the cladding of FS-SN- 3224 within the radial range of 2-4.2 p,m. They used a splicing point as one reflective mirror, and a cleaved fiber end as another reflective mirror, as shown in Figure 2.4. The fringe contrast of the interference may not be excellent due to the different reflectance from the two mirrors.
Figure 2.4 Structure of an FP cavity with different fiber cores.
Wang and coworkers  proposed another IFFPI sensor by using two identical mirrors based on splicing a section of a multimode optical fiber in two SMFs, that is, the singlemode-multimode-singlemode (SMS) structure, as shown in Figure 2.5. The fibers were Corning SMF-28 SMF and Corning InfiniCor 50-p.m MMF. The cavity length was controlled with an accuracy of ±70 |lm. The insertion loss was not a problem. When the light propagating in the SMF is launched into the MMF, it primarily excites the fundamental mode of the MMF.
By detecting the reflected light from the SMS structure, the interference fringes were recorded. The fringe contrast was about 15 dB, good enough for most physical parameter sensing. The average reflected light intensity was about -50 dB; compared with that, the Fresnel reflection was -13.8 dB. Therefore, the sensing signal is rather small. The authors demonstrated that the insertion loss was about 0.90 dB with a standard deviation of 0.17 dB by fabricating 50
Figure 2.5 IFFPI sensor based on the SMS structure.
such sensors. A multiplexing of 8 such SMS-IFFPI sensors was also demonstrated with cavity lengths between 553 and 5332 p,m.
Recently, Xuejin Li and coworkers further developed an IFFPI structure based on a section of microstructured optical fiber (MOF) fusion spliced to the SMF , as shown in Figure 2.6. The partial Fresnel reflection is from the interface of the two fibers and end face Fresnel reflection of the microstructured fiber. Similar to the previous two schemes, only cleaving and fusion splicing were required for the fabrication of such IFFPI structure. The fringe contrast can be as high as 20 dB when the microstructured fiber length is about 50 p,m. Due to the trend of the fringe contrast versus the fiber length, the fringe contrast can be further enhanced if shorter fiber length is used. The high-temperature sensing performance is good. Temperature up to 1000°C was tested and a sensitivity of 17.7 pm/°C was obtained.
Figure 2.6 IFFPI structure based on splicing a section of microstructured fiber to SMF. (a) Crosssection of the microstructured fiber and (b) detailed dimensions. (c) The fabricated IFFPI structure and (d) its schematic interrogation setup.
The most promising point of this kind of IFFPI sensor is the simple fabrication process; that is, only cleaving and fusion splicing are required. And it is good for high-temperature sensing as the whole structure was based on silica.