Singlemode FFPI Structures
Among all kinds of optical fibers, the SMF has the lowest transmission loss and also the lowest cost, thanks to the development of optical fiber communications, which make it an excellent candidate for the fabrication of FFPIs. One can use the fiber Bragg grating (FBG) pairs, reflective films to provide multi-beam interference, or the Fresnel reflection between the silica material of optical fibers and air to perform the two-beam interference, for the FFPIs. The principle of these FFPI sensors was described in Chapter 1.
Intrinsic FFPI Sensors
The intrinsic and extrinsic FFPIs (IFFPI, EFFPI) are differentiated by whether the material is the same as the fiber (silica) between the reflective surfaces. The IFFPIs use the same material, whereas the EFFPIs use different materials. The IFFPI sensors often consist of two reflective elements and a section of SMF in between. The reflective elements can be FBG pairs, reflective films, air holes, or surfaces formed by fusion splicing between different kinds of optical fibers.