One of the most classic EFFPI structures is obtained by aligning the two cleaved fiber end faces by a glass capillary. In 1991, Murphy et al.  proposed this structure, as shown in Figure 2. . A SMF was used as both the input and the output fiber. Its cleaved end face was one reflector. An MMF was used as another reflector that formed the EFFPI structure. The two fibers were aligned by a silica capillary. The Fresnel reflections were about 4% so that the first reflection dominates the interference and the subsequent reflections can be neglected. The interference fringes were mainly determined by the two-beam interference model, as discussed
Figure 2.7 Capillary-aligned EFFPI structure.
in Chapter 1. The far end of the MMF was shattered so that the influence on the low-finesse cavity can be reduced. In this structure, the two fibers were easy to be moved in the silica capillary to adjust the air cavity length. Since then, this kind of simple EFFPI structure has been widely used, especially for high-pressure sensing by using the silica capillary as the sensitive element and the air cavity as the detection device. The silica capillary can withstand the high pressure, thanks to its relatively large thickness.