Another commonly used interferometer based sensor is
the Fabry-Perot interferometric sensor (FFPI) and is
classified into two categories: Extrinsic Fabry-Perot
interferometer (EFPI) sensor and intrinsic Fabry-Perot
interferometer (IFPI) sensor. In an EFPI sensor, the FabryPerot
cavity is outside the fiber. Fiber guides the incident
light into to the FFPI sensor and then collects and the
reflected light signal from the sensor. In an IFPI sensor,
the mirrors are constructed within the fiber. The cavity
between two mirrors acts both as sensing element and
waveguide. In this case, the light never leaves the fiber
[19]. Figure 12 shows a typical EFPI sensor [19] based on
capillary tube.