Extrinsic FFPI Sensors
Although the IFFPI structure has the advantages of high mechanical performance and easy fabrication, two drawbacks limit their applications. One is the cross sensitivity to temperature, which makes the test of all the other parameters not accurate when there are evident temperature variations. The other is the fact that there is no access for biochemical samples to go into the FP cavity. Thus, the IFFPI sensors can only use the end surface to perform the biochemical sensing, which is based on the relative intensity detection and may limit the sensitivity.
The initial motivation of introducing the extrinsic FFPI (EFFPI) structure is to solve these problems. EFFPI structures are often sandwiched with an air cavity between two reflective surfaces. The air cavity is important to reduce temperature sensitivity, or even to make temperature-insensitive sensors. The air cavity also offers great potential for biochemical samples to be located in the EFFPI structure, enabling the efficient interaction between samples and light and enhancing the detection sensitivity.
The performance of EFFPIs is strongly dependent on the cavity length and propagation loss inside it . A strong loss of the cavity would enlarge the difference between the reflected intensities from the first and second reflective surface, and further degrade the interference fringe contrast. The loss of the EFFPI depends on the numerical aperture (NA) of the lead-in fiber, the air cavity length, the roughness, reflectance, and shape of the reflective surfaces. The NA of the lead-in fiber determines the divergent angle of light emitted out of the fiber. When cleaved optical fibers with flat surfaces are used to form an EFFPI, using lead-in fiber with lower NA should be better to reduce the propagation loss of light inside the cavity.
The cavity length of less than several hundreds of micrometers was often used to reduce the loss and obtain a fringe contrast better than 10 dB for high-performance sensing. Another way to enhance the performance is to use the Fizeau interferometer , which employs two surfaces with quite different reflectances with the far-end surface having a much higher reflectance. The high reflectance of the far-end surface can compensate the cavity loss and finally obtain a comparable reflected intensity from each surface. In this case, the cavity length can be extended up to several millimeters, greatly enhancing the multiplexing capability of the EFFPIs.
Up to now, the EFFPIs developed are mostly based on the two- beam interference by using the simple cleaved fiber end face. However, in some applications such as cavity quantum electrodynamics, high- finesse cavities are preferred. In these ultimate applications, flat- surface-based EFFPIs are not usable. Hunger et al.  fabricated high-finesse fiber FP cavities with concave, ultralow-roughness mirrors on the fiber end by using CO2 laser pulses. According to the beam propagation theory in optical cavities, concave mirrors are very helpful to construct a stable cavity with ultralow loss. After coating, the cavity can have a finesse as high as 105.
On the other hand, the cavity length is often used as a sensing parameter to detect strain, pressure, or acoustics and needs to be controlled or precisely measured . This part will be introduced in the interrogation and multiplexing section.