The most well-known FP interferometer configuration is based
on an air cavity formed between the optical fiber tip and a diaphragm
[5]. Usually, the device fabrication consists in three steps:
firstly, the splice between the SMF and the hollow core fiber is
made. Afterwards, the hollow core fiber is cleaved near the splice
and according to the desired cavity size. Finally, another splice
with a tiny section of SMF is cleaved forming a diaphragm. Different
applications were demonstrated over the last three decades. A
very interesting example is to apply pressure or acoustic sensing in
the diaphragm, inducing the simultaneous change of the cavity
length and phase [6]. Recently a vapor-sensitive polymer layer
was used as diaphragm, according to the scheme in Fig. 1, and
new solution for gas sensing was demonstrated [7]. Another configuration
well known by the scientific community is to apply a
thin film on the tip of the fiber. One of the most common examples
is to use it as a hydrogen sensor [8]. A different intrinsic FP interferometer
can be obtained simply by fusion splicing two fibers
with different core dimensions [9]. Multi-cavity FP interferometer
has also been explored as a biosensor [10]. Recently two new FPs
based on a diaphragm-free topology have been studied. The first
was based on a pure silica diaphragm-free hollow tube configuration
[11] and the second consisted on a hollow-core ring PCF [12].
Both configurations can be used to pressure sensing