Figure 12. Capillary tube based EFPI sensor.
One cleaved fiber end (lead-in) is inserted into a glass
capillary tube and another cleaved fiber end (target) is
inserted into the tube from the other end. Both lead-in and
target fibers are thermal fusion bonded with the tube. The
cavity length between the two fibers is controlled using a
precision optical positioner prior to the thermal fusion
bonding. One of the advantages of this EFPI strain sensor
is that its gauge length and cavity length can be different.
The strain sensitivity is determined by the gauge length,
while the temperature sensitivity is determined only by
cavity length since the fiber and tube have the same
thermal expansion coefficients. Hence, by making the
gauge length much longer than the cavity length, the
sensor temperature sensitivity becomes much less than
the strain sensitivity. So, no temperature compensation is
required.