The tunable laser of an optical sensing analyzer (si720, Micron
Optics) was coupled into the sensor through a 2 2 coupler
and the reflected light was routed back through the same
coupler to the receiver of the analyzer. A scan of the tunable
laser produced the spectrum information from 1520 to 1570 nm
with a 2.5-pm resolution. When the applied pressure changes,
the spectrum fringes shifts accordingly. A fringe valley traced
method [11] was used to process the fringes in order to demodulate
the air gap thickness. The air gap measurement exhibits a
resolution of 0.03 nm [12].