The change of interference spectrum with the change of humidity in the humidity control box can be observed by the OSA.
The wavelength shift of interference spectrum is monitored between the range from 35% to 85% RH and the measured data is recorded when the humidity is kept stable in the humidity control box.
When the moisture sensitive film absorbs more water molecules, the RI of the humidity sensitive film itself will be reduced and the intensity of the interference spectrum will be increased, which is completely consistent with  analysis.
Caption: Figure 5: The interference spectrum with the change of humidity in the external environment sensor.
During the light transmission process, the interference spectrum
will change periodically.
Note that there may be several cladding modes in the sensing fiber which will results in a relatively complex interference spectrum.
When the gold-coated FMI is inserted to the sealed oil tank, the mode indices of the cladding mode and the core mode of the sensing SMF will change due to the stress-optic effect , which consequently results in the wavelength shift of the interference spectrum of the FMI.
Hydrostatic pressure sensing is achieved by measuring the wavelength shift of the interference spectrum of the FMF-based hydrostatic pressure sensor.
An optical spectrum analyzer (OSA) with a resolution of 0.02 nm is used to detect the interference spectrum of the two modes of the FMF.
The wavelength spacing of the interference spectrum (around 1550nm) of the hydrostatic pressure sensor is about 8.9 nm, which is proportional to the length of the FMF.
When the pressure increases, the interference spectrum of the hydrostatic pressure sensor has a blue-shift.