(IEEE, 2022) Imas González, José Javier; Albert, Jacques; Del Villar, Ignacio; Ozcariz Celaya, Aritz; Ruiz Zamarreño, Carlos; Matías Maestro, Ignacio; Ingeniaritza Elektrikoa, Elektronikoaren eta Telekomunikazio Ingeniaritzaren; Institute of Smart Cities - ISC; Ingeniería Eléctrica, Electrónica y de Comunicación
The mode transition is a phenomenon observed in
thin film coated long period fiber gratings (LPGs) and singlemode multimode single-mode (SMS) fibers for certain values of
the coating thickness and refractive index, resulting in increased
sensitivity for sensing applications. It is shown here that mode
transitions occur simultaneously for a large number of mode
resonances in the transmission spectra of tilted fiber Bragg
gratings (TFBG) measured during the deposition of ~350nm thick
TiO2 coatings by Atomic Layer Deposition (ALD). In TFBGs, the
mode transition shows up as an acceleration of the resonance
wavelength shift vs thickness, but without fading of the resonance
amplitude. Furthermore, the results show that the mode transition
for cladding modes with predominantly “TE” polarization at the
cladding boundary is significantly sharper than that of
predominantly “TM” polarized modes and that it occurs at a
smaller coating thickness (<100 nm vs >200 nm). Finally, using a
separately determined coating refractive index (2.14, by
ellipsometry on witness flats deposited simultaneously) and
simulations of the resonance shifts of the TFBG with coating
thickness, it is demonstrated that a TFBG connected to a spectral
interrogation system can be used to measure the growth of a
coating on the surface of the fiber in real time.
