Gratings preserve the fiber integrity while providing a strong
coupling between the core-guided light and the cladding. LPFGs
consist in a periodic refractive index modulation of the fiber core
of a few hundreds of mm. They couple the forward-going core
mode into forward-going cladding modes. Their transmitted amplitude
spectrum is composed of a couple of wide resonances
(FWHM 20 nm or even more) distributed in a wavelength range
of a few hundreds of nm. TFBGs are short period (500 nm)
gratings with a refractive index modulation slightly angled with
respect to the perpendicular to the optical fiber axis. In addition to
the self-backward coupling of the core mode, they couple light
into backward-going cladding modes. Their transmitted amplitude
spectrum displays several tens of narrow-band cladding mode
resonances (FWHM 200 pm or even below) located at the lefthand
side of the Bragg resonance (or core mode resonance) corresponding
to the core mode self-coupling. According to phase
matching conditions, every cladding mode resonance possesses its
own effective refractive index and the maximum refractometric
sensitivity is obtained when this effective index tends to the surrounding
refractive index value. TFBGs act as spectral combs and
constitute the only optical fiber configuration able to probe simultaneously
but distinctively all the cladding modes supported
by an optical fiber [5].