To excite SPR from an optical fiber, light confined in the fiber
core has to be locally outcoupled and brought into contact with
the surrounding medium. In practice, this is achieved either by a
geometrical modification (polishing or etching of the cladding) so
as to expose the evanescent wave to the surrounding medium or
by using in-fiber gratings (refractive index modulations photoimprinted
in the fiber core along the propagation axis). Hence,
various architectures are available [2]: etched multimode optical
fibers, side-polished, D-shaped, tapered or U-bent optical fibers,
long period fiber gratings (LPFGs) and tilted fiber Bragg gratings
(TFBGs). Configurations based on cladding removal/decrease can
be quite easily achieved. The SPR is in this case spectrally manifested
by a broadband resonance (full width at half maximum
(FWHM) 50 nm or higher) in the transmitted amplitude spectrum.
Operation in reflection mode is possible by using a mirror
deposited on the cleaved fiber end face beyond the sensing region.
However, these configurations considerably weaken optical fibers
at the sensor head and may prevent their use in practical applications,
out of laboratory settings. For this reason, large core fibers
(unclad 200–400 mm core fibers) are the most spread in practice
[3]. These configurations operate at visible wavelengths, which
limits the extension of the evanescent wave in the surrounding
medium. Indeed, its penetration depth is proportional to the operation
wavelength (λ) and usually ranges between λ/5 and λ/2,
depending on the mode order [4]. Hence, operation at near-infrared
telecommunication wavelengths enhance the penetration
depth, which in turn improves the overall sensor sensitivity to
large-scale targets such as cells, as experienced in the following.
Such operation can be easily achieved with in-fiber gratings