Fig. 1 depicts a typical TFBG transmission spectrum where each
resonance corresponds to the coupling from the core mode to a
group of backward-going cladding modes. The spectral position of
a cladding mode resonance is related to its effective refractive
index, which in turn depends on the optical properties of the
medium surrounding the optical fiber outer surface. For instance,
the cladding mode resonance close to 1540 nm is characterized by
an effective refractive index value near 1.315, which is the refractive
index of water at this wavelength. In water, cladding mode
resonances at shorter wavelengths are therefore drastically attenuated
since the modes are no longer guided by the claddingwater
interface. Oppositely, resonances just above 1540 nm remain
guided. Their evanescent field penetrates the surrounding
medium over distances corresponding to several wavelengths. In
the jargon, the wavelength at which a mode is no longer guided is
the cut-off wavelength. In practice, resonances just above this
transition point are the most sensitive to surrounding refractive
index (SRI) changes [6]. They shift towards longer (shorter) wavelengths
as the SRI increases (decreases). For resonances at
longer wavelengths, the modes are increasingly confined into the
cladding and their SRI sensitivity decreases accordingly, reaching
complete insensitivity for the core mode resonance at the Bragg
wavelength. It is worth mentioning that the amplitude spectra
presented in Fig. 1 was recorded without polarization control of
the input light. We will see in the following that light polarization
is crucial in the case of gold-coated TFBGs.
TFBGs were manufactured in the same way as standard uniform
FBGs, i.e. through a lateral illumination of the fiber core using an
interference pattern of ultraviolet (UV) light usually around 240 nm.
In our work, 5 mm–1 cm-long TFBGs were manufactured into hydrogen-loaded
telecommunication grade single-mode optical fibre
using a 1090 nm period uniform phase mask and a frequency-doubled
Argon-ion laser emitting at 244 nm. Fiber hydrogenation was
done in a vessel containing pure hydrogen at a pressure of 200 atm
and temperature of 70 °C. This process enhances the fiber photosensitivity
to UV light used to produce a refractive index modulation
of the fiber core. TFBGs were obtained by a single scan of the laser
beam (width: 0.6 mm-averaged power: 60 mW) along the phase
mask at the speed of 20 mm/s. An external tilt angle ranging between
6° and 8° was chosen with respect to the perpendicular plane to the
optical fiber axis to ensure a strong coupling to cladding modes
characterized by an effective refractive index close to water (1.315).
Indeed, it is well known that for higher tilt angles, more coupling
happens to higher order cladding modes, for which the effective refractive index is close to the one of water. Right after the inscription
process, the gratings were annealing at 100 °C during 24 h to
remove the residual hydrogen and to stabilize their physical
properties.
A gold sheath was then plated on the optical fiber surface at the
grating location. In our experiments, we have used the sputtering
process to deposit a 50 nm gold sheath. The vacuum was obtained
in the chamber starting from ambient air. Two consecutive depositions
were made in the same conditions, with the optical fi-
bers rotated by 180° between the two processes to ensure that
100% of the surface is covered by gold. The gold thickness was
monitored in real time using a Quartz microbalance placed in the
sputtering chamber.
Finally, the gold-coated TFBGs were functionalized for biosensing
purposes. The chemistry involved in this process depends on the
target application. We usually rely on the antigen/antibody affinity.
Whatever the analyte to be detected, a self-assembled monolayer
(SAM) is manufactured. For this, gold-coated TFBGs were first thoroughly
rinsed with ethanol. They were then immersed in a solution
of thiols dispersed in ethanol. Thiols incubations were done during
12 h at room temperature in a 1 mm thick capillary tube sealed at
both ends to prevent solvent evaporation. At the end of the incubation,
the functionalized gold-coated TFBGs were removed from
the tube and again rinsed with ethanol, prior to the grafting of biomolecules
on the activated surface.
This four-step process yields the plasmonic immunosensor
sketched in Fig. 2. Depositing a gold or silver mirror on the cleaved
fiber end face upstream of the TFBG offers the convenient possibility
to operate in reflection mode.