2. Fabrication of plasmonic TFBG immunosensors
The immunosensor developed in our work consists of a TFBG
surrounded by a thin gold sheath to which biomolecules are
grafted.
TFBGs correspond to a refractive index modulation angled by a
few degrees relative to the perpendicular to the propagation axis
(inset of Fig. 1). In addition to the self-backward coupling of the
core mode at the Bragg wavelength, TFBGs redirect some light to
the cladding whose diameter is so large that several tens of
cladding modes can propagate, each with its own effective refractive
index neff,clad. The phase matching condition for the ith
cladding mode resonance is given by λclad,i¼(neff,coreþneff,clad,i)Λ
where neff,core is the effective refractive index of the core mode.
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
2. Fabrication of plasmonic TFBG immunosensorsThe immunosensor developed in our work consists of a TFBGsurrounded by a thin gold sheath to which biomolecules aregrafted.TFBGs correspond to a refractive index modulation angled by afew degrees relative to the perpendicular to the propagation axis(inset of Fig. 1). In addition to the self-backward coupling of thecore mode at the Bragg wavelength, TFBGs redirect some light tothe cladding whose diameter is so large that several tens ofcladding modes can propagate, each with its own effective refractiveindex neff,clad. The phase matching condition for the ithcladding mode resonance is given by λclad,i¼(neff,coreþneff,clad,i)Λwhere neff,core is the effective refractive index of the core mode.Fig. 1 depicts a typical TFBG transmission spectrum where eachresonance corresponds to the coupling from the core mode to agroup of backward-going cladding modes. The spectral position ofa cladding mode resonance is related to its effective refractiveindex, which in turn depends on the optical properties of themedium surrounding the optical fiber outer surface. For instance,the cladding mode resonance close to 1540 nm is characterized byan effective refractive index value near 1.315, which is the refractiveindex of water at this wavelength. In water, cladding moderesonances at shorter wavelengths are therefore drastically attenuatedsince the modes are no longer guided by the claddingwaterinterface. 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
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