Instrumentation & Measurement Magazine 24-9 - 71

Fig. 1. Common SPR configurations. (a) Kretschmann-Raether prism; (b) Gold-coated optical fiber sensing probe; (c) Geometry-modified optical fibers; and (d)
Fiber gratings. (Adapted from [1]).
a chemical etching process or by side-polishing, as shown in
Fig. 1c [1]. However, geometry-modified fibers can detrimentally
affect the mechanical stability of optical fiber sensors. As
an alternative to geometry-modified fibers, gratings photo-inscribed
in the core (normally by use of a high-power ultraviolet
light together with a phase mask) can be used to diffract part
of light into the cladding. Inscribed gratings minimally reduce
the mechanical strength of the optical fibers. We can benefit
from the fact that grating coupling is a resonant phenomenon
that only occurs at specific wavelengths in guided-wave
configurations.
In this paper, we introduce two well-known fiber grating
configurations. One configuration is the long period
grating (LPG), which can couple the forward propagating
core mode into forward propagating cladding modes (the
middle diagram of Fig. 1d) [1]. The refractive index modulation
period of a long period grating is typically in the
range 50~500 μm. The 20~50 μm-wide spectral resonances
of LPGs enable the coupling of SPP for surface biomedical
measurements. An alternative configuration is the tilted
fiber Bragg grating whose fringes are slightly tilted with respect
to the optical fiber propagation axis, as shown in the
bottom diagram in Fig. 1d [1]. A TFBG can couple the forward
propagating core mode into tens or even hundreds of
backward propagating cladding modes. Each of these cladding
modes is very narrow (bandwidth of around 0.1 nm),
thereby providing a high sensitivity for biosample measurements
[3]. Moreover, TFBG sensors rely on Bragg diffraction
of the core mode, which is immune to changes in the external
media. Since the TFBG's Bragg resonance spectrum is
only sensitive to strain and temperature, a TFBG sensor is
immune to the temperature crosstalk and the influence of
December 2021
light source fluctuations. Finally, the tilt of the grating plane
breaks the cylindrical symmetry of the fiber, thereby offering
a well-controlled polarization of cladding modes in both radial
and azimuthal directions, and provides high efficiency
SPR excitation in a gold film-coated fiber, as discussed in the
next section [4], [5].
How to Use Such Sensors
The SPR metallic substrate can be fabricated by coating the optical
fiber with a nano-scale metal film or nanoparticles (e.g.,
old or silver). However, the spectral characteristics of the SPR
are determined by the thickness, shape and uniformity of the
thin films on the surface of the optical fiber. Therefore, controlling
these three parameters allows the user to optimize the
performance for detecting the target molecular species. High
quality metal films are prepared via sputtering onto the surface
of the optical fiber. However, it is quite difficult to obtain
very uniform metal layers of the thicknesses required for optimum
SPR excitation, typically 50 nm.
A single-ended reflective optical fiber sensor can be
realized by depositing a gold coating onto the end of the TFBGbearing
fiber, resulting in a TFBG-SPR reflection spectrum
such as that shown in Fig. 2a. This simple two-pass configuration
can double the attenuation of each resonance. As shown
in Fig. 2b, the Bragg reflection now appears as a peak in the
measured power while the cladding resonances (with SPR
attenuation) still appear as troughs. An overall schematic diagram
of complete TFBG-SPR sensing system is shown in Fig.
2c. A broadband light source (BBS) with polarization controller
(PC) is used to excite SPR over the TFBG optical fiber sensor,
and its transmission spectrum is monitored with an optical
spectrum analyzer (OSA).
IEEE Instrumentation & Measurement Magazine
71

Instrumentation & Measurement Magazine 24-9

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