“Tilted” FBGs
Researchers have developed a new kind of
fiber grating sensor
that possesses all the
advantages of the well-established FBG technology in addition to being
able to excite cladding
modes resonantly: the
tilted FBG. The TFBG is
essentially an FBG whose
grating planes are angled
by a few degrees relative
to the plane perpendicular to the fiber axis. This grating tilt
has the effect of locally breaking the cylindrical symmetry of
the fiber in a way that allows strong coupling between the core
guided light and a large number of cladding modes.
Apart from tilt, everything we know about FBGs, including the many techniques used to fabricate them, can transfer
directly to TFBGs. Tilted gratings have also been studied for
almost as long as FBGs, with larger tilt angles for extracting
light from the fiber altogether. For example, TFBGs are used
in some niche telecommunications applications as all-fiber
polarizers, gain flattening filters for optical amplifiers and
channel monitors. It is only recently, however, that they have
been used in the sensing arena.
The figure below shows a typical transmission spectrum of
a TFBG. Each dip in the spectrum corresponds to light that
has been removed from the single-mode core and coupled to
one or several backward propagating cladding modes. In most
instances, the back propagating modes do not return all the
way to the fiber input because of the large attenuation of jacketed fibers, bends and so on (similar to the forward-coupled
cladding modes of LPGs).
00
- 5 – 5
Transmission (dB) Transmission [dBm]
- 10 – 10
- 15
– 15
- 20
– 20
Neff = 1.30
Neff = 1. 36
TFBG transmission spectrum with labels indicating the effective indices of selected modes.
1470 1480 1490 1500 1510 1520 1530 1540 1550 1560 1570
- 25
Neff = 1.447 Neff = 1. 40
Wavelength (nm)
10 degree tilt
Wavelength [nm]
– 25
The figure on the left shows how core guided light is extracted from the core by the grating and excites different cladding modes, selected according to wavelength (color) and polarization. Since the cladding modes each have a unique mode field shape and effective index (angle of incidence at the clad- ding boundary, as shown), they react differently to perturbations inside and outside of the fiber. A transmission spectrum such as the one shown in the
figure thus contains an incredible wealth of information that
begs to be “read,” provided that the transducing mechanisms
are well understood. In 2002, two independent research
groups (Ferdinand et al. in France and Lee et al. in Korea)
first recognized that the strong cladding mode resonances in
the transmission spectrum of TFBGs could be used to sense
refractive indices and bending, respectively.
Phil Saunders
Thermometers that are also
insensitive to temperature
Standard FBGs can sense two things very well—temperature
and strain—but only those two things. For anything else, a
transducing mechanism must be provided to transform the
required measurand into a change in temperature or strain.
This added mechanism most often involves packaging the FBG
into a structure that itself must respond to the perturbation
by straining or heating the fiber. This is not easy to do for biochemical sensing applications. Furthermore, the FBG responds
the same way to temperature and strain (a wavelength shift of
a narrow resonance) so it is normally impossible to distinguish
between the two perturbations with a single device.
The TFBG circumvents these two problems, thanks to the
simultaneous presence of a large number of resonances from a
single device. Since all resonances have the same temperature
dependence (roughly 10 pm/°C or one part in 105), the temperature sensitivity of any other sensing modality can be factored out
by using relative wavelengths instead of absolute ones. On the
other hand, the strain dependence of high order cladding modes
is quite different from that of the core mode. Therefore, a temperature desensitized axial strain sensor is obtained from a single
TFBG. Of course, it is also possible to use the temperature
sensitivity of any one of these modes as an in situ thermometer
and use other resonances to sense more parameters.
Bending and vibration
Each cladding mode “samples” the fiber cladding cross-section
and the medium adjacent to it differently: For instance, the
