Highly sensitive fiber Bragg grating refractive index sensors
Wei Liang, Yanyi Huang,
a兲
Yong Xu, Reginald K. Lee, and Amnon Yariv
Department of Applied Physics, California Institute of Technology, Pasadena, California 91125
共Received 31 January 2005; accepted 16 March 2005; published online 8 April 2005兲
We combine fiber Bragg grating 共FBG兲 technology with a wet chemical etch-erosion procedure and
demonstrate two types of refractive index sensors using single-mode optical fibers. The first index
sensor device is an etch-eroded single FBG with a radius of 3
m, which is used to measure the
indices of four different liquids. The second index sensor device is an etch-eroded fiber Fabry-Pérot
interferometer 共FFPI兲 with a radius of ⬃1.5
m and is used to measure the refractive indices of
isopropyl alcohol solutions of different concentrations. Due to its narrower resonance spectral
feature, the FFPI sensor has a higher sensitivity than the FBG sensor and can detect an index
variation of 1.4⫻10
−5
. Since we can measure the reflection signal, these two types of sensors can
be fabricated at the end of a fiber and used as point sensors. © 2005 American Institute of Physics.
关DOI: 10.1063/1.1904716兴
Since the early 1990s, fiber Bragg grating 共FBG兲 sensors
have been intensively developed due to their many desirable
advantages such as the small size, absolute measurement ca-
pability, immunity to electromagnetic interference, wave-
length multiplexing, and distributed sensing possibilities.
1–5
Thus far, the FBG sensors’ capability to measure physical
quantities such as the temperature, strain, pressure, etc., has
been studied extensively.
2–8
However, the use of FBG sen-
sors for detection of environmental refractive index change
has not been fully explored. Refractive index sensing is im-
portant for biological and chemical applications since a num-
ber of substances can be detected through measurements of
the refractive index.
2–4,8–13
For normal FBGs, removal of the
fiber cladding is required to increase the evanescent field
interaction with the surrounding environment. This concept
has been demonstrated using D-shaped fiber and side-
polished fiber.
9,11,12
In both cases, the strength and durability
of the sensor were greatly reduced. Special fiber was also
needed, which would raise the costs and limit the possible
applications. Long-period fiber gratings have also been dem-
onstrated to have high sensitivity to the refractive index of
the ambient media,
2,3,13–15
however, their multiple resonance
peaks and broad 共typically tens of nanometers兲 transmission
resonance features limit the measurement accuracy and their
multiplexing capabilities.
9
In addition, the relatively long
length of the grating limits their application as point sensor
devices.
In this letter, we first demonstrate a single etch-eroded
FBG sensor using standard single-mode telecommunication
fiber 共Corning SMF-28兲. Fiber Fabry-Pérot interferometers
共FFPIs兲 have also been widely used as sensors.
2,3,16,17
Com-
pared to a single FBG, the FFPI sensors possess narrower
resonance peaks and are more desirable for high accuracy
wavelength measurement.
2,3,9,18
To that end we propose and
demonstrate an etch-eroded FFPI sensor formed by two
FBGs. The FFPI sensor is used to measure the refractive
index of isopropyl alcohol 共IPA兲 solutions of different con-
centrations, exhibiting the capability of detecting very small
index variations of the ambient medium.
Fig. 1共a兲 shows the schematic of the FBG sensor and the
FFPI sensor. For a FBG sensor, the center wavelength of the
reflection band,
B
, is given by
2
B
=2·n
eff
⌳, 共1兲
where ⌳ is the period of the Bragg grating determined by the
phase mask, and n
eff
is the effective modal refractive index.
The index of the ambient material, n
ambient
, can be deter-
mined from the Bragg wavelength
B
, given the core index
n
co
and the radius a of the etched fiber.
19
For the etch-eroded FFPI formed by two FBGs, we as-
sume that the effective cavity length L
FP
is a constant, and
that the wavelength of a resonance mode,
R
, must satisfy
19
m
R
/2 = n
eff
L
FP
, 共2兲
where m is the modal number. When n
ambient
is changed, the
effective index is shifted by ⌬n
eff
and the resonance wave-
a兲
Author to whom correspondence should be addressed; electronic mail:
yanyi@caltech.edu
FIG. 1. 共a兲 Schematic of a single fiber Bragg grating 共FBG兲 sensor and a
fiber Fabry-Pérot interferometer 共FFPI兲 sensor. 共b兲 Scanning electron micro-
scope 共SEM兲 image of an etch-eroded fiber.
APPLIED PHYSICS LETTERS 86, 151122 共2005兲
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length of the mth mode is shifted by ⌬
R
. From Eq. 共2兲,we
can derive
⌬
R
/
R
=
␣
· ⌬n
eff
/n
eff
, 共3兲
where
␣
is the fraction of the optical length of the FFPI that
is etch eroded 关see Fig. 1共a兲兴.
In our experiment, the FBG is written in a hydrogen-
loaded Corning SMF-28 fiber using a KrF excimer laser 共248
nm兲 and an amplitude phase mask 共⌳ =524.58 nm兲. The
length of the FBG is 2.5 mm. The wet-chemical etching
setup is similar to the one shown in Ref. 20. We first etch the
fiber with a 52% hydrofluoric 共HF兲 acid solution for 30 min
to a fiber diameter below 15
m. Since the etching speed of
the Ge-doped core is faster than that of the silica cladding,
21
the etching solution is then replaced with a 13% HF solution
to slow down the etching process. The diameter of the final
etch-eroded fiber is controlled by monitoring the transmis-
sion loss in situ.
20
Figure 1共b兲 shows a scanning electron
microscope 共SEM兲 image of an etch-eroded fiber, which has
a very uniform thickness.
We then put the FBG sensor in different liquids and mea-
sure the reflection spectra by scanning the wavelength using
a tunable laser 共Agilent 8164A兲. The reflection spectra for
the FBG sensor in air 共n=1.00兲, methanol 共n=1.326兲, etha-
nol 共n=1.359兲, and isopropyl alcohol 共IPA兲共n=1.378兲 are
shown in Fig. 2共a兲. As expected, the reflection spectrum red-
shifts as the ambient refractive index increases. The Bragg
wavelength as a function of the ambient refractive index is
shown in Fig. 2共b兲 and fit to Eq. 共1兲 with ⌳ 共524.58 nm兲, n
co
共1.454兲, n
ambient
and an appropriate choice of the fiber radius
a. The effective modal index n
eff
in Eq. 共1兲 is obtained by
numerically solving the HE
11
modal equation of a step-index
cylindrical waveguide.
19
The fitting parameter a=2.975
m
agrees well with the measured value of 3.0
m as shown in
Fig. 1共b兲.
The FFPI sensor is formed by two FBGs written on a
hydrogen-loaded Corning SMF-28 fiber. The distance be-
tween the two FBGs is chosen to be ⬃5 mm so that there are
less than three resonance modes in the band gap of the FBG.
To eliminate the influence of the ambient media on the FBG
band gap in the measurements, we mask the fiber to selec-
tively etch the region between the two gratings without af-
fecting the gratings.
Compared to the FBG sensor, the FFPI sensor should
have a higher refractive index resolution due to its sharper
resonance feature. To demonstrate this, we use it to detect the
variation of the refractive indices of IPA-water solutions with
different IPA concentrations, which allows arbitrarily small
changes of index. Figure 3共a兲 shows the redshift of a reso-
nance mode due to the increase of the concentration of the
IPA-water solution. In Fig. 3共b兲 we plot the resonance wave-
length as a function of the mole ratio of the IPA in solution.
The refractive indices of the IPA solution shown in Fig. 3共b兲
are estimated based on the mole ratio of each component,
i.e., n
ambient
=关
·n
IPA
2
+共1−
兲·n
water
2
兴
1/2
,
22
where n
water
=1.333 is the refractive index of water, n
IPA
is the refractive
index of pure IPA, and
is the mole ratio of IPA in the
solution. The effective modal index n
eff
can then be calcu-
lated based on n
ambient
, n
co
共1.454兲, and the fiber radius a.
19
We fit the resonance wavelengths to Eq. 共3兲 as shown in Fig.
3共b兲, where the parameters are
␣
=0.9 共based on the sensor
geometry兲,
R
=1519.868 nm, n
eff
=1.42021 共based on the
FIG. 2. 共a兲共Color online兲 Reflection spectra of an etch-eroded single fiber
Bragg grating 共FBG兲 sensor immersed in different liquids. 共b兲 The Bragg
center wavelength as a function of the refractive index of ambient material.
The markers are Bragg center wavelengths extracted from 共a兲, the fitting
curve is calculated using Eq. 共1兲 with parameters n
co
=1.454 and a
=2.975
m.
FIG. 3. 共a兲共Color online兲 Reflection spectra of the resonance mode of a
fiber Fabry-Pérot interferometer 共FFPI兲 sensor in isopropyl alcohol 共IPA兲
solution with different concentrations. 共b兲 The resonance wavelength as a
function of the mole ratio of IPA solution. The circles are resonance wave-
lengths extracted from 共a兲, the numerical fitting is obtained using Eq. 共3兲
with parameters n
co
=1.454, a=1.475
m and
␣
=0.9.
151122-2 Liang
et al.
Appl. Phys. Lett. 86, 151122 共2005兲
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first experimental point兲, and a=1.475
m. The fiber radius
used in the fit agrees well with the measured radius of
1.5
m. Using the slope of the fitted curve, we can define the
sensor responsivity as ⌬/⌬n=71.2 nm. This number is an
order of magnitude higher than the one demonstrated in Ref.
9. The sensitivity of the sensor is limited by the stability of
the whole measuring system. In our experiment, the fluctua-
tion of the resonance wavelength is around 1 pm, which
indicates the sensor has the potential to detect an index varia-
tion of 1.4⫻10
−5
. It is also interesting to compare the per-
formance of the FFPI sensor with that of the FBG sensor. As
can be measured from Figs. 2共a兲 and Fig. 3共a兲, the resonance
spectra of the FFPI sensor have an approximate full width 共at
half extinction兲 of 0.025 nm as compared to 0.5 nm 共full
width at half maximum兲 for the FBG sensor. This means the
resolution of the FFPI sensor is enhanced by 20 times over
that of the FBG sensor.
Beside the high sensitivity, these etch-eroded devices
have some additional advantages. The maximum tensile
stress in a bent fiber of radius a is at the surface of the fiber
and is given by
23
⬀ E · a/R, 共4兲
where E is the Young’s modulus of the fiber and R is the
radius of the curvature. Thus, with the same bending curva-
ture, the induced tensile stress in the fiber decreases with the
reduction of the fiber radius. Since the radius of the etch-
eroded fiber sensor is on the order of 1
m, it has a higher
flexibility compared to a fiber sensor with a larger diameter,
and can be bent or coiled without being broken.
23,24
This
characteristic ensures the durability of the sensor devices. In
addition, since both the FBG and the FFPI sensors can be
used in reflection mode, they can be fabricated at the end of
a fiber and used as point detectors. Finally, by coating the
etch-eroded fiber surface with a thin layer of binding re-
agents, these sensors can be functionalized to detect specific
biological and/or chemical agents with high sensitivity.
In conclusion, we have combined FBG and wet-
chemical silica etching techniques to demonstrate a FBG re-
fractive index sensor and a FFPI refractive index sensor in
standard SMF-28 optical fiber. The FFPI sensor has a respon-
sivity of ⌬/⌬n=71.2 nm and displays the capability of de-
tecting an index variation of 1.4⫻ 10
−5
. These highly flexible
sensors add the function of refractive index sensing to the
established repertoire of FBG sensing techniques, and can be
applied to many fields, including biochemical sensing and
environmental monitoring.
The authors thank Will Green, John M. Choi and Joyce
Poon for helpful discussions. This work is supported by the
National Science Foundation 共DMR-0120967兲 and the De-
fense Advanced Research Projects Agency 共N00014-04-1-
0094兲.
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