1712IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 23, NO. 22, NOVEMBER 15, 2011
Miniaturized Metal-Dielectric-Hybrid Fiber Tip
Grating for Refractive Index Sensing
Jun-Long Kou, Sun-Jie Qiu, Fei Xu, Yan-Qing Lu, Ye Yuan, and Gang Zhao
Abstract—We fabricate a miniaturized metal-dielectric-hybrid
fiber tip grating (FTG) using a focused ion beam (FIB) method
with high accuracy for refractive index sensing applications. It is
the smallest fiber grating by now (nearly 10
at the fiber tip of 3
m in radius) and it has strong surface cor-
rugations with notches periodically. The grating shows refractive
(nearly zero) properties for reflection channels of different reso-
a multichannel sensor for simultaneous refractive index and tem-
perature/pressure measurements. Taking advantage of its flexible
design, tiny size, unique modal and spectral characteristics, the
metal-dielectric-hybrid FTG has great potential in fast-response
detection of ultrasmall objects.
m in length located
Index Terms—Grating, microfiber, sensor.
tiplexing/demultiplexing capability along a single fiber .
Fiber grating sensors have found a broad range of applications
and have been widely extolled in the research literatures on
refractive index, temperature, pressure measurements, and
so on –. Conventional fiber gratings are fabricated by
phase-mask-assisted ultraviolet (UV) light writing to generate
the periodic refractive index modulation in a typical silica
fiber. However, they have the limitations of weak available
evanescent field, small index modulation , and large size.
The evanescent field interacting with the outer environment is
important for refractive index sensing. Several approaches have
thus been proposed, e.g., polishing the fiber down to the core in
the grating region , etching the fiber to several micrometers
in diameter , , or employing D-shaped optical fiber .
However, the size is still limited by the grating length and
fiber diameter. Minimizing the size of a sensor head is a key
issue in some applications with special analyte, for example,
submicro-size bubbles, particles or droplets detection in ocean,
air or bio/chemical solutions. Small size usually also means fast
response time. A tapered fiber tip with ultrashort length (tens
HE advantages of optical fiber grating sensors are well
known as low cost, inherent self-referencing, and mul-
Manuscript received April 04, 2011; revised July 21, 2011; accepted August
17, 2011. Date of publication October 03, 2011; date of current version October
26, 2011. This work was supported by the National 973 Program under Con-
tract 2010CB327803 and Contract 2011CBA00205, and by the NSFC program
11074117 and 60977039.
The authors are with the College of Engineering and Applied Sciences and
National Laboratory of Solid State Microstructures, Nanjing University, Nan-
jing 210093, China (e-mail: firstname.lastname@example.org; email@example.com).
Color versions of one or more of the figures in this letter are available online
Digital Object Identifier 10.1109/LPT.2011.2166151
of micrometers) and ultrasmall head diameter (sub-micrometer
or tens of nanometers) is a perfect platform for ultracompact
fiber devices, and has been widely used in scanning near field
optical microscopy, subwavelength optical source, and sensing
With the recent advances in micro-machining techniques,
tremendous opportunities are shown to develop various
micro-devices in thin and small fiber nano-tip, such as
notch-like Fabry–Pérot cavity ,  and nano-scale optical
source through focused ion beam (FIB) drilling . In this
work, we fabricate a miniaturized metal-dielectric-hybrid fiber
tip grating (FTG) by FIB for refractive index sensing appli-
cations. It is the smallest fiber grating at present according
to our knowledge (nearly 10
fiber tip of 3
m in radius) and it has strong surface corru-
gations with notches (650 nm in depth) periodically. In the
metal-dielectric-hybrid waveguide exciting multiple modes, the
forward-propagating light can be backwardly coupled to sev-
eral modes simultaneously. Some modes have large evanescent
field and high sensitivity to outer environment while some have
small evanescent field and are insensitive to outer environment.
This kind of grating showing simultaneous refractive index
sensitive and insensitive properties for reflection channels of
different resonant modes can be used as a multichannel sensor
capable of simultaneous refractive index and temperature/pres-
sure measurements. The metal-dielectric-hybrid FTG presents
great potential in fast-response submicro-size object detection
due to the intriguing properties of flexible design, ultrasmall
size, unique modal and spectral characteristics.
m in length located at the
II. FABRICATION OF FTG
mercially available pipette puller (Sutter, model P2000). CO
laser power and pulling velocity are optimized to achieve an
optimum taper profile. The manufactured fiber tip has a long
pigtail to allow prompt link to other optical fiber component.
The fiber tip is
mm in length and is nonadiabatic. Then the
fiber tip is coated with a gold layer with thickness of 30 nm on
one side by magnetron sputtering. We choose gold due to its
relatively low absorption in the infrared and inertness to oxida-
tion when exposed in air. Then a grating is fabricated by FIB
milling at the fiber tip with local radius of
is completed using a Ga-ion-based FEI-201 FIB system in a
one-step process. A 30 kV, 58 pA cylindrical-symmetric beam
with diameter less than 20 nm is utilized in order to achieve
good milling quality. No other treatment is needed to improve
the sharpness or accuracy . Fig. 1 shows the scanning elec-
tron microscopic (SEM) picture of the periodic structures. The
m. The milling
1041-1135/$26.00 © 2011 IEEE
KOU et al.: MINIATURIZED METAL-DIELECTRIC-HYBRID FTG FOR REFRACTIVE INDEX SENSING1713
Fig. 1. SEM picture of the metal-dielectric-hybrid fiber tip grating ( 10
in length and
nm). Right: magnified picture of the grating.
Fig. 2. Measured reflection spectra of the FTG when immersed in air, acetone,
grating has shallow corrugations of period
17 periods. The total length is about 10 m, which is extremely
III. EXPERIMENTS AND DISCUSSION
Optical characterization of the FTG in Fig. 1 is performed
using an Ando AQ6317B optical spectrum analyzer (OSA) ac-
companied by an amplified spontaneous emission (ASE) source
(1525–1610 nm) and a fiber-optic circulator. We do not use po-
larized light tocharacterizethesensorbecausethebirefringence
of the grating is small according to our calculation. The experi-
mental setup is the same as shown in .
Fig. 2 shows the reflection spectra of the FTG immersed in
air, acetone, and isopropanol, respectively. The spectrum is
measured with reference to the light source directly because the
taper without grating indicates a reflection of
is nearly negligible , . The extinction ratio is about
10 dB. There are several valleys and peaks with different
characteristics in the spectral range of
when the outer environment changes from acetone to iso-
propanol. However, these valleys and peaks show larger shifts
at longer wavelengths, while those at shorter wavelength region
shift much less and almost stop at specific wavelengths. This
unique response to outer liquid refractive index comes from the
fact that the reflected light can be coupled to different modes.
In the micrometer-diameter metal-dielectric-hybrid fiber tip,
several modes are probably excited with similar propagation
constant because of the metal cladding , . Some modes
are well confined in the tip and have negligible field overlap
with the liquid while some modes are not. The different valleys
and peaks correspond to the coupling between these different
forward and backward propagating modes, with different
nm. They shift
Fig. 3. Calculated effective index of the surface guided mode and bound mode
as a function of the outer liquid refractive index
is assumed to be 3
m with a golden coating of 20 nm in thickness.
. The radius of the fiber tip
response properties for the outer environment. There is some
chirp because of the nonuniform taper. We have measured that
the diameter difference is less than 1 m in the grating region as
illustrated in Fig. 1. And according to our calculation, different
effective refractive index resulted from the variation of the
diameter will induce resonant wavelength shifts of
some small ripples in Fig. 2 probably come from the chirp.
The reflection resonant condition for the grating is :
backward modes, respectively.
that changes little with respect to the operating wavelength and
is proportional to the grating strength. At the same time, the
grating length and the grating strength significantly influence
the reflectivity. Because the period
coupled modes, the reflected mode with larger effective index
has a longer resonant wavelength from (1), provided that
a constant and is smaller compared with the reciprocal vector
provided by the grating.
For simplicity, we assume a theoretical model to explain our
experimental results which is simple and not perfectly matched
the device. Within the model of hybrid metal-cladding dielec-
tric multimode waveguide, the microfiber is 6
with uniform metal cladding (20nm inthickness).However,the
real device is much more complicated, with nonuniform metal
cladding and diameter. And if an asymmetrical mode field lies
mainly near the grating, leading to a larger modal overlap with
the grating, it may result in a higher sensitivity. Fig. 3 shows
function of outer liquid refractive index
index (corresponding to long resonant wavelength) than that of
the bound mode (corresponding to short resonant wavelength)
and has a larger overlap with the taper surface and the outside
environment, leading to a higher sensitivity to the surrounding
medium which is in coincidence with the spectra of Fig. 2.
The performance of resonant refractive index sensors can
be evaluated by using sensitivity
magnitude in shift of the resonant wavelength divided by the
and are the effective indices of the forward and
is the coupling coefficient
is the same for all these
m in diameter
. Due to the existence
, which is defined as the
1714IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 23, NO. 22, NOVEMBER 15, 2011
asterisks represent the experimental results with the solid line of linear fitting.
the sensitivity is measured by inserting the sensor in a beaker
containing mixtures of isopropanol and acetone, where the iso-
propanol component has the following ratios: 0, 1/7, 2/7, 3/7,
4/7 5/7, 6/7, and 1. These solutions are chosen with the objec-
tive of simulating aqueous solutions, having a refractive index
in the region around 1.33 at a wavelength
ratio is increased by adding small calibrated quantities of iso-
propanol to the solution at a position far from the sensor. The
refractive indices of pure isopropanol and acetone at 1.5 m are
1.3739 and 1.3577, respectively .
Fig. 4 displays measured resonant wavelength shifts of sev-
eral peaks and valleys and fitting of this FTG on the liquid re-
fractive index (a, b, c, d as marked in Fig. 2, (a) and (c) are
peaks,(b) and (d) arevalleys).Asthe refractive index increases,
the resonant wavelength shifts to longer wavelength. The sen-
sitivities of different modes change severely. It can be as high
as 125 nm/RIU (peak a, RIU is the acronym of refractive index
unit) or as low as 7 nm/RIU (valley (d)). For peak (a) (or valley
culation, we believe peak (a) (or valley (b)) corresponds to the
surface guided mode while peak (c) (or valley (d)) is the bound
zero by optimizing the tip grating profile and metal coating. Be-
cause of many different properties on the outer liquid refrac-
tive index, the metal-dielectric-hybrid FTG can be applied as a
multiparameter sensor and the index-insensitive channel can be
used to simultaneously measure temperature, pressure, and so
on. In our experiment, we didn’t measure the temperature sen-
sitivityandthemeasurementwastaken underroom temperature
with variation of less than 0.1 C. Conventionally, the tempera-
ture sensitivity of the microfiber grating is 10–20 pm/ C .
In conclusion, we demonstrate a miniaturized metal-dielec-
tric-hybrid fiber tip grating milled by FIB for refractive index
sensing applications. It might be the smallest fiber grating by
now. This kind of grating shows refractive index sensitive
(125 nm/RIU) and insensitive (nearly zero) properties for
different resonant modes from the metal-dielectric-hybrid
cladding structure. It can be used as a multichannel sensor with
simultaneous refractive index and temperature/pressure mea-
surements. Taking benefit of the flexible design, ultrasmall size,
unique modal and spectral characteristics, the metal-dielec-
tric-hybrid FTG has great potentials in fast-response ultrasmall
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