Slightly Tapered Optical Fiber with Inner Air-cavity for
Simultaneous Refractive Index and Temperature Sensing
D.N.Wang
1
, Lei Zhang
2
, Jibing Liu
3
and H. F. Chen
1
1
College of Optical and Electronic Technology, China Jiliang University, Hangzhou, China
2
Department of Electrical Engineering, Hubei Polytechnic University, Huangshi, China
3
College of Physics and Electronic Science, Hubei Normal University, Huangshi, China
Keywords: Optical Fiber Sensors, Interferometry, Microstructure Fabrication.
Abstract: A fiber in-line Mach-Zehnder interferometer based on inner air-cavities for simultaneous refractive index
and temperature sensing is presented. The inner air-cavities are fabricated by use of femtosecond laser
micromachining, fusion splicing and slightly tapering techniques. The transmission spectrum of the
interferometer exhibits a number of resonance wavelength dips corresponding to different orders of cladding
modes. By tracking the shift of two dip wavelengths, the changes of refractive index and temperature can be
obtained by use of a matrix method. The refractive index and temperature sensitivity achieved are 103.00
nm/RIU (refractive index unit) and 73.07 pm/Ԩ, respectively.
1 INTRODUCTION
A simultaneous refractive index (RI) and
temperature sensing is of great importance for many
applications in chemical industry, environmental
monitoring and biological sensing. One of the
techniques is to use optical fiber sensors because of
their convenient operation and many advantages
provided by optical fibers. A wide range of optical
fiber sensor configurations have been proposed, such
as the use of slanted multimode fiber Bragg grating
(FBG) (Zhao et al.), sampled FBG (Shu et al.),
birefringent FBG (Frazão et al.) cascaded long
period gratings (Zhang et al.), hybrid gratings (Chen
et al.), and different types of optical fiber
interferometers with hybrid structures (Kim et al.;
Lu et al.; Choi et al.; Li et al.; X. Chen et al.; Liao et
al.; Xiong et al.; Yao et al.; Meng et al.). The above
mentioned systems commonly are complex in
design, difficult in fabrication and of high cost.
Moreover, the sensor heads are large in size, which
makes it difficult to precisely determine the sensing
location.
Here we demonstrate a fiber in-line MZI based
on inner air-cavities for simultaneous RI and
temperature measurement. The proposed sensor is
formed by creating an inner air-cavity by use of
femtosecond (fs) laser micromachining together with
fusion splicing technique and followed by a slightly
tapering process (F. Chen et al.) The RI and
temperature can be simultaneously determined by
use of standard matrix inversion method. The
sensitivities achieved are 103.00 nm/RIU (refractive
index unit) and 73.07 pm/°C, respectively. The
device is robust, easy in operation and has high
sensitivity.
2 DEVICE FABRICATION AND
OPERATION PRINCIPLE
2.1 Device Fabrication
Fig. 1 shows the microscope image of the device
sensor head, fabricated by creating an inner-cavity
by fs laser micromachining together with fusion
splicing technique and then followed by a slightly
tapering process. The length and width of the air-
cavity inside the single mode fiber (SMF) are ~62
and ~80 μm, respectively. During the device
fabrication process, firstly, the fs laser pulses (800
nm) with pulse width of ~120 fs and energy of ~3 μJ
at the repetition rate of 1 kHz were focused onto a
cleaved single mode fiber (SMF) by a 20× objective
lens with a numerical aperture (NA) value of 0.50.
The SMF was mounted on a computer controlled X-
48
Wang, D., Zhang, L., Liu, J. and Chen, H.
Slightly Tapered Optical Fiber with Inner Air-cavity for Simultaneous Refractive Index and Temperature Sensing.
DOI: 10.5220/0005964900480052
In Proceedings of the 13th International Joint Conference on e-Business and Telecommunications (ICETE 2016) - Volume 3: OPTICS, pages 48-52
ISBN: 978-989-758-196-0
Copyright
c
 2016 by SCITEPRESS – Science and Technology Publications, Lda. All rights reserved

Y-Z translation stage with a 40 nm resolution. A
micro-hole with a diameter of ~12 μm and depth of
~25 μm was created at the center of the cleaved fiber
end facet. Secondly, a cleaved SMF without a
micro-hole was used to fusion splice the fiber tip
with a micro-hole. An inner air-cavity was created
due to the extremely high heating temperature. The
fusion splicer used was ERICSSON FSU975, and
the fusing current and fusing duration employed
were 16.2 mA and 2.0 s, respectively. Lastly, the
SMF with an inner air-cavity was slightly tapered by
using flame brushing technique to effectively excite
cladding mode in the optical fiber.
2.2 Principle of Operation
The schematic diagram of the sensor head is
illustrated in Fig. 2. The light launched into the inner
air-cavity is split into two parts: one passes through
the air-cavity and the other travels along the fiber
cladding. Finally, the interference is formed since
both of the two beams are recombined at the end of
air-cavity. The output of the MZI is given by
2
2cos( )
ac ac
Ln
II I II




(1)
where
a
I
and
c
I
represent the intensity of the light
passing through the air-cavity and traveling along
the fiber cladding, respectively,

is the
wavelength,
L is the cavity length, and
ca
nn n 
is the effective RI difference between the air-cavity
mode and the cladding mode. When the phase term
satisfies the condition:
2/(21)Ln m

  ,
where
m
is an integer, the intensity dip appears at
the wavelength
2
21
dip
Ln
m




(2)
The temperature sensitivity at
dip

can be written
as
2
[()]
21
dip
ca
nn
L
nL
Tm T TT
 




(3)
where T is the temperature. Assuming the thermo-
expansion coefficient is

and thermo-optical
coefficient is

, where
/
L
TL


and
/.
cc
nTn

 Since the effective RI of air-cavity is
nearly a constant,
/0
a
nT. Eq. (2) can be
simplified as
2
[]
21
dip
c
L
nn
Tm






(4)
The temperature sensitivity is mainly dependent on
the second term in Eq. (4) as the thermal-optical
coefficient

is much larger than the thermo-
expansion coefficient

in silica. For different
transmission modes in cladding, since the effective
RI and effective thermo-optical coefficient vary with
the mode order, the temperature sensitivities of
different mode orders are different.
Similarly, we can derive the dip wave shift due
to the change of RI of environment as
2
[]
21
dip
ca
nn
L
m








 
(5)
where

is the applied RI at the sensor device.
The transmission spectra of the device in air and
water respectively, are shown in Fig. 3, where four
dips appear. The two dips located at 1481.23 nm and
1580.63 nm in the water are selected for
simultaneously RI and temperature sensing. We
employ a matrix to present the dip wavelength shift
corresponding to the variation of RI of environment
and of temperature,
111
222
jk
T
jk





 


 



 
(6)
where
1


and
2


are the wavelength shifts,
1
j and
2
j represent the temperature sensitivities, and
1
k
and
2
k are the RI sensitivities of dip 1 and dip 2,
respectively.
T

and


denote the variation of
temperature and RI applied onto the sensor,
respectively.
3 EXPERIMENTAL RESULTS
AND DISCUSSIONS
To test the proposed device, the experimental set-up
as shown in Fig. 4 was used. The sensor head was
mounted between two fixed stages. The temperature
was controlled by a column oven with an accuracy
of ±0.1. To test the system response to the RI
change, the sensor head was immersed into a series
of RI liquids (from Cargille Laboratories), and after
each measurement, the fiber sensor head was rinsed
with methanol carefully until no residue liquid was
left on the sensor head surface and the original
spectrum could be restored. The fiber device was
Slightly Tapered Optical Fiber with Inner Air-cavity for Simultaneous Refractive Index and Temperature Sensing
49

connected to a broadband source (BBS) and an
optical spectrum analyzer (OSA: YOKOGAWA
6390) with 0.01 nm resolution. When the RI and
temperature were changed, the wavelength shifts of
dip 1 and dip 2 were recorded by the OSA.
To test the thermal response of the two dips, the
device was heated from 25 to 65°C with a step of
5°C in the air. As shown in Fig. 5, both the dip
wavelengths increase with the elevated temperature.
After employing linear fit to the experimental data,
the temperature responses of the two dips were
depicted in the Fig. 6. The sensitivities obtained are
73.07 pm/°C and 67.07 pm/°C, respectively.
The RI sensitivities were calibrated at the room
temperature of ~23°C, and tested within the range
between 1.33 and 1.38. As shown in Fig. 7, both the
dip wavelengths decrease with the increase of RI.
After employing linear fit to the experimental data,
the RI responses of the two dips were depicted in
Fig. 8. The RI sensitivities
1
k and
2
k obtained are -
103.00 and -78.94 nm/RIU, respectively.
According to the separate temperature and RI
measurements, the matrix in Eq. (6) can be written
as
1
2
0.07307 103.00
0.06707 78.94
T













(7)
By employing the matrix inversion method, we
can derive the change of temperature and RI on the
fiber device as
1
2
69.2417 90.3458
0.0588 0.0641
T






 


 
 
 

(8)
where the units of
T and


are in °C and RIU,
respectively, and that of
1


and
2

 are in nm.
Then, we can obtain the temperature and RI
information simultaneously as
r
TT and
r

  , where
r
T and
r

are reference
parameters, such as the parameters of room
temperature and the RI of water.
In the experiments, since the temperature was
controlled by a column oven, the fluctuation is
around ±0.1°C. Due to the 0.01 nm resolution of the
OSA, the RI and temperature resolutions can be
estimated to be ~9.7
× 10
-5
RIU and ~0.14°C,
respectively. Our proposed sensor head
demonstrates high sensitivity in RI (-103.00
nm/RIU) and temperature (73.07 pm/°C). Moreover,
since the size of MZI cavity is 62 μm, the sensing
location can be precisely determined.
The inner air-cavity itself also forms a Fabry-
Perot (FP) cavity, the corresponding free spectral
range (FSR) should be ~19.4 nm, however,
according to Fig. 3, the FSR of the fringe pattern
obtained is ~64.4 nm. Obviously, the FP effect can
be ignored.
4 CONCLUSIONS
In conclusion, we have proposed and demonstrated a
fiber in-line MZI based on inner air-cavities for
simultaneous RI and temperature sensing. The
proposed optical fiber sensor is ultra compact in size,
simple in structure and precise in sensing location.
The achieved refractive index and temperature
sensitivities are -103.00 nm/RIU and 73.07 pm/°C,
respectively. Such a sensing device has high
potential in chemical, environmental and biological
sensing applications.
ACKNOWLEDGEMENTS
This work was supported in part by National Natural
Science Foundation of China (61377094 and
61290313).
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APPENDIX
Figure 1: Microscope image of the optical fiber sensor
head.
Figure 2: Schematic diagram of the sensor head.
1450 1500 1550 1600 1650
-45
-40
-35
-30
-25
-20
-15
-10
Wavelength (nm)
(a)
Transmission (dB)
1450 1500 1550 1600 1650
-55
-50
-45
-40
-35
-30
-25
-20
Wavelength (nm)
(b)
Transmission (dB)

2
=1580.63nm

1
=1481.23nm
dip 2
dip 1
Figure 3 (a): Transmission spectrum of the sensor in air.
(b) Transmission spectrum of the sensor in water.
Figure 4: Experimental set-up.
1460 1470 1480 1490 1500
-42
-40
-38
-36
-34
-32
-30
-28
-26
-24
-22
-20
Wavelength (nm)
Transmission (dB)
1550 1560 1570 1580 1590 1600
-28
-26
-24
-22
-20
-18
-16
-14
Wavelength (nm)
Transmission (dB)
25

C
30

C
35

C
40

C
45

C
50

C
55

C
60

C
65

C
25

C
30

C
35

C
40

C
45

C
50

C
55

C
60

C
65

C
Figure 5: Dip wavelength shift with temperature.
Slightly Tapered Optical Fiber with Inner Air-cavity for Simultaneous Refractive Index and Temperature Sensing
51

30 40 50 60
1484
1484.5
1485
1485.5
1486
1486.5
1487
1487.5
Temperature (

C )
Wavelength (nm)
30 40 50 60
1584.5
1585
1585.5
1586
1586.5
1587
1587.5
Temperature (

C )
Wavelength (nm)
Linear Fitting
Experimetanl Result
Linear Fitting
Experimetanl Result
dip 2
Slope = 0.0671
nm/

C
R
2
=0.9902
dip 1
Slope = 0.0731 nm/

C
R
2
=0.9917
Figure 6: Dip wavelength shift with temperature by linear
fitting.
1450 1460 1470 1480 1490 1500 1510
-55
-50
-45
-40
-35
-30
-25
Wavelength (nm)
Transimission (dB)
1550 1560 1570 1580 1590 1600 1610
-50
-45
-40
-35
-30
-25
-20
Wavelength (nm)
Transimission (dB)
1.33
1.34
1.35
1.36
1.37
1.38
1.33
1.34
1.35
1.36
1.37
1.38
Figure 7: Dip wavelength shift with RI.
1.34 1.36 1.38
1475
1476
1477
1478
1479
1480
1481
1482
Refractive Index
Wavelength (nm)
1.34 1.36 1.38
1576.5
1577
1577.5
1578
1578.5
1579
1579.5
1580
1580.5
1581
Refractive Index
Wavelength (nm)
Linear Fitting
Experimetanl Result
Linear Fitting
Experimetanl Result
dip 2
slope=-78.94 nm/RIU
R
2
=0.9911
dip 1
solpe=-103 nm/RIU
R
2
=0.9734
Figure 8: Dip wavelength shift with RI by linear fitting.
OPTICS 2016 - International Conference on Optical Communication Systems
52