COL 12(2), 020603(2014) CHINESE OPTICS LETTERS February 10, 2014
DMSO-based photonic crystal fiber sensor with
enhanced sensitivity
Ali H. Abdulhadi
1
, Sun-jie Qiu
2
, and A. Hadi Al-Janabi
1∗
1
Institute of Laser for Postgraduate Studies, University of Baghdad, Iraq
2
College of Engineering and Applied Sciences, Nanjing University, Nanjing 210093, China
∗
Corresponding author: hadi.janabi@ilps.uobaghdad.edu.iq
Received August 12, 2013; accepted December 19, 2013; posted online January 27, 2014
We construct a dimethylsulfoxide (DMSO)-based photonic crystal fiber (PCF) temperature sensor with
enhanced sensitivity. A solid-core PCF with large mode area is employed to supply the in-line Mach–
Zehnder interference between the fundamental and cladding modes. Thus, temperature sensing can be
realized because of the shift of interference spectrum at different temperatures.The DMSO solvent is
infiltrated between the main sensor and a silica tube to increase the temperature sensitivity of the sensor.
The obtained sensitivity (0.315 nm/
◦
C) is one or two orders of magnitude higher than th at of prev iously
published results. The proposed sensor is adapted for high-temperature sensing.
OCIS codes: 060.0060, 060.2370, 060.5295, 060.2430.
doi: 10.3788/COL201412.020603.
In-line fiber modal interferometer is widely used in var-
ious sensing applications. Among them, temper ature
sensing by using a conventional optical fiber is a very
promising example
[1−3]
. Photonic crystal fiber (P C F)-
based in-line modal interferometer, which has unique
guiding mechanisms and modal properties, has shown
many advantages in these a pplications
[4,5]
. Different
PCF sensors based on modal interferometers are de-
signed and constructed to detect temperature, strain,
refractive index, and others
[6,7]
. High-sensitivity temper-
ature sensors based on PCF are still difficult to achieve
because of the low thermo-optic and thermal expansion
coefficient of fused silica (PCF material)
[8]
. A typical
PCF-based modal interferometer is obtained by sand-
wiching a PCF between two single mode fibe rs (SMFs).
Although different fabrication techniques, such as long
period grating, micro hole-collapsing, and tapering
[9−11]
,
have been attempted, these structures normally provide
low temperature sensitivity. One way to increase the
PCF temperature sensitivity is to infiltrate liquids with
high thermo-optic and thermal expansion coefficients
into the holes of PCF
[12]
. However, the infiltration of
liquids inside the holes of PCF remains a challenge be-
cause of splicing difficulty and evaporation effects of
solvents during splicing . Nevertheless, lo ng lengths of
exp ensive p olarization maintaining PCF have been used
to enhance tempe rature sensitivity
[13]
. To the best of
our knowledge, a ll liquids that infiltrate PCF tempera-
ture sensor are limited to temperatures b e low 100
◦
C.
Recently, Zhang et al.
[14]
proposed a Fabry–Perot in-
terferometer by using polarization-maintaining photonic
crystal fiber for high temperature sensing with max-
imum obtained sensitivity of 13.8 pm/
◦
C. Therefore,
developing a cost effective PCF-based sensor with oper-
ation temperatures over 100
◦
C with high sensitivity is
of great importance.
In this letter, we construct a PCF temperature sen-
sor base d on external infiltration of high thermo-optic
coefficient solvent, that is, dimethylsulfoxide (DMSO).
DMSO is a safe solvent for both synthesis and formula-
tion applications.This solvent has been used in medical
and chemical applica tio ns that could be found in several
nuclear magnetic resonance spec troscopes and micro-
electronic devices
[15]
. High boiling point (189
◦
C), low
evaporation effects at low temperature range, and low
toxicity contribute to the use of the DMSO in the exper-
iment. Thus, a new PCF sensor with high temperature
sensitivity is expected. This compact, rigid, responsive,
and highly sensitive device can be incorporated in alarm
systems and can withstand harsh environments.
In our exp e riment, a 2.3-cm-long large mode area
(LMA-8, NKT photonics, Denmark) PCF is used. The
PCF has an 8.4-µm core diameter and voids with an
average diameter of 2.17 µm and an average separa-
tion of 5.3 µm between voids (see Fig. 1(a)). This
kind of fiber has been used to build the in-fiber modal
interferometer
[11,12]
. The process to build the interfer-
ometer in the PCF by using a fusion splicer is simple.
A manual mode of a standard fusion splicer has been
selected because no special fusion splicer is available.
Optimal parameters for splicing have been chosen man-
ually. The separation of fusion splicer arc electrode is
controlled through the screen display of the fusion splicer.
The display indicates the exact distance. Thus, fusion
splicing can be carried out reliably by adjusting the elec-
trode tip. In our experiment, LMA-8 PCF is s pliced
in both sides with two conventional single mode fib e rs
(SMF-28, Corning, USA). The modal interference com-
prises a sandwich configuration of SMF–PCF–SMF. The
similar diameter of LMA-8 and SMF-28 makes the light
easier to be coupled in the splicing point
[16]
. To excite
the fundamental and cladding modes, a full collapse of
PCF air holes is achieved during fusion splicing. The
length of the collapsed region is fully controlled by ad-
justing the fusion splicer power and the fusion time. The
collapse of P CF holes creates the interferometer, and the
length of PCF affects the fringe visibility of the sensor.
The collapsed leng th of PCF was ∼34 0 µm. Fig ure 1(b)
shows the microscopic image of the PCF–SMF splicing
area.
1671-7694/2014/020603(4) 020603-1
c
2014 Chinese Optics Letters