This document describes research on optimizing the etching process for fabricating single mode fiber optic temperature sensors. It discusses how etching the fiber's cladding at a critical diameter of 11.2 μm produces the highest temperature sensitivity of 3.8 μW/°C. The sensor response is repeatable under temperature cycling and could be useful for transmitting analog signals over fiber. Maintaining a constant temperature during etching allows fabricating sensors more quickly while achieving a desired core-cladding diameter.

1. Ajay Kumar et al Int. Journal of Engineering Research and Applications
ISSN : 2248-9622, Vol. 4, Issue 2( Version 5), February 2014, pp.04-08
RESEARCH ARTICLE
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OPEN ACCESS
Optimize Etching Based Single Mode Fiber Optic Temperature
Sensor
Ajay Kumar*, Dr. Pramod Kumar**, Sachin Kumar**
*(Department of ECE, World College of Technology and Management, Gurgaon,)
** (Department of ECE, World College of Technology and Management, Gurgaon,)
***(Department of ECE, World College of Technology and Management, Gurgaon,)
ABSTRACT
This paper presents a description of etching process for fabrication single mode optical fiber sensors. The
process of fabrication demonstrates an optimized etching based method to fabricate single mode fiber (SMF)
optic sensors in specified constant time and temperature. We propose a single mode optical fiber based
temperature sensor, where the temperature sensing region is obtained by etching its cladding diameter over small
length to a critical value. It is observed that the light transmission through etched fiber at 1550 nm wavelength
optical source becomes highly temperature sensitive, compared to the temperature insensitive behavior observed
in un-etched fiber for the range on 30ºC to 100ºC at 1550 nm. The sensor response under temperature cycling is
repeatable and, proposed to be useful for low frequency analogue signal transmission over optical fiber by means
of inline thermal modulation approach.
Keywords - Optical fiber, temperature sensor, wet etching.
I. INTRODUCTION
Fiber of glass and plastic have used for the
communication of light carrier signal from one end to
another, this is basic of theoretical fiber optic
communication system. In recent years the optical
fibers have also been found for application as sensor
and have become interest to researchers because of
their high sensitivity, immunity to electromagnetic
interference and ease of operation in harsh
environments. Temperature sensing using fiber optics
is a commonly investigated problem which has wider
applications. Temperature sensor based on optical
attenuation in side-polished optical fiber has been
reported [1]. The use of a reference liquid with
known temperature dependent refractive index
characteristics, in a small part of fiber cladding has
been demonstrated for similar application [2].
Temperature sensing through absorption of
evanescent field in optical fiber is another reported
technique [3]. Splicing two different core fibers to
make a reflective mirror, has been demonstrated to
work as an intrinsic Fabry-Perot fiber-optic
temperature sensor [4]. The other techniques reported
for temperature sensing are based on interference of
selective higher-order modes [5] and liquid-core
optical fiber with small core-cladding refractive
index difference [6]. Resonance wavelength shift due
to evanescent field [7, 8] in side polished single mode
fiber has also been used for temperature sensing.
Fiber-optic temperature sensor using polarization
maintaining D fiber etched in dilute hydrofluoric acid
and immersed in immersion oil [9], show temperature
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dependent fiber insertion loss, which provide a
measure of surrounding temperature.
This paper presents the fabrication of single
mode fiber optic temperature sensor using controlled
wet etching of fiber cladding in hydrofluoric acid.
The SMF-28 fiber from corning, USA is used for this
work. Transmission loss in etched fiber to varying
diameters is measured at 1550 nm Newport source. It
is observed that the temperature sensitivity of fiber is
highest at a critical diameter obtained after etching.
Further the diameter from this critical value, lesser is
the temperature sensitivity of the sensor.
II. OPTIMIZE ETCHING PROCESS
FOR SINGLE MODE FIBER
Wet etching of optical fiber in hydrofluoric
acid is a simple technique, reported for making fiber
sensors. The wet etching technique though simple,
but is practically full of technical challenges. The
primary reason being small cladding dimension of
single mode fiber, which require precise control over
wet etching during the sensor fabrication process and
require precaution over breaking due to its flexibility.
Below figure-1 shows a block diagram which is used
for the fabrication of the SMF based sensors. Initial
fabrication step goes by removing some portion of
SMF plastic jacket (upto1 cm) with refractive index
of core and cladding 1.4682 and 1.4629 respectively,
after then it is fixed on a glass slide (area 7.5cm X
2.5cm) and placed above the temperature controller
which is used for maintaining the constant
temperatures.
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2. Ajay Kumar et al Int. Journal of Engineering Research and Applications
ISSN : 2248-9622, Vol. 4, Issue 2( Version 5), February 2014, pp.04-08
Figure 1: Fabrication setup for etched fiber sensor
On the exposed region of the SMF 40%
concentrated hydrofluoric acid is placed for etching
of the SMF at different temperature and by
maintaining the constant temperature (25, 35, 45, 55,
& 65ºC) we etched number of fiber with different
time period to achieve different core- clad dia of the
fiber as explain in figure-2.
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Figure 3: Etching Rate of SMF with changing
temperature
In the plot in figure 2 we have find how the
etching rate of the SMF response to the varying
temperature which is explained in figure-3 showing
exponential response with varying temperature
described as
0.031T
α(T) = 0.865e
III. ETCHED SINGLE MODE FIBER AS
SENSOR
A schematic diagram of etched SMF is
shown in figure-4 in which DCl is the cladding and
DCo is the core diameter it shows how the exposed
core-clad region has became after the etching of the
SMF.
Figure 2: Etched diameter of SMF with Time at
constant temperature.
The fiber diameter in etched portion is
measured under the Olympus optical microscope and
plotted as a function of etching time for various etch
temperatures. All the measured etched diameters (DE)
at a constant hotplate temperature (T) follow a linear
variation in etching time (t) as described by the
equation below
DE (T) = α(T) .t + α(T)
Here α(T) and β(T) are the etch rate
(µm/min) and fiber diameter at t = 0 (intercept on y
axis) respectively, for given temperature. At room
temperature the etch rate for fiber is relatively slow,
typically 1.69 µm/min, which rapidly increases to
6.62 µm/min at 65oC
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(a) Schematic of etched fiber region.
(b) Fiber image after etching
Figure 4: Sections of etched and un-etched portions
of single mode fiber
Using the above mention process for
fabrication of SMF sensor, we have tested many
sensor and find that etched fiber with core and thin
clad (upto 1 to 3µm, as shown in figure-4 ) responded
highly by interacting with the external medium,
producing change in the net optical power of the
fiber.
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3. Ajay Kumar et al Int. Journal of Engineering Research and Applications
ISSN : 2248-9622, Vol. 4, Issue 2( Version 5), February 2014, pp.04-08
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The fiber sensor is placed in the constant
heat zone of a Carbolite furnace with one port
connected to 1 mW, 1550 nm Agilent laser source
(HP-8153A) and, other port connected to optical
power meter, as shown in figure 5. The furnace is
programmed to vary from 35oC to 130oC with 3
o
C/minute ramp rate.
Figure 7: Temperature sensitivity dependence on
etched fiber diameter.
Figure 5: Experimental setup for Temperature
sensing
The corresponding monitored optical power
is recorded using the automated experimental set-up.
It is observed that the etched region surrounded by air
acts as a reliable temperature sensing device. The
temperature dependent transmitted optical power
variation in an un-etched and an 11.3 μm diameter
etched fiber are shown in figure 6. A linear
temperature dependent power variation is observed in
etched fiber with 3.2 μW/oC sensitivity, compared to
relatively temperature insensitive behavior in unetched fiber.
The sensitivity drops to less than 1.5 μW/oC
for diameter smaller than 8.5 μm and greater than
13.5 μm. Our experimental results highlight the high
temperature sensitivity region between „a‟ and „b‟,
shown by a Gaussian fit of data points and, the low
temperature-sensitivity region beyond „b‟ in figure 7. The „a‟ and „b‟ are 7.5 μm and 13.5 μm
respectively.
The temperature sensitivity (ST) is found
varying as a Gaussian function of etched diameter
and described by
𝐒 𝐓 = 𝟑. 𝟕𝟖𝟗 × 𝐞−
𝐝 𝐞𝐜𝐥 −𝟏𝟏.𝟐 𝟐
𝟐
.
Measured data points in the low temperaturesensitivity region fit in a power (P) equation
described by
P = 108.3 × d ecl-1.87
IV. RELIABILITY OF THE SENSOR
The fiber sensor with 11.2 μm etched
diameter is sealed inside 8 cm long borosilicate glass
capillary having 8 mm outer diameter, as shown in
figure 8.
Figure 6: Plot between optical power and
Temperature
Figure 8: Etched fiber temperature sensing region
inside a sealed glass tube
Measurements performed on all the
fabricated fiber sensors show similar behavior with
varying temperature sensitivity. The highest
temperature sensitivity 3.8 μW/oC has been observed
for 11.2 μm etched fiber, as shown in Figure 7.
(a)
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4. Ajay Kumar et al Int. Journal of Engineering Research and Applications
ISSN : 2248-9622, Vol. 4, Issue 2( Version 5), February 2014, pp.04-08
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behavior of an un-etched fiber. It is also observed
that the sensitivity rapidly falls to 1/e of its peak
value when the fiber diameter is 11.2 ± 2 μm. The
sensor response under temperature cycling is
repeatable and, proposed to be useful for low
frequency analogue signal transmission over optical
fiber by means of inline thermal modulation approach
VI.
(b)
Figure 9: (a) Measured temperature inside the
environmental chamber. (b) Optical response of
sensor under periodic temperature changes
The sensor is kept inside an environmental
chamber (ESPEC, USA) interfaced to Optical
component environmental test system (OCETS from
JDS, Canada) and programmed to undergo
temperature cycle from 30oC to 38oC. The
transmitted optical power is observed to follow the
temperature cycle of the chamber, as shown in Fig.
9(a) and 9(b), except the nonlinear sensor response in
the upper and bottom portions of curve. The sealed
sensor produces repeatable results but with small
reduction in temperature sensitivity. The cubical
chamber measuring 8 feet3 in volume is fitted with a
thermocouple at fixed location. The chamber takes
some time to attain uniform temperature, as measured
by the thermocouple. This causes a delayed response
from the sensor, particularly in the constant
temperature cycle zone.
V. CONCLUSION
As from the above sections we have shown
that etching of the SMF using hydrofluoric acid for
achieving desired diameter of the fiber sensor can be
done by controlling the etching process with time and
temperature which allows us to save time, rather
etching to time duration of hours we can etch it in
minutes. Maintaining a constant temperature value
for a constant time gives us the desired combine coreclad diameter of the SMF. For the purpose we have
etched number of fiber at constant temperature and
time. We have demonstrated that the effect of
temperature in the etching process makes the etching
rate to follow exponential path for the temperature
range 0° to 70°C without using any real time
monitoring of the power. We have fabricated the
single mode optical fiber based temperature sensor
using this etching technique. Our measurements
demonstrate the temperature sensitivity dependence
on etched fiber diameter. The sensor shows 3.8
µW/oC as the highest sensitivity for 11.2 µm etched
diameter in comparison to 0.2 µW/oC for 25 µm
diameter and, relatively temperature insensitive
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ACKNOWLEDGEMENTS
I sincerely thank Dr. Anuj Bhatnagar,
Scientist-E, SAMEER, IIT campus, Mumbai for their
extreme Guidance and valuable time throughout my
research.
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