Design and development of a high sensitive low cost fiber optic
refractometer.
P.V.N.Kishore, M. Sai Shankar and P. Vengal Rao
Department of Physics, National Institute of Technology-Warangal, Telangana-506004, India.
ABSTRACT
A simple and low-cost fiber optic refractometer has been designed and demonstrated. The sensor is configured by
etching a short region of a standard single-mode circularly bent fiber. Circularly bent three fiber loops are etched in equal
lengths and arranged parallel to each other. The working principle of the sensor is by detecting the evanescent field
modulation with respect to the change in ambient refractive index. In the present study glycerin solution with various
concentrations, possessing different refractive indices is chosen as the ambient.
Bent and etched optical fiber sensors have increased sensing capabilities but they are very much prone to fracture and
breakage. The proposed sensor is stable showing very less tendency towards any breakage or fracture. The sensitivity of
the sensor is tested for glycerin with different concentrations. It is evident from the experimental results that with the
increase of glycerin concentration the output intensity of the sensor is decreased linearly. The proposed sensor may find
applications in detecting various chemical species and biochemical applications.
Keywords: Refractometer, Evanescent fiber optic sensor, Single- mode fiber
1. INTRODUCTION
Measurement of Refractive index has great significance for estimating liquids in biotechnology applications, blood protein
concentration, salinity of urine, measuring concentrations of contagious chemicals, petrochemicals, various industrial
fluids etc., Optical refractometers such as Abbe and Rayleigh refractometers are prominent and well established
instruments in measuring the Refractive index. But they have some limitations as they are not small in size and inefficient
in remote sensing applications.
Recent days Fiber optic sensors are becoming very versatile, spanning in many areas of research and industrial
applications. They have very discrete advantages over conventional electrical and mechanical sensors such as small size,
light weight, immunity to electromagnetic interference, ease of fabrication, high sensitivity, inert to chemical corrosion,
remote sensing capability. Till date so many fiber optic based refractometers have been proposed such as fiber based fabryperot refractometer [1], multimode-single mode- multi mode structure based refractometer [2], fiber bragg grating [FBG]
based refractometer [3,4] abrupt taper Michelson interferometer based refractometer [5] and bent fiber based refractometer
[6,7]. They are very low cost, high sensitive, very susceptible to remote monitoring and inert to chemical contamination.
In the present article an alternative refractometer based on macro bending-based bare fiber refractometer is discussed. It
is of very low cost, easy to fabricate and offers competitive resolution.
Fiber optic evanescent sensors are one of the ideal sensors for remote monitoring of dangerous chemicals and industrial
processing. In general, light propagates in the core region of the fiber surrounded by cladding, by the phenomena known
as total internal reflection. While light propagating in the core, very minute portion of the light enters in to the cladding
region which is called Evanescent field of the fiber. By modification of the fiber structure, the amount of evanescent field
can be modified in the cladding. Number of studies are made on evanescent wave measurements [8-13]. To enhance the
sensitivity of the evanescent wave measurements, several approaches have been investigated. Sensitivity can be enhanced
by using a straight and short length fiber without cladding. Chemical etching of the fiber, polishing and heat removing
method are well known methods of removing the cladding layer, consequently increasing the evanescent field. While these
sensors provide reasonable signal from evanescent wave interactions with aqueous solutions, bending or coiling the unclad
section of the fiber can substantially enhance the signal. Murakami and Tsuchia [14] studied on the bending loses of the
optical fiber and demonstrated experimentally that the bending loses vary with the ambient refractive index of the fiber.
Later on Veldhuis etal made a simple optical wave guide based refractometer [15, 16].
Photonic Fiber and Crystal Devices: Advances in Materials and Innovations in Device Applications IX,
edited by Shizhuo Yin, Ruyan Guo, Proc. of SPIE Vol. 9586, 95860A · © 2015 SPIE
CCC code: 0277-786X/15/$18 · doi: 10.1117/12.2189436
Proc. of SPIE Vol. 9586 95860A-1
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In this paper, a novel fiber optic refractometer is proposed. Single mode fiber loops with three turns coiled with a selected
bending radius is formed. A small portion of the fiber coils arranged equal in length and parallel to each other, is etched
chemically. The fiber sensor is well suited for use with aqueous solutions and organic liquids. The sensor has the advantage
of compactness, easy design and capable of analyzing liquids with high sensitivity. The sensor head preparation and
experimental procedure are discussed in the subsequent sections.
2. SENSOR HEAD PREPARATION AND EXPERIMENTAL PROCEDURE:
2.1 Sensor Head Preparation:
Fig.1. Sensor Head
The sensor head is shown in the figure: 1. A circular hollow pipe of length 5 cm having a diameter of 1.9 cm which is
suitable diameter as proposed by pengfei Wang et al., [17], is taken and a small width of 5 mm slot along the length of the
pipe is cut. Buffer coating a standard single mode fiber (SMF28e) is stripped off at the region where it is to be etched and
is coiled on the circular pipe. The coating stripped off fiber regions of equal length is etched chemically using Hydro
fluoric acid (HF 40%) until the clad diameter reaches to 80 µm (125 µm before etching) which results in a better sensitivity
[6]. The etching process is carried as described in [18,19].
Circular Pipe
Fiber loops
BBS
SMF
……………….
……………..
……………..
……………..
……………..
……………..
……….…….
Etched
Portion
SMF
Computer
OSA
Glycerin
Fig 2. Schematic of the experimental setup. Cross sectional view of the sensor head is shown in the
figure.
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2.2 Experiment:
The schematic of experimental set up is shown in figure 2. The sensor head prepared as mentioned in section 2.1 is dipped
in various liquids of different refractive indices. Broad band light source having peak wavelength at 1550nm is used as
light source. Light is coupled into one end of the sensor head and the other end is connected to a detection system, Optical
Spectrum analyzer (OSA), which in turn connected to a computer to measure the output power.
To investigate the sensitivity of the sensor, liquids having different refractive indices are used to come into contact
with etched portion of the fiber. To obtain different refractive indices, 5%, 10%, 15% and 20% of glycerin solutions are
made by mixing glycerin with water in proper ratio and characterized by Abbe refractometer with a resolution of 10 -3. The
refractive indices of the glycerin solutions at 1550nm are given in table 1.
Table1. Refractive Indices of Glycerin concentrations at 1550nm.
Glycerin Concentration
Refractive index
5%
1.339
10%
1.347
15%
1.354
20%
1.361
Sensor with three turns is prepared as discussed above. In order to test the sensor response, the experiment is carried out
by dipping sensor separately in various glycerin concentrations as prepared by above procedure and data is collected.
3. RESULTS AND DISCUSSION
The refractive index of the solution affects the interaction of the light in the fiber with the solution, as expected for
evanescent wave-based devices. The closer the refractive index of the solution is to that of the fiber, the greater the
interaction of the evanescent wave with the sample. This condition leads to greater light loss as well as larger absorption
of signal. Thus the refractive index of the solution under analysis must be considered when measurements are taken. To
increase the interaction of the light propagating through the fiber with the medium in the sample region, curved fiber is
considered. Bending a fiber effectively converted the propagated modes to higher order modes that will have more
evanescent wave interaction with the medium surrounding the unclad or reduced core. To increase more interaction with
the fiber, three bent fiber loops with etched portions are used in the study. While the light propagating in the fiber coil, in
each turn at the etched region, light affected with the ambient refractive index which results in more decrease in the output.
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-18.78
1. Water
2.5% Glycerin
3.10% Glycerin
4.15% Glycerin
5.20% Glycerin
d8rü
-38.78
3
2
-58.78
1000
VI
5
-118.78
nm
1520.0
REM:
%/BW.
0.5 nm
55 kHz
Sens:
ST'.
rIr¿r.
4
10.0 nmldiv
1570.0
-35.43 dBm
583 ms
1620.0
In Vac
An
Off
Fig:3 OSA spectrums for water and various Glycerin
Concentrations.
The increase of the ambient refractive indices resulted an increase in the absorption of light but decrease in the fiber
transmission, as indicated by OSA spectrums in Fig.3. It shows a liner decrement of output power with the increase of the
refractive index which is illustrated in fig.4.
Equation
-70 -
y = a + b'x
Adj. R -Squar
0.98983
Value
-72
Standard Erro
--
Intercept
388.40502
23.45666
--
Slope
-344.0285
17.4161
-78 -
-80 1.330
1.335
1.340
1.345
1.350
1.355
1.360
1.365
Refractive Index
Fig.4.Loss in the fiber transmission corresponding to increase in the
refractive index of the fiber ambience.
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4. CONCLUSIONS
A simple fiber optic evanescent wave based sensor is proposed and demonstrated for the determination of refractive indices
of liquids. The sensor is inexpensive and have good sensitivity. No coating is used on the fibers, and it responded rapidly
to refractive index changes of its environment. The sensitivity of the sensor is found to be 313 dB/RIU. The sensitivity of
the device can be adjusted by varying the no. of turns the fiber and adjusting the radius of the fiber coil.
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