Download Temperature resilient measurement of refractive index for liquids

2013 Seventh International Conference on Sensing Technology
Temperature resilient measurement of refractive
index for liquids
Vijayakumar Narayanan
Sreehari H. and Sreedevi Nair S*
Fiber Optics & Photonics Lab
Government Engineering College, Barton Hill
Trivandrum, India 695 035
[email protected]
Department of Electronics & Communication Engineering
College of Engineering
Trivandrum, India 695 016
*
[email protected]
Abstract—The measurement of refractive index of
liquids is of great importance as it is its prime optical
property. There are several methods of refractive index
measurement for liquids. But the accuracy of
measurement is influenced by temperature fluctuations.
In this paper, a method is proposed for the
implementation of an accurate, portable and
temperature resilient, fiber based refractometer for
liquids. The analog front end of the refracto meter alone
is a cost effective adulteration detector. It can be
calibrated to measure the amount of adulteration or
absolute refractive index of liquids. The temperature
resilience in measurement is achieved
using an
instrumentation amplifier with high Common Mode
Rejection Ratio (CMRR). This refractometer is
implemented both using glass and plastic fibers.
the test cell, the refractive index as a function of
concentration is obtained. The refractive index of a liquid
depends on its density and the wavelength of the incident
light. Fluctuations in both temperature and concentration
will change the liquid density and hence the index.
Another
method,
the
Surface
Plasmon
Resonance(SPR) sensing system is based on the differential
reflectance. Changes of reflectance at two wavelengths are
proportional to the refractive index change of a sensed
medium. Differential of two reflectance gives two-folded
sensitivity. But all the existing methods require bulky
apparatus and stationary and stable measuring environment.
This reduces its portability and efficiency because it is
impossible to measure when and where we require.
The proposed method is intended to implement a
refractometer making use of the sensing capabilities of an
optical fiber. This method considers only the refractive
index measurement of liquids because of the large
commercial demand for liquid refractometers. The method
that we implement may also be used in gaseous index
measurement, with some modifications.
Keywords : refractometer; temperature resilient; fiber
based; adulteration in liquids.
I. INTRODUCTION
Fiber optic sensor technology has been growing
and widely used in the field of medicine, defense and
aerospace. Their advantages includes the ability to be
lightweight, of very small size, passive, low power, resistant
to electromagnetic interference, high sensitivity, wide
bandwidth and environmental ruggedness[1],[2].
The importance of refractive index measurement is
very high and several methods have been devised for the
same based on various principles and properties of
lightwaves [3-7]. Still most of these methods require
temperature resilient stable conditions for accurate
measurements. This problem arises due to the complete
dependence on geometrical optics wherein we use lenses
and other optical components that need to be arranged at
proper distances (eg: focal lengths). Apparatus based on ray
optics require high precision positioning of prisms and
lenses. As a result it becomes bulky thereby restricting the
implementation of a portable and convenient refractometer.
Techniques that measures the change in the refractive
index of the liquid based on its concentration have been
developed. In these methods if the test liquid concentration
is known, e.g., by measuring the mass of liquids added to
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II. THE PROPOSED METHOD
The proposed method is completely based on the fiber
gain variations as we change the refractive index of its
cladding material. For this the cladding has to be removed
and replaced with the liquid whose refractive index is to be
measured. The device would require only a fiber length of
10cm and few compact optoelectronic components resulting
in a portable and accurate hand-held refractometer.
A. Concept
An optical fiber consists of a very thin core surrounded
by a cladding of slightly lower refractive index that enables
the propagation of light through the fiber by means of total
internal reflection. The output power from an optical fiber
depends on its core and cladding refractive indices. Majority
of the power is transmitted through the core and only a very
small fraction of the total power propagates through the
cladding. Even this evanescent power level is completely
dependent upon the relative refractive index of the core and
the cladding.
The proposed method utilizes this dependence. Hence
the material used as the cladding will definitely influence
the net output power. So we can use this power variation to
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2013 Seventh International Conference on Sensing Technology
sense the refractive index of the material responsible.
B. Experimental Setup
We require an LED source followed by a 3dB splitter.
The 3dB splitter will ideally split the input power equally
and supplies to each of the two fiber cores. One fiber is used
as a reference while other is a clad less fiber immersed in
the liquid. A measure of relative refractive index will be
obtained in terms of light intensities at the photo-detectors.
The two identical photo-detectors convert the incident light
power into current which will then be amplified using an
instrumentation amplifier, that is stable under thermal
variation. Then a calibration will be done to get a measure
of refractive index. At the final stage microcontroller will be
programmed to interface a digital display that gives the
refractive index value. The basic block diagram of the
device is shown in Fig.1
Fig. 2(a): Satge I- Front end circuitry of the refractometerwith outputs Y1 and Y2
Figure1. Block Schematic of the Refractometer
The Components and tools used in the experiment set
up are listed in Table I below
Monomode fiber and ST Connectors
Tools : Splicing kit FSM 50S
Softwares : Microchip MPLAB IDE 8.6
50:50 optical (1X2) Splitter (NEST)
Tools : Connectorization kit
Softwares : The EAGLE Light Layout editor
Si Photodetector receptacles
Peripheral Interface Controller PIC16F876
LCD Display 16X2 LCD JHD162A
Table.I. Components and tools used inthe experiment
A low cost version of refractometer excluding the
digital display and the microcontroller can also be realized.
However it can be used for applications like detecting the
adulteration in a liquid, not for absolute measurements as the
output is not calibrated.
The front end circuitry of the refractometer is shown
in two distinct stages for convenience in figures 2(a) and
2(b). Stage I of the circuit constitutes two identical LED
sources, fiber with cladding
( Fiber A ) and the fiber
without cladding (Fiber B immersed in the liquid), followed
by photo-detectors.
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.
Fig. 2(b): Satge II- Front end circuitry of the
refractometer- which is an instrumentaion amplifier
amplifies the difference signal Y1-Y2 from Satge I.
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2013 Seventh International Conference on Sensing Technology
C. Procedure
In this method, the cladding of the glass fiber is etched
using Hydro Fluoric (HF) acid and is replaced with the
liquid whose refractive index is to be measured. Then the
output power will vary based on the relative refractive index
of this new fiber system.
The etched fiber will be placed in a hollow cylinder
and will be coupled with the source and detector at ends.
The liquid when filled around an etched fiber will act as a
cladding. When light is reflected at the boundary of a denser
and a rarer optical medium, the field associated with the
wave extends beyond the interface in the cladding region.
This field has an amplitude which decreases exponentially
with increasing distance from the boundary and is referred
to as an 'evanescent field'. When this field interacts with
cladding, it results in attenuation of the power of the
propagating wave. If P0 is the power transmitted by the
cladded fiber, then the power transmitted in the presence of
an absorbing liquid is given by [8],
index measurement treating temperature as a common mode
signal.
The optical power output (Y2) was measured using power
meter and the reading obtained before and after adding the
liquids which surrounds Fiber B are shown in figures 3
and 4. The reading were taken before calibration and they
do not indicate absolute refractive index.
P ( z )= P 0 . e− γz
(1)
where z is the distance along the unclad length and 'γ' is
the evanescent absorption coefficient of the medium.
Therefore the evanescent absorbance A of an unclad
fiber of length L surrounded by a fluid of evanescent
absorption coefficient γ is given by
A= log 10 [
P0
γL
]=
P ( z ) 2.303
Figure 3: Reading in optical power meter in dBm before
Calibration without adding liquid
(2)
But the effect of attenuation with respect to fiber length
will be negligible in the present scenario as the entire device
will be only a few centimetres long. The dependence of
output power with input power for the experiment that we
deal with is given by [9],
P= P 0 .
n2core − n 2liquid
2
2
n core− ncladding
(3)
Based on the detector readings, the device can be
calibrated to get refractive index for any given liquid, which
can then be used to measure the amount of adulteration or
any other application for that matter.
III. RESULTS AND DISCUSSION
The proposed method is an intrinsic intensity
modulated fiber optic refractometer based on the principle of
'evanescent wave absorption'. The refractometer was
implemented using glass fiber and also using plastic fiber,
due to the brittle nature of cladless glass fiber.
High Common Mode Rejection Ratio (CMRR) of
78dB was achieved in instrumentation amplifier. This really
brings about the temperature resilient nature of refractive
978-1-4673-5221-5/13/$31.00 ©2013 IEEE
Figure 4: Reading in optical power meter before
calibration and after adding liquid
The reading in optical power meter before and after
adding liquid is -46.3 dBm and -40.5 dBm respectively.
Using Fiber optic refractometer, the percentage
adulteration in gasoline (adulterated using kerosene) is
measured.. Adulterated mixtures will have refractive index
different from that of its component liquids. The clad
removed fiber is kept immersed in a measuring vessel. The
difference in reference and experimental fibers is boosted
using a instrumentation amplifier of high CMRR ( 78 dB ).
The sensitivity of the device is found to be linear with
adulteration.
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2013 Seventh International Conference on Sensing Technology
A measuring vessel of 30 ml capacity was fully filled
with both kerosene and gasoline. First sample was 3ml
kerosene with 27 ml gasoline (10 percent adulteration ) and
next sample used was 6 ml kerosene in 24 ml gasoline (20
percent adulteration) and so on. The gasoline refractive
index was varied in the range 1.38 to 1.42, as kerosene was
added to it, depending on the degree of adulteration.
The refractometer was implemented using both the
glass and plastic fibers as shown in Figures 6 and 7
respectively.
Figure 7: The refractometer implemented using plastic
fiber
Experiments shows that a better sensitivity is obtained
with refractometer using plastic fiber.
IV. CONCLUSION
Figure 5. The wiring diagram for the interface circuitry
A interface circuitry was designed to transform these
power meter reading to a convenient display which directly
shows the absolute refractive index.
A temperature resilient and accurate refractometer was
implemented. A low cost, portable purity sensor for liquids
was also materialized as the part of this work. This work
successfully utilized the sensing potential of optical fiber
and the temperature stability of instrumentation amplifier to
achieve accurate measurements of liquid refractive index.
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Figure 6: The refractometer implemented using glass fiber
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2013 Seventh International Conference on Sensing Technology
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