Attenuation Based Passive Fiber Optic Sensor – Characterization of Refractive Index at Wide Range of Temperatures Using a Tunable Light Source

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Attenuation Based Passive Fiber Optic Sensor – Characterization of Refractive Index at Wide Range of Temperatures Using a Tunable

Light Source

S. Srinivasulu

¹

&Dr. S. Venkateswara Rao

²

1&2

Department of Physics, JNTUH College of Engineering Hyderabad, JNTUniversity Hyderabad Telangana State, India - 500085

ABSTRACT:The study of refractive index is a topic of growing interest across the worlddue to their applications in numerous fields, ranging from consumer uses to the food, chemical, fragrant, military, medicine, beverages, bio- chemical, petro-chemical, pharmaceutical industries, etc. Many of the drawbacks found in conventional sensors were overcome by the use of optical fiber sensors operating at a new setof properties. The optical fiber sensors with their advantages such as low attenuation, light in weight, possibility of multiplexing, passive nature, immunity to EMI and RFI among others these sensors are alternative to electronic sensors. In the present work an evanescent wave absorption based fiber optic refractive index sensor operating at various wavelengths (630nm, 660nm, 820nm and 850nm) and at wide range of temperatures (10ôC to 60ôC) has been analyzed. The study of the sensor is taken up using two volatile liquid mixtures such as Toluene and Methanol as the liquid cladding in the region of sensing. By implementing the present sensor, the refractive index of any unknown liquid can be measured, whose value lies in the dynamic range between 1.305n_Dto 1.509n_D, temperature range of 10ôC to 60ôC and using the various wavelengths of 630nm, 660nm, 820nm and 850nm.

Keywords: Absorption, Dynamic Range, Evanescent wave, Refractive index sensor, Toluene and Methanol, Wide range of temperatures (10^oC to 60^oC).

INTRODUCTION

The optical fiber technology was revolutionized by the advent of LASERs in 1960s. The first optical fiber was demonstrated in 1963 to transmit the signals along glasses with extremely low attenuation [1]. The fiber optic sensing technology was to compete with the electric sensors, were very much established across the globe with showing new developments and much advanced technologies [2-6]. Volatile organic compounds are commonly vaporized at room temperatures and some of them can be breathed and some can cause adverse health hazardous [7- 8]. Due to the immunity to electromagnetic interference (EMI), the optical fibers can be used in electrical industry and they can also be implemented even as distributed sensors and also in optical time domine reflectometry (OTDR) [9-11]. Fiber optic sensors also can be used in the application such as food process control, detection of gases in liquids or presence of gases in blood [12]. The fiber optic sensing technology for the measurement of organic vapors began over last three decades ago and great improvements have been witnessed in the recent past [13-14]. The

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technology has been expanded to measure the environmental parameters that would be encountered by the man kind in almost all walks of life by designing the sensors variously and using the numerous functional mechanisms in the geometrical construction of the sensor. Thus optical fiber sensors can be used in wide range of applications ranging from civilian to scientific research and development and industrial applications. An important requirement of an optical fiber sensor is to be sensitive to the selected parameter and insensitive to the other parameter which arises generally to discriminate a specific gas among others. The wavelength of the light source is generally governs the selectivity of extrinsic optical fiber sensor. The selectivity of the sensor will be determined by the sensing material along with the physical and chemical properties of the matrix in case of intrinsic sensors. The efficiency of the sensor can be rated with the help of other important parameter i.e. response time of the sensor. For the development of a good quality sensor, the response time should be as low as possible for the online monitoring of parameters [15- 16].

EXPERIMENTAL DETAILS

The general configuration of the experimental setup of the optical fiber sensor consist mainly an opto-electronic light source connected to an insensitive part of the fiber (input fiber) using a suitable connector, the other end of the insensitive part of the fiber (output fiber) will be connected to a light detector using a another suitable connector, and at the middle of the fiber a sensing zone will be created depending upon the measurand to be measured. The main purpose of creating the sensing zone is to make the light traveling through the fiber interact with the measurand. The light can be made to interact with the measurand or analyte in two different methods. In the first method the interaction between light and measurand takes place outside the fiber, making the fibers both input and output fibers just to act as conduits. These sensors are called passive sensors or extrinsic sensors. In the other method the light will interact with the measurand within the fiber. They are called active sensors or intrinsic sensors.

In the present experiment an extrinsic fiber optic sensor has been designed using a uniform U-shaped glass rod of specific dimensions by connecting one end of the glass rod to input fiber and other end of the glass rod to the output fiber.

Thickness of rod: 0.5mm

Total height of the glass rod(H): 40mm Depth of the glass rod immersed in liquid(h): 30mm

Width between two prongs(Z): 5mm

Radius of the Curvature(X): 2.5mm

Depth of the Curvature(Y): 2.5mm

Total length of the glass rod immersed in liquid: 62.857mm

Fig.1: Geometrical parameters of U-shaped borosilicate glass rod.

In order to make the sensor versatile, to measure the refractive index of various liquids either transparent or dark at various temperatures ranging from 10^oC to 60^oC and also to conduct the measurement at various wavelengths of

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630nm, 660nm, 820nm and 850nm, a digital refractometer capable of measure the refractive index at temperature from 5^oC to 70^oC and a tunable light source respectively have been used in the experiment. A detector compatible to measure the output power at various wavelength have been used. Two chemicals, methanol (CH₄O) and toluene (C₇H₈) of refractive index 1.305n_Dto 1.509n_D respectively have been selected for the calibration of the sensor.

Chemical parameters of Toluene (C₇H₈) and Methanol (CH₄O):

Parameter Toluene (C₇H₈) Methanol(CH₄O)

Structure

CAS No. 108-88-3 67-56-1

Molar Mass (g/mole) 92.141 32.042

Refractive index (n) 1.4967 at 20^oC 1.3292 at 20^oC

Density (kg/m³) 0.8697×10³ at 20^oC 0.7940×10³ at 20^oC

Color Colorless Colorless

Boiling Point (ôC ) 110.6ôC 64.7ôC

Melting Point (ôC ) -94.9ôC -97.6ôC

The light launched from the source couples into the input fiber through SMA connector will enter into a U-shaped glass rod and couples out from the output fiber into the power meter and can be recorded. The U-shaped glass rod connected in the sensing zone having almost same thickness and refractive index of that of the insensitive part of the fiber will act as a core of the fiber. Different mixtures with combination of toluene and methanol at various proportions have been prepared using a two burette system. The refractive indices of all the mixtures have been determined using an automatic digital refractometer of modal number RX-7000i (Atago make, Japan) at temperatures ranging from 10ôC to 60ôC in steps of 5ôC below their boiling point and values are recorded [Table-1].

Table.1: Mole fraction of Methanol in Toluene + Methanol chemical mixture and Refractive indices of mixtures at various temperatures (10^oC to 60^oC).

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No.

Mole fraction of Methanol in Toluene + Methanol mixture

Refractive Index at various temperatures

10ôC 15ôC 20ôC 25ôC 30ôC 35ôC 40ôC 45ôC 50ôC 55ôC 60ôC

1 0.0000 1.50915 1.50591 1.50171 1.49770 1.49325 1.48974 1.48582 1.48293 1.47795 1.47509 1.47102 2 0.0406 1.49821 1.49519 1.49204 1.48908 1.48614 1.48305 1.48070 1.47721 1.47413 1.47102 1.46592 3 0.0869 1.48974 1.48695 1.48375 1.48070 1.47721 1.47470 1.47102 1.46821 1.46528 1.46230 1.45923 4 0.1403 1.47972 1.47645 1.47308 1.47041 1.46704 1.46402 1.46112 1.45831 1.45523 1.45241 1.44927 5 0.2025 1.46923 1.46689 1.46398 1.46004 1.45718 1.45382 1.45070 1.44702 1.44441 1.44128 1.43808 6 0.2758 1.45752 1.45441 1.45112 1.44888 1.44511 1.44176 1.43915 1.43608 1.43327 1.43021 1.42712 7 0.3636 1.44282 1.43915 1.43649 1.43379 1.43073 1.42754 1.42397 1.42118 1.41821 1.41529 1.41221 8 0.4706 1.42197 1.41821 1.41504 1.41221 1.40930 1.40618 1.40311 1.40054 1.39752 1.39464 1.39112 9 0.6038 1.39921 1.39602 1.39345 1.39001 1.38772 1.38499 1.38119 1.37824 1.37519 1.37224 1.36935 10 0.7742 1.36854 1.36541 1.36279 1.35984 1.35697 1.35348 1.35010 1.34749 1.34462 1.34154 1.33874 11 1.0000 1.33067 1.32802 1.32548 1.32228 1.31984 1.31652 1.31402 1.31241 1.31008 1.30822 1.30512

Initially the U-shaped glass rod is immersed in one of the liquid mixtures and light is launched from the source and collected from the detector. The light during the transmission through the U-shaped glass rod will interact with the liquid thereby it suffers some loss due to evanescence of part of the transmitted light through the glass rod into the liquidanddue to which a less amount of power will reach the detector. The amount of light reaching the detector becomes a function of concentration and hence the refractive index of liquid surrounding the U-shaped glass rod.

The output powers were recorded with the U-shaped glass rod into all the mixtures one by one at various operating wavelengths (630nm, 660nm, 820nm and 850nm) and at various temperatures of liquid ranging from 10^oC to 60^oC using an ice bath for low temperatures and a heating system for high temperatures.

Table.2: Mole fraction of Methanol in Toluene + Methanol chemical mixture and Output power at various temperatures (10^oC to 60^oC) for the operating wavelength of the source 630nm.

S.

No.

Output Power(dBm) at various temperatures

1 0.0000 -44.90 -44.53 -44.17 -43.73 -43.33 -42.97 -42.57 -42.27 -41.77 -41.47 -41.10 2 0.0406 -43.80 -43.50 -43.20 -42.87 -42.60 -42.33 -42.03 -41.73 -41.40 -41.10 -40.57 3 0.0869 -42.97 -42.67 -42.40 -42.03 -41.73 -41.43 -41.10 -40.83 -40.53 -40.23 -39.93 4 0.1403 -41.87 -41.60 -41.33 -41.07 -40.70 -40.43 -40.10 -39.83 -39.50 -39.20 -38.93 5 0.2025 -40.97 -40.63 -40.33 -40.00 -39.63 -39.33 -39.07 -38.70 -38.43 -38.00 -37.83 6 0.2758 -39.77 -39.40 -39.13 -38.87 -38.47 -38.13 -37.93 -37.60 -37.30 -37.07 -36.70

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7 0.3636 -38.27 -37.93 -37.67 -37.40 -37.10 -36.77 -36.37 -36.13 -35.83 -35.53 -35.23 8 0.4706 -36.20 -35.83 -35.50 -35.23 -35.00 -34.63 -34.33 -34.00 -33.70 -33.47 -33.10 9 0.6038 -33.90 -33.60 -33.30 -32.90 -32.63 -32.43 -32.10 -31.83 -31.53 -31.20 -30.93 10 0.7742 -30.80 -30.50 -30.27 -30.00 -29.73 -29.57 -29.30 -29.07 -28.80 -28.43 -28.20 11 1.0000 -27.40 -27.10 -26.80 -26.53 -26.10 -26.03 -25.70 -25.50 -25.40 -25.10 -24.80

S.

No.

1 0.0000 -45.23 -44.87 -44.47 -44.07 -43.63 -43.27 -42.87 -42.57 -42.03 -41.80 -41.40 2 0.0406 -44.13 -43.80 -43.50 -43.20 -42.90 -42.60 -42.30 -42.00 -41.73 -41.40 -40.87 3 0.0869 -43.27 -42.97 -42.70 -42.30 -42.00 -41.77 -41.40 -41.13 -40.83 -40.53 -40.23 4 0.1403 -42.17 -41.93 -41.60 -41.33 -41.03 -40.70 -40.40 -40.13 -39.83 -39.53 -39.23 5 0.2025 -41.23 -40.97 -40.60 -40.30 -40.00 -39.67 -39.37 -39.00 -38.73 -38.30 -38.10 6 0.2758 -40.07 -39.73 -39.43 -39.17 -38.80 -38.43 -38.23 -37.87 -37.63 -37.30 -37.00 7 0.3636 -38.57 -38.23 -37.93 -37.70 -37.37 -37.07 -36.67 -36.43 -36.13 -35.83 -35.53 8 0.4706 -36.50 -36.13 -35.80 -35.53 -35.30 -34.93 -34.63 -34.33 -34.03 -33.73 -33.40 9 0.6038 -34.20 -33.90 -33.60 -33.23 -33.00 -32.73 -32.40 -32.10 -31.83 -31.50 -31.20 10 0.7742 -31.10 -30.80 -30.57 -30.27 -30.03 -29.83 -29.57 -29.37 -29.07 -28.73 -28.47 11 1.0000 -27.67 -27.40 -27.13 -26.83 -26.47 -26.37 -26.00 -25.83 -25.70 -25.43 -25.10

S.

No.

1 0.0000 -46.87 -46.47 -46.17 -45.73 -45.30 -44.93 -44.57 -44.27 -43.77 -43.43 -43.07 2 0.0406 -45.80 -45.60 -45.23 -44.87 -44.60 -44.47 -44.00 -43.73 -43.37 -43.07 -42.57 3 0.0869 -44.93 -44.67 -44.53 -44.00 -43.73 -43.40 -43.07 -42.80 -42.47 -42.23 -41.93 4 0.1403 -43.87 -43.60 -43.30 -43.03 -42.73 -42.40 -42.10 -41.77 -41.50 -41.23 -40.93 5 0.2025 -42.97 -42.63 -42.30 -42.00 -41.63 -41.33 -41.07 -40.70 -40.43 -40.03 -39.80

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6 0.2758 -41.67 -41.40 -41.13 -40.87 -40.60 -40.13 -39.97 -39.57 -39.30 -39.00 -38.70 7 0.3636 -40.23 -39.97 -39.60 -39.40 -39.07 -38.77 -38.37 -38.13 -37.80 -37.50 -37.23 8 0.4706 -38.20 -37.80 -37.40 -37.23 -37.00 -36.57 -36.33 -36.10 -35.70 -35.40 -35.10 9 0.6038 -35.90 -35.60 -35.20 -34.90 -34.70 -34.40 -34.10 -33.83 -33.50 -33.17 -32.93 10 0.7742 -32.80 -32.50 -32.23 -31.97 -31.60 -31.40 -31.13 -30.90 -30.60 -30.33 -30.00 11 1.0000 -29.30 -29.00 -28.73 -28.43 -28.03 -27.93 -27.60 -27.43 -27.33 -27.00 -26.70

S.

No.

1 0.0000 -47.20 -46.87 -46.43 -46.00 -45.63 -45.23 -44.87 -44.53 -44.03 -43.77 -43.43 2 0.0406 -46.10 -45.83 -45.50 -45.20 -44.90 -44.60 -44.33 -44.00 -43.70 -43.43 -42.87 3 0.0869 -45.23 -44.97 -44.70 -44.33 -44.00 -43.73 -43.43 -43.10 -42.80 -42.50 -42.23 4 0.1403 -44.17 -43.90 -43.60 -43.30 -43.03 -42.70 -42.37 -42.10 -41.80 -41.50 -41.23 5 0.2025 -43.20 -42.93 -42.60 -42.33 -41.97 -41.67 -41.30 -41.00 -40.73 -40.30 -40.10 6 0.2758 -42.00 -41.73 -41.40 -41.17 -40.80 -40.40 -40.20 -39.87 -39.60 -39.30 -39.00 7 0.3636 -40.57 -40.20 -39.90 -39.70 -39.37 -39.10 -38.67 -38.43 -38.13 -37.83 -37.53 8 0.4706 -38.40 -38.13 -37.77 -37.53 -37.30 -36.93 -36.63 -36.30 -36.00 -35.70 -35.40 9 0.6038 -36.20 -35.87 -35.60 -35.30 -35.00 -34.60 -34.40 -34.10 -33.80 -33.50 -33.23 10 0.7742 -33.10 -32.80 -32.53 -32.27 -31.90 -31.73 -31.43 -31.20 -30.93 -30.60 -30.30 11 1.0000 -29.60 -29.33 -29.00 -28.77 -28.33 -28.20 -27.90 -27.73 -27.63 -27.30 -27.00

RESULTS AND DISCUSSION

In the present experiment various data have been collected to calibrate the sensor. By taking a small amount of liquid from each mixture the refractive index of all mixtures was determined using a digital refractometer at various temperatures ranging from 10^oC to 60^oC.In order to understand the relationship between mole fraction and refractive index, a graph is plotted between mole fraction and refractive index [fig. 2], and also relationship between refractive index and temperature are plotted in the form of a graph [fig.3].

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Fig.2: Relation between Mole fraction of Methanol in Toluene + Methanol solution and Refraction index.

Fig.3: Relation between Temperature and Refraction index of Toluene + Methanol solution.

As the variation of mole fraction, refractive index and temperature are related to one on another, a unified graph between all these three parameters have been shown in graph [fig.4].

Fig.4: Relation between Mole fraction of Methanol in Toluene + Methanol solution, Refraction index and Temperature.

The output power was recorded when the U-shaped glass rod immersed into liquid and maintaining at different temperatures using ice bath and heating system. A relationship is formed between mole fraction and output power of the liquid mixtures at the operating wavelengths 630nm, 660nm, 820nm and 850nm as shown in figures [fig.5-8].

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Fig.5: Relation between Mole fraction of Methanol in Toluene + Methanol solution and Output power for operating wavelength of the source 630nm.

The relation between the output power and the liquid temperature maintained between 10^oC to 60^oC has been shown in the graphical form [fig.9-12].

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Fig.9: Relation between Temperature and Output power of Toluene + Methanol solution for operating wavelength of the source 630nm.

Fig.10: Relation between Temperature and Output power of Toluene + Methanol solution for operating wavelength of the source 660nm.

A unified graph is plotted between temperature, refractive index and output power as their quantities vary with respect to one on another [fig.13-16].

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Fig.13: Relation between Refractive index, Output Power and Temperature of Toluene + Methanol solution for operating wavelength of the source 630nm.

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CONCLUSION

In the present experimentation toluene and methanol and their mixtures are used as analytes maintaining at the temperatures ranges between 10^oC to 60^oC by operating at the wavelengths of 630nm, 660nm, 820nm and 850nm to calibrate the sensor.

1. The refractive indices of chemical mixtures at various temperatures ranging between 10^oC and 60^oC have been recorded.

2. Output powers of all the mixtures at various temperatures and at various wavelengths have been determined.

3. The sensor is calibrated between temperatures from 10^oC to 60^oC and at operating wavelength 630nm, 660nm, 820nm and 850nm to measure the refractive index of unknown liquid either transparent or dark in the dynamic range of 1.305n_Dto 1.509n_D.

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AUTHOR:

S. SRINIVASULU [M.Sc.(Physics), M.Sc.(Maths), B.Ed., BLISc., CSIR-UGC NET, LMISCA], Ph.D. Research Scholar, UGC–Senior Research Fellow (SRF), Department of Physics, JNTUH–College of Engineering Hyderabad, Jawaharlal Nehru Technological University Hyderabad, Telangana State, India. He has published 21 research papers in reputed International Journals and presented 11 research papers in International Conferences. He was awarded with IARDO Young Scientist-2018, Active Young Researcher, Outstanding Research Scholar and Researcher of The Year-2019.