Temperature resilient measurement of refractive index for liquids

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1 Temperature resilient measurement of refractive index for liquids Vijayakumar Narayanan Fiber Optics & Photonics Lab Government Engineering College, Barton Hill Trivandrum, India 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. 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],[]. 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 Sreehari H. and Sreedevi Nair S * Department of Electronics & Communication Engineering College of Engineering Trivandrum, India * sreedevi.vipal@gmail.com 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. 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 /13/$ IEEE 369

2 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. (a): Satge I- Front end circuitry of the refractometerwith outputs Y1 and Y 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 :50 optical (1X) Splitter (NEST) Tools : Connectorization kit Softwares : The EAGLE Light Layout editor Si Photodetector receptacles Peripheral Interface Controller PIC16F876 LCD Display 16X LCD JHD16A 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 (a) and (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.. Fig. (b): Satge II- Front end circuitry of the refractometer- which is an instrumentaion amplifier amplifies the difference signal Y1-Y from Satge I /13/$ IEEE 370

3 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 P 0 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 (Y) 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 Figure 3: Reading in optical power meter in dbm before Calibration without adding liquid A= log 10 [ P 0 P(z) ]= γl.303 () 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. n core n core n liquid n cladding (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 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 dbm and 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 /13/$ IEEE 371

4 A measuring vessel of 30 ml capacity was fully filled with both kerosene and gasoline. First sample was 3ml kerosene with 7 ml gasoline (10 percent adulteration ) and next sample used was 6 ml kerosene in 4 ml gasoline (0 percent adulteration) and so on. The gasoline refractive index was varied in the range 1.38 to 1.4, 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. Figure 6: The refractometer implemented using glass fiber 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. REFERENCES [1] Argha Banerjee etal, Fiber optic sensing of liquid refractive index Science Direct, Sensors and actuators B, Vol 13, 007,pp [] Pengfei Wang etal, A macrobending singlemode fiber refractive index sensor for low refractive index liquids, Photonics Letters of Poland, Vol.(), 010, PP [3] Chenghua Sui, Pinghui Wu and Gaoyao Wei, A Fiber optic Evanescent Wave sensor for measuring refractive index change of liquids, PIERS Proceedings, Cambridge, USA, July 010, pp [4] S. S Patil and A D Shaligram, Refractometric fiber optic adulteration level detector for diesel International Journal of Advances in Engineering and Technology, September /13/$ IEEE 37

5 011, pp [5] D Sengupta, M. Sai Shankar, P. Saidi Reddy, R.L.N Sai Prasad and K Sreeman Narayana, A low cost fiber optic refractive index sensor Opto electronics & Advanced Materials Rapid Communications, Vol.4, No., February 010, pp [6] Wenjun Zhou etal, Compact refractometer based on extrinsic phase shift fiber Bragg grating, Sensors and actuators A: Physical, Vol.168, 011,pp [7] Rajan Jha et al, Refractometry based on a photonic crystal fiber interferometer, Optics Letters, Vol. 34, No.5, March 009, pp [8] Advances in Contemporary Physics and Energy, editors S.C. Kaushik, G.N. Tiwari, V.K. Tripathi, I.C. Goyal, A.Chandra [9] K.Thyagarajan, Novel refractometer using a tapered optical fiber Electronics Letters, Vol.0, No.13, /13/$ IEEE 373