International Journal of Electrical and Electronics Engineering Research (IJEEER) ISSN 2250-155X Vol. 3, Issue 2, Jun 2013, 81-86 Š TJPRC Pvt. Ltd.
FIBER OPTICS AS A CHEMICAL SENSOR BY MONITORING THE RATE OF WAVE ABSORBANCE UTTAM KUMAR GHOSH & BISWAJIT GHOSH Birbhum Institute of Engineering & Technology, Suri, India
ABSTRACT The paper briefly discusses the optical fiber chemical sensor based on wave absorbance technique and the experimental part measures this rate of wave absorption. The reaction we choose to study in the experiment was that between Potassium Iodide (KI) & Copper Sulphate (CuS04 ,5H2O).The uncladded portion(4cm) of the fiber was dipped in the solution. This resulted in the decrease of light intensity at the output of the fiber termination. It is due to the absorbance of lightwave as the reaction speeds up. Hence this gradual decrease accounts for the optical fiber sensing the progress of the reaction, i.e it acts as a chemical sensor. A curve of absorbance rate against time is then plotted.
KEYWORDS: Optical Fiber, Absorbance of Lightwave, Intensity of Light, Chemical Reaction INTRODUCTION An optical fiber is a very sensitive detection element, competitive in sensitivity, linearity and dynamic range with conventional sensors, and offering the advantages of no electrical connection for either power or data and no mechanical moving parts[1]. Perhaps the least exploited feature, to date, of optical fibers is their ability to translate minute changes is their mechanical/optical properties into significant changes in the path length of the guided light[2][3]. Although the major application of optical fibers has been in telecommunications, there is a growing application of optical fibers. In sensing applications for measurement of various physical and chemical variables, including pressure, temperature, magnetic field, current rotation, acceleration, displacement, chemical concentration, pH and so forth. Such field optic sensors are finding applications in industrial process control, the electrical power industry, automobiles and the defense sector[4][5]. One of the main advantages of a fiber optic sensor stems from the fact that optical fibers are purely dielectric and thus can be easily used in hazardous areas where conventional electrically powered sensors[6] would not be safe. In addition, fiber optic sensors are immune to electromagnetic interference, have greater geometric versatility (i.e., they can be configured into a variety of arrangements to suit the application), and should have a very short response time[7][8]. They can be multiplexed into various configurations and the information from various sensors can be transmitted over long distances by optical fiber. They can also be configured to provide spatially distributed measurements of external parameters[9]. In a typical optical fiber sensor, light from a source such as LASER diode or LED is guided by an optical fiber to the sensing region. Some property of the propagating light beam gets modulated by external measurand such as pressure, temperature, magnetic field and so forth[6][7][10]. The modulated light beam is then sent via another (or the same) optical fiber for detection and processing. The modulation could be in terms of the intensity, phase, state of polarization or frequency[11] [12]. Using multimode fiber[6] for sensing applications leads to less sensitive but simple, low cost solutions. On the other hand, with single mode fibers and laser diodes, orders of magnitude improvements in sensitivity are possible but require more sophisticated optical components as well as processing [7]. There are many different types of sensors using
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multimode as well as single mode fibers. Sensors based on single mode fibers are much more sensitive then multimode fiber sensors.
OPTICAL FIBER CHEMICAL SENSOR BASED ON WAVE ABSORBANCE Fiber optic chemical sensors (FOCS) can offer several advantages over traditional sensors[13][14]. The light weight and small size of fiber optic sensors are strongly complemented by their strong immunity to electromagnetic interference. Since the fiber sensors are made of glass they are environmentally rugged and can tolerate high temperatures, vibration, shock and they can operate in extremely harsh conditions[15][16]. In this paper, we would observe that the penetration of light wave of a total internal reflected light into and absorbing medium decreases the net amount of light guided through the fiber to its termination. The amount of absorption depends on the intensity of the light as well as the number of total internal reflections (TIR).The uncladded region of the fiber will enhance this absorption by the chemical solution. The design and use of a simple FOS working on the basis of wave absorbance for the study of time has been illustrated.
TIME DEPENDENT CHEMICAL ABSORBANCE An optical fiber is taken with a small portion of it has the cladding removed and immersed in a chemical solution. The power transmitted by it is expressed as P ' =P exp [-K (t)z] , where z is the distance along the uncladded region of the fiber, K(t) is the absorption per unit length,
Figure 1: Block Diagram to Monitor the Time Dependent Chemical Reaction Let P be the laser power transmitted in absence of the chemical solution and L is the length of the uncladded region of the fibre, Then P ' = P exp [-K( t(L] Or, K (t) = (2.303/L)log10[P/ P'] = (2.303/L) log10 [V/ V '] When a time dependent chemical reaction occurs in the medium such that X+Y=Z (t), & if the species Z strongly absorbs the radiation, then the absorption increases with time,& the light intensity at the output of fiber termination should decreases correspondingly. Thus the rate of temporal variation of light detected at the output end of the FOS will be the same as the formation rate of species Z and, hence, is related to the reaction rate.
Fiber Optics as a Chemical Sensor by Monitoring the Rate of Wave Absorbance
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EXPERIMENTAL SETUP In the experiment we used a multimode plastic clad silica fiber (250/380 μm) with its cladding removed from a region 4 cm in length. The uncladded region of the fiber which functions as the sensor element was immersed in the chemical solutions in a reaction cell, which was cylindrical glass tube of 5 cm diameter & 6 cm in length. The reaction we choose to study in this experiment was that between Potassium Iodide (KI) & Copper Sulphate (CuS0 4,5H2O). Figure 1.shows the generalized block diagram. A 1mW compound semiconductor laser (GaAs-GaP) with stabilized output was used as the source, and the transmitted power through the fiber was detected using a suitable detector. An Avalanche photo detector along with a digital millimeter was adequate for monitoring slow reactions. Equimolar solutions of potassium iodide & copper sulphate were taken in the cell. When KI is added to copper sulphate, cupric iodide is precipitated quantitatively and for each atom of copper present there is liberation of one atom of iodine. The reaction involved is as follows :– 2[CuSO4,5H2O] + 4KI =Cu2I2 + 2K2SO4 +I2↑+10H2O The evolution of iodine from this standard reaction was monitored using the above sensor. To enhance the absorption, 1% starch solution was added in the reaction cell.
Figure 2: Experimental Setup Used to Study the Time-Dependent Chemical Reaction Here the absorbing species that evolves during the reaction is iodine. To enhance the absorption at 7100 A (GaAs-GaP laser wavelength), starch solution was added to the medium and this resulted in an intense blue colour of the medium. As the reaction proceeds, blue colour deepens (in proportion to the iodine concentration) and hence the light intensity (milli volts) at the output end decreases at the same rate. We can actually watch this process happen in time by measuring the amount of light absorbed by the iodine, called the absorbance. The absorbance is proportional to the concentration of the reactants in the solution, so observing the absorbance as a function of time is essentially the same as observing the concentration as a function of time.
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EXPERIMENTAL DATA Table 1: Calculation of Absorbance per Unit Length as a Function of Time
Sl. No.
Time (min)
Voltage (mv)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26
0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48 50
20.00 18.61 16.11 15.00 13.95 12.67 11.52 10.10 9.28 8.63 8.03 7.30 6.47 6.02 5.34 4.85 4.52 4.20 3.95 3.45 3.15 2.93 2.66 2.60 2.42 2.25
Absorbance Per Unit Length, K 0 0.024 0.072 0.096 0.120 0.152 0.184 0.224 0.256 0.280 0.304 0.306 0.376 0.400 0.440 0.472 0.496 0.520 0.540 0.584 0.616 0.640 0.672 0.680 0.704 0.700
Figure 3: Rate of Absorbance Using the FOS Setup
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Fiber Optics as a Chemical Sensor by Monitoring the Rate of Wave Absorbance
RESULTS AND CONCLUSIONS Figure 3 shows the time variation of absorbance describing the evolution of iodine in the experiment. From this curve we can safely say that the decrease in light intensity at the output is due to the increase in the lightwave absorption with time. The rate at which the decomposition reaction is occurring is clearly related to the rate of change of the concentration, which is proportional to the slope of the graph. The slope can be estimated at each time in the data, taking the change in the absorbance divided by the change in time at each time step. The value of wave absorbance is also dependent on the length of the uncladded region.
REFERENCES 1. Culshaw Brian, “Fiber Optics in Sensing and Measurement” ,IEEE Journal of selected topics in quantum Electronics, Vol. 6, No. 6, November/December 2000. 2. Hocker G.B.,“ Fiber-optic sensing of pressure and temperature”,Applied Optics, Vol. 18, Issue9, pp.1445-1448 (1979). 3. Yao.S.K,Asawa.C.K., “Fibre optical intensity sensors”,IEEE J.Selected Areas in Communication,SAC-1/3,pp-562 -573,1983. 4. Keiser Gerd, “Optical Fiber Communications”, McGraw Hill International Editions,2000. 5. Pal Bishnu P.,”Fundamentals of Fibre Optics in Telecommunication & Sensor Systems, New Age Int.Limited,Delhi,2001. 6. Ghatak.Ajoy,Thyagarajan.K, “Introduction to Fibre Optics”,Cambridge University Press,1999. 7. Senior.John M., “Optical Fibre Communications”,Prentice Hall of India Private Ltd, 2nd Edition,2004. 8. Culshaw. Brian,Kersey.Alan, “Fiber-Optic Sensing: A Historical Perspective”, Invited Paper, Journal of Lightwave Technology, Vol. 26, No. 9, May 1, 2008. 9. Beck, W.J. ,Urbanczyk, W.; Barwicz, Andrzej, “Performance analysis of fiber-optic transducer for measuring low pressures”,Instrumentation and measurement Technology
Conference,1995,IMTC/95,Proceedings, Integrating
Intelligent Instrumentation and Control,IEEE,24-26 April 1995. 10. Sharma Anuj K., Jha Rajan, and Gupta B. D.,“Fiber-Optic Sensors Based on Surface Plasmon Resonance: A Comprehensive Review”, IEEE Sensors Journal,vol.7, No.8,August.2007. 11. H.A.Haus,Waves and Fields in Optoelectronics,Prentice Hall,Englewood Cliffs,NJ(1984), pp.99-103. 12. Wendeker, Kaminsk.M ,“ Development of a fiber-optic sensor for the measurement of dynamic cylinder pressure in spark ignition engine”Sensors,2005 IEEE ,Dept. of Internal Combustion Engines, Lublin Tech. Univ. ,Oct. 30 2005-Nov. 3,2005. 13. Yang, Cui-rong , Hangzhou Dianzi Univ., Hangzhou Pang, Quan; Fan, Ying-le; Xu, Ping ,“Fibre-optic Chemical Sensor Based on Characteristic Spectrum Recognition and its Application”, Control and Automation, 2007. ICCA 2007. IEEE International Conference , May 30 2007-June 1 2007, Page(s):802–805. 14. Klainer, Stanley M., “The Use of Fiber Optic Chemical Sensors for Monitoring Specific Parameters and Species in Aqueous Systems”,OCEANS '86,23-25 Sept. 1986,Page(s): 828–833.
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15. Ming Max,Liu Kang,”Principles and Applications of Optical Communications”,TMH. 16. Giallorenzi T.G.,”Optical fiber technology”,IEEE J.Quantum Electron,18(4),pp.626-666,1982.
AUTHOR’S DETAILS
Uttam Kumar Ghosh, born in India, received MSc(Physics)degree from I.I.T. Kharagpur in 1984 and M.Tech (Applied Optics) from I.I.T. Delhi in 1986. He has more than 15 years of industrial (PSU) experience in the field of Applied Optics. Presently he is a faculty of Electronics & Communication Engineering Department at Birbhum Institute of Engineering & Technology, Suri, India. His main research interests are in the area of Opto-Electronics and Optical Communication.
Biswajit Ghosh , born in India, obtained his M.Tech (ECE)in Microwave Engineering from the University of Burdwan, in 2008. He did his project on Broadband Microstrip Antenna from Bengal Engineering & Science University, Shibpur , Howrah. After that he joined Birbhum Institute of Engineering & Technology (Govt. Aided Institution), Suri,India. He is currently designated as Asst. Professor in the Electronics & Communication Department and in charge of up gradation of the Microwave Laboratory and is engaged in teaching & research in the areas of Microwaves, Electromagnetics and Antennas. He is a Life Member of the Society of the EMC Engineers (India),The Indian Society for Technical Educations (ISTE), Indian Society of Remote sensing (ISRS),The Indian Science Congress Association(ISCA),International Association of Engineers (IAENG), Indian Society of Electronics & Communication Engineers(ISECE),International Association of Engineers and Scientists(IAEST).