NURS 2010 CONFERENCE PROCEEDINGS – Dairo and Soge pp 70-75 70 Polymer Optical Fibre in Communications and Environmental Monitoring O. F. Dairo 1 and A. O. Soge Department of Physical Sciences, College of Natural Sciences, Redeemer’s University, Km 46 Lagos-Ibadan Expressway, Redemption City, Mowe, Ogun State 110115, Nigeria, E-MAIL: dairof@run.edu.ng Abstract This paper focuses on niche applications of polymer optical fibre (POF) in modern communication systems and other related areas. The electromagnetic isolation of POF and its relatively high sensitivity compared to conventional sensors has earned it a centre stage in sensing technology. The ability to detect and measure physical parameters such as temperature, humidity and wind-speed makes POF suitable for real time monitoring of environmental and climatic conditions. Other attractive features of POF include lower cost compared with glass optical fibres, better fracture resistance and flexibility. 1.0 Introduction Polymer optical fibres (POFs) have gained considerable interest in data communications and sensor applications, since they were introduced in 1960s [1]. Although they exhibit considerable greater optical signal attenuations than glass fibres, the toughness and durability of polymer allow POFs to be handled without special care. The high refractive-index differences that can be achieved between the core and cladding materials yield numerical aperture as high as 0.6, and hence coupling to sources can be done efficiently [2-3]. Additionally, the mechanical flexibility of fibre makes POFs to have large cores, with typical diameters ranging from 110 to 1400 µm. These properties allow the use of inexpensive large-area light-emitting diodes (LEDs), in combination with the less expensive polymer fibres, make an economically attractive system [2]. Presently, POFs are rapidly replacing copper cables in short-haul communication links mainly due to the advantages inherent to any optical fibre in relation to transmission capacity, immunity to interference and small weight. POFs also complement glass optical fibre (GOF) in short-haul communication links because they are easy to handle, flexible and economical. However, they are not used in long-haul communications because of their relatively high attenuation [1]. New types of polymer fibres have been developed in recent times for applications in data communications. For instance, a multi-core POF was developed for reduced attenuation and improved bandwidth performance compared to the standard POF of 1 mm core and a typical bandwidth of 50 – 100 MHz for 100 m [3]. A special type of polymer fibre known as a graded index (GI) polymer fibre which offers much higher bandwidth (GHz) has become commercially available. Fibre-Bragg gratings (FBGs) and micro-structured POF (MPOF) are fibre technologies that have been used to improve the transmission quality of POF. In-fibre photo-induced refractive index gradient using UV (laser) is known as fibre Bragg gratings. FBG formed in GOF is now possible in POF thereby extending the application of POF in diverse areas such as telecommunications, civil structure monitoring, environmental surveillance, seismic and geophysical study, undersea oil exploration, medical and biological testing and others. These technologies increase POF sensitivity to measurand such as strain or temperature [4-6]. 1 Corresponding author: Email: dairof@run.edu.ng NURS 2010 CONFERENCE PROCEEDINGS – Dairo and Soge pp 70-75 71 Over the years, different types of optical fibre sensors have been developed and commercialised for diverse applications. Some of the advantages of POF sensors are immunity to electromagnetic interference (EMI), high sensitivity, large bandwidth, lightweight, small size, and ease in signal light transmission [7]. 2.0 POF in Environmental Monitoring: Temperature Sensing The operation of a typical POF temperature sensor normally involves the integration of an absorber, liquid crystal, or phosphor in the fibre cladding, or using the fibre itself as the sensor. In the case of all fibre sensors, a temperature change ∆T produces a phase shift ∆ in the light propagating in the fibre. The phase shift can be detected with the aid of a fibre interferometer [8]. The relationship between the temperature change and the associated phase shift is given as: (1) where, λ is the wavelength of the propagating light, n is the refractive index of the medium, L is the fibre length, and t is the time duration of light propagation. The term in Equation (1) accounts for the fractional change in the fibre length with respect to time due to expansion or contraction of the optical fibre as the temperature varies. Similarly, the term is included in Equation (1) as a result of the temperature dependence of the refractive index. Extremely high sensitivities of 10 -8 o C are achievable with glass optical fibres which are not required for many applications where one degree accuracies are sufficient [8]. The use of oil as the thermo-sensitive material in optical fibre temperature sensor has been reported [9-10]. Gaston et al. [11] explored the linear variation of the refractive index of oils with temperature in fabricating optical fibre temperature sensors with improved performance. The transmission of the optical power along the fibre, when in contact with vegetable oils, was monitored as the temperature of the oils increased. Figure 1 shows the curves of optical power attenuation versus temperature obtained for a prototype optical fibre temperature sensor using olive and sunflower oils. The optical power loss varies linearly with the temperature. Fig. 1 Optical power loss of an optical fibre in contact with two different oils (olive and sunflower) against oil temperature variation at 1550 nm optical wavelength [11]. NURS 2010 CONFERENCE PROCEEDINGS – Dairo and Soge pp 70-75 72 2.1 POF in Environmental Monitoring: Relative Humidity Sensing Environmental factors, to a large extent, determine the needs and adaptation modes (of life) in any geographical location. Therefore, the knowledge of the climatic factors of a location is very important for everyday living and modelling of the performance of devices. POF has been demonstrated to interact with physical quantities in areas of metrology [12-17] and remote sensing [18]. Real-time humidity monitoring is practicable using POF due to the affinity for water of certain kinds of polymer or cellulose [19-20]. These materials usually swell in a humid atmosphere and show an increase or decrease in their refractive indices depending on the type of material. For instance, polymethylmethacrylate (PMMA) exhibits an increase in refractive index after absorbing moisture whilst the refractive index of hydroxyethylcellulose (HEC) film decreases in the presence of humid air. Muto el al., [19] reported that the refractive index of HEC film changed from 1.51 in the dry state to 1.48 in humid air with 80% relative humidity. This important feature of POF was applied to fabricate a simple optical humidity sensor with fast response and high sensitivity [19]. POF humidity sensors usually experience cross-sensitivity to temperature which has led to the emergence of dual parameter POF sensor. Zhang et al. [20] reported a compact optical fibre temperature and humidity sensor fabricated with both glass and polymer fibre Bragg gratings (FBGs). The operation of this dual parameter sensor is based on the Bragg wavelength shift recorded by the optical fibres when place in humid air. The response of the FBG sensors to humidity in the range of 50 – 95% at temperature 25 o C is shown in Fig. 2. A sensitivity of 35.2 ± 0.4 pm/ % was obtained for the POF sensor using linear regression (illustrated in Fig. 2). Similarly, the silica fibre showed some sensitivity to humidity, amounting to just 0.28 ± 0.01 pm/ %, probably due to the FBG being coated with polymer following inscription [10]. The temperature response of the two FBG sensors at a constant 50% humidity is also shown in Fig. 3. The sensitivities obtained by linear regression are 13.9 ± 0.3 pm/ o C for the silica FBG and -55 ± 3 pm/ o C for the POF FBG. The negative slope in the temperature response of POF is caused by the dominance of the contribution of the thermo-optic effect to the wavelength shift over that due to thermal expansion. Fig. 2 Humidity response of FBG sensors [20]. Fig. 3 Temperature response of FBG sensors [20]. NURS 2010 CONFERENCE PROCEEDINGS – Dairo and Soge pp 70-75 73 2.2 POF in Communications Cost, downtime and bandwidth consuming applications have always determined the use of communication systems. The high-capacity, low attenuation, electromagnetic interference immunity and light weight are the major contributors to the robust deployment of glass optical fibre in communications including cable television, CATV [21-22]. Glass multimode fibres have been optimised to support high throughput transmission at 850 and 1300nm wavelength [23]. Optical fibre in communication ensures high-performance, availability of a wide range of services and ease of management [24]. However, these benefits are not enjoyed in most cases by users where copper cable bridges the last communication miles. Polley et al. [25] have shown that the low-cost POF can economically and efficiently replace the present coaxial cable or twisted pair networks in homes and offices because it is easy to install [26]. High attenuation recorded in the order of 20dB for POF compared to 2.5dB for GOF at 850nm has been a major drawback in its wide application in telecommunication. It is noteworthy that optical coupling efficiency, which is very important, most especially for telecommunication applications, between the light source and fibre length, noted for POF is high over GOF, cost and installation time inclusive. New Cyclic Transparent Optical Polymer (CYTOP) with 15dBkm is available at 1300nm; Koike [27] noted that the calculated attenuation threshold (for the telecommunication window 850 and 1550nm) is not reached yet, due to extrinsic losses introduced during fabrication. In some cases, a maximum length of 300m within a building has been adopted. 3.0 Conclusion This paper has shown that POF application in communication is not exhaustive as well as in sensing. The prevailing need and requirements of a system under consideration will determine the application of POF features. However, it is clear that the cost advantage of POF over silica has made POF available for many experimental purposes and low cost designs. References [1] J. Zubia and J. Arrue, “Plastic Optical Fibers: An Introduction to Their Technological Processes and Applications,” Opt. Fiber Technol., vol. 7, pp. 101 – 140, 2001. [2] G. Keiser, Optical Fiber Communications. New York, USA: McGraw-Hill, 1991. [3] D. Kalymnios, “Plastic Optical Fibres (POF) in Sensing – Current Status and Prospects,” Proc. of 17 th International Conference on Optical Fibre Sensors, SPIE, vol. 5855, pp. ….. [4] P. J. Scully, D. Jones and D. A. 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NURS 2010 CONFERENCE PROCEEDINGS – Dairo and Soge pp 70-75 74 [11] A. Gaston, I. Lozano, F. Perez, F. Auza and J. Sevilla, “Evanescent Wave Optical-Fiber Sensing (Temperature, Relative Humidity, and pH Sensors),” IEEE Sensors Journal, vol. 3, No. 6, December 2003. [12] C. M. Tay, K. M. Tan, S. C. Tjin, C. C. Chan and H. Rahardjo, ”Humidity Sensing using Plastic Optical Fibers,” Microwave and Optical Technology Letters, Vol. 43, No. 5 December 2004 [13] E. Ito, J. Muramatu, T. Kanazawa, E. Nihei, Y. Koike, U. Yagi, T. Sobukawa and K. Takada, “Plastic Optical Fibre Thermosensor,” 3rd int. Conf. POF application and Applications, Yokokama (Japan), pp. 52-55, 1994. [14] P. J. Scully, R. Chandy, R. Edwards, E. Lewis, D. F. Merchant, R. Morgan, N. F. Schmitt and F. H. Zhang, “Plastic optical fibre sensors for environmental monitoring,” in Proc. 5th International Conference on Plastic Optical Fibres and Applications-POF’96, Paris(France), pp. 28-29, 1996. [15] K. Asada, and H. Yuuki, “Fiber Optic Temperature Sensor,” in Proc. Third International Conference on Plastic Optical Fibres and Applications-POF’94, Yokohama (Japan), pp. 49-51, 1994. [16] K. Asada, H. Yuuki and H. Hattori, “Application of POF in Temperature Sensors,” in proc. 4th int. Conf. on POF and Applications-POF’95, Boston, pp. 152-156, 1995. [17] K. T. V. Grattan and D. Kalymnios, “Fibre Optic Temperature Measurement – The possibilities with POF,” 7th Int. Plastic Optical Fibres Conf. ’98, Berlin, pp. 163-170, 1998. [18] K. H. Mertins and A. Meidinger, ‘‘Remote object sensor using plastic optical fibers and thick optical waveguides,’’ in Proc. Fourth International Conference on Plastic Optical Fibres and Applications_POF’95, Boston MA, pp. 137-139, 1995. [19] S. Muto, O. Suzuki, T. Amano and M. Morisawa, “A Plastic Optical Fibre Sensor for Real-time Humidity Monitoring,” IOP Journal of Measurement Science and Technology, vol. 14, No. 6, 2003 [20] C. Zhang, W. Zhang, D. J. Webb and G. D. Peng, “Optical Fibre Temperature and Humidity,” IET Electronics Letters, vol. 46, No. 9, April 2010. [21] H. H. Lu, C. L. Ying, W. I. Lin, Y. W. Chuang, Y. C. Chi, and S. J. Tzeng, “CATV/ROF transport systems based on light injection/optoelectronic feedback techniques and photonic crystal fiber,” Opt. Commun., vol. 273, pp. 389–393, 2007. [22] C. H. Chang, T. H. Tan, H. H. Lu, W. Y. Lin, and S. J. Tzeng, “Repeaterless hybrid CATV/16-QAM transport systems,” Prog. Electromagn. Res. Lett., vol. 8, pp. 171–179, 2009. [23] C. Lethien, C. Loyez , J. Vilcot, L. Clavier, M. Bocquet and P. A. Rolland, ” Indoor coverage improvement of MB-OFDM UWB signals with radio over POF system,” Optics Communication, 282, pp. 4706–4715, 2009. [24] J. M. Senior, Optical Fiber Communications: Principles and Practice, 3rd ed. Englewood [25] A. Polley, P. J. Decker, J. H. Kim, and S. E. Ralph, “Plastic optical fiber links: A statistical study,” presented at the Optical Fiber Communications, San Diego, CA, 2009. [26] A. Polley, P. J. Decker, and S. E. Ralph, “10 Gb/s, 850 nm VCSEL based large core POF links,” presented at the Conf. Lasers and Electro-Optics, San Jose, CA, 2008. Cliffs, NJ: Prentice-Hall, pp. 7–10, 2009. [27] Y. Koike, “Status of POF in Japan” Int. Conf. POF’96, Paris, October 22-24, pp. 1-8, 1996. Biographies O. F. Dairo is currently an Assistant Lecturer at the Redeemer’s University, Nigeria. He graduated in 2004 from Obafemi Awolowo University, Nigeria, with a BSc (Hons) in Electronic and Electrical Engineering. He obtained PgD. Communication Engineering and MSc Electronic Instrumentation Systems in 2006 and 2007 respectively from the University of Manchester, UK. His area of research is in Photonics applications and Digital Signal Processing. A. O. Soge is currently a Lecturer in the Department of Physical Sciences, Redeemer’s University, Nigeria. He holds MPhil degree in Electrical and Electronic Engineering from the University of Leicester, UK, and NURS 2010 CONFERENCE PROCEEDINGS – Dairo and Soge pp 70-75 75 BSc (Hons) Physics from the University of Ibadan, Nigeria. His areas of research interests are Photonics, Solid-State Devices and Electronic Instrumentation.