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International Journal of Engineering Research and Applications (IJERA) is an open access online peer reviewed international journal that publishes research and review articles in the fields of Computer Science, Neural Networks, Electrical Engineering, Software Engineering, Information Technology, Mechanical Engineering, Chemical Engineering, Plastic Engineering, Food Technology, Textile Engineering, Nano Technology & science, Power Electronics, Electronics & Communication Engineering, Computational mathematics, Image processing, Civil Engineering, Structural Engineering, Environmental Engineering, VLSI Testing & Low Power VLSI Design etc.

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  1. 1. Ajay Kumar et al Int. Journal of Engineering Research and Applications ISSN : 2248-9622, Vol. 4, Issue 2( Version 5), February 2014, pp.04-08 RESEARCH ARTICLE OPEN ACCESS Optimize Etching Based Single Mode Fiber Optic Temperature Sensor Ajay Kumar*, Dr. Pramod Kumar**, Sachin Kumar** *(Department of ECE, World College of Technology and Management, Gurgaon,) ** (Department of ECE, World College of Technology and Management, Gurgaon,) ***(Department of ECE, World College of Technology and Management, Gurgaon,) ABSTRACT This paper presents a description of etching process for fabrication single mode optical fiber sensors. The process of fabrication demonstrates an optimized etching based method to fabricate single mode fiber (SMF) optic sensors in specified constant time and temperature. We propose a single mode optical fiber based temperature sensor, where the temperature sensing region is obtained by etching its cladding diameter over small length to a critical value. It is observed that the light transmission through etched fiber at 1550 nm wavelength optical source becomes highly temperature sensitive, compared to the temperature insensitive behavior observed in un-etched fiber for the range on 30ºC to 100ºC at 1550 nm. The sensor response under temperature cycling is repeatable and, proposed to be useful for low frequency analogue signal transmission over optical fiber by means of inline thermal modulation approach. Keywords - Optical fiber, temperature sensor, wet etching. I. INTRODUCTION Fiber of glass and plastic have used for the communication of light carrier signal from one end to another, this is basic of theoretical fiber optic communication system. In recent years the optical fibers have also been found for application as sensor and have become interest to researchers because of their high sensitivity, immunity to electromagnetic interference and ease of operation in harsh environments. Temperature sensing using fiber optics is a commonly investigated problem which has wider applications. Temperature sensor based on optical attenuation in side-polished optical fiber has been reported [1]. The use of a reference liquid with known temperature dependent refractive index characteristics, in a small part of fiber cladding has been demonstrated for similar application [2]. Temperature sensing through absorption of evanescent field in optical fiber is another reported technique [3]. Splicing two different core fibers to make a reflective mirror, has been demonstrated to work as an intrinsic Fabry-Perot fiber-optic temperature sensor [4]. The other techniques reported for temperature sensing are based on interference of selective higher-order modes [5] and liquid-core optical fiber with small core-cladding refractive index difference [6]. Resonance wavelength shift due to evanescent field [7, 8] in side polished single mode fiber has also been used for temperature sensing. Fiber-optic temperature sensor using polarization maintaining D fiber etched in dilute hydrofluoric acid and immersed in immersion oil [9], show temperature dependent fiber insertion loss, which provide a measure of surrounding temperature. This paper presents the fabrication of single mode fiber optic temperature sensor using controlled wet etching of fiber cladding in hydrofluoric acid. The SMF-28 fiber from corning, USA is used for this work. Transmission loss in etched fiber to varying diameters is measured at 1550 nm Newport source. It is observed that the temperature sensitivity of fiber is highest at a critical diameter obtained after etching. Further the diameter from this critical value, lesser is the temperature sensitivity of the sensor. II. OPTIMIZE ETCHING PROCESS FOR SINGLE MODE FIBER Wet etching of optical fiber in hydrofluoric acid is a simple technique, reported for making fiber sensors. The wet etching technique though simple, but is practically full of technical challenges. The primary reason being small cladding dimension of single mode fiber, which require precise control over wet etching during the sensor fabrication process and require precaution over breaking due to its flexibility. Below figure-1 shows a block diagram which is used for the fabrication of the SMF based sensors. Initial fabrication step goes by removing some portion of SMF plastic jacket (upto1 cm) with refractive index of core and cladding 1.4682 and 1.4629 respectively, after then it is fixed on a glass slide (area 7.5cm X 2.5cm) and placed above the temperature controller which is used for maintaining the constant temperatures. 4|P age
  2. 2. Ajay Kumar et al Int. Journal of Engineering Research and Applications ISSN : 2248-9622, Vol. 4, Issue 2( Version 5), February 2014, pp.04-08 Figure 1: Fabrication setup for etched fiber sensor On the exposed region of the SMF 40% concentrated hydrofluoric acid is placed for etching of the SMF at different temperature and by maintaining the constant temperature (25, 35, 45, 55, & 65ºC) we etched number of fiber with different time period to achieve different core- clad dia of the fiber as explain in figure-2. Figure 3: Etching Rate of SMF with changing temperature In the plot in figure 2 we have find how the etching rate of the SMF response to the varying temperature which is explained in figure-3 showing exponential response with varying temperature described as 0.031T α(T) = 0.865e III. ETCHED SINGLE MODE FIBER AS SENSOR A schematic diagram of etched SMF is shown in figure-4 in which DCl is the cladding and DCo is the core diameter it shows how the exposed core-clad region has became after the etching of the SMF. Figure 2: Etched diameter of SMF with Time at constant temperature. The fiber diameter in etched portion is measured under the Olympus optical microscope and plotted as a function of etching time for various etch temperatures. All the measured etched diameters (DE) at a constant hotplate temperature (T) follow a linear variation in etching time (t) as described by the equation below DE (T) = α(T) .t + α(T) Here α(T) and β(T) are the etch rate (µm/min) and fiber diameter at t = 0 (intercept on y axis) respectively, for given temperature. At room temperature the etch rate for fiber is relatively slow, typically 1.69 µm/min, which rapidly increases to 6.62 µm/min at 65oC (a) Schematic of etched fiber region. (b) Fiber image after etching Figure 4: Sections of etched and un-etched portions of single mode fiber Using the above mention process for fabrication of SMF sensor, we have tested many sensor and find that etched fiber with core and thin clad (upto 1 to 3µm, as shown in figure-4 ) responded highly by interacting with the external medium, producing change in the net optical power of the fiber. 5|P age
  3. 3. Ajay Kumar et al Int. Journal of Engineering Research and Applications ISSN : 2248-9622, Vol. 4, Issue 2( Version 5), February 2014, pp.04-08 The fiber sensor is placed in the constant heat zone of a Carbolite furnace with one port connected to 1 mW, 1550 nm Agilent laser source (HP-8153A) and, other port connected to optical power meter, as shown in figure 5. The furnace is programmed to vary from 35oC to 130oC with 3 o C/minute ramp rate. Figure 7: Temperature sensitivity dependence on etched fiber diameter. Figure 5: Experimental setup for Temperature sensing The corresponding monitored optical power is recorded using the automated experimental set-up. It is observed that the etched region surrounded by air acts as a reliable temperature sensing device. The temperature dependent transmitted optical power variation in an un-etched and an 11.3 μm diameter etched fiber are shown in figure 6. A linear temperature dependent power variation is observed in etched fiber with 3.2 μW/oC sensitivity, compared to relatively temperature insensitive behavior in unetched fiber. The sensitivity drops to less than 1.5 μW/oC for diameter smaller than 8.5 μm and greater than 13.5 μm. Our experimental results highlight the high temperature sensitivity region between „a‟ and „b‟, shown by a Gaussian fit of data points and, the low temperature-sensitivity region beyond „b‟ in figure 7. The „a‟ and „b‟ are 7.5 μm and 13.5 μm respectively. The temperature sensitivity (ST) is found varying as a Gaussian function of etched diameter and described by 𝐒 𝐓 = 𝟑. 𝟕𝟖𝟗 × 𝐞− 𝐝 𝐞𝐜𝐥 −𝟏𝟏.𝟐 𝟐 𝟐 . Measured data points in the low temperaturesensitivity region fit in a power (P) equation described by P = 108.3 × d ecl-1.87 IV. RELIABILITY OF THE SENSOR The fiber sensor with 11.2 μm etched diameter is sealed inside 8 cm long borosilicate glass capillary having 8 mm outer diameter, as shown in figure 8. Figure 6: Plot between optical power and Temperature Figure 8: Etched fiber temperature sensing region inside a sealed glass tube Measurements performed on all the fabricated fiber sensors show similar behavior with varying temperature sensitivity. The highest temperature sensitivity 3.8 μW/oC has been observed for 11.2 μm etched fiber, as shown in Figure 7. (a) 6|P age
  4. 4. Ajay Kumar et al Int. Journal of Engineering Research and Applications ISSN : 2248-9622, Vol. 4, Issue 2( Version 5), February 2014, pp.04-08 behavior of an un-etched fiber. It is also observed that the sensitivity rapidly falls to 1/e of its peak value when the fiber diameter is 11.2 ± 2 μm. The sensor response under temperature cycling is repeatable and, proposed to be useful for low frequency analogue signal transmission over optical fiber by means of inline thermal modulation approach VI. (b) Figure 9: (a) Measured temperature inside the environmental chamber. (b) Optical response of sensor under periodic temperature changes The sensor is kept inside an environmental chamber (ESPEC, USA) interfaced to Optical component environmental test system (OCETS from JDS, Canada) and programmed to undergo temperature cycle from 30oC to 38oC. The transmitted optical power is observed to follow the temperature cycle of the chamber, as shown in Fig. 9(a) and 9(b), except the nonlinear sensor response in the upper and bottom portions of curve. The sealed sensor produces repeatable results but with small reduction in temperature sensitivity. The cubical chamber measuring 8 feet3 in volume is fitted with a thermocouple at fixed location. The chamber takes some time to attain uniform temperature, as measured by the thermocouple. This causes a delayed response from the sensor, particularly in the constant temperature cycle zone. V. CONCLUSION As from the above sections we have shown that etching of the SMF using hydrofluoric acid for achieving desired diameter of the fiber sensor can be done by controlling the etching process with time and temperature which allows us to save time, rather etching to time duration of hours we can etch it in minutes. Maintaining a constant temperature value for a constant time gives us the desired combine coreclad diameter of the SMF. For the purpose we have etched number of fiber at constant temperature and time. We have demonstrated that the effect of temperature in the etching process makes the etching rate to follow exponential path for the temperature range 0° to 70°C without using any real time monitoring of the power. We have fabricated the single mode optical fiber based temperature sensor using this etching technique. Our measurements demonstrate the temperature sensitivity dependence on etched fiber diameter. The sensor shows 3.8 µW/oC as the highest sensitivity for 11.2 µm etched diameter in comparison to 0.2 µW/oC for 25 µm diameter and, relatively temperature insensitive ACKNOWLEDGEMENTS I sincerely thank Dr. Anuj Bhatnagar, Scientist-E, SAMEER, IIT campus, Mumbai for their extreme Guidance and valuable time throughout my research. REFERENCES Journal Papers: [1] Yonghao Xu, Xianfeng Chen and Yu Zhu, “High sensitive temperature sensor using a liquid-core optical fiber with small refractive index difference between core and cladding materials”, Sensors, 8, pp. 18721878, 2008. [2] Saurabh Mani Tripathi, Student Member, OSA, Arun Kumar, Member, OSA, Emmanuel Marin, and Jean-Pierre Meunier, Member, IEEE, Member, OSA,” SidePolished Optical Fiber Grating-Based Refractive Index Sensors Utilizing the Pure Surface Plasmon Polariton”, Journal Of Lightwave Technology, Vol. 26, No. 13, July 1, 2008. [3] Enbang Li, Xiaolin Wang, Chao Zhang, “Fiber optic temperature sensor based on interference of selective higher-order modes”, Appl. Phys. Lett. 89, 091119, 2006. [4] Wei Liang, Yanyi Huang, Yong CSU, Reginald K. Lee, and Amnon Yariv,”Highly sensitive fiber Bragg grating refractive index sensors”, Applied Physics Letters 86, 151122 2005. [5] Sameer M. Chandani and Nicolas A. F. Jaeger, “Fiber-Optic temperature sensor using evanescent fields in D fibers”, IEEE Photon. Technol. Lett., vol.17, no. 12, pp.2706-2708, 2005. [6] J. Senosiain, I. Diaz, A. Gaston, and J. Sevilla, “High sensitivity temperature sensor based on side-polished optical fiber”, IEEE Trans. on Instrum. and Meas., vol. 50, no. 6, pp. 1656-1660, Dec. 2001. [7] Woo-Hu Tsai, Chun-jung Lin, “A novel structure for the intrinsic Fabry-perot fiberoptic temperature sensor”, J. of Lightwave Technol., vol. 19, no. 5, pp 682-686, 2001. [8] Woong-Gyu Jung, Sang-Woo Kim, KwangTaek Kim, Eung-Soo Kim, and Shin-Won 7|P age
  5. 5. Ajay Kumar et al Int. Journal of Engineering Research and Applications ISSN : 2248-9622, Vol. 4, Issue 2( Version 5), February 2014, pp.04-08 [9] [10] [11] Kang, “High sensitivity temperature sensor using a side-polished single mode fiber covered with the polymer planar waveguide”, IEEE Photon. Technol. Lett., vol. 13, no. 11, pp. 1209-1211, Nov. 2001. C. Gaffney and C. K. Chau, Am. J. Phys. 69, 821 (2001). Alberto Alvarez-Herrero, H. Guerrero, T. Belenguer, and D. Levy, “High-sensitivity temperature sensor based on overlay on side-polished fibers”, IEEE Photon. [12] [13] Technol. Lett., vol. 12, no. 8, pp. 1043-1045, Aug. 2000. Giovanni Betta and Antonio Pietrosanto, “An intrinsic fiber optic temperature sensor”, IEEE Trans. on Instrum. and Meas., vol. 49, no. 1, pp. 25-29, 2000. L. Falco, H. Berthou, F. Cochet, B. Scheja, and O. Parriaux, “Temperature sensor using single mode fiber evanescent field absorption”, in Proc. SPIE, vol. 586, pp. 114-119, 1986. 8|P age