Viscoelastic Behavior of Polymeric Optical Fiber
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Viscoelastic Behavior of Polymeric Optical Fiber Alejandro Sánchez, Karla Y. Guerra, Andrés V. Porta, Susana Orozco* Departamento de Física, Facultad de Ciencias, Universidad Nacional Autónoma de México, Av. Universidad 3000, Col. Universidad Nacional Autónoma de México, C. U., Delegación Coyoacán, C. P. 04510, D. F., México. * E-mail: [email protected]
ABSTRACT In this work, the viscoelastic behavior of a polymeric step-index optical fiber is studied, and the loss factors of their complex moduli are calculated. The loss factors of the Young and shear moduli were determined from the measurement of the damping ratio of a simple pendulum and a torsion pendulum respectively, using the Kelvin-Voigt model of the viscoelastic theory. The shear and Young complex moduli can be used to study the opticviscoelastic behavior of a polymeric step-index optical fiber.
INTRODUCTION The characterization of mechanical and thermal properties of optical fibers (OF) is essential to determine their sensibility and operation range in their applications as sensors or their stability in severe condition of optical transmission. On the other hand, the effect of thermally and mechanically induced stresses on optical fibers have been extensively studied, since the induced changes on the optical transmission can be used as a measure of an external agent intensity in the sensor technology. Usually the thermal or mechanical behavior of the OF is studied using the elastic theory [1]. Nevertheless, the polymeric optical fibers (POF) have viscoelastic behavior rather than elastic [2]. The characteristics of POF based sensors, being light weight, small in size, easily multiplexable, having high tensile strength and low cost have attracted considerable attention in recent years [3], therefore it is very important to determine the viscoelastic properties of these photonic materials. A viscoelastic material is characterized by several elastic complex moduli; each one has an accumulation term (storage modulus) and a dissipative term (loss modulus). The most widely used methods for determining the complex moduli of elasticity and the loss factor employ forced sinusoidal oscillations [4], and kinematic models based on springs and dashpots (Voigt cells) are commonly used to study the viscoelastic behavior of polymeric materials [2, 5, 6]. The advantage of this model consists of a minimum number of fitting parameters and a reduced complexity of mathematical analysis. In this work, the KelvinVoigt model is used to determine the shear and Young complex moduli of a polymeric stepindex optical fiber in the low frequency limit in which the moduli are independent of the frequency [2]. Input parameters of the model are obtained experimentally from the measurement of the damping ratio and the frequency in a torsion pendulum and a simple pendulum. In these experiments, the optical fiber is subjected to dynamic loading through 131
low frequency oscillations, where the internal friction in the fiber resists the exciting stress. The characterization of visc
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