A Hybrid Highly Birefringent Fiber Optic Sensing System for Simultaneous Strain and Temperature Measurement

A novel hybrid fiber optic sensor based on integration of polarimetric fiber sensors for simultaneous strain and temperature measurement is presented in this paper. Two types of polarimetric fibers are used: a side-hole fiber and a polarization maintaining photonic crystal fiber. The disadvantage of each sensor type can be overcome by using the sensors in a complementary manner to measure temperature independently from average strain. Highly birefringent (HB) polarization-maintaining (PM) fiber-based fiber-optic sensors are a new generation of sensors known as polarimetric fiber sensors which utilize polarization (phase) modulation within fibers to sense external perturbations [1]. HB polarimetric sensors can be made temperature insensitive but to measure strain they require means for setting a zero strain reference. Polarimetric optical fiber sensors based on highly birefringent polarization-maintaining fibers have attracted great interest during the last decade [1, 2]. In HB fibers, the difference between the phase velocities for the two orthogonally polarized modes is high enough to avoid coupling between these two modes. Fibers of this type have a built-in, well-defined, high internal birefringence obtained by designing the core and/or cladding with a noncircular (mostly elliptical) geometry, or by using anisotropic stress induced by stress elements running along the length of the fiber. The modal behaviour of the lowest-order mode HB fibers under various external deformations is of special interest for sensors and device applications. A number of physical quantities can be measured by sensors based on HB fibers: hydrostatic pressure, strain, vibration, temperature, acoustic waves, etc. A symmetric deformation effect (X) influences the propagation constant β in every mode because of the changes in the refractive indices of the core and the cladding. In a single-mode regime, this leads to the changes in the phase L ⋅ ∆ = ∆Φ β (where L is the fiber * E-mail: lesiak@if.pw.edu.pl length) between both polarizations of the fundamental LP01 ( ) X L X L X ∂ ∆ ∂ + ∂ ∂ ∆ = ∆Φ ) ( β β δ δ mode along the fiber [1]: , (1) where X stands for temperature (T), pressure (p) or longitudinal strain (ε) defined as: L L / ∆ = ε . The effect of longitudinal strain on mode coupling is to modulate the relative phase retardation between the two orthogonal polarizations in the LP01 mode. The general formula describing the birefringence sensitivity to strain can be expressed in terms of an experimental parameter Tε describing the amount of strain ε required to induce a 2π phase shift of polarized light observed at the output as [3]: L Tε π ε β ε β 2 ) ( 0 + ∆ = ∆ , (2) where 0 L β ∆ signifies the unperturbed modal (polarization) birefringence of a fiber. Under the influence of longitudinal strain the first term on the right-hand side of expression (1) is negligible with respect to the second, so that: ( ) ( ) ( ) ∂ ∆ ∂ ∆ ∆Φ ≅ = ∂ ∂ L L β β δ δε δ ε ε . (3) Hence the phase changes of the polarimetric responses are proportional to the absolute elongation δL and are independent of the length L of the sensing region. Under the influence of longitudinal axial strain, equation (3) can be approximated with the use of equation (2) in terms of the experimental parameter Tε only: ( ) ( ) 2 ∂ ∆ ∆Φ ≅ ⋅ ∂ sgn T ε δ π δε ε β . (4) The schematic of the hybrid sensing configuration is shown in Fig. 1. The phase change versus strain sensitivity of the polarimetric sensors is measured using a A hybrid highly birefringent fiber optic sensing system for simultaneous strain and temperature measurement Piotr Lesiak, Ginu Rajan, Yuliya Semenova, Gerald Farrell, Anna Boczkowska, Andrzej Domański, and Tomasz Woliński Faculty of Physics, Warsaw University of Technology, Warsaw, Poland; Photonics Research Centre, Dublin Institute of Technology, Dublin, Ireland; Faculty of Materials Science and Engineering, Warsaw University of Technology, Warsaw, Poland; 4 Faculty of Electronics and Information Technology, Warsaw University of Technology, Warsaw, Poland. Received September 28, 2010; accepted September 29, 2010; published September 30, 2010 doi: 10.4302/plp.2010. 3.15 PHOTONICS LETTERS OF POLAND, VOL. 2 (3), 140-142 (2010) http://www.photonics. pl/PLP © 2010 Photonics Society of Poland 141 polarimeter. A laser source with a wavelength of 1550nm is used as the input source. The fabricated composite material sample was 250mm-long, 35mm-wide and 2.5mm-thick. Both types of HB fibers are embedded in the same layer of the composite material (see Fig. 2). In the case of a side-hole fiber, both strain and temperature changes lead to a phase difference between both polarizations of the fundamental mode, while in the case of the HB PCF the phase difference originates only from longitudinal strain and is insensitive to temperature [4]. Thus by comparing outputs of the polarimetric sensors of both types, average strain and temperature can be obtained simultaneously. Fig. 1. Schematic of a hybrid sensing system in the intensity domain. To introduce strain in the composite material sample, the sample is subject to bend deformation by applying a force in the middle region of the sample (Fig. 3). The stress-induced (ε) elongation of the sample as a function of deflection is given by the following formula: