Distributed Strain and Temperature Discrimination in Unaltered Polarization Maintaining Fiber Mark Froggatt, Dawn Gifford, Steven Kreger, Matthew Wolfe, and Brian Soller Luna Technologies 3157 State Street Blacksburg, VA 24060 froggattm@lunatechnologies.com Abstract: A Rayleigh scatter-based distributed measurement technique is presented in which strain and temperature discrimination is achieved using standard polarization maintaining fiber as the sensor. High-sensitivity Optical Frequency Domain Reflectometry is used to measure the scatter. 1. Introduction The ability to distinguish between strain and temperature in fiber-optic sensing systems is critical to the large-scale success of any fiber sensing technique. Further, the ability to make numerous, distributed measurements in a single, standard fiber makes optical fiber sensing a practical and financially attractive alternative to conventional sensing methods. In this paper we present a technique that enables strain and temperature measurement and discrimination using a single, polarization-maintaining (PM) fiber. The measurement is performed using instrumentation and fiber that are standard products, readily available on the market. Early results show a temperature resolution of 3.5 C and a strain resolution of about 35 με with a spatial resolution of 2 cm over a potential range of 70 m (3500 sensing locations). Distributed sensing techniques currently exist to discriminate temperature and strain in which multiple fiber Bragg gratings are integrated in a single polarization maintaining (PM) fiber [1,2]. Obtaining cost-effective gratings, however, is a challenge. Manufacturing these gratings in PM fiber without compromising the mechanical integrity of the fiber is a further complication to the process. As a result, obtaining PM fiber with multiple fiber Bragg gratings at the desired locations is a daunting proposition, severely limiting the prospects for this technology, at least until a significant market has been proven. In this paper, we present a Rayleigh scatter-based technique for temperature and strain discrimination. Recent publications have demonstrated that the Rayleigh scatter inherently present in an optical fiber can be used to make distributed temperature or strain measurements using high-resolution, high-sensitivity Optical Frequency Domain Reflectometry (OFDR)[3-5]. The underlying physical principle for the measurement is identical to that of Bragg grating strain and temperature measurement, in which the applied temperature or strain effectively shifts the spectrum reflected from a particular grating, or, in the case of scatter, segment in the fiber. Because the mechanism is so similar, the same approach that can lead to strain and temperature decoupling in PM fiber with Bragg gratings can be used with the Rayleigh scatter approach. The significant advantage here is that standard, unaltered PM fiber can be used to make the measurement. The introduction of commercially available high sensitivity OFDR instrumentation means that both the sensing fiber and the interrogation apparatus required for the Rayleigh-based measurement are readily available, with no special modifications of either fiber or instrument required for application to fiber-optic sensing. The ability to discriminate temperature and strain in PM fiber is made possible by the unique characteristics of the fiber that make it polarization maintaining. Most PM fiber fabrication involves inducing residual stresses in the fiber by constructing the fiber out of materials with significantly different thermal expansion coefficients. A cross section of one common type of PM fiber is shown in Figure 1, where the two off-center regions are the areas of mismatched glass commonly referred to as the stress rods. As the fiber cools after drawing, these rods contract at a different rate than the pure silica glass around them, creating large, built-in stress that causes the index of refraction of the core—the circle in the center—to become polarization dependent. This polarization dependence of the core’s index of refraction is what causes the two polarization states in the fiber to propagate with different wavenumbers. In turn, the difference in wavenumbers prevents the coupling of light from one polarization mode of the fiber to the other. Given the physical basis for birefringence in PM fiber, it should not be surprising that the magnitude of the difference in the refractive index between the two polarization modes is temperature dependent and that, fortunate for this application, this same difference is only weakly affected by applied strain. This distinction in the response of the refractive index difference to temperature and strain enables discrimination between the two stimuli.