Spun Fiber Bragg Grating Sensors with Strong Resistance to Transverse Pressure

Fiber-Bragg-grating (FBG) based sensors, whose reflection wavelengths vary with the measurands such as temperature and strain, have all the advantages attributed to fiber-optic sensors. In addition, they can be easily wavelength-division-multiplexed in a single fiber, and are free from the normal fiber attenuation [1]. A traditional optical fiber is not ideally symmetric in its cross section; meanwhile, an external perturbation, such as transverse pressure and macro-banding, may further enhance the fiber asymmetry. As a result, an optical fiber always exhibits certain polarization dependence. In an FBG, laterally nonuniform refractive index distribution can form a certain polarization-dependent loss (PDL). These make traditional FBG sensors polarization-sensitive [2]. In many sensing applications, where polarized tunable lasers are adopted, a strong PDL may deteriorate the interrogation accuracy of FBG sensors. When a transverse pressure is further applied onto FBG temperature or longitudinal strain sensors, the FBGs may exhibit significant PDL, and severe distortions of FBG reflection spectra can even cause a failure of measurement. Spun fibers have attracted much attention in the fields of both optical communications and sensing [3,4]. It has been found that the FBGs imprinted in the spun fibers with millimeter spin periods and certain intrinsic birefringence have distinctive properties from those made of the conventional single-mode fibers [5,6]. In this work, we compared the influences of transverse pressure on FBGs written in Corning SMF-28 fiber and in our specially designed spun fibers. It is shown that the spun FBG sensors have stronger resistance to transverse pressure than the traditional FBG sensors.