1450 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 21, NO. 19, OCTOBER 1, 2009 Investigating Transverse Loading Characteristics of Microstructured Fiber Bragg Gratings With an Active Fiber Depolarizer Yiping Wang, Senior Member, IEEE, Hartmut Bartelt, Wolfgang Ecke, Kerstin Schroeder, Reinhardt Willsch, Jens Kobelke, Manfred Rothhardt, Ines Latka, and Sven Brueckner Abstract—In this letter, the transverse loading characteristics of Bragg gratings in microstructured optical fibers were investigated by use of an active fiber depolarizer. Increasing transverse load shifts the Bragg wavelength to longer wavelengths; its sensitivity to transverse load decreases with increasing volume of air holes around the fiber core. Such transverse loading characteristics were found to be dependent on the fiber orientations. Index Terms—Depolarizers, fiber Bragg gratings (FBGs), optical fiber sensors, photonic crystal fibers (PCFs). I. INTRODUCTION T HE transverse loading characteristic is an especially interesting sensing property for embedded optical fiber gratings [1]–[4]. Compared to the temperature and tensile strain characteristics of fiber Bragg gratings (FBGs), the transverse loading properties can be expected to be orientation dependent [1]–[3]. Microstructured fibers have been undergoing a rapid development over the past decade due to their unique mi- crostructures. Highly sensitive transverse loading sensors have been presented by inscribing an FBG in side-hole fibers [5]. We recently observed that FBGs in microstructured fibers exhibited an obvious polarization-dependent behavior due to intrinsic fiber birefringence, asymmetric laser irradiation during grating inscription, or/and externally mechanical perturbations [6]. Such polarization-dependent effects will unavoidably result in severe noise and systematic errors in the measurement of the Bragg wavelength [6], [7]. In this letter, we investigate the transverse loading character- istics, especially their sensitivity to fiber orientation, of FBGs in different types of microstructured fibers. An active fiber de- polarizer was developed to reduce the birefringence-induced wavelength fluctuations and improve the accuracy of the mea- sured transverse loading characteristics. Manuscript received March 11, 2009; revised June 14, 2009. First published July 31, 2009; current version published September 18, 2009. This work was supported in part by the Alexander von Humboldt Foundation, in part by the Thuringian Ministry of Education and Cultural Affairs, and in part by Hong Kong Special Administrative Region Government through a PolyU 5182/07E Competitive Earmarked Research Grant. Y. Wang was with the Institute of Photonic Technology, 07745 Jena, Ger- many. He is now with the Department of Electrical Engineering, The Hong Kong Polytechnic University, Kowloon, Hong Kong (e-mail: ypwang@china.com). H. Bartelt, W. Ecke, K. Schroeder, R. Willsch, J. Kobelke, M. Rothhardt, I. Latka, and S. Brueckner are with the Institute of Photonic Technology, 07745 Jena, Germany (e-mail: hartmut.bartelt@ipht-jena.de). Color versions of one or more of the figures in this letter are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/LPT.2009.2028308 Fig. 1. Schematic diagram of active fiber depolarizer. PZT: piezoelectric ce- ramic cylinder, V, kHz, V, and kHz. II. DEPOLARIZER Birefringence in optical fibers with FBGs may cause varia- tions of the Bragg wavelength due to environmentally induced birefringence fluctuations. For the stabilization of such mea- surements, depolarized light should be used, which can be brought about by averaging the state of polarization (SOP) of the source light over a measurement parameter. For broadband illumination, the light can be easily depolarized by means of a well-known passive fiber Lyot depolarizer, for which two cas- caded high-birefringence (Hi-Bi) fibers with a short length are employed to destroy the correlation between the two orthogonal SOPs [8]. However, for our measuring task of narrowband FBG reflection (half-width 0.1 nm), this approach is not practicable because it would require much longer Hi-Bi fibers to achieve effective Lyot depolarization. Therefore, we used an active fiber depolarizer for effectively averaging the SOP of the light over time by means of modu- lating the length of two pieces of Hi-Bi fiber asynchronously (Fig. 1) [7]. The active depolarizer consists of two segments of Hi-Bi fiber with a beat length of 1.2 mm at a wavelength of 632 nm. The two fiber segments were spliced under an angle of 45 between their birefringence axes. The first fiber segment with a length of 6.0 m was wound around a piezoelectric PZT cylinder with a diameter of 32 mm, and the second fiber segment with a length of 8.3 m was wound around another piezoelectric PZT cylinder with a diameter of 44 mm. PZT and PZT were independently driven by two asynchronous sinusoidal sig- nals with peak-to-peak voltages of and , respectively, in order to asynchronously modulate the length of the Hi-Bi fibers. As a result, all SOPs are rapidly swept through, and their effect on the measured Bragg wavelength is effectively averaged in the time domain. Thus, the measured Bragg wavelength fluctuation, resulting from slow distortions/perturbation in a fiber, will be ef- fectively reduced [7], [9]. Here, differences in the fiber length, the PZT diameter, and the applied sinusoidal signals between the two PZT units are to modulate the SOPs in the two Hi-Bi 1041-1135/$26.00 © 2009 IEEE