Investigations of bending loss oscillations in large mode area photonic crystal fibers Tadeusz Martynkien* a,b , Jacek Olszewski a , Waclaw Urbanczyk a , Tomasz Nasilowski b , Francis Berghmans b , and Hugo Thienpont b a Institute of Physics, Wroclaw University of Technology, W. Wyspianskiego 27, 50-370 Wroclaw, Poland b Vrije Universiteit Brussel, Pleinlaan 2, 1050 Brussels, Belgium ABSTRACT We demonstrate that bending loss in large mode area photonic crystal fiber (LMA PCF) reveals a strong dependence of the amplitude and location of the loss peaks on the bend radius and on the fiber angular orientation. To do so we measured bending induced loss in LMA PCF as a function of wavelength in a broad spectral range (800-1500 nm) for different bend radii and for different angular orientations of the fiber with respect to the bending plane. The oscillations of bending loss with the bend radius and orientation are particularly well pronounced when the radiative component of the fundamental mode is reflected from the flat boundary of the holey cladding. We also show the good agreement between our measurement results and earlier simulations relying on a finite element method with perfectly matched layers and equivalent index model. Keywords: photonic crystal fibers, bending loss. 1. INTRODUCTION Photonic crystal fibers (PCFs) are well known to exhibit particular properties compared to conventional optical fibers 1 . This report focuses on one of these properties and more specifically on the wavelength dependence of macro-bending loss as already previously investigated 2-10 . PCFs exhibit a short and long wavelength bending loss edge. The short wavelength bending loss edge is a consequence of the dispersion properties of the holey cladding. Recent numerical studies of bending loss relying on a finite element method with perfectly matched layers and equivalent index model 7-9 , revealed that real PCFs with finite micro-structured cladding dimensions exhibit properties similar to conventional double-clad fibers 11-14 and therefore give rise to periodic oscillations of the bending loss at the short wavelength loss edge. Reports on bending loss measurements in index guiding PCFs 2-6,10 did however not focus on these oscillations. The authors only mentioned that the spectral characteristics of the bending loss were affected by “some noise” for certain radii 3 . Furthermore, the bending loss in LMA-11 5 and in ESM-5 PCF 10 showed clearly visible but unexplained oscillations with wavelength. In this contribution, we present the results of a systematic measurement of bending loss in large mode area PCF (LMA- 20). We measured the loss in the spectral range of 0.8-1.6 µm as a function of bending radius and for different angular orientations of the fiber with respect to the bending plane. Our results show that both the depth and the period of the loss oscillations depend significantly on the angular orientation of the fiber. 2. MEASUREMENT METHOD We measured bending loss in large mode area PCF (LMA - 20) produced by Crystal Fibre A/S. The geometrical parameters of this fiber were as follows: pitch Λ = 13.2 µm, fill factor d/Λ = 0.485, and 7 layers of holes surrounding the core. We characterized the bending loss properties of the investigated fibers by recording the induced attenuation in 180° * tadeusz.martynkien@pwr.wroc.pl, Photonic Crystal Fibers II, edited by Kyriacos Kalli, Waclaw Urbanczyk, Proc. of SPIE Vol. 6990, 69900K, (2008) 0277-786X/08/$18 · doi: 10.1117/12.781493 Proc. of SPIE Vol. 6990 69900K-1 2008 SPIE Digital Library -- Subscriber Archive Copy