Israelsen, Stine Møller

Rottwitt, Karsten

The effect of an aircladding in a double cladding fiber designed to guide higher order modes is examined. For very high order symmetrical modes we find that the circular symmetry of the modes is broken

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Downloaded from orbit.dtu.dk on: Dec 15, 2017Effect of Aircladding on Bessel-Like ModesIsraelsen, Stine Møller; Rottwitt, KarstenPublication date:2013Link back to DTU OrbitCitation (APA):Israelsen, S. M., & Rottwitt, K. (2013). Effect of Aircladding on Bessel-Like Modes. Abstract from Australia andNew Zealand Conference on Optics and Photonics (ANZCOP 2013), Perth, Australia.Effect of Aircladding on Bessel-Like Modes Stine H. M. Larsen1, and Karsten Rottwitt1 1DTU Fotonik, Technical University of Denmark, 2800 Kgs. Lyngby, Denmark shml@fotonik.dtu.dk     The effect of an aircladding in a double cladding fiber designed to guide higher order modes is examined. For very high order symmetrical modes we find that the circular symmetry of the modes is broken. Micro-structured fiber; Higher order modes;  I.  ABSTRACT Higher order modes (HOMs) have recently attracted much attention. Especially Bessel-like modes, the LP0X modes, are interesting due to their properties such as diffraction free propagation and self-healing [1]. We investigate the effect of an air cladding on LP0X modes in a double cladding structure both numerically and experimentally to document the break of circular symmetry caused by aircladding and the impact of this effect on the diffraction free propagation distance.   We excite the HOMs in a double cladding fiber with an aircladding as the outer cladding using a long period grating (LPG) [2]. A microscope image and a sketch of the refractive index are seen in Fig. 1. (a)      (b)  Fig. 1: (a): Microscope image of the fiber. The core is single-moded with a cutoff at 760 nm. The aircladding contains 60 holes. (b): Sketch of the refractive index profile of the investigated fiber.                   Fig. 2: a) Image of LP016 generated with a LPG in the aircladding fiber. The camera is slightly saturated to image the outer rings of the mode. b) An ideal LP016 generated with a scalar mode solver. c) One of two LP016 modes generated with a full vectorial mode solver. We have excited LP016 with 90 % conversion efficiency. An image of the mode is depicted in Fig. 2a. Notice the bowtie shape in the mode which breaks the symmetry of LP0X-mode. This effect is not seen in the ideal solution, see Fig. 2b. The mode in Fig. 2a is one of two eigenmodes in between which one can shift by a slight perturbation of the fiber. The break of circular symmetry is verified with a full vectorial mode solver, see Fig. 2c. The effect is only observed for very high order HOMs. The onset of the effect is found using the full vectorial mode solver at LP010. A second effect observed in Fig. 2a is the ripples in the outer rings; this effect sets in for slightly higher order modes compared to the break of circular symmetry with an onset at LP012. Both effects may be caused by the shape of the aircladding.  The free space propagation of the non-symmetric LP016 and the ideal LP016 is investigated using a non-commercial Fourier propagation routine. The diffraction free propagation distance is defined as the distance where the maximum intensity has dropped to e-1 compared to the intensity at the end facet of the fiber. The simulated free space propagation along the axes indicated in Fig. 2b and Fig. 2c is plotted in Fig. 3.              Fig. 3: (a) Free space propagation simulation of the ideal LP016 which has a propagation distance of 216 µm and (b) of the one found with the full vectorial mode solver which has a propagation distance of 218 µm. In summary, we have shown that the aircladding in a double cladding fiber breaks the circular symmetry of high order LP0X modes. However the break of circulcar symmetry happens without reducing the diffraction free propagation distance. A. Acknowledgement       NKT Photonics is acknowledged for manufacturing the fiber. The group of Professor Siddarth Ramachandran at Boston University is acknowledged for facilitating the manufacturing of the LPGs.  REFERENCES [1] P. Steinvurzel, K. Tantiwanichapan, M. Goto, and S. Ramachandran: “Fiber-based Bessel beams with controllable diffraction-resistant distance,” Optics Letters 36(23), 4671-4673 (2011)  [2] H. G. Park, and B. Y. Kim: “Intermodal coupler using permanently photoinduced grating in two-mode optical fibre,” Electronics Letters 25(12), 797-798 (1989)  

Effect of Aircladding on Bessel-Like Modes

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Effect of Aircladding on Bessel-Like Modes

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