Coupling to circularly asymmetric modes via long-period gratings made in a standard straight fiber Radan Slavı ´k * Institute of Photonics and Electronics, Czech Academy of Sciences, Prague 18251, Czech Republic Received 16 January 2007; received in revised form 13 February 2007; accepted 14 February 2007 Abstract We demonstrate long-period fiber gratings made in a straight standard optical fiber via a CO 2 laser side irradiation that couples light between modes of different azimuthal symmetry. We show that such coupling can be even stronger than that to the azimuthally symmet- ric modes, provided the photoinduced asymmetry in the refractive index change across the fiber cross-section is high enough. This sug- gests that some previously-observed phenomena with CO 2 -written LPGs not observed with other types of LPGs may be attributed to LPG resonances that couple light into the cladding modes with higher azimuthal symmetry. We show it on an example of bend-induced LPG resonance position tuning with the resonance depth maintained constant. Ó 2007 Elsevier B.V. All rights reserved. 1. Introduction Within an optical fiber, a diffraction grating (made as a periodic refractive index variation along the axis of propa- gation) can assist coupling of light between various modes. Most popular are fiber Bragg gratings (FBG), where the coupling is made between the co- and counter propagating light of the fiber core mode [1] and long-period fiber grat- ings (LPGs) [2], where the coupling is made between the two modes propagating in the same direction, typically the fiber core mode and a cladding mode. To get a coupling between the two modes via an LPG, the coupling coeffi- cient j, which is proportional (for a step-index grating) to the field overlap between the core mode of the unper- turbed fiber and the cladding mode of the perturbed fiber, needs to be other than zero. The fiber refractive index distribution across a standard fiber cross-section is circularly symmetric and thus, when considering circularly symmetric (photoinduced) refractive index change, only coupling between modes of the same azi- muthal symmetry is allowed [2]. However, most currently- used methods for writing diffractive structures in optical fibers are based on a side-illumination [3], in which certain level of azimuthally asymmetric refractive index change is inevitable. When the refractive index is changed only within the tiny fiber core (e.g., by a UV illumination of a fiber with UV-photosensitive core, which is the case for most cur- rently-made gratings in optical fibers), the core mode field profile changes only slightly, resulting in a small birefrin- gence [3]. At the same time, the field profile of the cladding mode remains almost unchanged, as only a small fraction of its power propagates through the fiber core. The slight bire- fringence causes that the LPG resonance dip observed for the light propagating along the two axes of birefringence have slightly different resonant wavelengths. Let us now consider methods that cause the refractive index changes across the entire fiber cross-section – this can be observed, e.g., when bending the fiber with an in-written LPG [4,5] or when considering a side-inscribed LPG with a method that changes the refractive index in the entire fiber cross-sec- tion. The last-mentioned example could occur for LPGs induced mechanically [6], written with an arc [7] or CO 2 laser irradiation [8]. It is worth noting that CO 2 laser was reported to form LPGs in two ways. For lower CO 2 dose, the local heat releases the stress (causing a change in the refractive index through the elasto-optic effect) that was induced during drawing of the fiber that consists of two 0030-4018/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2007.02.064 * Tel.: +420 266773515; fax: +420 28468 0222. E-mail address: slavik@ufe.cz. www.elsevier.com/locate/optcom Optics Communications 275 (2007) 90–93