Polarization dependent loss of mechanically induced long-period fibre gratings G. Rego * UOSE, INESC-Porto, R. Campo Alegre 687, 4169-007 Porto, Portugal Escola Superior de Tecnologia e Gesta ˜o-IPVC, 4900-348 Viana do Castelo, Portugal Received 12 June 2007; received in revised form 14 August 2007; accepted 31 August 2007 Abstract We present results on the measurements of polarization dependent loss (PDL) of mechanically induced long-period fibre gratings. It is shown that a proper choice of the fabrication parameters leads to a considerable reduction of PDL values. Ó 2007 Elsevier B.V. All rights reserved. Keywords: Long-period fibre gratings; Mechanically induced; Polarization dependent loss 1. Introduction Mechanically induced long-period fibre gratings (MLPFGs) have been attracting the fibre optic community due to their simplicity, flexibility and inexpensiveness [1,2]. MLPFGs can be performed in any kind of singlemode fibres [2–4] or even in holey fibres [5,6] without the need to remove its coating, and their attenuation loss can be controlled in real time what makes them very promising for spectral filtering and gain equalization of fibre amplifi- ers [7–9]. However, the MLPFGs are intrinsically polariza- tion sensitive and exhibit polarization mode dispersion (PMD) due to linear birefringence caused by external pres- sures and/or microbending. These properties may depend on a particular fabrication technique and despite the huge number of proposed techniques only few authors have characterized the produced gratings in terms of the intrin- sic polarization dependent loss (PDL). In this paper, we present results on PDL measurements of MLPFGs produced by winding a string around a fibre/ grooved tube set. We have investigated the influence of the fabrication parameters on gratings PDL. Some modifica- tions of the setup were introduced and their effect on PDL was addressed. A simple birefringence compensation method is also discussed. 2. MLPFGs fabrication Gratings were induced in the Corning SMF-28 fibre using the technique described in [2]. The MLPFGs were fabricated placing the fibre on top of a cylindrical tube with grooves separated by 0.6 mm and having a depth of 0.15 mm. Afterwards, a 0.25-mm diameter nylon string under tension, caused by a mass of 41.8 g, was wound around the fibre/tube set 70 times. Fig. 1a shows the trans- mission spectrum of the grating as a function of the exter- nal loads applied to it. The effect of loading a MLPFG performed in an uncoated fibre is shown in Fig. 1b. The general behaviour observed, i.e., the increase of the cou- pling strength with loads increase, may be due to the rein- forcement of the periodic microbending (geometric deformation and stress) induced by pressure [10–12]. As it can be seen for weights above 378 g back-coupling occurs, i.e., the energy is coupled back from the cladding modes to the core mode. It should be noted that to achieve an attenuation loss of 9 dB we need to load the pristine 0030-4018/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2007.08.063 * Address: UOSE, INESC-Porto, R. Campo Alegre 687, 4169-007 Porto, Portugal. Tel.: +351 220402301; fax: +351 220402437. E-mail address: gmrego@fc.up.pt www.elsevier.com/locate/optcom Available online at www.sciencedirect.com Optics Communications 281 (2008) 255–259