PDL and DGD Measurements of Mechanically Induced Long-period Fiber Gratings G. Rego 1,2,3 , M. Morais 4 , J. L. Santos 2,5 , H. M. Salgado 2,3 1 Escola Superior de Tecnologia e Gestão – Instituto Politécnico de Viana do Castelo, 4900-348 Viana do Castelo, Portugal 2 INESC Porto – UOSE, Rua Campo Alegre, 687, 4169-007 Porto, Portugal 3 Departamento de Engenharia Electrotécnica e de Computadores, FEUP, Rua Dr. Roberto Frias, s/n 4200-465 Porto, Portugal 4 Instituto Electrotécnico Português, Rua de S. Gens, 3717, 4460-409 Senhora da Hora, Portugal 5 Departamento de Física, Faculdade de Ciências, Universidade do Porto, Rua Campo Alegre, 687, 4169-007 Porto, Portugal Abstract: We have investigated the polarization dependent loss (PDL) and the differential group delay (DGD) of mechanically induced long-period fiber gratings. A birefringence compensation method is also presented. 1. Introduction. Recently, several techniques to mechanically induce long-period fiber gratings (MLPFGs) have been proposed [1]-[3]. These technologies share the simplicity, the flexibility and the inexpensiveness. The MLPFGs can be performed in any kind of singlemode fibers, without the need to remove its coating, and their attenuation loss can be controlled in real time what makes them very attractive for EDFAs gain equalization [3]-[4]. However, the MLPFGs are intrinsically polarization sensitive and exhibit polarization mode dispersion (PMD) due to linear birefringence caused by external pressures [5]. These properties may depend on a particular fabrication technique. In this paper, we present results of further investigations on MLPFGs produced by winding a string around a fiber/grooved tube set [1]. PDL and DGD measurements are also presented and a birefringence compensation method is discussed. 2. MLPFGs Fabrication. Gratings were fabricated placing the fiber 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 fiber/tube set 70 times. Fig. 1 shows the transmission spectrum of the grating as a function of the external loads applied to it. The effect of loading a MLPFG performed in an uncoated fiber is shown in Fig. 2. As it can be seen for weights above 378 g back-coupling occurs, i.e., the energy is coupled to other cladding modes. 1515 1520 1525 1530 1535 1540 1545 1550 1555 -10 -9 -8 -7 -6 -5 -4 -3 -2 -1 0 0 g 192 g 378 g 670 g Transmission (dB) Wavelength (nm) Fig. 1. Transmission spectrum of the grating, induced in a coated fiber, as a function of the applied loads. 1515 1520 1525 1530 1535 1540 1545 1550 1555 -13 -12 -11 -10 -9 -8 -7 -6 -5 -4 -3 -2 -1 0 0 g 192 g 378 g 670 g Transmission (dB) Wavelength (nm) Fig. 2. Transmission spectrum of the grating, induced in an unc oated fiber, as a function of the applied loads.