Testing polarization-dependent gain of erbium-doped fiber amplifiers

Abstract

EDFAs are broadband optical amplifiers, which are increasingly replacing conventional optoelectronic regenerators. In high-speed data transmission they have to be designed for low polarization dependence to prevent a rise in the bit error rate. Long links like undersea or transcontinental cables contain many EDFAs, this way multiplying their polarization dependencies. Therefore, EDFAs have to be characterized carefully for polarization mode dispersion (PMD) and polarization dependent gain (PDG). In this paper, the PMD measurement is achieved with a modified Stokes vector method. In this method, the polarization controller is used to generate a number of input polarization states, all of which arelocated on an unit circle on the the North Pole of Poincare sphere. For each state, a small wavelength step is applied. Using the Polarization Analyzer, the arc on the Poincare sphere, which is caused by changing the wavelength, is determined. The arc is known to vary sinusoidally with the input polarization state. The widest arc is the measure for the PDG. The measuring system includes HP8164A Tunable Laser, HP8509B Polarization Analyzer, HP 8169A polarization controller and HP8153A Power Meter etc. Four dedicated polarization states (0° lin., 45° lin., 90° lin., circular) are set and the power without device-under-test (DUT) is measured. Then the EDFA is inserted and the power is measured again. This delivers the first row of the Mueller-Matrix from which the PDG can be calculated.