This part of IEC 61280 provides uniform methods of measuring polarization mode dispersion (PMD) of single-mode installed links. An installed link is the optical path between transmitter and receiver, or a portion of that optical path. These measurements may be used to assess the suitability of a given link for high bit rate applications or to provide insight on the relationships of various related transmission attributes. The principles of this document are aligned with those of the optical fibre and optical fibre cable test method, IEC 60793-1-48 (see Bibliography), which focuses on aspects related to the measurement of factory lengths. Instead, this document focuses on the measurement methods and requirements for measuring long lengths that might be installed – and that might also include other optical elements, such as amplifiers, DWDM components, multiplexers, etc. PMD is a statistical parameter. The reproducibility of measurements depends on the particular method, but is limited also by the PMD level of the link. Gisin [3]5) derived a theoretical limit to this reproducibility, by assuming an infinite range of measured wavelengths and ideal measurement conditions. NOTE 1 Test methods for factory lengths of optical fibres and optical fibre cables are given in IEC 60793-1-48. NOTE 2 Test methods for optical amplifiers are given in IEC 61290-11-1 and IEC 61290-11-2. NOTE 3 Test methods for passive optical components are given in IEC 61300-3-32. NOTE 4 Guidelines for the calculation of PMD for links that include components such as dispersion compensators or optical amplifiers are given in IEC 61282-3. With the exception of Method D, all methods in this document may be used to measure the PMD in the gain band of links that include pumped optical amplifiers. For these links, amplified spontaneous emission (ASE) noise can generate depolarized spectral energy in the neighbourhood of the measurement wavelength. This will, in general, reduce the accuracy of the measurement. For Methods A, B, C, E and F, this effect can be moderated by implementing an optical or electrical filter at the receive end. However, optical filtering will not remove the ASE right under the signal spectrum. The accuracy will then be limited by a lower degree of polarization (DOP), if the spectral width of the filter cannot be sufficiently reduced as with a broadband source. Lower DOP may require the signal to be integrated longer to be meaningful or the result will become too noisy and interpretation will be erroneous. None of the methods is suitable for measuring the PMD of links with polarization dependent loss (PDL) in excess of 10 dB. Links with PDL values less than 1 dB can be measured with reasonable accuracy. Measurement accuracy may be compromised by the presence of PDL in excess of 1 dB.