However, the interferometric method is the only method appropriate for measurements of cabled fiber that may be moving or vibrating such as is found in the field. Īll methods use a linearly polarized source at the FUT input and are suitable for laboratory measurements of factory lengths of fiber and cable.Other methods or analyses may exist but they are generally not standardized or are limited in their applications. Three methods are generically used for measuring PMD. Test methods for polarization mode dispersion This maximum DGD may then be used to define the quality of service that would tolerate values lower than this maximum DGD. However, if a known distribution such as the Maxwell distribution may be fit to the DGD distribution probability, then a mean (or average) value of the DGD 〈Δ τ〉 may be correlated to the RMS value and used as a system performance predictor in particular with a maximum value of the DGD distribution associated to a low probability of occurrence. In this case, the spreading can be related to the RMS (root mean square) of DGD values 〈Δ τ 2〉 1/2. For broadband transmission, the DGD statistically varies as a function of wavelengths or frequencies and result in an output pulse that is spread out in the time domain (see Figures 13a–c). On the contrary, the maximum pulse spread due to PMD occurs when both PSPs are equally excited, and is related to the difference in their group delays, the DGD associated with the two PSPs.
PMD COMPENSATION IN OPTISYSTEM BLOCK PSP
These asymmetry characteristics vary randomly along the fiber and in time, lead to a statistical behavior of PMD.įor a deployed cabled fiber at a given time and optical frequency, there always exist two PSPs such that the pulse spreading due to PMD vanishes, if only one PSP is excited.
The environmental conditions may stress the deployed cable and affect the fiber.Įach time the fiber undergoes local stresses and consequently birefringence. The core may present localized clusters of dopants and The core may be subjected to microbending
The core is not perfectly concentric with the cladding Ī real fiber is not perfectly circular.(c) The DGD Δ τ is large enough even at low bit rate to make the pulse spreading and creating impairment. (b) The pulse is spread by the DGD Δ τ but the bit rate is high enough to create an impairment. (a) The pulse is spread by the DGD Δ τ but the bit rate is too low to create an impairment. PMD effect on the pulse broadening and its possible pulse impairment. The optical signal is split into fast and slow components and the fast component is delayed properly before being recombined with the slow component.įigure 13. Traditional electrical equalization after the optical signal is detected can be used for compensation but the highest bit-rate achievable is limited by the speed of electronics. Therefore, the amount of PMD can vary with time for a given link, which means that dynamic compensation is necessary. PMD can originate from the imperfections in the fiber or environmental factors like temperature and mechanical pressure. It is usually manifested as the splitting of a pulse at the receiver even after chromatic dispersion compensation and causes intersymbol interference. Polarization mode dispersion (PMD) means that the group velocity dispersion of the two orthogonal polarizations in the fiber are different and cannot be simultaneously compensated by a single DCF. Chou, in Encyclopedia of Modern Optics, 2005 Polarization Mode Dispersion