ABSTRACT
Single-mode optical ˜bers employed in long-distance telecommunication systems usually do not preserve the state of polarization (SOP) of the launched optical signal. ™us, the SOP of an optical signal emerging from a ˜ber-optic cable is usually unknown and may even ¤uctuate randomly with time [1]. Optical components at the receiving end of a ˜ber-optic link are therefore required to operate with all possible polarization states. However, certain optical elements, such as polarization demultiplexers or polarization-mode dispersion compensators [2,3], are inherently sensitive to the signal’s SOP. If such an element is employed at the receiving end of a ˜ber, then the signal emerging from the ˜ber most likely needs to be transformed into the preferred input SOP of the polarization-sensitive element. ™is transformation may be accomplished by inserting an adjustable polarization controller before the polarization-sensitive element [4]. Moreover, if the input SOP to the controller ¤uctuates randomly with time, the polarization transformation needs to be continuously adjusted to compensate for the changes in the input SOP. Such adaptive transformation of a time-varying input SOP into a ˜xed output SOP is known as automatic polarization stabilization [5]. Figure 24.1 shows a typical setup for an automatic polarization stabilization system, in which the polarization controller is adjusted via a feedback signal received from an optical monitor detector that is connected to the output of the polarization-sensitive element [6].
