Controlling the state of polarization (SOP) of optical signals in optical communication systems is an important and challenging problem that has been the subject of research for many years. The control of SOP is required in many applications such as polarization mode dispersion (PMD) compensation, polarization-division multiplexing (Pol-Mux) transmission, coherent detection, and the like. The birefringence of optical fiber supports two polarization modes, each having different propagation velocities, giving rise PMD. PMD is a limiting factor for high speed optical communication systems, i.e. 10 Gb/s and beyond. For a first order approximation, the PMD of fiber can be described by the parameters of differential group delay (DGD) and principal states of polarization (PSPs). An input optical pulse whose state of polarization (SOP) is aligned with one of the fiber's two input PSPs comes out of the fiber without distortion. An input optical pulse aligned with neither input PSP emerges as two orthogonally polarized pulses, separated in time by the fiber's DGD. Both DGD and PSPs of optical fiber vary randomly with time due to environmental factors and the like. Optical PMD compensators have been proposed to mitigate the effects of PMD. These compensators usually include polarization controller(s) and fixed or variable DGD component(s). A control loop is usually set up to control the polarization controller to minimize the PMD effect based on some error signal.
Polarization can be used as a degree of freedom of an electromagnetic field. Polarization multiplexing can double the capacity of a wavelength channel and spectral efficiency by transmitting two optical signals at the same wavelength with orthogonal SOPs. At the end of the transmission line and before the receiver, the two optical signals with orthogonal SOPs need to be de-multiplexed with a polarization splitter. Because the SOPs of the optical signals after fiber transmission are random and time varying, polarization control is needed before the polarization splitter to transfer the SOPs of signals to the required SOP by the polarization splitter. Coherent detection is a scheme that allows a complete retrieval of both amplitude and phase of optical signals by mixing the input optical signal with a local optical signal called an oscillator before optical detection. Therefore high receiver sensitivity can be achieved with coherent detection. Traditional coherent detection requires a match of SOPs between the signal and the local oscillator, which again requires the control of SOP.
All these applications of the polarization controlling demand the feature of “endless control”, which means the optical components acting on the SOP have to be unlimited. When the SOP of the input signal drifts in one direction monotonically, the polarization controller should be able to track the SOP evolution without discontinuity. If reset is needed for the controlling parameters somewhere during the SOP tracking, the reset process should not cause any SOP discontinuity as well. Polarization controlling can be achieved through many tunable mechanically or electrically driven components based on variety of materials or configuration, for example, cascaded electromagnetic fiber squeezers, endlessly rotatable optical retardation plates, liquid crystal polarization rotators or guided-wave electro-optic devices. All of these components can be modeled as wave plates with variable orientation angles and/or variable retardation angles. The basic configurations can be divided into three classes: wave plates with variable orientation angles; wave plates with variable retardation angles (or called linear retarders), and wave plates with both variable orientation angles and variable retardation angles (such as Soleil-Babinet compensators). Regarding the polarization controlling speed, the fastest polarization controlling device so far is the integrated electro-optic controller based on x-cut, z-propagation lithium niobate which includes a cascade of integrated polarization transform stages, each acting as a variable retardation wave-plate with adjustable orientation angle.
There is a problem referred to herein as Glitch that is a loss of polarization tracking due to a dithering algorithm failure that can occur in certain configurations of a polarization controlling device. There exists a need for polarization control systems and methods utilizing an algorithm that can provide “endless control” without ever losing polarization tracking.
USPTO Backgrounds
USPTO Backgrounds