Endless Tracking Polarization Controller

In response to the need for larger capacities in long-haul optical digital transmission, greater channel density has been achieved by wavelength division multiplexing (WDM), and the bit rate for each channel is also being increased. At 40 Gbps, which is in the next generation of transmission rates, polarization mode dispersion (PMD), which does not present a problem at bit rates of 10 Gbps and below, can impose significant limitations on the distance over which transmission is possible. Further, at the distances served by 10-Gbps transmission, which is currently becoming the mainstream technology, there are concerns that using fibers installed in previous years may, depending on the amount of PMD, cause problems in terms of system configuration. In recent years there have been many programs to study PMD compensators . In PMD compensators, however, changes in distributed birefringence--induced either by increasing the core eccentricity of single-mode fibers or by means of internal stress due to changes in lateral pressure or temperature--give rise to a phenomenon known as differential group delay (DGD), in which two orthogonal polarization modes, which should, in theory, be degenerating in fact diverge. In PMD compensators, a polarization controller is required to track changes in polarization due to distributed birefringence in the transmission path . These changes in polarization are, due to variations in the transmission path environment, random with respect to time, with the result that in order to maintain the compensated state, it is necessary that the polarization controller track continuously all changes in polarization without becoming saturated (referred to in this paper as “endless tracking”). In particular, when a phase shifting device is used, attention must be paid to its control due to the limited range of phase shifting, a problem that has already been investigated . In wave plates that are mechanically rotated or waveguide-type polarization controllers using Lithium niobate (LN), which move in an equivalent manner , it is the optical axis (eigen polarization axis) that is rotated, rather than the phase shift, so that there is no need to pay attention to saturation, but there are problems with the response speed in the former case and optical characteristics in the latter. Phase-shifting devices can all be configured in the fiber, and fiber squeezers having superior optical characteristics have been widely studied in the past , but since they apply direct lateral pressure to the fiber they raise the problem of reliability as an in-line device. Accordingly we report the development of an endless tracking polarization controller using variable Faraday rotators (VFRs), which offers superior optical characteristics and the promise of higher reliability. In the process of device design, it was necessary to investigate two issues: (1) whether the range of phase shifting of the VFR, being less than that of the fiber squeezer, was adequate or not; and (2) the control method and tracking stability. However, it is difficult to investigate these two issues if some of the polarization conversions of the polarization controller are only discussed as examples on the Poincare sphere, as in previously published work. In the present work, we used expression 7) of the rotation group to express the polarization conversion itself, and since we presented a mathematical treatment of the two issues above, we will also report on our methodology. Endless Tracking Polarization Controller