Abstract: The effects of polarization dependent hole burning and/or polarization dependent loss are reduced by modulating the state of polarization (SOP) of an optical signal being launched into the transmission path periodically between first and second states of polarization of at least one pair of orthogonal states of polarization. Ideally, the launched modulated signal should spend equal time intervals in both states of the orthogonal pair. In one exemplary embodiment, the SOP is modulated such that it moves along a great circle on the Poincare sphere. In another exemplary embodiment, the SOP is modulated such that it traces a complete great circle on the Poincare sphere. In a preferred embodiment, a complete great circle is traced at a uniform speed on the Poincare sphere.

Abstract: The effects of polarization dependent hole burning and polarization dependent loss are reduced by modulating the state of polarization (SOP) of an optical signal being launched into an optical transmission path periodically between first and second states of polarization of at least one pair of orthogonal states of polarization. Preferably, the SOP is modulated at a rate that is substantially higher than 1/t.sub.s, where t.sub.s is the anistropic saturation time of the optical amplifier. Ideally, the state of polarization of the launched optical signal should be modulated such that it traces a complete great circle on the Poincare sphere. In addition, the effects of polarization dependent loss are further reduced by controllably selecting the particular great circle being traced on the Poincare sphere.

Abstract: An on-line technique which allows the monitoring and adjustment of switching node control parameters within an optical network. More specifically, a technique wherein a low-frequency modulation is induced upon an optical signal as it passes through a node within a switch or sub-system by varying the control voltage applied to that node about what is assumed to be the optimum control voltage level, the optical signal at position downstream from the node is analyzed to ascertain the amplitude, frequency, and phase characteristics of the induced low-frequency modulation, and a determination is made, on the basis of the amplitude, frequency, and phase information, as to the whether the control voltage which was assumed to be optimal is indeed at the correct level to insure proper operation of the node. The amplitude of the induced low-frequency modulation may be maintained at low level so as not to interfere with the transmission of the primary information and data carried by the optical signal.

Abstract: A performance monitoring technique which allows faults within an optical communication system to be detected and located. More specifically, a technique employing a signal generator to controllably insert maintenance signals at the input and output of each module within an optical communication system, and a single monitor to receive the inserted maintenance signals at a fixed point within the system, and, in response, determine the location of faulty modules. The monitor analyzes the received inserted maintenance signals, and locates faults by pinpointing modules from which a maintenance signal inserted at the module output is received uncorrupted by the monitor, but which return a corrupted maintenance signal when the insertion is made at the module input. A particular embodiment of the invention enables the maintenance signals to be inserted via pre-existing control drivers within system modules.

Abstract: Wide optical bandwidth and broad wavelength tuning range are achieved in a reset-free, optical, automatic polarization controller by combining three controllable fractional wave elements in cascade and further by controlling the orientations of both outermost fractional wave elements to differ by a prescribed angular amount which is maintained substantially constant. The prescribed angular amount is defined to be between 0 and 2.pi., inclusively. Synchronous control of both outermost fractional wave elements maintains the prescribed angular difference to be maintained constant during operation of the polarization controller. The three fractional wave elements are described as an endlessly rotatable half-wave element and two synchronously rotatable quarter-wave elements wherein the half-wave element is placed between the quarter-wave elements. Each fractional wave element varies the orientation of linear birefringence along its optical wavepath and introduces a specified phase retardation.

Abstract: Cascadability and simplicity of design and operation are primary attributes of a novel electrooptic polarization transformer for reset-free endless polarization control that allows general polarization transformations from any arbitrarily varying input and into any arbitrarily varying output polarization by producing adjustment elliptical birefringence of constant total phase retardation in a single-mode waveguide. Here, a particular transformation is obtained by adjusting the azimuth of linear birefringence and the ratio of linear to circular birefringence. In this integrated-optic realization, the endless polarization transformer includes at least one cascadable transformer section comprising a first TE.rarw..fwdarw.TM mode converter followed by a first TE/TM phase shifter followed by a second TE.rarw..fwdarw.TM mode converter and a second TE/TM phase shifter. The sections are formed over a birefringent waveguide capable of supporting propagation of TE and TM optical signal modes.