At&t Bell Laboratories

Lucent Technologies Inc.

At+t Corp.

Fred L Heismann

Mohammad Taghi Fatehi

Steven K Korotky

Daniel A Fishman

David L Wilson

Robert L Rosenberg

Rodney C Alferness

Edmond J Murphy

John J Veselka

Glenn D Bergland

A distortion analyzer for generating a distortion signal based on an input signal. For example, the distortion signal may be generated by adding a first signal corresponding to the entire spectrum of the input signal to a second signal corresponding to a low-frequency component of the input signal and then subtracting from the result of addition a third signal corresponding to a specific frequency band of the input signal. The effect of polarization mode dispersion that an optical signal experiences as it propagates through an optical transmission fiber may be compensated for at a receiver using a birefringent compensator, in which the distortion analyzer continuously examines the signal outputted by the compensator and supplies the distortion signal to the compensator, which causes the compensator to reduce the effect of such polarization mode dispersion based on the level of the distortion signal.

Distortion analyzer

The effect of polarization mode dispersion that an optical signal experiences as it propagates through an optical transmission fiber is compensated for at a receiver using a birefringent compensator, in which a distortion analyzer repeatedly examines the signal outputted by the compensator and causes the compensator to reduce the effect of such polarization mode dispersion based on the results of such examination.

Distortion analyzer for compensation apparatus of first order polarization mode dispersion (PMD)

An optical device having a first substrate having a fast optical axis and a slow optical axis and a first end and a second substrate having a fast optical axis and a slow optical axis and a second end, the second end is positioned adjacent to the first end such that the fast optical axis of the first substrate is coupled to the slow optical axis of the second substrate and the slow optical axis of the first substrate is coupled to the fast optical axis of the second substrate, thereby permitting an optical signal to pass between the first and second substrates. The first substrate and the second substrate have substantially equal length to thereby substantially eliminate polarization dispersion.

Polarization dispersion compensation for optical devices

A double-periodic grating is described for obtaining polarization-independent filtering and coupling. The grating may be viewed as a combination of two gratings with slightly different grating periods. The two periods are carefully chosen to match the difference in propagation constants of the TE and TM polarization modes such that both modes are coupled at the same wavelength. A novel method of obtaining a double-periodic grating is described.

Polarization-independent optical wavelength selective coupler

An interferometric modulator comprising a substrate of titanium-doped lithium niobate (Ti:LiNbO.sub.3) having top and bottom surfaces and a waveguide formed in the top surface is disclosed. A conductive layer is formed on the bottom surface of the substrate and electrically coupled to a top-surface ground plane or simply to ground potential. The bottom surface conductive layer functions to thermally stabilize the modulator during operation.

Electrooptic device structure and method for reducing thermal effects in

The effects of polarization dependent hole burning and/or polarization dependent loss are reduced by modulating the state of polarization (SOP) of an arbitrarily polarized optical signal being launched into the transmission path periodically through a predetermined sequence of polarization states. The sequence of polarization states is selected such that on average the launched modulated signal excites substantially all possible polarization states with substantially equal probability, independent of the input SOP to the polarization modulator. In one exemplary embodiment, the SOP is modulated by a combination cascade of a rotating half-wave plate followed by a quarter-wave plate, which rotates at a different angular speed. In another exemplary embodiment, the SOP is modulated by a combination cascade of two variable phase retarders oriented at fixed angles of 0.degree. and 45.degree., where the modulation frequencies of the two retarders are substantially different from each other.

Apparatus and method employing polarization modulation to reduce effects

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.

Apparatus and method employing fast polarization modulation to reduce

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. In one example, the particular great circle being traced is selected such that a predetermined parameter, for example, the signal to noise ratio, of the optical information signal being received at the remote end of the optical transmission network is maintained at a prescribed value, e.g., a maximum value. In an embodiment of the invention, the great circle is traced at a uniform speed such that the launched optical signal spends equal time intervals in both states of any pair of orthogonal states of polarization on the selected great circle on the Poincare sphere.

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. Analysis of the amplitude, frequency, and phase of the modulated signal may be performed at a single fixed location within the network. Furthermore, multiple switching nodes may be monitored simultaneously from a single location within the network by modulating each node at a unique frequency. This allows each modulated signal received to be differentiated from all others, and readily identified as having originated at a particular node.

Automatic on-line monitoring and optimization of optical network

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. This allows optical losses with the system to be minimized by eliminating the need for introduction of additional optical couplers or attenuators into the system.

Performance monitoring and fault location for optical equipment, systems

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. Embodiments of the polarization controller are realized using either distributed bulk optic devices or integrated electrooptic waveguide devices. Rotation of the elements is afforded by a feedback control circuit which monitors the output optical polarization and derives appropriate electrical drive signals to achieve the proper rotation of the elements.

Automatic polarization controller having broadband, reset-free operation

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.

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Freehold, NJ, United States of America

Fred L Heismann