Lucent Technologies Inc.

Fitel USA Corporation

Lynn E Nelson

Robert Meachem Jopson

Herwig Werner Kogelnik

James Power Gordon

Thomas Andrew Strasser

Andrew Roman Chraplyvy

In accordance with the invention, an optical fiber transmission system is provided with multiple order PMD compensation to provide enhanced compensation at high bit rates and across a range of frequencies. Specifically, PMD is compensated by a concatenated series of components, each component configured to compensate for the effects of a successively higher order term of a PMD Taylor series approximation. Advantageously, each component comprises a polarization controller and a differential dispersion element of specified order. In an exemplary embodiment, the first order differential dispersion element can be a standard differential group delay (DGD) element. The element of second order can be a differential group-velocity dispersion element, and the third order element can be a differential dispersion slope element. These differential dispersion elements of various orders can be fixed or tunable elements in different embodiments.

Multiple order PMD compensator for optical fiber transmission system

In one method, two light signals, of the same optical frequency, but having orthogonal states of polarization, are transmitted through an optical device and the mean signal delay of each of the light signals is measured. Calculations, based upon disclosed relationships, provide the polarization-independent delay (&tgr; ) through the optical device based upon the mean signal delays (&tgr; and &tgr; ) of each of the light signals. By comparing &tgr; at adjacent wavelengths, the chromatic dispersion of the optical device can be accurately measured even in the presence of PMD. In a second, similar method, four light signals of non-degenerate polarizations states that span Stokes space are utilized. In a modification of the above-described methods based on the measurement of pulse delays, the methods are adapted to the measurement of phase delays of sinusoidally modulated signals.

Chromatic dispersion measurement

Four different light signals, all of the same optical frequency, but, having different states of polarization, are transmitted through an optical device and the mean signal delay of each of the light signals is measured. Calculations, based upon the relationship, &tgr; &equals;&tgr; &minus;&frac12; {right arrow over (&OHgr;)}&middot;{overscore (s)}, describing the polarization dependence of &tgr; (a measured mean signal delay) through the device as a function of &tgr; (a polarization independent delay component of the device), {right arrow over (&OHgr;)} (the PMD vector at the device input) and {overscore (s)} (the input Stokes vector of the light signal), yield the PMD of the device. Also, by comparing data taken at adjacent wavelengths, the chromatic dispersion of the optical device can be accurately measured even in the presence of PMD.

Polarization mode dispersion measurement

In an optical fiber transmission system, higher order PMD compensation is realized with a sweeper device at the input to the fiber which converts the polarization of the light beam into a frequency dependent polarization whose rate of change is similar to the rate of change of one of the PSPs of the fiber. The frequency dependent polarization of the light beam is then aligned with one of the frequency-dependent PSPs at the input of the fiber. Furthermore, differential group delay dispersion for a given frequency can be reduced by employing a chromatic dispersion compensator prior to the receiver end of the fiber transmission system. Control of the polarization of the light beam can be facilitated by monitoring PMD in the system, or alternatively, monitoring an effect of PMD in the system, such as bit error rates.

Method and apparatus for compensating for higher order PMD in a fiber transmission system

In an optical fiber transmission system, higher order PMD compensation is realized at the output of the transmission fiber. A compensator is placed at the output of the transmission fiber whose PMD vector is equal in magnitude and opposite in direction to the PMD vector at the output of the transmission fiber. The compensator PMD vector sweeps at a rate that matches the frequency sweep rate of the PMD vector at the fiber output. To compensate for second order PMD while avoiding the introduction of higher order PMD effects, a planar sweep is advantageously employed. A polarization pair controller is employed in advance of the compensator to align the PMD vector at the compensator input with the PMD vector at the fiber output so that the two cancel as well as to align the rotational axis of the compensator PMD vector with the rotational axis of the fiber PMD vector. The system may also include a monitoring device to monitor the compensation of fiber PMD, to determine the need for adjustments to the system.

Method and apparatus for compensating for higher order paid PMD in a fiber transmission system at the fiber output

The present invention consists of a method and apparatus for measuring first and higher order PMD vectors in optical fibers. For each first-order PMD vector determination, two distinct polarization states are sequentially injected into an optical device under test for each of a pair of frequencies &ohgr; and &ohgr;&plus;&Dgr;&ohgr; . A Stokes vector s representing the first polarization state must not be parallel or anti-parallel to a Stokes vector s representing the second polarization state, but the relative angle between s and s need not be known. The frequency interval &Dgr;&ohgr; is large to obtain a high signal-to-noise ratio. Thus, four light beams are injected, the first at frequency &ohgr; and polarization s , the second at frequency &ohgr; and polarization s , the third at frequency &ohgr;&plus;&Dgr;&ohgr; and polarization s , and the fourth light at frequency &ohgr;&plus;&Dgr;&ohgr; and polarization s . The output polarizations of these beams are measured and four corresponding output Stokes vectors are determined. A first-order PMD vector is then calculated from the four input Stokes vectors and four output Stokes vectors. Each subsequent PMD vector determination is then made using a pair of frequencies which differs from the previous pair of frequencies by a frequency interval &Dgr;&ohgr; which is small compared to &Dgr;&ohgr; . Since &Dgr;&ohgr; can be small, high spectral resolution of the PMD vector can be obtained, while still maintaining good signal-to-noise ratio provided by using a large &Dgr;&ohgr; . The good signal-to-noise ratio and high spectral resolution of the PMD vector allow the accurate determination of second- and higher-order PMD vectors.

Method for measurement of first-and second-order polarization mode dispersion vectors in optical fibers

Resonances in cross-phase-modulation (CPM) impairment occur in wavelength-division-multiplexed (WDM) transmission systems, having multiple equal dispersion-length amplifier spans, when channels have an integral number of bit walk-throughs per amplifier span. The CPM resonances are reduced by mis-matching the amplifier span lengths (and/or dispersion in each span) so an integral number of bit walk-throughs do not occur in successive amplifier spans. The CPM resonances are reduced by adding different lengths of dispersion-compensating fiber to each span, using different modulation bit rates and/or clock phase delay for each channel, and using different wavelength-selective clock phase delays for each channel.

Wavelength division multiplexed system having reduced cross-phase modulation

An apparatus and method for simultaneously taking measurements that are used for determining PMD vectors. This reduces the time interval over which all measurements are taken and reduces inaccuracy caused by PMD variation during the time measurement interval. The apparatus and method may be used in conjunction with any technique for calculating PMD, such as the Poincare Sphere Technique or Jones Matrix Eigenanalysis. The apparatus simultaneously produces multiple light beams. To distinguish each light beam from the others, each beam is given a distinct modulation. All the beams are then combined and passed through the optical device under test. A polarization measuring device then measures the output polarization of the combined beam and outputs one or more composite electrical signals that describe the Stokes components of the output polarization of the combined beam and that have the same modulations present in the original combined beam. Using modulation filters having the same modulations as given to each original light beam, electrical signals may be extracted from the composite electrical signals that describe the Stokes components of the output polarization corresponding to each original light beam. Also, measurement and control of the frequency difference between light beams used for determining PMD vectors is improved by receiving light beams of two distinct frequencies using a detector that then outputs a signal having a frequency equal to the difference in frequencies of the light beams. In addition, a feedback loop is used to better control the frequency difference between two light sources. Also, a frequency shifter is used to generate two light beams separated by a specific frequency interval.

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Monmouth Beach, NJ, United States of America

Lynn E Nelson