Optical waveguides – Polarization without modulation
Reexamination Certificate
1999-12-16
2001-12-11
Schuberg, Darren (Department: 2872)
Optical waveguides
Polarization without modulation
C385S028000, C359S199200, C359S199200, C359S199200, C359S199200
Reexamination Certificate
active
06330375
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to optical transmission systems and more particularly relates to dealing with so-called polarization mode dispersion in such systems.
BACKGROUND OF THE INVENTION
Polarization Mode Dispersion (PMD) occurs in an optical fiber as a result of a small residual birefringence that is introduced in the fiber core by asymmetric internal stress as well as random polarization coupling due to external forces acting upon the fiber. It is well-known that PMD may severely impair the transmission of a signal in an optical fiber network. This is especially the case in modern digital lightwave systems that operate at bit rates of at least 10 Gb/s per transmitted wavelength channel.
U.S. Pat. No. 5,930,414, which issued Jul. 27, 1999 to D. A. Fishman et al, discloses different apparatus for mitigating signal impairments due to PMD. Such apparatus employs a variable optical birefringence element which introduces a differential optical time delay between at least two selectable mutually orthogonal polarization states, i.e., the “Principal States of Polarization” (PSP) in the transmission fiber. An optical distortion analyzer coupled to the output of the variable birefringence element generates a control signal for that element.
The compensation apparatus shown in
FIG. 4
of the referenced '414 patent generates a continuously variable Differential Group Delay (DGD) between two principal states of polarization to compensate for first-order PMD in the transmission fiber. One difference between this arrangement and the time-delay line shown in
FIG. 1
of the referenced '414 patent is that the PSPs of the variable birefringence element in the apparatus of
FIG. 4
of the '414 patent is frequency dependent. Such frequency dependence (which is an important aspect of a so-called second-order PMD effect) may lead to excessive distortion in the optical signal, since the first-order PMD in the fiber can only be compensated for over a certain optical bandwidth. Moreover, the PMD may even increase in certain parts of the optical spectrum.
Signal distortion caused by the second-order PMD effect in conventional non-return-to-zero (NRZ) and return-to-zero (RZ) digital optical signals may be observed in the electrical spectrum of the received optical signal as a narrowband “tone” of spectral energy at the certain frequencies related to the signal bit rate, e.g., 10 GHz for NRZ and 20 GHz for RZ signals.
SUMMARY OF THE INVENTION
We decrease substantially the distortion caused by second-order PMD effects in a variable birefringence element of PMD compensation apparatus by modifying the control signal applied to the variable birefringence element such that the optical signal experiences minimal second-order PMD. In particular, the control signal generated by the distortion analyzer of
FIG. 2
or
8
of the above-referenced '414 patent is modified by subtracting from the control signal in a weighted fashion the output signal of a bandpass filter tuned to the narrowband tone outputted by the distortion analyzer. The resulting modified signal, V
f
, is then supplied to the polarization controllers in the variable birefringence element to reduce the level of the distortion that occurs as result of the aforementioned second order effect. The modified V
f
, more particularly, causes the PMD compensator to operate at a point where the overall second-order distortions in both the transmission fiber and PMD compensator are minimal. The inventive feature particularly orients the PSPs of the compensator to offset any second-order PMD distortion that may be generated by the transmission fiber. That is, the second-order effect is subtracted from, rather than added to, the distortion produced by the transmission fiber.
These and other aspects of our invention are set forth in the following detailed description, corresponding drawings and ensuing claims.
REFERENCES:
patent: 5473457 (1995-12-01), Ono
patent: 5930414 (1999-07-01), Fishman et al.
patent: 6130766 (2000-10-01), Cao
Fishman Daniel A.
Heismann Fred L.
Wilson David L.
Assaf Fayez
Lucent Technologies - Inc.
Mendelsohn Steve
Schuberg Darren
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