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Method and system for equalizing PMD using incremental delay switching
| Details |
Inventors: Robinson, Andrew Niall; Fee, John A.;
Assignee: MCI Communications Corporation ()
Primary Examiner: Negash; Kinfe-Michael
Assistant Examiner:
Attorney, Agent or Firm:
A polarization mode compensation system and method using optical switch elements to establish incremental delays between different polarization modes of an optical data signal is provided. A polarization mode separator separates the optical data signal into first and second orthogonally polarized optical signals. A first variable switching delay element provides a first incremental propagation delay for the first polarized optical signal. A second variable switching delay element provides a second incremental propagation delay for the second polarized optical signal. The first and second variable switching delay elements consist of a series of optical switches optically interconnected by different incremental lengths of optical fiber. For example, 2.times.2 optical switches are provided for switching between a reference fiber segment and a respective delay fiber segment to provide a relative incremental propagation delay. A controller controls optical switches in the first and second variable switching delay elements to set first and second incremental propagation delays. In particular, the first and second polarized optical signals are incrementally delayed relative to one another so as to compensate for polarization mode dispersion. A beam combiner then combines the first and second polarized optical signals to form an optical output data signal which can be detected accurately by a receiver without the effects of polarization mode dispersion. In this way, optical data signals can be reliably transmitted over greater distances along a long-haul fiber optic dispersive medium at even greater bit-rates and bandwidth. |
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DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention is a PMD compensator based upon delay elements that use optical switching to add well-defined increments of delay to either polarization of an optical signal.
The apparatus of the invention is depicted in FIGS.
3A and 3B of the attached drawings.
FIG.
3A shows a typical end-to-end network path.
A transmitter 300 transmits an optical data signal through an optical fiber 310 for detection by a receiver 350.
For example, transmitter 300 can be a DFB laser modulator or any other type of modulated light source for producing an optical data signal.
Optical fiber 310 is a long-haul, single-mode, dispersion-shifted fiber approximately 100 kilometers in length.
More generally, any type of optical fiber can be used.
Additional fibers, line amplifiers, and/or repeaters can also be included between the transmitter 300 and receiver 350.
Receiver 350 can be any suitable photodetector for detecting the modulated optical data signal.
A polarization-mode dispersion (PMD) compensator 320 is provided along optical fiber 310 near the receiver 350.
According to the present invention, the PMD compensator 320 equalizes PMD which further increases bandwidth and transmission range.
For example, given the presence of PMD compensator 320, transmitter 300 can transmit modulated laser data on the order of 1 to 100 Gb/s (gigabits/sec), or more, over a 100 km.
single-mode fiber without symbol confusion.
Receiver 350 can detect the output reliably and accurately in a telecommunication environment.
FIG.
3B shows a block diagram of the components of the PMD compensator 320 in accordance with one embodiment of the present invention.
Optical paths are indicated generally by a loop along the transmission path.
The optical data signal traveling through optical fiber 310 enters beam splitter 322 as optical input 315.
A portion of the optical input 315 is diverted to a delay detector 327.
The delay detector detects delays between two orthogonal polarization modes of the detected light
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Optical Systems 
