Optical: systems and elements – Deflection using a moving element – Using a periodically moving element
Reexamination Certificate
2000-03-03
2003-06-10
Negash, Kinfe-Michael (Department: 2733)
Optical: systems and elements
Deflection using a moving element
Using a periodically moving element
C359S199200, C359S199200, C359S199200
Reexamination Certificate
active
06577413
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates generally to systems for polarization demultiplexing within optical transmission systems, and specifically to systems for polarization demultiplexing a higher-speed multiplexed optical signal into lower-speed polarized tributary signals using lower-speed electro-optics to measure the tributary autocorrelation value for use in optimally demultiplexing the higher-speed signal.
In the field of optics and optical transmission systems, multiplexing different data streams for transmission within a system is common. Typically, two or more lower-speed tributary signals are combined or multiplexed together in time slots to form a higher-speed multiplexed signal. For example, two 10 Gbit/sec (lower-speed) tributary signals may be bit-interleaved or multiplexed in alternating time slots to form a 20 Gbit/sec (higher-speed) multiplexed signal. In this way, a single optical path can support transmission of data from multiple sources to multiple receivers.
One way to increase the transmission capacity of such a system is to use optical time domain multiplexing (OTDM) and optical time domain demultiplexing (OTDD). However, the multiplexed transmission capacity of many optical transmission systems is typically limited by the speed of available electro-optics. Multiplexing lower-speed tributary signals into a higher-speed multiplexed optical signal and then demultiplexing the tributary signals out again usually requires wideband electro-optics capable of running at the higher-speed of the multiplexed signal. As the speed of the multiplexed signal increases, the availability of electro-optics that operate at this increased speed unfortunately diminishes and can be a problem for optical transmission system designers.
Applicant has observed that a problem with most OTDD techniques is that they require gating signals at the same or higher-speed as the multiplexed signal. In the previously mentioned example, a gating signal of at least 20 GHz is usually required to handle demultiplexing two 10 Gbit/sec optical tributary signals from a 20 Gbit/sec multiplexed signal. This may be cost prohibitive or impractical as the speed of the multiplexed signal increases.
Furthermore, many existing demultiplexing methods are polarization sensitive, such as techniques using LiNbO
3
modulators or conventional four-wave mixing (FWM) to demultiplex higher-speed signals. In these systems, wideband or high-speed driving signals and high-speed electro-optics are often still required to effectively demultiplex the tributary signals from the multiplexed signal.
Patents and publications have described general polarization multiplexing and demultiplexing of tributary signals within optical transmission systems. For example, in an article authored by F. Heismann, P. B. Hansen, S. K. Korotky, G. Raybon, J. J. Veselka and M. S. Whalen entitled “Automatic Polarization Demultiplexer for Polarization-Multiplexed Transmission Systems” and published in Proceedings, Vol. 2 of 19
th
European Conference on Optical Communication, published on Sep. 12, 1993 (hereinafter “the Heismann article”), the authors describe multiplexing two orthogonally polarized optical signals into a single fiber and then demultiplexing them using an automatic polarization demultiplexer. More particularly, the Heismann article describes using a polarization transformer in combination with a simple polarization splitter within the demultiplexer.
Additionally, in an article authored by M. L. Dennis, I. N. Duling III, and M. F. Arend entitled “Soliton Loop Mirror Demultiplexer with Polarization-Multiplexed Signal and Control” and published in Optical Fiber Communication '96 Technical Digest Series, Vol. 2 on Feb. 25, 1996 (hereinafter “the Dennis article”), the authors generally describe a nonlinear optical loop mirror-based demultiplexer using orthogonally polarized signals and control streams while operating in the soliton regime. The Dennis article further states that polarization multiplexing of control and signal ensures a high ON/OFF extinction ratio while allowing single wavelength operation.
SUMMARY OF THE INVENTION
In accordance with the invention as embodied and broadly described herein, in one aspect, an apparatus is described for polarization demultiplexing a multiplexed optical signal into optical tributary signals within the context of an optical transmission system. In general, the apparatus includes a polarization beam splitter for separating one of the optical tributary signals from the other optical tributary signals. Upon receiving the multiplexed optical signal, separation into tributaries is based upon a polarization relationship. This relationship is preferably an orthogonal relationship between the tributaries in order to provide low crosstalk on the receiving end. Typically, the polarization beam splitter has a first output providing one optical tributary signal and a second output providing another optical tributary signal.
The apparatus also includes a feedback unit which receives one of the optical tributary signals. The feedback unit is optically coupled to an input of the polarization beam splitter. The feedback unit adjusts a polarization state of the multiplexed optical signal based upon an autocorrelation value of one of the optical tributary signals. The autocorrelation value is measured by the feedback unit. Adjustments are typically made to the polarization state depending upon an extinction ratio calculated using autocorrelation values for the signal.
Additionally, the feedback unit typically includes an autocorrelator and a polarization adjustment device. The autocorrelator has input optically coupled to one of the optical tributary signals and provides the autocorrelation value of that optical tributary signal on a low-speed output. The polarization adjustment device has an optical input for receiving the multiplexed optical signal, an optical output coupled to the input of the polarized splitter, and a control input coupled to the low-speed output of the autocorrelator. In this configuration, the polarization adjustment device can adjust the polarization state of the multiplexed optical signal --.--.
The polarization adjustment device may be operative to maximize the autocorrelation value by adjusting the polarization state of the multiplexed optical signal. The polarization adjustment device may also determine an extinction ratio of one of the optical tributary signals based upon the autocorrelation value and maximize the extinction ratio by adjusting the polarization state of the multiplexed optical signal.
The polarization adjustment device typically includes a processing unit and a polarization controller. The processing unit is coupled to the low-speed output of the autocorrelator and provides a feedback signal on its feedback output based upon the autocorrelation value. The polarization controller has an optical input for receiving the multiplexed optical signal, an optical output coupled to the input of the polarized splitter, and a feedback input coupled to the feedback output of the processing unit. In this configuration, the polarization controller can adjust the polarization state of the multiplexed optical signal based upon the value of the feedback signal.
The processing unit may be operative to maximize the autocorrelation value by altering the feedback signal provided to the polarization controller so that the polarization controller can responsively sweep the polarization state of the multiplexed optical signal over a portion of a Poincare sphere. Furthermore, the processing unit may be further operative to determine an extinction ratio of one of the optical tributary signals based upon the autocorrelation value and to maximize the extinction ratio by altering the feedback signal to sweep the polarization state of the multiplexed optical signal over the portion of the Poincare sphere.
In another aspect, a polarization multiplexed optical transmission system is described. In general, the system includes a polarization multiplexer, a high-speed
Finnegan Henderson Farabow Garrett & Dunner L.L.P.
Negash Kinfe-Michael
Pirelli Cavi e Sistemi S.p.A.
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