Bi-directional all-optical regenerator

Optical: systems and elements – Deflection using a moving element – Using a periodically moving element

Reexamination Certificate

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C359S199200

Reexamination Certificate

active

06317232

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates generally to methods and systems for regenerating optical signals, and more particularly to a bi-directional all-optical regenerator.
DESCRIPTION OF THE PRIOR ART
Optical fiber systems have become the physical transport medium of choice in long distance telephone and data communication networks. However, a problem with optical fiber systems is dispersion, which causes the optical pulses to spread. The original optical fiber systems include, in addition to a light transmitter and a light receiver connected by optical fiber, repeaters at various points along the optical fiber path. Repeaters are optical-electrical devices that include a receiver and a transmitter in series with circuitry for amplifying, reshaping, and retiming the signal. The receiver part of the repeater converts the signal on the optical fiber from the optical domain to the electrical domain and the transmitter converts the signal from the electrical domain back to the optical domain. The retiming and reshaping circuitry processes the signal prior to retransmission.
Recently, optical network operators have proposed and have begun to introduce all-optical systems. An all-optical system does not include electro-optical repeaters. Rather, all-optical systems use optical line amplifiers, such as rare earth-doped fiber amplifiers, to amplify the optical signals along the route.
Optical amplifiers simply amplify the signal and do not include any means for reshaping or retiming the signal. Accordingly, dispersion can be a severe problem in all-optical systems. One solution to chromatic dispersion in all-optical systems is disclosed in U.S. Pat. No. 5,430,822, which discloses dispersion compensating optical fibers. By inserting an appropriate length of dispersion compensating optical fiber into an optical system, dispersion related signal degradation can be compensated.
In addition to dispersion compensating fiber, there has been disclosed in U.S. Pat. No. 5,369,520 an optical regenerator. A regenerator differs from a line amplifier in that it not only strengthens the amplitude of the signal, but also reshapes the pulses and removes timing jitter. The regenerator of the '520 patent includes an electrical-optical clock recovery stage and a Sagnac loop optical gate stage. The clock recovery stage generates a periodic optical pulse that matches the clock signal that underlies the incoming data signal. The clock signal is used in the optical gate to generate a retimed regenerated output signal.
The purpose of the Sagnac loop of the '520 patent is to use the on-off state of the data signal to meter out single pulses of the clock signal. The clock signal recovered by the electrical-optical clock recovery stage enters the Sagnac loop through a coupler where it is split evenly and traverses the loop in both directions. The signal halves from each direction reconverge at the same optical coupler, and because of their phasing and the fact that they have passed through identical paths, they recombine to couple all of the energy back into the original input port. As long as the loop is kept symmetrical and there is no data signal, the other port from the coupler does not output any clock pulses. The data signal is propagated over a portion of the loop. As a data “one” pulse propagates through a portion of the loop, it travels along side one of the clock signal halves and imparts a phase shift, due to non-linearity of the shared fiber material known as the Kerr effect. The counter-propagating clock signal half is essentially unaffected by the data signal. When the clock signal halves recombine at the coupler, the imbalance introduced by the data signal causes the clock pulse to emerge from the output port of the coupler. Thus, a data signal is used to gate out high quality clock pulses.
Recently, there has been proposed an all-optical regenerator that includes an optical clock recovery stage and an optical gate stage. The clock recovery stage is an optical ring with an amplifier and a variable delay line. The size of the ring is selected so that a light pulse makes a complete cycle through the ring during one bit period of the expected incoming data signal, or an integral multiple thereof. The variable delay line is used to fine tune the ring delay with respect to the incoming signal. The optical ring and amplifier form a ring laser that is modulated into a circulating pulse by copropagating it with the incoming data signal. The incoming data signal is amplified and coupled into the clock recovery ring where it shares paths with a portion of the ring laser through a section of optical fiber. The circulating clock and passing data signals are amplified to sufficient levels to cause the material in the shared fiber path to exhibit a non-linear refractive index the Kerr effect. The non-linearity provides a venue for cross modulation. The clock recovery stage includes two outputs. One is a strong, idealized pulse stream from the ring laser representing the recovered clock signal. The other is a sample of the data signal after going through a portion of the ring. Both of these signals are fed into the optical gate stage, which is a Sagnac loop or non-linear optical loop mirror (NOLM).
An all-optical regenerator includes several expensive, specialized optical components and acts only on a single optical carrier channel. Each channel requires a separate set of equipment. Additionally, to regenerate carriers traveling in opposite directions, two complete regenerator sets occupying two spaces in an equipment rack are required for each carrier. Thus, regenerators are expensive in terms of both cost and space. It is an object of the present invention to reduce the number of optical components required to regenerate more than one optical carrier.
SUMMARY OF THE INVENTION
The present invention provides an all-optical system for regenerating a first optical signal carried in a first direction on an optical transmission medium and a second optical signal carried in a second direction on the optical transmission medium. The system includes a bi-directional clock recovery loop and a bi-directional optical gate. The bi-directional clock recovery loop includes a first optical clock recovery circuit for recovering a first clock signal from the first optical signal and a second optical circuit for recovering a second clock signal from the second optical signal. The first and second optical circuits of the clock recovery loop share at least some common optical circuit elements. The bi-directional optical gate includes a first non-linear optical light mirror circuit for producing a first regenerated signal based on the first optical signal and the first clock signal and a second non-linear optical light mirror circuit for producing a second regenerated signal based on the second optical signal and the second clock signal. The first and second non-linear optical light mirror circuits share at least some common optical circuit elements.
The bi-directional clock recovery loop includes a first optical signal input arranged to receive the first signal and a second optical signal input arranged to receive the second signal. The clock recovery loop outputs the first clock signal at a first recovered clock signal output and the second clock signal at a second recovered clock signal output. The clock recovery loop also outputs the first optical signal at a first optical signal output and the second optical signal at a second optical signal output.
The bi-directional optical gate includes a first recovered clock signal input coupled to the first recovered clock signal output of the bi-directional clock recovery loop and a second recovered clock signal input coupled to the second recovered clock signal output of the bi-directional clock recovery loop. The optical gate also includes a first optical signal input coupled to the first optical signal output of the bi-directional clock recovery loop and a second optical signal input coupled to the second optical signal output of the bi-directional clock rec

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