Optical: systems and elements – Optical amplifier – Correction of deleterious effects
Reexamination Certificate
1999-04-14
2001-04-24
Buczinski, Stephen C. (Department: 3662)
Optical: systems and elements
Optical amplifier
Correction of deleterious effects
C359S341430, C359S339000, C359S348000
Reexamination Certificate
active
06222669
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to high bit-rate optical transmission systems and, in particular, to an apparatus for correcting timing jitter in a stream of soliton pulses.
BACKGROUND OF THE INVENTION
In order to enable the transmission of optical signals at rates exceeding a few dozen gigabits per second (Gbps), much research has been done in the area of pulse shaping and materials fabrication. In particular, it has been found that traditional bandwidth limitations imposed by chromatic dispersion can be overcome by fabrication of the optical transmission medium such that the induced chromatic dispersion is a function of pulse amplitude. If, in addition, the pulse shape and amplitude are carefully chosen, then an original pulse will maintain the chosen shape and amplitude as it travels along the fiber. Such specially shaped pulses are known as solitons and can be transmitted at intervals as low as 10 picoseconds (ps) between pulses.
In an ideal soliton transmission system, each soliton is at the center of the corresponding symbol interval. Unfortunately, in a practical system, timing jitter influences the position of a soliton relative to the center of the symbol interval and an error occurs when the pulse is shifted too far off center. The timing jitter has three main sources: Gordon-Haus jitter due to the addition of optical noise from upstream optical amplifiers, soliton-soliton interaction from the symbol pattern and interaction between channels of different wavelengths sharing the same fiber in a wavelength division multiplexed (WDM) system. A discussion of timing jitter can be found in the summary paper “Soliton WDM Transmission” by Bruce M. Nyman and S. G. Evangelides, presented at the conference on optical fiber communication (OFC) in 1995 and incorporated by reference herein.
To overcome the error-inducing effects of timing jitter in practical systems, it is necessary to install very high bit-rate regeneration units every five hundred kilometres or less. Since full regeneration units are relatively expensive components, it would be desirable to increase the distance which could be travelled by solitons without requiring their full regeneration. One possible approach is to place partial regeneration units, which are considerably less expensive than full regeneration units, at various points along the optical path.
One known method of partially regenerating solitons is described in an article entitled “Optical Retiming Regenerator Using 1.5 um Wavelength Multielectrode DFB LDs” by M. Jinno and T. Matsumoto, which appeared in Vol. 25, No. 20 of the journal “Electronics Letters”, published on Sep. 28
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, 1989 and incorporated by reference herein. The approach disclosed therein is based on a self-pulsating circuit to extract an optical clock from an input signal; the input signal is then combined with the clock signal and injected into a bistable multi-electrode distributed feedback laser diode which then reproduces the original optical data, retimed using the extracted clock. While this method possesses some desirable features, it is limited to a frequency range below 200 MHz and it is not feasible to construct such a circuit to operate at frequencies that are higher by several orders of magnitude.
Another relevant technique involves the use of an electro-absorptive modulator with a recovered clock, as described in WIPO International Patent Application WO 96/27956, published Sep. 12
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, 1996, which is incorporated by reference herein. While the clock recovery system disclosed therein provides re-centering of optical symbols in a desired manner, it is extremely difficult to implement the modulator and the clock recovery apparatus at bit rates on the order of 100 Gbps or more.
SUMMARY OF THE INVENTION
It is an object of the present invention to mitigate or obviate one or more disadvantages of the prior art.
The invention may be summarized according to a first broad aspect as a regenerator for correcting timing jitter of a stream of data pulses travelling along an optical path in a direction of interest. The regenerator comprises a saturable absorber connected in the optical path, for presenting a region of low loss to optical signals of an intensity higher than a threshold and absorbing optical signals of an amplitude lower than the threshold. The regenerator also comprises a resonant optical circuit coupled to the saturable absorber, for controlling the saturable absorber to provide a plurality of successive regions of low loss at a controllable resonant rate.
The data pulses may be soliton pulses. The resonant optical circuit may comprise an optical amplifier, an input fiber connected between the West port of the saturable absorber and the input of the amplifier and an output fiber connected between the input of the amplifier and the East port of the saturable absorber. There may also be couplers connected to the input and output fibers and to the main optical path, for coupling the data pulses into resonant pulses propagating in the resonant circuit.
According to a second broad aspect, the invention may be summarized as a method comprising the steps of providing the saturable absorber, controlling the saturable absorber to provide a plurality of successive regions of low loss at a controllable resonant rate, adjusting the resonant rate of said regions of low loss to a multiple of the rate of said pulses and passing the pulses through the saturable absorber for re
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-timing and re-shaping.
The invention can be summarized according to a third broad aspect as a regenerator for regenerating an input optical signal defining data symbols occupying respective symbol intervals of a common duration. The regenerator comprises an input for receiving the input optical signal, an output for transmission of a regenerated optical signal, a main optical path connected between the input and output of the regenerator and a saturable optical element connected in the main optical path.
The regenerator also comprises a first coupler connected in the main optical path between the input of the regenerator and the saturable optical element and a second coupler connected in the main optical path between the saturable optical element and the output of the regenerator. An important component of the regenerator is a resonant optical circuit coupled to the main optical path by the first and second couplers.
One of the couplers is arranged to couple light from the saturable optical element to the resonant optical circuit. The other one of the couplers is arranged to couple light from the resonant optical circuit to the saturable optical element. In addition, one of the couplers is arranged to couple no significant light from the resonant optical circuit to the output of the regenerator. Finally, the main optical path and the resonant optical circuit are arranged so as to introduce a delay from the saturable optical element through the resonant optical circuit and back to the saturable optical element substantially equal to an integer multiple of the symbol interval duration.
In use, optical noise or an optical signal provided by a low-power laser is converted by the resonant circuit into a continuous sequence of resonant optical pulses having an even spacing equal to the delay around the resonant circuit such that the resonant optical pulses are substantially centered within respective symbol intervals.
According to another broad aspect, the regenerator comprises a closed resonant optical circuit having an entry point and an exit point and arranged to introduce a round-trip delay substantially equal to an integer multiple of the symbol interval duration. The entry point of the resonant optical circuit is coupled to the first coupler by an entry fiber and the exit point of the resonant optical circuit is coupled to the second coupler by an exit fiber. In accordance with the invention, the first coupler is arranged to couple light either from the main optical path or from the saturable optical element to the entry fiber and the second coupler is arranged to couple light from th
Hardcastle Ian
Roberts Kim B.
Buczinski Stephen C.
Nortel Networks Limited
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