Dispersion tolerant optical data transmitter

Details Dispersion tolerant optical data transmitter Dispersion tolerant optical data transmitter

2002-02-08

2003-09-23

Pascal, Leslie (Department: 2633)

Optical communications

Transmitter

C398S183000, C398S190000

Reexamination Certificate

active

06623188

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to optical data transmitters. In particular the invention relates to optical data transmitters that have relatively high tolerance to effects of fiber dispersion and nonlinearity compared with conventional NRZ fiber-optic transmitters.
BACKGROUND OF THE INVENTION
In the information age, the demand for data networks of higher and higher data capacities, at lower and lower costs is constantly increasing. This demand is fueled by many different factors, such as the tremendous growth of the Internet and the World Wide Web. The increasing numbers of on-line users of the Internet and the World Wide Web have greatly increased the demand for bandwidth because of the proliferation of bandwidth-intensive applications such as audio and video streaming and file transfer.
Optical fiber transmission has played a key role in increasing the bandwidth of telecommunications networks. Optical fiber offers much higher bandwidths than copper cables and is much less susceptible to various types of electromagnetic interference and other undesirable effects. As a result, it is the preferred medium for transmission of data at high data rates and over long distances.
At very high data rates, chromatic dispersion in optical fiber transmission lines causes waveform deterioration and becomes a limiting factor in standard single-mode optical fiber. Although dispersion-shifted optical fiber exists, which exhibits very low dispersion at optical fiber transmission wavelengths, there is a large installed base of standard signal-mode optical fiber. Thus, there is a great demand for dispersion tolerant data transmission systems.
Correlative coding techniques can be used to enhance tolerance to fiber dispersion and other non-linear effects. Correlative coding techniques, also known as partial response signaling, were developed in the 1960s. One type of correlative coding technique is called duobinary signaling. Duobinary coding was first published in 1963 by A. Lender in “Duobinary Technique for High Speed Data Transmission,” IEEE. Trans. Commun. Electron., vol. CE-82, pp. 214-218, May 1963.
A duobinary (DB) signal is created by delaying a binary bit sequence by one full bit and then adding the delayed binary bit sequence to the original bit sequence. See, for example, U.S. Pat. No. 5,917,638 issued to Franck et al. The DB signal can be expressed as follows:
DB
i
=m
i
+m
i−1
.  (1)
The DB signal is a three level sequence with one half of the bandwidth of the binary bit sequence m. Duobinary coding reduces the signal bandwidth by mapping a binary data signal having two levels to be transmitted into a three-level signal having three meaningful values or levels. See, for example, U.S. Pat. No. 5,867,534 issued to Price et al. The signal received by the receiver is interpreted in terms of three levels rather than two levels. The reduction in signal bandwidth reduces the waveform deterioration caused by chromatic dispersion.
Duobinary coding has been implemented with optical signals using a Mach-Zehnder interferometric modulator biased at the quadrature point and a three level intensity detector as the receiver. See for example, X. Gu and L.C. Blank, “10 GB/s unrepeatered three-level optical transmission over 100 km of standard fibre,” Electronics Letters Vol. 29 No. 25 pp 2209-2210 (received Oct. 8, 1993).
An optical duobinary transmission system has been proposed that uses a two-level (on, off) approach. See, for example, K. Yonenaga, S. Kuwano, S. Norimatsu and N. Shibata, “Optical duobinary transmission system with no receiver sensitivity degradation,” Electronics Letters Vol. 31 No. 4 pp 302-304 (received Dec. 7, 1994). Since typical optical detectors respond to optical intensity as opposed to amplitude, decoding is automatically achieved at the detector and duobinary decoding is not necessary. The system requires that the phase of the “on” state signal take the values of either ‘0’ or ‘&pgr;’. The two ‘on’ states correspond to the ‘+1’ and ‘−1’ states of the duobinary signal, and the ‘off’ state corresponds to the ‘0’ state of the duobinary signal.
The optical duobinary signal is generated by driving a dual-drive Mach-Zehnder modulator with push-pull operation. Two duobinary signals for driving the Mach-Zehnder are generated from original binary signals by using two duobinary encoders. The two duobinary signals are applied to two electrodes of the Mach-Zehnder modulator. The ‘0’ state of the duobinary signal is equal to the zero level. The ‘+1’ and ‘−1’ states have the same magnitude and opposite signs for push-pull operation.
SUMMARY OF THE INVENTION
The dispersion tolerant optical data transmitter of the present invention performs preceding. The precoding can be accomplished either at the line rate or at a lower rate if a multiplexer is used. Decoding is performed at the receiver by a square law detector. In one embodiment, a delay of less than a full bit period is used.
The dispersion tolerant optical data transmitter of the present invention is approximately a factor of four less sensitive to chromatic dispersion than conventional optical transmitters. Also, the dispersion tolerant optical data transmitter of the present invention is less sensitive to fiber nonlinearities and can transmit at higher power levels, and therefore, longer distances, because the carrier is suppressed.
Accordingly, in one aspect, the present invention is embodied in an optical data transmitter including a precoder that converts an input data signal to a binary precoded data signal and to a complementary binary precoded data signal at an output and a complementary output, respectively. In one embodiment, the precoder is a serial precoder.
In another embodiment, the precoder is a parallel precoder having n sets of parallel data inputs that receive n sets of parallel data. The parallel precoder generates n sets of parallel precoded data at n sets of parallel outputs from the n sets of parallel data. The parallel precoder also includes a multiplexer having n sets of parallel data inputs that are coupled to the n sets of parallel outputs of the parallel precoder, respectively. The multiplexer generates the binary precoded data signal and the complementary binary precoded data signal at the output and the complementary output, respectively.
The optical data transmitter also includes a delay element coupled to one of the output and the complementary output of the precoder. The delay element delays one of the complementary binary precoded data signal and the binary precoded data signal relative to the other at an output of the delay element, by a time corresponding to less than one bit period of the binary precoded data signal.
In one embodiment, the delay element delays one of the complementary binary precoded data signal and the binary precoded data signal relative to the other by a time corresponding to between 0.4 and 0.9 of the bit period of the binary precoded data signal. In one embodiment, the delay element includes a variable delay element. In one embodiment, the delay element is selected to increase dispersion tolerance of a communication system that includes the optical data transmitter.
The optical data transmitter further includes a differential amplifier having a first input that is coupled to an output of the delay element and a second input that is coupled to one of the output and the complementary output of the precoder. The differential amplifier converts the binary precoded data signal and the complementary binary precoded data signal to a four-level data signal and to a complementary four-level data signal at a differential output and a complementary differential output, respectively.
In one embodiment, the four-level data signal includes a minimum amplitude, a first intermediate amplitude, a second intermediate amplitude, and a maximum amplitude. An average of the minimum amplitude and the maximum amplitude is substantially equal to an average of the first intermediate amplitude and the second intermediate amplitude.
The op

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