# Patent

## Title
Half rate precoded data RZ transmitter

## Abstract

An optical duobinary transmitter. The transmitter uses a half-rate precoder, half-rate non-linear modulation drive circuits and a multiplex modulator for generating duobinary modulation on an optical signal from which full-rate data can be detected without decoding. The intensity of the optical signal is modulated to be zero between data symbols.

## Background
CROSS-REFERENCE TO RELATED PATENT APPLICATIONS
This application is a continuation-in-part of application Ser. No. 10/299,425 filed Nov. 18, 2002 now U.S. Pat. No. 6,804,472. The present invention and the invention of application Ser. No. 10/299,425 were owned by the same entity at the time the inventions were made.


BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention is generally related to duobinary optical signal generation and more particularly to an optical transmitter and method using half rate data streams for generating full rate modulation in a duobinary optical signal.
2. Description of the Prior Art
Recently, optical duobinary techniques have attracted attention for narrowing the spectrum of a transmitted optical signal and reducing the waveform distortion that is induced by optical fiber chromatic dispersion. The spectrum of the transmitted signal is reduced by a factor of about two by mapping a binary data signal to be transmitted into a three-level duobinary signal, with redundancy within the three levels, to represent the binary data. While there are several techniques for implementing duobinary mapping onto an optical carrier, all of the techniques result in the transmission of equivalent optical signals that take on one of three possible optical electric-field amplitude values, with certain normalization, of {−1, 0, 1}.
The transmitters for generating these optical signals have electronic circuits for generating signals for driving an optical modulator. One important limitation for these electronic circuits is data rate. In general, the higher the date rate, the more difficult it is to design the circuits and the more expensive they are to manufacture. A second limitation is linearity. In general, it is less difficult and less expensive, and higher data rates are possible, when the electronic circuits are not required to be linear.
The U.S. Pat. No. 5,867,534 by Price and Uhel; and papers “Reduced Bandwidth Optical Digital Intensity Modulation with Improved Chromatic Dispersion Tolerance” published in Electronics Letters, vol. 31, no. 1, in 1995 by A. J. Price and N. Le Mercier, and “210 km Repeaterless 10 Gb/s Transmission Experiment through Nondispersion-Shifted Fiber Using Partial Response Scheme” published in the IEEE Photonics Technology Letters in 1995 by A. J. Price, L. Pierre, R. Uhel and V. Havard report the usage of a low-pass filter to generate the three-level duobinary signal and an optical duobinary technique where a redundancy is given to optical phase. However, because the input of the low-pass filter is the full-rate non-return-to-zero (NRZ) data, full-speed electronic circuits are required.
The U.S. Pat. No. 5,543,952; and papers “Optical Duobinary Transmission System with no Receiver Sensitivity Degradation” published in Electronic Letters in 1995 by K. Yonenaga, S. Kuwano, S. Norimatsu and N. Shibata, and “Dispersion-Tolerant Optical Transmission System using Duobinary Transmitter and Binary Receiver” published in the Journal of Lightwave Technology in 1997 by K. Yonenaga and S. Kuwano report the usage of a delay-and-add circuit to generate the three-level duobinary signal and an optical duobinary technique where a redundancy is given to optical phase. Again, because the input of the delay-and-add circuit is the full-rate NRZ data, full-speed electronic circuits are required.
In both the U.S. Pat. Nos. 5,543,952 and 5,867,534, electronic modulator drivers may operate at a bandwidth less than one-half the system data rate. However, the modulation drivers are required to be linear in order to handle the three levels of the duobinary signal.
The U.S. Pat. Nos. 5,917,638 and 6,188,497 by Franck et al., and a paper by T. Franck, P. B. Hansen, T. N. Nielsen, and L. Eskildsen entitled “Duobinary Transmitter with Low Intersymbol Interference” published in IEEE Photonics Technology Letters in 1998 report a duobinary transmitter having dual binary modulation signals for driving a modulator. In a simplified view, an optical modulator is used as an adder for the delay-and-add circuit used in the U.S. Pat. No. 5,543,952. However, full-rate circuits are again required as both modulation signals have the same data rate as the optical signal.
The U.S. Pat. No. 6,337,756; and papers “A Dual-Drive Ti:LiNbO3 Mach-Zehnder Modulator Used as an Optoelectronic logic gate for 10-Gb/s Simultaneous Multiplexing and Modulation” published in IEEE Photonics Technology Letters in 1992 of P. B. Hansen and A. H. Gnauck, and “Prechirped Duobinary Modulation” published in IEEE Photonics Technology Letters in 1998 by A. Djupsjobacka report the usage of a dual-drive modulator as both a multiplexer and a modulator. Each of the dual modulator drive signals operates at one half of the optical data rate. However, no method is proposed or successfully demonstrated for preceding the data for providing the modulator drive signals or for recovering the original data from the duobinary optical signal by symbol-by-symbol detection.
There is need for a duobinary optical transmitter using electronic circuits at low data rates without a requirement to be linear where the original data is recoverable with an optical receiver by symbol-by-symbol detection.
SUMMARY OF THE INVENTION
The present invention is a method and optical transmitter using electronic circuits operating at one-half data rate where the circuits operate without a requirement of linearity for generating an optical signal having full-rate duobinary modulation and where the original data is recoverable with an optical receiver by symbol-by-symbol detection.
Briefly, a preferred embodiment of an optical transmitter of the present invention includes a precoder and a multiplex modulator. The precoder uses two exclusive-OR gates and a one symbol delay component for calculating two cumulative cross parities for two input data streams. The multiplex modulator includes a one-half symbol delay component, modulation drivers and a dual-drive optical modulator. The one-half symbol delay component delays one of the cumulative cross parity streams by one-half symbol time with respect to the other. The modulation drivers amplify the cumulative cross parities either before or after the one-half symbol delay for driving the optical modulator. The optical modulator modulates an optical signal with a modulation drive signal corresponding to the difference between the one-half symbol delayed cumulative cross parity stream-stream and the other cumulative cross parity stream for providing a duobinary optical signal having an optical electric field having an intensity that may be detected symbol-by-symbol for recovering the original data in the two input data streams.
An advantage of the present invention is that half-rate precoder and modulator driver circuits are used for generating full-rate duobinary modulation on an optical signal from which the original data can be simply detected without decoding. Because the modulator drive signals are binary, another advantage is that the modulation drivers can be operated as nonlinear amplifiers.
A duobinary optical signal has three states—a low (zero) field state, a positive field state having a phase angle of 0 radians, and a negative field state having a phase angle of π radians. This signal is sometimes called a phase duobinary signal in order to distinguish it from an amplitude duobinary signal having three amplitudes all at the same phase. A rapid transition between phase states of an optical signal may cause frequency chirp. Frequency chirp is undesirable because it spreads the frequency band of signal energy. However, a conventional phase duobinary optical system avoids this frequency chirp by using a balanced modulator drive signal composed of two simultaneous signals for driving a dual-drive modulator. The dual-drive modulator uses the simultaneous signals for modulating two portions of an optical carrier simultaneously in equal and opposite directions of phase rotation and combines the two portions for providing the duobinary optical signal. Alternatively, a single drive balanced Mach-Zehnder modulator can internally split a single drive signal input between two waveguide arms. Each of the waveguide arms modulates a portion of the optical signal. The effect of the equal and opposite phase rotation is to cancel the optical signal during the transitions between phase states so that there is little or no phase change during the transition except when there is zero intensity at the instant in time when the duobinary optical signal flips between phase states. Because there is little no phase change except when there is zero intensity, there is little or no energy spread by the frequency chirp in a conventional duobinary system.
The present invention of a phase duobinary optical system also uses a modulator drive signal composed of two signals for driving a dual-drive modulator. However, in the present invention the two signals may occur one at a time. The dual-drive modulator uses the two signals independently for modulating two portions of an optical carrier for making independent transitions from one phase state to another and combines the two portions for generating the duobinary output optical signal. Because the drive signals may occur one at a time, the present invention does not avoid frequency chirp by canceling the optical signal in the conventional manner with equal and opposite phase rotations during the state transitions.
In order to prevent frequency chirp spreading for the present invention, a preferred embodiment of a multiplex modulator includes a return-to-zero (RZ) modulator. The RZ modulator uses a half rate clock drive signal for providing an RZ light signal to the dual-drive modulator or single drive balanced modulator. The dual drive modulator modulates the RZ light signal with modulation drive signal, as described above, corresponding to the difference between the one-half symbol delayed cumulative cross parity stream-stream and the other cumulative cross parity stream for providing an RZ duobinary optical signal having an optical electric field having an intensity that may be detected symbol-by-symbol for recovering the original data in the two input data streams. The clock drive signal is timed so that the RZ duobinary output optical signal has full intensity during mid-symbol times and little or no intensity during the state transitions, thereby minimizing the spreading effect of the frequency chirp.
Therefore, an advantage of the present invention is that half-rate data processing and nonlinear modulator drivers are used for generating a full rate data duobinary optical signal from which the original data can be simply detected without decoding, while at the same time frequency chirp is avoided.
These and other objects and advantages of the present invention will no doubt become obvious to those of ordinary skill in the art after having read the following detailed description of the best mode which is illustrated in the various figures.



IN THE DRAWINGS
FIG. 1 is a block diagram showing a duobinary transmitter of the present invention using half-rate signal processing for providing a full-rate duobinary optical signal;
FIG. 1A is a block diagram showing a return-to-zero embodiment of a duobinary transmitter of the present invention using half-rate signal processing for providing a full-rate RZ duobinary output optical signal;
FIG. 1B is a block diagram of showing an amplitude modulator for providing an RZ light signal for the duobinary transmitter of FIG. 1A;
FIGS. 2A-B are time charts of first and second exemplary half-rate input data streams to the duobinary transmitters of FIGS. 1 and 1A;
FIGS. 2C-D are time charts of first and second cumulative cross parity streams in the duobinary transmitters of FIGS. 1 and 1A for the input data streams of FIGS. 2A-B;
FIGS. 2E-F are time charts of first and second modulator drive signals in the duobinary transmitters of FIGS. 1 and 1A for the input data streams of FIGS. 2A-B;
FIG. 2G is a time chart of a duobinary optical electric field provided by the duobinary transmitter of FIG. 1 for the input data streams of FIGS. 2A-B;
FIG. 2H is a time chart of an intensity of the duobinary optical electric field of FIG. 2G;
FIG. 3A is a transfer characteristic for the optical electric field of a dual-drive modulator of the duobinary transmitters of FIGS. 1 and 1A;
FIG. 3B is a transfer characteristic for the intensity of the optical electric field of a dual-drive modulator of the duobinary transmitters of FIGS. 1 and 1A;
FIG. 4 illustrates an experimental setup for verifying the multiplexing and modulating functions of the duobinary transmitters of FIGS. 1 and 1A;
FIG. 5 illustrates measured waveforms for the experimental setup of FIG. 4;
FIG. 6A is a time chart of a full rate clock signal of the duobinary transmitter of FIG. 1A;
FIG. 6B is a time chart of an RZ light signal of the duobinary transmitter of FIG. 1A for the clock signal of FIG. 6A;
FIG. 6C is a time chart of an RZ duobinary optical electric field provided by the duobinary transmitter of FIG. 1A for the RZ optical signal of FIG. 6B and the input data streams of FIGS. 2A-B;
FIG. 6D is a time chart of an intensity of the duobinary optical electric field of FIG. 6C; and
FIG. 6E is a transfer function of the amplitude modulator of FIG. 1B.



DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
FIG. 1 shows a block diagram of a duobinary transmitter 10 of the present invention having a precoder 11 and a multiplex modulator 12. The precoder 11 uses first and second exclusive-OR gates 20 and 21, respectively, and a one-symbol delay component 22 for receiving first and second binary half-rate input data streams D1 (t), denoted by 25a, and D2(t), denoted by 25b, respectively, and computing first and second cumulative cross parity streams P1(t), denoted by 26a, and P2(t), denoted by 26b, respectively. The first and second data streams D1(t) 25a and D2(t) 25b taken together carry the full-rate data that is to be transmitted.
The first exclusive-OR gate 20 provides the first cumulative cross parity stream 26a P1(t) equal to P2(t−T)+D1(t)mod 2, and the second exclusive-OR gate 21 provides the second cumulative cross parity stream 26b P2(t) equal to P1(t)+D2(t)mod 2, where the T is a half-rate input symbol time corresponding to the symbols in the half-rate input data-streams D1(t) 25a and D2(t) 25b. Recursive operation of the first exclusive-OR gate 20 results in the first cumulative cross parity stream 26a P1(t) of D1(t)+D2(t−D)+D1(t−T)+D2(t−2T)+D1(t−2T)+D2(t−3T)+D1(t−3T)+ . . . modulo 2 as the cumulative cross parity of the first data stream D1(t) 25a and the second data stream 25b one symbol delayed D2(t−T). It should be noted that the first cumulative cross parity stream 26a P1(t) is the cross parity of the first data stream D1(t) 25a and the one symbol delayed second data stream D2(t−T) plus the previous first cumulative cross parity.
Similarly, recursive operation of the second exclusive-OR gate 21 results in the second cumulative cross parity stream 26b P2(t) of D2(t)+D1(t)+D2(t−T)+D1(t−T)+D2(t−2T)+D1(t−2T)+D2(t−3T)+D1(t−3T)+ . . . modulo 2 as the cumulative cross parity of the second data stream D2(t) 25b and the first data stream 25a. It should be noted that the second cumulative cross parity stream 26b P2(t) is the cross parity of the second data stream D2(t) 25b and the first data stream D1(t) plus the previous second cumulative cross parity. Filters may be inserted for filtering the cumulative cross parity streams P1(t) 26a and P2(t) 26b between the precoder 11 and the multiplex modulator 12.
The multiplex modulator 12 includes a dual-drive Mach-Zehnder optical modulator 27 and a light source 28. The light source 28 provides input light 29 to the optical modulator 27. The optical modulator 27 modulates the input light 29 with first and second modulator drive signals V1(t), denoted by 40a, and V2(t), denoted by 40b, respectively. A first modulator driver 41a amplifies the first precoder output (first cumulative cross parity stream) 26a for providing the first modulator drive signal 40a. A second modulator driver 41b amplifies the second precoder output (second cumulative cross parity stream) 26b before or after the second precoder output 26b is delayed by a one-half symbol delay component 42 by half the input symbol time (T/2). Because the modulator drive signals V1(t) 40a and V2(t) 40b are binary, the modulator drivers 41a and 41b may be limiting, saturated, or nonlinear amplifiers without a linearity requirement. The optical modulator 27 is biased with a bias voltage Vb, denoted by 45, for providing a modulator output signal 50. The bias voltage Vb 45 is set so that the modulator output signal 50 is minimized when the voltages of the first and second modulator drive signals 40a and 40b are equal.
FIG. 1A is a block diagram of a duobinary transmitter of the present invention having a reference number 10A. The duobinary transmitter 10A includes the precoder 11 described above and a multiplex modulator 12A. The multiplex modulator 12A includes the dual-drive Mach-Zehnder optical modulator 27, the light source 28, the first modulator driver 41a, the second modulator driver 41b, and the one-half symbol delay component 42 described above. The multiplex modulator 12A also includes a return-to-zero (RZ) amplitude modulator 30. The modulator 30 amplitude modulates the input light 29 with a clock signal C(t) (FIG. 6A) in on-off cycle for providing an RZ light signal having a pulsed intensity INZ(t) (FIG. 6B). The functions of the light source 28 and the RZ modulator 30 may be combined in an RZ light source 31. The optical modulator 27 modulates the RZ light signal with the modulator drive signals V1(t) 40a and V2(t) 40b for providing a return-to-zero (RZ) duobinary output signal 50A having an optical electric field EO(t) (FIG. 6C) and an output optical intensity IO(t) (FIG. 6D).
FIG. 1B shows the RZ modulator 30 as a zero-chirp balanced Mach-Zehnder device having a drive range Vπ. The modulator 30 can be constructed as a single drive balanced Mach-Zehnder Interferometer (MZI) device with an X-cut using LiNbO3 for receiving the clock signal C(t) at a single input. Alternatively, the MZI device can be constructed with a dual input drive for receiving the clock signal C(t) at a both inputs. The clock signal C(t) operates at the rate of the first and second half rate data streams D1(t) and D(t) and is synchronized with the data streams D1(t) and D1(t). The peak-to-peak amplitude of the clock signal C(t) is nominally twice the range Vπ of the RZ modulator 30. The RZ Mach-Zehnder modulator 30 is biased with a voltage VbNZ for providing pulses for the intensity INZ(t) at twice the rate of the clock signal C(t) according to a transfer function shown in FIG. 6E. In another implementation, a clock signal C2(t) operates a twice the rate of the first and second half rate data streams D1(t) and D1(t) (twice the rate of the clock signal C(t) shown in FIG. 6A) and is synchronized with the data streams D1(t) and D1(t). In this implementation, the peak-to-peak amplitude of the clock signal C2(t) is nominally the same as the range Vπ of the RZ modulator 30 and the modulator 30 is biased at a voltage VbNZ+Vπ/2 or a voltage VbNZ−Vπ/2 for providing pulses for the intensity INZ(t) at the same rate as the clock signal C2(t) according to the transfer function shown in FIG. 6E.
FIGS. 2A and 2B show exemplary first and second binary input data streams 25a D1(t) and 25b D2(t), respectively, versus time t. The time t is shown in units of the half-rate input symbol time T. FIGS. 2C and 2D show the first and second cumulative cross parities streams (first and second precoder output symbol streams) 26a P1(t) and 26b P2(t), respectively, responsive to the exemplary input data streams 25a and 25b, versus the time t. FIGS. 2E and 2F show the first and second modulator drive signals V1(t) 40a and V2(t) 40b, respectively, responsive to the exemplary input data streams 25a and 25b, versus the time t. The modulator drive signals 40a and 40b have a timing offset of T/2 (one-half the half-rate input symbol time), versus the time t.
FIGS. 2G and 2H show the optical signal 50 (FIG. 1) in the form of an optical electric field E(t), denoted by 50a, and an optical intensity I(t), denoted by 50b, respectively, responsive to the exemplary input data streams 25a and 25b, versus the time t. Note that the beginning time t from 0 to T/2 of the signals 40b, 50a, and 50b cannot be derived from the input data streams 25a and 25b. Importantly, it should be noted that the optical intensity I(t) 50b corresponds to the multiplexed data in the combination of the first and second data input data streams D1(t) 25a and D2(t) 25b, thereby enabling symbol-by-symbol recovery by an intensity detector of the full-rate original data.
FIG. 3A shows an electric field transfer characteristic Eout(t)/Ein(t), denoted by 60a, of the optical modulator 27 with respect to the difference V1(t)−V2(t) between first and second modulator drive signals V1(t) 40a and V2(t) 40b. FIG. 3B shows an intensity transfer characteristic Iout(t)/Iin(t) denoted by 60b, of the optical modulator 27 with respect to the difference V1(t)−V2(t) between first and second modulator drive signals V1(t) 40a and V2(t) 40b. The peak-to-peak signal swing for each of the modulator drive signals 40a and 40b is equal to the maximum peak input Vπ specified for the modulator 27.
Using the transfer characteristic 60a in FIG. 3A in terms of an optical electrical field, the modulator output signal 50 has the optical electrical field of 50a that is shown in FIG. 2G. The optical electrical field of 50a is a duobinary signal with the following properties: a) the signal has the same sign if there are even number of zeros in between; b) the signal changes sign if there are odd number of zeros in between; c) there is no direct transition from positive to negative electrical field and vice versus without first through the zero state.
Using the transfer characteristic 60b in FIG. 3B in term of intensity, the modulator output signal 50 has the intensity of 50b that is shown in FIG. 2H. Comparing the waveform of the output intensity 50b with the input data streams 25a and 25b, it is seen that the intensity 50b is a multiplexed signal of both 25a and 25b. If a photodetector is used to detect the intensity of 50b, no decoder but a demultiplexer is required to recover the original data in the waveforms of 25a and 25b. Importantly, the intensity waveform of 50b has twice the data-rate of the input data streams of 25a and 25b. 
FIG. 4 illustrates an experimental setup for demonstrating the function of the multiplexing modulator 12 using two 10 Gb/s pattern generators 100a and 100b to give two independent 2331−1 pseudo-random-bit-sequences (PRBS). The output 50 of the multiplexing modulator 12 is passed to a photodetector 101 followed by an oscilloscope 102.
FIG. 5 shows measured eye-patterns 110a, 110b and 111, respectively, at the oscilloscope 102, when the bias voltage Vb 45 (FIG. 1) is properly adjusted for each individual case. The eye-pattern 110a is recorded when the pattern generator 110a is operating and the pattern generator 100b is not operating. The eye-pattern 110b is recorded when the pattern generator 100b is operating and the pattern generator 100a is not operating. The eye-pattern 111 is recorded when both the pattern generator 110a and 100b are operating. Comparing eye-patterns 110a, 110b and 111 confirms the operation of the multiplex modulator 12.
FIG. 6A shows the clock signal C(t). FIG. 6B shows the RZ light signal 11z(t). FIG. 6C shows the optical electric field signal EO(t) for the RZ duobinary output signal 50A. The optical electric field signal EO(t) carries information symbols in three states—a low (zero) level field 52e, a high (non-zero) field of a positive polarity 53e, and a high (non-zero) field of a negative polarity 54e. The timing of the clock signal C(t) is controlled so that the optical electric field EO(t) has an intersymbol low (zero) level field 51e during phase transitions between the symbols. FIG. 6D shows the intensity IO(t) of the RZ duobinary output signal 50A. The intensity IO(t) has a low (zero) level intensity 51i (FIG. 6D) corresponding in time to the low level field 51e between the informational states, a low level intensity 52i (FIG. 6D) corresponding in time to the low (zero) field 52e, a high level intensity 53i (FIG. 6D) corresponding in time to the high level positive field 53e, or a high level intensity 54i (FIG. 6D) corresponding in time to the high level negative field 54e. 
FIGS. 6C and 6D show the optical symbol states carrying information in the optical signal 50A in the form of an optical electric field EO(t) and an optical intensity IO(t) responsive to the exemplary input data streams 25a and 25b, versus the time t. Note that the beginning time t from 0 to T/2 of the signals 40b and 50A cannot be derived from the input data streams 25a and 25b. Importantly, it should be noted that the optical intensity IO(t) corresponds to the multiplexed data in the combination of the first and second data input data streams D1(t) 25a and D2(t) 25b, thereby enabling symbol-by-symbol recovery by an intensity detector of the full-rate original data.
FIG. 6E is a transfer function 55 of the RZ modulator 30. The transfer function 55 shows intensity output Ioutput versus the drive voltage input Vinput of the modulator 30. The VbNZ bias and the amplitude of the clock signal C(t) are set so that the modulator 30 generates pulses at twice the frequency of the clock signal C(t).
Although the present invention has been described in terms of the presently preferred embodiments, it is to be understood that such disclosure is not to be interpreted as limiting. Various alterations and modifications will no doubt become apparent to those skilled in the art after having read the above disclosure. Accordingly, it is intended that the appended claims be interpreted as covering all alterations and modifications as fall within the true spirit and scope of the present invention.

## Claims

1. An optical transmitter, comprising:
a precoder for precoding two half-rate data streams having half-rate input symbol times into two cumulative cross parity streams, said half-rate data streams carrying data equivalent to a single full-rate data stream; and
a multiplex modulator for using said parity streams for providing a return-to-zero (RZ) optical signal carrying duobinary RZ modulation having intensity corresponding to said full-rate data stream; and wherein:
a current parity of a first of said parity streams is a cross parity of a current symbol of a first of said half-rate data streams with a delayed second half-rate data stream, plus a last previous parity of said first parity stream, where a second of said half-rate data streams is delayed by said input symbol time for providing said delayed second half-rate data stream; and
a current parity of a second of said parity streams is a cross parity of a current symbol of said second half-rate data stream with a current symbol of said first half-rate data stream, plus a last previous parity of said second parity stream.

2. The transmitter of claim 1, wherein:
said duobinary modulation corresponds to a difference between a half-delayed second parity stream and said first parity stream where said second parity stream is delayed by one-half said input symbol time with respect to said first parity stream for providing said half-delayed second parity stream.

3. An optical transmitter, comprising:
a precoder for precoding two half-rate data streams having half-rate input symbol times into two cumulative cross parity streams, said half-rate data streams carrying data equivalent to a single full-rate data stream; and
a multiplex modulator for using said parity streams for providing a return-to-zero (RZ) optical signal carrying duobinary RZ modulation having intensity corresponding to said full-rate data stream; and wherein:
the precoder includes a symbol delay component for delaying a second of said parity streams by said input symbol time for providing a delayed parity stream; a first gate for exclusive-OR combining a first of said half-rate data streams and said delayed parity stream for providing a first of said parity streams; and a second gate for exclusive-OR combining a second of said half-rate data streams with said first parity stream for providing said second parity stream.

4. The transmitter of claim 3, wherein:
the multiplex modulator includes a one-half symbol delay component for delaying said second parity stream by one-half said input symbol time with respect to said first parity stream for providing a half-delayed second parity stream, and an optical modulator for modulating an RZ light signal with a modulation drive corresponding to a difference between said half-delayed second parity stream and said first parity stream for providing said duobinary modulation as an optical electric field having said intensity.

5. The transmitter of claim 4, wherein:
the multiplex modulator includes an RZ light source for providing said RZ light signal, a dual-drive optical modulator for multiplexing said half-delayed second parity stream and said first parity stream onto said RZ light signal for forming said optical electric field.

6. An optical transmitter, comprising:
a precoder for precoding two half-rate data streams having half-rate input symbol times into two cumulative cross parity streams, said half-rate data streams carrying data equivalent to a single full-rate data stream; and
a multiplex modulator for using said parity streams for providing a return-to-zero (RZ) optical signal carrying duobinary RZ modulation having intensity corresponding to said full-rate data stream; and wherein:
said RZ optical signal has an optical electric field having intersymbol low field levels and field states including low field states and non-zero field states, said intersymbol low field levels interleaved between said field states, said intensity greater in said non-zero field states than in said intersymbol low field levels and said low field states; and
said optical electric field has one of said non-zero field states having a first field sense when there are an even number of said low field states following a last one of said non-zero field states; has a change in said field sense when there are an odd number of said low field states following said last non-zero field state; and has no change in said field sense until at least one of said low field states after said last non-zero field state.

7. A method for transmitting an optical signal, comprising:
precoding two half-rate data streams having half-rate input symbol times into two cumulative cross parity streams, said half-rate data streams carrying data equivalent to a single full-rate data stream; and
multiplexing representations of said parity streams onto a return-to-zero (RZ) optical signal as duobinary RZ modulation having intensity corresponding to said full-rate data stream; and wherein:
a current parity of a first of said parity streams is a cross parity of a current symbol of a first of said half-rate data streams with a delayed second half-rate data stream, plus a last previous parity of said first parity stream, where a second of said half-rate data streams is delayed by said input symbol time for providing said delayed second half-rate data stream; and
a current parity of a second of said parity streams is a cross parity of a current symbol of said second half-rate data stream with a current symbol of said first half-rate data stream, plus a last previous parity of said second parity stream.

8. The method of claim 7, wherein:
said duobinary modulation corresponds to a difference between a half-delayed second parity stream and said first parity stream where said second parity stream is delayed by one-half said input symbol time with respect to said first parity stream for providing said half-delayed second parity stream.

9. A method for transmitting an optical signal, comprising:
precoding two half-rate data streams having half-rate input symbol times into two cumulative cross parity streams, said half-rate data streams carrying data equivalent to a single full-rate data stream; and
multiplexing representations of said parity streams onto a return-to-zero (RZ) optical signal as duobinary RZ modulation having intensity corresponding to said full-rate data stream; and wherein:
the step of precoding includes delaying a second of said parity streams by said input symbol time for providing a delayed parity stream; exclusive-OR combining a first of said half-rate data streams and said delayed parity stream for providing a first of said parity streams; and exclusive-OR combining a second of said half-rate data streams with said first parity stream for providing said second parity stream.

10. The method of claim 9, wherein:
the step of multiplexing includes delaying said second parity stream by one-half said input symbol time with respect to said first parity stream for providing a half-delayed second parity stream; and modulating an RZ light signal with a modulation drive corresponding to a difference between said half-delayed second parity stream and said first parity stream for providing said duobinary RZ modulation as an optical electric field having said intensity.

11. The method of claim 10, wherein:
the step of multiplexing includes generating an RZ light signal and using a dual-drive optical modulator for multiplexing said half-delayed second parity stream and said first parity stream onto said RZ optical light signal for forming said optical electric field.

12. A method for transmitting an optical signal, comprising:
precoding two half-rate data streams having half-rate input symbol times into two cumulative cross parity streams, said half-rate data streams carrying data equivalent to a single full-rate data stream; and
multiplexing representations of said parity streams onto a return-to-zero (RZ) optical signal as duobinary RZ modulation having intensity corresponding to said full-rate data stream; and wherein:
said RZ optical signal has an optical electric field having intersymbol low field levels and field states including low field states and non-zero field states, said intersymbol low field levels interleaved between said field states, said intensity of said non-zero field states greater than said intersymbol low field levels or said low field states; and
the step of multiplexing includes modulating said optical signal for said optical electric field having one of said non-zero field states having a first field sense when there are an even number of said low field states following a last one of said non-zero field states; having a change in said field sense when there are an odd number of said low field states following said last non-zero field state; and having no change in said field sense until at least one of said low field states after said last non-zero field state.