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.

Description:
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:LiNbO 3  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 D 1  (t), denoted by  25   a , and D 2 (t), denoted by  25   b , respectively, and computing first and second cumulative cross parity streams P 1 (t), denoted by  26   a , and P 2 (t), denoted by  26   b , respectively. The first and second data streams D 1 (t)  25   a  and D 2 (t)  25   b  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  26   a  P 1 (t) equal to P 2 (t−T)+D 1 (t)mod 2, and the second exclusive-OR gate  21  provides the second cumulative cross parity stream  26   b  P 2 (t) equal to P 1 (t)+D 2 (t)mod 2, where the T is a half-rate input symbol time corresponding to the symbols in the half-rate input data-streams D 1 (t)  25   a  and D 2 (t)  25   b . Recursive operation of the first exclusive-OR gate  20  results in the first cumulative cross parity stream  26   a  P 1 (t) of D 1 (t)+D 2 (t−D)+D 1 (t−T)+D 2 (t−2T)+D 1 (t−2T)+D 2 (t−3T)+D 1 (t−3T)+ . . . modulo 2 as the cumulative cross parity of the first data stream D 1 (t)  25   a  and the second data stream  25   b  one symbol delayed D 2 (t−T). It should be noted that the first cumulative cross parity stream  26   a  P 1 (t) is the cross parity of the first data stream D 1 (t)  25   a  and the one symbol delayed second data stream D 2 (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  26   b  P 2 (t) of D 2 (t)+D 1 (t)+D 2 (t−T)+D 1 (t−T)+D 2 (t−2T)+D 1 (t−2T)+D 2 (t−3T)+D 1 (t−3T)+ . . . modulo 2 as the cumulative cross parity of the second data stream D 2 (t)  25   b  and the first data stream  25   a . It should be noted that the second cumulative cross parity stream  26   b  P 2 (t) is the cross parity of the second data stream D 2 (t)  25   b  and the first data stream D 1 (t) plus the previous second cumulative cross parity. Filters may be inserted for filtering the cumulative cross parity streams P 1 (t)  26   a  and P 2 (t)  26   b  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 V 1 (t), denoted by  40   a , and V 2 (t), denoted by  40   b , respectively. A first modulator driver  41   a  amplifies the first precoder output (first cumulative cross parity stream)  26   a  for providing the first modulator drive signal  40   a . A second modulator driver  41   b  amplifies the second precoder output (second cumulative cross parity stream)  26   b  before or after the second precoder output  26   b  is delayed by a one-half symbol delay component  42  by half the input symbol time (T/2). Because the modulator drive signals V 1 (t)  40   a  and V 2 (t)  40   b  are binary, the modulator drivers  41   a  and  41   b  may be limiting, saturated, or nonlinear amplifiers without a linearity requirement. The optical modulator  27  is biased with a bias voltage V b , denoted by  45 , for providing a modulator output signal  50 . The bias voltage V b    45  is set so that the modulator output signal  50  is minimized when the voltages of the first and second modulator drive signals  40   a  and  40   b  are equal. 
       FIG. 1A  is a block diagram of a duobinary transmitter of the present invention having a reference number  10 A. The duobinary transmitter  10 A includes the precoder  11  described above and a multiplex modulator  12 A. The multiplex modulator  12 A includes the dual-drive Mach-Zehnder optical modulator  27 , the light source  28 , the first modulator driver  41   a , the second modulator driver  41   b , and the one-half symbol delay component  42  described above. The multiplex modulator  12 A 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 I NZ (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 V 1 (t)  40   a  and V 2 (t)  40   b  for providing a return-to-zero (RZ) duobinary output signal  50 A having an optical electric field E O (t) ( FIG. 6C ) and an output optical intensity I O (t) ( FIG. 6D ). 
       FIG. 1B  shows the RZ modulator  30  as a zero-chirp balanced Mach-Zehnder device having a drive range V π . 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 D 1 (t) and D(t) and is synchronized with the data streams D 1 (t) and D 1 (t). The peak-to-peak amplitude of the clock signal C(t) is nominally twice the range V π  of the RZ modulator  30 . The RZ Mach-Zehnder modulator  30  is biased with a voltage V bNZ  for providing pulses for the intensity I NZ (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 C 2 (t) operates a twice the rate of the first and second half rate data streams D 1 (t) and D 1 (t) (twice the rate of the clock signal C(t) shown in  FIG. 6A ) and is synchronized with the data streams D 1 (t) and D 1 (t). In this implementation, the peak-to-peak amplitude of the clock signal C 2 (t) is nominally the same as the range V π  of the RZ modulator  30  and the modulator  30  is biased at a voltage V bNZ +V π /2 or a voltage V bNZ −V π /2 for providing pulses for the intensity I NZ (t) at the same rate as the clock signal C 2 (t) according to the transfer function shown in  FIG. 6E . 
       FIGS. 2A and 2B  show exemplary first and second binary input data streams  25   a  D 1 (t) and  25   b  D 2 (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)  26   a  P 1 (t) and  26   b  P 2 (t), respectively, responsive to the exemplary input data streams  25   a  and  25   b , versus the time t.  FIGS. 2E and 2F  show the first and second modulator drive signals V 1 (t)  40   a  and V 2 (t)  40   b , respectively, responsive to the exemplary input data streams  25   a  and  25   b , versus the time t. The modulator drive signals  40   a  and  40   b  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  50   a , and an optical intensity I(t), denoted by  50   b , respectively, responsive to the exemplary input data streams  25   a  and  25   b , versus the time t. Note that the beginning time t from 0 to T/2 of the signals  40   b ,  50   a , and  50   b  cannot be derived from the input data streams  25   a  and  25   b . Importantly, it should be noted that the optical intensity I(t)  50   b  corresponds to the multiplexed data in the combination of the first and second data input data streams D 1 (t)  25   a  and D 2 (t)  25   b , thereby enabling symbol-by-symbol recovery by an intensity detector of the full-rate original data. 
       FIG. 3A  shows an electric field transfer characteristic E out (t)/E in (t), denoted by  60   a , of the optical modulator  27  with respect to the difference V 1 (t)−V 2 (t) between first and second modulator drive signals V 1 (t)  40   a  and V 2 (t)  40   b .  FIG. 3B  shows an intensity transfer characteristic I out (t)/I in (t) denoted by  60   b , of the optical modulator  27  with respect to the difference V 1 (t)−V 2 (t) between first and second modulator drive signals V 1 (t)  40   a  and V 2 (t)  40   b . The peak-to-peak signal swing for each of the modulator drive signals  40   a  and  40   b  is equal to the maximum peak input V π  specified for the modulator  27 . 
     Using the transfer characteristic  60   a  in  FIG. 3A  in terms of an optical electrical field, the modulator output signal  50  has the optical electrical field of  50   a  that is shown in  FIG. 2G . The optical electrical field of  50   a  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  60   b  in  FIG. 3B  in term of intensity, the modulator output signal  50  has the intensity of  50   b  that is shown in  FIG. 2H . Comparing the waveform of the output intensity  50   b  with the input data streams  25   a  and  25   b , it is seen that the intensity  50   b  is a multiplexed signal of both  25   a  and  25   b . If a photodetector is used to detect the intensity of  50   b , no decoder but a demultiplexer is required to recover the original data in the waveforms of  25   a  and  25   b . Importantly, the intensity waveform of  50   b  has twice the data-rate of the input data streams of  25   a  and  25   b.    
       FIG. 4  illustrates an experimental setup for demonstrating the function of the multiplexing modulator  12  using two 10 Gb/s pattern generators  100   a  and  100   b  to give two independent 23 31 −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  110   a ,  110   b  and  111 , respectively, at the oscilloscope  102 , when the bias voltage V b    45  ( FIG. 1 ) is properly adjusted for each individual case. The eye-pattern  110   a  is recorded when the pattern generator  110   a  is operating and the pattern generator  100   b  is not operating. The eye-pattern  110   b  is recorded when the pattern generator  100   b  is operating and the pattern generator  100   a  is not operating. The eye-pattern  111  is recorded when both the pattern generator  110   a  and  100   b  are operating. Comparing eye-patterns  110   a ,  110   b  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  11   z (t).  FIG. 6C  shows the optical electric field signal E O (t) for the RZ duobinary output signal  50 A. The optical electric field signal E O (t) carries information symbols in three states—a low (zero) level field  52   e , a high (non-zero) field of a positive polarity  53   e , and a high (non-zero) field of a negative polarity  54   e . The timing of the clock signal C(t) is controlled so that the optical electric field E O (t) has an intersymbol low (zero) level field  51   e  during phase transitions between the symbols.  FIG. 6D  shows the intensity I O (t) of the RZ duobinary output signal  50 A. The intensity I O (t) has a low (zero) level intensity  51   i  ( FIG. 6D ) corresponding in time to the low level field  51   e  between the informational states, a low level intensity  52   i  ( FIG. 6D ) corresponding in time to the low (zero) field  52   e , a high level intensity  53   i  ( FIG. 6D ) corresponding in time to the high level positive field  53   e , or a high level intensity  54   i  ( FIG. 6D ) corresponding in time to the high level negative field  54   e.    
       FIGS. 6C and 6D  show the optical symbol states carrying information in the optical signal  50 A in the form of an optical electric field E O (t) and an optical intensity I O (t) responsive to the exemplary input data streams  25   a  and  25   b , versus the time t. Note that the beginning time t from 0 to T/2 of the signals  40   b  and  50 A cannot be derived from the input data streams  25   a  and  25   b . Importantly, it should be noted that the optical intensity I O (t) corresponds to the multiplexed data in the combination of the first and second data input data streams D 1 (t)  25   a  and D 2 (t)  25   b , 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 I output  versus the drive voltage input V input  of the modulator  30 . The V bNZ  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.