Abstract:
A method and apparatus for generating multilevel amplitude and phase encoded signals using a pair of 1:N beam splitter/combiner(s) having a pre-defined splitting distribution and an array of zero-biased, Mach-Zehnder Modulators configured for phase modulation.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates generally to the field of photonics and in particular to methods and apparatus for generating multilevel amplitude and phase encoded signal(s).  
       BACKGROUND OF THE INVENTION  
       [0002]     High performance optical communications systems require transmitters capable of modulating both the amplitude and phase of an optical signal having a high data rate with multi-level precision. In addition, optical modulation formats used for such high performance systems must provide high spectral efficiency, data-rate agility and high receiver sensitivity.  
         [0003]     A 16-state modulation format based upon differential quadrature-phase-shift keying (DQPSK) and a 4-level pulse amplitude modulation format (PAM4) has been recently demonstrated. (See, e.g., K. Sekine et al.,  Proposal And Demonstration Of  10- G symbol/sec,  16- ary  (40  Gb/s )  Optical Modulation/Demodulation Scheme ,” which appeared in Proceedings of European Conference on Optical Communication (ECOC&#39;04), paper We3.4.5, 2004). As is known, the DQPSK-PAM4 format encodes data at 4 bits/symbol and is typically realized by driving a single Mach-Zehnder Modulator (MZM) with a 4-level electronic signal, which in turn is produced by power-combing two signal tributaries of different amplitudes. Unfortunately, this DQPSK-PAM4 scheme exhibits a number of infirmities.  
         [0004]     More specifically, and in addition to being overly complex, this scheme results in a less-than-optimal amplitude spacing for the PAM4 modulation since the transfer function of the MZM is pre-determined and cannot be used to produce an optimal spacing (or constellation). Additionally, to approach the optimal spacing, the saturation regime of the MZM transfer function cannot be reached, thereby resulting in additional optical loss and a larger inter-symbol interference (ISI) with a limited modulator bandwidth. Finally, the generation of the multi-level RF signal causes higher ISI in the RF domain and the amplification of the multi-level RF signal requires an amplifier having a large bandwidth and high linearity, which is much harder to implement than a saturating amplifier that is commonly used for binary modulation.  
       SUMMARY OF THE INVENTION  
       [0005]     We have developed a method and accompanying apparatus that produces a superior constellation of multilevel phase and amplitude modulated signals and does not suffer from the deficiencies associated with the prior art. Advantageously, our inventive method and apparatus employs an optical splitter/combiner pair—which exhibits preferable splitting ratio(s)—and an array of Mach-Zehnder Modulators configured for phase modulation.  
     
    
     BRIEF DESCRIPTION OF THE DRAWING  
       [0006]     A more complete understanding of the present invention may be realized by reference to the accompanying drawing in which:  
         [0007]     FIGS.  1 ( a ) and  1 ( b ) are constellation diagrams of DQPSK-PAM4 and DQPSK+PAM8 formats, respectively wherein signal average power is normalized to unity;  
         [0008]      FIG. 2  is a schematic of a modulator apparatus for modulating DQPSK-PAM2N signals according to the present invention;  
         [0009]      FIG. 3  is a constellation diagram of a QAM16 signal wherein signal average power is normalized to unity;  
         [0010]      FIG. 4  is a schematic of a modulator apparatus for modulating QAM2N signals according to the present invention; and  
         [0011]      FIG. 5  is a flowchart depicting our inventive method. 
     
    
     DETAILED DESCRIPTION  
       [0012]     With initial simultaneous reference to  FIG. 1 ( a ) and  FIG. 1 ( b ), there is shown preferred constellation diagrams for DQPSK-PAM4 and DQPSK-PAM8 formats, respectively. As is known, such constellation diagrams are representations of digital modulation scheme(s) in a complex plane. By convention, the real and imaginary axis are oftentimes called the in-phase or I-axis and the quadrature or Q-axis, respectively.  
         [0013]     With continued reference to  FIG. 1 ( a ) and  FIG. 1 ( b ), preferred relative amplitudes of the PAM levels for the DQPSK-PAM2N format(s) are approximately: 
 
 C ·(0.9+[0 1 2 . . . 2 N −1]) 
 
 where C is a normalizing factor. Note that the “amplitude” here refers to the amplitude in the field domain, which is the square root of intensity. The optimal value 0.9 for the lowest amplitude level is determined from the bit-error rate (BER) estimation with random Gaussian noise in the optical field. 
 
         [0014]     A preferred constellation of a multilevel signal may be determined by substantially equalizing the performance of all of the tributaries comprising the signal. As can be appreciated all of the symbols in a PAM constellation may be reached by the summation and subtraction among the following N terms, namely C·2 N−2 , C·2 N−3 , . . . C·2 −1 , on top of the averaged term, C·(0.4+2 N−1 ).  
         [0015]     For example, the 8-PAM levels of the DQPSK-PAM8 format, 0.9+[0 1 2 3 4 5 6 7], can be reached by 4.4±2±1±0.5. More specifically, a first data tributary can be encoded by the most significant modulation arm which provides the (±2) component, a second data tributary can be encoded by the second most significant modulation arm which provides the (±1) component, and the last data tributary can be encoded by the least significant modulation arm which provides the (±0.5) component. Note that an appropriate pre-coding can be used so that received signal tributaries can be independently recovered. Without modulating the least significant tributary or tributaries, the overall data rate of the generated multilevel signal will be reduced, but the signal will be more immune to noise. This provides a flexibility of data-rate agile transmission, in which the overall signal data-rate can be varied depending on the quality of the transmission link, e.g., the worse the link quality, the lower the data rate to ensure acceptable transmission performance. This data-rate agile transmission can be readily realized with the invention.  
         [0016]     With reference now to  FIG. 2  there is shown a schematic of a modulator for the DQPSK-PAM2N format, incorporating the teachings of the present invention. More particularly, laser  230  is followed by a DPQSK Modulator  240 , whose output of is split through the effect of power splitter  270 .  
         [0017]     As can be seen from this  FIG. 2 , the power splitter  270  includes N+1 output arms,  277 [ 0 ], . . . ,  277 [N], which exhibit split power distributions of (0.4+2 N+1 ), (2 N−2 ), (2 N−3 ), (1), (0.5), respectively. Consequently, and as can be readily appreciated by those skilled in the art, when modulated laser light output by modulator  240  is presented to the power splitter  270 , it is split into a number of signals, the number being equal to the number of individual output arms. As indicated earlier, each of the split signals exhibits a different power which, according to the present exemplary embodiment, is (0.4+2 N−1 ), (2 N−2 ), (2 N−3 ), . . . , (1), and (0.5) for each of the output arms  277 [ 0 ], . . . ,  277 [N], respectively.  
         [0018]     In this regard, our inventive structure employs two, “unequal splitters”, namely, the power splitter  270  and a power combiner  280 . For the DQPSK-PAM2 N  format shown and described here, we use two, 1:(1+N) splitters, wherein one of the arms is not modulated. As we shall describe later, our inventive structure(s) may be extended to the QAM-2 N  format with quadrature modulation in addition to the in-phase modulation described herein.  
         [0019]     Continuing with our discussion of this preferred embodiment for the generation of a DQPSK-PAM2 N  format, the output arm indicated by reference  277 [ 0 ], which has a power of (0.4+2 N−1 ), is un-modulated. A phase control unit  278  is used to ensure that the reference is aligned in phase with the modulated signals. Each of the other output arms  277 [ 1 ], . . . ,  277 [N], includes a Mach-Zehnder Modulator  278 [ 1 ], . . . ,  278 [N], biased at null, to act as a phase modulator. The optical signal(s) traversing each of the individual output arms  277 [ 1 ], . . . ,  277 [N], and its respective, null biased MZM  278 [ 1 ], . . . ,  278 [N], may be either unchanged, or changed by exactly π, by modulating the individual MZM with a signal tributary, thereby realizing the “summation” and “subtraction” functions that are required to reach all of the symbol states.  
         [0020]     The optical signals output from each of the MZM  278 [ 1 ], . . . ,  278 [N] positioned in a respective output arm  277 [ 1 ], . . . ,  277 [N], are recombined into a common signal through the effect of power combiner  280 . The power combiner  280 , is a mirror image of the power splitter  270 , having the same splitting ratio(s). The output of the power combiner  280 , is combined, DQPSK-PAM2 N  formatted signal, having (N+2) bits/symbol.  
         [0021]     Note that since the signal optical power is proportional to the square of the signal optical field amplitude, the cascaded use of the beam splitter and combiner having the same splitting ratios produces desired signal power levels.  
         [0022]     With these inventive teachings in place, we can extend our invention to quarature-amplitude modulation (QAM). Referring now to  FIG. 3 , there is shown a constellation diagram for a QAM16 signal. As is shown in that  FIG. 3 , the average power of the signal is normalized to unity.  
         [0023]     With reference now to  FIG. 4 , there is shown a schematic of a QAM2N modulator constructed according to our inventive teachings. To produce a QAM2N signal, there are a total of N arms interconnecting power splitter  420  and power combiner  430 , namely  440 [ 1 ], . . . ,  440 [N/2] and  445 [ 1 ], . . . ,  445 [N/2].  
         [0024]     As can be seen in this  FIG. 4 , N/2 of the arms ( 440 [ 1 ], . . . ,  440 [N/2] are for the in-phase(I) tributaries and the other N/2 arms ( 445 [ 1 ], . . . ,  445 [N/2]) are for the quadrature (Q) tributaries. Accordingly, as light emanating from laser  410  is directed into power splitter  420 , it is split into N separate signals, each traversing a separate output arm,  440 [ 1 ],  440 [N/2], and  445 [ 1 ], . . . ,  445 [N/2], and each having a particular power level effected by the power splitting distribution of the power splitter  420 .  
         [0025]     In this exemplary embodiment, the power splitting of the respective output arms is 2 N/2−1 , 2 N/2−2 , . . . , 1 for in-phase arms  440 [ 1 ], . . . ,  440 [N/2], and 2 N/2−1 , 2 N/2−2 , . . . , 1, for quadrature arms  445 [ 1 ], . . . ,  445 [N/2], respectively. Each signal traversing the arm(s) are modulated, through the effect of Mach-Zehnder Modulator(s)  450 [ 1 ], . . . ,  450 [N/2], and  455 [ 1 ], . . . ,  455 [N/2] configured for phase modulation and their respective tributary(ies). For the quadrature branches,  445 [ 1 ], . . . ,  445 [N/2], there is a π/2 phase offset  460 [ 1 ],  460 [N/2], between those quadrature branches and the in-phase branches  440 [ 1 ],  440 [N/2]. More specifically, the first I(Q) data tributary can be encoded by the most significant modulation arm which provides the (±2 N/2−1 ) component, the second I(Q) data tributary can be encoded by the second most significant modulation arm which provides the (±2 N/2−2 ) component, and the last I(Q) data tributary can be encoded by the least significant modulation arm which provides the (±1) component. Similar to the case with DQPSK+PAM signals, without modulating the least significant tributary or tributaries, the overall data rate of the generated QAM signal will be reduced to make the signal more immune to noise. This provides a flexibility of data-rate agile QAM transmission, in which the overall signal data-rate can be varied depending on the quality of the transmission link, e.g., the worse the link quality, the lower the data rate to ensure acceptable transmission performance.  
         [0026]     The signals traversing the output branches are subsequently combined, through the action of power combiner  430  which, as before, mirrors the power-splitting of power splitter  420 . Subsequently, a QAM2N signal, having N bits/symbol is output  470  from the power combiner  430 .  
         [0027]     As can now be appreciated, with our inventive design(s) shown and described, there is no modulator bandwidth-limitation induced optical phase pattern dependence since the MZMs used provides the exact changes in addition to the splitting ratio(s) being fixed. Additionally, and as we have shown, our inventive structures may be readily extended into alternative formats, such as the QAM2N format shown.  
         [0028]     At this point, while we have discussed and described our invention using some specific examples, those skilled in the art will recognize that our teachings are not so limited. Accordingly, our invention should be only limited by the scope of the claims attached hereto.