In order to adapt to the dynamics in the future optical networks, we propose a flexible high-order QAM transmitter using a tandem in-phase/quadrature (IQ) modulators to synthesize different high-order quadrature amplitude modulation (QAM) formats, such as 16QAM, 32 or 36QAM and 64QAM. To generate high-order QAMs, an offset-QAM is firstly generated using an IQ modulator driven by electronics with reduced modulation-level, and then mapped to other quadrants through another following IQ modulator configured as a standard quadrature phase-shift keying (QPSK) modulator. All of the embedded sub-Mach-Zehnder modulators are operated in push-pull configurations to avoid introducing excess phase chirp. In contrast with the schemes based on a single IQ modulator driven by multilevel electronics or a highly-integrated parallel modulator, by deploying commercially-available optical modulators and driving electronics with reduced modulation-level, the transmitter complexity in optics and electronics is well-balanced. In the case of generating optical 64QAM, different from another tandem scheme deploying dual-drive IQ modulator driven by independent four binary streams, less phase chirp is observed in our proposed scheme, and comparable implementation penalty is obtained even without applying additional specific compensation algorithm in the coherent receiver. Moreover, thanks to the tandem structure and the deployment of QPSK modulator, the obtained high-order QAM is naturally differentially coded, which is helpful to solve the phase ambiguity at coherent receiver. We experimentally demonstrate the generations of these high-order QAMs including 16QAM, 32/36QAM and 64QAM, and confirm the error-free operations with comparable BER performance to the “electrical” approach based on a single IQ modulator.
©2013 Optical Society of America
High-order quadrature-amplitude modulation (QAM), such as 16QAM, 32 (or 36) QAM and 64QAM, has raised a lot of research attention as a candidate for scaling up the capacity and spectrum efficiency in optical links. For example, in order to generate 64QAM, several schemes have been reported, such as a single in-phase/quadrature (IQ) modulator with eight-level driving electronics [1, 2], or integrated three IQ modulators in parallel driven by binary electronics . Although the “electrical” solution based on a single IQ modulator [1, 2] has a simple optical hardware, it requires sophisticated technique for preparing superior-quality eight-level electronics by either combining three binary electrical signals  or deploying high-speed digital-to-analog converters (DACs) . The operation symbol-rate is restricted by the DACs’ resolution, linearity of driver amplifiers or bandwidth of electrical components. On the other hand, the “optical” synthesis approach based on highly-integrated parallel modulators  could minimize the complexity in driving electronics, but it takes time to be commercialized, and shows poor performance even with back-to-back configuration, e.g. exhibiting error floor at bit-error rate (BER) of around 10−2 when generating 64QAM . Recently the tandem-modulator approach, a serial combination of IQ modulator with a dual-drive Mach-Zehnder modulator (MZM) , a phase modulator , or another IQ modulator [6, 7], has been utilized for synthesizing various multilevel optical signals such as 8-ary phase-shift keying (8PSK), 8QAM and 16QAM, where only binary electronics were deployed to drive each modulator. If extending this scheme to generate higher-order QAM, like 32/36QAM and 64QAM, we can also benefit from the reduced complexity in driving electronics. Instead of preparing superior-quality six- or eight-level electronics for generating 32/36QAM and 64QAM, just binary and three- or four-level electronics are required. The overall complexity of the transmitter could be effectively traded-off between electrical and optical parts in the proposed scheme. In contrast with the “electrical” [1, 2] and “optical” [3, 8] approaches, the proposed scheme is referred to as “hybrid” approach here. We also noticed that, quite recently, another 64QAM transmitter was reported using tandem two IQ modulators where at least one of them must be with four drive arms (dual-drive) driven separately by four binary streams in order to synthesize offset 16QAM . However, since the embedded sub-MZMs in the dual-drive IQ modulator are not operated in push-pull configurations, significant phase chirp is introduced to the generated 64QAM signals. Return-to-zero pulse carving is indispensable to suppress the strong phase chirp. Moreover, although binary driving electronics are deployed, the transmitter is sensitive to the bias and driving conditions in order to ensure the equal spacing between symbols in the resultant constellation. An additional specially-developed equalization algorithm at coherent receiver is required to compensate for the symbol deviation in the constellation.
In this paper, we propose and experimentally demonstrate a flexible high-order QAM transmitter using two tandem IQ modulators with balanced complexity in electronics and optics. The complexity in driving electronics is significantly reduced by deploying electronics with fewer levels, compared with the “electrical” approach. Meanwhile, it is more practical than the “optical” approach, as just commercially-available IQ modulators are deployed. With fixed configuration in optical part, i.e. two tandem IQ modulators, optical 16-, 32 (36)-, 64-QAMs are successfully synthesized by feeding binary, three- and four-level driving electronics, respectively. When generating 64QAM using the proposed scheme, different from another tandem transmitter scheme driven by binary electronics , less phase chirp is introduced in the proposed scheme, and no additional specific algorithm is required to compensate for the symbol deviation from the desired position and unequal distances among symbols in the resultant constellation. Error-free operation is experimentally confirmed for these formats. Since the “electrical” approach has been widely used for generating high-order QAM, the performance of high-order QAMs generated from a single IQ modulator driven by multi-level electronics is also experimentally evaluated for performance comparison. The measured BER results of the proposed scheme show comparable performance to the “electrical” approach. Thanks to the tandem structure in the transmitter and the use of standard QPSK modulator, the coding of the generated QAM signals naturally follows the differential coding rule, which is helpful to solve the phase ambiguity at coherent receiver. As the proposed transmitter could be re-configured to generate several different multilevel QAM signals using the same optical hardware, it could be deployed as a flexible QAM transmitter to adaptively generate the desired QAM formats including cross and square QAMs in the future dynamic optical networks. The proposed tandem IQ scheme features versatility, flexibility and simplicity.
2. Operation principle
The operation principle of the proposed high-order QAM transmitter is depicted in Fig. 1 . It consists of two tandem IQ modulators. As well known, if biasing the nested sub-MZM at null point and introducing 90-degree phase-offset between two branches in an IQ modulator, QPSK, 9QAM or 16QAM could be synthesized by feeding binary, 3- or 4-level driving electronics to each sub-MZM in IQ modulator. With a certain amount of bias offset from null point (e.g. quadrature point) and a reduced peak-to-peak driving voltage (<Vπ) for each sub-MZM, these QAM symbols would be squeezed to one quadrant in complex IQ plane, referred to as offset-QAM here. Another following IQ modulator acts as a standard QPSK transmitter by configuring the bias point at null point and driving signals with 2Vπ. After combing these two IQ modulators in series, the obtained offset-QAMs from the first IQ modulator will be mapped to other quadrants, thereby obtaining a complete 16-, 36- or 64-QAM constellation. Obviously the complexity in electrical part is drastically decreased albeit two commercially-available standard IQ modulators are deployed. In principle, the 3- or 4-level driving electronics for generating the offset-QAM could be synthesized either by combining several binary electrical signals using broadband combiners or by deploying DACs. It is beneficial to use DACs since pre-equalization could be applied to the driving electronics in order to compensate the distortions caused by the nonlinearity in both driving amplifiers and modulators. Automatic bias controller with feedback circuits could be simply deployed to stabilize the transmitter performance. Note that, different from the scheme in , the deployed IQ modulator in our proposed scheme could be either a single-drive one or a dual-drive one in a push-pull driving condition, where the phase chirp could be well managed.
Usually, the problem of the phase ambiguity which exists in the recovered carrier signal at coherent receiver can be solved by the use of differential encoding. Fortunately, in our proposed transmitter, the deployment of standard QPSK modulator and tandem configuration makes offset-QAM in the first quadrant map to other quadrants through phase rotation around the center in complex plane. Thereby, the resultant coding of the generated QAM naturally follows the differential coding, which helps overcome the four-fold phase ambiguity in the decoder at receiver side. As an example, the bit to symbol mapping for the obtained 64QAM is illustrated in Fig. 2 . Here we assume natural coding is applied in each IQ modulator. The offset 16QAM in the first quadrant is generated from IQ1. The corresponding coding is represented by I2Q2I1Q1. The two most significant bits (MSBs, I3Q3) are determined by the following standard QPSK modulator, i.e. the relative quadrant location in the complex plane. The other least significant bits (LSBs, I2Q2I1Q1) in other quadrants are obtained by rotating offset 16QAM in the first quadrant by corresponding phase degrees. It clearly shows that the resultant bit to symbol mapping happens to be coded in a differential manner.
Another tandem transmitter scheme has been reported to generate 64QAM . Two tandem IQ modulators are also deployed. However, in order to synthesize offset 16QAM, at least one of them should be dual-drive IQ modulator and the embedded sub-MZMs are separately driven by four binary streams, which is equivalent to four parallel phase modulators. To study the performance difference between these two tandem schemes when generating 64QAM, numerical simulation is performed to investigate the constellations. Figure 3 shows numerically-simulated offset-16QAM and 64QAM constellation diagrams generated by our proposed scheme and the binary-tandem scheme reported in . In order to see the trajectories among symbols, low-pass Bessel filtering with 3-dB bandwidths of 0.75 times the symbol rate was applied at the driving electronics. It is seen that the transitions between the symbols are straight lines for the offset-16QAM in our proposed transmitter since all of the embedded sub-MZMs are driven in push-pull configurations. Although cascade structures are deployed in both schemes, compared with our proposed scheme, it is obvious that more bended transitions among symbols are seen in the resultant offset-16QAM and 64QAM constellations in the binary-tandem scheme reported in . It indicates that our proposed scheme has less phase chirp and it is less sensitive to bandwidth limitations in the electronics, and more tolerant against dispersion. This is mainly attributed to the push-pull configurations in the embedded sub-MZMs in our proposed scheme.
3. Experiment and results
We performed experiments to verify the feasibility of the proposed high-order QAM transmitter by demonstrating the generations of optical 16QAM, 32QAM, 36QAM and 64QAM. Figure 4 shows the experimental setup for performance evaluation of the proposed transmitter. A fiber laser (FL) at ~1552.4 nm with a line-width of around 10 kHz was deployed as a laser source. The proposed high-order QAM transmitter is composed of two tandem IQ modulators (IQ-1 and IQ-2). The deployed IQ modulators used in the experiment have 3-dB optical bandwidths of around 25 GHz, and ~3.5-V half-wave voltages (Vπ). To generate offset-QAMs, IQ-1 was driven by electrical signals from an arbitrary waveform generator (AWG, Tektronix AWG7122B) operated at a sampling rate of 12 GSamples/s. The electrical driving signals had around 1-V peak-to-peak swing. Binary, 3- or 4-level electrical driving signals could be synthesized by programming the AWG. They were originally generated from de-correlated pseudo-random binary sequences (PRBS) with a length of 215-1. These multilevel electronics could be used to drive IQ modulator to generate optical offset QPSK, 8 (or 9) QAM, and 16QAM. Two nested sub-MZMs in IQ-1 were biased at the point with around 1-V offset from the null point of the modulator. In the second IQ modulator, IQ-2, two sub-MZMs were fully-driven by two de-correlated binary 215-1 PRBS streams with peak-to-peak voltage of ~7V and biased at null point in order to generate a QPSK. A variable optical delay line (∆T) was inserted between these two cascaded IQ modulators for time synchronization. For performance comparison, the “electrical” approach was also performed for generating high-order QAMs. A single IQ modulator with the nested sub-MZMs biased at null points and driven by electrical 4-, 6-, 8- level signals with 1-V peak-to-peak driving voltage was used for synthesis of optical 16-, 32-(36-), and 64-QAM.
At the receiver side, an attenuator was placed in the front of an EDFA followed by an optical band-pass filter to adjust the optical signal-to-noise ratio (OSNR) of the received signal. The signal was then detected by a phase-diversity intradyne coherent receiver, including a local oscillator (LO), an optical 90-degree hybrid and two balanced photo-detectors (PDs). The LO was generated from another FL with a line-width of around 10 kHz. After detection using balanced PDs, the data were digitized at 50 GSamples/s by using a digital storage oscilloscope with a 12.5-GHz analog bandwidth (Tektronix DPO71254). The captured data were then off-line processed for reconstructing constellation and measuring BER. The deployed digital signal processing (DSP) flow is illustrated in Fig. 4. It is mainly composed of compensation of skew, power and IQ imbalance, data resampling, carrier recovery, linear equalization by finite impulse response (FIR) filtering and final hard-decision circuits. Note that it is straightforward to apply the proposed transmitter to the polarization multiplexing scenario. However, due to the unavailability of hardware, here we only demonstrate the single-polarization application. The detailed experiment results for generating 16-, 32-, 36- and 64QAM will be presented separately in the following sub-sections.
In order to generate optical 16QAM, only binary electronics are required to drive each IQ modulator in the proposed tandem transmitter. As shown in Fig. 5 , an offset-4QAM (Fig. 5(a)) and a standard QPSK (Fig. 5(b)) are generated respectively in the two cascaded IQ modulators. Finally 16QAM (Fig. 5(c)) could be obtained after the proposed transmitter. The measured intensity eye diagrams and corresponding constellations are shown in Fig. 5. The feasibility of generating 16QAM using the proposed tandem scheme has been verified by measuring the constellation at 40Gbaud in , but BER has not measured yet. Here, the noise tolerance of the generated optical 16QAM is quantitatively evaluated by measuring BER under different OSNR (noise bandwidth: 0.1nm). Figure 6 shows the plotted BER curves under different OSNR conditions. For performance comparison, the “electrical” scheme using a single IQ modulator driven by 4-level electronics is also experimentally demonstrated by measuring BER performance. As shown in Fig. 6, the proposed scheme and the “electrical” scheme exhibit similar BER performance. The required OSNR at BER of 3.8x10−3, which corresponds to the 7%-overhead pre-forward error correction (pre-FEC) BER threshold, is measured to be 13 dB. In contrast to the theoretical limit, around 1.5 dB implementation penalties are observed at BER of 3.8x10−3. The inset (a) and (b) in Fig. 6 show the measured optical eye diagrams of the obtained 16QAM using the proposed scheme and the “electrical” scheme, respectively.
3.2 32QAM and 36QAM
Recently, optical 32 or 36QAM has been used to realize high spectrum-efficient Nyquist wavelength-division multiplexing transmission (WDM) with spectral efficiency of up to 8.37b/s/Hz [10, 11]. Usually optical 32 or 36QAM is synthesized using a single IQ modulator driven by 6-level driving electronics, i.e. “electrical” approach. Using the proposed tandem structure, 32 or 36 QAM could also been generated if one of the IQ modulators is driven by 3-level electronics. Obviously the complexity in electronics is simplified compared with the “electrical” approach. Figure 7 depicts the measured constellation and optical intensity eye diagrams. Offset 8QAM (Fig. 7(a)) or 9QAM (Fig. 7(c)) is firstly generated from an IQ modulator driven by 3-level electronics. The obtained 8QAM or 9QAM illustrates 5 or 6 levels in optical intensity from the measure eye diagrams. After cascading offset 8- or 9QAM with a standard QPSK, 32- or 36QAM could be simply obtained, which is shown in Fig. 7(b) and Fig. 7(d), respectively.
BER performance of the obtained 32- and 36QAM is measured and shown in Fig. 8 and Fig. 9 , respectively. For both 32QAM and 36QAM, comparable BER results are obtained by the proposed “hybrid” scheme (blue squares) and the “electrical” scheme (red circles). For the obtained 32QAM, compared with the theoretical BER approximation, around 2-dB implementation penalty is observed at BER of 3.8x10−3. In the experiment of the square 36QAM, for simplicity, each symbol is decoded into 5 bits for BER measurement, same to the cross 32QAM. As shown in Fig. 9, around 2.3 dB implementation penalty is obtained at BER of 3.8x10−3, in contrast to the theoretical BER result.
Figure 10 shows the measured optical intensity eye diagrams (top) and constellations (bottom). If only activating the first IQ modulator, IQ-1, and driving the modulator using 4-level driving electronics, offset-16QAM constellation is observed, shown in Fig. 10(a). After cascading with a QPSK, optical 64QAM are finally synthesized (Fig. 10(b)). The optical spectrum of the obtained 60-Gbit/s 64QAM is shown in the inset of Fig. 4 with a bandwidth resolution of 0.01 nm. The measured BER as function of OSNR is plotted in Fig. 11 . The theoretical curve is also presented for reference. Around 2.8 dB and 4.3 dB implementation penalties are observed at BERs of 2.4x10−2 and 3.8x10−3, respectively. The implementation penalty is comparable to that obtained in the “electrical” scheme , where a single IQ modulator driven by eight-level electronics, and less than that observed in another tandem transmitter scheme where a dual-drive IQ modulator is driven by binary electronics , even without deploying additional specially-designed equalization algorithm. The observed eye diagrams of the proposed scheme and “electrical” scheme are shown in the inset of Fig. 11. Since the 8-level electronics from AWG7122B is used to drive the single IQ modulator, due to the limited bandwidth of the AWG, the observed eye diagram in the “electrical” approach is severely deteriorated compared with that in the proposed scheme.
In the proposed tandem transmitter structure, a specific hierarchical equalization algorithm would be helpful to separately compensate the distortion caused by the imperfect biasing in different modulators in the transmitter. Further improvement is expected when applying more sophisticated pre- or post- equalization technique in the system. Note that, in , a phase-folded decision-directed (PFDD) equalization algorithm was applied at the coherent receiver to partially overcome the transmitter imperfections. Since the offset-16QAM was obtained by using a dual-drive IQ modulator fed by four binary electronics in , it was much easier to have the symbols shifted from the desired positions due to phase chirp and the imperfect bias and driving conditions. Therefore, the PFDD algorithm was helpful to compensate the constellation distortion. However, in our “hybrid” scheme, the offset-16QAM is obtained by driving an IQ modulator with 4-level electronics, where the equal symbol distances within 16QAM subset could be naturally ensured. As shown in Fig. 11, even without deploying such algorithm, better BER performance could be obtained in our proposed scheme.
In our proposed scheme, to properly generate offset-QAMs, the embedded sub-MZMs in IQ-1 are driven by a small voltage, and biased at points with an offset from null points. The insertion loss of the deployed IQ modulator in the experiment is around 7dB. Except the insertion loss, the input light experiences additional 9-dB loss to generate offset-QAMs. Thanks to the tandem structure in our proposed scheme, EDFA could be inserted between two tandem modulators to maintain the signal’s OSNR from the transmitter.
Instead of using a highly-integrated parallel optical modulator or complicated multi-level driving electronics, we demonstrate a flexible high-order QAM transmitter using a “hybrid” approach, i.e. two tandem IQ modulators driven by driving electronics with reduced modulation-level. Since only commercially-available modulators and electronics with fewer levels are deployed, the transmitter complexity in optical and electrical parts is well-balanced. In the case of generating 64QAM, in contrast with another tandem scheme using binary driving electronics , phase chirp is well-managed and no additional specific algorithm is required to compensate the distortion in the constellation. It could be configured as high-order QAM transmitter for generating 16QAM, 32 (or 36) QAM, and 64QAM. Error-free operations have been experimentally confirmed for all of these formats with comparable BER performance to the “electrical” approach based on a single IQ modulator.
G.-W. Lu would like to thank Mr. Nobuyuki Onodera from Tektronix Japan for kind help and technical support. The work was supported in part by Grant-in-Aid for Young Scientist (B) (23760352) from the Ministry of Education, Science, Sports and Culture, Japan.
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