JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 29, NO. 17, SEPTEMBER 1, 2011 2545 A Method for Generating Arbitrary Optical Signal Constellations Using Direct Digital Drive Yossef Ehrlichman, Ofer Amrani, and Shlomo Ruschin Abstract—A digitally operated optical quadrature-amplitude modulation (QAM) modulator based on a single multielectrode Mach–Zehnder modulator is presented. Generation and perfor- mance of 64QAM are studied in detail. Simulation results show that a single modulator with 13 electrodes, each of which is driven by either one of the voltages 0 or , provides close-to-ideal 64QAM constellation despite the inherent nonlinearity of the modulator. Moreover, employing a sufficient number of elec- trodes, close to ideal error performance can be obtained for any constellation order, or shape. Simulations results are demon- strated for several different square constellations: 16QAM and 256QAM, and nonsquare constellations: 32QAM and 128QAM. A brief discussion on the utilization of the proposed scheme as a predistorter is also given. Index Terms—Direct digital drive, Mach–Zehnder, quadrature amplitude modulation (QAM). I. INTRODUCTION M ODULATION schemes such as binary phase-shift keying (PSK) and quadrature PSK (QPSK) are typi- cally limited to transmitting a few tens of gigabits per second on a fiber-optic channel. Approaching the 100 Gb/s range with differential-QPSK modulation would require a high signaling rate of 50 GBaud (or 25 GBaud if polarization multiplexing is used). To alleviate this problem, multilevel modulation schemes, such as M-ary quadrature amplitude modulation (QAM) (or multiphase modulation such as M-PSK) can be used. QAM is a modulation scheme that conveys data by means of modulating both the amplitude and the phase of a sinusoid carrier, thus providing spectral efficiencies in excess of 2 bit/symbol. A widely accepted approach for generating multilevel sig- naling is by employing two parallel Mach–Zehnder modulators (MZMs) making the optical equivalent of an IQ modulator [1], [2]. According to this approach, the required number of control voltages increases as the number of levels in the M-ary modu- lation scheme is increased. For example, a 64QAM modulator may require as many as 128 distinct control voltage levels. Ho and Cuei [3] proposed a method for generating QAM sig- nals using a single dual-drive modulator with a single electrode on each arm. Although the hardware saving associated with this Manuscript received December 27, 2010; revised April 04, 2011, June 06, 2011; accepted June 13, 2011. Date of publication June 27, 2011; date of current version August 17, 2011. The authors are with the School of Electrical Engineering, Tel-Aviv Univer- sity, Tel-Aviv 69978, Israel (e-mail: syos@eng.tau.ac.il; ofera@eng.tau.ac.il; ruschin@eng.tau.ac.il). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/JLT.2011.2160449 approach appears attractive, the set of distinct voltage levels re- quired to drive the electrodes increases with the constellation size. Kametani et al. [4] demonstrated the generation of 16QAM constellation with a dual-drive MZM. This modulator employs a symbol mapper comprising of a lookup table (LUT) that feeds two digital-to-analog converters (DAC) which in turn drive the two electrodes. It is shown that a driving voltage of is sufficient for covering the electric field required for ideal 16QAM modulator with a two 6-bit DACs. A limiting factor associated with this approach is the speed of the electrical DAC. Multilevel electrical driving signals are difficult to obtain at high data rates as their implementation requires high-speed DAC. Hence, modulator configurations that rely on binary electrical driving signals are highly attractive. Yamazaki et al. [5] demonstrated generation of 64QAM mod- ulation providing 60 Gb/s. The modulator consisted of 12 high- speed phase modulators integrated with a hybrid configuration of silica planar lightwave circuits and LiNbO . Siemetz [6] discussed a scheme that generates square 16QAM constellation using binary driving signals. The scheme consisted of an IQ modulator followed by a QPSK modulator, which was implemented by two phase modulators. This imple- mentation is rather quite complex as it requires synchronizing four modulators. Sakamoto et al. [7] demonstrated modulation of 16QAM at 50 Gb/s using a multiarm MZM. According to this approach, QAM signals were synthesized from binary electric signals by using a multiparallel modulator. In each arm of the modulator, binary-PSK was generated by the MZM and every two arms were paired to form QPSK. The QPSKs created in the multi- parallel modulator have different amplitudes. To create -level QAM, superposing sets of the QPSKs was required. For the generation of a 16QAM constellation, four integrated MZMs are needed, and for 64QAM constellation, six integrated MZMs are needed. Kang [8] presented a hybrid integrated modulator using an array of four AIGaInAs/InP electroabsorption modulators (EAM) integrated with an array of semiconductor optical amplifiers to compensate for optical coupling losses. 50 Gb/s QPSK was demonstrated and it was pointed out that the modu- lator is capable of generating QAM signals. Doerr [9] showed a compact InP 16QAM modulator which employed four EAMs modulators using a four-arm interferometer. Lu et al. [10] demonstrated a 40 Gbaud 16QAM transmitter using tandem IQ modulators driven by four separate binary electrical signals. Herein, we present a simple method for generating arbitrary M-ary constellations by introducing the concept of direct digital driving, which facilitates the use of only two voltage levels for 0733-8724/$26.00 © 2011 IEEE