16QAM and Hexagonal 16QAM Optical Transmitter Using a Dual-Driven IQ Modulator Driven by 4 Binary Signals

Thanks to the advances of digital signal processing, high-order modulation formats signals with coherent detection provide more spectrally efficient optical transmission systems. Dual polarization (DP) quadrature phase shift keying (QPSK) operating at 100 Gb/s with receiver digital signal processing (DSP) are now commercially available [1]. 16-ary quadrature amplitude modulation (16-QAM), doubling the spectral efficiency (SE) further to 4 bit/s/Hz [2], become the potential candidate for next generation optical transmission system beyond 100Gb/s per channel.

Consequently, extensive researches have been done for 16-QAM signal generation. Several methods have been demonstrated to realize 16-QAM transmitter. 16-QAM signals can be generated serially with binary diving signals by cascading IQ modulator, Mach-Zehnder Modulators (MZM) and phase modulators (PM) [3], or cascading two IQ modulators [4]. Such transmitter composed of several discrete modulators, thus a large insert loss is induced and the long term stability becomes a big issue. Another approach for 16-QAM generation is to integrate several modulators in parallel structure, such as two IQ modulators, or several MZM modulators [5][6]. However, the complex modulator structures make it hard to fabricate. Another practical method to realize 16-QAM transmitter is to use four-level driving signals to drive an commercially-available signal-drive IQ modulator[2][7][8]. In such approach, the four-level driving signals are relatively difficult to generate and processing, comparing to typical binary driving signals. Thus, expensive digital-to-analog converters (DAC) are usually used for four-level signal generation.

In this paper, we propose and experimentally demonstrate a 16-QAM tra...

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...6QAM generation using a new synthesizing method”, ECOC, 2009. pp. 1-2.

[4] G.-W. Lu et al., “40-Gbaud 16-QAM transmitter using tandem IQ modulators with binary driving electronic signals”, OPT EXPRESS 18(22), 23062-23069 (2010).

[5] A. Chiba, et al., “16-level quadrature amplitude modulation by monolithic quad-parallel mach-zehnder optical modulator”, ELECTRON LETT 46(3), 227-228 (2010).

[6] G. W. Lu et al., “16-QAM Transmitter using Monolithically Integrated Quad Mach-Zehnder IQ Modulator”, ECOC, 2010, Paper Mo.1.F.3 2010.

[7] P. J. Winzer et al., “Spectrally efficient long-haul optical networking using 112-Gb/s polarization-multiplexed 16-QAM”, J LIGHTWAVE TECHNOL 28(4), 547–556 (2010).

[8] M. S. Alfiad et al.. “11×times, 224-Gb/s POLMUX-RZ-16QAM Transmission Over 670 km of SSMF With 50-GHz Channel Spacing”, IEEE PHOTONIC TECH L 22(15), 1150-1152( 2010).