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In this paper, we consider the design and analysis of a coherent optical OFDM (CO-OFDM) transmitter using an optical in-phase/quadrature (I/Q) modulator with limited extinction ratio (ER) under four modulation formats. The transmitter-side digital signal processing (DSP), including hard clipping and predistortion, is introduced to combat the penalty induced by the limited ER of the modulator and the quantization noise of the digital-to-analog converter (DAC). The DAC resolution needed for transmitters using modulators with various ER values are examined. The peak-to-peak driving voltage (Vpp) and the clipping ratio (CR) of the transmitted signals are also optimized with respect to minimizing the Q-factor penalty for the modulator with different ER values. The simulation results show that, unlike previous work using modulators with an infinite ER, the optimum Vpp and CR values vary with different ERs and modulation levels.

Design and Analysis of a CO-OFDM Transmitter With Limited Modulator Extinction Ratio