Xingwen Yi

Comparison of ICI Reduction and Fiber Nonlinearity Tolerance for DCS-OFDM and Conventional OFDM With Equal Spectrum Efficiency

Digital coherent superposition (DCS) for optical orthogonal frequency-division multiplexing (OFDM) subcarrier pairs with Hermitian symmetry can simultaneously reduce the inter-carrier interference (ICI) from laser phase noise and cancel the fiber nonlinearity to the first order. In this paper, we study an interesting scenario, where we upgrade the conventional OFDM with 4-quadrature amplitude modulation (QAM) to DCS-OFDM with 16-QAM, which have equal spectrum efficiency and require no change on hardware. We conduct a simulation to compare the tolerance to the laser phase noise and fiber nonlinearity, which are the main concerns for such an upgrading. First, the 16-QAM DCS-OFDM relaxes the laser linewidth requirement and performs better with a smaller fast Fourier transform size. Second, the nonlinear tolerance of 16-QAM DCS-OFDM is better when the nonlinear effects appear. The nonlinear tolerance of DCS-OFDM depends on the dispersion compensation scheme. The maximum SNR improvement is 3.4 dB with full post-dispersion compensation, and it can increase to 6.6 dB with symmetrical dispersion compensation at the launch power of 4 dBm. Finally, we verify that the maximum SNR improvement of 16-QAM DCS-OFDM with full post-dispersion compensation is 3.7 dB at the launch power of 6.2 dBm in the experiment.

Phase Noise Effects on Phase-Modulated Coherent Optical OFDM

One of the primary drawbacks of orthogonal frequency-division multiplexing (OFDM) is the large peak-to-average ratio (PAPR). It continues to contribute nonlinear degradations after it was introduced into optical fiber transmissions. However, phase-modulated OFDM can reduce the PAPR to the minimum, since its amplitude can be constant. In this paper, we conduct the theoretical and experimental study on the laser phase noise effect in phase-modulated coherent optical OFDM systems. Due to the laser phase noise, the phase transformation at the receiver is prone to errors due to phase wrapping. Consequently, we propose a mean-phase-compensation (MPC) method, which compensate for the mean phase offset for each OFDM symbol that has a Gaussian-distributed histogram. This method can avoid the problematic unwrapping function in phase transformation. We derive the closed-form expressions for the phase noise spreading and the corresponding signal-to-noise ratios (SNRs) based on the Wiener phase noise model. Unlike conventional coherent optical OFDM, the phase noise spreading to each OFDM subcarrier is decided by the reciprocal of the square of the subcarrier frequency, and the SNR can be improved by using a stronger PM modulation. We also discuss the SNR improvement by discarding a few affected subcarriers closing to dc. In a simulation, we demonstrate that our closed-form expressions are accurate. Finally, we experimentally verify our MPC method and our analysis.

Signal Degradation and Excess Insertion Loss of Optical Mach–Zehnder Modulators in the Presence of Electronic Dispersion Compensation

Mach–Zehnder (MZ) modulators are widely used in optical transmitters to generate high-speed optical signals. With electronic dispersion compensation (EDC), the driving signals to MZ modulators change from digital to analog, or continuous waveforms. Consequently, there is signal degradation from their sinusoidal transfer curves. In this paper, we theoretically and numerically investigate the signal degradation in optical transmitters using MZ modulators in the presence of EDC. We show that the temporal waveforms follow the Gaussian distribution when the EDC is large, and therefore, the modulation index needs to be optimized between the output SNR and the excess insertion loss of MZ modulators. With varying modulation index, we quantify the output SNR and excess insertion loss of optical IQ modulators that consist of two parallel MZ modulators. The calculations are also extended to the scenarios where the sinusoidal transfer curves are compensated for. All the analytical results are verified in a simulation of QPSK transmissions, and they are useful for finding an optimized modulation index for a tradeoff between the output SNR and excess insertion loss.

Colorless reception of a single channel from 64 WDM-OFDM channels using coherent balanced receivers

Abstract. It is desirable to remove the optical de-multiplexer in reconfigurable optical add/drop multiplexers (OADM) and use coherent receivers instead for channel selection. We set up a wavelength division multiplexed orthogonal frequency division multiplexed system with a raw bit rate of 66.7  Gb/s per channel and a transmission distance of 2,240 km. We show that the OADM can broadcast more than 64 WDM channels to a coherent balanced receiver and can select out a single channel with less than 1 dB of Q-factor degradation.

Wavelet packet transform–based optical orthogonal frequency-division multiplexing transmission using direct detection

Abstract. As an alternate to fast Fourier transform-based orthogonal frequency-division multiplexing (OFDM), wavelet packet transform (WPT)-based OFDM (WPT-OFDM) does not require cyclic prefix to avoid inter-symbol-interference. The wavelet has many varieties and therefore, it can provide more freedom for system design to suit different applications. We propose a real-valued WPT-OFDM that uses intensity modulation/direct detection. We also conduct an experiment to verify its performance through a 75-km standard single-mode fiber.

Significant overhead reduction of multi-band Tb/s coherent optical OFDM systems

The timing and optical laser phase of individual bands in multi-band OFDM are correlated. We significantly reduce the OFDM overhead through using FFT window synchronization for channel estimation and sharing pilot subcarriers for phase estimation.