Mingyue Zhu

Hilbert superposition and modified signal-to-signal beating interference cancellation for single side-band optical NPAM-4 direct-detection system

We propose a Hilbert superposition and modified signal-to-signal beating interference (SSBI) cancellation scheme in an optical single side-band (SSB) modulation and direct-detection system. The optical SSB signal is generated by a relatively low-cost dual-drive Mach-Zehnder modulator (DDMZM). The two driving signals are a pair of Hilbert signals with Nyquist pulse-shaped four-level pulse amplitude modulation (NPAM-4). In addition to the transmitted baseband signal, both its Hilbert transform and the SSBI can also be detected by direct-detection, which introduce the interference to the transmitted signal. We use the first-stage Hilbert superposition cancellation (HSC) to cancel the unwanted Hilbert transform signal and a modified single-stage linearization filter which contains the second-stage HSC to deduct the SSBI in the receiver. We experimentally demonstrate that 40 Gb/s optical SSB NPAM-4 signal transmission over 80 km standard single mode fiber (SSMF).

56-Gb/s Optical SSB PAM-4 Transmission over 800-km SSMF Using DDMZM Transmitter and Simplified Direct Detection Kramers-Kronig Receiver

We propose and experimentally demonstrate a simplified direct detection Kramer-Kronig (DD-KK) receiver followed by a Volterra filter to mitigate the SSBI and fiber nonlinearity in a 56-Gb/s optical SSB PAM-4 transmission over 800-km SSMF.

Hilbert superposition based on direct-detection for single side-band optical NPAM-4 signal

We propose and demonstrate a Hilbert Superposition (HS) scheme in an optical single side-band (SSB) modulation and direct-detection system. The optical SSB signal is generated by a relatively low-cost dual-drive Mach-Zehnder modulator (DDMZM) biased at the quadrature point. The two driving signals are a pair of Hilbert signals with Nyquist pulse-shaped four-level pulse amplitude modulation (NPAM-4). In addition to the transmitted signal, both its Hilbert transform and the signal-to-signal beating interference (SSBI) can also be detected by the direct-detection, which introduce the interference to the transmitted signal. We use the HS before demodulation in the receiver to cancel the unwanted Hilbert transform term signal and deduct the SSBI to mitigate the nonlinear distortion. We demonstrate that the optical SSB NPAM-4 signal of 100 Gb/s over 80 km standard single mode fiber (SSMF) with bit error rate (BER) at 2.4×10−3 by simulation and 25 Gb/s signal with BER at 1.3×10−4 after 80 km transmission by experiment, respectively. We also find that the complexity of the equalization operation is reduced after HS.

Sparse volterra model based on single side-band optical NPAM-4 direct-detection system

Signal-to-signal beating interference (SSBl) is one of the main drawbacks of direct-detection based optical transmission systems. Volterra filter is a common equalization method to mitigate the nonlinear distortion. However, the computational complexity may be unpractical as the transmission capacity increases. In this paper, we demonstrate that the sparse Volterra can reduce the complexity significantly in a single-side band optical Nyquist pulse-shaped four-level pulse amplitude (NPAM-4) system. The experimental results show that sparse Volterra and full Volterra have comparable performance, while sparse Volterra only needs half kernels of full Volterra.

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.