Control of a Nonlinear Mach-Zehnder Interferometer for Regeneration Using Digital Signal Processing

Abstract

Transmission impairments degrade the quality of optical signals and limit the transmission reach. Phase modulated signals are susceptible to both amplitude and phase noises. Optical regenerators based on nonlinear Mach Zehnder interferometers (NMZIs) are able to suppress them both. To use a NMZI as a regenerator, the frequency of the pump signal must be kept close to that of the data carrier and the phase difference between the data and pump signals needs to be aligned with the phase transfer curve. A digital signal processing (DSP) based control method is proposed and demonstrated in this thesis.

The method uses a coherent receiver with DSP to estimate the frequency offset and phase difference between the input data and pump signals. A control signal is generated according to the estimation results for a frequency and phase shifter to adjust the pump signal. System performance of regenerators with the control method for 43 Gb/s DPSK signals is evaluated by simulation. The sampling rate of the analog-to-digital converter (ADC) in coherent receiver is 335.94 MSa/s, which largely reduces the complexity and cost. The simulations assume all the samples are obtained at bit centers for simplicity. The validity of this assumption is demonstrated.

Experimental 40 Gb/s DPSK signal is detected using an optical modulation analyzer (OMA). Signals are collected at an OSNR of 20.7 dB, corresponding to an error free condition. Frequency offset and phase estimation is performed using the collected data with both the fast Fourier transform (FFT) and moving average method at 40 GSa/s and the proposed method at 312.5 MSa/s. The two methods give results of minor difference. After frequency offset correction and interpolation, the experimental data are used as the input DPSK signals in the simulations to evaluate the system performance.

Cascadability of the regenerator with the proposed control method is demonstrated in recirculating loop simulations using both simulation and experimental DPSK signals. The experimental data are collected when no noise is added. The error free transmission distance is greatly increased by using the NMZI-based regenerators with the proposed control method.