Optical Phase Modulation Format Demodulation And Coherent Detection

The high development of optical fiber communications is excited by the rapid increasing demand of network services. Advanced phase modulation formats have been attracting more and more attentions both from academic and industry as the higher bit rate and transmission length are required. The advantages of phase modulation format are:the sensitivity to receive phase modulated signal is superior to traditional intensity modulation by3dB. Phase modulation format can tolerate higher fiber nonlinearity effect in the fiber transmission. The deployment of Quadrature Phase Shift Keying (QPSK) decreases the signal baud rate and releases the requirement of high speed electronic devices. Therefore, phase modulated signal generation, demodulation, and conversion become very important research issues. Optical coherent detection has made revolutionary progress in recent years. Phase modulation formats are used more widely because the phase signal can be received by the coherent detection. Besides the transmission impairments can be compensated thanks to the powerful digital signal processing techniques.The optical signal generation, conversion and optical coherent detection with respect to the phase modulation formats are theoretical investigated and experimentally studied in this thesis. The major research achievements and contributions of this dissertation are summarized as follows:(1) The optics modulation principle is investigated. The generation, demodulation, and detection of Differential Phase Shift Keying (DPSK), QPSK and high level Quadrature Amplitude Modulation (QAM) are studied.(2) We propose and demonstrate combining DPSK and duobinary (DB) transmission for the downstream in40Gb/s long-reach wavelength division multiplexed-passive optical networks (WDM-PONs) in order to provide robust transmission performance in the backhaul section and simple detection at the Optical Network Units (ONUs). DPSK is deployed in the trunk span as it provides stronger robustness to fiber nonlinearity. DB is used in the access span where its higher chromatic dispersion tolerance relieves the need for dispersion compensation.4channel all-optical modulation format conversion from DPSK to duobinary is realized in a single Mach-Zehnder delay interferometer (MZDI) in the Remote Node (RN) to decrease system cost.(3) A novel silicon based dual polarization DPSK (DP-DPSK) demodulation device is proposed. The polarization separation is realized based on the mode conversion and the multi-mode interference (MMI) structure. The orthogonal polarization DPSK signals are demodulated based on only one microring resonator. We first time demonstrate the80Gb/s DP-DPSK polarization de-multiplexing and demodulation on chip.(4) Ultra-wideband (UWB) generation based on the microring-coupled Mach-Zehnder Interferometer (MZI) is proposed. Microring-coupled MZI plays the role of high sensitive linear discriminator. Two polarity Monocycle pulses are generated from Gaussian modulated phase signal using silicon microring-coupled MZI device.(5) Optical coherent detection principle and coherent receiver are investigated. Digital signal processing (DSP) algorithms for different modulation formats are studied in principle and simulation analysis. The performances of algorithms are presented for the experimental data recovery. We propose a novel cascaded adaptive blind equalizers based on decision-directed modified least mean square (DD-MLMS) algorithm for polarization separation and carrier phase recovery. The algorithm is compatible with various square-QAM formats. The experimental results of DP-QPSK and DP-16QAM back-to-back transmission show that the performance is very close to the general algorithm but with a benefit of the reduced operation complexity.(6) We propose9-QAM data recovery for DP-QPSK signal in presence of strong filtering to approach Nyquist bandwidth in order to increase spectrum efficiency. The decision-directed least radius distance (DD-LRD) algorithm for blind equalization is used for9-QAM recovery and inter-symbol interference (ISI) compression.11×112Gb/s DP-QPSK25-GHz spaced Nyquist-WDM experiment result shows optical signal-to-noise ratio (OSNR) tolerance is improved by0.5dB at a bit error rate (BER) of1×10-3compared to constant modulus algorithm (CMA) plus post-filter algorithm.3x112Gb/s DP-QPSK over1120km transmission is demonstrated. The BER can achieve1×10-4when the bandwidth per channel is strongly compressed to18GHz.