Minghao Li

Dynamic Analysis of All-Optical Wavelength Conversion of Differential Phase-Shift Keyed Signals Based on Semiconductor Optical Amplifier Mach–Zehnder Interferometer

All-optical wavelength conversion of differential phase-shift keyed (DPSK) signals based on semiconductor optical amplifier Mach-Zehnder interferometer (SOA-MZI) is simulated and analyzed. The results show that, to obtain both good quality of the converted signal and high conversion efficiency, phase difference between the upper and lower arms of MZI should be near ¿, which suggests strong cross-phase-modulation (XPM) and cross-gain-modulation (XGM) for optimized operation of this wavelength converter, while weak XPM and XGM will lead to sacrifice of conversion efficiency. The results also show larger wavelength up-conversion range, and suggest non-return-to-zero (NRZ) format for 10 Gb/s operation while return-to-zero (RZ) format for 40 Gb/s operation. Besides, short carrier lifetime is preferred for high-speed applications and appropriate linewidth enhancement factor can be utilized to mitigate amplitude fluctuation of the converted signal if the carrier lifetime is not short enough.

Noise Suppression Mechanisms in Regenerators Based on XGC in an SOA With Subsequent Optical Filtering

A theoretical model is presented to analyze the noise suppression characteristics in cross-gain-compression (XGC)-based regenerators, where an optical bandpass filter (BPF) is cascaded after a semiconductor optical amplifier (SOA). Analytical expressions of variance are obtained for in-phase and quadrature noises after SOA and after filter. The result shows that the physical mechanism behind XGC-based noise suppression is that gain change in the SOA is out of phase with the input power fluctuation, which leads to a negative cross-correlation term in the expression of in-phase noise variance at SOA output. On the other hand, nonlinear phase noise introduced by refractive index change in the SOA will contributes to quadrature noise, which is proportional to the square of the linewidth enhancement factor ~ αN2. It is proposed that a slightly blue-shifted optical BPF can convert phase/quadrature noise-related frequency chirp into amplitude fluctuation that can cancel a part of the amplitude fluctuation after SOA, thus lead to further reduction of in-phase noise. This mechanism can be used to alleviate the negative effect of SOA-induced nonlinear phase noise and enhance the overall noise suppression performance of XGC-based regenerators, which is experimentally and numerically demonstrated at 10 Gbit/s.

Improving the performance of a crosspoint-switch-based optical buffer by using a differential phase-shift keying payload

We experimentally demonstrate a novel decimal optical buffer scheme based on a multiloop configuration and a single switch element—an optical crosspoint switch (OXS) matrix. Its variable-delay range is of 1 to 999 times the basic delay unit. The buffer dynamic reconfiguration can be achieved at nanosecond switching speed. Our results show that by using a differential phase-shift keying (DPSK) payload in the buffer it can outperform an on-off keying payload with 4-dB sensitivity improvement owing to its alleviation of patterning-induced degradation, clearly validating DPSK as a promising modulation format to overcome nonlinear impairments and to extend number of hops in all-optical packet-switching network.

The study of optical FSK modulation for 40-Gb/s WDM-PON network with centralized lightwave source

This paper proposes and numerically investigated a novel high-speed wavelength-division-multiplexed passive optical network (WDM-PON) architecture with colorless user terminals based on the use of a different modulation scheme for downstream and upstream transmission. In the central office (CO), based on the carrier suppressing functionality of Mach-Zehnder modulator (MZM) and differential-phase-shift-keying to amplitude-shift-keying conversion using Mach-Zehnder delay interferometer, 40-Gb/s optical frequency shift keying (FSK) is generated and employed for transmitting the downstream data. In the remote node (RN) or optical network unit (ONU), the upstream data is re-modulated at 2.5-Gb/s by an intensity modulator and sent back to CO with the same fiber. Since only one light source is needed for each WDM channel, and the MZM to generate carrier suppressed signal can be shared by all the channels, a centralized 40-Gb/s WDM-PON access system with low-cost configuration is realized. Error free transmission over 20-km SMF can be observed for both downstream and upstream signals in our simulation.

40 Gbit/s FSK all-optical wavelength conversion and NOT gate using periodically poled lithium niobate waveguides

All-optical wavelength conversion is considered to be a key technology in future high-speed large-capcity dense wavelength-division-multiplexed (DWDM) optical networks. Transparent wavelength conversion is highly desired especially for various advanced modulation formats. We report a novel 40 Gbit/s all-optical transparent frequency-shift keying (FSK) wavelength conversion and logic NOT gate by exploiting cascaded second-harmonic generation and difference-frequency generation (cSHG/DFG) in a periodically poled lithium niobate (PPLN) waveguide. In addition, transparent wavelength conversions for optical duobinary (ODB) and alternate-mark inversion (AMI) modulation formats are demonstrated in the experiment.

A novel scheme of high bit rate optical FSK transmitter

We propose a novel scheme of high bit rate optical frequency shift-keying transmitter. Base on the periodic notch spectral properties of Mach-Zehnder delay interferometer and the carrier suppressing functionality of Mach-Zehnder modulator, a high-speed optical FSK signal can be simply generated with another phase modulator and a single wavelength laser source. The transmission characteristics of this FSK signal are investigated under varying dispersion management. Simulated results show that 40Gb/s FSK signal gives only 1dB penalty after 80 km SMF transmission link under the post-compensation management scheme.

All-optical 40-Gb/s RZ-DPSK to phase-incorporated ASK-Manchester format conversion

Based on a Mach-Zehnder interferometer and optical temporal time delay (OTDL), we propose and demonstrate experimentally an all-optical 40-Gb/s RZ-DPSK to ASK-Manchester format conversion. This obtained ASK-Manchester signals have the phase changes due to the demodulation of RZ-DPSK, which is called as phase-incorporated ASK-Manchester signal. Compared with pure Manchester signal, the simulated results show that the phase-incorporated ASK-Manchester signal has a compact optical spectrum and lower power at the low radio frequency (RF) frequencies, which presents high tolerance on chromatic dispersion and little the cross-talk between payload and label. Moreover, the efficiency and the BER performance of conversion and the dynamic range of fiber input power, transmission length, and chromatic dispersion tolerance of the converted ASK-Manchester are detailed discussed. Less than 1-dB power penalty over 50-km SMF transmission is observed.

Numerical simulation of all-optical wavelength conversion of DPSK signal based on SOA in a Mach-Zehnder configuration

All-optical wavelength conversion of differential phase-shift keyed (DPSK) signals based on SOA in a Mach-Zehnder interferometer (SOA-MZI) configuration is simulated and analyzed using the transfer function of MZI and a wideband dynamic model of SOA. The operation principle is analyzed and operation point selection, influence of SOA physical parameters, different signal format and operation speed are discussed in detail. The results of 10Gb/s operation show that SOA-MZI is compatible with both non-return-to-zero (NRZ) and return-to-zero (RZ) formatted signals. However, the conversion performance is sensitive to the operation point of the involved SOAs. To maximize the Q value of the demodulated conversion signal, the power and wavelength of the original DPSK signal and the probe light should be optimized to obtain approximately π phase difference between the upper and lower arms of MZI in the middle of each bit. Besides, SOA with short carrier lifetime and large linewidth enhancement factor is preferred for wavelength conversion applications. 40Gb/s operation is also simulated with SOA carrier lifetime of 100ps, and the results strongly suggests 40Gb/s operation with RZ formatted signals and relatively large input powers of the clock signal.

Transparent wavelength conversion for a novel orthogonal FSK/IM modulation format at 40-Gb/s based on FWM effect of SOA

We numerically investigate a transparent wavelength conversion for a high-speed orthogonal frequency-shift keying and intensity-modulated (FSK/IM) modulation format based on four-wave-mixing (FWM) in semiconductor optical amplifier (SOA). The inter-band and intra-band carrier dynamics, such as gain saturation, carrier density pulsation (CDP), carrier heating (CH) and spectral hole burning (SHB) have all been taken into account in our in-house SOA model. Simulation results show that about 3.5-dB and 1.5-dB power penalty at BER of 1e-9 can be achieved for the 40-Gb/s FSK payload and 2.5-Gb/s IM label after conversion. With fixed signal wavelength, the 1-dB sensitivity penalty range of the pump wavelength for the payload and label can be observed at the value of 4-nm and 5.6-nm respectively. Additionally, the dynamic range of IM modulation depth from 0.3 to 0.8 can also be observed after conversion.

Study of a novel high-speed all-optical orthogonal modulation scheme based on frequency-shift keying and intensity-modulated labeling

We propose and demonstrate a novel orthogonal optical la- beling scheme based on 40-Gbit/s optical frequency-shift keying FSK payload and 2.5-Gbit/s intensity-modulated IM labeling. Using the technology of carrier-suppressed modulation and conversion of differen- tial phase-shift keying to IM, only one light source is needed to generate the optical FSK signal. The system performance, including range of IM modulation depth, bit error ratio, and dispersion limitation, is carefully investigated by numerical simulation. With IM modulation depth of 0.4, the 40 Gbit/s FSK payload and 2.5-Gb/s IM label are transmitted over a 50-km standard single mode fiber SMF with 0.87 and 0.92 dB penalty and show immunity to input power range upto 11 and 15 dB, respec- tively. Optimal methods to improve the system performance are also proposed and discussed. After optimization, the IM modulation depth can be reduced to 0.2.