Jianji Dong

Ultrawideband monocycle generation using cross-phase modulation in a semiconductor optical amplifier

We propose a novel scheme to generate ultrawideband (UWB) monocycle pulses based on cross-phase modulation (XPM) of a semiconductor optical amplifier (SOA). The proposed system consists of a SOA and an optical bandpass filter (OBF). Due to the XPM, a continuous wave (CW) probe signal is phase modulated by another optical Gauss pulse in the SOA. The OBF will convert the phase modulation to intensity modulation. A pair of polarity-reversed UWB monocycle pulses is achieved by locating the probe carrier at the positive and negative linear slopes of the OBF. Both cases conform to the UWB definition of the Federal Communications Commission.

Ultrafast All-Optical Signal Processing Based on Single Semiconductor Optical Amplifier and Optical Filtering

We propose and demonstrate several all-optical signal-processing technologies, including wavelength conversion (WC), leading/trailing edge detection, and logic gates based on a semiconductor optical amplifier (SOA) and a detuning optical bandpass filter (OBF). The system is based on pump-probe scheme. When the pump signal is return-to-zero (RZ) format, both inverted WC and noninverted WC at 40 Gb/s are obtained, and the polarity variation is highly dependent on the OBF detuning. When the pump signal is nonreturn-to-zero (NRZ) format, both the inverted WC and leading/trailing edge detection are obtained. In the inverted WC, the OBF detuning is small, and it helps to accelerate the amplitude recovery. However, in the noninverted WC and pulse edge detection, the OBF detuning is relatively large, and it acts as frequency-amplitude conversion. Finally, we present all- optical reconfigurable logic gates based on various nonlinearities in an SOA. The logic gates including AND, NOR/NOT, and OR are obtained by four-wave mixing, cross-gain modulation, and transient cross-phase modulation in the SOA, respectively. The NOR and OR gates are capable of operating at 40 Gb/s with the help of the OBF detuning. The XNOR gate is implemented by combining the AND output and NOR output.

High-speed all-optical differentiator based on a semiconductor optical amplifier and an optical filter.

We demonstrate a novel all-optical differentiator that carries out the first-order temporal derivation of optical intensity variation at high speed. It consists of a semiconductor optical amplifier (SOA) and an optical filter (OF) serving as an optical phase modulator and a frequency discriminator, respectively. A polarity-reversed derivative pair with a certain bias can be obtained by locating the probe wavelength at the positive or negative slope of the OF. Differentiations of super-Gaussian and Gaussian signals are obtained at various data rates. Defined as the mean absolute deviation of the measured result from the ideal result, total average errors of less than 0.12 are observed in all cases. Input power dynamics as well as control wavelength dependence are investigated and show that the cross-gain modulation in the SOA is detrimental to the differentiation performance.

Simultaneous demonstration on all-optical digital encoder and comparator at 40 Gb/s with semiconductor optical amplifiers

We proposed and experimental demonstrated all-optical two line-four line encoder and two bit-wise comparator of RZ data streams at 40Gb/s based on cross gain modulation (XGM) and four wave mixing (FWM) in three parallel SOAs. Five logic functions for digital encoder and comparator between two signals A and B: AB, AB, AB, AB and AOmicronB, were achieved simultaneously. The first three optical logics are realized based on XGM in SOAs, the fourth is realized with FWM, and the fifth is the mixing result of the first and the fourth. A detuning filter is employed to improve the output performance. The output extinction ratio (ER) for the XGM operation is above 10dB, and the ER for FWM operation is around 8 dB. Wide and clear eye patterns for the five logic outputs can be observed.

Reconfigurable All-Optical Logic Gates for Multi-Input Differential Phase-Shift Keying Signals: Design and Experiments

Differential phase-shift keying (DPSK) signals are promising candidate for the long-haul transmission systems. However, the development of the all-optical signal processing techniques for the DPSK signals is still in its infancy, especially the all-optical logic operations. In this work, a general scheme for reconfigurable logic gates for multi-input DPSK signals with integration possibility is proposed. Benefiting from the optical logic minterms developed by two kinds of optical devices, i.e., optical delay interferometers and semiconductor optical amplifiers (SOAs), target logic functions can be realized by combining specific minterms together. The scheme is reconfigured by changing the phase control of the delay interferometers or the input wavelengths. The latter approach was adopted in the experimental trials. Although the outputs of the scheme are on-off keying (OOK) signals, the data format is compatible with all-optical decision circuits where OOK format is preferred. Two- and three-input experiments are carried out at 20 Gbit/s with nonreturn-to-zero DPSK signals. Various logic operations are demonstrated, including full sets of two- and three-input minterms, AND, NOR, XOR, and XNOR logic operations where the AND and NOR logic are derived simultaneously and the XOR and XNOR logic are convertible. The optical SNR as well as the Q-factor of the two- and three-input results are measured and compared. It shows that the input powers to the SOAs are critical in achieving good extinction ratio and the Q-factor of logic results degrades when several minterms are combined. The recovery time of the SOAs need to be optimized as well. Finally, the scaling issues of the scheme are discussed.

All-optical ultrawideband monocycle generation utilizing gain saturation of a dark return-to-zero signal in a semiconductor optical amplifier

We demonstrate a simple and compact scheme to generate ultrawideband (UWB) monocycle pulses utilizing gain saturation of a dark return-to-zero (RZ) signal in a semiconductor optical amplifier (SOA). When optical pulses with a dark RZ format propagate through the SOA, the output power at the rising edge will be overamplified compared with that at other durations due to the gain unsaturation. As a result, the output pulses are monocyclelike. The UWB frequency spectra at different injected currents, different input pulse widths, and different input wavelengths are analyzed. Our experiments show that the monocycle generation has good tolerance to the SOA bias current and the input signal wavelength.

Comparison analysis of optical frequency comb generation with nonlinear effects in highly nonlinear fibers

We compare several schemes for broadband optical frequency comb (OFC) generation based on several nonlinear effects in highly nonlinear fiber (HNLF). Cascaded four wave mixing (CFWM) and self-phase modulation (SPM) processes in HNLF are proved to be effective ways for spectrum broadening. We investigate some parameters affecting the performance of the output OFC in detail. When only CFWM occurs in the HNLF, broadband OFC can be generated with poor power flatness. When only SPM occurs in the HNLF, we obtain a 10 GHz OFC of 103 comb lines within 5-dB power deviation. When both CFWM and SPM simultaneously occur in the HNLF, we obtain a 10 GHz OFC of 143 comb lines within 4.5-dB power deviation. All the OFC generation schemes have the advantages of tunability of central wavelength and repetition frequency.

40 Gb/s all-optical NRZ to RZ format conversion using single SOA assisted by optical bandpass filter

We propose a novel all-optical format conversion from nonreturn-to-zero (NRZ) to return-to-zero (RZ) based on single semiconductor optical amplifier (SOA) and a detuning optical bandpass filter (OBF). By adopting the ultrafast SOA model associated with intraband mechanism, the 40Gb/s NRZ-to-RZ format conversion is successfully demonstrated with numerical simulation. We investigate the influence of the filter detuning, extinction ratio, conversion efficiency, output pulsewidth, and filter bandwidth. These simulation results confirm the feasibility of our scheme. The proposed scheme is robust and potential for applications in future optical networks.

A proposal for two-input arbitrary Boolean logic gates using single semiconductor optical amplifier by picosecond pulse injection

We propose theoretically two-input arbitrary Boolean logic (NOR, OR, AND, XOR, XNOR, NAND) using single semiconductor optical amplifier (SOA) assisted by several detuning optical filters. The probe spectrum is broadened by picosecond pulse injection in the SOA, and four consequent optical Gaussian filters are used to select different frequency components to acquire logic NOR, OR, AND, XOR, respectively. Then two additional logic gates, XNOR and NAND, are realized by combining two logic channels. The power penalty, Q-factor, and extinction ratio are measured for all logic gates. It is shown that the output logic with dark return-to-zero (RZ) format has a large power penalty. The Q-factor is larger than 6 and the extinction ratio is larger than 6.3 dB for all logic gates within 16 nm wavelength range.

All-optical differentiator based on cross-gain modulation in semiconductor optical amplifier.

A novel scheme for carrying out all-optical intensity differentiation is proposed and demonstrated based on the cross-gain modulation (XGM) in a semiconductor optical amplifier (SOA). Because of the XGM, the mathematical definition of differentiation can be expressed by the combined power of the amplified pump pulse and the delayed modulated probe pulse at the output of the SOA. The final waveform and error evolution versus relative time delay between two pulses is investigated. Interestingly enough, it is shown that the acquisition of good differentiation is possible under zero time delay. A further study of zero time delay has been performed for various data rates and shows that the carrier recovery time of the SOA is a speed-limiting factor of the scheme.