Transient Cross-phase Modulation Research Articles

A study of 100 Gb/s 2R regeneration for return-to-zero code signal

Quality of optical signal can be severely degraded after a long distance optical fiber transmission, therefore all-optical 2R (re-amplification, re-shaping) regeneration is required for its low power consumption and good potential of high-speed operation. Semiconductor optical amplifier (SOA) is a very promising candidate for regeneration due to its relatively low nonlinear switching power threshold and possibility of integration. However, speeds of the existing 2R schemes based on SOAs are limited by pattern effects introduced by the long carrier recovery time. So far, most of experimental results for SOA-based regeneration schemes have been limited to 40 Gb/s, only a few demonstrations at 80 Gb/s are available. An effective method to cope with the pattern effects in the SOA is to employ cross gain compression (XGC) effect. Simultaneously injecting two signals of different wavelengths with complementary logics and balanced powers into the SOA leads to an almost constant ability to make the XGC effect intrinsically immune to the pattern effects. Previously, 2R experimental results were demonstrated at 10 Gb/s and 40 Gb/s with XGC respectively, but no further results of higher speed have been presented to date. In this study, we improve the previous two-stage configuration for XGC-based regeneration by introducing a transient cross phase modulation (T-XPM) in the first SOA for generating the high-quality logic-inverted signal, which, as we find, is essential to facilitating the high-speed XGC operation in the next SOA. Firstly, a numerical model of the photon-electron dynamics in an SOA is built with considering the ultrafast intra-band processes, amplified spontaneous emission (ASE) noise at multiple wavelengths, and device segmentation along propagation direction. The quality of the logic-inverted signal with different offsets of the filter wavelength for T-XPM is studied with the model. It is found that the appropriate blue-shift detuning of the filter wavelength greatly helps to improve the quality of the logic-inverted signal. In experiment, the influence of the filter offset on the quality of the logic-inverted signal is also investigated systematically and the best quality and the largest eye-opening of the logic-inverted signal are achieved with a blue-shift of 0.72 nm, which are consistent with the simulation result. With the best logic-inverted signal, XGC effect is deployed in the second SOA. Effective reduction of the amplitude fluctuation can be observed by comparing the eye diagrams of the input degraded with output regenerative signals. Bit error rates (BERs) are also measured for all four tributaries of the degraded and the regenerated signals and the receiving sensitivity at a BER of 10-9 is improved by 1.7-2 dB. Such results show that the XGC-based 2R regeneration scheme is effective even at a speed of as high as 100 Gb/s with the help of high-quality logic-inverted signal. Degraded signals at different wavelengths (from 1535 nm to 1555 nm) are successfully regenerated with Q-factor improvement, demonstrating the wavelength-independent regeneration capability of the XGC-based 2R regenerator.

Experimental demonstration and devices optimization of NRZ-DPSK amplitude regeneration scheme based on SOAs.

We propose and experimentally demonstrate a novel scheme which can simultaneously realize wavelength-preserving and phase-preserving amplitude noise compression of a 40 Gb/s distorted non-return-to-zero differential-phase-shift keying (NRZ-DPSK) signal. In the scheme, two semiconductor optical amplifiers (SOAs) are exploited. The first one (SOA1) is used to generate the inverted signal based on SOA's transient cross-phase modulation (T-XPM) effect and the second one (SOA2) to regenerate the distorted NRZ-DPSK signal using SOA's cross-gain compression (XGC) effect. In the experiment, the bit error ratio (BER) measurements show that power penalties of constructive and destructive demodulation at BER of 10-9 are -1.75 and -1.01 dB, respectively. As the nonlinear effects and the requirements of the two SOAs are completely different, quantum-well (QW) structures has been separately optimized. A complicated theoretical model by combining QW band structure calculation with SOA's dynamic model is exploited to optimize the SOAs, in which both interband effect (carrier density variation) and intraband effect (carrier temperature variation) are taken into account. Regarding SOA1, we choose the tensile strained QW structure and large optical confinement factor to enhance the T-XPM effect. Regarding SOA2, the compressively strained QW structure is selected to reduce the impact of excess phase noise induced by amplitude fluctuations. Exploiting the optimized QW SOAs, better amplitude regeneration performance is demonstrated successfully through numerical simulation. The proposed scheme is intrinsically stable comparing with the interferometer structure and can be integrated on a chip, making it a practical candidate for all-optical amplitude regeneration of high-speed NRZ-DPSK signal.

Wavelength Conversion of 10-Gb/s RZ-OOK Using Filtered XPM in a Passive GaAs–AlGaAs Waveguide

Wavelength conversion of a 10-Gb/s return-to-zero on-off-keyed (RZ-OOK) signal has been achieved using transient cross-phase modulation in a passive GaAs-AlGaAs bulk-waveguide, followed by a detuned filter. The converted RZ-OOK signal showed a power penalty of <1 dB at a bit-error-rate of 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-9</sup> relative to the baseline RZ-OOK signal.

Suppression of Nonlinear Patterning Effect in Wavelength Conversion Based on Transient Cross-Phase Modulation in Semiconductor Optical Amplifier Assisted with a Detuning Filter

Nonlinear patterning (NLP) effect in wavelength conversion based on transient cross-phase modulation (XPM) in semiconductor optical amplifier (SOA) assisted with a detuning filter is theoretically investigated. A non-adiabatic model is used to estimate the ultrafast dynamics of gain, phase and electron temperature in the SOA. Simulation results show that the NLP can be greatly suppressed by introducing an assist light, especially for the probe wavelength distant from gain peak. Furthermore, the results also indicate that the improvement is more evident for long wavelength probe light and assist light in counter-propagating configuration.

High speed reconfigurable logic gates based on single semiconductor optical amplifier

The nonlinearities of a semiconductor optical amplifier (SOA) are exploited to realize various all-optical signal processing at ultrahigh speed. We demonstrate reconfigurable logic gates with XNOR, AND, NOR, OR, and NOT functions based on various nonlinearities of single SOA, including four-wave mixing, cross gain modulation and transient cross phase modulation. Since the modulation speed of the SOA is limited by the recovery time of carrier density, a detuning optical bandpass filter with 0.32nm-bandwidth follows the SOA to enhance the modulation speed. Multi-logic gates at 40Gbit/s are obtained with single SOA.

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

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> 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 <emphasis emphasistype="smcaps">and</emphasis>, <emphasis emphasistype="smcaps">nor</emphasis>/<emphasis emphasistype="smcaps">not</emphasis>, and <emphasis emphasistype="smcaps">or</emphasis> are obtained by four-wave mixing, cross-gain modulation, and transient cross-phase modulation in the SOA, respectively. The <emphasis emphasistype="smcaps">nor</emphasis> and <emphasis emphasistype="smcaps">or</emphasis> gates are capable of operating at 40 Gb/s with the help of the OBF detuning. The <emphasis emphasistype="smcaps">xnor</emphasis> gate is implemented by combining the <emphasis emphasistype="smcaps">and</emphasis> output and <emphasis emphasistype="smcaps">nor</emphasis> output. </para>

10 Gbit/s RZ-OOK to RZ-BPSK format conversion using SOA and synchronous pulse carver

Conversion of return-to-zero on-off keying (RZ-OOK) to RZ binary phase-shift keying (RZ-BPSK) by transient cross-phase modulation in a semiconductor optical amplifier (SOA), and a synchronous pulse carver, is demonstrated for the first time using commercially available components. A receiver sensitivity gain of ∼1.5 dB was achieved for RZ-BPSK relative to baseline RZ-OOK.

40 Gbit/s reconfigurable photonic logic gates based on various nonlinearities in single SOA

40 Gbit/s reconfigurable photonic logic gates with XNOR, AND, NOR, OR and NOT functions are demonstrated, based on various nonlinearities of a single semiconductor optical amplifier (SOA), including four-wave mixing (FWM), cross-gain modulation (XGM) and transient cross-phase modulation (T-XPM). Detuning optical bandpass filters are employed to enhance the SOA modulation bandwidth.

Investigation on inverted and non-inverted wavelength conversion based on transient cross phase modulation of semiconductor optical amplifier

All-optical wavelength conversion based on transient cross phase modulation of semiconductor optical amplifier (SOA) is a promising approach to achieve high-speed all-optical signal processing. We experimentally demonstrate inverted and non-inverted wavelength conversion (WC) based on single SOA and an optical bandpass filter with 0.4 nm bandwidth. When the central wavelength of the following filter is blue shifted by 0.25 nm or red shifted by 0.05 nm with respect to the wavelength of probe signal, the WC is inverted. When the filter detuning is blue shifted 0.29 nm or red shifted 0.25 nm, the WC is non-inverted. The polarity evolution from inverted WC to non-inverted WC is also presented with numerical simulation. The obtained results are in good agreement with the experiment.

Analytical Solution for SOA-Based All-Optical Wavelength Conversion Using Transient Cross-Phase Modulation

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> All-optical wavelength conversion (WC) based on transient cross-phase modulation (T-XPM) of semiconductor optical amplifier (SOAs) is a promising approach to accelerate the gain recovery of SOAs. An analytical solution is deduced to explain the polarity variation of WC based on T-XPM of SOAs from the viewpoint of the impulse response functions. We conclude that both inverted and noninverted WCs can be realized when the central wavelength of the optical bandpass filter is either blue-shifted or red-shifted with respect to the wavelength of the probe signal. We are able to validate the formula against our experimental results recently published. </para>

Experimental demonstration of both inverted and non-inverted wavelength conversion based on transient cross phase modulation of SOA

We demonstrate experimentally ultrafast inverted and noninverted wavelength conversion (WC) based on a semiconductor optical amplifier (SOA) and an optical bandpass filter (OBF). In the case of small detuning, the WC is inverted regardless if the OBF is blue- or red-shifted with respect to the central wavelength of the converted signal. However the WC is non-inverted when the filter detuning is relatively large. An analytical formula for the transient cross phase modulation is applied to reveal the polarity variation of WC with respect to the OBF detuning. The theoretical detuning values are in good agreement with our experimental results.