Non-return-to-zero Differential Phase Shift Keying Research Articles

Performance Comparison of Different Modulation Schemes in High-Speed MDM Based Radio Over FSO Transmission Link Under the Effect of Atmospheric Turbulence Using Aperture Averaging

In this work, we investigate the performance comparison of different modulation schemes viz. non return-to-zero (NRZ), alternate mark inversion (AMI), return-to-zero differential phase shift keying (RZ-DPSK), and non return-to-zero differential phase shift keying (NRZ-DPSK) in a 2 × 40 Gbps–40 GHz Radio over free space optics (RoFSO) transmission link incorporating mode division multiplexing (MDM) of two distinct Hermite Gaussian (HG) modes (HG00 and HG01) under the influence of different atmospheric turbulence conditions. Further, the effect of increasing receiver antenna aperture diameter on the link performance is investigated and compared for all four modulation schemes under different turbulence conditions. It is observed that as the strength of the atmospheric turbulence increases, the performance of the link degrades whereas the Bit error rate and the Quality Factor (Q Factor) of the received signal are significantly improved by increasing the receiver aperture diameter. Among the four modulation schemes, link with NRZ-DPSK modulation scheme gives the best performance under the influence of atmospheric turbulence followed by RZ-DPSK, AMI, and NRZ scheme. Also, the proposed NRZ-DPSK scheme based MDM-RoFSO link is investigated under different weather conditions and the results are compared with the previously reported works in the literature.

On the performance of blue–green waves propagation through underwater optical wireless communication system

The present study proposed a high-data-rate underwater optical wireless communication (UOWC) system to propagate the laser blue–green waves through water. The presented study not only focuses on analysis of challenges in UOWC link including attenuation, absorption, scattering and turbulence model, but also investigates the performance of the proposed system using two different methods of balanced modulation schemes. Spectrum efficiency of the system can be improved by using appropriate modulation formats. Return-to-zero differential phase shift keying (RZ-DPSK) and non-return-to-zero differential phase shift keying (NRZ-DPSK) schemes are two modulation formats that we investigate them to improve the characteristics of the proposed UOWC system. The paper explains a real model and exhaustive analysis for advanced UOWC works by using channel model and modulation formats for presented underwater link. Performance of the proposed system under different modulation schemes and physical aspects of UOWC is studied with several parameters like max quality factor, min bit error rate (BER) and eye diagram. For clear ocean, the performance of the proposed system is good and min BER less than 10−90 for two modulation formats. Generally, results at different condition show that the operation of NRZ-DPSK modulation has better performance than RZ-DPSK scheme.

Investigation on Maximum Available Reach for Different Modulation Formats in WDM-PON Systems

Abstract Considering the growing demand for broadband of access networks, in the present paper we investigate various modulation formats as a way of increasing the performance of optical transmission systems. Non-return-to-zero (NRZ) on-off keying, return-to-zero (RZ) OOK, carrier suppressed RZ (CSRZ) OOK, duobinary (DB), NRZ differential phase shift keying (NRZDPSK), RZ-DPSK and CSRZ-DPSK formats are compared using the maximal achievable reach with bit error rate less than 10−9 as a criterion. Simulations are performed by using OptSim software tool. It is shown that using the transmission system without dispersion compensation the best results are shown by duobinary and CSRZ-OOK modulation formats, but with the system using dispersion compensating fiber (DCF) the longest transmission distance is achieved by RZ-DPSK modulation format. By investigating the influence of channel spacing for best-performed modulation formats, network reach decrease for transmission systems with DCF fiber has been observed due to channel crosstalk.

Ultrafast gain recovery in a QW-SOA and its application for 40 Gb/s regenerative format conversion from NRZ-DPSK to RZ-OOK

We propose a scheme to realize gain recovery of the order of a few picoseconds in a typical quantum well semiconductor optical amplifier (QW-SOA) using a pump signal with amplitude dips. In the scheme, the gain recovery is determined by the fast carrier density depletion and carrier temperature heating processes. The ultrafast gain recovery is numerically verified by a completed model combining the QW band structure calculation with rate equations in the QW-SOA. In practice, a pump signal with amplitude dips can be partly characterized by a non-return-to-zero differential phase-shift keying (NRZ-DPSK) signal generated by a Mach–Zehnder modulator. Therefore, we experimentally demonstrate 40 Gb/s wavelength-converting and regenerative format conversion from NRZ-DPSK to return-to-zero on–off keying (RZ-OOK), which may be a key function to connect different kinds of modulation formats at gateway nodes between long-haul backbone networks and metro area networks.

40 Gb/s reconfigurable optical logic gates based on FWM in silicon waveguide.

Here we experimentally demonstrate reconfigurable logic gates via four-wave-mixing (FWM) in silicon waveguide with an operating speed of up to 40 Gb/s. After demodulated by a 40-GHz delay interferometer (DI), four non-return-to-zero differential phase shift keying (NRZ-DPSK) signals with carefully selected wavelengths are launched into the waveguide at the same time. Thanks to the effective FWM in silicon nano-waveguide, a full set of the two-input logic minterms can be generated simultaneously, and arbitrary combinational logic functions are able to be realized by properly combining these minterms.

On-chip WDM mode-division multiplexing interconnection with optional demodulation function.

We propose and fabricate a wavelength-division-multiplexing (WDM) compatible and multi-functional mode-division-multiplexing (MDM) integrated circuit, which can perform the mode conversion and multiplexing for the incoming multipath WDM signals, avoiding the wavelength conflict. An phase-to-intensity demodulation function can be optionally applied within the circuit while performing the mode multiplexing. For demonstration, 4 × 10 Gb/s non-return-to-zero differential phase shift keying (NRZ-DPSK) signals are successfully processed, with open and clear eye diagrams. Measured bit error ratio (BER) results show less than 1 dB receive sensitivity variation for three modes and four wavelengths with demodulation. In the case without demodulation, the average power penalties at 4 wavelengths are -1.5, -3 and -3.5 dB for TE₀-TE₀, TE₀-TE₁ and TE₀-TE₂ mode conversions, respectively. The proposed flexible scheme can be used at the interface of long-haul and on-chip communication systems.

Experimental Investigation of All-Optical NRZ-DPSK to RZ-DPSK Format Conversion Based on TOAD

We propose and experimentally demonstrate a novel all-optical non-return-to-zero differential-phase-shift-keying (NRZ-DPSK) to return-to-zero differential-phase-shift-keying (RZ-DPSK) format conversion scheme. This scheme is based on the terahertz optical asymmetric demultiplexer (TOAD). A 10 Gb/s converted RZ-DPSK signal is obtained with a wide duty cycle tuning range from 16% to 66%. For all converted RZ-DPSK signals, the receiver sensitivities at BER of 10−9 are 0.4 to 1.7 dB higher compared with the original NRZ-DPSK signal. The clear and open eye diagrams are presented to demonstrate the high quality format conversion performance. Moreover, the optical spectra show that this conversion is in a wavelength-preserving operation.

Energy efficiency in WDM fiber-optical links considering OADM/OXC nodes

In this paper, authors discuss the energy consumption for 1bit transmission as a function of spectral efficiency (SE, bps/Hz), aggregated traffic amount (C, bps), and overall transmission distance (LP2P, km) in wavelength division multiplexing (WDM) systems with optical add-drop multiplexer/ optical cross-connect (OADM/OXC) nodes. The choice of bitrate and modulation formats was limited to three options: 10G non-return-to-zero on-off keying (NRZ-OOK), 40G NRZ differential phase-shift keying (NRZ-DPSK) and 100 dual polarization quadrature phase-shift keying (DP-QPSK). The reasons for that is the legacy in transmission networks and high energy efficiency of 3R (re-amplification, re-timing, re-shaping) regenerators and transponders, respectively. In addition, these formats are commonly used for 10-40-100G mixed-line rate solutions in core networks. It is found out that 100G DP-QPSK is the best solutions among two other considered in terms of both energy consumption per bit and spectral efficiency. Finally and foremost, it is estimated that for the WDM channels added at the OADM/OXC node, energy consumption per 1 transmitted bit could growth more than two times as compared to wavelengths transmitted over point-to-point fiber-optical links and then dropped at the receiving node.

Experimental investigation of all-optical nonreturn-to-zero differential phase-shift keying to return-to-zero DPSK format conversion based on nonlinear polarization rotation of semiconductor optical amplifier

A novel all-optical nonreturn-to-zero differential phase-shift keying (NRZ-DPSK) to return-to-zero DPSK (RZ-DPSK) format conversion scheme is proposed and experimentally demonstrated. This conversion is based on nonlinear polarization rotation of a semiconductor optical amplifier. Experimental results show that a 10 Gb/s RZ-DPSK signal with an extinction ratio over 10 dB can be converted with a tunable duty cycle from 33% to 66%, and the ER of the converted signal decreases with the increase in the duty cycle. For all cases of different duty cycles, the converted signals experience a -0.4 to -1.2 dB power penalty at a bit error rate of 10(-9) compared with the original signal. In addition, the spectra show that this format conversion is a wavelength-preserved operation.

All-optical encryption/decryption for nonreturn-to-zero differential phase-shift keying signals using four-wave mixing in semiconductor optical amplifier

By experiment we demonstrate an all-optical encryption/decryption scheme for nonreturn-to-zero dif- ferential phase-shift keying (NRZ-DPSK) signals at 10 Gbit∕s using all-optical exclusive-OR (XOR) logic. The key bit stream is performed by a pseudorandom bit stream. The all-optical XOR logic is achieved by nonde- generate four-wave mixing (FWM) in a semiconductor optical amplifier (SOA), which allows high data rate oper- ation and asymmetric optical powers of the two input bit streams. The gain dynamics and pattern effect associated with the SOA carrier lifetime are alleviated due to the constant envelope of the NRZ-DPSK signals.

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.

Simultaneous multichannel wavelength multicasting and XOR logic gate multicasting for three DPSK signals based on four-wave mixing in quantum-dot semiconductor optical amplifier.

In this paper, we experimentally demonstrate simultaneous multichannel wavelength multicasting (MWM) and exclusive-OR logic gate multicasting (XOR-LGM) for three 10Gbps non-return-to-zero differential phase-shift-keying (NRZ-DPSK) signals in quantum-dot semiconductor optical amplifier (QD-SOA) by exploiting the four-wave mixing (FWM) process. No additional pump is needed in the scheme. Through the interaction of the input three 10Gbps DPSK signal lights in QD-SOA, each channel is successfully multicasted to three wavelengths (1-to-3 for each), totally 3-to-9 MWM, and at the same time, three-output XOR-LGM is obtained at three different wavelengths. All the new generated channels are with a power penalty less than 1.2dB at a BER of 10(-9). Degenerate and non-degenerate FWM components are fully used in the experiment for data and logic multicasting.

Polarization-Independent All-Optical Regenerator for DPSK Data

We demonstrate polarization-independent simultaneous all-optical phase-preserving amplitude regeneration and wavelength conversion of NRZ differential phase shift keying (DPSK) data by four-wave mixing (FWM) in a semiconductor optical amplifier (SOA). The dependence upon polarization state of the signals is eliminated by using a co-polarized dual-pump architecture. Investigation on the regenerative capability vs. pumps detuning shows significant BER threshold margin improvement over 6 nm conversion range.

All-Optical Wavelength-Shift-Free NRZ-DPSK to RZ-DPSK Format Conversion with Pulsewidth Tunability by an SOA-Based Switch

We demonstrate an all-optical non-return-to-zero differential phase shift keying (NRZ-DPSK) to return-to-zero differential phase shift keying (RZ-DPSK) format conversion with wavelength-shift-free and pulsewidth tunable operations by using a semiconductor optical amplifier (SOA)-based switch. An NRZ-DPSK signal is injected into the SOA-based switch with an RZ clock, and is converted to RZ-DPSK signal owing to the nonlinear effects inside the SOA. In this scheme, the wavelength of the converted RZ-DPSK signal is maintained as the original wavelength of the input NRZ-DPSK signal during the format conversion. Moreover, the pulsewidth of the converted signal is tunable in a wider operating range from 30 to 60 ps. The format conversion with pulsewidth tunability is based on cross-phase modulation (XPM) and cross-gain modulation (XGM) effects in the SOA. The clear eye diagrams, optical spectra and the bit-errorrate (BER) characteristics show high conversion performance with the wide pulsewidth tuning range. For all cases of the converted RZ-DPSK signal with different pulsewidths, the receiver sensitivities at a BER of 10−9 for the converted RZ-DPSK signal were 0.7 to 1.5 dB higher than the receiver sensitivity of the input NRZ-DPSK signal. key words: Differential phase shift keying (DPSK), format conversion, wavelength-shift-free, pulsewidth tunable operation, cross-phase modulation (XPM), cross-gain modulation (XGM)

Silicon based polarization insensitive filter for WDM-PDM signal processing

We propose and fabricate a novel circuit that combines two two-dimensional (2D) grating couplers and a microring resonator (MRR). According to the polarization states, one 2D grating coupler first splits the input signals into two orthogonal paths, which co-propagate in the loop and share a common MRR, and then the two paths are combined together by the other 2D grating coupler. The proposed circuit is polarization insensitive and can be used as a polarization insensitive filter. For demonstration, the wavelength division and polarization division multiplexing (WDM-PDM) non return-to-zero differential-phase-shift-keying (NRZ-DPSK) signals can be demodulated successfully. The bit error ratio measurements show an error free operation, reflecting the good performance and the practicability.

All-optical amplitude regeneration of non-return-to-zero differential-phase-shift-keying signal

We propose and experimentally demonstrate amplitude regeneration for non-return-to-zero differential-phase-shift-keying (NRZ-DPSK) signal at 40Gb/s with two cascaded semiconductor optical amplifiers (SOAs) and a subsequent delay interferometer (DI). The amplitude regeneration can reduce the accumulation of nonlinear phase noise (NPN), which is one of the most severe impairments resulting from the intensity fluctuations of the signal in the long haul transmission. The regeneration course includes three stages, which are the NRZ-DPSK to return-to-zero DPSK (RZ-DPSK) format conversion achieved by the cross phase modulation (XPM) effect in a SOA, the amplitude regeneration of RZ-DPSK realized by using the gain saturation effect in a second SOA and the format conversion from RZ-DPSK back to NRZ-DPSK based on a constructive interferometer with a DI. The regenerative performance of the system is characterized in terms of bit-error rate against received power. A power penalty improvement up to 1.2dB is obtained in our experiment.

Simultaneous multi-channel RZ-OOK/DPSK to NRZ-OOK/DPSK format conversion based on integrated delay interferometers and arrayed-waveguide grating

Format conversion is enabling function at the interface of different networks with different optimal modulation formats. Meanwhile, multi-channel signal processing function has its special significance for dense wavelength division multiplexing networks. In this paper, we designed and fabricated a delay interferometer cascaded with an arrayed-waveguide grating based on silicon on insulator. The 4-channel return-to-zero on-off keying (RZ-OOK) to non-return-to-zero on-off keying (NRZ-OOK) and return-to-zero differential phase shift keying (RZ-DPSK) to non-return-to-zero differential phase shift keying (NRZ-DPSK) format conversions at 40 Gbit/s were realized with this integrated device, the output eye diagrams showed a good conversion performance. Thanks to the mature fabrication technology, this device shows great potential for its small size, low power consumption and possibility of monolithic integration.

Fiber-based optical parametric amplif ier for 40-Gb/s NRZ-DPSK signal transmission system employing QC-LDPC codes

In this letter, we investigate quasi-cyclic low-density parity-check (QC-LDPC) codes in a 40-Gb/s nonreturn-to-zero differential phase-shift keying (NRZ-DPSK) signal transmission system based on a fiber-based optical parametric amplifier (FOPA). A constructed algorithm of QC-LDPC codes according to the optimizing set of shift values on the circulant permutation matrix (CPM) of the basis matrix is proposed. Simulation results prove that the coding gain in the encoded system can be realized at 10.2 dB under QC-LDPC codes with a code rate of 5/6 when the bit error rate (BER) is 10-9. In addition, the error-floor level originating from the uncoded system is suppressed.

Chromatic Dispersion Monitoring for NRZ-DPSK System Using Asynchronous Amplitude Histogram Evaluation

We propose and demonstrate a chromatic dispersion (CD) monitoring technique for a nonreturn-to-zero differential phase-shift keying (NRZ-DPSK) signal generated with a Mach-Zehnder modulator at 40 Gb/s by analyzing the waveform overshoot after transmission. Without demodulating the NRZ-DPSK signal, the CD up to 120 ps/nm is monitored experimentally via the amplitude of the overshoot. Furthermore, based on the asynchronous amplitude histogram (AAH) evaluation method, a novel histogram processing method, which is more convenient, is proposed and investigated. Experimental results show that the monitoring scheme for CD is independent of optical signal-to-noise ratio (OSNR) within a certain range. This technique needs neither demodulating the signal nor extracting the synchronous clock, and it can be applied to high-order modulation formats at different bit rates.

A novel duplex WDM-PON with DPSK modulated downstream and re-modulation of the downlink signal for OOK upstream

We experimentally demonstrate and analyze a 10 Gbit/s full duplex wavelength division multiplexing passive optical network (WDM-PON) system. A non-return-to-zero differential phase shift keying (NRZ-DPSK) modulation technique is first utilized for downlink direction, and then the downlink signal is re-modulated for the uplink direction using intensity modulation technique of on-off keying (OOK) with a data rate of 10 Gbit/s per channel. An effective colorless WDM-PON full duplex transmission system is achieved for the data rate of 10 Gbit/s per channel with a channel spacing of 60 GHz over the distance of 25 km with low power penalty.