Return-to-zero On-off Keying Research Articles

EDFA based adaptive RZ-OOK transmission for FSO communication

This paper proposes an erbium-doped fiber amplifier (EDFA) based adaptive return-to-zero on-off keying (RZ-OOK) transmission for free-space optical (FSO) communication. EDFA has the characteristics of average power limitation. Therefore, the transmitted RZ-OOK duty cycle is varied for accomplishing pulse power variations relying on determined average signal-to-noise ratio of received signal. Thus, the atmospheric turbulent effects can be compensated by this type of adaptive RZ-OOK transmission. The channel models with different turbulence effects are established, and the proposed technique is validated in simulation. The simulation results illustrate that the communication quality of system is considerably enhanced by the proposed technique compared with fixed threshold decision and adaptive threshold decision.

Self-Pilot Tone Based Adaptive Threshold RZ-OOK Decision for Free-Space Optical Communications

This paper studies a novel self-pilot tone based adaptive threshold return-to-zero on-off keying (RZ-OOK) decision for free-space optical (FSO) communications. RZ-OOK has the characteristics of impulse series in the spectrum. Therefore, these impulses can be utilized as the pilot tones of the transmitted signal to convey the channel state information (CSI) of FSO links. Then, the CSI signal is extracted using a local oscillator (LO) with the frequencies of the impulse series and low pass filter. Finally, the adaptive threshold decision (ATD) is realized by assigning optimized weight factors into the extracted CSI signal. The proposed adaptive threshold RZ-OOK decision was studied in simulation under various pulse durations of RZ-OOK signal and frequencies of LO. Simulation results demonstrated that RZ-OOK with the proposed impulse tone-extracted CSI signal under optimized weight factor performs close to the conventional ATD under precise CSI knowledge.

Transmissive Fiber Bragg Grating-Based Delay Line Interferometer for RZ-OOK to NRZ-OOK Format Conversion

We propose a return-to-zero on-off keying (RZ-OOK) to non-return-to-zero (NRZ) OOK conversion scheme based on a transmissive phase-modulated fiber Bragg grating (PM-FBG). The PM-FBG has a spectrum similar to the combination of a delay line interferometer and a narrow band optical filter, which is designed and synthesized using numerical optimization algorithm. The coupling strength of the PM-FBG is almost uniform and the grating period varies along the fiber length according to the optimization method. The designed PM-FBG has been fabricated using advanced ultraviolet laser inscription technique. Experimental results show that such a PM-FBG can perform RZ-OOK to NRZ-OOK format conversion successfully.

Signal reshaping and noise suppression using photonic crystal Fano structures.

We experimentally demonstrate the use of photonic crystal Fano resonances for reshaping optical data signals. We show that the combination of an asymmetric Fano resonance and carrier-induced nonlinear effects in a nanocavity can be used to realize a nonlinear power transfer function, which is a key functionality for optical signal regeneration, particularly for suppression of amplitude fluctuations of data signals. The experimental results are explained using simulations based on coupled-mode theory and also compared to the case of using conventional Lorentzian-shaped resonances. Using indium phosphide photonic crystal membrane structures, we demonstrate reshaping of 2 Gbit/s and 10 Gbit/s return-to-zero on-off keying (RZ-OOK) data signals at telecom wavelengths around 1550 nm. Eye diagrams of the reshaped signals show that amplitude noise fluctuations can be significantly suppressed. The reshaped signals are quantitatively analyzed using bit-error ratio (BER) measurements, which show up to 2 dB receiver sensitivity improvement at a BER of 10-9 compared to a degraded input noisy signal. Due to efficient light-matter interaction in the high-quality factor and small mode-volume photonic crystal nanocavity, low energy consumption, down to 104 fJ/bit and 41 fJ/bit for 2 Gbit/s and 10 Gbit/s, respectively, has been achieved. Device perspectives and limitations are discussed.

An integrated nonlinear optical loop mirror in silicon photonics for all-optical signal processing

The nonlinear optical loop mirror (NOLM) has been studied for several decades and has attracted considerable attention for applications in high data rate optical communications and all-optical signal processing. The majority of NOLM research has focused on silica fiber-based implementations. While various fiber designs have been considered to increase the nonlinearity and manage dispersion, several meters to hundreds of meters of fiber are still required. On the other hand, there is increasing interest in developing photonic integrated circuits for realizing signal processing functions. In this paper, we realize the first-ever passive integrated NOLM in silicon photonics and demonstrate its application for all-optical signal processing. In particular, we show wavelength conversion of 10 Gb/s return-to-zero on-off keying (RZ-OOK) signals over a wavelength range of 30 nm with error-free operation and a power penalty of less than 2.5 dB, we achieve error-free nonreturn to zero (NRZ)-to-RZ modulation format conversion at 10 Gb/s also with a power penalty of less than 2.8 dB, and we obtain error-free all-optical time-division demultiplexing of a 40 Gb/s RZ-OOK data signal into its 10 Gb/s tributary channels with a maximum power penalty of 3.5 dB.

Q-factor improvement of degenerate four-wave-mixing regenerators for ASE degraded signals

All-optical regenerators can be used to suppress amplified spontaneous emission (ASE) noise introduced by cascaded erbium doped fiber amplifiers (EDFAs) in optical fiber communication systems and lead to the improvement of optical receiver sensitivity. By introducing the Q-factor transfer function (QTF), we evaluate the Q-factor performance of degenerate four-wave mixing (DFWM) regenerators with clock pump and reveal the differences between the optimal input powers determined from the static and dynamic power tranfer function (PTF) and the QTF curves. Our simulation shows that the clock-pump regnerator is capable of improving the Q-facor and receiver sensitivity for 40 Gbit/s ASE-degraded return-to-zero on-off keying (RZ-OOK) signal by 2.58 dB and 4.2 dB, respectively.

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.

All-optical pseudorandom bit sequences generator based on TOADs

Abstract A scheme for all-optical pseudorandom bit sequences (PRBS) generator is demonstrated with optical logic gate ‘XNOR’ and all-optical wavelength converter based on cascaded Tera-Hertz Optical Asymmetric Demultiplexer (TOADs). Its feasibility is verified by generation of return-to-zero on–off keying (RZ-OOK) 263-1 PRBS at the speed of 1 Gb/s with 10% duty radio. The high randomness of ultra-long cycle PRBS is validated by successfully passing the standard benchmark test.

Theoretical study of all-optical RZ-OOK to NRZ-OOK format conversion in uniform FBG for mixed line-rate DWDM systems

In this work we study all-optical multi-channel return-to-zero (RZ)–on-off keying (OOK) to nonreturn-to-zero (NRZ)–OOK format conversion in single uniform fiber Bragg grating (FBG) for mixed line-rate dense wavelength-division multiplexing systems using mathematical simulations. Forty and 20 Gbit/s RZ–OOK signals with 33% and 50% duty cycles are converted to NRZ–OOK signals in single uniform FBG with 21% reflectivity. Impact of amplitude noise from FBG contrast profile on modulation format conversion efficiency is also studied.

Self Optical Pulsation Based RZ-BPSK and Reused RZ-OOK Bi-Directional OC-768 Transmission

The generation and reusing of a self-pulsated optical RZ carrier is demonstrated to achieve bi-directional OC-768 transmission with downstream return-to-zero binary phase-shift keying (RZ-BPSK) and reused upstream return-to-zero ON-OFF keying (RZ-OOK) data formats at 40-Gbit/s. The self-started optical RZ carrier is generated by linking a single-mode laser with a nonlinearly biased Mach-Zehnder modulator based self-feedback loop, which is externally modulated to transmit the downstream RZ-BPSK and reused to transmit the upstream RZ-OOK. By lengthening the true-time delay in the self-feedback loop with a 900-m long SMF, the self-started 40-GHz photonic-microwave clock with an ultralow SSB phase noise of -160 dBc/Hz at an offset frequency of 100 kHz could suppress its relative timing jitter to 2 fs for obtaining a Q-factor as high as 5 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">16</sup> . Both the photon-microwave clock and optical RZ carrier can be triggered to achieve bi-directional downstream RZ-BPSK and upstream reused RZ-OOK transmissions with receiving power sensitivities of -15.7 and -12.3 dBm, respectively, at a requested BER of 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-9</sup> . The bi-directional downstream RZ-BPSK data reveals a comparable BER performance with the uni-directional case by a power penalty of only 0.5 dB, whereas the power penalty between uni- and bi-directional upstream RZ-OOK data is up to 3 dB.

All-optical modulation based on silicon quantum dot doped SiO x :Si-QD waveguide

All-optical modulation based on silicon quantum dot doped SiOx:Si-QD waveguide is demonstrated. By shrinking the Si-QD size from 4.3 nm to 1.7 nm in SiOx matrix (SiOx:Si-QD) waveguide, the free-carrier absorption (FCA) cross section of the Si-QD is decreased to 8 × 10−18 cm2 by enlarging the electron/hole effective masses, which shortens the PL and Auger lifetime to 83 ns and 16.5 ps, respectively. The FCA loss is conversely increased from 0.03 cm−1 to 1.5 cm−1 with the Si-QD size enlarged from 1.7 nm to 4.3 nm due to the enhanced FCA cross section and the increased free-carrier density in large Si-QDs. Both the FCA and free-carrier relaxation processes of Si-QDs are shortened as the radiative recombination rate is enlarged by electron–hole momentum overlapping under strong quantum confinement effect. The all-optical return-to-zero on-off keying (RZ-OOK) modulation is performed by using the SiOx:Si-QD waveguides, providing the transmission bit rate of the inversed RZ-OOK data stream conversion from 0.2 to 2 Mbit/s by shrinking the Si-QD size from 4.3 to 1.7 nm.

RZ-OOK to NRZ-OOK format conversion based on a single fiber Bragg grating

We propose a return-to-zero on-off keying (RZ-OOK) to non-return-to-zero (NRZ) OOK conversion scheme based on a single custom-designed fiber Bragg grating (FBG). The custom-made FBG is designed and syn­thesized using discrete layer-peeling algorithm. It is shown that such a FBG can replace the combination of interferometer and the cascaded filter that are invariably employed together in the reported schemes for ­RZ-OOK to NRZ-OOK format conversion. Simulation results show that the input 20-Gb/s RZ-OOK signals with different duty cycles can be converted into NRZ-OOK signals with high Q-factor.

基于光子晶体光纤中简并四波混频效应的高效可调谐偏振不敏感波长变换器

A high-efficiency tunable polarization-insensitive all-optical wavelength convertor for 10 Gb/s return-to-zero on-off keying (RZ-OOK) signals using degenerate four-wave mixing (FWM) in a highly nonlinear photonic crystal fiber (PCF) is demonstrated. The residual birefringence in the 50 m dispersion-flattened PCF with 11 W-1mkm-1 nonlinear coefficient guarantees the FWM-based wavelength conversion to be polarization-insensitive when the pump polarization is exactly at 45d to the birefringent axes of the PCF. Experimental results show that with 45d pump launch, the polarization dependence of FWM in the PCF can be decreased to less than 0.6 dB over the entire 25 nm conversion bandwidth. The optical signal-to-noise ratios (OSNR) of the converted signals are better than 40 dB and the conversion efficiencies are better than -15 dB over the conversion range with 10 Gb/s signal polarization-scrambled.

All-optical amplitude-phase transmultiplexing of RZ-OOK and RZ-BPSK to RZ-QPSK by polarization-insensitive XPM using a nonlinear birefringent AlGaAs waveguide

Polarization-insensitive (PI) phase-transmultiplexing (PTM) of a 10-Gb/s return-to-zero ON-OFF keying (RZ-OOK) pump and a 10-Gb/s RZbinary phase-shift keying (RZ-BPSK) probe to 20-Gb/s RZ-quadrature-PSK (RZ-QPSK) has been successfully demonstrated for the first time in a passive, birefringent AlGaAs waveguide, utilizing PI cross-phase modulation (PI-XPM). For differential QPSK (DQPSK)-detection, a 10 − 9-BER pre-amplified receiver sensitivity penalty of ≈ 2.5 dB for the in-phase component and ≈ 4.9 dB for the quadrature component were found. The penalties were relative to the FPGA-precoded RZ-DQPSK baseline for a pump-probe detuning of ≈ 12 nm, when the probe state of polarization was scrambled and the pump was launched off-axis into the waveguide.

Data Transfer From RZ-OOK to RZ-BPSK by Polarization-Insensitive XPM in a Passive Birefringent Nonlinear AlGaAs Waveguide

The polarization-insensitive data transfer of 10-Gb/s return-to-zero ON-OFF keying (RZ-OOK) to RZ-binary phase-shift keying (RZ-BPSK) has been successfully carried out for the first time, in a passive birefringent nonlinear Al0.18Ga0.82As waveguide, utilizing polarization-insensitive cross-phase-modulation (PI-XPM). A 10-9 bit-error-rate (BER) pre-amplified receiver sensitivity penalty of ≈2.2 dB relative to baseline RZ-BPSK was measured for PI-XPM, when the RZ-OOK pump was polarization-scrambled and the RZ-probe was launched at 40°. The probe's optimal launch angle for PI-XPM was significantly influenced by the anisotropy of the Al0.18Ga0.82As waveguide.

640 GBd Phase-Correlated OTDM NRZ-OOK Generation and Field Trial Transmission

We generate a 640 GBd phase-correlated optical time-division multiplexed (PC-OTDM) return-to-zero ON-OFF keying (RZ-OOK) signal based on cross-phase-modulation-induced cross-polarization rotation in an Al highly nonlinear fiber and a 640 GBd PC-OTDM nonreturn-to-zero (NRZ)-OOK signal by RZ-to-NRZ conversion using passive filtering. This is employed in a 1.19 Tb/s PDM-NRZ-OOK field transmission with BER <; 3.8 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> for all 128 tributaries. As a benefit of phase coherence between adjacent TDM channels, a PC-OTDM signal can be very tolerant to strict filtering and the bandwidth of the 640 GBd PC-OTDM NRZ-OOK signal can be restricted to 700 GHz. In addition, with a narrower spectrum a PC-OTDM NRZ-OOK signal is more tolerant to chromatic dispersion and polarization-mode dispersion compared to a conventional (i.e., phase-uncorrelated) OTDM RZ signal.

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.

All-optical broadcast and multicast technologies based on PPLN waveguide

All-optical 1×4 broadcast and 1×3 multicast experiments of a 40-Gb/s return-to-zero on-off keying (RZ-OOK) signal based on a periodically poled lithium niobate (PPLN) waveguide are demonstrated in this letter. Clear opened eye diagrams and error-free performance are achieved for the broadcast signals at 1541.3, 1543.7, 1548.5, and 1550.9 nm. Multicast technology uses cascaded second-harmonic generation and difference-frequency generation in a Ti:PPLN waveguide. An error-free operation with a negligible power penalty is achieved for the three wavelength-division multiplexing multicast signals at 1533, 1537, and 1541 nm.

Generation and transmission of dispersion tolerant 10-Gbps RZ-OOK signal for radio over fiber link

We proposed and demonstrated the generation and transmission of 10-Gbps return-to-zero ON/OFF keying (RZ-OOK) signal using a new technique without pulse carving at transmitter. The new technique is characterized by a 3 dB built-in gain with better tolerance for chromatic dispersion in standard single mode fiber (SSMF). Fiber Bragg grating (FBG) is used as chromatic dispersion compensating device to investigate the tolerance of the proposed scheme. The simulation model of wavelength division multiplexing (WDM) based on OptiSystem.v.8.0 is presented. Simulation results show that there are error free transmission performance in a distance of 600 km with negligible power penalty and improved receiver sensitivity compared to conventional pulse carving approach.

Timing misalignment monitoring between the data modulator and the pulse carver in return-to-zero on-off keying systems using half-symbol delay tap sampling

A novel method using half-symbol delay tap sampling technique for monitoring timing misalignment between data modulator and pulse carver in 33% duty-cycle return-to-zero on-off keying (RZ-OOK), 50% duty-cycle RZ-OOK, and carrier-suppressed RZ-OOK optical transmitters is proposed and experimentally demonstrated. The presented scheme is able to not only identify three RZ-OOK signals but also determine both the timing misalignment magnitude and direction. The effectiveness of the proposed scheme is validated by simulation and experiment in 10-Gbps optical systems, and its robustness against imperfections of the transmitter is also analyzed.