OSNR Monitoring Technique for DPSK/DQPSK Signals Based on Self-Heterodyne Detection

We describe a new method for monitoring the “in-band” optical signal-to-noise ratio (OSNR) of differential phase-shift-keying (DPSK) and differential quadrature phase-shift-keying (DQPSK) signals. This technique estimates the OSNR by analyzing the radio frequency spectrum obtained by the self-heterodyne detection. The results show that the performance of the proposed technique is not sensitive to the effects of chromatic dispersion and polarization-mode dispersion. In addition, this technique can be used for monitoring the polarization-scrambled signals. For a demonstration, we measure the OSNR of the polarization-scrambled 10-Gb/s DPSK and 20-Gb/s DQPSK signals in a 640-km-long transmission link.      

OSNR Monitoring Method for OOK and DPSK Based on Optical Delay Interferometer

We describe a simple method to measure the optical signal-to-noise ratio (OSNR) of on-off-keying (OOK) and differential phase-shift-keying (DPSK) signals by using an optical delay interferometer (ODI) having a sinusoidal and tunable passband. This OSNR monitoring method is independent of chromatic dispersion, polarization-mode dispersion, and noise polarization. We show experimentally that accurate OSNR measurements are made for a 10-Gb/s OOK signal by using a 1-bit ODI and a 40-Gb/s DPSK signal by using a partial-bit ODI with the OSNR ranging from 5 to 25 dB.     

Novel modulation and detection for bandwidth-reduced RZ formats using duobinary-mode splitting in wideband PSK/ASK conversion

This paper proposes a novel duobinary-mode-splitting scheme that uses wideband phase-shift-keying (PSK)/amplitude-shift-keying (ASK) conversion for modulation and detection of bandwidth-reduced return-to-zero (RZ) modulation formats. We have first demonstrated that the proposed scheme greatly simplifies the modulation process of the duobinary carrier-suppressed RZ format (DCS-RZ) based on baseband binary nonreturn-to-zero (NRZ) modulation. We also proposed carrier-suppressed RZ differential-phase-shift-keying format (CS-RZ DPSK) as a novel bandwidth-reduced RZ format by applying the proposed scheme in the detection process. These novel RZ formats are shown to be very useful for dense wavelength-division multiplexed (DWDM) transport systems using high-speed channels, over 40 Gb/s, with spectrum efficiencies higher than 0.4 b/s/Hz. We demonstrate that the proposed modulation and detection scheme greatly simplifies the DWDM transmitter and receiver configuration if the periodicity of the optical PSK/ASK conversion filter equals the WDM channel spacing. The large tolerance of the formats against several fiber nonlinearities and their wide dispersion tolerance characteristics are tested at the channel rate of 43 Gb/s with 100-GHz spacing. The novel CS-RZ DPSK format offers higher nonlinearity tolerance against cross-phase modulation than does the DCS-RZ format.     

差分相移键控的非归零到归零格式转换研究

提出了一种全新的基于相位-强度混合调制和色散补偿的光差分相移键控(DPSK)信号的非归零(NRZ)到归零(RZ)格式转换器,理论分析了转换器参数对转换的影响,数值研究了恶化条件下的10 Gb/s的NRZ-DPSK到RZ-DPSK的格式转换。实验展示了10 Gb/s的DPSK信号格式转换及解调后的误码性能。计算结果表明,通过设计转换器参数可获得低占空比RZ-DPSK信号,且转换后信号质量较高。实验结果表明格式转换功率代价较低,转换后RZ-DPSK信号时间抖动较原NRZ-DPSK信号减小。该格式转换器还适合光四相差分相移键控(DQPSK)的非归零到归零格式转换及多波长操作。     

WDM PON using 10-Gb/s DPSK downstream and re-modulated 10-Gb/s OOK upstream based on SOA

A signal remodulation scheme of 10-Gb/s differential phase-shift keying(DPSK) downstream and 10-Gb/s on-off keying(OOK) upstream using a semiconductor optical amplifier(SOA) and a Mach-Zehnder intensity modulator(MZ-IM) at the optical networking unit(ONU) side for wavelength division multiplexed passive optical network(WDM PON) is proposed.Simulation results indicate that error-free operation can be achieved in a 20-km transmission,and the receiver sensitivity of return-to-zero differential phase-shift keying(RZ-DPSK) is higher than nonreturn-to-zero differential phase-shift keying(NRZ-DPSK) in the proposed scheme.     

Duty-ratio control of nonlinear phase noise in dispersion-managed WDM transmissions using RZ-DPSK modulation at 10 Gb/s

The authors compare analytical and numerical estimates, showing that the nonlinear phase noise of short optical pulses associated with the coupling between amplified spontaneous emission noise and fiber nonlinearity may be controlled by adjusting the duty cycle of the return-to-zero (RZ) signal modulation format. The impact of this effect in the optimization of the performance of 10-Gb/s dispersion-managed wavelength division multiplexed (WDM) systems using RZ-differential phase-shift keying (DPSK) modulation is discussed. By extensive numerical simulations, it is shown that the transmission quality of ultradense WDM systems using the RZ-DPSK modulation format may be significantly enhanced by optimizing the duty cycle of the RZ pulses.     

All-Optical Clock Recovery From NRZ-DPSK Signal

All-optical clock recovery (CR) from 20-Gb/s nonreturn-to-zero differential phase-shift-keying (NRZ-DPSK) signal is demonstrated, with an all-fiber delay interferometer (DI) and a mode-locked semiconductor optical amplifier (SOA) fiber laser. The tunable DI serves as an all-optical DPSK demodulator and the phase-modulated NRZ-DPSK signal is converted into the intensity-modulated pseudoreturn-to-zero (PRZ) signal, with the enhancement of the clock component. Followed SOA fiber-laser is used to achieve CR from the PRZ signal. Fixed bit pattern and 2³¹-1 pseudorandom binary sequence NRZ-DPSK signals are used to test the performance of the proposed system. It is shown that the recovered clock signal with the extinction ratio over 10 dB and the root-mean-square timing jitter of 800 fs can be achieved     

RZ/CSRZ-DPSK and chirped NRZ signal generation using a single-stage dual-electrode Mach-Zehnder modulator

This letter presents a novel configuration for return-to-zero (RZ) differential phase-shift keyed (DPSK), carrier-suppressed (CS) RZ DPSK, and chirped nonreturn-to-zero (CNRZ) signal generation, which only requires a single-stage dual-electrode Mach-Zehnder modulator (DE-MZM). RZ-DPSK, CSRZ-DPSK, or CNRZ signals can be generated via the same configuration by changing the DE-MZM operating conditions. Analytical derivation and simulation on signal generation and wavelength-division-multiplexed transmission was also carried out to justify the equivalence of the proposed scheme and conventional methods. The proposed scheme is expected to be more cost-effective due to the reduction in required modulator number.     

A novel frequency-offset monitoring technique for direct-detection DPSK systems

We propose and demonstrate a novel technique to monitor the frequency offset between the optical source and delay interferometer (DI) for direct-detection differential phase-shift-keying (DPSK) systems. In this scheme, a phase-modulated tone is applied to DPSK signals at the transmitter and then detected after being converted into an amplitude-modulated tone at the DI to be used for the monitoring signal. Our experimental demonstration shows that the monitoring range and sensitivity of the proposed scheme are measured to be /spl plusmn/2 GHz and /spl sim/10 MHz, respectively, which we believe are good enough to be used either to generate alarm signals for the frequency offset monitoring or to control the feedback loop of the DI.     

A WDM Passive Optical Network With Polarization-Assisted Multicast Overlay Control

We propose a novel wavelength-division-multiplexed passive optical network which supports simultaneous delivery of 10-Gb/s point-to-point downstream and upstream data as well as 10-Gb/s downstream multicast data. The multicast overlay control is achieved by a polarization-assisted scheme at the optical line terminal (OLT). A separate lightpath is provided for the downstream multicast differential phase-shift keying (DPSK) data without additional light sources. The upstream amplitude-shift keying signal at the optical network unit is superimposed onto the received multicast DPSK signal before being transmitted back to the OLT.      

All-Optical Chromatic Dispersion Monitoring for Phase-Modulated Signals Utilizing Cross-Phase Modulation in a Highly Nonlinear Fiber

We propose and experimentally demonstrate an all-optical chromatic dispersion (CD) monitoring technique for phase-modulated signals utilizing the cross-phase-modulation effect between the input signal and the inserted continuous-wave probe. The probe's optical spectrum changes with the accumulated CD on the input signal, indicating that the optical power variations can be measured for monitoring. The experimental results show that this technique can monitor up to 120 ps/nm of CD for a 40-Gb/s return-to-zero differential phase-shift keying (RZ-DPSK) transmission system, with the maximum measured optical power increment of 16.5 dB. The applicability of this monitoring technique to higher bit-rate phase-modulated signals, such as 80-Gb/s RZ differential quadrature phase-shift keying and 80-Gb/s polarization-multiplexed RZ-DPSK, is also investigated via simulation.      

Wavelength Conversion of High-Speed Phase and Intensity Modulated Signals Using a Highly Nonlinear Chalcogenide Glass Chip

We report all-optical wavelength conversion of high-speed differential phase-shift keyed (DPSK) and on–off keyed (OOK) signals using the nonlinear Kerr-effect in an optical chip. This was enabled by four-wave mixing (FWM) of the signal with a continuous-wave (CW) pump laser in a 7-cm-long dispersion-shifted planar rib waveguide in highly nonlinear $hbox{As}_{2} hbox{S} _{3}$ glass. Both conversion of 40-Gb/s DPSK and 160-Gb/s OOK signals by 33 and 15 nm, respectively, are shown. These are first demonstrations of signal processing by CW-pumped FWM in a chalcogenide waveguide, highlighting its capability to perform phase-preserving operations at high bit rates in chip-scale devices.      

Rayleigh Backscattering Noise-Eliminated 115-km Long-Reach Bidirectional Centralized WDM-PON With 10-Gb/s DPSK Downstream and Remodulated 2.5-Gb/s OCS-SCM Upstream Signal

We propose a novel scheme of Rayleigh backscattering noise-eliminated, long-reach, single-fiber, full-duplex, centralized wavelength-division multiplexed passive optical network with differential quadrature phase-shift keying (DPSK) downstream and remodulated upstream using an optical carrier-suppressed subcarrier-modulation (OCS-SCM) technique and optical interleaver. The error-free transmissions of 10-Gb/s downstream and 2.5-Gb/s upstream signals are experimentally demonstrated over 115-km single-fiber bidirectional SMF-28 with less than 0.5 and 1.9 dB power penalties, respectively.      

Experimental study on optically band-limited 40-Gb/s RZ signals with optically time-division demultiplexing receiver

The impact of optical filtering on 40-Gb/s return-to-zero (RZ) signals was experimentally investigated with an optically time-division multiplexing (OTDM) receiver. Through the evaluation of the signal performance by changing the interpulse-phase conditions of the symmetrically band-limited 40-Gb/s signals, we have confirmed that similar performance was obtained regardless of the interpulse-phase condition, owing to the pulse-reshaping capability of an OTDM receiver. A performance comparison was also conducted between symmetrically and asymmetrically filtered 40-Gb/s RZ signals. It was found that the symmetrically filtered signal was more tolerant for the dispersion-compensation error, while the asymmetrically filtered signal was more tolerant for fiber nonlinearity with optical filters that have a 3-dB bandwidth of 45 GHz.     

Highly efficient multichannel wavelength conversion of DPSK signals

This paper demonstrates a multichannel wavelength conversion of differential phase-shift-keyed (DPSK) signals using four-wave mixing in a highly nonlinear fiber. The wavelengths of three 10-Gb/s nonreturn-to-zero (NRZ) DPSK channels are simultaneously converted without incurring the cross-gain modulation penalty usually associated with on-off-keyed signals. A maximum conversion efficiency of 85% was achieved for both NRZ and return-to-zero DPSK signals.     

Advanced data modulation techniques for WDM transmission

This article discusses the various schemes for generating typical advanced modulation formats in terms of configuration and cost, and particularly presents some recently proposed configurations for optical data generation with better performance or reduced components, including pulsed multichannel source generation for return-to-zero (RZ)-based WDM application, multichannel dual-mode pulse source for carrier-suppressed RZ (CSRZ)-based WDM application, CSRZ and CSRZ differential phase shift keyed (DPSK) signal generation using a single Mach-Zehnder modulator (MZM) together with an electrical mixer, chirped RZ generation with reduced modulator number, and RZ/CSRZ-DPSK generation using a single dual-drive MZM and optical minimum shift keying (MSK).     

基于马赫-曾德尔调制器的先进调制格式的产生

提出了一种基于差分马赫-曾德尔调制器(MZM)产生80 Gbit/s高速率差分相移键控归零码(RZ-DPSK)、差分相移键控载波抑制归零码(CSRZ-DPSK)、差分正交相移键控归零码(RZ-DQPSK)、差分正交相移键控载波抑制归零码(CSRZ-DQPSK)的新方法.在采用两个差分MZM级联产生数据速率为80 Gbit/s的RZ/CSRZ-DPSK光信号的基础上,仅需增加一个双驱动MZM,就可以产生RZ/CSRZ-DQPSK信号,说明提出的方法有一定的扩展性,并简化了高速RZ/CSRZ-DQPSK光信号的产生过程.对产生的80 Gbit/s RZ/CSRZ-DPSK和RZ/CSRZ-DQPSK信号进行的仿真结果表明,CSRZ-DPSK信号比RZ-DPSK信号的频谱结构更加紧凑,差分正交相移键控(DQPSK)光谱形状与差分相移键控(DPSK)相同,只是由于RZ/CSRZ-DQPSK在码元速率下传输数据,得到的光谱在频域被压缩.     

Superposition of DQPSK over inverse-RZ for 3-bit/symbol modulation-demodulation

We successfully demonstrated overwriting of differential quadrature phase-shift keying (DQPSK) on inverse return-to-zero (RZ) pulses for simple 3-bit/symbol operation at a 10-Gb/s symbol rate (30-Gb/s bit rate). We adopted cross-gain modulation (XGM) in a semiconductor optical amplifier (SOA) for inverse-RZ generation, which allows both low and high levels of RZ optical signal to have a finite pulse energy in a bit time slot. We verified a wide tolerance of 20% of the bit-slot for time slot alignment between amplitude-shift keying and differential phase-shift keying modulation in the proposed scheme. We also demonstrated wide dynamic range characteristics at the extinction ratio for both 2- and 3-bit/symbol operation, compared to the conventional scheme. The proposed scheme allows a cross-modulation penalty, due to the intensity to phase modulation, of less than 1.5 dB in 2-bit/symbol and less than 5 dB in 3-bit/symbol operation.     

100-Gb/s DQPSK Transmission Experiment Without OTDM for 100G Ethernet Transport

In order to realize a future 100-Gb Ethernet (100 GbE) transport, 100-Gb/s transmission without 100-GHz-class electronics and optical time-division-multiplexing technique was demonstrated. By using a differential quadrature phase-shift-keying (DQPSK) modulation format and commercially available electronics, 2- and 50-km transmissions of 100-Gb/s signal were successfully achieved over a standard single mode fiber. The receiver sensitivity, chromatic dispersion, and differential group delay tolerances of 100-Gb/s DQPSK signal were also evaluated. Through these evaluations, the possibility of DQPSK modulation for future 100-GbE transport is verified     

100-Gb/s Packet Signal Generation With Spectral Efficiency Larger Than 1 bit/Hz/s

We have experimentally demonstrated how to generate 100-Gb/s packet signals with spectral efficiency higher than 1bit/Hz/s for the first time. The optical packet with 3.125-Gb/s label and 100-Gb/s return-to-zero differential quadrature phase-shift-keying payload are generated by using optical carrier-suppression and separation and vestigial sideband filtering techniques. The performance of transmission and label erasure has also been evaluated.