Tolerance of laser frequency offset in optical minimum-shift keying transmission systems

A mismatch between laser frequency and delay interferometer (DI) phase is found to be the most critical impairment for the receiver performance in a practical phase-modulated (PM) system. This paper investigates the receiver performance degradation caused by frequency offset between optical signal and DI in a 10 Gb/s minimum-shift keying (MSK) system, and compares it with the conventional PM formats, optical differential-phase-shift keying (DPSK) and differential-quadrature-phase-shift keying (DQPSK), which are nonreturn-to-zero (NRZ) and 50% duty cycle return-to-zero (RZ). Results show that the MSK system is about double times and six times more robust to frequency offset than the DPSK and DQPSK systems operating at the same bit rate, respectively.     

Linear and nonlinear distortion effects in direct detection 40 Gb/s MSK modulation formats multi-span optically amplified transmission

The generation of minimum shift keying (MSK) requires a linear variation of the phase, hence a constant frequency of the optical carrier. However, the generation of the optical phase may be preferred by driving an optical modulator using sinusoidal signal for practical implementation. Thus a nonlinear variation of the carrier phase, hence some distortion effects are produced. In this paper, we investigate the use of linear and nonlinear phase shaping filtering and their impacts on MSK modulated optical signals transmission over optically amplified long haul communications system. The evolution of the phasor of the in-phase and quadrature components is illustrated for lightwave-modulated signal transmission. The distinct features of three different MSK modulation formats: linear MSK, weakly nonlinear MSK and strongly nonlinear MSK and their transmission are simulated. Transmission performance obtained indicates the resilience of the MSK signals in transmission over multi-optically amplified multi-spans.     

Generation, detection and characterization of optical minimum shift keying data format

This paper presents the detailed principle on the generation and detection of optical minimum shift keying (MSK) format for high-speed and high spectral efficiency WDM systems. 10.7 Gb/s optical MSK data is successfully generated by experiment using the proposed scheme. The results show that optical MSK signal exhibits very compact optical spectrum, which is promising for high spectral efficiency WDM applications. Simulation results on dispersion and nonlinear tolerance comparison confirm that MSK signal has better dispersion tolerance and nonlinear tolerance compared with 50% duty cycle return-to-zero differential phase shift keying (RZ-DPSK) and 50% duty cycle RZ on-off-keying (RZ-OOK) formats.     

Multi-amplitude minimum shift keying modulation format for optical communications

We present the multi-amplitude level minimum shift keying (M-ary MSK) modulation scheme for long haul optically amplified transmission systems. New configurations of optical M-ary MSK transmitters using two cascaded electro-optic phase modulators are proposed and its detailed operation principles are expressed. The optical receiver for optical multi-amplitude MSK modulation format requires both amplitude direct-detection and differential phase balanced-detection. Numerical results on spectral characteristics, dispersion tolerance and the relationship between amplitude and phase with launched average power for long-haul transmission of multi-level MSK modulation format are presented. The dispersion tolerance shows that the 2-amplitude MSK at 80 Gb/s is well compared with 40 Gb/s binary MSK modulation format.     

Data-aided channel estimation and frequency domain equalization of minimum-shift keying in optical transmission systems

We demonstrate a digital optical communication system based on minimum shift keying （MSK） signal transmission with coherent detection. 5-Gb/s MSK signal can transmit over a 160-km standard single mode fiber （SSMF） without phase compensation. At the receiver, we use data-aided channel estimation and frequency domain equalization （FDE） techniques in the digital signal processing （DSP） algorithm, then analyze its performance characteristics compared with quadrature phase shift keying （QPSK） format. The simulation results show that the MSK format will be a potential candidate for next-generation access network.     

差分相移键控色散管理孤子多扰动系统的相位抖动

差分相移键控(DPSK)调制方式和色散管理孤子传输方式的结合能抵制噪声和非线性损伤,在高速(40 Gbit/s以上)多信道系统中具有突出的优点。采用变分法分析了多波长信道的放大自发辐射(ASE)噪声、信号间的非线性串扰(ISI)等多种扰动因素引起差分相移键控色散管理孤子系统的均方根相位抖动,给出了扰动的作用区域以及各扰动的大小。研究发现,放大自发辐射引起的抖动与传输距离成三次方的关系,而交叉相位调制(XPM)引起的抖动与距离近似成线性关系。通过优化选择色散管理图强度范围1.5~3.5,各种扰动得到了抑制,而以放大自发辐射扰动抑制为最大,此时要远低于交叉相位调制引起的抖动,然后分别是交叉相位调制-放大自发辐射扰动和交叉相位调制,从而波分复用系统主要来自于增加信道数这一客观限制。     

Comparison of polarization-mode dispersion tolerances in polarization-multiplexing systems with different modulation formats

The polarization-mode dispersion (PMD) tolerance of 10 Gb/s polarization-multiplexing (PM) system is investigated. Using the importance sampling (IS) method, the outage probabilities of the PM systems with three modulation formats, including on-off keying (OOK), differential phase-shift keying (DPSK) and differential quadrature phase-shift keying (DQPSK), are quantified. When the amplified spontaneous emission (ASE) noise is assumed to be dominant, we evaluate the optical power penalties caused by the PMD effect at bit error rate (BER) of 10⁻¹². The performance of compensated PM systems with variable optical delay line is also described. The simulation results indicate that the OOK signal with higher duty cycle (DC) performs better in the PM systems with PMD compensation. It is found that the higher-order PMD impairs seriously the performance of the PM system, and phase-keying formats are more sensitive to the PMD than the OOK.     

All-optical demultiplexing of 4-ASK optical signals with four-wave mixing optical gates

A new all-optical technique to demultiplex quaternary-amplitude-shift keying (4-ASK) signals into two on-off keying (OOK) signals is proposed. We show that such quaternary-to-binary conversion may be performed with two types of optical gates (OGs); a first one that implements an S-shaped optical power transfer function (TF), which is also the characteristic TF of optical pulse reshaping devices, and a second type that implements a U-shaped TF. Furthermore, we describe a heuristic approach to design such OGs by using the fiber four-wave mixing effect in highly non-linear dispersion-shifted fibers. The performance of the technique is evaluated by simulating the demultiplexing of 4-ASK signals with these OGs. Results indicate that the proposed devices are able to provide binary signals with acceptable BERs even after the 4-ASK signals are propagated through a cascade of eight fiber links of 50 km followed by optical amplifiers with a 4.5-dB noise figure.     

一种新的自由空间光通信调制方式——圆偏振位移键控

自由空间光通信(FSO)在开放的大气链路中传输光信号, 不可避免地会受到大气扰动和背景光噪声的影响, 导致系统的可靠性降低。为了抑制大气扰动等对FSO系统造成的不利影响, 提出一种新的光信号调制方式——圆偏振位移键控(CPOLSK)。该调制方式利用圆偏振光的两种旋光状态进行数据的传送, 接收端结合差分方法进行光信号接收。给出了CPOLSK的装置模型, 并对其性能进行了分析。与目前广泛采用的开关键控(OOK)、脉冲位置调制(PPM)方式相比, CPOLSK很好地抑制了背景光噪声的影响, 同时对光电探测器内部噪声也有一定的抑制效果。     

基于串行最小频移键控的8进制调制的研究

提出了一种新型的基于串行结构的最小频移键控调制技术,产生了相位连续,频谱宽度窄,每码元时间有两次π/2相移,类似于传统最小频移键控调制信号,给出了该调制方案的理论推导,并将其应用于8进制的多维多阶调制系统中。理论和仿真分析了不同的8进制调制方案在频谱特性,残留色散容限,自相位调制容限的不同,还分析了其受到系统滤波带宽的影响。最后仿真实现了通过350km的色散完全补偿的光纤120Gb/s的数据传输。     

Analysis of a DPSK soliton transmission system

We present a closed form expression to analyse the error performance of a differential phase-shift keying (DPSK) digital modulation format in an optical soliton communication system. In the analysis we include the effect of Gordon-Haus jitter, which is mainly due to the amplified spontaneous emission noise of the amplifiers. Gordon-Haus jitter is found to be less significant when the bit period is 15 times greater than the jitter variance and becomes more significant when the signal-to-noise ratio is greater than 20 dB. Due to a good bit error rate (BER) performance (BER ≈ 10⁻²⁰at SNR= 21 dB), the DPSK modulation format has a potential application for a high-bit-rate and low-noise soliton communication system.     

Performance analysis of FSO links under strong atmospheric turbulence conditions using various modulation schemes

Strong atmospheric turbulence is a major hindrance in wireless optical communication systems. In this paper, the performance of a wireless optical communication system is analyzed using different modulation formats such as, binary phase shift keying-subcarrier intensity modulation (BPSK-SIM), differential phase shift keying (DPSK), differential phase shift keying-subcarrier intensity modulation (DPSK-SIM), M-ary pulse position modulation (M-PPM) and polarization shift keying (PolSK). The atmospheric channel is modeled for strong atmospheric turbulences with combined effect of turbulence and pointing errors. Novel closed-form analytical expressions for average bit error rate (BER), channel capacity and outage probability for the various modulation techniques, viz. BPSK-SIM, DPSK, DPSK-SIM, PolSK and M-PPM are derived. The simulated results for BER, channel capacity and outage probability of various modulation techniques are plotted and analyzed.     

Performance comparison for RZ-DPSK signal in DCF-based and CFBG-based dispersive transmission system

The impact of the phase noise induced by self-phase modulation and intrachannel nonlinear effect for return-to-zero differential phase-shift keying (RZ-DPSK) in long haul 40 Gb/s transmission systems where dispersion is compensated by chirped fiber Bragg grating (CFBG) is analyzed and numerical evaluated, and it is compared with what is derived from the conventional DCF-based phase-modulated system. Our work also provides a clear physical picture of how the transmission performance is affected by CFBG, which is instructive for further research on CFBG compensated phase-modulated formats.     

Improvement of switching properties and cascadability of an ultra-fast reconfigurable optical crosspoint switch matrix using DPSK payloads

In this paper, we demonstrate mitigation of pattern-induced degradation in an optical crosspoint switch (OXS) matrix by utilizing differential phase shift keying (DPSK) modulation format. We experimentally demonstrate 4 × 4 unicast optical packet switching and dynamic reconfiguration for 4-channel, 200 GHz spacing of RZ-DPSK payloads. Reconfigurable time as fast as 2 ns is achieved owing to the optimized control circuit and device fabrication. The power and wavelength dependence are obtained for the RZ-DPSK payload. We also investigate the cascadability of the OXS based on re-circulating loops. Due to the great suppression of the pattern effect in OXS, DPSK has shown dramatical improvement of switching properties compared to conventional ON-OFF keying (OOK) signal. The DPSK payload can outperform OOK for 3.2 dB after 9 hops optical switching.     

Proposal and simulation for all-optical format conversion between differential phase-shift keying signals based on cascaded second-order nonlinearities

We propose and simulate simple realizations of all-optical format conversion between differential phase-shift keying (DPSK) signals based on cascaded second-order nonlinearities in a periodically poled lithium niobate (PPLN) waveguide. Four kinds of 40 Gb/s all-optical format conversion from non-return-to-zero differential phase-shift keying (NRZ-DPSK) to return-to-zero differential phase-shift keying (RZ-DPSK) are investigated based on cascaded second-harmonic generation and difference-frequency generation (cSHG/DFG) or cascaded sum- and difference-frequency generation (cSFG/DFG). The optical spectra, temporal waveforms, eye diagrams, constellation diagrams, and time-related phase distribution are analyzed, which indicate successful implementation of NRZ-DPSK-to-RZ-DPSK format conversion. The obtained results also confirm the phase preservation characteristic of PPLN.     

Analysis of chromatic dispersion effect on coherent optical CPFSK system and performance improvement with the use of combinations of various optical fibers

The performance of a coherent optical continuous-phase frequency shift keying (CPFSK) system is evaluated theoretically including the combined effect of fiber chromatic dispersion, non-zero laser line width and receiver noises. The system limitations imposed by chromatic dispersion and laser phase noise are depicted. The system performance can be improved by shifting the zero-dispersion wavelength to the operating wavelength of the system and this shifting can be achieved efficiently by using a suitable combination of different types of optical fibers. Dispersion shifting characteristics are demonstrated using four kinds of fibers, namely: standard single-mode fiber (SMF), dispersion shifted fiber (DSF), dispersion compensated fiber (DCF) and large core fiber (LCF). The effect of dispersion shifting on the system performance is evaluated in terms of transmission distance and bit rate.     

Simultaneous format conversion of parallel multichannel based on FWM in symmetric highly nonlinear fiber loop

A parallel multichannel format conversion scheme for elastic optical networking based on four wave mixing （FWM） in symmetric highly nonlinear fiber loop （S-HNLF-L） is proposed and the performance is evaluated and discussed. Parallel four channels format conversion from quadrature phase shift keying （QPSK） to binary phase shift keying （BPSK） signals at 40 Gb/s is theoretically analyzed and simulated. The results are helpful for the format-adaptive elastic optical networks.     

Semiconductor optical amplifier used as regenerator for degraded differential phase-shift keying signals

Phase and amplitude regeneration are necessary for degraded differential phase-shift keying communication systems.This paper proposes a regenerator based on semiconductor optical amplifier for differential phase-shift keying signals.The key regeneration mechanism is theoretically analysed.The effectiveness of semiconductor optical amplifier based regenerator is demonstrated by comparing the bit error rate and eye diagrams before and after regeneration for 40-Gbit/s differential phase-shift keying 1080-km transmission systems.The results show that regeneration effects are very well.Bit error rate is less than 10 12 with the regenerator.     

相位调制信号的全光再生技术

介绍了差分相移键控信号的全光再生技术.研究了多进制相位调制信号的全光再生技术.对适用于正交相移键控和差分正交相移键控信号的全光再生器进行了仿真.研究结果表明,这种再生器可以显著抑制信号的幅度噪音和相位噪音,使40 Gb/s RZ差分正交相移键控系统在误码率为10－12条件下的传输距离延长至2 000 km以上.     

Cascaded SOA configuration for NRZ-OOK to RZ-QPSK format conversion

We propose and experimentally demonstrate an all-optical format conversion from two independent non-return-to-zero on-off keying (NRZ-OOK) signals to a return-to-zero quadrature phase shift keying (RZ-QPSK) signal using two cascaded SOAs. Within each SOA, the cross phase modulation (XPM) process is properly controlled by an assistant light. After evaluating its effect on XPM, a suitable power range of assistant light is chosen for the NRZ-OOK to RZ-QPSK format conversion. In the experiment, a RZ-QPSK signal is successfully obtained with opened demodulation eye-diagrams.