A generalized suboptimum unequally spaced channel allocationtechnique. II. In coherent WDM systems

For pt.I see ibid., vol.46, no.8, p.1027-37 (1998). Four-wave mixing (FWM) in dispersion-shifted optical fiber is a major problem associated with low optical input power levels in optical wavelength-division multiplexed (WDM) systems. To reduce the crosstalk caused by FWM, a generalized suboptimum unequally spaced channel allocation (S-BISCA) technique has been proposed. While the S-USCA technique reduces the PWM power substantially, it also reduces the minimum channel spacing compared to conventional equal channel spacing (ECS) systems when the same number of carrier channels are accommodated in a fixed optical bandwidth. This results in more interchannel interference (ICI) when employing the S-USCA scheme. The power penalty of the ECS and the S-USCA systems caused by crosstalk and frequency drift are investigated and compared in this paper. The superior system performance region, where S-USCA systems out perform ECS systems, is also quantified. For 20-channel systems using an amplitude-shift keying (ASK) heterodyne detection scheme, for instance, results show that the S-USCA technique pays less power penalty up to bit rates of 5.5, 7.5, and 9.5 Gb/s, when all channels have identical states of polarization and the launched input power per channel P_in, equals to -6, -3, and 0 dBm, respectively     

A generalized suboptimum unequally spaced channel allocationtechnique. I. In IM/DD WDM systems

Four-wave mixing (FWM) is the most serious fiber nonlinearity associated with low-input optical power levels in long-haul multichannel optical systems employing dispersion-shifted fiber. To reduce the crosstalk due to FWM, a generalized suboptimum unequally spaced channel allocation (S-USCA) technique is proposed and investigated. Even though the developed technique is useful in combating FWM crosstalk in wavelength division multiplexing (WDM) lightwave systems with up to 12 channels, its main virtue is in designing multichannel WDM lightwave systems with more than 12 channels. Comparisons of power penalty due to FWM between equal channel spacing (ECS) systems and the S-USCA systems are presented. It is shown that for an intensity modulation/direct detection (IM/DD) transmission system operating in an optical bandwidth of 16 nm with 0 dBm (1 mW) peak optical input power per channel, while a conventional ECS WDM system with 0.84-nm channel spacing cannot even achieve a bit-error rate (BER)=10^-9, the suboptimum technique developed in this paper, for the same minimum channel spacing, can achieve a BER=10^-9 with an FWM crosstalk power of less than 1 dB at the worst channel in a 20-channel WDM system     

Design and analysis of all-optical up- and down-wavelength converter based on FWM of SOA-MZI for 60 Gbps RZ data signal

In this paper, the design and analysis of the all-optical up- and down-wavelength converter based on four-wave mixing (FWM) effect of semiconductor optical amplifier Mach–Zehnder interferometer (SOA-MZI) have been presented. The return-to-zero (RZ) modulated data signal at a bit rate of 60 Gbps has been evaluated for error-free operation to show the feasibility of proposed system at different pump wavelength. The converted signal power and quality factor are investigated as the function of variable signal power and pump power. The optimized operating input signal power of ?5 dBm with Q-factor of \(\sim \)28 dB for RZ modulated signal by using SOA-MZI structure with enhanced FWM effect. The important contribution of these investigations that it is possible to expand the optical network with limited available channel bandwidth by utilizing the wavelength converter and gives an approach to implement wavelength converter for future hybrid optical access networks.     

Investigations on combined XPM and FWM effects on optical pulse propagation in dynamic IP traffic over WDM networks

The influence of IP bursty traffic on combined nonlinear effects of XPM (cross phase modulation) and FWM (four-wave mixing) in IP over WDM networks are investigated and calculated by solving the nonlinear Schrodinger equation (NLSE) using a novel method under two conditions: on–off Poisson distributed IP traffic and self-similar traffic. Different eye diagrams are obtained under various IP traffic types and input optical powers. When the input power of a single channel is larger than 3 dBm, the effect of IP bursty traffic will deteriorate eye diagrams dramatically in a 40-channel WDM network. We also calculate the FWM powers and interchannel power distribution under different IP traffic loads. Based on the interchannel power distribution, we could find out which channels affect the probe channel seriously by the FWM effect. All these numerical results are useful for the Quality-of-Service (QoS) design, traffic grooming, lightpath routing, and wavelength assignment of IP over WDM networks.     

Multiple-frequency basestation RoF system based on polarization multiplexed FWM in SOA

An approach of multiple-frequency millimeter-wave(mm-wave)signal generation is proposed for radio-over-fiber(RoF)system with multiple-frequency basestations(MFBSs).Two groups of orthogonally polarized signals are injected into a semiconductor optical amplifier(SOA),and subsequently ten new different wavelengths are generated via four-wave mix-ing(FWM)effect.At each MFBS,different wavelengths are filtered out using demultiplexer and then input to a photodiode(PD)to generate the mm-wave signals with the frequencies from 52 GHz to 68 GHz at the interval of 2 GHz.Simulation results verify that the proposed multiple-frequency generation for MFBS RoF system can work properly.     

Bidirectional hybrid OFDM based Wireless-over-fiber transport system using reflective semiconductor amplifier and polarization multiplexing technique

In this paper, a bidirectional hybrid OFDM based Wireless-over-fiber architecture has been investigated and demonstrated to transmit 10 Gbps as well as 6.25 Gbps OFDM data for downlink transmission and 5 Gbps as well as 2.5 Gbps OFDM data for uplink transmission over 50-km single mode fiber (SMF) employing polarization multiplexing technique (POLMUX) at optical line terminal (OLT) and optical network unit (ONU). The POLMUX technique is exercised by polarization beam splitters and polarization beam combiners. Mach-Zehnder modulator and RSOA have been used for modulation at OLT and ONU respectively. Transmission performances are observed by constellation diagrams, EVM and BER values. For 10 Gbps, 6.25 Gbps down-link signal and 5 Gbps, 2.5 Gbps up-link signal the power penalty of 3 dBm, 2.3 dBm and 4 dBm, 3.2 dBm at a BER of 10⁻⁹ between back-to-back and over 50-km SMF plus 10-m and 5-m wireless link, are observed respectively. For 32-QAM < 10.5% EVM and for 16-QAM < 13% EVM are recorded. Our architecture is a prominent alternative, not only due to its have potential of both optical and wireless technology but also it is offers a powerful platform to communicate high data rates and support various type of future unforeseen applications and services.     

New techniques for the suppression of the four-wave mixing-induced distortion in nonzero dispersion fiber WDM systems

The performance of a wavelength-division multiplexing (WDM) optical network can be severely degraded due to fiber nonlinear effects. In the case where nonzero dispersion (NZD) fibers are employed, the four-wave mixing (FWM) effect sets an upper limit on the input power, especially in the case of narrow channel spacing. In order to reduce FWM-induced distortion two new techniques, the hybrid amplitude-/frequency-shift keying (ASK/FSK) modulation and the use of prechirped pulses are investigated. It is shown that both techniques can greatly improve the Q-factor in a 10 Gb/s WDM system. This happens even for very high input powers (/spl sim/10 dBm), where the degradation of the conventional WDM system is prohibitively high. The proposed methods are also applied and tested in higher bit rates (40 Gb/s). It is deduced that although the hybrid ASK/FSK modulation technique marginally improves the system performance, the optical prechirp technique can still be used to greatly increase the maximum allowable input power of the system.     

Economic and system impact of hybrid Raman–EDFA amplification in a 40 × 40 Gbps optical transmission network with DPSK modulation

Demands of modern high-bandwidth services drive the need to constantly improve existing optical amplification technology beyond its current bounds. In this paper, we demonstrate a hybrid broadband amplification scheme which is capable of improving the system performance of a wavelength-division-multiplexed (WDM) network. We present the study of optical signals with differential-phase-shift keying (DPSK) modulation at 40 Gbps and its transmission in a 50-GHz spaced, 40-channel WDM system over an 80-km link with hybrid optical amplification. A comparison of the system and cost impacts of a Raman-only amplification scheme with two hybrid Raman–erbium doped fiber amplifier schemes (Hybrids I and II) is performed. It is shown that one of the proposed hybrid schemes (Hybrid II) outperforms the other by (i) improving the tolerance to signal input power by 17 dB and (ii) increasing the system reach by 55 km for input signal power of 5 dBm, for a bit error rate (BER) performance of 10⁻¹².     

Four-wave-mixing suppression effect of dispersion varying fiber

This letter describes the four-wave-mixing (FWM) suppression effect of dispersion varying fiber (DVF) whose chromatic dispersion increases (or decreases) along its length. The FWM suppression performance is investigated for the dispersion variation rate and its cycle number in the DVF. Moreover, it is clarified experimentally that the DVF functions efficiently in wavelength division multiplexing (WDM) systems with an input power of more than 10 dBm/ch     

WDM systems with unequally spaced channels

Crosstalk due to four-wave mixing (FWM) is the dominant nonlinear effect in long-haul multichannel optical communication systems employing dispersion-shifted fiber. A method is discussed to find non-uniform channel separations for which no four-wave mixing product is superimposed on any of the transmitted channels, therefore suppressing FWM crosstalk. The residual crosstalk, due to channel power depletion only, is analytically evaluated for intensity-modulated repeaterless wavelength-division-multiplexed (WDM) systems and compared to experimental results. The theory includes the effect of the channel depletion on the amplitude of each phase-matched FWM wave. The probability of error is evaluated including the statistics of the pattern dependent channel depletion. The BER curve computed for an 8-channel WDM system is found to be in good agreement with experimental results. In the experiment, repeaterless transmission of eight 10 Gb/s WDM channels over 137 km (11 Tb/s-km) of dispersion-shifted fiber was demonstrated and error-free operation was achieved over a wide range of input powers using unequally spaced channels. The same system with equally spaced channels could not achieve a probability of error lower than 10^-6. The use of unequal channel spacing allowed fiber input power to be increased by as much as 7 dB, which could be translated into a fivefold increase of the bit rate per channel (and therefore of the system capacity), or to an increase in the system length of about 30 km     

Four-wave mixing reduction technique based on smart filter approach

This paper proposed a new technique to suppress the four-wave mixing (FWM) effect by using a smart filter technique. The behaviour of FWM and the performance of wavelength division multiplexing systems with 4 and 16 channels were simulated in the presence of the proposed technique. The simulation was also performed under different parameters such as input power, number of channels and channel spacing. The FWM power drastically decreases by 12 and 19 dB for the 4 and 16 channels, respectively, when the smart filter is used as compared with the conventional system. In terms of system performance, the suggested approach for 4 and 16 channels at the first channel offers low bit error rate (BER) values of 3.23 × 10^?23 and 1.7 × 10^?21, respectively. The smart filter with the channel spacing variation for the 4-channel system subsequently improved the BER value at the fourth channel. Results confirm that the smart filter approach is an active solution that can suppress the FWM effect in optical transmission systems.     

A 60-GHz-Band Analog Optical System-on-Package Transmitter for Fiber-Radio Communications

We developed an analog optical system-on-package (SoP) transmitter for a 60-GHz-band radio-over-fiber (RoF) link. The SoP transmitter consisted of an electroabsorption modulator, radio frequency amplifiers, and a bandpass filter. The 60-GHz RoF wireless link was prepared to measure the performance of the SoP transmitter. The transmission characteristics of 64-quadrature amplitude modulation (64-QAM) data of the 60-GHz RoF wireless link, including the SoP transmitter, were investigated by measuring the error vector magnitude (EVM) and signal-to-noise ratio (SNR) with a baseband frequency. The EVM of the 60-GHz RoF wireless link was between 2.25% and 2.80%, and the SNR was between 27.36 and 29.31 dB from 140 and 770 MHz, at input baseband power of -9 dBm. The noise figure had the minimum of 8.44 dB at 500 MHz. We successfully transmitted digital community antenna television (CATV) system signals through the 60-GHz RoF wireless link, including the SoP transmitter. Digital CATV signals of 86 channels could be transmitted through the 60-GHz RoF wireless link, and the total throughput was found to be 2.61 Gb/s.     

Bidirectional and simultaneous transmission of baseband and wireless signals over RSOA based WDM radio-over-fiber passive optical network using incoherent light injection technique

Reflective semiconductor optical amplifier (RSOA) based wavelength division multiplexed radio-over-fiber passive optical network (WDM−RoF−PON) has been proposed and demonstrated, to transmit 2.5 Gbps baseband (BB) and 1.25 Gbps wireless data in downstream and 1 Gbps BB data signal in upstream over 25-km single-mode fiber (SMF), and wireless downstream signal over 25-km SMF as well as 5.2 m free space in air. In the downstream, 2.5 Gbps BB data and 1.25 Gbps wireless data are modulated using single-electrode Mach–Zehnder modulator (SD-MZM) based on double-sideband with optical carrier suppression (DSBCS) scheme and simultaneously transmitted by incoherent light injection technique and employing fiber Bragg grating (FBG) at the base station. RSOA is utilized at the user end to reuse the carrier for uplink transmission. High receiver sensitivity, low bit-error-rate (BER) and excellent eye-diagram, eye height are achieved in our proposed network system and the results affirm the acceptability of proposed RSOA based WDM−RoF−PON.     

RZ、CSRZ码在CBG色散补偿系统中的传输

全部利用线性啁啾光纤布拉格光栅（CBG）作色散补偿模块和在线通道滤波顺，在2500km超长距离的G．652光纤上实现10Ghps归零码（RZ）、载波抑制归零码（CSRZ）光信号的无电中继传输，并在2080km和2560km处分别对2种信号的传输性能进行了测试。CSRZ在上述2处的功率代价分别为～1dBm和～3dBm（BER-10^12，PRBS=10^23-1），RZ的功率代价分别为～3dBm和～5dBm，验证了在相同系统平台下CSRZ光信号比RZ光信号有更好的性能.     

基于光纤四波混频效应的2R全光再生方法研究

提出了一种基于光纤中四波混频(FWM)效应实现对光信号的再放大和再整形的2R再生功能的方法,分析了该方法的工作原理,设计了一种基于光纤FWM效应实现光信号2R再生的光纤传输仿真系统。仿真结果表明,2R再生信号良好,系统Q值达到21.59,验证了该方法的可行性。所设计的2R再生方案中,泵浦光的波长与功率是影响FWM效率和2R再生性能的主要因素,对其进行了扫参研究,得出泵浦波长为1 548nm、功率为40dBm时,2R再生性能最优。     

The effect of four-wave mixing in fibers on opticalfrequency-division multiplexed systems

The limiting effects of four-wave mixing on optical frequency-division multiplexing (OFDM) systems are described. The optical nonlinearity in a single-mode fiber imposes a fundamental limitation on the capacity of optical frequency-division multiplexed systems. In particular, four-wave mixing (FWM) crosstalk may severely degrade the system performance when the fiber input powers are large and/or the channel spacing is too small. Theoretical and experimental results of the effects of FWM in OFDM systems are presented. The theoretical results demonstrate the dependence of FWM on various system parameters. An analysis of FWM in both undirectional and bidirectional transmission systems is included. The receiver sensitivity degradation from FWM crosstalk is measured in a 16-channel coherent system     

光纤通信系统色散补偿方案的优化

为了优化光纤通信系统色散补偿方案,采用软件仿真的方法设计了一个用色散补偿光纤进行色散补偿的单信道通信系统,利用光纤环形镜的全反射特性使该系统的色散补偿方案得到了优化,补偿效果良好,并节约了成本。对色散补偿及光纤环形镜的工作原理进行了理论分析和仿真验证,取得了系统在2.5Gbit/s和10Gbit/s下Q参量和眼图的仿真数据,分别找出了两个信号速率下的系统最佳输入功率。结果表明,系统在2.5Gbit/s下的最佳输入功率为13dBm,此时Q参量达到了172.88;系统在10Gbit/s下的最佳输入功率为6dBm,其相应Q参量为45.96。这一结果对实际应用中光纤通信系统的色散补偿是有帮助的。     

基于相位调制器产生光毫米波的全双工光纤无线通信系统

提出并实验研究了一种基于相位调制器产生光毫米波信号的全双工光纤无线通信(RoF)系统。在中心站采用相位调制器结合滤波的方法产生重复频率为40GHz的载波抑制双边带毫米波信号,利用交叉复用器分离开毫米波信号的上下边带,其中的一个边带强度调制数据速率为2.5Gbit/s的下行基带信号,另一个边带被发送到基站调制上行传输的基带数据。该系统抗色散效果好,在经过40km标准单模光纤上/下行传输数据速率2.5Gbit/s的基带信号后,双向的传输功率代价都小于0.5dBm。在光纤无线通信系统中采用相位调制器结合滤波的方法产生光毫米波,同时基于波长重用技术再生上行光载波信号,可以简化中心站和基站配置,节约系统成本。     

Improved aquila optimization-based parameter evaluation framework and meta-heuristic algorithm in radio-over-fiber SpatialMux multi-input multi-output long-term evolution system

In heterogeneous access network, Multiple-Input Multiple-Output (MIMO) radio-over-fiber (RoF) system is an efficient approach for multiple signal transmission with low cost and complexity. The performance of RoF fronthaul system in MIMO system will be varied with different nonlinear effects. By adjusting various transmission parameters, such as the input signal power or the laser bias current, the nonlinear impacts produced by the RoF system can be reduced. In this paper, a novel algorithm Improved Aquila Optimization (IAO) is proposed to optimize transmission circumstances of MIMO RoF system. It determines the appropriate bias current for both lasers and Radio Frequency (RF) signal power in a short period. The input signals are wavelength multiplexed with Intensity Modulation and Direct Detection (IM/DD) applied. The carrier as well as transmission frequency is governed by the MIMO-Long-Term Evolution (LTE) standard. The proposed system is implemented in MATLAB, and the performance is evaluated. The experimental results show that fast convergence and trade-off between noise and nonlinearity are obtained with varying bandwidth. In the experimental scenario, the maximum Error Vector Magnitude (EVM) of 1.88, 3.14, and signal-to-noise ratio (SNR) of 3.204, and 2.698 was attained for both quadrature phase shift keying (QPSK) and quadrature amplitude modulation (QAM) modulation. [Correction added on 24 April 2023, after first online publication: the SNR values were corrected in the preceding sentence.] For 100 iterations, the processing time was reduced to 0.137 s. When compared with the conventional state-of-the-art approaches, the accuracy and computational complexity of the proposed approach are improved.     

Inter-BSs virtual private network for privacy and security enhanced 60 GHz radio-over-fiber system

A novel inter-basestations (inter-BSs) based virtual private network (VPN) for the privacy and security enhanced 60 GHz radio-over-fiber (RoF) system using optical code-division multiplexing (OCDM) is proposed and demonstrated experimentally. By establishing inter-BSs VPN overlaying the network structure of a 60 GHz RoF system, the express and private paths for the communication of end-users under different BSs can be offered. In order to effectively establish the inter-BSs VPN, the OCDM encoding/decoding technology is employed in the RoF system. In each BS, a 58 GHz millimeter-wave (MMW) is used as the inter-BSs VPN channel, while a 60 GHz MMW is used as the common central station (CS)–BSs communication channel. The optical carriers used for the downlink, uplink and VPN link transmissions are all simultaneously generated in a lightwave-centralized CS, by utilizing four-wave mixing (FWM) effect in a semiconductor optical amplifier (SOA). The obtained results properly verify the feasibility of our proposed configuration of the inter-BSs VPN in the 60 GHz RoF system.