Extended Method of Fast Allocation of USC’s for Millimeter-Wave DWDM-ROF Transmission Over ZDSF

Four-wave mixing (FWM) may happen efficiently and introduce crosstalk at the signal channels in dense-wavelength-division- multiplexing (DWDM) systems employing zero-dispersion-shifted fiber (ZDSF). This paper presents extended method of fast allocation of unequally spaced channels (USC’s) to minimize FWM crosstalk in millimeter-wave (MMW) DWDM radio-over-fiber (ROF) systems. When all of the MMW sub-carriers are at same frequency, by spacing the channels unequally using the extended method, all FWM products except the ones bound to be generated at the signal frequencies, can be excluded from the signal channels. While when the MMW sub-carriers are allowed to be different in frequencies, all of the FWM products can be excluded from the signal channels by spacing the channels unequally. Transmission of 622 Mbit/s BPSK data over 40 kilometers ZDSF, with and without wavelength interleaving, are simulated, for both schemes with equally and unequally spaced channels. Results comparison shows that the link performances can be clearly improved, when the channels are unequally spaced using the extended method.     

Four-Wave Mixing Crosstalk Measurement in a Highly Doped L-Band Erbium-Doped Fiber Amplifier by Using Half of the Signal Channels

We propose a four-wave mixing (FWM) crosstalk measurement method for a signal with 50 GHz spaced wavelength channels in a highly doped L-band erbium-doped fiber amplifier (EDFA). The method uses the crosstalk measured by using a signal with 100 GHz spaced wavelength channels. This approach is based on analyses of the channel spacing dependence of FWM signal generation and the effect of the gain evolution of each signal channel along the EDF during FWM signal generation at a specific frequency. It is shown that reducing the channel spacing from 100 to 50 GHz has little impact on the efficiency of FWM generation if the gain evolution is the dominant term in FWM generation. It is also shown that the contributing ratio of the gain evolution of each channel is almost the same for the two signals. These results enable us to develop a measurement method for estimating the FWM crosstalk in an L-band EDFA applied to a wavelength-division multiplexing system with a 50 GHz spaced signal by using a 100 GHz spaced signal and a compensation factor. The validity of the measurement method is successfully demonstrated experimentally.      

Analysis of repeated unequally spaced channels for FDM lightwavesystems

In long-haul optical frequency-division-multiplexing (FDM) systems, transmission characteristics are degraded by four-wave mixing (FWM). To overcome this problem, repeated unequally spaced (RUS) channels have been recently proposed as a new frequency allocation. In this paper, frequency distribution and intensity of generated FWM lights, and a total bandwidth of signal lights of RUS channels are compared with those of already known equally spaced (ES) and unequally spaced (US) channels. It is found that intensities of generated FWM lights of RUS are less than those of ES when the number of channels and a total bandwidth of signals are common in both channels. It is also revealed that RUS has a narrower total bandwidth than US when the number of channels and the minimum channel spacing are common in both channels. Since RUS simultaneously satisfies a low FWM light intensity and a narrow signal bandwidth, it is considered that RUS is suitable for FDM lightwave transmission systems     

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     

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     

A novel 1×4 coupler-multiplexer permutation switch for WDMapplications

A novel 1×4 coupler multiplexer permutation switch (CMPS) is proposed for applications in wavelength-division-multiplexing (WDM) optical networks. The structure of the CMPS integrates the multiplexing and switching functions into a single compact device. It consists of a single-mode/multimode-waveguide grating-assisted, backward-coupler multiplexer followed by a 1×4 digital optical switch (DOS). The specific design uses an InP-based 1×4 CMPS with InGaAsP-InP multiple-quantum-well (MQW) DOS. The calculated values of crosstalk for the coupler multiplexer and the DOS are <-25 dB and -23 dB, respectively, giving an overall crosstalk <-21 dB for channel bandwidths of 10-13 GHz. The device channels are unequally spaced, which reduces unwanted four-wave mixing (FWM), but are fitted to the ITU standard wavelength grid     

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     

Effects of Frequency Allocations and Zero-Dispersion Frequencies on FDM Lightwave Transmission Systems

In long-haul frequency-division-multiplexing lightwave transmission systems, transmission characteristics are degraded by four-wave mixing (FWM) generated in optical fibers. To overcome this problem, modified repeated unequally spaced (RUS) allocations such as equally spaced RUS (ERUS) and unequally spaced RUS (URUS) allocations have already been examined. In addition, it has been shown that FWM noises decrease by separating signal frequencies and a zero-dispersion frequency in equally spaced (ES) allocations. In this paper, ES, ERUS, and URUS allocations are studied from the viewpoint of the position of the zero-dispersion frequency relative to the signal frequencies. It is revealed that FWM noises are reduced in ES, ERUS, and URUS, with an increase in the separation between the signal frequencies and the zero-dispersion frequency, and FWM noises in ERUS and URUS are much lower than FWM noises in ES.     

Influence of Frequency Allocations and Optical Filters on FDM Optical Fiber Communications

In long-haul frequency-division-multiplexing (FDM) lightwave transmission systems, transmission characteristics are degraded by four-wave mixing (FWM) generated in optical fibers. To date, equally spaced (ES), unequally spaced (US), and repeated US (RUS) allocations have been demonstrated in FDM lightwave transmission systems. It has already been theoretically and experimentally revealed that intensities of generated FWM lights with f_FWM=f_i for RUS are lower than those for ES, and a total bandwidth of signal lights for RUS is narrower than that for US, where f_FWM is a frequency of an FWM light, and f_i is a frequency of a signal light with a channel index i. Moreover, to overcome RUS, ES RUS (ERUS) and US RUS (URUS) have been proposed as modified RUSs and theoretically analyzed. In this paper, ES, RUS, ERUS, and URUS allocations are studied from the viewpoint of a transmission bandwidth of an optical filter. It is revealed that FWM noises are reduced in URUS more than in ES, RUS, and ERUS with a decrease in a transmission bandwidth of an optical filter. The upper limit of a filter transmission bandwidth to achieve a bit error rate of 10^-9 is also obtained.     

由5个AWG组成的600个通道的光波分复用器

本文介绍了有 6 0 0个通道、通道间隔为 2 5 GHz的光波分复用器。该波分复用器是由 1个 8通道间隔为 1.875 THz的母阵列波导光栅 (AWG)和 4个 12 8× 12 8的通道间隔为 2 5 GHz的子阵列波导光栅级联而成的。这种设计因为其体积小而在超密集的波分复用器的发展中更富有竞争力     

Crosstalk reduction in N×N WDM multi/demultiplexers bycascading small arrayed waveguide gratings (AWG's)

This paper shows a new scheme which improves the crosstalk performance of large optical multi/demultiplexers, a key component in wavelength division multiplexing (WDM) systems. This scheme uses arrayed waveguide gratings (AWG's) of various sizes and requires no additional equipment. It is well known that a large multi/demultiplexer can be constructed by cascading small multi/demultiplexers. We have studied the impact of the number and size of AWG stages on crosstalk performance. This paper proves that to obtain a multistage multi/demultiplexer with minimum crosstalk, the total channel number of each AWG stage must be minimized. For example, cascading 10-channel AWG's and 11-channel AWG's improves the crosstalk performance of a 110-channel multi/demultiplexer by about 7.5 dB. Furthermore, the crosstalk performance degradation due to fabrication error is theoretically investigated taking channel bandwidth into account. Optimum design parameters of multistage AWG's are introduced: When the AWG suppression ratio is 30 dB and the ratio of channel bandwidth to channel spacing is about 0.24, the degradation in crosstalk performance due to fabrication error is minimized. The tradeoff between the crosstalk performance and the efficiency in terms of hardware and wavelength are also discussed. It is discovered that this simple scheme can yield a crosstalk-free WDM router. Crosstalk reduction obtained by this scheme allows the realization of flexible multiwavelength networks based on wavelength routing     

Reduction of four-wave mixing crosstalk in WDM systems usingunequally 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 technique to design the channel frequency allocation in order to minimize the crosstalk due to FWM is presented. It is shown that suitable unequal channel separations can be found for which no four-wave mixing product term is superimposed on any of the transmitted channels. This is obtained at the expense of some expansion of the system bandwidth. Simulations are presented to show the effectiveness of this technique in a 10-channel, 10-Gb/s per channel, system     

Design and performance of an optical path cross-connect systembased on wavelength path concept

This paper describes the system design and performance of an optical path cross-connect (OPXC) system based on wavelength path concept. The (OPXC) is designed to offer 16 sets of input and output fiber ports with each fiber transporting eight multiwavelength signals for optical paths. Each optical path has a capacity of 2.5 Gb/s. Consequently, the total system throughput is 8×16×2.5=320 Gb/s and the OPXC features high modularity and expandability for switch components. By exploiting planar lightwave circuit (PLC) technologies, four sets of (8×16) delivery-and-coupling-type optical switches (DC-switches) are developed for the 320 Gb/s throughput OPXC system. The DC-switch offers the average insertion-loss of 12.6 dB and ON/OFF ratio of 42.1 dB. The PLC arrayed-waveguide gratings are confirmed to successfully demultiplex the eight directly modulated signals, multiplexed at a spacing of 1 nm, with a crosstalk of under -25 dB. Eight wavelength-division multiplexing signals, directly modulated at 2.5 Gb/s, are confirmed to be transported over 330 km via a cross-connection node in the test-bed system that simulates five-node network. The experimental performances demonstrated In this paper ensures full scale implementation of the proposed optical path cross-connect system with 320 Gb/s throughput and high integrity     

基于SOI的16通道200GHz阵列波导光栅

设计并制作了基于绝缘体上硅(SOI)材料的1×16阵列波导光栅(AWG).该AWG器件的中心波长为1 550 nm,信道间隔为200 GHz,采用了脊型波导结构.首先确定了波导的结构尺寸以保证单模传输,并利用束传播法(BPM)模拟了波导间隔、弯曲半径和锥形波导长度等参数对器件性能的影响,对器件结构进行了优化,同时也利用BPM方法模拟了器件的传输谱.模拟结果显示:器件的最小信道损耗为4.64 dB,串扰小于-30 dB.根据优化的器件结构,通过光刻等半导体工艺制作了AWG,经测试得到AWG器件的损耗为4.52～8.1 dB,串扰为17～20 dB,能够实现良好的波分复用/解复用功能.     

800 Gbit/s (80×10.66 Gbit/s) transmission over 3200 km ofnon-zero dispersion shifted fibre with 100 km amplified spans employingdistributed Raman amplification

80×10.66 Gbit/s wavelength division multiplexing transmission over 3200 km fibre with 100 km amplifier spacing and 50 GHz channel spacing is demonstrated. Error-free transmission of all 80 channels is achieved by employing distributed Raman amplification, forward error correction and low dispersion slope TrueWave^R fibre     

Low‐Loss Compact Arrayed Waveguide Grating with Spot‐Size Converter Fabricated by a Shadow‐Mask Etching Technique

This paper describes a low‐loss, compact, 40‐channel arrayed waveguide grating (AWG) which utilizes a monolithically integrated spot‐size converter (SSC) for lowering the coupling loss between silica waveguides and standard single‐mode fibers. The SSC is a simple waveguide structure that is tapered in both the vertical and horizontal directions. The vertically tapered structure was realized using a shadow‐mask etching technique. By employing this technique, the fabricated, 40‐channel, 100 GHz‐spaced AWG with silica waveguides of 1.5% relative index‐contrast showed an insertion‐loss figure of 2.8 dB without degrading other optical performance.     

Analysis of Optical Singal Transmission Characteristics in AWG Multi/Demultiplexer with Electromagnetic Field Theory

Based on the electromagnetic field theory,the optical signal transmission characteristics in input/output waveguides,slab waveguides and arrayed waveguides of the arrayed waveguide grating(AWG) multi/demultiplexer are analyzed.The relationship between the physical parameters such as geometry sizes and relative refractive index in AWG multi/demultiplexer and the optical signal transmission characteristics are discussed.This theoretical study can be used for optimizing the design and improving the performance of the AWG multi/demultiplexer.     

FSK Modulation Scheme for High-Speed Optical Transmission

In this paper,we describe the generation,detection,and performance of frequency-shift keying (FSK) for high-speed optical transmission and label switching.A non-return-to-zero (NRZ) FSK signal is generated by using two continuous-wave (CW) lasers,one Mach-Zehnder modulator (MZM),and one Mach-Zehnder delay interferometer (MZDI).An RZ-FSK signal is generated by cascading a dual-arm MZM,which is driven by a sinusoidal voltage at half the bit rate.Demodulation can be achieved on 1 bit rate through one MZDI or an array waveguide grating (AWG) demultiplexer with balanced detection.We perform numerical simulation on two types of frequency modulation schemes using MZM or PM,and we determine the effect of frequency tone spacing (FTS) on the generated FSK signal.In the proposed scheme,a novel frequency modulation format has transmission advantages compared with traditional modulation formats such as RZ and differential phase-shift keying (DPSK),under varying dispersion management.The performance of an RZ-FSK signal in a 4 × 40 Gb/s WDM transmission system is discussed.We experiment on transparent wavelength conversion based on four-wave mixing (FWM) in a semiconductor optical amplifier (SOA) and in a highly nonlinear dispersion shifted fiber (HNDSF) for a 40 Gb/s RZ-FSK signal.The feasibility of all-optical signal processing of a high-speed RZ-FSK signal is confirmed.We also determine the receiver power penalty for the RZ-FSK signal after a 100 km standard single-mode fiber (SMF) transmission link with matching dispersion compensating fiber (DCF),under the post-compensation management scheme.Because the frequency modulation format is orthogonal to intensity modulation and vector modulation (polarization shift keying),it can be used in the context of the combined modulation format to decrease the data rate or enhance the symbol rate.It can also be used in orthogonal label-switching as the modulation format for the payload or the label.As an example,we propose a simple orthogonal optical label switching technique based on 40 Gb/s FSK payload and 2.5 Gb/s intensity modulated (IM) label.     

16 channel 200 GHz arrayed waveguide grating based on Si nanowire waveguides

A 16 channel arrayed waveguide grating demultiplexer with 200 GHz channel spacing based on Si nanowire waveguides is designed.The transmission spectra response simulated by transmission function method shows that the device has channel spacing of 1.6 nm and crosstalk of 31 dB.The device is fabricated by 193 nm deep UV lithography in silicon-on-substrate.The demultiplexing characteristics are observed with crosstalk of 5-8 dB,central channel’s insertion loss of 2.2 dB,free spectral range of 24.7 nm and average channel spacing of 1.475 nm.The cause of the spectral distortion is analyzed specifically.     

基于硅纳米线波导的16通道200GHz阵列波导光栅

设计了基于硅纳米线波导的16通道,通道间隔为200GHz的阵列波导光栅(AWG)。传输函数法模拟了器件传输谱,结果表明器件的通道间隔为1.6nm,通道间串扰为31dB。器件利用SOI材料,由193nm深紫外光刻工艺制备。光谱测试结果分析表明,通道串扰为5-8dB,中心通道损耗2.2dB,自由光谱区长度24.7nm,平均信道间隔1.475nm。详细分析了器件谱线畸变的原因。