WDM ring network using a centralized multiwavelength light sourceand add-drop multiplexing filters

We propose and experimentally demonstrate a wavelength division multiplexed (WDM) ring network employing a centralized multiwavelength light source to supply all nodes with optical carriers of precise wavelength spacing. This approach overcomes the problem of monitoring and controlling the wavelengths of optical sources dispersed throughout the network. A four-node test bed using optical lattice-type add-drop multiplexing filters and a multiwavelength light source based on resonant cavity-enhanced four wave mixing in a Fabry-Perot laser diode was constructed to demonstrate the feasibility of the proposed method. The effects of coherent crosstalk in such networks are analyzed, and methods such as phase scrambling or the use of a pulsed optical source are shown to significantly reduce the effects of coherent crosstalk. The use of four-port add-drop multiplexer (ADM) filters instead of 1:n optical wavelength multiplexer/demultiplexers to implement the wavelength add-drop function at each node is shown to lead to a broader transmission bandwidth for each channel and also easier compatibility with the coherent crosstalk suppression mechanisms described     

A 4-wavelength optical multi/demultiplexer for WDM subscriber loop systems using analog baseband video transmission

This paper describes the design principles and performance of optical multi/demultiplexers (MUX/DEMUX's) in wavelength-division multiplexing (WDM) subscriber-loop systems over a 50- μm core diameter, graded-index (GI), multimode fiber, which employ analog baseband video transmission using laser diodes (LD's). In this WDM arrangement, requirements for MUX/DEMUX's are: 1) low insertion loss; 2) no signal degradation caused by optical interchannel crosstalk; 3) only a small amount of analog baseband signal degradation caused by the use of MUX/DEMUX; 4) a small size and simple structure capable of multiplexing three or four wavelengths; and 5) good stability. The newly developed 4-wavelength MUX/DEMUX satisfies the preceding requirements and is suitable for application to WDM subscriber-loop systems using analog baseband signals along with digital signals.     

An array of photonic filtering advantages:arrayed-waveguide-grating multi/demultiplexers for photonicnetworks

The author introduces the principles, fabrication techniques, and recent progress of planar-type arrayed-waveguide-grating (AWG) multi/demultiplexers, which have been developed for wavelength division multiplexing (WDM)-based photonic networks. The AWG has already been used in point-to-point WDM systems and is a key component in the construction of flexible and large-capacity WDM networks. This is because, compared with conventional filters consisting of thin-film interference filters and micro-optics, the AWG offers the advantages of low loss, high port counts, and mass productivity. Further progress on the AWG is expected to contribute greatly to the construction of future photonic networks including optical add/drop multiplexing systems and optical crossconnect systems     

Optical multi/demultiplexers for single-mode fiber transmission

This paper describes two types of optical multi/demultiplexers using a single-mode fiber for a two-way transmission system. One type, the filter type, consists of four bandpass filters and a short wavelength pass filter or a long wavelength pass filter. The other, the filter-grating type, consists of two bandpass filters and a grating. The present multi/demultiplexers have four channels with 1.05, 1.15, 1.3, and 1.5 μm wavelengths. Total insertion losses are less than 2.3 and 3.2 dB for the filter type and the filter-grating type, respectively. Cross-talk between channels is less than -40 dB.     

Tunable optical filters for dense WDM networks

WDM is currently taking over as the leading technology in point-to-point transmission links. For optical implementation of WDM networks, logical functionalities such as wavelength (channel) selection should be carried out in the wavelength domain; thus, the development of dynamic optical devices is required. One key device is a tunable optical filter. Important features of such a filter include low insertion loss, narrow bandwidth, high sidelobe suppression, large dynamic range, fast tuning speed, a simple control mechanism, small size, and cost effectiveness. Here, an extensive overview of the different technologies used to produce tunable optical filters is presented. Among them, fiber filters such as fiber Bragg gratings and fiber Fabry Perot are the most commercialized, yet inherently limited in their dynamic speeds. For high demanding dynamics, micro-machined and acousto-optic filters can offer a good solution for microsecond tuning speeds. Faster tunable devices, in nanosecond tuning speeds, might emerge out of microresonators, electrooptic filters, and active DBR filters     

无源光网络的初步升级方案

利用 13 10nm/ 15 5 0nm粗波分复用器 (CWDM)与 15 5 0nm光纤型带内耦合器 (OFIBC) ,提出了无源光网络(PON)的初步升级方案 ,CWDM与OFIBC复用器大的通带间隔与温度不敏感性保证了可以采用现行的无制冷的F P腔激光器 ,实验表明根据OFIBC的通带 ,选择激光器中心波长 ,系统可以达到具有较小的功率损耗与串扰。     

Optical multiplexing in fiber networks: progress in WDM and OTDM

WDM systems are maturing and are in use in real point-to-point applications. The technology has also become of interest in the development of all-optical networks, and in this context the performance of WDM can be more critical. In some networks, multiple-access demands that a large number of channels be deployed over long distances, and in these cases the performance of WDM may be degraded by crosstalk arising from fiber nonlinearity. Other problems arise from non-ideal components such as optical amplifiers, switches, and multiplexers/demultiplexers, Much activity in WDM has been understanding its limitations and improving component performance, Some of the limitations imposed may be overcome by the use of OTDM, and research in this area continues to be of interest and importance. However, OTDM is less mature, and much activity is concerned with the realization of robust subsystems such as sources and demultiplexers. It is likely that in the longer term networks will employ a combination of WDM and OTDM     

Key technologies for terabit/second WDM systems with high spectralefficiency of over 1 bit/s/Hz

To fully utilize a limited gain bandwidth of about 35 nm (4.4 THz) in an erbium-doped fiber amplifier, an increase in signal spectral efficiency is required. In this paper, we investigate the key technologies to achieve terabit/second wavelength-division multiplexing (WDM) systems with over 1 bit/s/Hz spectral efficiency. Optical duobinary signals, which have narrower optical spectra than conventional intensity modulation signals, were applied to such dense WDM systems. The measured minimum channel spacing for 20-Gbit/s optical duobinary signals was 32 GHz and a spectral efficiency of over 0.6 bit/s/Hz was reached. By using polarization interleave multiplexing, spectral efficiency was expected to reach 1.2 bit/s/Hz in an ideal case with no polarization dependencies along the transmission lines. In such ultradense WDM systems with narrower channel spacing, stabilizing the wavelengths of laser diodes is an important issue for achieving stable operation over long periods. To do this, we developed a simple and flexible wavelength stabilization system which uses a multiwavelength meter. The wavelengths for 116 channels with 35-GHz spacing were stabilized within ±150 MHz. The stabilization system is applicable to ultradense WDM signals with a spectral efficiency of over 1 bit/s/Hz by employing wavelength interleave multiplexing and an optical selector switch. On the basis of these investigations, we demonstrated a 2.6-Tbit/s (20 Gbit/s×132 channels) WDM transmission by using optical duobinary signals. In addition, 1.28-Tbit/s (20 Gbit/s×64 channels) WDM transmission with a high spectral efficiency of 1 bit/s/Hz was achieved by using polarization interleave multiplexing     

Optical beat-induced crosstalk of an acousto-optic tunable filterfor WDM network application

Acousto-optic tunable filters (AOTF) using TE-TM mode conversion are attractive for wavelength routers, such as WDM add/drop multiplexers or WDM cross-connect switching fabrics, due to their multichannel selectivity. However, their multichannel selection creates optical beat-induced crosstalk, the so called “coherent crosstalk”, due to the interaction of the lightwave with several acoustic waves. This paper evaluates the transmission characteristics of WDM systems employing AOTF's. First, we develop an analytical model of coherent crosstalk based on the coupled mode theory. Next, we examine coherent crosstalk induced BER degradation both theoretically and experimentally for optical WDM systems and show that the analytical studies well support the experimental results. Finally, maximum AOTF cascade number is estimated based on these results for WDM based wavelength routing networks     

A tunable wavelength demultiplexer using logarithmic filter chains

This paper proposes a novel wavelength division multiplexer (WDM) demultiplexer design for dense WDM networks. The proposed demultiplexer consists of rectangular shaped periodic frequency filters connected in series. These rectangular filters can be formed with an apodized one-dimensional (l-D) photonic crystal structure on a ridged semiconductor waveguide. A design example is given for such filters. In such designs, we can use a moderate refractive index contrast, in discrete groups of many layers, to achieve the same stop-band width that we would get with a large contrast. Apart from being compact in size, to demultiplex an arbitrary wavelength from N wavelengths, only log₂ N switches and filter stages are required. This type of filter has a large potential to be used in integrated photonic implementation and packet switched dense WDM applications     

Dispersion of cascaded fiber gratings in WDM lightwave systems

Fiber gratings operating in the transmission mode can provide high dispersion at wavelengths close to the Bragg resonance. When multiple gratings are cascaded for wavelength division multiplexing (WDM) applications, the net dispersion between the stop bands of any two consecutive gratings is significantly modified. We discuss the dispersion characteristics of such cascaded fiber gratings and propose a dispersion compensator for simultaneous compensation of group-velocity dispersion (GVD) for multiple channels of a WDM lightwave system. We also discuss the impact of the dispersion possessed by cascaded gratings on grating based add-drop multiplexers     

Modeling of four-wave mixing and gain peaking in amplified WDMoptical communication systems and networks

A theoretical model is presented for analyzing the propagation of densely spaced WDM optical signals through a cascade of erbium-doped fiber amplifiers and single-mode optical fibers with nonuniform chromatic dispersion. By combining a numerical solution for the EDFA and an analytical expression for FWM components generated through the cascade, the model allows a realistic system analysis which includes gain peaking effect, amplified spontaneous emission accumulation and the effect of dispersion management on the four-wave mixing efficiency. The FWM power distribution at the end of the multi-amplifier transmission link is computed taking into account the phase relation between FWM light amplitudes generated within different sections of the link. The transmission of many WDM channels, evenly spaced around 1547.5 nm, has been analyzed for various dispersion management techniques and propagation distances. Numerical results point out the importance of such a model for a realistic design of WDM optical communication systems and networks. A proper choice of chromatic dispersion, amplifier characteristics, span length, input signal powers and wavelengths, combined with the use of gain equalizing filters, allows to maximize the transmission distance ensuring acceptable signal-to-noise ratio (SNR) and limited SNR variation among channels     

Low-loss wavelength division multiplexing (WDM) devices for single-mode systems

We report here on single-mode microoptic wavelength division multiplexing (WDM) devices with two channels located at 1275 and 1345 nm, respectively. Data are presented for four multiplexers and four demultiplexers. The average insertion loss for the multiplexers was 0.5 ± 0.2 and 1.0 ± 0.3 dB for the short and long wavelength channels, respectively. For the demultiplexers, the average losses were 0.8 ± 0.2 dB and 1.0 ± 0.1 dB, respectively. The full channel widths of the demultiplexers for 0.5 dB of additional loss were about 22 nm. All measurements included the presence of prototype precision single-mode connectors. The multiplexers were based on interference filters and GRIN lenses with identical single-mode fibers used for inputs and output. The demultiplexers employed diffraction gratings and GRIN lenses with 50- μm core graded-index output fibers. In addition three of the demultiplexers included a third output channel centered at 1521 nm. The 28- dB dynamic range of the monochromator-based test appartus was insufficient to evaluate the crosstalk performance. Measurements on two demultiplexers, using a 1295-nm laser, yielded values of -33 and -38 dB, respectively for the crosstalk in the 1345-nm channel.     

Optical signal processing for lightwave communications networks

A number of optical signal processing functions that might be potentially important for future lightwave communication networks are described. An optical network with a distribution capacity of 100 HDTV channels is considered along with how such a network can be implemented using the following functional subsystems: frequency converters; transmitter banks; modified (wavelength division multiplexing) WDM demultiplexers; and tunable optical receivers. Discussed are the key network-level issues: the power budget, the channel separation, and the overall rationale for selection of multiplexing techniques. A hardware implementation of the functional subsystems using three basic building blocks-tunable amplifiers/filters, phase locked loops, and comb generators-is discussed     

WDM在SDH传输网中应用的技术问题

波分复用技术 ( WDM)在中国电信同步数字体系 SDH光传输网中的广泛运用 ,必将极大地满足宽带长途传输和接入的需求。通过分析波分复用技术相对于时分复用技术( TDM)的优越性及其在 SDH传输网中的应用 ,讨论了 SDH网络中使用 WDM的关键技术 ,并探讨了 WDM与 SDH网络管理一体化的技术方案和优势     

WDM系统信道串话分析

Ｆａｂｒｙ－Ｐｅｒｏｔ光滤汉器是波分复用（ＷＤＭ）系统中实现信道解复用的关键器件，本文根据Ｆ－Ｐ腔光滤波器特性，给出了ＷＤＭ系统串话模型，并分析了信道误码率和信道串话造成的的功率代价。     

Optical-transmission characteristics of optical-fiber cables and installed optical-fiber cable networks for WDM systems

Many cables containing 1.3-/spl mu/m zero-dispersion single-mode (SM) optical fibers are installed in trunk and access networks. Recently, there have been a number of studies on wavelength-division-multiplexing (WDM) systems designed to increase transmission capacity and flexibility. If we can construct WDM systems using SM optical-fiber cable networks designed to transmit using wavelengths in the 1.3-/spl mu/m window (O-band), this will prove very effective in reducing construction costs. It is therefore important to examine the wavelength dependence of the transmission characteristics of SM optical-fiber cables and networks that have already been installed and in which several optical fibers are joined. In this paper, we describe the measured optical characteristics of SM optical-fiber cables and installed optical-fiber cable networks at various wavelengths. The optical characteristics were stable in the 1.46 to 1.625-/spl mu/m wavelength range and we confirmed that the installed SM optical-fiber cable networks could be used for WDM system applications.     

Uni- and bidirectional 4λ×560 Mb/s transmissionsystems using WDM devices based on wavelength-selective fusedsingle-mode fiber couplers

Four-wavelength-channel wavelength-division multiplexing (WDM) lab transmission system experiments with buried heterostructure (BH) lasers at 1200-, 1240-, 1280-, and 1320-nm wavelengths, all-fiber WDM devices, and 20-km single-mode link fiber at a 560-Mb/s bit rate demonstrated that unidirectional and bidirectional WDM transmission systems could be operated successfully by using all-fiber 4λ multiplexing, 4λ demultiplexing, or 4λ multiplexing/demultiplexing devices with a low insertion loss per wavelength channel (2.1-4.7 dB), enough optical far-end crosstalk attenuation (18-37 dB), and high optical near-end crosstalk attenuation (43-49 dB). It is concluded that the four-wavelength-channel WDM lab transmission system at 560 Mb/s mainly used as a test bed is not representative of future unidirectional trunk WDM systems. Such systems favor distributed feedback (DFB) lasers in the 1500-1560-nm wavelength range where fiber attenuation is lower than in the 1200-1320 nm wavelength range and where 1500-nm DFB lasers with a smaller linewidth do not limit the repeater distance as much because of mode partition noise     

1550-nm volume holography for optical communication devices

The two-lambda method can provide a strategic approach to implement all-optical devices for communication wavelength division multiplexing (WDM) signal processing based on volume holography. By writing holograms at 488 nm in LiNbO/sub 3/:Fe and reading them in the third window of optical communication systems (1550 nm), the feasibility of WDM demultiplexers and holographic memories for digital bytes is here demonstrated.     

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