Coherent and incoherent crosstalk in WDM optical networks

The impact of coherent and incoherent crosstalk on an optical signal passing through optical cross-connect nodes (OXC's) in wavelength division multiplexing (WDM) optical networks is studied, and the analytical expressions are given. Such crosstalk will be generated when the optical propagation delay differences of optical paths in an OXC do not exceed the coherent time of the lasers. While causing fluctuation of signal power, coherent crosstalk may cause noise or not, depending on the relationship between the optical propagation delay differences and the time duration of one bit of the signal. Incoherent crosstalk may cause very high noise power, because it can be a coherent combination of crosstalk contributions. The statistical impact of all crosstalk contributions on signal is studied by simulation, and the concept of quantile is proposed to relax the crosstalk specification requirement for components. The crosstalk specification requirements are then obtained for components used in WDM optical networks with different scales     

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     

Performance of WDM ring and bus networks in the presence ofhomodyne crosstalk

This paper examines the effects of coherent and incoherent homodyne crosstalk in wavelength division multiplexed (WDM) ring and bus networks using reconfigurable optical add-drop multiplexers (OADM's). It is widely understood that incoherent homodyne crosstalk causes power penalties at the receivers in these networks. We show that coherent homodyne crosstalk causes a range of possible received powers, and that coherent and incoherent crosstalk together lead to a range of possible power penalties. A Monte Carlo simulation is used to examine the probability distribution of power penalties due to homodyne crosstalk under various conditions. We find that increasing the switch and multiplexer crosstalk within each OADM, and increasing the number of WDM channels, all produce increased probabilities of large power penalties. However, the number of nodes through which a signal is transmitted does not affect the power penalty distribution     

A comparison of the homodyne crosstalk characteristics of opticaladd-drop multiplexers

We compare the homodyne crosstalk characteristics of a number of different optical add-drop multiplexer (OADM) structures, in the context of wavelength-division multiplexed (WDM) ring or bus network links. Nine fixed-wavelength and rive reconfigurable OADM structures are examined. For the fixed-wavelength structures, we find considerable variation in the levels of incoherent crosstalk, and generally low coherent crosstalk levels. For the reconfigurable structures, incoherent crosstalk levels are quite similar, but the coherent crosstalk levels differ considerably. The range of homodyne crosstalk-induced power penalties is calculated for all structures, and the probability distribution is found for each reconfigurable OADM. The effects of the number of OADMs in the link and the number of wavelength channels in the network are also studied. We find that the best choice of OADM depends on the application     

高能光纤激光器光束合成技术

高能光纤激光器光束合成技术是近年来高能激光器尤其是定向能源应用中的研究热点,可突破单根单模光纤激光的输出功率限制,为高功率高光束质量的激光武器应用奠定了理论基础。介绍了光纤激光非相干合成和相干合成的国内外研究现状,给出了非相干合成技术中光束重叠和光谱合成的基本合成原理,重点介绍了国内外多家研究机构光谱合成近年来所达到的技术水平;介绍了国内外相干合成技术的最新研究进展,对相干合成等效大口径激光阵列输出中几种不同的透射式相干合成阵列输出和反射式相干合成阵列输出的关键合成装置,以及相干合成单一孔径输出中的核心光学元件进行详细分析。最后简要对比了高能光纤激光器光束相干合成技术和非相干合成技术的优缺点和应用范围。     

Noise and crosstalk limitations in optical cross-connects withreshaping wavelength converters

Noise and crosstalk are two important limiting factors in optical networks with optical cross-connects (OXC's). If reshaping wavelength converters are used in the OXC nodes, noise and crosstalk are transformed into timing jitter which accumulates in the network and limits the performance. A method to evaluate the influence of jitter caused by noise and crosstalk in cascaded OXC's is presented and applied to three different OXC architectures, taking both coherent and incoherent crosstalk into account. In the analyzed architectures, the losses in the OXC and the coherent crosstalk is found to be the most important limitations     

WDM光网络系统相干串扰与非相干串扰

同频串扰可以分为相干串扰与非相干串扰,本文提出了WDM光网络系统串扰相干性分析的数学模型,分析提出了相等的相干串扰代价和非干串扰代价下引起的WDM光网络系统功率代介的一致性,并与模拟相干串扰和非相干串扰结果进行了比较验证。实验结果表明,对应1dB功率代价下,WDM光网络系统对应相干串扰和非相干串扰代价容限步于－20dB和理论结果一致。串扰对于网络规模有一定的限制。     

Mutual Injection-Locking and Coherent Combining of Two Individual Fiber Lasers

In this paper, mutual injection-locking and coherent combining are demonstrated with two individual erbium-doped fiber lasers that were coupled by two fiber splitters. Mutual injection-locking theory of two lasers is analyzed. In the free-running state, the far-field beam profile is a simple intensity superposition, as expected of two incoherent beams. Under mutual injection-locking, interference fringes with high contrast ratio are obtained, and the two fiber lasers lase at the same wavelength with a stable output power. We have found that the two fiber lasers are always in out-of-phase mode, which is consistent with theoretical analysis. Coherent beam combining by mutual injection-locking is realized without the need for length or amplitude control. This method can be easily scaled to combine more beams.     

Monolithic multiple wavelength surface emitting laser arrays

A two-dimensional (2-D) surface emitting laser array emitting 140 unique, nonredundant, uniformly separated, single-mode wavelengths in the 980-nm regime is described. The wavelength separation between neighboring lasers is as small as 0.3 nm. A large total wavelength span of 43 nm was obtained without compromising the performance of the lasers. All 140 lasers have nearly the same threshold currents, voltages, and resistances. The techniques used are generic and can be readily extended to both longer and shorter wavelength lasers. The authors also report the first wavelength division multiplexing system experiment using part of this laser array. A BER (bit-error ratio) of 10 ^-9 at 155 Mb/s was obtained with simultaneous operation of four lasers at a wavelength separation of 1.5 nm. Negligible optical and electrical crosstalk was observed between the lasers     

Investigation on the spectral characteristics of DFB lasers withdifferent grating configurations made by electron-beam lithography

The single-mode stability for distributed-feedback (DFB) lasers with various electron-beam-written grating configurations has been investigated theoretically and experimentally, for both as-cleaved and AR-coated lasers. Other laser properties interesting for coherent and multichannel communications systems, such as linewidth and tunability, have also briefly been investigated. Lasers with more sophisticated grating structures, such as an optimized multiple phase-shifted or a corrugation-pitch-modulated grating, did not exhibit performance significantly superior to that of λ/4-shifted DFB lasers with an appropriate coupling coefficient. Antireflection (AR)-coating of the end facets proved indispensable for obtaining a high yield for lasers with single-mode operation at high output power and for reducing the large chip-to-chip variation seen for the as-cleaved lasers. A theoretical investigation of the effect of end reflections on the stopband and of the problem of determining the coupling coefficient was also made     

Channel capacity optimization of chirp-limited dense WDM/WDMAsystems using OOK/FSK modulation and optical filters

Frequency chirp in semiconductor lasers that are directly modulated through carrier injection not only induces dispersion in long-haul single-mode fiber communication systems, but also causes energy loss and signal distortion when frequency selective elements such as Fabry-Perot filters are present. This effect is significant for high-bit-rate channels in a dense wavelength-division-multiplexed system in a LAN/MAN environment which requires narrow-band filters to suppress crosstalk from adjacent channels. A simulation approach is used to determine the channel capacity in the presence of chirp and crosstalk. The results show that a minimal channel spacing of less than 37 and 10 GHz with each channel operating at 2 Gb/s using OOK and FSK, respectively, can be achieved by a dense wavelength division multiple access system. It is also shown that this system performance can be optimized through adjusting various parameters of the laser and the filter     

Advanced semiconductor lasers

Recent research activities in the field of advanced semiconductor lasers are reviewed with emphasis on highly stable single-wavelength lasers and surface-emitting (SE) lasers for wideband lightwave communication systems and optical parallel information processing. The operational characteristics of DSM (dynamic single-mode) lasers are summarized and requirements for high-performance operation as light sources for high-speed transmission or coherent communications are described. A type of DSM laser called the distributed-reflector (DR) laser is described as an advanced DSM laser which enables high efficiency, high power, and narrow linewidth operations. Specific features and the potential of SE lasers are summarized. Research activities and remaining problems to be solved for a breakthrough in optical parallel information processing are presented. The potential of multidimensional quantum-well structures, such as QW lasers and quantum-box lasers, is discussed in terms of superior characteristics in both stationary and dynamic operations. The present fabrication technologies for realizing high-performance lasers based on multidimensional QW structures are also presented     

Design and fabrication of low-loss and low-crosstalk polarization-maintaining optical fibers

Low-loss and low-crosstalk polarization-maintaining optical fibers (called PANDA fibers), which are potentially useful for coherent optical communication systems, have been fabricated based on the optimum design. Transmission loss of 0.22 dB/km, comparable to that of commercially available single-mode fibers, and crosstalk of -27 dB in a 5-km length, corresponding to the polarization-mode-coupling coefficient of4.0 times 10^{-7}m^-1, have been demonstrated in the 1.56-μm wavelength.     

FBG-based bidirectional optical cross connects for bidirectional WDM ring networks

This paper proposes a new bidirectional optical cross connect (BOXC) using fiber Bragg gratings (FBGs) and optical circulators for bidirectional wavelength-division-multiplexing ring networks. Dynamic and independent wavelength routing is achieved by employing cascaded tunable FBGs. The proposed BOXC requires a small tuning range equal to wavelength channel spacing for tunable FBGs. Therefore, the wavelength count of the proposed BOXC could be increased easily without imposing constraint on the tuning range of FBGs. Coherent and incoherent crosstalk arising in the proposed BOXC and their impact on the power penalty are studied in detail. The Monte Carlo simulation is used to characterize the probability distribution of the power penalty due to both coherent and incoherent crosstalk under various conditions. The specification requirements are then obtained for the FBGs used in BOXC nodes with a different number of wavelengths. The study in this paper reveals that the requirement for the isolation of FBGs is more stringent than that for the reflectivity of FBGs at the Bragg wavelength. It is also shown that a large BOXC can be built based on the Benes network structure, and a method to reduce the complexity is also presented.     

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     

Electrical signal processing techniques in long-haul fiber-opticsystems

The potential for electrical signal processing to mitigate the effect of intersymbol interference in long-haul fiber-optic systems is discussed. Intersymbol interference can severely degrade performance and consequently limit both the maximum distance and data rate of the system. Several techniques for reducing intersymbol interference in single-mode fiber systems with single-frequency lasers are presented, and those techniques which are appropriate at high data rates in direct coherent detection systems are identified. The performances of linear equalization (tapped delay lines), nonlinear cancellation (variable threshold detection), maximum-likelihood detection, coding, and multilevel signaling are analyzed. The results for a simulated binary 8-Gb/s system show that simple techniques can be used to reduce intersymbol interference substantially, thereby increasing the system margin by several decibels. A six-tap linear equalizer increases the dispersion-limited distance (due to chromatic or polarization dispersion) by 20% (or reduces the optical power penalty by as much as a factor of two) in direct detection systems, even when the distortion is nonlinear. A nonlinear cancellation technique (adjusting the decision threshold in the detector based on previously detected bits) can more than double the dispersion-limited distance and/or data rate     

Laterally coupled InGaAsP/InP distributed feedback lasers at 1.5μm for chemical sensing applications

Data is presented on stable, tunable single-mode operation from laterally coupled InGaAsP/InP ridge waveguide distributed feedback lasers at ~1.5 μm, suitable for chemical sensing, realised by a greatly simplified fabrication process requiring a single MOCVD growth step. Completed lasers show sidemode suppression ratios as high as 47 dB and wavelength shifts with injection current of 0.035 nm/mA (or 4.5 GHz/mA)     

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