Strictly nonblocking directional-coupler-based switching networksunder crosstalk constraint

Directional-coupler (DC)-based switching systems can switch signals at the rate of several terabits per second. Such switches can also transmit signals with multiple wavelengths simultaneously. Despite these advantages, DCs suffer from an intrinsic crosstalk problem that must be overcome in building a robust switching system. In this paper, the principles of constructing strictly nonblocking DC-based photonic switching systems under various crosstalk constraints are explored. We demonstrate how crosstalk adds a new dimension to the theory of switching systems. We find the sufficient nonblocking condition for photonic networks under crosstalk constraints and demonstrate that some well-known nonblocking networks can tolerate a stricter crosstalk constraint while retaining their hardware complexity. The theory developed in the paper can guide us in making the design tradeoff between the level of crosstalk and the amount of hardware     

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     

Nonlinear crosstalk and two countermeasures in SCM-WDM opticalcommunication systems

We investigate, theoretically and experimentally, crosstalk between wavelengths in subcarrier-multiplexed (SCM) wavelength-division multiplexed (WDM) optical communication systems. Crosstalk arises mainly from interactions between subcarriers on one wavelength and the optical carrier of another wavelength. In a dispersive fiber, crosstalk can be attributed to stimulated Raman scattering (SRS) and cross-phase modulation (XPM) combined with group velocity dispersion (GVD). We investigate the phase relationship between SRS-induced and XPM-induced crosstalks. Crosstalks induced by SRS and XPM add in the electrical domain and can interfere constructively or destructively. Experimental results show that the combined crosstalk level can be as high as 40 dBc after 25 km of SMF with two wavelengths and 18 dBm per wavelength of transmitted power. We propose two crosstalk countermeasures. The first countermeasure uses parallel fiber transmission. We show theoretically that both SRS-induced and XPM-induced crosstalks can be cancelled to the first order. We present an experimental demonstration of concept which has achieved 15 dB of crosstalk cancellation over 200 MHz. The second countermeasure uses optical carrier suppression. We show, theoretically and experimentally, that by suppressing the optical carrier, we can significantly reduce crosstalk while maintaining the same link budget and carrier-to-noise ratio (CNR) at the receiver, 20 dB of crosstalk reduction over 2 GHz has been demonstrated experimentally     

Crosstalk performance of coherent time-addressed photonic CDMAnetworks

We report a theoretical investigation of the crosstalk performance of photonic code-division multiple access (CDMA) networks that are based on coherent matched filtering of optical pulses. We describe the importance of time gating in the reduction of noise in spread-time CDMA schemes. We give guidelines for the selection of codes in coherent matched filtering, and give a code set that produces low crosstalk. We present calculated bit-error rates (BERs) that show for individual user data rates in the gigabit per second range that crosstalk limits aggregate bit rates to the tens of gigabits per second range. This level of performance is a significant improvement over purely incoherent spread-time approaches. Such low crosstalk suggests that this scheme may be the first spread-time photonic CDMA scheme that is not crosstalk-limited     

Investigation of adjacent channel crosstalk in multichannelmonolithically integrated 1.55 μm photoreceiver arrays

We have investigated adjacent channel crosstalk in 3-, 8-, and 16-channel InP-based monolithically integrated p-i-n/HBT photoreceiver arrays, with a channel bandwidth of 11 GHz. By using a novel monolithically integrated radiation shield, we have been able to reduce the crosstalk to -35 dB at 10 GHz. These parameters represent the best performance in multichannel integrated photoreceiver arrays. The two main components of crosstalk are found to be radiation crosstalk and electrical crosstalk and these are separately dependent on interchannel spacing and single- or dual-source biasing schemes. An electromagnetic full-wave solution shows that the measured crosstalk in arrays without the radiation shield could be dominated by radiation crosstalk, which can be modeled as a capacitive coupling between adjacent channels. Similarly, electrical crosstalk can be modeled by equivalent parasitic resistive and inductive elements. Values of these circuit elements have been determined by analyzing experimental data     

Statistical analysis on stimulated Raman crosstalk in dispersion-managed fiber links

Power impairments due to stimulated Raman scattering (SRS) in dispersion-managed (DM) fiber links are evaluated theoretically. We extend previous work on the statistical analysis of SRS crosstalk to the case of multiple fiber segments. Closed-form formulas are derived, and the applicable range is presented by comparison with simulation results. The SRS crosstalk in DM fiber links is evaluated using derived formulas, and the preferable configuration of DM fiber links for suppressing SRS crosstalk is discussed. We also evaluate the exact power penalty induced by the SRS crosstalk in consideration of the log-normal waveform distribution due to SRS and non-Gaussian noise and clarify the system bounds in some typical DM fiber links. The developed approach provides a design rule for DM fiber links from the viewpoint of SRS crosstalk suppression.     

Experimental comparison of the effect of discrete and distributedpath inband crosstalk on system performance: application to predictingsystem performance penalties

The system performance degradations of inband crosstalk produced by distributed Rayleigh scattering and a single discrete time delayed path have been measured in the same system. Rayleigh scatter crosstalk degrades performance more than equal amounts of discrete crosstalk. By parameterizing the system bit error rate (BER) and received power, we have separated the optical and electrical noise contributions to the BER based on their intensity and crosstalk scaling. We observed behavior consistent with earlier models, but found an unexpected increase in the total optical noise. This excess noise had a very regular linear scaling with crosstalk power. Because of its well defined crosstalk and intensity scaling, this easily measured noise term can be used as a correction to established models to more accurately estimate system performance at high optical powers and low crosstalk levels     

Linear crosstalk in 4×4 semiconductor optical amplifier gateswitch matrix

We report a comprehensive crosstalk investigation of a packaged InGaAsP/InP 4×4 semiconductor optical amplifier gate switch matrix, experimentally as well as theoretically. For a fully loaded switch with the same wavelength on all four inputs, all possible switching combinations are analyzed, thus yielding realistic crosstalk figures. Coherent and incoherent crosstalk phenomena are identified, and a switch crosstalk less than -40 dB has been measured     

Performance implications of component crosstalk in transparentlightwave networks

We find both theoretically and experimentally that crosstalk due to small component imperfections in transparent lightwave networks will tend to generate large performance degradations. The most severe degradations, which arise from interference between signal and crosstalk electric fields, are far larger than those predicted from simple power-addition without field interference. Systems corrupted by crosstalk from a single source of optical-field leakage appear to require component crosstalk levels ϵ⩽-20 dB, if conventional receivers are used. Since the degradations are believed to depend on N√ϵ, where N is the number of crosstalk generators, each producing a crosstalk power level of ϵ, the problem is expected to grow rapidly severe with network size     

Improved linear crosstalk tolerance in an un-amplified ring PON

There is increased interest in ring-based passive optical network (PON) architectures due to their local area network (LAN) capability among end users and inherent protection and restoration schemes. The ring architecture includes optical filters and switches with non-ideal channel isolation, leading to the generation of linear crosstalk, which can significantly degrade the system performance if it is not sufficiently suppressed. We use simulation to analyze the effects of linear crosstalk in an un-amplified TDM optical ring and show how we can improve the tolerance of the crosstalk in our architecture that employs optimized tap split ratios as a means of increasing the number of ONUs served. Our approach is based on simulation of the entire network using a combination of wavelength-domain and time-domain techniques. Typically, PON networks use two or three wavelengths that are separated by more than 50 nm to maintain the cost of components low and as such, it is logical that any crosstalk will be negligible. However, the subpar performance of such components motivated our study into the impact of crosstalk. We show that the tap ratio optimization which increases the number ONUs that can be served in the ring can also exacerbate the effects of linear interchannel crosstalk at the ONU’s downstream signal receivers. We also show that surplus power at the last ONU can be utilized to counter this degraded crosstalk performance by careful tap ratio selection, thereby improving the overall tolerance to linear crosstalk in the system.     

Joint spectrum management and constrained partial crosstalk cancellation in a multi-user xDSL environment

In modern DSL systems, crosstalk is a major source of performance degradation. Crosstalk cancellation techniques have been proposed to mitigate the effect of crosstalk. However, the run-time complexity of these crosstalk cancellation techniques grows with the square of the number of lines. Therefore one has to be selective in cancelling crosstalk to reduce complexity. Secondly, crosstalk cancellation requires signal-level coordination between transmitters or receivers, which is not always available. Because of accessibility constraints, crosstalk between certain lines cannot be cancelled and so has to be mitigated through spectrum management. After a complexity study, this paper presents a solution for the joint spectrum management and constrained partial crosstalk cancellation problem. The complexity of the partial crosstalk cancellation part of the problem is reduced based on a line selection and user independence observation. However, to fully benefit from these observations, power loading has to be applied in the spectrum management part. We therefore also consider ON/OFF power loading, which has a low complexity and shows only a minor performance degradation compared to normal power loading. The resulting algorithm will be compared to currently available algorithms for independent spectrum management and partial crosstalk cancellation.     

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     

Inband crosstalk analysis of optical cross-connect architectures

In this paper, inband crosstalk in several important classes of optical cross-connect (OXC) architectures is studied. We derive the expression for the power penalty imposed by crosstalk so that the various OXC architectures can be evaluated in a systematic way. This, in turn, enables us to study the relation between crosstalk and the component requirement.     

Profile detection in multiuser digital subscriber line systems

Multiuser transmission methods for digital subscriber line (DSL) systems have become of interest with the potential for increased data rate and loop reach. These methods often assume that the set of crosstalk interferers, called the crosstalk profile, and their associated channel responses are known. For DSL systems, the interferers are often uncoordinated, so that in a dynamic environment where DSL transmitters can energize and deenergize, the crosstalk profile cannot be transmitted to the user of interest. While the crosstalk channel estimation problem in a dynamic environment can be intractable for general transmission systems, channel and crosstalk analysis can make use of the specific DSL environment. Namely, the physical channels in a DSL system do not change rapidly, and hence estimates of the crosstalk channel can be saved for future reference. For this reason, we introduce the concept of a channel profile. We develop several algorithms to detect the crosstalk profile and investigate the asymptotic behavior of the new algorithms. Simulations show that for typical crosstalk interference scenarios, the observation time to determine the correct crosstalk profile at probability of error less than 10^-3 can be less than 2 ms     

Validation and test issues related to noise induced by parasitic inductances of VLSI interconnects

Shows the results of studies of noise induced by various combinations of parasitic capacitances and inductances. Interconnects are simulated with parameters obtained from a 0.18 /spl mu/m process. The four kinds of noise addressed are (i) crosstalk pulse; (ii) crosstalk speedup and slowdown; (iii) oscillatory noise; (iv) combination of oscillatory noise and crosstalk pulse. The crosstalk effects induced by a combination of mutual capacitance and mutual inductance can be larger than those induced by mutual capacitance alone, even if capacitive crosstalk dominates. For certain interconnects that are capacitively and inductively coupled, transitions in the same direction on an aggressor and victim line can cause speedup or slowdown, depending on timing parameters. A similar observation holds for transitions in opposite directions. We also observe that oscillatory noise can combine with crosstalk pulse under certain skew conditions and give rise to a large magnitude of noise. We show that inductance induced noise can be a problem in medium length interconnects. Because such interconnects can occur in combinational logic blocks, the generation of suitable vectors for test and validation of such logic blocks is of concern.     

Effects of bursty crosstalk in DSL

We propose a simple model for the bursty operation of digital subscriber line (DSL) modems. Based on this model, we derive the autocorrelation and the power spectrum density (PSD) of the bursty crosstalk. We analyze the effects of bursty operation in both the crosstalk PSD and the training of the DSL modem. This analysis suggests that both the duty cycle and the bursty activity rate must be optimized when bursty crosstalk is used to increase the performance of crosstalk-limited systems     

Spatial correlation of alien crosstalk in MIMO DSL systems

We statistically characterize the spatial correlation of measured far-end crosstalk collected on 2 kft of 26 AWG twisted-pair copper cable. We then develop and verify a parameterized model of this spatial correlation and use it to investigate the behavior of the spatial correlation of far-end crosstalk in a twisted-pair environment.     

Crosstalk Characterization in Geiger-Mode Avalanche Photodiode Arrays

Following our work on Geiger-mode avalanche photodiode arrays, we have recently been dealing with the crosstalk issue in newly developed dense arrays with a minimum distance between pixel centers of 84 mum. In this paper, we present our crosstalk measurement approach, including the experimental setup and the offline calculation methods. Different characterizations of the crosstalk probability P_CT versus capacitance have been performed to extrapolate the P_CT when no measurement setup loads the pixels. We also present results regarding the crosstalk probability versus pixel distance and bias. Moreover, by adopting a slightly different approach, the probability density decay time has been measured to investigate about crosstalk origin.     

Convergence behavior of a PS-FSE in the presence of acyclostationary crosstalk interference

The convergence and steady-state behavior of a phase-splitting fractionally spaced equalizer (PS-FSE) are analyzed in the presence of a cyclostationary crosstalk interference. It is shown that the FSE in the presence of cyclostationary crosstalk has a different eigenstructure compared to the one in the presence of stationary noise. We also show that the convergence characteristic of the PS-FSE varies depending on the relative clock phase between the signal and the crosstalk     

Impact of crosstalk in an arrayed-waveguide multiplexer onN×N optical interconnection

The impact of crosstalk in an arrayed-waveguide N×N wavelength multiplexer is investigated precisely in relation to its application to wavelength-routing N×N all optical networks. In such systems multiple crosstalk light which has the same wavelength as the signal results in signal-crosstalk beat noise. We confirm that the noise is Gaussian and obtain the relation between crosstalk and power penalty. It is shown that the crosstalk must be less than -38 dB for a 16×16 system to keep the power penalty below 1 dB at a bit error rate of 10^-9