Crosstalk-induced Power Penalties Research Articles

In-band crosstalk-induced power penalties in various signal modulation formats.

A simple equation for the estimation of in-band crosstalk-induced power penalty has been derived by considering the symbol peak power as well as the impact of the average intersymbol distance in various modulation formats of optical signals. The derived equation can easily estimate the in-band crosstalk-induced penalty by considering the level of crosstalk and Q-value. Among various formats of signals, the in-band crosstalk-induced penalties in on-off keying (OOK), differential quadrature phase shift keying, and pulse amplitude modulation-4 (PAM-4) signals have been estimated for comparison. From the results, we reconfirmed that the OOK non-return-to-zero signal would be 9 dB more tolerant to in-band crosstalk than the PAM-4 signal. The estimated results agreed well with the measured results.

Integrated Optical Add-Drop Multiplexer in SOI Based on Mode Selection and Bragg Reflection

We present a compact optical add-drop multiplexer in silicon-on-insulator based on combining mode selection with Bragg reflection. The add-drop channel has a 3-dB bandwidth of 2 nm, an extinction ratio of 30 dB, and an interferometric crosstalk of less than –42 dB. We characterize the inter-channel and interferometric crosstalk performance using BER measurements for 10-Gb/s NRZ data and obtain crosstalk induced power penalties less than 3.5 dB.

Silicon Photonic Four-Channel Optical Add-Drop Multiplexer Enabled by Subwavelength Grating Waveguides

Wavelength-division multiplexing over optical links provides an effective solution for the bandwidth challenge of off-chip and on-chip communications. We demonstrate a compact silicon photonic four-channel optical add-drop multiplexer enabled by subwavelength-grating-based contradirectional couplers. Passbands of the device show on-chip insertion losses below 1.8 dB with wide 3 dB bandwidth of ∼6.7 nm suitable for coarse wavelength-division Multiplexing in short-reach optical interconnect applications. Transmission of 10 Gb/s data stream through different channels of the multiplexer results in negligible power penalties, whereas interferometric crosstalk-induced power penalties are below 2.8 dB.

Crosstalk Penalty in Microring-Based Silicon Photonic Interconnect Systems

We examine inter-channel and intra-channel crosstalk power penalties between non-return-to-zero on-off keying (NRZ-OOK) wavelength-division-multiplexing (WDM) channels for microring-based silicon photonic interconnects. We first propose a new model that relates the crosstalk power penalty to the interfering signals power, the extinction ratio of the non-return-to-zero, OOK modulated “victim” channel, and finally the bit-error-ratio that the power penalty is referenced to. As for inter-channel crosstalk, the proposed model agrees well with our recent experimental measurements. We leverage this model to quantify crosstalk induced power penalties in a microring based WDM receiver. We also propose an optimization procedure to equilibrate the power penalty across channels. We then compare our model with intra-channel crosstalk measurements, where two NRZ channels are at the same wavelength and are simultaneously routed to different paths by two cascaded ring resonators. We remark that intra-channel crosstalk is very sensitive to the data rate of NRZ channels. As data rate increases, the observed disturbances exceed what models predict. Based on these observations, we propose an empirical modification of the original model for estimating intra-channel crosstalk power penalties in high ( $>$ 20 Gb/s) data rate situations.

Low crosstalk Bragg grating/Mach-Zehnder interferometer optical add-drop multiplexer in silicon photonics.

We characterize the interferometric crosstalk and system performance of two optical add-drop multiplexer (OADM) designs based on Bragg grating/Mach-Zehnder interferometers implemented in silicon-on-insulator. Both OADM designs exhibit low crosstalk and negligible crosstalk-induced power penalties over their 3 dB bandwidths. The devices are tolerant to wavelength drift and misalignment between the transmitter and OADM; moreover, their designs can be optimized further to enable high performance operation in WDM systems.

Performance analysis of coherently detected orthogonal wavelength division multiplexing scheme

In this paper, a highly spectrally efficient scheme orthogonal wavelength division multiplexing with coherent detection (Co-OWDM) is analyzed. We study the effect of channel spacing on inter-channel crosstalk induced power penalty and on decision threshold Q-factor; effect of number of channels on Q-factor is also discussed. Results obtained with coherent detection are compared with direct detection technique. We also analyzed Co-OWDM with quadrature phase shift keying (QPSK) technique, passive micro-optics based 90° optic is used for coherent detection in this technique. QPSK based Co-OWDM system performance is analyzed in terms of bit error rate (BER) and quality factor.

Dependence of In-Band Incoherent Crosstalk-Induced Penalty on Seed Source in RSOA-Based WDM-PONs

We have investigated the dependence of in-band incoherent crosstalk-induced power penalties on seed source in reflective semiconductor optical amplifier (RSOA)-based wavelength-division-multiplexed passive optical networks (WDM-PONs). The in-band crosstalk penalties in an RSOA-based WDM-PON were measured with two different seed sources; a broadband light source (BLS) and a distributed feedback laser diode (DFB-LD). The 1-dB power penalties with a BLS seeded and a DFB-LD seeded RSOA source were observed when the crosstalk-to-signal ratios were ~ -14 and -16 dB, respectively. From the results, we found that the directly-modulated RSOA sources with a BLS or a DFB-LD seed source were more tolerant to the in-band crosstalk than an externally-modulated DFB-LD source. In addition, we confirmed that the in-band crosstalk-induced power penalty in the RSOA-based WDM-PONs could be calculated with a proper estimation of an equivalent optical source bandwidth.

DWDM self-healing access ring network with cost-saving, crosstalk-free and bidirectional OADM in single fiber

A cost-saving, crosstalk-free and bidirectional optical add-drop multiplexer (B-OADM) based on cascaded thin-film optical add-drop filters (TFOADFs) with optical circulators as well as the optical protection function promptly restored at the access node for a single-fiber self-healing access ring network are proposed and demonstrated. Using this B-TFOADM configuration, no crosstalk appears on the dropped and added channels, respectively. The bit-error rate performances in this B-OADM are investigated in a 2.5-Gb/s DWDM system, showing no intraband and interband crosstalk-induced power penalties. Such crosstalk-free and bidirectional architecture greatly simplifies the hub complexity as well as the low-cost components at each node are employed.

Effects of In-Band Crosstalk in Wavelength-Locked Fabry–PÉrot Laser-Diode-Based WDM PONs

We investigated the impact of in-band crosstalk on the system's performance in wavelength-locked Fabry-Perot laser-diode (FP-LD)-based wavelength-division-multiplexed passive optical networks (WDM PONs). As expected, wavelength-locked FP-LD, which was injected by using an incoherent broadband light source, was more tolerant to in-band crosstalk than distributed-feedback (DFB) laser diode. A 1-dB power penalty in the wavelength-locked FP-LD-based PON system was observed when the crosstalk-to-signal ratio was ~-9 dB. We also compared the measured power penalties with the calculated power penalties. It has been shown that the in-band crosstalk-induced power penalty in wavelength-locked FP-LD-based WDM PON could be estimated properly by taking into account both effects of power addition and signal-crosstalk beat noise.

Incoherent in-band crosstalk induced power penalty in amplified WDM networks: a comparative study using Gaussian and chi-squared probability density functions

An analytical expression for power penalty (PP) from the formula for bit-error rate is derived by employing saddle point approximation in the chi-squared probability distribution function. This theory differs from the earlier ones in that it takes into account the dominant signal-amplified spontaneous emission beat noise applicable for optically pre-amplified systems. Imperfect extinction ratio accounting for higher power penalties is also considered in the analysis. Results have been compared with the available experimental results for a low number of crosstalk sources and an excellent agreement has been found. Although Gaussian probability density function (pdf) is simple to analyse, PP calculated from this pdf for a low number of crosstalk sources fails to match with the experimental results.

Multi-degree ROADM based on wavelength selective switches: Architectures and scalability

Multi-degree implementations of the reconfigurable optical add–drop multiplexer (ROADM) are analyzed in terms of in-band crosstalk performance with an emphasis on the reduction in the number of wavelength selective switches used in the module. It is shown experimentally that an 8-degree ROADM using the wavelength selective switches for channel adding and optical splitters for channel dropping performs closely to one that uses switches for both operations. A new modular architecture based on the wavelength selective switches is proposed for implementations applicable to large degree nodes. Performance analysis of this architecture indicates node scalability to 256 degree with crosstalk-induced power penalties below 0.5 dB.

Analysis of the effect of thermal chirp on interferometric homodyne and heterodyne crosstalk in optical communication systems employing directly Modulated DFB lasers

A closed-form expression is derived for the probability density function (pdf) of the beat noise created by homodyne and heterodyne interferometric crosstalk in optical communication systems employing directly modulated distributed feedback lasers at bit rates between 155 Mb/s and 2.5 Gb/s. Thermal chirp is shown to be the predominant chirp mechanism affecting homodyne-crosstalk-induced penalty at bit rates up to 2.5 Gb/s. Theoretical calculations of the crosstalk-induced power penalty are verified experimentally.

Optical crosstalk in fiber-radio WDM networks

This paper investigates the impact of optical crosstalk in fiber-radio systems incorporating wavelength division multiplexing, (WDM). We present a simple analytical model that allows the impact of optical crosstalk in such networks to be assessed and validate the results via experiment for both in-band and out-of-band optical crosstalk. We show that crosstalk-induced power penalties in fiber-radio WDM networks are reduced compared to baseband modulation for the case of in-band crosstalk. In addition, in contrast to baseband modulated optical links, the crosstalk channel in fiber-radio systems can be filtered electrically if the crosstalk signal carries a different wireless frequency. However, a power penalty is still observed for the case of in-band crosstalk, even for perfect electrical filtering of the crosstalk channel.

A comparison of the homodyne crosstalk characteristics of optical add-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.

Low-crosstalk and compact optical add-drop multiplexer using a multiport circulator and fiber Bragg gratings

Three kinds of low-crosstalk and compact optical add-drop multiplexers (OADMs) based on a multiport optical circulator (MOC) with fiber Bragg gratings are proposed and demonstrated. For the MOC-based structure, there is a significant intraband crosstalk reduction of about 37 and 16 dB on the dropped and added channels, respectively, for the best proposed MOC-based structure as compared with the conventional structure. Bit-error rate performance and both intraband and interband crosstalk-induced power penalties of these MOC-based OADMs are examined in a 10 Gb/s system demonstration. Such a low-crosstalk, miniaturized, and compact OADM may find important applications in dense wavelength-division multiplexing networks.

Experimental verification of a new model describing the influence of incomplete signal extinction ratio on the sensitivity degradation due to multiple interferometric crosstalk

Larger optical penalties than predicted by a Gaussian crosstalk model are found both in our experiments and in the literature when investigating signals including multiple interferometric crosstalk contributions. We attribute this to an imperfect signal extinction ratio. In this letter, simple analytical relations for crosstalk induced power penalties are derived taking the signal extinction ratio into account and excellent agreement with 10-Gb/s experiments is obtained. Both theory and experiments show the importance of the signal extinction ratio in connection with interferometric crosstalk.

Analysis of interferometric crosstalk in optical switch blocks using moment generating functions

A detailed statistical model describing the detection process in case of interferometric crosstalk is derived. The complexity of the model is independent of the number of crosstalk channels, opposite to the complexity of a modified Gaussian approximation that increases with the number of crosstalk channels. Using our new model, we analyze crosstalk induced power penalty in optical switch blocks as a function of switch size, crosstalk levels and receiver decision thresholds.