P.J. Legg

Solution paths to limit interferometric noise induced performance degradation in ASK/direct detection lightwave networks

Interferometric noise, arising on the optical interference of the desired information signal and parasitic crosstalk waveforms at the photodetector, afflicts practically all lightwave communication networks, inducing unacceptable power penalties and bit error rate floors. In this paper, the induced performance degradation is quantified, both experimentally and analytically, and solution paths are identified. It is concluded that the total crosstalk level of noise generating parasitics in a generalized optical network must be held below -25 dB for a penalty of less than 1 dB-a further 2 to 4 dB may lead to network failure; otherwise, means of suppressing the noise by RF rejection at the receiver must be invoked. A number of approaches to achieving a reduction in the level of interferometric noise are presented and contrasted.

On the various time constants of wavelength changes of a DFB laser under direct modulation

The temporal behavior of the optical frequency emitted by several DFB lasers under direct square wave modulation was measured using an all-fiber implementation of a Mach-Zender interferometer with an imbalance of 30 ps. The impulse response of the optical frequency to injection current modulation was found to contain a time constant as short as 10-20 ns, together with a few longer ones. The existence of such a short time constant is consistent with a thermal analysis of a laser structure with finite thermal impedance of the active region and should be taken into consideration in various wide bandwidth applications of direct modulated semiconductor lasers.

2/spl times/2 buffered switch fabrics for traffic routing, merging, and shaping in photonic cell networks

An approach to optical packet switching is discussed, which uses small, simplified optical elements for traffic routing, merging, and shaping. The elements are constructed from 2/spl times/2 switches and optical delay lines, and may be implemented in a variety of technologies. They are designed for use with deflection routing, and even when using only six switches in a module, a deflection probability of 2.8/spl times/10/sup -7/ is possible with a load of 0.8. The modules may also be used as 2/spl times/1 mergers where a deflection probability of 10/sup -12/ is possible with six switches and a total load of 0.8. The BER performance of the modules is simulated with respect to crosstalk, with even relatively poor switch devices of -18.5 dB isolation yielding a power penalty of less than 1 dB. A networking strategy radically different from todays is discussed, driven by the need to reduce hardware, software and operating costs.

Inter-channel crosstalk phenomena in optical time division multiplexed switching networks

The nature of inter-channel crosstalk arising in OTDM switching networks formed from imperfectly isolated 2/spl times/2 crosspoints and optical delay lines is investigated and a novel classification is identified. It is shown that the mixing of crosstalk and signal waveforms which are either, a) mutually coherent or, b) incoherent and (optical) frequency matched to within the receiver bandwidth, may result in intensity noise and far greater performance degradation than for c) incoherent signals whose optical beat frequency exceeds the receiver bandwidth. Initial experimental studies indicate that crosspoint isolation <-15 dB is required if undilated networks containing more than four crosspoints are to be realised.<<ETX>>

Interferometric noise reduction through intrabit frequency evolution of directly modulated DFB lasers

A new technique is demonstrated that improves the performance of optical networks subject to interferometric noise. Every transmitter comprises a selected DFB laser that is NRZ ASK directly modulated with a large modulation depth. Under modulation, the center frequency of the DFB laser is found to vary over the duration of each bit, falling during mark bits and rising during space bits with an exponential-like dependency of characteristic time constant /spl sim/20 ns. The optical frequency at the midpoint of each bit interval is not constant but depends upon the sequence of the preceding bits. Therefore, on the interference of a delayed-replica parasitic crosstalk waveform the interferometric noise generated on detection may be reduced by RF filtering according to the difference in center frequency of the interfering bits. This noise suppression which requires no additional hardware has been successfully modeled and experiment demonstrates little performance gain for a single interferer, as predicted. However, in the presence of multiple interferers significant improvement is predicted and observed at sub-Gb/s rates.

Sequence dependence of phase-induced intensity noise in optical networks that employ direct modulation.

Phase-induced intensity noise in optical networks that employ directly modulated laser sources is observed to be bit-sequence dependent. This dependence is explained by optical frequency variations that are due to the heating history of the laser chip and is accurately modeled. This effect may permit suppression of phase-induced intensity noise in many types of fiber system with multipaths.

Interferometric noise reduction in crosstalk corrupted optical WDM and TDM switching fabrics

Interferometric noise due to crosspoint crosstalk in optical WDM and TDM switching fabrics is substantially reduced by manipulation in the time and/or wavelength domains. Furthermore, providing sufficient discrete wavelengths are available, or sufficient time compression can be realized, almost all noise may be eliminated at the expense of additional equipment. Practical realizations of the proposed scheme are discussed.<<ETX>>

Modeling the performance of optical time division multiplexed (TDM) switching structures in the presence of interferometric noise due to interchannel crosstalk

Crosstalk induced interferometric noise has been found to constrain the performance of optical switching networks. A computer design and simulation package has been used to analyze the effect of this noise on optical TDM crossconnects. This software tool is able to calculate all possible routes that a signal may traverse within a crossconnect for a given input/output assignment, and also incorporates a theoretical analysis of interferometric noise. It has been found that for a crossconnect capable of switching 512 independent channels, the constituent 2 x 2 crosspoints must possess isolations better than -20 dB, and the switch architecture must be dilated.

Interferometric noise analysis in coherence-multiplexed optical fiber communication systems for local area networks

Coherence multiplexing (CM) exploits the coherence properties of optical sources so that many users may be multiplexed onto a single physical channel giving a secure, asynchronous mode of transmission, free of central network control. In this paper, for the first time, the CM system performance limitations owing to the impact of interference from unwanted users is presented and the suitability of LED, single mode Fabry-Perot laser and DFB sources are considered. It is concluded that the single mode Fabry-Perot is preferred, offering readily achievable interferometer length tolerances and near complete RF rejection of interferometric noise at the receiver. However, interferometric noise still limits the network capacity: a linewidth of lnm permits 17 users to be simultaneously offered a single video channel with a BER<10-9.

Interferometer noise analysis in coherent optical CDM systems for LAN applications

A novel optical technique, combining the coherence property of optical sources and the mathematical properties of digital code families is demonstrated. This technique not only exploits the large bandwidth available in single mode optical fibers but also provides a secure, asynchronous communication system.