Differential-phase-shift keying (DPSK) has recently been used to reach record distances in long-haul lightwave communication systems. This paper will review theoretical, as well as implementation, aspects of DPSK, and discuss experimental results.

Optical phase-shift-keyed transmission

Demand for network bandwidth is growing at unprecedented rates, placing growing demands on switching and transmission technologies. Wavelength division multiplexing will soon make it possible to combine hundreds of gigabit channels on a single fiber. This paper presents an architecture for Burst Switching Systems designed to switch data among WDM links, treating each link as a shared resource rather than just a collection of independent channels. The proposed network architecture separates burst level data and control, allowing major simplifications in the data path in order to facilitate all-optical implementations. To handle short data bursts efficiently, the burst level control mechanisms in burst switching systems must keep track of future resource availability when assigning arriving data bursts to channels or storage locations. The resulting Lookahead Resource Management problems raise new issues and require the invention of completely new types of high speed control mechanisms. This paper introduces these problems and describes approaches to burst level resource management that attempt to strike an appropriate balance between high speed operation and efficiency of resource usage.

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Terabit burst switching

Fiber-optic communication systems form the high-capacity transport infrastructure that enables global broadband data services and advanced Internet applications. The desire for higher per-fiber transport capacities and, at the same time, the drive for lower costs per end-to-end transmitted information bit has led to optically routed networks with high spectral efficiencies. Among other enabling technologies, advanced optical modulation formats have become key to the design of modern wavelength division multiplexed (WDM) fiber systems. In this paper, we review optical modulation formats in the broader context of optically routed WDM networks. We discuss the generation and detection of multigigabit/s intensity- and phase-modulated formats, and highlight their resilience to key impairments found in optical networking, such as optical amplifier noise, multipath interference, chromatic dispersion, polarization-mode dispersion, WDM crosstalk, concatenated optical filtering, and fiber nonlinearity

Advanced Optical Modulation Formats

We address the issue of how to provide basic quality of service (QoS) in optical burst-switched WDM networks with limited fiber delay lines (FDLs). Unlike existing buffer-based QoS schemes, the novel offset-time-based QoS scheme we study in this paper does not mandate any buffer for traffic isolation, but nevertheless can take advantage of FDLs to improve the QoS. This makes the proposed QoS scheme suitable for the next generation optical Internet. The offset times required for class isolation when making wavelength and FDL reservations are quantified, and the upper and lower bounds on the burst loss probability are analyzed. Simulations are also conducted to evaluate the QoS performance in terms of burst loss probability and queuing delay. We show that with limited FDLs, the offset-time-based QoS scheme can be very efficient in supporting basic QoS.

QoS performance of optical burst switching in IP-over-WDM networks

We investigated through simulations the performance of Nyquist-WDM Terabit superchannels implemented using polarization-multiplexed phase shift-keying based on 2 (PM-BPSK) and 4 (PM-QPSK) signal points or polarization-multiplexed quadrature amplitude modulation based on 8 (PM-8QAM) and 16 (PM-16QAM) signal points. Terabit superchannels are obtained through the aggregation of multiple subcarriers using the Nyquist-WDM technique, based on a tight spectral shaping of each subcarrier which allows very narrow spacing. We first studied the optimum transmitter/receiver filtering in a back-to-back configuration. Then we investigated the maximum reach for different spectral efficiencies, after nonlinear propagation over uncompensated links with lumped amplification. Performance for systems based on both standard single-mode fiber (SSMF) and large effective area non-zero dispersion-shifted fiber (NZDSF) has been analyzed. Assuming SSMF with 25-dB span loss, we found that PM-BPSK can reach 6480 km at a net capacity of 4 Tb/s across the C band. Conversely, PM-16QAM can deliver 27 Tb/s, but over 270 km only. Note that a lower span length, the use of Raman amplification and/or pure silica-core fibers (PSCFs) can significantly increase the maximum reach, but without changing the hierarchy among the performance of modulation formats. We also show that the maximum reachable distance is approximately 2/3 of the one achievable in linear propagation at the optimum launch power, regardless of the modulation format, spacing and fiber type. As additional results, we also verified that the optimum launch power per subcarrier linearly depends on the span loss, varies with the fiber type, but it is independent of the modulation format, and that the relationship between the maximum reachable distance and the span loss is almost linear.

On the Performance of Nyquist-WDM Terabit Superchannels Based on PM-BPSK, PM-QPSK, PM-8QAM or PM-16QAM Subcarriers

A 1-Tb/s aggregate capacity (50 channels each at 20 Gb/s) was transmitted through 55 km of nonzero-dispersion fiber. Fifty channels were generated by polarization multiplexing 25 wavelengths.

1-Tb/s transmission experiment

One Tb/s aggregate capacity (50 channels each at 20 Gb/s) was transmitted through 55 km of non-zero-dispersion fiber. The fifty channels were generated by polarization multiplexing 25 wavelengths.

One Terabit/s Transmission Experiment

We demonstrate that channel spacing can be reduced to values smaller than the Nyquist channel spacing over transoceanic distance. Modulation memory induced by constrained transmitter bandwidth together with multisymbol detection can reduce intersymbol interference for systems with sub-Nyquist channel spacing. We transmit 198 × 100 G bandwidth constrained polarization-division-multiplexed return-to-zero quaternary phase shift keying channels with 400% spectral efficiency over 6860 km using 52 km spans of 150 μ m2 fiber and simple single-stage erbium-doped fiber amplifiers without any Raman amplification. We also show that 100 G coherent nonlinear performance scales differently with distance on uncompensated dispersion maps compared with direct detection transmission.

20 Tbit/s Transmission Over 6860 km With Sub-Nyquist Channel Spacing

We transmitted 96×100G pre-filtered PDM-RZ-QPSK channels with 300% spectral efficiency over 10,608km using 52km spans of 150μm2 fiber and simple single-stage EDFAs. We also achieved 400% spectral efficiency over 4,368km using similar techniques.

A.J. Lucero

Papers

1-Tb/s transmission experiment

One Terabit/s Transmission Experiment

20 Tbit/s Transmission Over 6860 km With Sub-Nyquist Channel Spacing

Transmission of 96×100G pre-filtered PDM-RZ-QPSK channels with 300% spectral efficiency over 10,608km and 400% spectral efficiency over 4,368km

Fast Power Transients in Optically Amplified Multiwavelength Optical Networks