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 demonstrate two techniques to reduce the effects of fiber chromatic dispersion in fiber-wireless systems incorporating external modulators. We theoretically and experimentally show that the achievable link distance can be increased by varying the chirp parameter of the modulator to give large negative chirp using a dual-electrode Mach-Zehnder modulator (MZM) biased at quadrature. In addition, we show that dispersion can be almost totally overcome by implementing a simple method using the dual-electrode MZM to generate an optical carrier with single sideband (SSB) modulation. We demonstrate the transmission of a 51.8-Mb/s pseudorandom bit sequence (PRBS) at 12 GHz over 80 km of standard single-mode fiber using the SSB generator and measure a bit-error-rate (BER) power penalty due to fiber dispersion of less than 0.5 dB for a BER equal to 10/sup -9/.

Overcoming chromatic-dispersion effects in fiber-wireless systems incorporating external modulators

Hybrid fiber-wireless networks incorporating WDM technology for fixed wireless access operating in the sub-millimeter-wave and millimeter-wave (mm-wave) frequency regions are being actively pursued to provide untethered connectivity for ultrahigh bandwidth communications. The architecture of such radio networks requires a large number of antenna base-stations with high throughput to be deployed to maximize the geographical coverage with the main switching and routing functionalities located in a centralized location. The transportation of mm-wave wireless signals within the hybrid network is subject to several impairments including low opto-electronic conversion efficiency, fiber chromatic dispersion and also degradation due to nonlinearities along the link. One of the major technical challenges in implementing such networks lies in the mitigation of these various optical impairments that the wireless signals experience within the hybrid network. In this paper, we present an overview of different techniques to optically transport mm-wave wireless signals and to overcome impairments associated with the transport of the wireless signals. We also review the different designs of subsystems for integrating fiber-wireless technology onto existing optical infrastructure.

Fiber-Wireless Networks and Subsystem Technologies

Advanced optical modulation formats have become a key ingredient to the design of modern wavelength-division-multiplexed (WDM) optically routed networks. In this paper, we review the generation and detection of multigigabit/second intensity- and phase-modulated formats and highlight their resilience to key impairments found in optical networking, such as optical amplifier noise, chromatic dispersion, polarization-mode dispersion, WDM crosstalk, concatenated optical filtering, and fiber nonlinearity

Advanced Modulation Formats for High-Capacity Optical Transport Networks

Publisher Summary Polarization mode dispersion (PMD) is a linear effect that can be compensated in principle. In an ideal circularly symmetric fiber, the two orthogonally polarized modes have the same group delay. However, in reality, fibers exhibit a certain amount of birefringence because of imperfections in the manufacturing process or mechanical stress on the fiber after manufacture. It is noted that fluctuations in the polarization mode and fiber birefringence produced by the environment lead to dispersion that varies statistically with time and frequency. PMD causes different delays for different polarizations and when the difference in the delays approaches a significant fraction of the bit period, it leads to pulse distortion and system penalties. Environmental changes— including temperature and stress—cause the fiber PMD to vary stochastically in time. PMD, illustrating the basic concepts, the measurement techniques, the PMD measurement, the PMD statistics for first- and higher orders, the PMD simulation and emulation, the system impairments, and the mitigation methods has been summarized in the chapter. Both the optical and the electrical PMD compensations are considered.

Polarization-Mode Dispersion

The authors propose a modulation format in which the phase of the signal pulses alternates, in order to reduce the intrachannel four-wave mixing. They demonstrate numerically that the performance of a 40-Gb/s transmission link can be substantially improved.

Reduction of intrachannel four-wave mixing using the alternate-phase RZ modulation format

The authors have analyzed and measured the dispersion effects of residual chirp arising from asymmetry in field overlap in Mach-Zehnder structures. A clear influence of chirp was seen at 7 Gb/s and a propagation length of 75-km nondispersion shifted fiber at lambda =1.55 mu m. It is shown that a MZ-modulator with field overlap in one arm only and operated in the blue-shift mode is the best choice for single coplanar stripline (CPS) electrode MZ-modulators in this case. Calculations have shown good agreement with measured results. This indicates that a simple model for fiber mode propagation have shown good agreement with measured results. This can be used together with a FFT-algorithm for calculating the effect of residual chirp from external modulators in multigigabit dispersive fiber-optic systems. >

Residual chirp in integrated-optic modulators

A novel linearization concept to reduce the distortion products from integrated-optic modulators is proposed and analyzed. The proposed linearization concept could be used on any type of modulator with an odd transfer function. A dual parallel Mach-Zehnder modulator was linearized, and the results are compared to those obtained using a conventional linearization concept. The proposed concept gives a 40% higher modulation index compared to a concept based on reduction of the cubic nonlinearity of the modulator. >

A linearization concept for integrated-optic modulators

Field trial over 820 km installed SSMF and its potential Terabit/s superchannel application with up to 57.5-Gbaud DP-QPSK transmission

The proposed modulation scheme enables us to combine the advantages of the duobinary modulation, i.e., reduced transmission bandwidth compared to traditional intensity modulation, together with the advantages of prechirped modulation. Below, a theoretical investigation is presented where four different methods of modulating a double electrode Mach-Zehnder modulator is compared. It will be shown that the proposed prechirped duobinary modulation scheme will have the best performance in terms of power penalty.

Anders Djupsjöbacka

Papers

Reduction of intrachannel four-wave mixing using the alternate-phase RZ modulation format

Residual chirp in integrated-optic modulators

A linearization concept for integrated-optic modulators

Field trial over 820 km installed SSMF and its potential Terabit/s superchannel application with up to 57.5-Gbaud DP-QPSK transmission

Prechirped duobinary modulation