Raman amplifiers are being deployed in almost every new long-haul and ultralong-haul fiber-optic transmission systems, making them one of the first widely commercialized nonlinear optical devices in telecommunications. This paper reviews some of the technical reasons behind the wide-spread acceptance of Raman technology. Distributed Raman amplifiers improve the noise figure and reduce the nonlinear penalty of fiber systems, allowing for longer amplifier spans, higher bit rates, closer channel spacing, and operation near the zero-dispersion wavelength. Lumped or discrete Raman amplifiers are primarily used to increase the capacity of fiber-optic networks, opening up new wavelength windows for wavelength-division multiplexing such as the 1300 nm, 1400 nm, or short-wavelength S-band. As an example, using a cascade of S-band lumped amplifiers, a 20-channel, OC-192 system is shown that propagates over 867 km of standard, single-mode fiber. Raman amplifiers provide a simple single platform for long-haul and ultralong-haul amplifier needs and, therefore, should see a wide range of deployment in the next few years.

Raman amplifiers for telecommunications

Progress in optical networking has stimulated interest in optical performance monitoring (OPM), particularly regarding signal quality measures such as optical signal-to-noise ratio (SNR), Q-factor, and dispersion. These advanced monitoring methods have the potential to extend fault management and quality-of-service (QoS) monitoring into the optical domain. This paper reviews OPM applications and techniques, while examining the role of OPM as an enabling technology for advances in high-speed and optically switched networks.

Optical performance monitoring

We report a procedure to prepare highly monodisperse copper telluride nanocubes, nanoplates, and nanorods. The procedure is based on the reaction of a copper salt with trioctylphosphine telluride in the presence of lithium bis(trimethylsilyl)amide and oleylamine. CuTe nanocrystals display a strong near-infrared optical absorption associated with localized surface plasmon resonances. We exploit this plasmon resonance for the design of surface-enhanced Raman scattering sensors for unconventional optical probes. Furthermore, we also report here our preliminary analysis of the use of CuTe nanocrystals as cytotoxic and photothermal agents.

CuTe Nanocrystals: Shape and Size Control, Plasmonic Properties, and Use as SERS Probes and Photothermal Agents

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

We investigate new connection-provisioning algorithms to efficiently provide signal-quality-guaranteed connections in an optical wavelength-division-multiplexing (WDM) mesh network operating with high-speed wavelength channels. In an optical network, a connection is set up to carry a data signal via an all-optical channel (lightpath) from its source to destination node. The optical signal transmitted along the lightpath may need to travel through a number of crossconnect switches (OXCs), optical amplifiers, and fiber segments. While the signal propagates toward its destination, the optical components would continuously degrade the signal quality by inducing impairments. When the signal degradation is so severe that the received bit-error rate (BER) becomes unacceptably high, the lightpath would not be able to provide good service quality to a connection request. Such a lightpath, which has poor signal quality due to transmission impairments in the physical layer, should not be used for connection provisioning in the network layer. With increasing channel bit rate to 10 Gb/s or higher, fiber linear and nonlinear impairments become prominent factors, which affect the signal quality. Thus, new techniques in both physical layer and network layer are necessary for mitigating impairments to accommodate high-speed traffic. Therefore, to ensure service quality of high-speed connections, we develop intelligent impairment-aware routing and wavelength assignment (RWA) algorithms, which automatically consider the effects of high-speed transmission impairment when setting up a lightpath. The main contribution of our paper is that we investigate a novel hierarchical RWA model for high-speed connection provisioning where the optical signal-to-noise ratio (OSNR) and polarization mode dispersion (PMD) effect are estimated in the physical layer, and regarded as metrics for lightpath computation in the network layer. The performance of the proposed connection-provisioning strategies is demonstrated to be promising through illustrative numerical examples.

Connection provisioning with transmission impairment consideration in optical WDM networks with high-speed channels

We report wavelength-division-multiplexed (WDM) systems experiments with polarization multiplexing at 40 Gb/s per channel and 0.8 bit/s/Hz spectral efficiency designed to explore the worst case impairments caused by first-order polarization-mode dispersion (PMD) in the fiber. Both polarization-division multiplexing using two polarizations at each wavelength and polarization interleaving where adjacent channels have orthogonal polarizations were examined. Measured system penalties show that polarization multiplexing increases the PMD sensitivity by approximately a factor of five due to coherent crosstalk.

Observation of PMD-induced coherent crosstalk in polarization-multiplexed transmission

The Muller matrix method enables the low-noise, high-resolution measurement of second-order polarization-mode-dispersion (PMD) depolarization and related parameters required for systems modeling. We report experimental observations of second-order PMD statistical dependencies and scaling for fibers with mean differential group delay (DGD) ranging from 1.3 to 17.0 ps. Measurements of 10 different fibers confirm that the depolarization scales with the mean DGD while the polarization-dependent chromatic dispersion scales with the square of the mean DGD.

Measurement of depolarization and scaling associated with second-order polarization mode dispersion in optical fibers

We demonstrate a new technique for monitoring channel wavelength and power in a wavelength-division-multiplexed lightwave system. This technique uses a dispersive delay line to map wavelength into time and data correlation to determine the time shift of the channels. The technique was experimentally demonstrated for three 2.5-Gb/s channels spaced by 100 GHz using a set of concatenated fiber Bragg gratings as the dispersive delay.

Optical monitoring using data correlation for WDM systems

Cross-phase modulation impairment resonances exist in wavelength-division-multiplexed lightwave systems when channels are spaced such that there occurs an integral number of bit walkthroughs per amplifier span. We have observed these resonances in transmission through 624-km standard single-mode fiber systems having amplifier span lengths of 40, 80, or 120 km. Q measurements of a 2.5-Gb/s five-channel system over 8/spl times/80 km with +7 dBm/channel predict on-resonance bit-error rates (BER's) that are several orders of magnitude higher than off-resonance BER's.

Resonances in cross-phase modulation impairment in wavelength-division-multiplexed lightwave transmission

Random birefringence causing transmission impairments in optical fibers is characterized by first- and second-order polarisation mode dispersion (PMD) vectors. These are measured by time domain or frequency domain techniques. Results showing low-noise high-resolution PMD data are described.

L.E. Nelson

Papers

Observation of PMD-induced coherent crosstalk in polarization-multiplexed transmission

Measurement of depolarization and scaling associated with second-order polarization mode dispersion in optical fibers

Optical monitoring using data correlation for WDM systems

Resonances in cross-phase modulation impairment in wavelength-division-multiplexed lightwave transmission

PMD characterization techniques