The synthesis of arbitrarily shaped femtosecond pulses by spectral filtering in a temporally nondispersive grating apparatus is demonstrated. Spectral filtering is accomplished by utilizing spatially patterned masks to modify the amplitude and the phase of the optical frequency components that are spatially dispersed within the apparatus.We are able to pattern spectra over a large dynamic range (approaching 104) and with unprecedented resolution.We illustrate the power of this technique by synthesizing a number of femtosecond waveforms, including femtosecond tone bursts with terahertz repetition rates, picosecond square pulses with 100-fsec rise times, and highly complex pseudonoise bursts produced by spectral phase encoding.

In a wavelength-routed optical network, a transmitted signal remains in the optical domain over the entire route (lightpath) assigned to it between its source and destination nodes. The optical signal may have to traverse a number of crossconnect switches (XCS's), fiber segments, and optical amplifiers, e.g., erbium-doped fiber amplifiers (EDFA's). Thus, while propagating through the network, the signal may degrade in quality as it encounters crosstalk at the XCS's and also picks up amplified spontaneous emission (ASE) noise at the EDFA's. Since these impairments continue to degrade the signal quality as it progresses toward its destination, the received bit error rate (BER) at the destination node might become unacceptably high. Previous work on the lightpath routing and wavelength assignment (RWA) problem assumed an ideal physical layer and ignored these transmission impairments. The main contribution of our work is to incorporate the role of the physical layer in setting up lightpaths by employing appropriate models of multiwavelength optical devices (XCS's and EDFA's) such that the BER of a candidate lightpath can be computed, in advance, to determine if this lightpath should be used for the call. Features from existing RWA algorithms are integrated with our on-line BER calculation mechanism. Our simulation studies indicate that employing BER-based call-admission algorithms has a significant impact on the performance of realistic networks.

This article reviews arguments that, in the process of action formation and ascription, the relative status of the participants with respect to a projected action can adjust or trump the action stance conveyed by the linguistic form of the utterance. In general, congruency between status and stance is preferred, and linguistic form is a fairly reliable guide to action ascription. However incongruities between stance and status result in action ascriptions that are at variance with the action stance that is otherwise conveyed in the turn. This argument is presented, first, in relation to epistemic status and stance where the process is argued to be both fundamental and universal across all declarative and interrogative utterances. Some consequences of this way of viewing action are discussed. The argument is then briefly extended to deontics and benefactives.

We present evidence of soliton propagation by 185-fsec dark pulses at a wavelength of 0.62 /zm in a 1.4-m length of single-mode optical fiber. Our experiments utilize specially shaped, antisymmetric input pulses, which closely correspond to the form of the fundamental dark soliton. At appropriate power levels the dark pulses propagate without broadening. Our measurements are in quantitative agreement with numerical solutions to the nonlinear Schrodinger equation and constitute the first clear observation of the fundamental dark soliton in optical fibers. PACS numbers:42.50.Qq, 42.65.Re, 42.81.Dp Although soliton phenomena arise in many distinct areas of physics, the single-mode optical fiber has been found an especially convenient medium for their study. Hasegawa and Tappert proposed in 1973 that the nonlinear refractive index in glass optical fibers could be utilized to compensate for group velocity dispersion (GVD), resulting in optical solitons which could propagate without distortion. 1 Since then, soliton propagation of bright optical pulses has been verified in a number of elegant experiments performed in the negative GVD region of the spectrum (X>\.3jjmin standard singlemode fibers) 2 ; most recently, transmission of 55-psec optical pulses through 4000 km of fiber was achieved, by use of a combination of nonlinear soliton propagation to avoid pulse spreading and Raman amplification to overcome losses. 3 For positive dispersion (A,<1.3//m), bright pulses cannot propagate as solitons, and the interaction of the nonlinear index with GVD leads to spectral and temporal broadening of the propagating pulses. These effects form the basis for the fiber-and-grating pulse compressor, 4 ' 5 which was utilized to produce the shortest optical pulses (6 fsec) ever reported. 6 For both signs of GVD, the experimental results are in quantitative agreement with the predictions of the nonlinear Schrodinger equation (NLSE).Although bright solitons are allowed only for negative dispersion, the NLSE admits other soliton solutions for positive GVD. 1,7 These solutions are "dark-pulse solitons," consisting of a rapid dip in the intensity of a broad pulse or a cw background. The fundamental dark soliton, for which we report here the first experimental observation, is predicted to be an antisymmetric function of time, with an abrupt K phase shift and zero intensity at its center. Other dark solitons with a reduced contrast and a lesser, more gradual phase modulation also exist. Throughout the text we will use the terms "black" and "gray" soliton, respectively, to refer to the fundamental and to the lower-contrast dark-soliton solutions.As a result of difficulty in generating the required input dark pulses, previous experimental evidence for dark-soliton propagation in fibers is limited. Krokel et al* reported the evolution of an even-symmetry, 300fsec dark pulse into a complementary pair of lowcontrast dark pulses, which they interpreted as gray solitons. Emplit et al. 9 performed experiments utilizing odd-symmetry dark pulses -...

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