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We characterize the nonlinear propagation of picosecond pulses in chalcogenide [As.sub.2][S.sub.3] single-mode fiber using a pump-probe technique. The cross-phase modulation (XPM)-induced sideband broadening and stimulated Raman scattering (SRS)-induced sideband amplification are measured in order to map out the Raman gain spectrum of this glass across the C-band. We extract the Raman response function from the Raman gain spectrum and determine the power and polarization dependence of the SRS. In contrast to previous work using [As.sub.2][Se.sub.3] fiber, we find that the [As.sub.2][S.sub.3] fiber does not suffer from large two-photon absorption (TPA) in the wavelength range of the telecommunications band. We achieved a 20 dB peak Raman gain at a Stokes shift of 350 [cm.sup.-1] in a 205 mm length of [As.sub.2][S.sub.3] single-mode fiber. The Raman gain coefficient is estimated to be 4.3 x [10.sup.-12] m/W and the threshold pump peak power is estimated to be 16.2 W for the 205 mm [As.sub.2][S.sub.3] fiber. We also demonstrate that we can infer the dispersion of the [As.sub.2][S.sub.3] fiber and justify the Raman response function by comparing simulation and experimental results. [c] 2009 Optical Society of America OCIS codes: 060.0060, 060.4370, 190.0190, 190.4370.

We present a technique for measuring the modal filtering ability of single mode fibers. The ideal modal filter rejects all input field components that have no overlap with the fundamental mode of the filter and does not attenuate the fundamental mode. We define the quality of a nonideal modal filter [Q.sub.f] as the ratio of transmittance for the fundamental mode to the transmittance for an input field that has no overlap with the fundamental mode. We demonstrate the technique on a 20 cm long mid-infrared fiber that was produced by the U.S. Naval Research Laboratory. The filter quality [Q.sub.f] for this fiber at 10.5 gm wavelength is 1000 [+ or -] 300. The absorption and scattering losses in the fundamental mode are approximately 8 dB/m. The total transmittance for the fundamental mode, including Fresnel reflections, is 0.428 [+ or -] 0.002. The application of interest is the search for extrasolar Earthlike planets using nulling interferometry. It requires high rejection ratios to suppress the light of a bright star, so that the faint planet becomes visible. The use of modal filters increases the rejection ratio (or, equivalently, relaxes requirements on the wavefront quality) by reducing the sensitivity to small wavefront errors. We show theoretically that, exclusive of coupling losses, the use of a modal filter leads to the improvement of the rejection ratio in a two-beam interferometer by a factor of [Q.sub.f]. OCIS codes: 060.2390, 060.2430, 060.2270.

The mid-wave IR and long-wave IR laser potential of rare-earth ions in chalcogenide glass fiber is examined.

We have used telluride glass fibers fabricated in house to measure the evanescent-absorption spectra of water, methanol, ethanol, isopropanol, acetone, ethanoic acid, hexane, and chloroform. Furthermore, detection limits of less than 2 vol. % solute were obtained for mixtures of water and methanol, ethanol, isopropanol, acetone, and ethanoic acid. Techniques to reduce the detection limits are discussed. Key words: Chalcogenide glass fiber, remote IR spectroscopy, evanescent absorption, fiber-optic sensors.

Sources and origins of scattering centers in As2S3 glasses and optical fibers are analyzed using scattering theory. Particle size, relative refractive index and the wavelength of light influence the scattering loss in the optical fibers. The strongest scattering is due to bubble because of its high refractive index and As2S3 crystal are the weakest scatterers. The scattering capacity is analyzed for various impurities.