Generation of enhanced coherent anti-Stokes Raman spectroscopy signals in liquid-filled waveguides

Optical limiting, pulse reshaping, and stabilization with a nonlinear absorptive fiber system

Optical limiting, pulse reshaping, and stabilization effects have been demonstrated based on a two-photon absorption mechanism with a dye-solution-filled hollow fiber system. The nonlinear absorptive medium is the solution of a new dye, trans-4-[p-(N-hydroxyethyl-N-methylamino)styryl]-N-methylpyridinium iodide (ASPI) in dimethyl sulfoxide, with which we filled a 20-cm-long quartz hollow fiber of 100-mum internal diameter. The input optical signal was a laser pulse train that contained ~30 pulses of 130-ps pulse width. When the input peak intensity reached 400-1000-MW/cm(2) levels, obvious optical limiting could be observed and the envelope of the transmitted pulse train became flatter and broader. By using another new dye solution, 4-[N-(2-hydroxyethyl)-N-(methyl)amino phenyl]-4?-(6-hydroxyhexyl sulfonyl)-stilbene (APSS) in benzyl alcohol, which interacted with a series of ~800-nm laser pulses of ~8-ns pulse width, we obtained a much higher nonlinear absorption coefficient and a superior optical peak-power stabilization effect.

Two-photon resonance-enhanced refractive-index change and self-focusing in a dye-solution-filled hollow fiber system

The general behavior of two-photon absorption-enhanced refractive-index change in a third-order nonlinear optical medium is briefly described. The nonlinear medium was the solution of a new dye, trans-4-[p-(N-hydroxyethyl-N-methylamino)styryl]-N-methyl pyridinium iodide (ASPI) in dimethyl sulfoxide as the solvent, that was poured into a 20-cm-long quartz hollow fiber of 100-mum internal diameter. This dye solution has a strong two-photon absorption and subsequent upconversion fluorescence emission when excited with 1064-nm laser radiation. When the input peak intensity reached 500-1500-MW/cm(2) levels, obvious changes in beam profiles of the output IR laser beam were observed due to a self-focusing or self-trapping process occurring inside the fiber system. As a result of this process, highly directional frequency-upconverted superradiant lasing output was obtained with a beam size ~5 times smaller than that of a linearly transmitted He-Ne probe laser beam. The demonstrated mechanism can be useful for fiber laser-amplifier and fiber-integrated optics devices.

Broadband, multiwavelength stimulated-emission source based on stimulated Kerr and Raman scattering in a liquid-core fiber system

A novel broadband and multiwavelength stimulated-emission source that consists of a Kerr liquid-filled hollow-fiber system pumped with a monochromatic laser beam is described. The device operates by multiorder stimulated Raman scattering (SRS) and stimulated Kerr scattering (SKS) that result from the high local optical intensity and long gain length provided by the liquid-core hollow-fiber system. A number of transparent liquids have been investigated; CS(2) is identified as the medium that most efficiently generates SKS and SRS. The overall efficiency from the pump input to the coherent output of the system is measured as a function of the fiber parameters (internal diameter and length), inputcoupling focusing length, and pump conditions (pulse duration, spectral width, and wavelength). A number of other experimental results, such as optimized working conditions, output spectral structure, total spectral-covering range, pump-threshold requirements, and the maximum capacity for high pump operation are also presented and discussed.