Experimental evidence for cross-phase modulation, induced-phase modulation, and self-focusing of picosecond pulses in optical fibers

Abstract

The effects of cross-phase modulation (XPM), induced-phase modulation (IPM), and self-focusing on 532-nm 25-ps pulses propagating in short lengths of optical fibers have been investigated experimentally. The Raman spectra were found strongly marked by XPM effects generated by co propagating pump laser pulses. XPM salient features are (1) ΔωRaman/Δωpump ≈ 2.9, (2) Stokes broadening larger than anti-Stokes broadening, (3) fast spectral modulation, and (4) slight shift of the Raman peak toward the lowest frequencies. Induced-phase modulation effects generated by strong 1064-nm control pulses on weak 532-nm pulses were also characterized. Broadening of 532-nm pulses and intensity peak shifts were observed. Both processes for the probe pulse increased linearly with the control pulse intensities. Moreover, a strong energy conversion from 1064 to 532 nm occurred from nonmatched four-photon mixing. Finally, self-focusing of 25-ps Raman pulses was observed in a large-core optical fiber. For intense input pulses, a continuum of Stokes frequencies and Raman light was generated in a small-ring waveguide structure. The Raman beam diameter was 10 times smaller than the beam diameter of the laser at low intensities. The ring structure was attributed to an induced-gradient index profile arising from the nonlinear refractive index.