When two optical beams copropagate inside a single-mode fiber, the intensity-dependent refractive index couples the two beams through a nonlinear phenomenon known as cross-phase modulation (XPM). Such an XPM-induced interaction between the two waves can destabilize the steady state and lead to temporal modulations (self-pulsing) in the presence of group-velocity dispersion. This phenomenon is analogous to the modulation instability of a single beam occurring in the anomalous-dispersion regime of the fiber. The XPM-induced modulation instability can occur even in the normal-dispersion regime. The features of XPM-induced modulation instability are discussed in detail. This phenomenon may be useful for generating ultrashort pulses (<100 fs) in the visible region of the optical spectrum.

We have studied the nonlinear absorption spectra and optical limiting properties of 2,2′-(9,9-dihexyl) bifluorene. Measurements were done in chloroform solution, in the visible range (450–650 nm), and for nanosecond time-duration pulses. The two-photon absorption (TPA) spectrum, obtained by the up-conversion fluorescence method, shows a resonance at λ=534 nm with a cross-section σTPA=60 10−50 cm4⋅s/photon-molecule. Semiempirical quantum chemistry calculation on fluorene and bifluorene suggests an enhancement of the bifluorene TPA due to coupling effects between monomers. In nonlinear transmission measurements, two-photon absorption is reinforced by the excited-state absorption that occurs during the pulse duration. At resonance, the three-photon absorption coefficient is α3=14 000 cm3/GW2 for a bifluorene concentration of 600 g/L. This strong nonlinear absorption leads to an efficient optical power limiting in the green and blue parts of the spectrum. The maximum transmitted energy is lower than 10 μJ for an input energy of up to 200 μJ in a F/5 optical geometry.