Time domain measurement of the nonlinear refractive index in optical fibers and semiconductor films

A new technique to measure the nonlinear refractive index n 2 in optical fibers and semiconductor films has been developed. It is based on the time delay two-beam coupling of very intense picosecond laser pulses that have been self-phase modulated in the nonlinear optical medium. The two beams are coupled in a slow responding medium that is sensitive to time dependent phase distortions. We determine that the amount of phase distortion experienced by the pulse is proportional to the nonlinear refractive index of the medium. This time domain approach can also be applied to optical fiber amplifiers in the presence of gain and to semiconductor films. Because the technique is based on pure refraction the measurement of n 2 is insensitive to nonlinear absorption, thermal effects, and surface roughness. With this technique we have measured n2 in 20-m length of Silica-glass, Ytterbium-doped, and Erbium-doped optical fibers at 1.064-mum. Also we have measured the change of n 2 at 1.064-mum in the presence of a 980-nm pump laser in Yb 3+-doped and Er3+-doped fibers. Finally we have extended the technique to measure n2 in 2-mm thick samples of GaAs, CdTe and ZnTe semiconductors. In the language of ultrafast spectroscopist, if the best tool to characterize an ultrashort optical pulse is the pulse itself, then the best tool to characterize an optical nonlinear medium is a pulse that has been modified by the medium.