Abstract

Results on third order optical nonlinearity characterization are presented using a recent technique [1] based on nonlinear spectral evolution of femtosecond pulses The method consists in a first step of pulse stretching in a prisms line of variable dispersion; afterwards, pulses propagate across the nonlinear sample under test experiencing χ⁽³⁾ nonlinearity. At the output the pulse spectrum analysis reveals two different types of spectral evolution depending on the relative signs of the prisms line dispersion with respect to that of the nonlinearity: spectral compression or spectral broadening. The amplitude of these spectral changes, when the initial dispersion is continuously varied from negative to positive values, is directly related to the Kerr nonlinear coefficient of the material to be characterized. This technique has been called D-scan [1], for "dispersive-scan", in reference to the well-known Z-scan technique [2] that operates in a very similar way in the spatial domain. Our method is applicable at all wavelengths. It doesn't require complex detection system nor interferometric adjustment. It is a very linear and sensitive technique. Because of the use of femtosecond pulses, it is only sensitive to fast response nonlinearity. D-scan is also able to perform measurement of the imaginary part of the third order nonlinear susceptibility by plotting the overall transmitted energy versus input dispersion. D-scan has proven to be a useful experimental method for the characterisation of optical waveguides such as a monomode silica fiber [1]. Here we show that D-scan is also applicable to bulk material characterization. For this, we have measured the nonlinear coefficient n₂ of a SF 57 glass piece of 5 mm thickness. In the case of bulk material D-scan offers the advantage of being almost insensitive to sample inhomogeneity and surface optical quality.

© 2000 IEEE