Abstract

In the course of attempting to increase the accuracy with which we could make measurements of the nonlinear index of refraction (n₂) of optical materials by the z-scan¹ technique, we found a number of advantages that result from relay knag-tag the input beam, as well as some surprising observations that permit a substantial fraction of a Gaussian beam to be converted to a near flat-top or other smoothly varying distribution. The method we are using is actually a modification of the "top-hat" z-scan technique,² which has the advantages described in Ref. 2 of higher sensitivity and smaller uncertainties introduced by beam-quality considerations than the Gaussian-beam technique.¹ We find, however, that we .obtain comparable n₂ measurements by the two techniques. The additional modification we have made to the top-hat technique, which uses an apertured expanded beam rather than a Gaussian as the input beam, is to place the defining aperture for the top hat at the front focal plane of the lens that focuses the beam into the sample and then reimaging the input aperture with a second lens onto a ccd camera (Fig. 1). Reimaging eliminates diffraction fringes from the aperture and provides a stationary image even for a wedged sample; recording the entire image permits minimization of spurious effects such as varying interference fringes.

© 1996 Optical Society of America