Pii: 0030-4018(79)90392-4

During the past decade the application of light pulses in the picosecond time domain has received increasing attention. In these investigations it is of primary interest to know the duration of the ultrashort light pulses. Two methods to measure the pulse duration have found frequent application: The two-photon fluorescence technique, TPF [I] and the streak camera [2]. The autocorrelation pattern of the TPF technique gives direct information on the pulse duration and the coherence properties of the pulse [3] but the difficult alignment, the high-power requirement, and the weakly emitting fluorescent pattern become a serious hindrance for wide application. The commercially available streak cameras are very expensive, show a jitter of the order of 100 ps of more, and have a temporal resolution of several picoseconds (except very near to the special cut-off frequency). In 1966 it was suggested [4,5] and experimentally demonstrated [4] that the second harmonic signal generated by two equal pulses in a nonlinear crystal allows the determination o f the autocorrelation function giving a value for the duration of the ultrashort light pulse. The technique has found recent application for the studies of the pulse duration of passively mode-locked CW dye lasers or synchronously pumped CW dye lasers. The autocorrelation function is obtained by introducing quasi-continuously a delay between the two interacting pulses [6]. More recently, it was noticed that the spatial width of the second harmonic beam (SHB) may be used to obtain the autocorrelation function [7,8], i.e., the du\ , / /