Rise-fall ambiguities and their removal from frequency-domain nonlinear-optical techniques

Most frequency-domain nonlinear-optical techniques measure only the magnitude of the Fourier transform of a temporal response and, hence, do not uniquely determine the response. We show that, for the commonly used response, h(t) = A exp(-t/tau(f)) + B exp(-t/tau(s)),where A identical with alpha/tau(f) and B identical with (1 - alpha)/tau(s), the spectral line shape can always be fitted by two different values of alpha. A measurement of the optical Kerr transient of carbon disulfide illustrates this ambiguitv. We also demonstrate a single-scan method that is free from such ambiguities. It involves adding coherent background with a nonzero quadrature-phase wavelength.