Nonlinear contributions in intracavity dispersion measurements

Current state-of-the-art Ti:sapphire laser systems operate in a dispersion-limited regime at around 10 fs pulsewidth. This remarkable progress has been made as a result of intracavity dispersion optimization, as initial results in self-focusing modelocking produced pulses of ~90 fs duration. The recently developed technique of Frequency-Domain Dispersion (FDD) can provide measurements of the complete intracavity group delay under operating conditions. Since dispersion is a linear optical property, one might wonder whether it is meaningful to measure the dispersion inside a nonlinear system while it is operating. In the paper, the author discusses the magnitude and origin of nonlinear corrections to the measured intracavity group delay in FDD measurements. The author also raises the general questions: (a) since a modelocked laser is a nonlinear optical system, is it correct to measure its dispersion when it´s lasing ?; and (b) since a modelocked laser is a nonlinear system, is it correct to measure the dispersion when it´s not lasing?