Frequency-dependent mode size in Kerr-lens mode-locked lasers

Summary form only given. We examine the connection between wavelength and spatial mode that occurs in Kerr-lens mode-locking (KLM) lasers because self-phase modulation and self-focusing have the same underlying origin. We demonstrate that a laser generating 10-fs pulses exhibits a strong wavelength dependence in its output beam; that is it has a frequency-dependent mode size (FDMS). The measurements were performed on a KLM Ti:sapphire laser in the "X" configuration generating a bandwidth>100 nm. KLM achieves mode-locking by producing a spatial mode at high intensity that is better matched to the gain region (or some other aperture) than for low intensity. The fact that spectral broadening by self-phase modulation produces frequency extrema with less than optimum focusing therefore means that these extrema also experience less gain, i.e. the intensity dependent mode size that is responsible for KLM also acts as a spectral filter, limiting the bandwidth.