We consider second harmonic generation (SHG) of ultrashort pulses in the case of strong phase- and group-velocity mismatch. Spectral fringes appear in the second harmonic related to two delayed replicas of the fundamental pulse in the time domain. The fringe separation can be used to evaluate the group-velocity and refractive index of nonlinear crystals at extreme wavelengths. Experimental results with femtosecond pulses in SrB(4)O(7) (SBO) are used to refine the Sellmeier equation describing the n(c) refractive index down to 160 nm, essential for the use of this unique nonlinear crystal for random quasi-phase-matching in the VUV.