Efficient harmonic generation with a broad-band laser

Harmonic generation using a broad-band input beam is studied theoretically and experimentally. Angular spectral dispersion is imposed on the input beam to improve the phase matching and thereby increase the conversion efficiency. The coupled amplitude equations are derived for a grating-dispersed input beam, and it is shown that, in the parameter range of interest, a simple theoretical model based on the intuitive concept of instantaneous frequency suffices to describe the nonlinear interaction. A laser having a frequency-chirped output with a 25 A bandwidth is used to demonstrate the technique for third-harmonic generation in the small-signal regime. A factor of 2.5 improvement in the conversion efficiency is observed when an angular spectral dispersion is imposed on the input beam, in excellent agreement with the predictions of the simple model.