A novel common-path scheme for fourth harmonic generation by ultrashort laser pulses

A new compact common-path scheme for Fourth-Harmonic Generation (FHG) of femtosecond laser pulses is proposed here. Guidelines for selection of non-linear crystals and birefringent crystals used for delay pre-compensation are derived based on approximation of non-saturated sum-frequency mixing of Gaussian pulses. We experimentally demonstrate the scheme and study its performance for a different combinations of BBO nonlinear crystals for the case of generation of sub-200 nm deep ultraviolet pulses by FHG of pulses delivered by a Ti:Sapphire ultrafast laser. Pulses with duration between 130 fs and 370 fs and pulse energy of 19 to 85 mu J were generated. It is shown, that in addition to its compactness and intrinsic stability, the scheme allows a superior spectrum and pulse quality.