Efficient second-harmonic generation of tunable femtosecond pulses into the blue in periodically poled KTP

We report efficient single-pass second-harmonic generation of tunable femtosecond pulses into the blue in periodically poled KTP (PPKTP). Using a 2 mm crystal with 4 μm grating period and femtosecond fundamental pulses at 76 MHz, we show that average powers of up to 320 mW in the blue at conversation efficiencies as high as 20% are attainable despite the limitations of group velocity mismatch and spectral acceptance in PPKTP. Accurate measurements of phase-matching acceptance in PPKTP have been performed and excellent agreement between experimental data and calculations is confirmed. Using temperature phase matching, a significant tuning range of 422-427 nm in the blues is also demonstrated. Cross-correlation measurements of the fundamental and second-harmonic pulses in a 500 μm thick β-BaB₂O₄ result in blue pulse durations of 700-800 fs for 130 fs fundamental pulses.