Efficient frequency doubling of femtosecond pulses with BIBO in an external synchronized cavity

• Efficient Second harmonic generation (SHG) of very low energy (~1.4nJ/pulse) femtosecond pulses is demonstrated using a BIBO crystal.
• The crystal is placed in an external ring cavity, synchronized with an input mode-locked laser working at 78 MHz repetition rate.
• A frequency doubling efficiency of 53% is obtained, which is, the highest value ever reported for such a low input energy pulses.
• The cavity losses, mode matching and impedance matching are also studied using a SHG model, showing a good agreement between theory and experiment.
• This study may have applications in non-linear frequency conversion, enhancement cavities and experimental quantum optics.

We experimentally demonstrate the second harmonic generation (SHG) of infrared femtosecond pulses using a BIBO crystal placed in an external ring cavity, synchronized with an input mode-locked laser at 78 MHz. A frequency doubling efficiency of 53% is achieved which is, to the best of our knowledge, the highest value ever reported for a low energy input beam of 1.4 nJ/pulse. Theoretical analysis of cavity related issues such as design, fundamental mode characteristics and fidelity against misalignments are also presented. The modeling of SHG cavity enables us to estimate the cavity losses and the mode matching visibility. Such synchronized SHG cavities in pulse domain, having higher SHG conversion efficiencies compared to their continuous wave counterparts, may find potential applications in scientific areas such as in photonics, and in quantum optics.