Generation of green second harmonic radiation in LBO, BiBO, KTP, and PPLN crystals using passively Q-switched sub-nanosecond microchip laser
A sub-nanosecond green laser source has big demand in such fields like spectroscopy, micromachining, fluorescence imaging, and laser displays. Most often green light is produced by frequency doubling of pulsed laser sources based on Nd3+-ion or Yb3+-ion doped gain media that oscillate in the near-IR range. For creating compact and relatively small source of green light suitable for broad type of applications a passively Q-switched Nd:YAG microchip laser (Teem Photonics) operating at 1064 nm with 6.9 kHz repetition rate was chosen. It delivers 560 ps long pulses with 10 μJ energy, corresponding to an average output power of 69 mW. Crystals of BiBO, KTP, LBO, and PPLN were chosen for frequency doubling. Main goal of this work was to study the characteristics of these crystals under similar experimental conditions and to select the most efficient one for this task. To optimize second harmonic generation (SHG) process, different focusing conditions were used during the experiments. In this work we measured the second harmonic output power as a function of the incident power, beam profile for fundamental and second harmonic radiation for all crystals, and second harmonic output power as a function of temperature for PPLN crystal. Crystal of PPLN was found to be the most suitable for SHG process and produced it with up to 60% conversion efficiency.