Temporally stretched Q-switched pulses in the 2 µm spectral range

Mid-infrared Q-switched laser radiation around λ ≈2 μm is important in material processing and numerous medical applications. However, in order to transmit pulse energies >; 50 mj through optical fibres it is advantageous to stretch the pulse duration towards the μs time regime to prevent fiber damage at high irradiances. At higher pulse energy peak power density can exceed the threshold for fiber fragmentation of about 30 MW/cm² . To prevent damage of intracavity optical elements and fiber damage the pulse duration has to be stretched when energies in excess of 50 mj are needed. Overcoupled intracavity frequency conversion seems to be a promising method to generate Q-switched laser pulses in the microsecond time regime. If the conversion efficiency becomes much higher than that necessary to generate maximum second harmonic peak power (overcoupling regime), the fundamental peak power is limited by nonlinear losses and the remaining inversion is slowly depleted in an extended pulse. Therefore the overcoupling effect provides a method to adjust the pulse duration without significant loss of pulse energy. In the case investigated here this effect is used to stretch the fundamental wavelength, and the frequency conversion only acts as an additional loss. Therefore, in contrast to previous works, not only the second harmonic but also the fundamental wavelength is additionally coupled out. This reduces the intracavity intensity and therefore the SHG efficiency and thus the performance of the overcoupling effect. In oder to exploit this effect a Ho:YAG laser Q-switched by an AOM at a repetition rate of 300 Hz was designed.