Nd:YAG/Cr:YAG microchip-based system for laser-induced damage threshold measurement

Abstract

A compact system for measuring laser-induced damage threshold (LIDT) was developed. As a source of linearly polarized 1064 nm testing radiation, the special Q-switched microchip laser was used. This laser was based on monolith crystal composed of Nd:YAG active laser part (0.9 at.% Nd/Y, 20 mm long, pump mirror deposited) and Cr:YAG saturable absorber (initial transmission 60 %, 1.8 mm long, output coupler with reflection 60 % @ 1064 nm deposited). The laser generates 3.9 ns long pulses with an energy of 0.42 mJ and repetition rate of 50 Hz. The probe beam spatial structure has Gaussian profile (M² = 1.1). At the focal point of the focusing aspheric lens, it is possible to achieve a peak energy density of up to 900 J/cm² (the beam radius at the focal spot was 5.6 μm). The LIDT measurement uses computer-controlled attenuator and sample 3D positioner. To calibrate the spatial parameters of the probe beam, the knife-edge method is used. The system was designed to test transparent samples with a diameter of up to 25 mm. The measurement takes place automatically. First, the probe beam is characterized. Next, the LIDT estimate is determined, and then the LIDT measurement is performed using the S-on-1 method. The system was tested for silica glass and YAG crystal. The optical part of the device has dimensions of 45 × 60 cm. It is assumed that this system will be used for routine control of LIDT of dielectric layers on laser crystals and mirrors.