Frequency-doubled power-scaled Pr:YAlO3 laser generation at 373.5 nm wavelength

Summary form only given. Frequency-doubled Pr:YAP laser emission has already been described under both flash-lamp [1] and GaNdiode pumping [2,3]. Nevertheless, the output power of commercially affordable pump GaN-laser-diodes is not so simple to be increased (unlike e.g. the OPS lasers), which limits the output of GaN-diode pumped laser systems. Thus, to overcame partially this disadvantage, the power-scalled laser system architecture has to be employed. Following our results concerning the second harmonic generation from the GaN-diode pumped Pr:YAP laser [2], power-scaled continuous-wave violet-generation by intracavity frequency doubling of the near-infrared emitting Pr:YAP laser is described.In our experiment, 0.6 at. % doped Pr:YAP active medium φ5 mm x 5 mm in dimensions grown by the Czochralski method and cut along the b-axis (according to Pbnm notation) was used. For a waste heat removal, the laser crystal was mounted on a water-cooled copper heat sink which temperature was kept at 15°C. For active medium pumping, two GaN laser diodes (LD) from NICHIA Corporation delivering output power of up to 1W at 448 nm wavelength each was employed. Laser diode beam collimation was performed by an antireflection coated (400-600 nm) aspherical lens with a focal length of 4.5 mm. To improve the LD beam quality, beam shaping optics consisting of antireflection coated (350-700 nm) cylindrical lens pair in Galileo´s-telescope arrangement were employed. As a nonlinear medium for intracavity frequency-doubling, a BBO crystal was chosen. The crystal 6 x 6 x 10 mm3 in size was designed for the I-type of conventional birefringent phase matching. Both BBO´s facets were antireflection-coated around 750 and 375 nm wavelength. 4-mirror resonator (see Fig.1) was formed by flat pump mirrors (M1, M2) and folded mirrors (M3, M4) with 50 mm radii of curvature. All mirrors were highly reflective at the fundamental laser wavelength (747 nm). The reason for the “bre- k V-folded” cavity was effort to use the same arm dimensions (M1-M3 and M3-M4) as described in [2] while preserving the 60 mm focal-length of the focusing lenses. The M1-M3 and M3-M4 resonator lengths were about 70 mm and 86 mm, respectively. The active medium was situated as near as possible to the pump mirror (M1), i.e., in the location of the cavity beam waist, whereas the nonlinear BBO crystal was placed in the second waist of the cavity. In order to minimize crystal exposure against the double-frequency-converted radiation with the aim to prevent any possible crystal solarization, the blue laser radiation was almost coupled out using both mirrors M3 and M4 with a non-optimal transmissivity value of T~70 % at 373.5 nm. The maximal output power reached at 373.5 nm wavelength was 46.6 mW with 275 mW oscillation threshold of the pump power absorbed. Taking into account the maximal output power of 256 mW which was extracted from the 4-mirror resonator without BBO crystal and with the 98% reflectivity (at 747 nm) of the M4 mirror, the resulting near-infrared-to-violet conversion efficiency amounted to 18.2 %.