Characterization of PPSLT crystals for efficient frequency doubling of infrared laser radiation
Fraunhofer Institute for Laser Technology ILT
Periodically poled crystals are used for frequency doubling of pulsed infrared radiation. Through the principle of quasi-phase-matching, very high effective non-linearities can be used for this purpose. What’s more, the radiation is not subject to walk-off and as a result long crystals can be used without negatively affecting the beam quality. The conversion efficiency, however, depends considerably on the quality of the poling structure produced. Periodically poled stoichiometric lithium tantalate (ppSLT) exhibits a higher resistance to photorefractive damage than lithium niobate. In this study ppSLT crystals from four different manufacturers are compared under reproducible conditions.
All the crystals used have a length of 10 mm and a poling period of 8 μm for frequency doubling from 1064 nm to 532 nm and are operated in a thermostat for the tests. An adjustment mechanism permits high-resolution translation of the sample along the transversal and the longitudinal axis and detuning of the angle of incidence in polar and azimuthal direction. A microchip laser with an average output power of 40 mW and a pulse peak output of 6 kW is used as the pulsed radiation source. For the case of optimal phase matching and central positioning of the laser beam with respect to the crystal aperture the conversion efficiency for various focus radii is measured. In addition, the temperature and angle acceptances as well as the transversal homogeneity of the crystals are examined.
The maximum conversion efficiencies achieved are, depending on the crystal, between 60 percent and 80 percent. Additionally, distinct differences in the homogeneity of the poling structure were found. The variances in conversion efficiency along both transversal coordinates are between 1 percent and 30 percent. The temperature and angle acceptances exhibit deviations of 1 percent and 10 percent from the theoretical reference values.
Converters based on ppSLT can be used up to an average output power of a few watts for doubling the frequency of laser radiation of low pulse power in the visible wavelength range.