CaF<inf>2</inf>:Dy<sup>2+</sup>lasers

Abstract

The fabrication and operation of CaF 2 :Dy2+lasers are considered in detail. Laser action occurs at 4238.5 cm-1(extrapolated to 0°K) between the states T 1 (5I 7 ) \rightarrow T_{2} (5I 8 ).Chemical or electrolytic reduction techniques produce the most stable laser rods. The fluorescent lifetime of the (5I 7 ) states depends primarily upon the total Dy concentration in the crystal and varies from over 50 ms at low concentrations to 10 ms at 0.2 percent dysprosium. This lifetime is essentially temperature independent. The operation of the laser in a magnetic field is analyzed. By aligning the magnetic field and the laser axis parallel to the crystalline [100] axis, modulation of the laser output can be obtained at rates up to 1 MHz with a few tens of gauss modulating field. The saturation in laser output power at high levels is due to the finite lifetime of the (5I 7 ) multiplet, which is estimated to be several ms. The fluorescent linewidth of the laser transition varies from 4 cm-1at 200°K to 0.024 cm-1at 4.2°K. The absolute energy of the transition shifts 2.0 cm-1over the same temperature range with a slope of -0.009 cm-1/°K at 77°K. A technique of cooling the crystal with flowing, nonboiling liquid N 2 permits high-level laser operation near 77°K. Power outputs up to 1.21 watts have been obtained from CaF 2 :Dy2+when the crystal is pumped optically with two 1-kW tungsten lamps in a double ellipse. Under these conditions, the crystal temperature rises 15 to 20° above the coolant due to the high thermal resistivity of the crystal-liquid nitrogen interface. Typical pulse thresholds at 77°K are 20 joules into an FT524 lamp. The threshold drops rapidly at lower temperatures, but is strongly dependent on the frequency separation between the narrow fluorescent line and the cavity modes. Measurements at 4.2°K are particularly difficult because of this effect. Continuous operation at 27°K has been obtained at electrical inputs to the tungsten lamp as low as 20 watts. Laser output consists of a continuous output modulated at 20 to 100 kHz, with spherical reflectors or a pulsed, on-off output with flat reflectors. With spherical reflectors, magnetic feedback can be applied to reduce the amplitude of the modulation to less than 10 percent. The same type of magnetic modulation can be used to obtain a series of output pulses phase locked to the modulating frequency. The modulating frequency can be varied from a few kHz up to almost a MHz. The most effective modulation was obtained with an inhomogeneous modulating magnetic field and with the laser axis parallel to the crystalline [100] direction. At low modulation frequencies and with high fields, quasi- Q - switched operation was obtained. Q -switching was also observed with an external rotating mirror, which gave a series of pulses of 20 ns rise time. These did not have the characteristic high power of Q -switched pulses but peak powers of only a few tens of watts. This is assumed to be caused by the absorption of the metastable (5I 7 )level. The single-pass gain of a laser amplifier was measured; it corresponded to \alpha=0.039 cm-1.