Example 1
[0058] In order to examine laser damage and optical damage caused by ultraviolet light, various kinds of single crystal materials were evaluated with respect to their light resistance using a laser having a light source of third harmonics of a YAG laser. The result is shown in Table 1. The crystal materials evaluated herein were a TeO2 crystal that had been used conventionally, and LN, MgO:LN, Li2B4O7, (GdY)1Ca4O(BO3)3, and CsLiB6O10 that were used for the acoustooptic device of the present invention. TABLE 1 Absolute Value Presence or of Laser Damage Relative Value Absence of Threshold of Laser Damage Optical Material (kW/mm2) Threshold Damage TeO2 29 1 Absent LN 87 3 Present MgO:LN 57-87 2-3 Absent Li2B4O7 At least 120 At least 4 Absent (GdY)1Ga4O(BO3)3 At least 120 At least 4 Absent CsLiB6O10 At least 120 At least 4 Absent
[0059] From the result, it is understood that among these materials, the TeO2 crystal has the lowest relative value of the laser damage threshold and therefore is the most susceptible to the laser damage. In this case, the “laser damage” denotes the state where the crystal surface was damaged by a laser beam and a concave portion was formed at the surface. Particularly, in the case of using TeO2, a metal Te was observed when the concave portion and its surrounding portion were analyzed using an X-ray microanalyzer. This conceivably is because of cleavage of chemical bonds caused by absorption of strong ultraviolet light and heat. Consequently, the TeO2 crystal is not suitable for the use in which high power is used. LN and MgO:LN showed the laser damage thresholds that are about twice to triple as high as that of TeO2. Regarding MgO:LN, the amount of MgO used therein was preferably in the range of 0.5 to 7 mol. %, and an acoustooptic medium made of MgO:LN containing more than 7 mol. % of MgO as a dopant was considerably susceptible to laser damage. In the Li2B4O7 crystal, the (GdY)1Ca4O(BO3)3 crystal, and the CsLiB6O10 crystal, the laser damage threshold was at least four times as high as that of the TeO2 crystal, and no damage was measured in this evaluation.
[0060] The above-mentioned results showed that LN, MgO:LN, and oxide single crystals containing boron as the main component have higher damage thresholds than that of the TeO2 crystal that has been used conventionally.
[0061] Next, the acoustooptic media made of the above-mentioned materials were evaluated with respect to the optical damage. The evaluation was carried out under the conditions that an argon laser was used as a light source, and the laser intensity at the sample position was 1.8 kW/mm2. As is known conventionally, optical damage (the distortion in a beam pattern) was found in the acoustooptic medium made of the LN crystal that was not doped with MgO. No optical damage, however, was found under the same conditions in the acoustooptic media made of the TeO2 crystal, the MgO:LN crystal, the Li2B4O7 crystal, the (GdY)1Ca4O(BO3)3 crystal, and the CsLiB6O10 crystal. As the result of the optical damage, the laser beam pattern was deformed considerably into an ellipse or was not uniform.
[0062] As described above, among LN crystals, particularly a LN crystal doped with MgO is subjected to less optical damage. Hence, a LN crystal doped with 0.5 to 7 mol. % of MgO that is subjected to less optical damage as well as less laser damage conceivably is suitable for the acoustooptic medium. Since no optical damage was found in the Li2B4O7 crystal, the (GdY)1Ca4O(BO3)3 crystal, and the CsLiB6O10 crystal, they are considered to be adaptable to both the case where peak power is high and the case where continuous light is used.
[0063] Next, ultraviolet acoustooptic devices like the one shown in FIG. 1 were produced using various acoustooptic media, and the acoustooptic effects of the various acoustooptic media were checked. In this case, the acoustooptic performance is not always reflected as it is since the acoustic impedances of the transducer unit 2 and the acoustooptic medium 3 and the electrical impedances of the radio-frequency signal generator and the transducer unit 2 were not optimized. However, when using third harmonics of a pulsed NdYAG laser with a wavelength of 355 nm that was employed as a light source, the diffraction efficiency was about 5% to 20% as shown in Table 2, with the power of incoming radio-frequency signals being 2 to 3 W. Furthermore, in this case, the acoustooptic devices did not require to be water-cooled or the like.
[0064] Particularly, by covering the acoustooptic media with a thermal conductive sheet, it was possible to obtain ultraviolet acoustooptic devices in which no defocus nor drift of laser beams occurs. In this connection, a graphite sheet was particularly useful as the thermal conductive sheet since it had a thermal conductivity that was twice that of copper. TABLE 2 Diffraction Material Efficiency (%) LN 20 MgO:LN 20 Li2B4O7 5 (GdY)1Ga4O(BO3)3 6 CsLiB6O10 5
[0065] It is to be understood that an impedance matching circuit may be provided between the radio-frequency signal generator and the transducer unit 2, although it was not used here.
