Table 2. Comparison of Copolymer Samples and PEMA with Previously *Previously reported data converted to equivalent values for filtered Gaussian beam profile. It is apparent that our ability to reduce the impurity content (as indicated by the number of scattering centers observed) was not good for the 11 copolymers and PEMA shown here. We have shown previously10,14 that there is a strong correlation between scattering centers and the damage thresholds. However, within the above restriction it also seems that varying the copolymer content does little to influence the damage resistance of the sample. S-61 and S-50 have increasing amounts of n-butylacrylate (nRA) (5% and 11% by weight, respectively), and S-58, S-53 and S-52 have increasing amounts of isodecyl methacrylate (IDM) (5.2%, 10% and 20% by weight, respectively), but neither copolymer conclusively demonstrates an increasing damage threshold. In fact, the opposite is true for the NBA copolymers and nothing can be concluded for the IDM copolymers. IDM does have the largest and heaviest side group, thus it will spread apart the polymer chains and make the entire polymer more elastic. This is a desired effect, because it will improve the viscoelastic properties of the sample, and thus should increase the damage threshold of the material as indicated by Dyumaev, et. al. The same comparison should be true for the nBA (S-61 or S-62) copolymers vis-a-vis the ethyl acrylate (EA) copolymers (S-56 or S-57) but does not hold. It is probable that the inclusion content is so large that only a big change (i.e., nBA or EA to IDM) in the copolymer will have a significant effect. A comparison of polymerization methods was attempted with S-56 (thermal) and S-57 (uv) and also with S-61 (thermal) and S-62 (uv) with the expected result that S-57 and S-62 would exhibit improved thresholds based on our previous work with PMMA.12 This is not true, but it should also be noted that the particulate content of S-57 and S-62 is higher than their thermally polymerized counterparts, even though all samples were prepared under conditions as identical as possible. It is clear that the inclusion content has the greater influence and no trend can be established for these samples. The two samples (S-54 and S-55) of PEMA recorded were the initial attempts at making these samples. The inhibitor was not removed, and initiator was added to S-55. It is clear that adding the initiator added inclusions and decreased the damage thresholds of S-55. This was expected. PEMA is interesting however, because it is still relatively hard, but is softer than PMMA. PEMA should therefore have improved viscollastic properties and greater damage resistance. The S-63 is a copolymer of MMA and cyclohexyl methacrylate (CHMA). Both materials alone make a hard polymer and PCHMA additionally tends to crystallize and become milky in appearance. copolymer sample studied was slightly cloudy, and there is little in its results to indicate any improvement over the other samples. In fact, its single-shot behavior is relatively worse than the other samples, while its multiple-shot behavior is not much better than several other samples. Finally, a comparison of the samples with PMMA of similar inclusion content is shown in table 2. It appears that the copolymer material, or PEMA, has a somewhat higher single-shot damge threshold. However, the differences are not very great, and certainly no sample had damage thresholds greater than some of the PMMA samples previously reported.12 It is thus unclear whether copolymers or a different methacrylate polymer will have a significantly improved laser damage threshold. 5. Conclusions The apparent improper characterization of the sombrero function beam's peak fluence yields a significantly lower damage threshold than those obtained with a properly determined Gaussian peak fluence. The high inclusion density present in our samples probably masks the bulk material properties that would influence the damage threshold. The copolymers and PEMA shows a modest improvement in single-shot threshold level over PMMA, which agrees with the Soviet literature.6,7 The multi-shot threshold improvement of copolymers over that of PMMA is inconclusive for our samples. We did not observe the improved multi-shot threshold performance of the copolymers as was reported by Dyumaev, et. al. Finally, due to the large number of inclusions in our samples, work in this area needs to be continued with better prepared samples. 6. References [1] [2] [3] [4] [5] [6] [7] [8] K.M. Dyumaev, A. A. Manenkov, A. P. Maslyukov, G. A. Matyushin, V. S. Nechitailo, and D. A. Gromov, K. M. Dyumaev, A. A. Manenkov, A. P. Maslyukov, G. A. Matyushin, V. S. L. D. Merkle, M. Bass, and R. T. Swimm, "Multiple Pulse Laser-Induced Bulk Damage in R. M. O'Connell, T. F. Deaton, and T. T. Saito, "Single- and Multiple-Shot R. M. O'Connell, T. T. Saito, T. F. Deaton, K. E. Siegenthaler, J. J. McNally, and A. A. Shaffer, "Laser Damage in Plastics at the Frank J. Seiler Research Laboratory, presented at the Fifteenth Annual Symposium on Optical Materials for high power lasers, Boulder, Colorado, November 14-16, 1983 NBS Spec. Pub. (to be published). B. M. Emel 'yanova, T. F. Ivanova, M. P. Votinov, V. M. Ovchinnikov, V. D. Piterkin, and Z. A. Smirnova, "Optical Strength of Copolymers of Methyl Methacrylate and Butyl Acrylate," Sov. Tech. Phys. Lett., 3 (7), 280 (1977). V. I. Bezrodnyy, 0. V. Przhonshaya, Ye. A. Tikhonov, M. V. Bondar, M. T. Shpak, "Polymer Active and Passive Laser Elements Based on Organic Dyes," Kvantovaya Electronika, 9 (12), 2455 (1982). R.M. O'Connell, A.B. Romberger, A.A. Shaffer, T.T. Saito, T.F. Deaton, and K.E. Siegenthaler, "Improved Laser-Damage-Resistant Polymethyl methacrylate," JOSA B, 1, 853. (1984) [9] [10] [11] [12] M. Mauck, "Knife-Edge Profiling of a Q-Switched Nd:YAG Laser Beam and Waist," Applied B.W. Mullins and A.B. Romberger, "Real Time Laser Damage Detection in Bulk Materials by Strehl Intensities," Proc. Southwest Conference on Optics: 1985, R.S. McDowell and J.B. Gerardo, editors, accepted for publication, (1985). B.W. Mullins and B.A. Richert, "Strehl Ratio Measurements of Laser Damaged Plastics," Proc. 15th Annual Symposium on Laser Induced Damage in Optical Materials, A. Guenther and H. Bennet, editors, accepted for publication. R.M. O'Connell, R.V. Ellis, A.B. Romberger, T.F. Deaton, K.E. Siegenthaler, A.A. Manuscript Received 12-19-85 Improvement of The Bulk Laser Damage Threshold of A. Yokotani, T. Sasaki. K. Yoshida. T. Yamanaka and C. Yamanaka Institute of Laser Engineering. Osaka University. Potassium dihydrogen phosphate (KDP) crystals were grown under the irradiation of ultraviolet light. The bulk laser damage threshold was improved two to three times (15 20 J/cm) compared to the case of crystals grown by conventional methods. Microbes such as germs and bacteria are frequently generated in the KDP solution with the usual growth method. causing a low damage threshold. A main reason of the low damage threshold is due to the organic materials such as microbes or their carcass which are incorporated in the growing KDP crystals. Key words: potassium dihydrogen phosphate; crystal: harmonic generation: high power laser: laser fusion: damage threshold: ultraviolet light: microbes: germs; bacteria: pasteurization: dissociation. 1. Introduction Potassium dihydrogen phosphate (KDP) crystals which can withstand the irradiation over 10 J/cm with a 1 ns pulse at 1.053 μm laser wavelength are necessary for the harmonic generation of fusion lasers. Typically as-grown crystals have the bulk laser damage threshold of 6-9 J/cm.[1.2] Swain showed that the baking and pulsed laser irradiation of KDP crystals improved the laser damage threshold without describing the reason. [3] The KDP crystals are usually grown from the aqueous solution at the temperature range between 60°C and 20°C. Microbes are sometimes observed in the KDP solution during the long growth term in spite of stirring of the solution. If these microbes or their carcass are incorporated into the growing crystals. the damage threshold will show the lower values than the intrinsic value. because the damage threshold of the organic material is much lower compared to the inorganic material. To prevent the creation of the microbes in the KDP solution we have grown KDP crystals under the irradiation of ultraviolet (UV) light. The KDP crystals which have been grown by this method showed the increase of the damage threshold two to three times ( 15 - 20 J/cm2 ). We have also performed the experiment of the effect of the ultraviolet irradiation on the bulk laser damage threshold of KDP crystals after growth. 2. Damage threshold of KDP crystals grown under the UV irradiation 2.2. Crystal growth unit Figure 1 shows the crystal growth unit. The inner vessel was made of glass which was durable against the UV radiation. Low pressure mercury lamps with Suprasil quartz tubes of which input power were 10 and 20 W were used to pasteurize the KDP solution. The mother liquor was 9 liters and the temperature falling method was adopted to grow crystals. The starting temperature was 45 C. The solution was filtered with 0.2 um membrane before starting of crystal growth. Z-cut seed crystals of which cross section was 5cm x 5cm were used. The growth rate of Z-axis was 3 - 8 mm/day. The technique of crystal habit control by metal impurities such as Cr and Al was used. [4] The growth term was approximately one month. 2.2. Investigation of microbes in KDP solution The microbes in KDP solution were studied by using Water Sampler from Millipore Limited ( SPC Total-Count Sampler ). This is a standard method for total bacteria count in water. In the case of solution unpasteurized by the UV lamps, a number of colonies of gel-like germs appeared on the sampler after one or two days of incubation. Figure 2 illustrates a typical photograph of such germs. The gathering of white strings are certain kind of mold which generated in the air atmosphere from the gel-like germs. We could not observe such germs from the solution pasteurized with the UV lamps even after four or five days of incubation. 2.3. Laser damage test facility Figure 3 shows the experimental setup of glass laser damage test facility. The measurement of the bulk laser damage threshold was performed using a Q-switched YLF laser (1.053 μm) of transverse and longitudinal single mode with a 1 nsec pulse. The pulse was focused into Z-cut KDP samples by a lens of which focus length was 3.5 cm. The surfaces of KDP crystals were finished with combination of diamond turning and wet polishing with 0.05 μm alluminun oxide powder. The damage was observed by eyes. A He-Ne laser was also used to find the small damage spots by utilizing the scattering from the damage spots. The focus spots was moved shot by shot. 2.4. Results of damage test The results of the damage threshold measurement are shown in figure 4. The abscissa is damage threshold and the ordinate shows number of samples. The empty squares illustrate the samples grown without the UV irradiation. The shaded squares shows the samples with the UV irradiation. In the case without the UV irradiation. most samples showed the damage thresholds of 6-10 J/cm2, whereas, for the samples with the UV irradiation showed considerably higher damage thresholds (1022 J/cm). There is a tendency that larger the intensity of UV irradiation is. higher the damage threshold becomes. 2 In a few cases without the UV irradiation. very high damage thresholds ( 20 J/cm ) were also observed. We consider that one reason of the wide spread of the damage thresholds may be due to the differences of the initial conditions of the KDP powders. We did not investigate the quantity of microbes or other organic impurities in the KDP powder before dissolving in pure water. 3. Effect of the UV irradiation on KDP crystals after growth We have investigated the effect of the UV irradiation on KDP crystals after growth. Lowrence Livermore Laboratory reported that the repetitive Xenon flashes were not effective. [2] We used the UV light from continuous high pressure Xenon lamp of which input power was 500 W. The experimental setup is shown in figure 5. The KDP sample whose dimension was 1.5 cm x 1.5 cm x 4 cm was set at the distance of 5 cm from the Xenon lamp. This sample was cut from the crystal grown at the growth rate of 8 mm/day with addition of Cr and Al ions to the solution. The bulk damage threshold were measured as a function of the distance from the UV irradiated surface, which is illustrated in figure 6. Exposure time of the Xe lamp was 3 hours. The damage threshold decreased almost exponentially with the distance. The temperature of the sample became almost 50°C due to the irradiation of Xe lamp but there was no temperature distribution in the sample. Therefore we can say that the increase of the damage threshold was induced mainly by the UV light and not by the thermal effect. Figure 7 shows the damage threshold as a function of the UV exposure time. The data were taken at a distance of 5mm from the UV irradiated surface. Although we tried to measure the difference of the absorption spectra of KDP crystals grown with and without the UV lamps. it was too small to measure. Instead. we have measured the change of the absorption spectra of a KDP solution which was kept for a long term ( approximately one month at room temperature and had much microbes. The results are shown in Figure 8. A continuous high pressure Xe lamp whose input power was 500 W again used for the UV irradiation. The transmittance increased only by filteration of the solution with 0.22 um membrane and more |