Next Generation Laser Glass for Nuclear Fusion. B. PENG* and Teturo IZUMITANI* (Received April 28, 1993)

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1 (1234) Next Generation Laser Glass for Nuclear Fusion December 1993 Laser Original Next Generation Laser Glass for Nuclear Fusion B. PENG* and Teturo IZUMITANI* (Received April 28, 1993) The stable borosilico-phosphate new base glass which has good chemical durability, low expansion coefficient, low nonlinear refraction index and can dope high concentration of dopant has been developed. Yb3+-Er3+ codoped borosilico-phosphate glass probably is candidates for economical nuclear fusion system pumping by diode laser because it can satisfy the requirements of K. Naito (Osaka Univ.) for economical nuclear fusion system which are (a) a thermal shock parameter large than 3 W/cm, (b) saturation parameter of 10 J/cm2, (c) fluorescence lifetime longer than 4 ms, (d) a nonlinear refraction index small than 3 X esu. The strong fluorescence intensity in Yb3+-Er3+ codoped borosilico-phosphate glass at 1.54 Đm also can be used for eye safe range finder laser. The Yb3+ doped special phosphate glass which has a high concentration of modifier was developed. It also may be used for economical nuclear fusion system since it has some advantages below: a)the lifetimes are almost 5 times longer than Nd3+ doped phosphate glass (LHG8) which is using for nuclear fusion system, b)the emission intensities are stronger than LHG8, c)the emission wavelength are close that of LHG8. The energy transfer and energy backtransfer in Yb3+ Er3+ and Yb3+-Nd3+ systems were investigated. Key Word: Laser glasses for nuclear fusion, Yb3+ Er3+ doped glasses, Yb3+ doped glasses, Nd3+-Yb3+ doped glasses, Judd-Ofelt theory, Fluorescence properties, Various glasses, Energy transfer analysis. 1. Introduction Nd3+ doped phosphate glass LHG8 and HAP4 are well known for nuclear fusion laser glass. But the lifetime of Nd3+ doped glasses is about 300 Đs and too short for economical nuclear fusion system. According to the requirements of Nait1), new solid state laser material pumped with diode laser for nuclear fusion should have the following properties: a) a thermal shock parameter larger than 3W/cm, b) saturation paramater of 10J/cm2, c) a fluorescence lifetime longer than 4ms, d) a nonlinear index of refractive smaller than 3 X esu, d) it can be pumped by diode laser. For reaching this purpose, the rare earth elements, ex. Yb3+, Er3+, Tm3+ and Ho3+ were examined. The laser wavelength of Tm3+ and Ho3+, 1.8 Đm and 2 Đm respectively, are too long for getting visible laser wavelength using non-linear optical crystals. Yb3+ and Er3+ ions satisfy the conditions suggested by Naito. In this work, the stable borosilico-phosphate new base glass which has good chemical property, good thermal property, low nonlinear refraction index and can dope high concentration of dopant has been developed. The fluorescence properties of Yb3+ and Er3+ ions in various glasses * Izumitani Special Lab., R&D Center, HOYA Corporation (3-3-1 Musashino, Akishimashi, Tokyo 196, Japan) \

2 Vol.21, No. 12 The Review of Laser Engineering (1235) Table I The composition of base glasses in this study were systematically studied. The sensitizers Nd3+ and Yb3+ are discussed and the energy transfer analysis for Yb3+ Er3+ system and Nd3+-Yb3+ system were investigated. According to the research of Yb3+ in various glasses and recent research of Yb3+ in Apatite2,3,4), we found that the spontaneousemission probablity and emission cross section of Yb3+ were mainly controlled by the polorizability of base glass and the higher polorized base material gives higher emission cross section for Yb3+. Therefore, the Yb3+ doped special phosphate glass which has a high concentration of modifier has been developed. 2. Experiment Procedure The batch compositions of the glasses are listed in Table I. Highly pure materials were used to eliminate the contamination by impurities. Fluoride and fluorophosphate glasses were prepared by melting the batch materials in glassy carbon crucible in nonreactive gas atomsphare inside a silica tube. After melting, the glass was rapidly cooled to the glass transition temperature and annealed. Aluminate and gallate glasses were melted in a platinum crucible under dry nitrogen atomsphare control. Then glasses were casted in graphite mould and annealed. Silicate, germanate and phosphate air. The melt was poured into graphite mould and annealed. All of samples were fabricated to size of 25 ~25 ~5 mm3 and optically polished. The absorption spectra were measured by HITACHI330 spectrophotometer at room temperature. The emission spectra were obtained by exciting the samples with LD804nm, LD980 nm and Xenon Lamp. The light from the light source was chopped at 80 Hz and focused to the 5X25 cm2 face of sample. The position of 1 mm from the edge was excited to minimize the reabsorption of emission. The emission from the sample was focussed to a monochromator and detected by Ge detector cooled at 77K. The signal was intensified with a lock-in amplifier and processed by a computer. The fluorsecnce lifetimes were measured by exciting the samples with a Nd: YAG pulse laser and a Xenon lamp and detecting by S-1 photomultiplier tube and Ge detector. The fluorescence decay curves were recorded and averaged with a computer controlled transient digitizer. The energy transfer efficiencies were calculated by Reisfeld equation5): where r is a donor's lifetime in donor and accepter codoped material, Ė0 is the lifetime of donor without an accepter. glasses were melted in a platinum crucible in

3 (1236) Next Generation Laser Glass for Nuclear Fusion December 1993 Table II Comparison of laser spectroscopic properties of Nd3+4F3/2 level in various glasses Table III Comparison of laser spectroscopic properties of Yb3+4F5/2 level in various glasses Table IV Comparison of laser spectroscopic properties of Er3+13/2 level in various glasses 3. Results and discussion 3.1 Er3+ laser glass Er3+ is an attractive element for glass laser, since it generates radiation of the eye-safe wavelength, 1.53,ƒÊm6). But the efficiency of only Er3+ doped glass laser is low because the Er3+ has weak absorption band in the visible range and acts as a three level system. Table II to IV presented the comparison of laser spectroscopic properties of Nd3+4F3/2 level, Yb3+ 2F 5I2 level and Er3+13/2 level in various glasses. The spontaneous-emission probability(ar) and emission cross section (ƒð) of Er3+ and Nd3+ were calculated by Judd-Ofelt theory7'8) based on absorption spectra. The spontaneousemission probability (AR) and emission cross section of Yb3+ were calculated by Fuchtbauer- Ladenburg equation9) and Judd-Ofelt theory8). From the tables, it can be seen that the spontaneous-emission probability (AR) decreases in the order of Nd3+ >Yb3+ >Er3+ and

4 Vol.21, No.12 The Review of Laser Engineering (1237) Table V Yb3 +'s lifetime (1.02ƒÊm) for Yb3+ and Yb3+ +Er3+ doped glasses Er3+ are very efficient in phosphate glass and silicate glass. Figure 1 shows the dependence of Er3+ 13/2 15/2 emission intensities on the concentrations of Yb3+ and Er3+ ions in silicate and phosphate glasses. It can be seen that the Er3+13/2 15/2 emission intensity is strongly dependent on the concentration ratio of Er3+ /Yb3+. The Yb3+ Er3+ codoped phosphate glass gives the highest emission intensity for 13/2 4/15/2 transition. The Yb3+ ions has only one excited 4f maniford (2F5/2 level) located at approximately cm-1. When the excitation Fig. 1 The emission intensity of Er3+ (13/2 15/2) in Yb3+ Er3+ doped silicate and phosphate is performed to the 2F5/2 level of Yb3+ with diode laser operating at 980 nm, the observed glasses. lifetime increases in the order of Nd3+ < Yb3+ Er3+. The change is very remarkable. There- < fore, the higher spontaneous-emission probability(ar) will be obtained from Nd3+, Yb3+ ions and the longer lifetime(ƒñm) will be given by Yb3+, Er3+ ions. Since the energy backtransfer from Er3+ to Nd3+ is very high 10), we investigated Yb3+ Er3+ codoped various glasses. The fluorescence lifetimes of 2F5/2 2F7/2 (Yb3+) transition for Yb3+ doped, Yb3+ Er3+ codoped various glasses and energy transfer efficiencies from Yb3+ to Er3+ in various glasses calculated by equation 1 were shown in Table V. The energy transfer from Yb3+ to Wavelength (nm) Fig. 2 The emission spectrum of Yb3 + (2F5/2 2F7/2) and the absorption spectrum of Er3+ (15/2 11/2) in phosphate glass.

5 (1238) Next Generation Laser Glass for Nuclear Fusion December 1993 Fig. 3 The emission spectrum of Yb3+ (2F5/2 2F7/2) and the absorption spectrum of Er3+ (15/2 11/2) in silicate glass. Fig. 5 The emission spectrum of Er3+ (11/2 15/2) and the absorption spectrum of Yb3+ (2F7/2 2F5/2) in silicate glass. backtransfer from Er3+ to Yb3+ occured at the same time. The fluorescence lifetimes of 11/2 13/2 (Er3+) transition for only Er3+ doped, Er3+ Yb3+ codoped phosphate and silicate glasses were presented in Figure 6-7. The energy backtransfer efficiencies from Er3+ to Yb3+ in phosphate and silicate glasses calculated by equation 1 were also shown in Figure 6-7. It can be seen that the energy backtransfer efficiency Fig. 4 The emission spectrum of Er3+ (11/2 15/2) and the absorption spectrum of Yb3+ (2F7/2 F5/2) in phosphate glass. emission of Yb3+ 2F5/2 2F7/2 transition just overlaps the Er3+ absorption band for 15/ 11/2 transition as shown in Figure 2-3. Q It means that the energy transfer from Yb3+ to Er3+ is possible. But the emission of Er3+ 11/2 15/2 transition also overlaps the Yb3+ absorption band for 2F7/2 2F5/2 transition as shown in Figure 4-5. Therefore, the energy Fig. 6 The lifetimes of Er3+ 11/2 level in Er3+ doped, Yb3+ Er3+ codoped phosphate glasses and energy transfer from Er3+ to Yb3+ in phosphate \

6 Vol.21, No.12 The Review of Laser Engineering (1239) Fig. 7 The lifetimes of Er3+ 11/2 level in Er3+ doped, Yb3+ Er3+ codoped silicate glasses and energy transfer from Er3+ to Yb3+ in silicate glass. Fig. 8 The emission intensity of Er in Er, Yb copdoped P-Si-B glass pumped by diode laser at 980 nm. from Er3+ to Yb3+ in phosphate glass is lower than that in silicate glass and when Er3+ concentration is low the energy backtransfer can be neglected in phosphate glass. These results can explain the phenomenon in Figure 1. When Er3+ concentration is below 0.5cat% the Er3+ 1 3/2 15/2 emission intensity increases with the Er3+ concentration. As the Er3+ concentration increases, the energy backtransfer occurs and the Er3+ 13/2 15/2 emission intensity decreases through the maximum in phosphate glass. Since the energy backtransfer occurs in silicate glass even the Er3+ concentration is low, the Er3+ 13/2 15/2 emission intensity in Yb3+ Er3+ codoped silicate glass is low than that of phosphate glass althrough the energy transfer from Yb3+ to Er3+ are almost the same for silicate glass and phosphate glass. Because the Er3+ absorption band for 15/2 11/2 transition is weak, the Yb3+ concentration should be higher as possible as we can for enhancing efficient Er3+ 13/2 15/2 transition. Therefore, the stable borosilico-phosphate new base glass has been developed which has good chemical property, low expansion coefficient, Fig. 9 The emission spectra of Er3+ (13/2 15/2) doped B-Si-P glass and LEG30. low nonlinear refraction index and can dope high concentration of dopant. Figure 8 shows the dependence of the Er3+ 13/ 15/2 emission intensity on Yb3+ concentrations when Er3+ concentration is a constant in Yb3+ Er3+ codoped borosilico-phosphate glasses. It can be seen that when Yb3+ concentration increases the the Er3+ 13/2 15/2 emission intensity increases. The properties of Yb3+ Er3+ codoped borosilico-phosphate

7 (1240) Next Generation Laser Glass for Nuclear Fusion December 1993 glass will be summarized in Table VIII-IX. The Yb3+ Er3+ codoped borosilico-phosphate glass satisfies the requirements of Naito. It may be the candidate for economical nuclear fusion system. The strong emission intensity at 1.53 Đ m in Yb3+ Er3+ codoped borosilico-phosphate glass also can be used for eye safe range finder laser. Figure 9 shows the emission spectra of Yb3+ Er3+ codoped borosilico-phosphate glass and commercial 1.53 Đm Er3+ laser glass(leg30). The emission intensity of Yb3+ Er3+ codoped borosilico-phosphate glass - is 1.6 times higher than LEG Yb3+ 1aser glass From Table III, it can be seen that aluminate Fig. 11 The dependence of spontanneous emission probability on polarizability of base glass, M:mol. weight, d:density, n:refractive index. glass and phosphate glass have higher spontaneous-emission probability (AR) and emission cross section. Since the aluminate glass cannot be made large enough for application because it is not very stable, the Yb3+ doped phosphate glasses were researched. As discussed previously, the Yb3+ ions has only one excited 4f maniford (2F5/2 level) located at approximately cm-1. Therefore, the effect of concentration quenching should be small. Figure 10 shows the dependence of the Yb3+ 2F5/2 Fig. 12 The emission spectra of Yb doped P-Si-B glass (LY12), high polarizability phosphe glass (ADY18) and Nd doped phosphate glass (LHG8). 2F 7/2 emission intensity on Yb3+ concentrations in borosilico-phosphate glass. It can be observed that when Yb3+ concentration increases the Yb3+ 2F5/2 2F7/2 emission intensity Fig. 10 The emission intensity of Yb in P-Si-B glass pumped by diode laser at 980 nm. increases. Recently, the efficent laser performance of Yb3+: Apatite crystal was \

8 Vol.21, No.12 The Review of Laser Engineering (1241) Table VI Nd3+'s lifetime (1.06 um) for Nd and Nd +Yb dopedg lasses The strong polarizability feature gives rise to a high emission cross section3). According to our research, it also was found that the spontaneous-emission probability (AR) and emission cross section of Yb3+ is determined by polarizability of base glass and the higher spontaneous-emission probability (AR) and emission cross section come from higher polarized base glass as shown in Figure 11. Therefore, the Yb3+ doped special phosphate glass with a high concentration of modifiers was developed. The glass composition is PO2.5 50%, (BO1.5+ALO1.5) 6%, (BaO+SrO+ZnO) 38 % and YbO1.5 6% in cat%. Since the concentration of modifier is very high, the oxygen polarizability of glass is very high. The Yb3+ doped high modifier phosphate glass has higher spontaneous-emission probability(ar), higher emission cross section and higher emission intensity than that of Yb3+ doped common oxide glasses as shown in Figure 11 and Figure 12. Figure 12 also shows the emission spectra of Yb3+ doped high polarized phosphate glass, Yb3+ doped borosilico-phosphate glass and Nd3+ doped phosphate glass (LHG8). Yb3+ doped high polarized phosphate glass has higher emission intensity than that of Nd3+ doped phosphate glass (LHG8). The emission intensity of Yb3+ doped borosilico-phosphate glass is almost the same as Nd3+ doped phosphate glass (LHG8). The properties of Yb3+ doped high polarized phosphate glass and Yb3+ doped borosilico-phosphate glass are shown in Table Although the saturation paramater of them (about 19 J/cm2) are larger than Naito's requirement and lifetime (about 1.6 ms) are shorter than Naito's requirement, they have some advantages below: a) the lifetimes are almost 5 times longer than Nd3+ doped phosphate glass (LHG8) which is using for nuclear fusion system, b) the emission intensities are stronger or the same comparing with LHG8, c) the emission wavelength are close that of LHG8. They also may be used for economical nuclear fusion system. The spontaneous-emission probability (AR) and emission cross section of Yb3+ were mainly controlled by the polorizability of base glass. The higher polorized base material gives higher emission cross section. Since the spontaneous-emission probability- (AR) and emission cross section of Nd3+ are larger than that of Yb3+ as discussed before, we examined the possibility of Nd3+ Yb3+ codoped glasses for economical nuclear fusion system. Table IV shows the fluorescence lifetimes of 4F3/2 11/2 (Nd3+) transition for Nd3+ doped and Nd3+ Yb3+ codoped various glasses, and energy transfer efficiencies from Nd3+ to Yb3+ in various glasses calculated by equation 1. The energy transfer from Nd3+ to Yb3+ is very efficient in aluminate glass, but it cannot borosilico-phosphate glass has good chemical

9 (1242) Next Generation Laser Glass for Nuclear Fusion December 1993 Fig. 13 The emission intensity of Yb in Nd, Yb copdoped P-Si-B glass pumped by Xenon lamp. Fig. 14 The emission spectra of Nd-Yb doped P-Si- B glass (LNY3) and Nd doped phosphate glass (LHG8), pumped by Xe Lamp. make big one for application. Since the property, good thermal property, low nonlinear refraction index and can dope high concentration of dopant, we investigated Nd3+ Yb3+ codoped borosilico-phosphate glasses. Figure -13 shows the dependence of Yb3+ (2F5/2 2F7/2) emission intensity on concentration ratio of Nd3+ and Yb3+ in borosilico-phosphate glass. The Yb3+ 16cat% and Nd3+ 1.6cat% doped borosilicophosphate glass has the highest Yb3+ emission intensity when pumped by Xenon Lamp. The Yb3+ emission spectrum of Yb3+ 16cat% and Nd3+ 1.6cat% doped borosilico-phosphate glass was compared with the Nd3+ emission spectrum of Nd3+ doped phosphate glass (LHG8) as shown in Figure 14. The Yb3+ emission intensity of Yb3+ 16cat% and Nd3+ 1.6cat% doped borosilico-phosphate glass is higher than Nd3+ emission intensity of Nd3+ doped phosphate glass (LHG8). The energy transfer efficiency from Nd3+ to Yb3+ and energy backtransfer efficiency from Yb3+ to Nd3+ in Yb3+ 16cat% and Nd3+ 1.6cat% doped borosilico-phosphate glass were calculated by equation 1 based on fluorescence lifetime data. The results were listed in Table VII. It can be seen that the energy transfer efficiency from Nd3+ to Yb3+ is efficient (80%), but the energy backtransfer is also high (34%). The Yb3+ emission spectrum of Yb3+ 16cat% and Nd3+ 1.6cat% doped borosilico-phosphate glass and Nd3+ emission spectrum of Nd3+ doped phosphate glass (LHG8) pumped by diode laser at 804 nm were illustrated in Figure 15. The Yb3+ emission intensity of Yb3+ 16cat% and Nd3+ 1.6cat% doped borosilico-phosphate glass is only one third Nd3+ emission intensity of Nd3+ doped phosphate glass (LHG8) due to the high energy backtransfer. In case of pumped by Xenon Lamp, the Yb3+ emission intensity of Yb3+ 16cat% and Nd3+ 1.6cat% doped borosilicophosphate glass is higher than Nd3+ emission intensity of Nd3+ doped phosphate glass (LHG8) because the glass was pumped both Nd3+ and Yb3+. The Yb3+ emission is added from excited Yb3+ pumped by Xenon Lamp. The Yb3+ emission intensity of Yb3+ 16cat% and Nd3+ 1.6cat% doped borosilico-phosphate

10 Vol.21, No.12 The Review of Laser Engineering (1243) Table VII-1 Yb3+'s lifetime (1.02ƒÊm) for Yb and Nd+Yb doped P-Si-B glasses Table VII-2 Nd3+'s lifetime (1.06ƒÊm) for Nd and Nd+Yb doped P-Si-B glasses. The properties of new developed glasses were summarized in Table VIII-IX. 4. Conclusion The Yb3+ Er3+ codoped borosilico-phosphate glass satisfy the requirements of Naito for economical nuclear fusion system which are (a) a thermal shock parameter large than 3 W/cm, (b) saturation parameter of 10 J/cm2, (c) fluorescence lifetime longer than 4ms, (d) a nonlinear refraction index small than 3 ~10-13 esu. It also can be used for eye safe range finder laser. Althrough the saturation paramater of Fig. 15 The emission spectra of Nd-Yb doped P-Si- B glass (LNY3) annd Nd doped phosphate glass (LHG8), pumped by LD804, nm (50 mw). glass is higher than that of Nd3+ doped phosphate glass (LHG8) when pumped by Xenon Lamp. It might be possible to use in replacing Nd3+ glass laser by Xenon Lamp pumping. Yb3+ doped high polarized phosphate glass (about 19 J/cm2) is bigger than Naito's requirement and the lifetime (about 1.6 ms) is shorter than Naito's requirment, It has some advantages below:a) the lifetime is almost 5 times longer than Nd3+ doped phosphate glass (LHG8) which is using for unclear fusion system, b) the emission intensity is stronger than LHG8, c) the emission wavelength is close that of LHG8. It also may be used for economical nuclear fusion \

11 (1244) Next Generation Laser Glass for Nuclear Fusion December 1993 Table VIII The properties of some new laser glasses. Table IX The comparison of new laser glasses for nuclear fusion with Nd3+ doped YAG and phosphate glasses (LHG8, HAP4). Reference 1) K. Naito, T. Kanabe, S. Nakai et al.: Japan J. Appl. Phys. 31 (1992) ) S. A. Payne, W. F. Krupke, L. K. Smith, L. D. DeLoach, W. L. Kawy: Technical Digest of CLEO (1992) ) L. D. DeLoach, S. A. Payne, W. F. Krupke, L. K. Smith, W. L. Kawy, J. B. Tassano, OSA: Proceeding on Advanced Solid-State Lasers (1993) ) L. K. Smith, S. A. Payne, W. F. Krupke, L. D. DeLoach, W. L. Kawy, B. H. T. Chai: OSA Proceeding on Advanced Solid-State Lasers (1993) ) R. Reisfeld, Y. Kolisky, Chem. Phys. Lett. 80 (1981) ) V. P. Gapontsev, S. M. Matitsin, A. A. Isinev, V. B. Kravchenko: Opt. and Laser Technol. Aug. (1982) ) W. T. Canall, P. R. Felds, K. RaJank: J. Chem. Phys. 49 (1968) ) M. J. Weber: Phys. Rev. 157 (1967) ) W. F. Krupke: IEEE J. Quantum Electron. 80 (1974) ) X. Zhou, T. Izumitani: J. Ceramic Society of Japan 101 (1993) 84.