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Preparation techniques for phosphate laser glasses
Journal of Non-Crystalline Solids 80 (1986) 623-629 North-Holland, Amsterdam
623
PREPARATION T E C H N I Q U E S FOR PHOSPHATE LASER GLASSES
JIANG Yasi, Z H A N G Junzhou, XU Wenjiun, MA Zhongtang, YING Xingxin, MAO Hanfen, MAO Sen and LI Jie Shanghai Institute of Optics and Fine Mechanics, Academia Sinica, Shanghai, PRC
To secure the special technological properties of phosphate laser glasses, a series of preparation techniques, including a hermetic platinum melting system with a special stirrer, removal of water from the glass melt, and the forming process have been developed to produce high quality glass blanks of large dimensions for building high-power laser systems. The results are given.
1. Introduction
Nd phosphate laser glasses have been widely used in lasers, especially in high-power laser systems for investigating fusion and laser interaction with plasma [1-4], because of a large stimulated emission cross section, low non-linear refractive index, and good physical, chemical, and mechanical properties. Several kinds of phosphate glasses for different purposes were produced abroad [5,6]. In the early 1960's laser output was obtained from phosphate glass by one of the authors while searching for laser glass hosts. Great attention has been paid to developing phosphate glasses for highpower lasers in the 1970's, and several types of phosphate laser glasses with different performances have been made [7]. High power lasers require that the glassy active medium have large dimensions, high optical homogeneity, and high quantum efficiency. The glass disks used in 30 cm aperture laser amplifiers measure 60 cm along the major axis of the ellipse, and the glass rods are up to 9 cm in diameter and 60 cm in length. The refractive index difference within a glass blank should be less than 2 x 10 6; and the absorption coefficient due to OH-groups at 3.5 ~m should be less than 10 cm -1. Phosphate glass melts are technologically characterized by high volatility, low viscosity at the melting temperature, high tendency towards divitrification, difficult removal of water from melts, and rapid change of viscosity with temperature, etc. There have been some investigations of the technology of silicate laser glasses [8,9], but no systematic reports on phosphate laser glasses. It is necessary to study special preparation tech0022-3093/86/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
624
Jiang Yasi et al. / Preparation techniques]:orphosphate laser glasses
niques, different from those of ordinary glass, to satisfy the demands of developing high power laser systems in China. Based on the previous preparation techniques of optical glasses, a set of platinum melting systems has been developed to produce phosphate laser glass blanks of large dimensions and high quality.
2. General considerations (1) Remelting. Using raw materials with low iron content, the batch material is melted in a highly purified fused quartz crucible to obtain the homogeneous cullet for the sequential melting. The culler is remelted in a platinum crucible in order to avoid the corrosion effect of the batch on the platinum. (2) Platinum system melting. All the melting vessel contacting the glass melt is made of platinum to avoid contamination from the refractory, and to eliminate easily the striae in the melt so that large homogeneous glass blanks with low loss can be produced. (3) Periodic melting. It is not necessary to select continuous melting, since the yield of laser glass is not so large. Loading, fining, homogenizing and cooling can be carried out in the same crucible. (4) Tall crucible. The tall crucible, of which the ratio of height to diameter is over 2, is employed to decrease the gas-melt interface, and reduce volatilization. It will benefit the elimination of striae. (5) Bubbling. During the fining period, dried gas is bubbled through the melt to remove the residual OH-groups and water. (6) Hermetic melting. A platinum dome is put at the lop of the crucible after loading. All the moving parts are sealed with the glass melt to prevent the influence of the outside atmosphere and reduce contamination, so the non-active loss and OH-group content could be decreased. (7) Flow-casting forming. After cooling to a certain temperature, the glass melt flows from the crucible through an extruder tube set at the bottom of the crucible, into a preheated iron mould. Large blanks could be formed in this manner.
3. Model experiment A model experiment, including the flow process of the viscous liquid through the extruder tube from the crucible and the elimination of striae, was conducted~ Based on hydrodynamical principles, the effects of the shape of the crucible, the diameter of the extruder tube, and the liquid level on the flow of liquid were analysed [10,11] and the vessel was designed for practical melting. The flow-casting manner differs from other forming processes in the
Jiang Yasi et al. / Preparation techniques [or phosphate laser glasses
625
requirement of homogenization of the melt. Some local inhomogeneous regions in the forming box of an optical glass continuous tank, or in the ordinary crucible casting process, are allowed to be maintained. In the flow-casting process, most of the melt in the crucible should be homogeneous, and the glass extruded from the crucible must be completely striae-free since the glass is to be cast into a mould. A series of model stirrers, e.g. frames, propellers, helical ribbons and their combinations, were selected for model experiments in the tall crucible in order to observe the motion of the viscous liquid along the horizontal and vertical directions and the elimination of striae during stirring. The experimental results show that the blade of the stirrer used in the tall crucible for homogenizing the glass should be continuous to avoid layer inhomogeneity. A stirrer promoting the flow of glass along the vertical direction would be of benefit to the elimination of striae, and increasing the blade area or raising the rotation speed of the stirrer would be advantageous to homogenization.
4. Experimental melting According to general considerations and the model experimental results described above, a l-liter platinum melting facility shown schematically in fig. 1 was built. Technological issues concerning the furnace structure, the drying of gas and bubbling gas through the melt, the temperature-time regime, hermetization of the melting chamber by using the glass melt, the flow of glass from the crucible, the casting method and motion of the
if
lI Fig. 1. Schematicdiagramof l-litre melter.
Fig. 2. Stress birefringencepattern.
626
Jiang Yasi et al. / Preparation techniques for phosphate laser glasses
casting mould, etc, were settled. A monolithic striae-free glass equal to two thirds of the load of glass in the crucible could be obtained. In addition to the phosphate glass, good results were also achieved in making fluorophosphate glass using this facility. Based on the experimental results for 1 liter melting, a set of moderate-scale melting equipment containing approximately 45 kg of molten glass was constructed. The N21 and N24 type laser glasses of China have been manufactured in this equipment. T h e glasses are fined and bubbled at a viscosity of about 10 P, stirred at 50 P, and formed at 100 P. 70% of the load of glass can be cast to form a monolithic glass without any striae.
5. The quality o| laser glass Some of the glass performance characteristics depend on the glass composition, others are decided by the preparation technology used. T h e quality factors of N21 are as follows: (l) Water content. T h e water content can be determined by the absorption coefficient at 3.5/~ due to OH-groups. It is generally less than 10 cm-1; it can even be 6 cm -1 for some melts. (2) Fluorescence life. T h e lifetime of N2106 (Nd203 = 0.6 wt%) is about 340/~s. For some glasses it can be up to 365 ~s. T h e lifetime of N2122 is 320-340/~S. (3) Stress birefringence. After special annealing, the birefringence was determined at the points symmetrically located at the edge for a 50 x 22 x 5 cm 3 block of glass and an elliptical disk. All the stress birefringence values measured are less than 1 nm/cm. T h o u g h it is difficult to measure the magnitude of the stress birefringence for rods of glass, the stress distribution could be observed by placing the rod between two crossed polaroids. A symmetrical distribution of the stress birefringence is shown in fig. 2 for a 55 x 9 x 8 cm 3 glass block. (4) Optical homogeneity. Using a Zygo interferometer with a H e - N e laser as light source, an elliptical disk of 40 x 20 x 5 cm 3 was used for the test. T h e interference pattern and the three-dimensional optical path difference diagram (fig. 3a, b) show that the peak-valley value of the wave form distortion is 0.061 A for 5 cm optical paths, and the refractive index variation is estimated to be about ] x l0 -6. A 10 cm aperture disk amplifier which consists of 6 elliptical disks of 2.5 cm in thickness was tested by a shearing interferometer. T h e shearing interferogram shown in fig. 4 displays a symmetrical waveform without local distortion. (5) Amplification performance of laser. T h e performance of the laser as an amplifier depends on the laser configuration, optical pumping, and the properties and quality of the laser glass. As an experimental result, fig. 5 shows the relation between the gain coefficient and the optical pumping density for a 10 cm aperture disk amplifier.
Jiang Yasi et al.
Preparation techniques for phosphate laser glasses
627
b Fig. 3. Interference pattern and optical path difference diagram.
Fig. 4. Shearing interferogram of 10 cm aperture disk amplifier.
Jiang Yasi et al. / Preparation techniques for phosphate laser glasses
628
/3 (cm 1)
0.10
0.08 /
0.06
/ --
i /
./ 0.04
-
./ 0.02 - /
/
/
./
/
o/
I
20
I
40
I
60
I
80
I 100
w (J/cm:9
Fig. 5. The relation between gain coefficient /3 and pumping energy density W. O, 2 0 c m amplifier; C), 10 cm amplifier.
6. C o n c l u s i o n
The technology for preparing large phosphate laser glass blanks with good quality has been developed and a moderate-scale melting facility has been constructed. It is characterized by remelting cullet in a platinum system, hermetical melting, special crucible and stirrer, bubbling dry gas through the molten glass, and flow-casting forming. A lot of phosphate laser glasses produced in this manner have been experimentally used in rod laser amplifiers of 7 cm in diameter and disk amplifiers of 20 cm in aperture, and will be used for building high power laser systems in China. Besides being used in the production of phosphate laser glasses and phosphate optical crown glasses, according to the technical characteristics described above, this preparation technique may be suitable for producing optical glasses with volatility, low viscosity and easy devitrification, such as dense barium crown, lanthanum crown, lanthanum flint, fluorophosphate glass and fluoride glass. References [1] 1. Bunkenberg et al., IEEE. J. Quant. Electron. QE-17 (1981) 1620. [2] J.M. McMahon et al., IEEE. J. Quant. Electron. QE-17 (1981) 1629.
Jiang Yasi et al. / Preparation techniques [or phosphate laser glasses [3] [4] [5] [6] [7] [8]
629
C. Yamanaka et al., IEEE. J. Quant. Electron. QE-17 (1981) 1639. I.N. Ross et al., IEEE. J. Quant. Electron. QE-17 (1981) 1653. Optics Laser Techn. 14 (1982) 185. Hoya Glasses for High-power Laser systems, Hoya Corporation (1978). Jiang Zhonghong, Song Xiuyu and Zhang Junzhou, Silicate 3 (198 I) 1. E.N. Bloembergen, Fundamentals of Damage in Laser Glass, Nat. Mater. Adv. Board Publ., NMAB-271, Washington D.C. (July 1970). [9] R.F. Woodcock, Laser Unconvent. Optics J. No. 26 (1970) I. [10] Zhang Junzhou and Jiang Yasi, Bull. Chinese Sil. Soc. 2 (1983) 1. [11] Jiang Yasi, Zhang Junzhou and Ying Xingxin, Glass and Enamel 12 (1984) 8.
623
PREPARATION T E C H N I Q U E S FOR PHOSPHATE LASER GLASSES
JIANG Yasi, Z H A N G Junzhou, XU Wenjiun, MA Zhongtang, YING Xingxin, MAO Hanfen, MAO Sen and LI Jie Shanghai Institute of Optics and Fine Mechanics, Academia Sinica, Shanghai, PRC
To secure the special technological properties of phosphate laser glasses, a series of preparation techniques, including a hermetic platinum melting system with a special stirrer, removal of water from the glass melt, and the forming process have been developed to produce high quality glass blanks of large dimensions for building high-power laser systems. The results are given.
1. Introduction
Nd phosphate laser glasses have been widely used in lasers, especially in high-power laser systems for investigating fusion and laser interaction with plasma [1-4], because of a large stimulated emission cross section, low non-linear refractive index, and good physical, chemical, and mechanical properties. Several kinds of phosphate glasses for different purposes were produced abroad [5,6]. In the early 1960's laser output was obtained from phosphate glass by one of the authors while searching for laser glass hosts. Great attention has been paid to developing phosphate glasses for highpower lasers in the 1970's, and several types of phosphate laser glasses with different performances have been made [7]. High power lasers require that the glassy active medium have large dimensions, high optical homogeneity, and high quantum efficiency. The glass disks used in 30 cm aperture laser amplifiers measure 60 cm along the major axis of the ellipse, and the glass rods are up to 9 cm in diameter and 60 cm in length. The refractive index difference within a glass blank should be less than 2 x 10 6; and the absorption coefficient due to OH-groups at 3.5 ~m should be less than 10 cm -1. Phosphate glass melts are technologically characterized by high volatility, low viscosity at the melting temperature, high tendency towards divitrification, difficult removal of water from melts, and rapid change of viscosity with temperature, etc. There have been some investigations of the technology of silicate laser glasses [8,9], but no systematic reports on phosphate laser glasses. It is necessary to study special preparation tech0022-3093/86/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
624
Jiang Yasi et al. / Preparation techniques]:orphosphate laser glasses
niques, different from those of ordinary glass, to satisfy the demands of developing high power laser systems in China. Based on the previous preparation techniques of optical glasses, a set of platinum melting systems has been developed to produce phosphate laser glass blanks of large dimensions and high quality.
2. General considerations (1) Remelting. Using raw materials with low iron content, the batch material is melted in a highly purified fused quartz crucible to obtain the homogeneous cullet for the sequential melting. The culler is remelted in a platinum crucible in order to avoid the corrosion effect of the batch on the platinum. (2) Platinum system melting. All the melting vessel contacting the glass melt is made of platinum to avoid contamination from the refractory, and to eliminate easily the striae in the melt so that large homogeneous glass blanks with low loss can be produced. (3) Periodic melting. It is not necessary to select continuous melting, since the yield of laser glass is not so large. Loading, fining, homogenizing and cooling can be carried out in the same crucible. (4) Tall crucible. The tall crucible, of which the ratio of height to diameter is over 2, is employed to decrease the gas-melt interface, and reduce volatilization. It will benefit the elimination of striae. (5) Bubbling. During the fining period, dried gas is bubbled through the melt to remove the residual OH-groups and water. (6) Hermetic melting. A platinum dome is put at the lop of the crucible after loading. All the moving parts are sealed with the glass melt to prevent the influence of the outside atmosphere and reduce contamination, so the non-active loss and OH-group content could be decreased. (7) Flow-casting forming. After cooling to a certain temperature, the glass melt flows from the crucible through an extruder tube set at the bottom of the crucible, into a preheated iron mould. Large blanks could be formed in this manner.
3. Model experiment A model experiment, including the flow process of the viscous liquid through the extruder tube from the crucible and the elimination of striae, was conducted~ Based on hydrodynamical principles, the effects of the shape of the crucible, the diameter of the extruder tube, and the liquid level on the flow of liquid were analysed [10,11] and the vessel was designed for practical melting. The flow-casting manner differs from other forming processes in the
Jiang Yasi et al. / Preparation techniques [or phosphate laser glasses
625
requirement of homogenization of the melt. Some local inhomogeneous regions in the forming box of an optical glass continuous tank, or in the ordinary crucible casting process, are allowed to be maintained. In the flow-casting process, most of the melt in the crucible should be homogeneous, and the glass extruded from the crucible must be completely striae-free since the glass is to be cast into a mould. A series of model stirrers, e.g. frames, propellers, helical ribbons and their combinations, were selected for model experiments in the tall crucible in order to observe the motion of the viscous liquid along the horizontal and vertical directions and the elimination of striae during stirring. The experimental results show that the blade of the stirrer used in the tall crucible for homogenizing the glass should be continuous to avoid layer inhomogeneity. A stirrer promoting the flow of glass along the vertical direction would be of benefit to the elimination of striae, and increasing the blade area or raising the rotation speed of the stirrer would be advantageous to homogenization.
4. Experimental melting According to general considerations and the model experimental results described above, a l-liter platinum melting facility shown schematically in fig. 1 was built. Technological issues concerning the furnace structure, the drying of gas and bubbling gas through the melt, the temperature-time regime, hermetization of the melting chamber by using the glass melt, the flow of glass from the crucible, the casting method and motion of the
if
lI Fig. 1. Schematicdiagramof l-litre melter.
Fig. 2. Stress birefringencepattern.
626
Jiang Yasi et al. / Preparation techniques for phosphate laser glasses
casting mould, etc, were settled. A monolithic striae-free glass equal to two thirds of the load of glass in the crucible could be obtained. In addition to the phosphate glass, good results were also achieved in making fluorophosphate glass using this facility. Based on the experimental results for 1 liter melting, a set of moderate-scale melting equipment containing approximately 45 kg of molten glass was constructed. The N21 and N24 type laser glasses of China have been manufactured in this equipment. T h e glasses are fined and bubbled at a viscosity of about 10 P, stirred at 50 P, and formed at 100 P. 70% of the load of glass can be cast to form a monolithic glass without any striae.
5. The quality o| laser glass Some of the glass performance characteristics depend on the glass composition, others are decided by the preparation technology used. T h e quality factors of N21 are as follows: (l) Water content. T h e water content can be determined by the absorption coefficient at 3.5/~ due to OH-groups. It is generally less than 10 cm-1; it can even be 6 cm -1 for some melts. (2) Fluorescence life. T h e lifetime of N2106 (Nd203 = 0.6 wt%) is about 340/~s. For some glasses it can be up to 365 ~s. T h e lifetime of N2122 is 320-340/~S. (3) Stress birefringence. After special annealing, the birefringence was determined at the points symmetrically located at the edge for a 50 x 22 x 5 cm 3 block of glass and an elliptical disk. All the stress birefringence values measured are less than 1 nm/cm. T h o u g h it is difficult to measure the magnitude of the stress birefringence for rods of glass, the stress distribution could be observed by placing the rod between two crossed polaroids. A symmetrical distribution of the stress birefringence is shown in fig. 2 for a 55 x 9 x 8 cm 3 glass block. (4) Optical homogeneity. Using a Zygo interferometer with a H e - N e laser as light source, an elliptical disk of 40 x 20 x 5 cm 3 was used for the test. T h e interference pattern and the three-dimensional optical path difference diagram (fig. 3a, b) show that the peak-valley value of the wave form distortion is 0.061 A for 5 cm optical paths, and the refractive index variation is estimated to be about ] x l0 -6. A 10 cm aperture disk amplifier which consists of 6 elliptical disks of 2.5 cm in thickness was tested by a shearing interferometer. T h e shearing interferogram shown in fig. 4 displays a symmetrical waveform without local distortion. (5) Amplification performance of laser. T h e performance of the laser as an amplifier depends on the laser configuration, optical pumping, and the properties and quality of the laser glass. As an experimental result, fig. 5 shows the relation between the gain coefficient and the optical pumping density for a 10 cm aperture disk amplifier.
Jiang Yasi et al.
Preparation techniques for phosphate laser glasses
627
b Fig. 3. Interference pattern and optical path difference diagram.
Fig. 4. Shearing interferogram of 10 cm aperture disk amplifier.
Jiang Yasi et al. / Preparation techniques for phosphate laser glasses
628
/3 (cm 1)
0.10
0.08 /
0.06
/ --
i /
./ 0.04
-
./ 0.02 - /
/
/
./
/
o/
I
20
I
40
I
60
I
80
I 100
w (J/cm:9
Fig. 5. The relation between gain coefficient /3 and pumping energy density W. O, 2 0 c m amplifier; C), 10 cm amplifier.
6. C o n c l u s i o n
The technology for preparing large phosphate laser glass blanks with good quality has been developed and a moderate-scale melting facility has been constructed. It is characterized by remelting cullet in a platinum system, hermetical melting, special crucible and stirrer, bubbling dry gas through the molten glass, and flow-casting forming. A lot of phosphate laser glasses produced in this manner have been experimentally used in rod laser amplifiers of 7 cm in diameter and disk amplifiers of 20 cm in aperture, and will be used for building high power laser systems in China. Besides being used in the production of phosphate laser glasses and phosphate optical crown glasses, according to the technical characteristics described above, this preparation technique may be suitable for producing optical glasses with volatility, low viscosity and easy devitrification, such as dense barium crown, lanthanum crown, lanthanum flint, fluorophosphate glass and fluoride glass. References [1] 1. Bunkenberg et al., IEEE. J. Quant. Electron. QE-17 (1981) 1620. [2] J.M. McMahon et al., IEEE. J. Quant. Electron. QE-17 (1981) 1629.
Jiang Yasi et al. / Preparation techniques [or phosphate laser glasses [3] [4] [5] [6] [7] [8]
629
C. Yamanaka et al., IEEE. J. Quant. Electron. QE-17 (1981) 1639. I.N. Ross et al., IEEE. J. Quant. Electron. QE-17 (1981) 1653. Optics Laser Techn. 14 (1982) 185. Hoya Glasses for High-power Laser systems, Hoya Corporation (1978). Jiang Zhonghong, Song Xiuyu and Zhang Junzhou, Silicate 3 (198 I) 1. E.N. Bloembergen, Fundamentals of Damage in Laser Glass, Nat. Mater. Adv. Board Publ., NMAB-271, Washington D.C. (July 1970). [9] R.F. Woodcock, Laser Unconvent. Optics J. No. 26 (1970) I. [10] Zhang Junzhou and Jiang Yasi, Bull. Chinese Sil. Soc. 2 (1983) 1. [11] Jiang Yasi, Zhang Junzhou and Ying Xingxin, Glass and Enamel 12 (1984) 8.