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Growth and characterization of large KDP crystals for high power lasers
Optical Materials 30 (2007) 88–90 www.elsevier.com/locate/optmat
Growth and characterization of large KDP crystals for high power lasers C. Maunier
a,*
, P. Bouchut b, S. Bouillet a, H. Cabane c, R. Courchinoux a, P. Defossez c, J.-C. Poncetta a, N. Ferriou-Daurios a
a
c
CEA-Centre d’e´tudes scientifiques et techniques d’Aquitaine, BP2 - 33114 Le Barp, France b CEA-LITEN – 17, rue des martyrs, 38054 Grenoble, France Saint-Gobain Crystals & Detectors – 104, route de Larchant, 77140 St. Pierre-les-Nemours, France Available online 14 December 2006
Abstract Large KDP crystals with good optical quality were grown using a rapid growth process. The management of trivalent impurities in solution was studied trying to influence the geometrical behaviour of the crystals, and then improve the production yield. The crystals obtained were systematically controlled to check how the growth parameters affect the optical quality of the material, and then guide the work on the growth process. 2006 Elsevier B.V. All rights reserved. PACS: 81.10.Dn; 42.70.Mp; 42.62.Eh Keywords: Crystal growth; Crystal quality; KDP; Laser; ICF
1. Introduction Laser Megajoule (LMJ) is a high power laser dedicated to plasma physics and inertial confinement fusion (ICF) [1]. For optical switching and frequency conversion applications, LMJ requires large KH2PO4 (KDP) plates, approximately 400 · 400 · 10 mm3, with excellent optical quality for optimal beam propagation. Depending on their use, the plates are cut with a specific orientation regarding the crystal structure. Thus, large crystals with dimensions at least 550 · 550 · 500 mm3 are required. Around 1990, a rapid growth technique has been developed to obtain single crystals from a supersaturated solution with high growth rates [2,3]. This technique has been improved during the 90s to produce large KDP crystals for National Ignition Facility, which is the US laser for ICF [4]. Using this process, one can produce a LMJ-sized *
Corresponding author. Tel.: +33 5 57 04 69 23; fax: +33 5 57 04 53 41. E-mail address: [email protected] (C. Maunier).
0925-3467/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2006.11.019
single crystal within 60 days, instead of 2 years with the traditional growth technique used for KDP. If this process leads to a considerable reduction of the production period, its drawback is a lack of material homogeneity, caused by the polysectorial nature of the crystal growth. The simultaneous growth of several adjoining sectors results in the appearance of boundaries [5], but also in a variation of chemical composition—consequently of refractive index—within the crystal. Such variation is observed because impurities as trivalent ions (Al3+, Fe3+, Cr3+,. . .) are more adsorbed by prismatic sectors than by pyramidal ones [6,7]. This heterogeneity has effect on the transmitted wavefront quality, and therefore on the optical switch function and the frequency conversion efficiency. Other problem is related to the production yield and height-to-width ratio of the crystal. Actually, owing to the cut orientation of the plates, the higher the crystal for a given width, the better the production yield [4]. Following sections describe the efforts undertaken to improve the crystal growth process and illustrate how the characteriza-
C. Maunier et al. / Optical Materials 30 (2007) 88–90
tion of the material can help to optimize the growth parameters. 2. Growth of large KDP single crystals The rapid growth technique and equipment have already been detailed in [4], and will not be discussed here again. The KDP salt used here was ultra-pure regarding some impurities which are an issue for crystal quality (for example, Fe3+, Cr3+ were <200 ppb while Al3+ was <500 ppb). It is well known that trivalent ions as Fe3+, Cr3+ and 3+ Al are inhibitors for the growth of prismatic sectors [2,8]. Burnham et al. have shown that Al3+, unlike Fe3+, does not affect laser damage threshold for levels less than 2000 ppb [9]. Thus, it seems to be a relevant parameter to enhance the aspect ratio, by reducing the prismatic growth rate without lowering the pyramidal one. The first part of this study (1998–2003) was performed with a borosilicate glass growth tank, which is slightly dissolved at high temperatures in the acid KDP solution. It results in dissolution of glass components in the solution, including Al3+ ions [7]. Al3+ content was daily estimated for each growth, based on its initial content in the solution, the crystal dimensions, an empirical segregation coefficient, and the mean dissolution rate which was estimated around 80 ppb per day. An example of those calculations, compared with the sampling analysis performed during the growth, is shown in Fig. 1. During the first-third of the growth, the Al3+ content increased owing to the dissolution of the tank and small size of the crystal. Then, the area of prismatic sides became larger, and more Al3+ was adsorbed while the dissolution of the tank decreased due to temper1400
89
ature lowering. Thus, concentration of Al3+ in the solution was gradually reduced. In those conditions, the only parameter available to drive the ratio was the initial Al3+ content in the solution. Even with just one parameter, its evolution during the growth did not fit exactly the model for each crystal. It has been attributed to a modification of the mean dissolution rate depending on the wearing of the glass tank: due to the dissolution of the surface, the chemical composition of the glass on contact with the solution was slightly different from one growth to the other. This slight modification was also observed between two different glass tanks. Lately (2004), the glass tank was replaced with a polymer one, which is not deteriorated by the solution. It facilitates the driving of the ratio during the growth, because solution became the only provider of impurities, and their content could be easily controlled by preparing it. Al3+ content was initially adjusted to the desired value, and thereafter it was monitored by several samplings during the growth. Then, if needed, it can be increased by doping the solution. Nevertheless, the previous work with the glass tank has shown that Al3+ must be restricted to values less than 1100–1200 ppb in order to avoid the forming of prismatic inclusions (see the open circles on Fig. 1) that deteriorates the optical quality of KDP. The reason is that inclusions forming is favoured by adsorption of particles (which can be Al-containing aggregates) on the growth layer [10]. But even if a polymer tank facilitates the control of impurities and the growth, a clear relation between Al3+ content and aspect ratio is not evident. As it can be seen in Fig. 1, the content was almost the same for two successive growths in a polymer tank (named PT-a and PT-b), while Fig. 2 indicates that aspect ratio was approximately 6% higher for PT-b during the second half of the growth. The initial growth conditions (raw material, Al3+ content,
1.0
1000
PT (2004) PT-a (2005) PT-b (2005)
800 aspect ratio
600
3+
Al in solution (ppb)
1200
400
estimation sampling GT (2003) PT (2004) PT-a (2005) PT-b (2005)
200
0.9
0.8
0 0
10
20
30 40 growth day
50
60
Fig. 1. Al3+ content in solution estimated (solid and broken lines) and measured (symbols) during growths in glass (GT) and polymer (PT) tanks. Open circles are for the days when prismatic inclusions where observed during the growth with glass tank.
0.7 15
20
25
30
35 40 45 growth day
50
55
60
Fig. 2. Evolution of the aspect ratio for crystals grown in polymer tanks with control of the Al3+ content.
90
C. Maunier et al. / Optical Materials 30 (2007) 88–90
component surface (%)
100
80
60
40
3+
GT (1999) - 1200 ppb initial Al 3+ GT (2002) - 800 ppb initial Al 3+ PT (2004) - 1000 ppb constant Al
20 20 40 60 80 140 160 deviation to mean phasematching angle (µrad) Fig. 3. Dhpm measurements for SHG plates cut from crystals grown with glass (GT) and polymer (PT) tanks.
preparing of the solution, supersaturation) were the same for both crystals, except the seed. During the study, both of the growth tanks have allowed us to obtain several KDP crystals as large as 600 · 600 · 500 mm3. Some of those crystals were cut to produce 400 · 400 mm2 switch and SHG plates. 3. Characterization of the grown material Two characteristics are particularly of interest regarding the operating of the laser and considering the heterogeneity of rapid grown material: frequency conversion efficiency and residual birefringence. We have developed a technique that allows a 9 · 9 points mapping of frequency conversion efficiency over the whole surface of a full scale SHG plate, for a given temperature. For each point, it measures the deviation between the mean phase matching direction hpm and laser beam propagation. This deviation is noted Dhpm. KDP plates cut from the crystals grown with glass and polymer tanks were measured to understand the benefit of impurity management on frequency conversion yield. Fig. 3 illustrates that maximal Dhpm can be reduced by a factor of two, switching from glass to polymer tank. It also displays that to a given Dhpm corresponds a higher percentage of the SHG surface if it has been grown in a polymer tank. One can deduce that controlling the impurity content in solution (using a polymer tank) leads to better hpm homogeneity over the whole plate. As birefringence can reduce the efficiency of a Pockels cell by lowering its extinction ratio, some of the switch
plates produced have been controlled with a 60 cm ‘‘crossed-polarizers’’ device. Residual birefringence up to 14 ± 2 nm cm 1 has been measured on polished parts cut from crystals grown with glass tanks. A difference of 2– 3 nm cm 1 was usually detected between two adjoining pyramidal–pyramidal or prismatic–prismatic growth sectors, while it was 4–7 nm cm 1 on both sides of the prismatic–pyramidal boundaries. The assembled Pockels cells where then controlled on a specific bench, and in spite of this residual birefringence, they have shown no significant loss of extinction ratio. In conclusion, residual birefringence induced by the growth process, either in the worse case (glass tank), is not an issue for our need. All the experiments performed give a useful feedback on how to use growth parameters for aspect ratio driving, without deteriorating the optical properties of the material. It is interesting to note that some controls performed (Dhpm, transmitted wavefront) are also used as data in a model called Miro which simulates and predicts the performances of the laser line [11]. 4. Conclusion The past five years were extremely active to stabilize the growth process, and make it suitable for LMJ industrial production. The main challenge was trying to enhance both yield and material quality. Despite the successful growths performed, some questions are still remaining. Work is going on this way, and any new interesting improvement to the growth technique could be integrated in the future into the industrial process. At last, in addition to the good results obtained with the measurements of the plates, the qualification and well operating of the LMJ prototype (named LIL), currently available for plasma physic experiments [12], is also a remarkable proof of the KDP production quality. References [1] C. Cavailler et al., in: IFSA 2003 Proceedings, American Nuclear Soc. Inc., 2004, p. 523. [2] L.N. Rashkovich, KDP-family single crystals, Adam Hilger, Bristol – UK, 1991. [3] N. Zaitseva et al., Sov. Phys. Crystallogr. 36 (1) (1991) 113. [4] N. Zaitseva et al., Prog. Cryst. Growth Charact. Mater. (2001) 1. [5] B. Dam et al., J. Cryst. Growth 74 (1986) 118. [6] S.A. de Vries et al., Phys. Rev. Let. 80 (10) (1998) 2229. [7] N. Zaitseva et al., J. Cryst. Growth 204 (1999) 512. [8] H.V. Alexandru, J. Cryst. Growth 258 (2003) 149. [9] A. Burnham et al., in: SPIE Proceedings, vol. 4347, 2001, p. 373. [10] I. Smolski et al., J. Cryst. Growth 169 (1996) 741. [11] O. Morice, Optical Eng. 42–6 (2003) 1530. [12] J.-M. Di Nicola et al., in: IFSA 2003 Proceedings, American Nuclear Soc. Inc., 2004, p. 558.
Growth and characterization of large KDP crystals for high power lasers C. Maunier
a,*
, P. Bouchut b, S. Bouillet a, H. Cabane c, R. Courchinoux a, P. Defossez c, J.-C. Poncetta a, N. Ferriou-Daurios a
a
c
CEA-Centre d’e´tudes scientifiques et techniques d’Aquitaine, BP2 - 33114 Le Barp, France b CEA-LITEN – 17, rue des martyrs, 38054 Grenoble, France Saint-Gobain Crystals & Detectors – 104, route de Larchant, 77140 St. Pierre-les-Nemours, France Available online 14 December 2006
Abstract Large KDP crystals with good optical quality were grown using a rapid growth process. The management of trivalent impurities in solution was studied trying to influence the geometrical behaviour of the crystals, and then improve the production yield. The crystals obtained were systematically controlled to check how the growth parameters affect the optical quality of the material, and then guide the work on the growth process. 2006 Elsevier B.V. All rights reserved. PACS: 81.10.Dn; 42.70.Mp; 42.62.Eh Keywords: Crystal growth; Crystal quality; KDP; Laser; ICF
1. Introduction Laser Megajoule (LMJ) is a high power laser dedicated to plasma physics and inertial confinement fusion (ICF) [1]. For optical switching and frequency conversion applications, LMJ requires large KH2PO4 (KDP) plates, approximately 400 · 400 · 10 mm3, with excellent optical quality for optimal beam propagation. Depending on their use, the plates are cut with a specific orientation regarding the crystal structure. Thus, large crystals with dimensions at least 550 · 550 · 500 mm3 are required. Around 1990, a rapid growth technique has been developed to obtain single crystals from a supersaturated solution with high growth rates [2,3]. This technique has been improved during the 90s to produce large KDP crystals for National Ignition Facility, which is the US laser for ICF [4]. Using this process, one can produce a LMJ-sized *
Corresponding author. Tel.: +33 5 57 04 69 23; fax: +33 5 57 04 53 41. E-mail address: [email protected] (C. Maunier).
0925-3467/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2006.11.019
single crystal within 60 days, instead of 2 years with the traditional growth technique used for KDP. If this process leads to a considerable reduction of the production period, its drawback is a lack of material homogeneity, caused by the polysectorial nature of the crystal growth. The simultaneous growth of several adjoining sectors results in the appearance of boundaries [5], but also in a variation of chemical composition—consequently of refractive index—within the crystal. Such variation is observed because impurities as trivalent ions (Al3+, Fe3+, Cr3+,. . .) are more adsorbed by prismatic sectors than by pyramidal ones [6,7]. This heterogeneity has effect on the transmitted wavefront quality, and therefore on the optical switch function and the frequency conversion efficiency. Other problem is related to the production yield and height-to-width ratio of the crystal. Actually, owing to the cut orientation of the plates, the higher the crystal for a given width, the better the production yield [4]. Following sections describe the efforts undertaken to improve the crystal growth process and illustrate how the characteriza-
C. Maunier et al. / Optical Materials 30 (2007) 88–90
tion of the material can help to optimize the growth parameters. 2. Growth of large KDP single crystals The rapid growth technique and equipment have already been detailed in [4], and will not be discussed here again. The KDP salt used here was ultra-pure regarding some impurities which are an issue for crystal quality (for example, Fe3+, Cr3+ were <200 ppb while Al3+ was <500 ppb). It is well known that trivalent ions as Fe3+, Cr3+ and 3+ Al are inhibitors for the growth of prismatic sectors [2,8]. Burnham et al. have shown that Al3+, unlike Fe3+, does not affect laser damage threshold for levels less than 2000 ppb [9]. Thus, it seems to be a relevant parameter to enhance the aspect ratio, by reducing the prismatic growth rate without lowering the pyramidal one. The first part of this study (1998–2003) was performed with a borosilicate glass growth tank, which is slightly dissolved at high temperatures in the acid KDP solution. It results in dissolution of glass components in the solution, including Al3+ ions [7]. Al3+ content was daily estimated for each growth, based on its initial content in the solution, the crystal dimensions, an empirical segregation coefficient, and the mean dissolution rate which was estimated around 80 ppb per day. An example of those calculations, compared with the sampling analysis performed during the growth, is shown in Fig. 1. During the first-third of the growth, the Al3+ content increased owing to the dissolution of the tank and small size of the crystal. Then, the area of prismatic sides became larger, and more Al3+ was adsorbed while the dissolution of the tank decreased due to temper1400
89
ature lowering. Thus, concentration of Al3+ in the solution was gradually reduced. In those conditions, the only parameter available to drive the ratio was the initial Al3+ content in the solution. Even with just one parameter, its evolution during the growth did not fit exactly the model for each crystal. It has been attributed to a modification of the mean dissolution rate depending on the wearing of the glass tank: due to the dissolution of the surface, the chemical composition of the glass on contact with the solution was slightly different from one growth to the other. This slight modification was also observed between two different glass tanks. Lately (2004), the glass tank was replaced with a polymer one, which is not deteriorated by the solution. It facilitates the driving of the ratio during the growth, because solution became the only provider of impurities, and their content could be easily controlled by preparing it. Al3+ content was initially adjusted to the desired value, and thereafter it was monitored by several samplings during the growth. Then, if needed, it can be increased by doping the solution. Nevertheless, the previous work with the glass tank has shown that Al3+ must be restricted to values less than 1100–1200 ppb in order to avoid the forming of prismatic inclusions (see the open circles on Fig. 1) that deteriorates the optical quality of KDP. The reason is that inclusions forming is favoured by adsorption of particles (which can be Al-containing aggregates) on the growth layer [10]. But even if a polymer tank facilitates the control of impurities and the growth, a clear relation between Al3+ content and aspect ratio is not evident. As it can be seen in Fig. 1, the content was almost the same for two successive growths in a polymer tank (named PT-a and PT-b), while Fig. 2 indicates that aspect ratio was approximately 6% higher for PT-b during the second half of the growth. The initial growth conditions (raw material, Al3+ content,
1.0
1000
PT (2004) PT-a (2005) PT-b (2005)
800 aspect ratio
600
3+
Al in solution (ppb)
1200
400
estimation sampling GT (2003) PT (2004) PT-a (2005) PT-b (2005)
200
0.9
0.8
0 0
10
20
30 40 growth day
50
60
Fig. 1. Al3+ content in solution estimated (solid and broken lines) and measured (symbols) during growths in glass (GT) and polymer (PT) tanks. Open circles are for the days when prismatic inclusions where observed during the growth with glass tank.
0.7 15
20
25
30
35 40 45 growth day
50
55
60
Fig. 2. Evolution of the aspect ratio for crystals grown in polymer tanks with control of the Al3+ content.
90
C. Maunier et al. / Optical Materials 30 (2007) 88–90
component surface (%)
100
80
60
40
3+
GT (1999) - 1200 ppb initial Al 3+ GT (2002) - 800 ppb initial Al 3+ PT (2004) - 1000 ppb constant Al
20 20 40 60 80 140 160 deviation to mean phasematching angle (µrad) Fig. 3. Dhpm measurements for SHG plates cut from crystals grown with glass (GT) and polymer (PT) tanks.
preparing of the solution, supersaturation) were the same for both crystals, except the seed. During the study, both of the growth tanks have allowed us to obtain several KDP crystals as large as 600 · 600 · 500 mm3. Some of those crystals were cut to produce 400 · 400 mm2 switch and SHG plates. 3. Characterization of the grown material Two characteristics are particularly of interest regarding the operating of the laser and considering the heterogeneity of rapid grown material: frequency conversion efficiency and residual birefringence. We have developed a technique that allows a 9 · 9 points mapping of frequency conversion efficiency over the whole surface of a full scale SHG plate, for a given temperature. For each point, it measures the deviation between the mean phase matching direction hpm and laser beam propagation. This deviation is noted Dhpm. KDP plates cut from the crystals grown with glass and polymer tanks were measured to understand the benefit of impurity management on frequency conversion yield. Fig. 3 illustrates that maximal Dhpm can be reduced by a factor of two, switching from glass to polymer tank. It also displays that to a given Dhpm corresponds a higher percentage of the SHG surface if it has been grown in a polymer tank. One can deduce that controlling the impurity content in solution (using a polymer tank) leads to better hpm homogeneity over the whole plate. As birefringence can reduce the efficiency of a Pockels cell by lowering its extinction ratio, some of the switch
plates produced have been controlled with a 60 cm ‘‘crossed-polarizers’’ device. Residual birefringence up to 14 ± 2 nm cm 1 has been measured on polished parts cut from crystals grown with glass tanks. A difference of 2– 3 nm cm 1 was usually detected between two adjoining pyramidal–pyramidal or prismatic–prismatic growth sectors, while it was 4–7 nm cm 1 on both sides of the prismatic–pyramidal boundaries. The assembled Pockels cells where then controlled on a specific bench, and in spite of this residual birefringence, they have shown no significant loss of extinction ratio. In conclusion, residual birefringence induced by the growth process, either in the worse case (glass tank), is not an issue for our need. All the experiments performed give a useful feedback on how to use growth parameters for aspect ratio driving, without deteriorating the optical properties of the material. It is interesting to note that some controls performed (Dhpm, transmitted wavefront) are also used as data in a model called Miro which simulates and predicts the performances of the laser line [11]. 4. Conclusion The past five years were extremely active to stabilize the growth process, and make it suitable for LMJ industrial production. The main challenge was trying to enhance both yield and material quality. Despite the successful growths performed, some questions are still remaining. Work is going on this way, and any new interesting improvement to the growth technique could be integrated in the future into the industrial process. At last, in addition to the good results obtained with the measurements of the plates, the qualification and well operating of the LMJ prototype (named LIL), currently available for plasma physic experiments [12], is also a remarkable proof of the KDP production quality. References [1] C. Cavailler et al., in: IFSA 2003 Proceedings, American Nuclear Soc. Inc., 2004, p. 523. [2] L.N. Rashkovich, KDP-family single crystals, Adam Hilger, Bristol – UK, 1991. [3] N. Zaitseva et al., Sov. Phys. Crystallogr. 36 (1) (1991) 113. [4] N. Zaitseva et al., Prog. Cryst. Growth Charact. Mater. (2001) 1. [5] B. Dam et al., J. Cryst. Growth 74 (1986) 118. [6] S.A. de Vries et al., Phys. Rev. Let. 80 (10) (1998) 2229. [7] N. Zaitseva et al., J. Cryst. Growth 204 (1999) 512. [8] H.V. Alexandru, J. Cryst. Growth 258 (2003) 149. [9] A. Burnham et al., in: SPIE Proceedings, vol. 4347, 2001, p. 373. [10] I. Smolski et al., J. Cryst. Growth 169 (1996) 741. [11] O. Morice, Optical Eng. 42–6 (2003) 1530. [12] J.-M. Di Nicola et al., in: IFSA 2003 Proceedings, American Nuclear Soc. Inc., 2004, p. 558.