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UV laser-induced damage tolerance measurements of CsB3O5 crystals and its application for UV light generation
Optical Materials 31 (2008) 461–463
Contents lists available at ScienceDirect
Optical Materials journal homepage: www.elsevier.com/locate/optmat
UV laser-induced damage tolerance measurements of CsB3O5 crystals and its application for UV light generation D. Rajesh a,b,*, M. Yoshimura a,c, T. Eiro a, Y. Mori a,c, T. Sasaki a,c, R. Jayavel b, T. Kamimura d, T. Katsura e, T. Kojima e, J. Nishimae e, K. Yasui e a
Graduate School of Engineering, Osaka University, 2-1 Yamada-Oka, Suita, Osaka 565 0871, Japan Crystal Growth Centre, Anna University, Chennai 600 025, India c JST, CREST, 2-1 Yamada-oka, Suita, Osaka 565-0871, Japan d Department of Electronics, Information and Communication Engineering, Osaka Institute of Technology, 5-16-1 Ohmiya, Asahi-ku, Osaka 535-8585, Japan e Advanced Technology R&D Center, Mitsubishi Electric Corporation, 8-1-1 Tsukaguchi-Honmachi, Amagasaki, Hyogo 661-8661, Japan b
a r t i c l e
i n f o
Article history: Received 18 January 2008 Received in revised form 23 April 2008 Accepted 3 July 2008 Available online 28 August 2008 PACS: 42.65.k 42.70.Mp 42.65.Ky 42.62.Cf 42.72.Bj
a b s t r a c t Cesium triborate, CsB3O5 (CBO), is an excellent nonlinear optical crystal for ultraviolet light generation. CBO crystals can be grown by top seeded solution growth from self flux solutions. The growth of large-size crystal is difficult because of the multi-nucleation and dissolution of seed during growth. These problems have been avoided and large size crystals (159.2 g) of size 50 45 45 mm3 (a b c) have been grown. The bulk laser-induced damage threshold (LIDT) of CBO was determined using an 1-on-1 technique by the third-harmonic (355 nm) light of longitudinal single-mode Q-switch Nd:YAG laser. The bulk LIDT was found to be two times that of fused quartz for a 6 ns pulse. The as-grown CBO crystal has been used to generate the highest ever reported third-harmonic power of 103 W for the case of a single-pass frequency-conversion system using a high-power Nd:YAG laser. Ó 2008 Elsevier B.V. All rights reserved.
Keywords: Top-seeded solution growth Borates Nonlinear optic materials Harmonic generators UV sources Industrial applications
1. Introduction Solid-state lasers emitting in the deep-UV light are in great demand because of the ease in operation and cost effectiveness. The properties of the nonlinear optical crystals play an important factor in the performance of such solid-state UV lasers. Borate crystals are usually used for high-power UV generation because of their relatively high tolerance to laser-induced damage threshold, large nonlinear optical coefficients and adequate transparency in the UV region. Cesium triborate, CsB3O5 (CBO), is one such important nonlinear optical borate crystal mainly used for the third-harmonic generation (THG) of the Nd:YAG laser [1]. This crystal shows superior properties than the widely used lithium triborate (LBO) crystal [2]. It has a wide optical transparency range (167–3400 nm) [3], * Corresponding author. Present address: Graduate School of Engineering, Osaka University, 2-1 Yamada-Oka, Suita, Osaka 565 0871, Japan. E-mail addresses: [email protected], [email protected] (D. Rajesh). 0925-3467/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2008.07.008
and large effective nonlinear optical coefficient for THG process. CBO crystals can be grown from self-flux solutions using topseeded solution growth (TSSG) technique. From our previous investigations we found that for stable growth, near-stoichiometric solutions are suitable [4]. However growth of large-size crystals poses a serious problem because of the undesirable spontaneous nucleation, seed dissolution caused by heavy evaporation of the cesium-rich component and due to the high viscosity of the CBO solution. There are only few reports on the growth and studies of CBO crystals. The largest ever reported CBO crystal was grown by Chang et. al. (162 g) using submerged-seeded growth technique [5]. The CBO crystals grown by the method might have inclusions because of the effect of platinum dipped inside the solution along with the seed. Also it is difficult to control the growth direction of the crystal by the seed orientation in submerged-seeded growth as in TSSG. This type of controlled growth by using a-axis seed was possible using TSSG, so this method was chosen for growth. Using
462
D. Rajesh et al. / Optical Materials 31 (2008) 461–463
TSSG, large CBO crystal of 50 45 45 mm3 (a b c) and weighing about 159.2 g has been grown. CBO is mainly used for the third harmonic of the Nd:YAG laser but there are no reports on the damage tolerance data at these wavelengths. The LIDT measurements were carried out for the CBO crystals at 355 nm and the LIDT was found to be about two times that of fused quartz. The asgrown CBO crystal has been used to generate the highest ever reported third-harmonic power of 103 W by single-pass configuration using a fundamental Nd:YAG laser power of 295.2 W. 2. Experiments and discussion 2.1. Growth of large size crystals CBO crystals can be grown from various compositions of selfflux solutions by TSSG technique. We attempted the growth of CBO crystals from 74 mol% B2O3 (near stoichiometric) solution as this composition was found to be appropriate for stable growth from our previous investigations [4]. The starting materials Cs2CO3 and B2O3 (3 N purity) in the ratio of 74 mol% B2O3 were dissolved in pure water to release carbon dioxide by the neutralization process. The dried growth material was charged in a platinum crucible and heated in steps of various temperatures. The homogenized charge was heated to about 30 °C higher than the saturation temperature and stirred with a platinum paddle for a few hours. The solution was cooled to saturation temperature and crystal growth was started. A seed-rotation rate of 60 rpm and cooling rate of 0.1 °C/day were used for the growth of CBO crystals. When growth was attempted we found that the main problems during the growth of large-size CBO crystals are evaporation of the cesiumrich component leading to spontaneous nucleation and the dissolution of the seed during growth. The cesium-rich volatiles posed a problem by attacking the seed crystal and caused the dissolution of the seed. Within a period of one week the seed got dissolved and the growing crystals fell into the solution. During growth the seed crystal was covered with a platinum foil so that no part of the seed except that covered with the platinum foil was exposed to the harmful volatiles. This reduced the dissolution of the seed crystals. By using a-axis seed and by adjusting the temperature profile in the furnace, large size optical quality crystals were grown. This control of growth by seed direction is not possible in the case of submerged-seeded technique. Using TSSG, large CBO crystal of size 50 45 45 mm3 (a b c) and weighing about 159.2 g has been grown (Fig. 1).
There is no report on the bulk LIDT data of the CBO crystals in the ultraviolet region. The damage threshold has been evaluated using an 1-on-1 technique by the third-harmonic (355 nm) light of longitudinal single-mode Q-switch Nd:YAG laser (Continuum, Powerlite Plus). We measured the damage tolerance of CBO and fused quartz (Shin-Etsu, OX) with the same experimental setup. By using reference data for fused quartz, the absolute damage tolerance value of CBO was estimated. The setup for the damage threshold is as shown in Fig. 2. The laser beam was incident normal to the (0 1 1) face of an optically polished CBO crystal and was polarized along the a-axis. The laser beam was focused to a point 5 mm inside the crystal and prior to every single shot, the irradiation site was checked with a He–Ne laser to ensure that there was no newly created scatters at the focal point. Damage was considered to have occurred when the scatters appeared in the crystal when illuminated with a He–Ne laser. The crystal was moved 1 mm each time a new laser pulse was irradiated. The damage threshold of CBO crystal was found to be about two times that of fused quartz. The damage threshold of fused quartz (OX) at 355 nm for 0.85 ns pulse is reported as 13 J/cm2 (15.3 GW/cm2) [6]. The pulse width of the 355 nm light was 6 ns, pffiffiffi and the bulk LIDT of the CBO crystal by using the s scaling rule 2 [7] was estimated to be about 11 GW/cm for 355 nm.
2.2. LIDT measurements at 355 nm
2.3. High power UV light generation: 103 W
As the CBO crystal is mostly used for THG of the Nd:YAG laser it is very important to evaluate the damage tolerance at this wavelength as this would be practical for many industrial applications.
Optical devices were cut from as-grown crystals along the angle of (h, u) = (90° and 43.3°) for type-II THG of 1064 nm light. For type-II THG the effective nonlinear coefficient of CBO is
Fig. 1. Large-size optical quality CBO crystal grown by TSSG.
Photo Photo Detector
3 3ω
Oscilloscope
Nd:YAG laser
2 2ω
Single shot mode
KDP crystal
KTP crystal
Lens
5 mm
f= 100 mm
He-Ne laser
CBO crystal Fig. 2. Setup for the LIDT measurement of CBO crystals.
D. Rajesh et al. / Optical Materials 31 (2008) 461–463
high-power 355 nm laser light. The setup for high power UV generation is as shown in Fig. 3. It has a maximum output of 300 W at a pulse repetition rate of 20 kHz. The beam quality factor M2 was found to be 1.3. The pulse duration was 58 ns. A type-I 15mm-long LBO crystal was used to generate second-harmonic generation (SHG). A 10-mm-long CBO crystal was used for the THG. Two dielectric mirrors separate the generated UV beam from residual fundamental and second-harmonic beams. The power was detected by thermopile power meter (OPHIR, 1000 W-V1-SH). Fig. 4 shows the generated THG output as a function of fundamental laser power. The as-grown CBO crystal has been used to generate the highest ever reported third-harmonic power of 103 W for the case of a single-pass frequency-conversion system. A high conversion efficiency of 34.6% was obtained from x to 3x. At different focusing condition the stability of the output was confirmed at 100 W over 20 min duration.
Q-switched diode-pumped Nd:YAG MOPA system Output power: ~300W Polarization: Liner Beam quality: M2=1.2 Repetition rate: 20kHz
THG crystal (TYPE-II CBO) 10 mm
355nm
SHG crystal (TYPE-I LBO) 15 mm
532nm
463
1064nm
Fig. 3. Setup for the high power UV generation.
3. Conclusions In summary we reported on the growth of large-size CBO single crystals by TSSG method. We were able to reduce the formation of multi-nucleation and also the seed dissolution and large CBO crystal of 50 45 45 mm3 (a b c) and weighing about 159.2 g has been grown by TSSG. The LIDT of CBO at 355 nm for a 6 ns pulse was found to be two times that of fused quartz. The as-grown CBO crystal has been used to generate the third-harmonic power of 103 W by using a fundamental Nd:YAG laser power of 295.2 W. Our results suggest that CBO is a potential nonlinear optical crystal for high power UV generation.
355-mm output power (w)
120
100
80
60
Acknowledgements
40
20
0 0
50
100
150
200
250
300
Fundamental laser power (w) Fig. 4. The THG power generated using CBO crystals as a function of the fundamental frequency.
deff = 1.15 pm/V, its angular acceptance is 2.7 mrad cm and its walk off angle is 16.2 mrad [2]. These crystals were optically polished and were uncoated. The surface hygroscopic property of CBO is more than that of LBO. We have suppressed the hygroscopic surface problem of CBO by treating in an inert ambient and heating process. The experiments are in progress and will be published in another paper. A high-power diode-pumped Nd:YAG laser developed by Mitsubishi Electric corporation was used to generate
One of the authors (D. Rajesh) would like to thank the Ministry of Education, Culture, Sports, Science and Technology, Japan for the award of Japanese Government Scholarship through Ministry of Human Resource development, India. He is also thankful to Anna University, India to carry out this work in Japan. The authors are grateful for the support from New Energy and Industrial Technology Development Organization (NEDO). References [1] Y. Wu, T. Sasaki, S. Nakai, A. Yokotani, H. Tang, C. Chen, Appl. Phys. Lett. 62 (1993) 2614. [2] H. Kitano, T. Matsui, K. Sato, N. Ushiyama, M. Yoshimura, Y. Mori, T. Sasaki, Opt. Lett. 28 (2003) 263. [3] Y. Kagebayashi, Y. Mori, T. Sasaki, Bull. Mater. Sci. 22 (1999) 971. [4] T. Saji, N. Hisaminato, M. Nishioka, M. Yoshimura, Y. Mori, T. Sasaki, J. Cryst. Growth 274 (2005) 183. [5] F. Chang, P. Fu, Y. Wu, G. Chen, Z. Yu, C. Chen, J. Cryst. Growth 277 (2005) 298. [6] N. Kuzuu, K. Yoshida, H. Yoshida, T. kamimura, N. Kamisugi, Appl. Opt. 38 (1999) 2510. [7] W. Koechner, Solid-State Laser Engineering, fifth ed., Springer, Berlin, 1999. p. 681.
Contents lists available at ScienceDirect
Optical Materials journal homepage: www.elsevier.com/locate/optmat
UV laser-induced damage tolerance measurements of CsB3O5 crystals and its application for UV light generation D. Rajesh a,b,*, M. Yoshimura a,c, T. Eiro a, Y. Mori a,c, T. Sasaki a,c, R. Jayavel b, T. Kamimura d, T. Katsura e, T. Kojima e, J. Nishimae e, K. Yasui e a
Graduate School of Engineering, Osaka University, 2-1 Yamada-Oka, Suita, Osaka 565 0871, Japan Crystal Growth Centre, Anna University, Chennai 600 025, India c JST, CREST, 2-1 Yamada-oka, Suita, Osaka 565-0871, Japan d Department of Electronics, Information and Communication Engineering, Osaka Institute of Technology, 5-16-1 Ohmiya, Asahi-ku, Osaka 535-8585, Japan e Advanced Technology R&D Center, Mitsubishi Electric Corporation, 8-1-1 Tsukaguchi-Honmachi, Amagasaki, Hyogo 661-8661, Japan b
a r t i c l e
i n f o
Article history: Received 18 January 2008 Received in revised form 23 April 2008 Accepted 3 July 2008 Available online 28 August 2008 PACS: 42.65.k 42.70.Mp 42.65.Ky 42.62.Cf 42.72.Bj
a b s t r a c t Cesium triborate, CsB3O5 (CBO), is an excellent nonlinear optical crystal for ultraviolet light generation. CBO crystals can be grown by top seeded solution growth from self flux solutions. The growth of large-size crystal is difficult because of the multi-nucleation and dissolution of seed during growth. These problems have been avoided and large size crystals (159.2 g) of size 50 45 45 mm3 (a b c) have been grown. The bulk laser-induced damage threshold (LIDT) of CBO was determined using an 1-on-1 technique by the third-harmonic (355 nm) light of longitudinal single-mode Q-switch Nd:YAG laser. The bulk LIDT was found to be two times that of fused quartz for a 6 ns pulse. The as-grown CBO crystal has been used to generate the highest ever reported third-harmonic power of 103 W for the case of a single-pass frequency-conversion system using a high-power Nd:YAG laser. Ó 2008 Elsevier B.V. All rights reserved.
Keywords: Top-seeded solution growth Borates Nonlinear optic materials Harmonic generators UV sources Industrial applications
1. Introduction Solid-state lasers emitting in the deep-UV light are in great demand because of the ease in operation and cost effectiveness. The properties of the nonlinear optical crystals play an important factor in the performance of such solid-state UV lasers. Borate crystals are usually used for high-power UV generation because of their relatively high tolerance to laser-induced damage threshold, large nonlinear optical coefficients and adequate transparency in the UV region. Cesium triborate, CsB3O5 (CBO), is one such important nonlinear optical borate crystal mainly used for the third-harmonic generation (THG) of the Nd:YAG laser [1]. This crystal shows superior properties than the widely used lithium triborate (LBO) crystal [2]. It has a wide optical transparency range (167–3400 nm) [3], * Corresponding author. Present address: Graduate School of Engineering, Osaka University, 2-1 Yamada-Oka, Suita, Osaka 565 0871, Japan. E-mail addresses: [email protected], [email protected] (D. Rajesh). 0925-3467/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2008.07.008
and large effective nonlinear optical coefficient for THG process. CBO crystals can be grown from self-flux solutions using topseeded solution growth (TSSG) technique. From our previous investigations we found that for stable growth, near-stoichiometric solutions are suitable [4]. However growth of large-size crystals poses a serious problem because of the undesirable spontaneous nucleation, seed dissolution caused by heavy evaporation of the cesium-rich component and due to the high viscosity of the CBO solution. There are only few reports on the growth and studies of CBO crystals. The largest ever reported CBO crystal was grown by Chang et. al. (162 g) using submerged-seeded growth technique [5]. The CBO crystals grown by the method might have inclusions because of the effect of platinum dipped inside the solution along with the seed. Also it is difficult to control the growth direction of the crystal by the seed orientation in submerged-seeded growth as in TSSG. This type of controlled growth by using a-axis seed was possible using TSSG, so this method was chosen for growth. Using
462
D. Rajesh et al. / Optical Materials 31 (2008) 461–463
TSSG, large CBO crystal of 50 45 45 mm3 (a b c) and weighing about 159.2 g has been grown. CBO is mainly used for the third harmonic of the Nd:YAG laser but there are no reports on the damage tolerance data at these wavelengths. The LIDT measurements were carried out for the CBO crystals at 355 nm and the LIDT was found to be about two times that of fused quartz. The asgrown CBO crystal has been used to generate the highest ever reported third-harmonic power of 103 W by single-pass configuration using a fundamental Nd:YAG laser power of 295.2 W. 2. Experiments and discussion 2.1. Growth of large size crystals CBO crystals can be grown from various compositions of selfflux solutions by TSSG technique. We attempted the growth of CBO crystals from 74 mol% B2O3 (near stoichiometric) solution as this composition was found to be appropriate for stable growth from our previous investigations [4]. The starting materials Cs2CO3 and B2O3 (3 N purity) in the ratio of 74 mol% B2O3 were dissolved in pure water to release carbon dioxide by the neutralization process. The dried growth material was charged in a platinum crucible and heated in steps of various temperatures. The homogenized charge was heated to about 30 °C higher than the saturation temperature and stirred with a platinum paddle for a few hours. The solution was cooled to saturation temperature and crystal growth was started. A seed-rotation rate of 60 rpm and cooling rate of 0.1 °C/day were used for the growth of CBO crystals. When growth was attempted we found that the main problems during the growth of large-size CBO crystals are evaporation of the cesiumrich component leading to spontaneous nucleation and the dissolution of the seed during growth. The cesium-rich volatiles posed a problem by attacking the seed crystal and caused the dissolution of the seed. Within a period of one week the seed got dissolved and the growing crystals fell into the solution. During growth the seed crystal was covered with a platinum foil so that no part of the seed except that covered with the platinum foil was exposed to the harmful volatiles. This reduced the dissolution of the seed crystals. By using a-axis seed and by adjusting the temperature profile in the furnace, large size optical quality crystals were grown. This control of growth by seed direction is not possible in the case of submerged-seeded technique. Using TSSG, large CBO crystal of size 50 45 45 mm3 (a b c) and weighing about 159.2 g has been grown (Fig. 1).
There is no report on the bulk LIDT data of the CBO crystals in the ultraviolet region. The damage threshold has been evaluated using an 1-on-1 technique by the third-harmonic (355 nm) light of longitudinal single-mode Q-switch Nd:YAG laser (Continuum, Powerlite Plus). We measured the damage tolerance of CBO and fused quartz (Shin-Etsu, OX) with the same experimental setup. By using reference data for fused quartz, the absolute damage tolerance value of CBO was estimated. The setup for the damage threshold is as shown in Fig. 2. The laser beam was incident normal to the (0 1 1) face of an optically polished CBO crystal and was polarized along the a-axis. The laser beam was focused to a point 5 mm inside the crystal and prior to every single shot, the irradiation site was checked with a He–Ne laser to ensure that there was no newly created scatters at the focal point. Damage was considered to have occurred when the scatters appeared in the crystal when illuminated with a He–Ne laser. The crystal was moved 1 mm each time a new laser pulse was irradiated. The damage threshold of CBO crystal was found to be about two times that of fused quartz. The damage threshold of fused quartz (OX) at 355 nm for 0.85 ns pulse is reported as 13 J/cm2 (15.3 GW/cm2) [6]. The pulse width of the 355 nm light was 6 ns, pffiffiffi and the bulk LIDT of the CBO crystal by using the s scaling rule 2 [7] was estimated to be about 11 GW/cm for 355 nm.
2.2. LIDT measurements at 355 nm
2.3. High power UV light generation: 103 W
As the CBO crystal is mostly used for THG of the Nd:YAG laser it is very important to evaluate the damage tolerance at this wavelength as this would be practical for many industrial applications.
Optical devices were cut from as-grown crystals along the angle of (h, u) = (90° and 43.3°) for type-II THG of 1064 nm light. For type-II THG the effective nonlinear coefficient of CBO is
Fig. 1. Large-size optical quality CBO crystal grown by TSSG.
Photo Photo Detector
3 3ω
Oscilloscope
Nd:YAG laser
2 2ω
Single shot mode
KDP crystal
KTP crystal
Lens
5 mm
f= 100 mm
He-Ne laser
CBO crystal Fig. 2. Setup for the LIDT measurement of CBO crystals.
D. Rajesh et al. / Optical Materials 31 (2008) 461–463
high-power 355 nm laser light. The setup for high power UV generation is as shown in Fig. 3. It has a maximum output of 300 W at a pulse repetition rate of 20 kHz. The beam quality factor M2 was found to be 1.3. The pulse duration was 58 ns. A type-I 15mm-long LBO crystal was used to generate second-harmonic generation (SHG). A 10-mm-long CBO crystal was used for the THG. Two dielectric mirrors separate the generated UV beam from residual fundamental and second-harmonic beams. The power was detected by thermopile power meter (OPHIR, 1000 W-V1-SH). Fig. 4 shows the generated THG output as a function of fundamental laser power. The as-grown CBO crystal has been used to generate the highest ever reported third-harmonic power of 103 W for the case of a single-pass frequency-conversion system. A high conversion efficiency of 34.6% was obtained from x to 3x. At different focusing condition the stability of the output was confirmed at 100 W over 20 min duration.
Q-switched diode-pumped Nd:YAG MOPA system Output power: ~300W Polarization: Liner Beam quality: M2=1.2 Repetition rate: 20kHz
THG crystal (TYPE-II CBO) 10 mm
355nm
SHG crystal (TYPE-I LBO) 15 mm
532nm
463
1064nm
Fig. 3. Setup for the high power UV generation.
3. Conclusions In summary we reported on the growth of large-size CBO single crystals by TSSG method. We were able to reduce the formation of multi-nucleation and also the seed dissolution and large CBO crystal of 50 45 45 mm3 (a b c) and weighing about 159.2 g has been grown by TSSG. The LIDT of CBO at 355 nm for a 6 ns pulse was found to be two times that of fused quartz. The as-grown CBO crystal has been used to generate the third-harmonic power of 103 W by using a fundamental Nd:YAG laser power of 295.2 W. Our results suggest that CBO is a potential nonlinear optical crystal for high power UV generation.
355-mm output power (w)
120
100
80
60
Acknowledgements
40
20
0 0
50
100
150
200
250
300
Fundamental laser power (w) Fig. 4. The THG power generated using CBO crystals as a function of the fundamental frequency.
deff = 1.15 pm/V, its angular acceptance is 2.7 mrad cm and its walk off angle is 16.2 mrad [2]. These crystals were optically polished and were uncoated. The surface hygroscopic property of CBO is more than that of LBO. We have suppressed the hygroscopic surface problem of CBO by treating in an inert ambient and heating process. The experiments are in progress and will be published in another paper. A high-power diode-pumped Nd:YAG laser developed by Mitsubishi Electric corporation was used to generate
One of the authors (D. Rajesh) would like to thank the Ministry of Education, Culture, Sports, Science and Technology, Japan for the award of Japanese Government Scholarship through Ministry of Human Resource development, India. He is also thankful to Anna University, India to carry out this work in Japan. The authors are grateful for the support from New Energy and Industrial Technology Development Organization (NEDO). References [1] Y. Wu, T. Sasaki, S. Nakai, A. Yokotani, H. Tang, C. Chen, Appl. Phys. Lett. 62 (1993) 2614. [2] H. Kitano, T. Matsui, K. Sato, N. Ushiyama, M. Yoshimura, Y. Mori, T. Sasaki, Opt. Lett. 28 (2003) 263. [3] Y. Kagebayashi, Y. Mori, T. Sasaki, Bull. Mater. Sci. 22 (1999) 971. [4] T. Saji, N. Hisaminato, M. Nishioka, M. Yoshimura, Y. Mori, T. Sasaki, J. Cryst. Growth 274 (2005) 183. [5] F. Chang, P. Fu, Y. Wu, G. Chen, Z. Yu, C. Chen, J. Cryst. Growth 277 (2005) 298. [6] N. Kuzuu, K. Yoshida, H. Yoshida, T. kamimura, N. Kamisugi, Appl. Opt. 38 (1999) 2510. [7] W. Koechner, Solid-State Laser Engineering, fifth ed., Springer, Berlin, 1999. p. 681.