- Email: [email protected]
Performance of a new picosecond KTP optical parametric generator and amplifier
1 July 1998
Optics Communications 152 Ž1998. 347–350
Performance of a new picosecond KTP optical parametric generator and amplifier L. Carrion, J.P. Girardeau-Montaut
)
Laboratoire de Sciences et Ingenierie des Surfaces, UniÕersite´ Claude Bernard – Lyon I, ´ 43 Bd du 11 NoÕembre 1918, 69622 Villeurbanne Cedex, France Received 16 October 1997; accepted 25 March 1998
Abstract Generation and amplification of picosecond laser pulses based on a double pass through potassium titanyl phosphate ŽKTP. crystals pumped by the second harmonic of a mode-locked Nd:YAG laser at 532 nm are reported. This new arrangement allows to obtain broadly tunable pulses from visible Ž700 nm. to infrared Ž2.8 mm. with a higher conversion efficiency Žup to 14%. comparatively to the previous devices. q 1998 Elsevier Science B.V. All rights reserved. PACS: 42.65.Ky
Parametric interactions between different waves propagating through a nonlinear media have been first investigated by Armstrong et al. w1x. Since then, optical parametric devices have been widely developed and represent one of the most successful ways to obtain a widely tunable source of intense radiation. In particular, pulsed optical parametric generators ŽOPG. and amplifiers ŽOPA. can work with low repetition rates. In spectral domains from far UV to near infrared ŽNIR., several groups have developed OPGs by using a one- or multi-pass system principally with b-barium borate ŽBBO. w2x, lithium ŽLBO. w3x, and potassium titanyl phosphate ŽKTP. w4–7x. The use of KTP for NIR applications is advantageous because of its high optical damage, wide transparency range w8x Žfrom 0.35 to 4 mm., and large figure of merit w9x as well as its lack of hygroscopic susceptibility. Generation and amplification of IR pulses has been previously made with different devices. Nevertheless, a maximum conversion effi-
)
E-mail: [email protected]
ciency of less than 12% w4–6x was reached with high pumping intensity Žover 10 mJrpulse. at 532 nm. Gragson et al. obtained near 20% efficiency with a single-pass KTP OPA seeded by white light continuum w7x. In this paper, we report on the performance of a 532 nm pumped picosecond OPGrOPA new device based on a double pass through KTP. Our goal was to reach conversion efficiency higher than those obtained previously by using moderately low pumping energy and reduced size crystals. As noted by Vanherzeele w4x, the gain of parametric amplification over an interaction length l is proportional to l I p , where I p is the pump power flow per unit area. Consequently, it is obvious that decreasing the crystal length and the pump beam size in the same proportions will not change the efficiency of the OPA. This procedure preserves also the angular aperture of parametric interactions given by drl Žwhere d is the diameter of the pump beam., and consequently the output divergence and the spectral width Žcharacterized by noncollinear phase matching. will remain the same. Furthermore, the device we conceived differs from those previously made in that the first pass Žduring which the generation of signal and idler from quantum noise takes place. and the second pass
0030-4018r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII S 0 0 3 0 - 4 0 1 8 Ž 9 8 . 0 0 1 7 7 - 1
(
348
L. Carrion, J.P. Girardeau-Montautr Optics Communications 152 (1998) 347–350
Fig. 1. Schematic representation of the KTP OPGrOPA; BS: beamsplitter; DM: dichroic mirrors; M IR : protected gold mirror; M: mirrors R MA X at 532 nm; Pr: BK7 prisms; P: BK7 plate.
Žamplification. are made with different pump pulses but through the same crystals. This allows to keep reasonable interaction length even for limited crystal size and limited
pump energies. In addition to advantageous financial aspects, this device can be interesting because it takes a minimum of space. We obtained IR picosecond pulses tunable from 0.67 to 2.854 mm with a maximum energy conversion efficiency close to 14%. The experimental device conceived for this work is depicted in Fig. 1. The 1064 nm output of a mode-locked Nd:YAG laser generates s-polarized, 532 nm pump pulses by frequency doubling in a KDP crystal. The pulse duration is 25 ps and the repetition rate is 10 Hz. An attenuator made by two BK7 plates rotating each with opposite angles allows us to modify and to regulate the pumping intensity. A telescope with a compression ratio of 3:1 reduces the beam diameter from 6 mm to 2 mm in order to limit the angular aperture of the laser pump in the crystals. A 50:50 beamsplitter is used to split up the pump pulse into two components. The two flux-grown KTP crystals used here have an aperture of 9 = 5 mm2 with an interaction length of 6 mm. The laser damage threshold of flux-grown KTP being about 10 GWrcm2 w9x, the end faces were uncoated. They are cut for a type II Žo ™ e q o. configuration in the Ž x,z . plane Ž u s 08., with an angle f between the entrance face normal and the Ž x . optical axis equal to 558. The two crystals have opposite angles f and are tilted with opposite angles in such a way that the walk-off effects on the signal in the first and the second crystal are compensated w10x. The two KTP crystals are separated by a distance of 20 cm so that only the central part of the generated pulses overlaps the pump pulse in the second crystal and takes part in the amplification. The part of the pump pulse reflected by the beamsplitter makes a
Fig. 2. Angle tuning curve for amplification in KTP as a function of the crystal orientation Žangle between the surface normal and the incident pump wave vector. and the corresponding effective nonlinear coefficient. The pump wavelength is 532 nm and propagates in the Ž x,z . plane. Although a little discrepancy is observed for higher wavelengths, agreement between theoretical curve and experimental data Ždots. seems quite good.
L. Carrion, J.P. Girardeau-Montautr Optics Communications 152 (1998) 347–350 Table 1 Sellmeier equation coefficients for the optical axes of KTP
nX nY nZ
349
A
B
C
D
The generated signal and idler wavelength satisfy the two collinear phase matching conditions involving energy and momentum conservation:
3.0065 3.0333 3.3134
0.03901 0.04154 0.05694
0.04251 0.04547 0.05658
0.01327 0.01408 0.01682
lp
1
first pass through the crystal and generates signal Žppolarized. and idler Žs-polarized. pulses from quantum noise. After the first pass, a dichroic mirror reflects back the residual pump, while generated pulses are transmitted towards a protected gold mirror Ž R ) 95% from 600 nm to 10 mm.. Then, they are reflected back and amplified through the same crystals by the second part of the pump properly delayed. The tunable output is transmitted through a dichroic mirror. It is spectrally diagnosed by an IR monochromator associated with a lead selenide ŽPbSe. photodetector, and its energy is measured with a joulemeter. Fig. 2 shows the theoretical and experimental angular phase-matching curve of the OPGrOPA, as well as the effective nonlinearity d eff of KTP given by w11x: d eff s d 24 sin u ,
1 s
Ž1.
where d 24 s 3.64 pmrV w12x, and u is the angle between the wave vector of the pulses in the crystal and the Ž x . optical axis.
1 q
ls
li
,
np
lp
ns
s
ls
q
ni
li
,
Ž2.
where l and n denote the wavelength and the refractive indices seen by the propagating pulses. The subscripts ‘p’, ‘s’, and ‘i’ stand for pump, signal and idler, respectively. The refractive indices are calculated from a modified form of the Sellmeier equations given by w13x n2 s A q
B 2
l yC
y Dl2 ,
Ž3.
where l is the wavelength in micrometers. The constants A, B, C, and D are given for the three optical axes of flux grown KTP in Table 1. The total efficiency of the device is given in Fig. 3. The whole irradiance of the pump is 2.5 GWrcm2 , i.e., 1.25 GWrcm2 Ž1 mJ per pulse. for each pass. The loss at the entrance face of each crystal Ž R f 10%. is neglected. This value of irradiance is adequate because it corresponds to the saturation regime for optical parametric generation, and it is less than the optical damage threshold of the components. Maximum total output energy Ž267 mJ per pulse. is obtained for lsignal s 900 nm and lidler s 1.324 mm. This gives a maximum conversion efficiency of 13.6%. Referring to the tuning angle behaviour of the output energy Žright top of Fig. 3., we observe that the efficiency remains
Fig. 3. OPGrOPA total output energy Žsignal q idler. as a function of idler wavelength. The total pump irradiance is 2.5 GWrcm2 . The inset shows the output energy as a function of crystal orientation.
350
L. Carrion, J.P. Girardeau-Montautr Optics Communications 152 (1998) 347–350
higher than 90% of its maximum value and remains stable over a wide angular range D u s 358 Žbetween y98 and y448.. That is, the signal wavelength between 0.815 and 0.991 mm, and the idler wavelength between 1.148 and 1.532 mm will benefit the best conditions for generation and amplification. We observe that the gain decreases sharply for wavelengths above 1.5 mm, and becomes practically zero above 2.8 mm. Also, the effective nonlinearity of KTP decreases as the crystal orientation increases Žcf. Fig. 1.. As a matter of fact, working with limited pumping energy may represent a great disadvantage for far-IR wavelengths as far as it gives far-IR pulses with very poor energy Ž- 50 mJ for li ) 2.3 mm.. Although, conversion efficiencies can be compared to those previously obtained in this spectral range w4–7x. Because of the limited aperture of the crystals, we notice a lowering near the degeneracy point: between 1.02 mm and 1.12 mm there are no generated pulses because the pump pulse cannot propagate through the entire effective length. Nevertheless, some corrections due to dichroic mirrors should be taken into account over the whole spectral range, in particular the fact that their transmission decreases from 90% to 45% above 2.7 mm. These corrections are not accounted for in the results because this effect cannot be cancelled experimentally. Moreover, the efficiency of the OPA is limited by another effect known as grey-tracks w14x. This kind of damage occurs in flux-grown KTP for pumping energies above 1 GWrcm2 and is related to the presence of Ti 3q centers w15x. As a consequence, optical absorption in KTP becomes important near 532 nm and therefore the transmission of pump pulses is reduced. However, grey tracks disappear within a few minutes after the end of irradiation. An investigation of the consequences on generation and amplification is in progress. In conclusion, we have investigated a new double-pass, 532-nm-pumped KTP OPGrOPA system. We obtained tunable NIR pulses from 0.67 to 2.854 mm. Using reduced size crystals Žtotal interaction length of 24 mm. and less pumping energy Ž1 mJ per pulse for each pass., the device conceived allowed us to achieve conversion efficiencies Ž14%. which compares favourably with those of previous
systems. Nevertheless, the performance of such a device is limited by the formation of grey tracks in KTP. Furthermore, it gives very poor energy for far-IR pulses. However this can be sufficient for the kind of applications based on time-resolved spectroscopic methods using sum frequency generation from surfaces and interfaces. Some additional investigations about the spectral width of the output pulses and a comparison with results given by theoretical models will be undertaken.
References w1x J.A. Armstrong, N. Bloembergen, J. Ducuing, P.S. Pershan, Phys. Rev. 127 Ž1962. 1918. w2x J.Y. Huang, J.Y. Zhang, Y.R. Shen, C. Chen, B. Wu, Appl. Phys. Lett. 57 Ž1990. 1961. w3x S. Lin, J.Y. Huang, J. Ling, C. Chen and Y.R. Shen, Appl. Phys. Lett 59 Ž1991. 2805; J.Y. Zhang, J.Y. Huang, Y.R. Shen and C. Chen, J. Opt. Soc. Am. B 10 Ž1993. 1758; X. Liu, Z. Xu, B. Wu and C. Chen, Appl. Phys. Lett. 66 Ž1995. 1446; Z. Xu, X. Liu, D. Deng, Q. Wu, L. Wu, B. Wu, S. Lin, B. Lin and C. Chen, J. Opt. Sov. Am. B 12 Ž1995. 2222. w4x H. Vanherzeele, Appl. Optics 29 Ž1990. 2246. w5x H. He, Q. Zhao, J. Guo, Y. Lu, Y. Liu, J. Wang, Optics Comm. 87 Ž1992. 33. w6x S.A. Reid, Y. Tang, Appl. Optics 35 Ž1996. 1473. w7x D.E. Gragson, D.S. Alavi, G.L. Richmond, Optics Lett. 20 Ž1995. 1991. w8x R.F. Belt, G. Gashurow, Y.S. Liu, Laser Focus 21 Ž1985. 110. w9x J.D. Bierlein, H. Vanherzeele, J. Opt. Soc. Am. B 6 Ž1989. 622. w10x W.R. Bosenberg, W.S. Pelouch, C.L. Tang, Appl. Phys. Lett. 55 Ž1989. 1952. w11x V.G. Dmitriev, C.G. Gurzadyan and D.N. Nikogosyan, Handbook of Nonlinear Optics Crystals, Springer-Verlag, Berlin, 1991, Chap. 3.2.7 w12x H. Vanherzeele, J.D. Bierlein, Optics Lett. 17 Ž1992. 982. w13x CASIX Crystal Guide 1995, Crystals and Materials – Laser Accessories, 24. w14x G.M. Loiacono, D.N. Loiacono, T. McGee, M. Babb, J. Appl. Phys. 72 Ž1992. 2705. w15x M.G. Roelofs, J. Appl. Phys. 65 Ž1989. 4976.
Optics Communications 152 Ž1998. 347–350
Performance of a new picosecond KTP optical parametric generator and amplifier L. Carrion, J.P. Girardeau-Montaut
)
Laboratoire de Sciences et Ingenierie des Surfaces, UniÕersite´ Claude Bernard – Lyon I, ´ 43 Bd du 11 NoÕembre 1918, 69622 Villeurbanne Cedex, France Received 16 October 1997; accepted 25 March 1998
Abstract Generation and amplification of picosecond laser pulses based on a double pass through potassium titanyl phosphate ŽKTP. crystals pumped by the second harmonic of a mode-locked Nd:YAG laser at 532 nm are reported. This new arrangement allows to obtain broadly tunable pulses from visible Ž700 nm. to infrared Ž2.8 mm. with a higher conversion efficiency Žup to 14%. comparatively to the previous devices. q 1998 Elsevier Science B.V. All rights reserved. PACS: 42.65.Ky
Parametric interactions between different waves propagating through a nonlinear media have been first investigated by Armstrong et al. w1x. Since then, optical parametric devices have been widely developed and represent one of the most successful ways to obtain a widely tunable source of intense radiation. In particular, pulsed optical parametric generators ŽOPG. and amplifiers ŽOPA. can work with low repetition rates. In spectral domains from far UV to near infrared ŽNIR., several groups have developed OPGs by using a one- or multi-pass system principally with b-barium borate ŽBBO. w2x, lithium ŽLBO. w3x, and potassium titanyl phosphate ŽKTP. w4–7x. The use of KTP for NIR applications is advantageous because of its high optical damage, wide transparency range w8x Žfrom 0.35 to 4 mm., and large figure of merit w9x as well as its lack of hygroscopic susceptibility. Generation and amplification of IR pulses has been previously made with different devices. Nevertheless, a maximum conversion effi-
)
E-mail: [email protected]
ciency of less than 12% w4–6x was reached with high pumping intensity Žover 10 mJrpulse. at 532 nm. Gragson et al. obtained near 20% efficiency with a single-pass KTP OPA seeded by white light continuum w7x. In this paper, we report on the performance of a 532 nm pumped picosecond OPGrOPA new device based on a double pass through KTP. Our goal was to reach conversion efficiency higher than those obtained previously by using moderately low pumping energy and reduced size crystals. As noted by Vanherzeele w4x, the gain of parametric amplification over an interaction length l is proportional to l I p , where I p is the pump power flow per unit area. Consequently, it is obvious that decreasing the crystal length and the pump beam size in the same proportions will not change the efficiency of the OPA. This procedure preserves also the angular aperture of parametric interactions given by drl Žwhere d is the diameter of the pump beam., and consequently the output divergence and the spectral width Žcharacterized by noncollinear phase matching. will remain the same. Furthermore, the device we conceived differs from those previously made in that the first pass Žduring which the generation of signal and idler from quantum noise takes place. and the second pass
0030-4018r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII S 0 0 3 0 - 4 0 1 8 Ž 9 8 . 0 0 1 7 7 - 1
(
348
L. Carrion, J.P. Girardeau-Montautr Optics Communications 152 (1998) 347–350
Fig. 1. Schematic representation of the KTP OPGrOPA; BS: beamsplitter; DM: dichroic mirrors; M IR : protected gold mirror; M: mirrors R MA X at 532 nm; Pr: BK7 prisms; P: BK7 plate.
Žamplification. are made with different pump pulses but through the same crystals. This allows to keep reasonable interaction length even for limited crystal size and limited
pump energies. In addition to advantageous financial aspects, this device can be interesting because it takes a minimum of space. We obtained IR picosecond pulses tunable from 0.67 to 2.854 mm with a maximum energy conversion efficiency close to 14%. The experimental device conceived for this work is depicted in Fig. 1. The 1064 nm output of a mode-locked Nd:YAG laser generates s-polarized, 532 nm pump pulses by frequency doubling in a KDP crystal. The pulse duration is 25 ps and the repetition rate is 10 Hz. An attenuator made by two BK7 plates rotating each with opposite angles allows us to modify and to regulate the pumping intensity. A telescope with a compression ratio of 3:1 reduces the beam diameter from 6 mm to 2 mm in order to limit the angular aperture of the laser pump in the crystals. A 50:50 beamsplitter is used to split up the pump pulse into two components. The two flux-grown KTP crystals used here have an aperture of 9 = 5 mm2 with an interaction length of 6 mm. The laser damage threshold of flux-grown KTP being about 10 GWrcm2 w9x, the end faces were uncoated. They are cut for a type II Žo ™ e q o. configuration in the Ž x,z . plane Ž u s 08., with an angle f between the entrance face normal and the Ž x . optical axis equal to 558. The two crystals have opposite angles f and are tilted with opposite angles in such a way that the walk-off effects on the signal in the first and the second crystal are compensated w10x. The two KTP crystals are separated by a distance of 20 cm so that only the central part of the generated pulses overlaps the pump pulse in the second crystal and takes part in the amplification. The part of the pump pulse reflected by the beamsplitter makes a
Fig. 2. Angle tuning curve for amplification in KTP as a function of the crystal orientation Žangle between the surface normal and the incident pump wave vector. and the corresponding effective nonlinear coefficient. The pump wavelength is 532 nm and propagates in the Ž x,z . plane. Although a little discrepancy is observed for higher wavelengths, agreement between theoretical curve and experimental data Ždots. seems quite good.
L. Carrion, J.P. Girardeau-Montautr Optics Communications 152 (1998) 347–350 Table 1 Sellmeier equation coefficients for the optical axes of KTP
nX nY nZ
349
A
B
C
D
The generated signal and idler wavelength satisfy the two collinear phase matching conditions involving energy and momentum conservation:
3.0065 3.0333 3.3134
0.03901 0.04154 0.05694
0.04251 0.04547 0.05658
0.01327 0.01408 0.01682
lp
1
first pass through the crystal and generates signal Žppolarized. and idler Žs-polarized. pulses from quantum noise. After the first pass, a dichroic mirror reflects back the residual pump, while generated pulses are transmitted towards a protected gold mirror Ž R ) 95% from 600 nm to 10 mm.. Then, they are reflected back and amplified through the same crystals by the second part of the pump properly delayed. The tunable output is transmitted through a dichroic mirror. It is spectrally diagnosed by an IR monochromator associated with a lead selenide ŽPbSe. photodetector, and its energy is measured with a joulemeter. Fig. 2 shows the theoretical and experimental angular phase-matching curve of the OPGrOPA, as well as the effective nonlinearity d eff of KTP given by w11x: d eff s d 24 sin u ,
1 s
Ž1.
where d 24 s 3.64 pmrV w12x, and u is the angle between the wave vector of the pulses in the crystal and the Ž x . optical axis.
1 q
ls
li
,
np
lp
ns
s
ls
q
ni
li
,
Ž2.
where l and n denote the wavelength and the refractive indices seen by the propagating pulses. The subscripts ‘p’, ‘s’, and ‘i’ stand for pump, signal and idler, respectively. The refractive indices are calculated from a modified form of the Sellmeier equations given by w13x n2 s A q
B 2
l yC
y Dl2 ,
Ž3.
where l is the wavelength in micrometers. The constants A, B, C, and D are given for the three optical axes of flux grown KTP in Table 1. The total efficiency of the device is given in Fig. 3. The whole irradiance of the pump is 2.5 GWrcm2 , i.e., 1.25 GWrcm2 Ž1 mJ per pulse. for each pass. The loss at the entrance face of each crystal Ž R f 10%. is neglected. This value of irradiance is adequate because it corresponds to the saturation regime for optical parametric generation, and it is less than the optical damage threshold of the components. Maximum total output energy Ž267 mJ per pulse. is obtained for lsignal s 900 nm and lidler s 1.324 mm. This gives a maximum conversion efficiency of 13.6%. Referring to the tuning angle behaviour of the output energy Žright top of Fig. 3., we observe that the efficiency remains
Fig. 3. OPGrOPA total output energy Žsignal q idler. as a function of idler wavelength. The total pump irradiance is 2.5 GWrcm2 . The inset shows the output energy as a function of crystal orientation.
350
L. Carrion, J.P. Girardeau-Montautr Optics Communications 152 (1998) 347–350
higher than 90% of its maximum value and remains stable over a wide angular range D u s 358 Žbetween y98 and y448.. That is, the signal wavelength between 0.815 and 0.991 mm, and the idler wavelength between 1.148 and 1.532 mm will benefit the best conditions for generation and amplification. We observe that the gain decreases sharply for wavelengths above 1.5 mm, and becomes practically zero above 2.8 mm. Also, the effective nonlinearity of KTP decreases as the crystal orientation increases Žcf. Fig. 1.. As a matter of fact, working with limited pumping energy may represent a great disadvantage for far-IR wavelengths as far as it gives far-IR pulses with very poor energy Ž- 50 mJ for li ) 2.3 mm.. Although, conversion efficiencies can be compared to those previously obtained in this spectral range w4–7x. Because of the limited aperture of the crystals, we notice a lowering near the degeneracy point: between 1.02 mm and 1.12 mm there are no generated pulses because the pump pulse cannot propagate through the entire effective length. Nevertheless, some corrections due to dichroic mirrors should be taken into account over the whole spectral range, in particular the fact that their transmission decreases from 90% to 45% above 2.7 mm. These corrections are not accounted for in the results because this effect cannot be cancelled experimentally. Moreover, the efficiency of the OPA is limited by another effect known as grey-tracks w14x. This kind of damage occurs in flux-grown KTP for pumping energies above 1 GWrcm2 and is related to the presence of Ti 3q centers w15x. As a consequence, optical absorption in KTP becomes important near 532 nm and therefore the transmission of pump pulses is reduced. However, grey tracks disappear within a few minutes after the end of irradiation. An investigation of the consequences on generation and amplification is in progress. In conclusion, we have investigated a new double-pass, 532-nm-pumped KTP OPGrOPA system. We obtained tunable NIR pulses from 0.67 to 2.854 mm. Using reduced size crystals Žtotal interaction length of 24 mm. and less pumping energy Ž1 mJ per pulse for each pass., the device conceived allowed us to achieve conversion efficiencies Ž14%. which compares favourably with those of previous
systems. Nevertheless, the performance of such a device is limited by the formation of grey tracks in KTP. Furthermore, it gives very poor energy for far-IR pulses. However this can be sufficient for the kind of applications based on time-resolved spectroscopic methods using sum frequency generation from surfaces and interfaces. Some additional investigations about the spectral width of the output pulses and a comparison with results given by theoretical models will be undertaken.
References w1x J.A. Armstrong, N. Bloembergen, J. Ducuing, P.S. Pershan, Phys. Rev. 127 Ž1962. 1918. w2x J.Y. Huang, J.Y. Zhang, Y.R. Shen, C. Chen, B. Wu, Appl. Phys. Lett. 57 Ž1990. 1961. w3x S. Lin, J.Y. Huang, J. Ling, C. Chen and Y.R. Shen, Appl. Phys. Lett 59 Ž1991. 2805; J.Y. Zhang, J.Y. Huang, Y.R. Shen and C. Chen, J. Opt. Soc. Am. B 10 Ž1993. 1758; X. Liu, Z. Xu, B. Wu and C. Chen, Appl. Phys. Lett. 66 Ž1995. 1446; Z. Xu, X. Liu, D. Deng, Q. Wu, L. Wu, B. Wu, S. Lin, B. Lin and C. Chen, J. Opt. Sov. Am. B 12 Ž1995. 2222. w4x H. Vanherzeele, Appl. Optics 29 Ž1990. 2246. w5x H. He, Q. Zhao, J. Guo, Y. Lu, Y. Liu, J. Wang, Optics Comm. 87 Ž1992. 33. w6x S.A. Reid, Y. Tang, Appl. Optics 35 Ž1996. 1473. w7x D.E. Gragson, D.S. Alavi, G.L. Richmond, Optics Lett. 20 Ž1995. 1991. w8x R.F. Belt, G. Gashurow, Y.S. Liu, Laser Focus 21 Ž1985. 110. w9x J.D. Bierlein, H. Vanherzeele, J. Opt. Soc. Am. B 6 Ž1989. 622. w10x W.R. Bosenberg, W.S. Pelouch, C.L. Tang, Appl. Phys. Lett. 55 Ž1989. 1952. w11x V.G. Dmitriev, C.G. Gurzadyan and D.N. Nikogosyan, Handbook of Nonlinear Optics Crystals, Springer-Verlag, Berlin, 1991, Chap. 3.2.7 w12x H. Vanherzeele, J.D. Bierlein, Optics Lett. 17 Ž1992. 982. w13x CASIX Crystal Guide 1995, Crystals and Materials – Laser Accessories, 24. w14x G.M. Loiacono, D.N. Loiacono, T. McGee, M. Babb, J. Appl. Phys. 72 Ž1992. 2705. w15x M.G. Roelofs, J. Appl. Phys. 65 Ž1989. 4976.