We have demonstrated the generation of a high-energy green laser pulse using large aperture CsLiB6O10 (CLBO) crystals for the first time to our knowledge. A pulsed energy of 25 J at 532-nm was generated using the 1064-nm incident Nd:glass laser radiation with an energy of 34 J. High conversion efficiency of 74 % at intensities of only 370 MW/cm2 was obtained using a two-stage crystal architecture. This result represents the highest green pulse energy ever reported using the CLBO crystals.
© 2002 Optical Society of America
Nonlinear optical crystals provide a means of extending the frequency range of available laser sources [1,2]. High power green lasers based on second- harmonic generation (SHG) of near-infrared solid-state lasers are promising for use as pump sources for Ti:sapphire chirped pulse amplification (CPA) systems [3–5]. At present the major crystals commonly used for SHG of neodymium-doped lasers are KTiOPO4 (KTP), LiB3O5 (LBO), ß-BaB2O4 (BBO) and KH2PO4 (KDP). Though the first three crystals, KTP, LBO and BBO have large effective nonlinear coefficients,[1,2] their small crystal sizes (~1 cm3) do not permit SHG of high energy lasers for which a large laser beam diameter is typical. The output SHG energies of the systems using these crystals are therefore limited to several hundred milijoules. The KDP crystals can be grown to a large size (> 30 cm aperture) possessing high optical quality . KDP is therefore, still widely used in most high energy (tens of Joules or more) laser systems with a sacrifice of lower conversion efficiency mainly because of its small effective nonlinear coefficient. In order to obtain a modest conversion efficiency of about 50 % using the Type II KDP crystal, the high input laser intensity level of several GW/cm2 is required and thus close to the damage threshold of optical materials .
The CsLiB6O10 (CLBO) crystal is a recently developed borate crystal which can be easily grown to large sizes . The Type I CLBO crystal is transparent below 200 nm and has therefore been used for generation of fourth and fifth harmonics of the neodymium-doped lasers. Both Type I and Type II CLBO crystals are also phase matchable for SHG of neodymium-doped lasers. Type II CLBO crystal offers some attractive nonlinear properties as compared with Type II KDP crystal.[2,10] For Type II phase matching at a pump wavelength of 1064-nm, CLBO has a larger effective nonlinear coefficient (deff) and temperature bandwidth. The deff for CLBO and KDP are 0.95 pm/V and 0.38 pm/V, respectively. The temperature bandwidths for CLBO and KDP are 43.1 °C-cm and 19.1 °C-cm, respectively. A larger deff value reduces the input laser intensity for a given SHG efficiency and also enables a shorter crystal to be used in which minimizes angular dephasing. For example, a SHG conversion efficiency of over 50 % with a green pulse energy of 1.55 J has been reported using the CLBO crystal with an input laser intensity of 360 MW/cm2 . This intensity is much lower than the several GW/cm2 levels used in previous work using the KDP crysta l and is also much lower than the damage threshold of the CLBO crystal as the CLBO exhibits a high bulk laser induced damage threshold of 26 GW/cm2. The large temperature bandwidth enables the crystal to be used at high average powers without significant degradation of performance. The KDP crystal has a wide angular bandwidth of 3.4 mrad-cm as compared with 1.7 mrad-cm for the CLBO crystal. The small angular bandwidth of CLBO is not, however, a limitation as its large deff value allows the use of a larger low divergence input beam at lower intensities. A CLBO crystal with dimension 14 × 11 × 11 cm3 can be grown by the top-seeded Kyropoulos method in three weeks  and thus an extra large crystal dimension such as a KDP crystal with tens of Centimeters aperture or more is technically possible with current growth technologies. The CLBO crystal has therefore, the potential to generate high energy SHG of large aperture neodymium-doped lasers with high conversion efficiency.
In this paper we report on the generation of a high green pulse energy of 25 J with an incident energy of 34 J of Nd:glass laser using large aperture CLBO frequency doublers, corresponding to an energy conversion efficiency of 74 %. To the best of our knowledge this is the highest green output pulse energy ever produced using CLBO crystals.
2. Experimental setup
The SHG experiments were carried out using a custom-built high power flash-lamp pumped Nd:silicate glass laser system, operated at a few shots per hour. This laser has a single-pass master oscillator power amplifier (MOPA) architecture. In the system a long-cavity single-longitudinal-mode 1064-nm Nd:YAG master oscillator generated pulses of about 25 ns (FWHM) duration and 200 mJ energy that were then shaped by a soft aperture to flat-top spatial profile. The shaped pulses were then amplified to 800 mJ by a Nd:YAG rod preamplifier of 9 mm diameter. The energy was further increased in a chain of 16, 25 and 45 mm diameter Nd: silicate glass rod amplifiers to approximately 60 J. Spatial filters were used at appropriate locations in the amplifier chain in order to reduce the intensity nonuniformity due to Fresnel diffraction. A pair of Faraday rotators were also used to prevent pulses from propagating backward down the laser chain, placed between the pre-amplifier and the 16 mm glass rod amplifier, and between the 16 mm and 25 mm glass rod amplifiers, respectively. The output temporal profile was observed to be smooth and near-Gaussian, and the spatial profile almost flat-top.
We employed a two-stage CLBO crystal architecture in order to achieve high conversion efficiency and to minimize back-conversion . A dichroic mirror placed between the two crystals separated the high power green output of the first crystal from the unconverted fundamental beam in which avoided Fresnel reflection losses of the high peak power green output in the second crystal. The dual green outputs enable the Ti: sapphire amplifier to be pumped from both sides of the Ti:sapphire. The individual linear polarized beams are then suitably rotated using λ/2 plates for correct orientation to the Ti:sapphire crystal in order to obtain sufficient absorption. The first and second CLBO crystals (KOGAKUGIKEN Co., Ltd) each had a cross-section of 30 mm × 30 mm and their lengths were 11.5 mm and 15.5 mm, respectively. These lengths were optimized using a numerical model based on the coupled wave equations . Both crystals had no anti-reflection coatings. Each crystal was orientated for Type II SHG of the 1064-nm input fundamental laser (i.e., the second-harmonic output beam is an extraordinary ray) and was housed in a heater fitted with a proportional-integral-derivative (PID) controller. The crystals were maintained constantly at 160 °C with an accuracy of 0.1 °C and were argon gas purged in order to avoid their degradation due to stresses introduced by crystal hydration, cutting, polishing, and thermal shock owing to laser power absorption . The temperature ramping rate was fixed at 2.3 °C/min. No degradation has been observed for more than 6 months without any maintenance, which means that the CLBO crystal has maintained the crystalline condition in this crystal housing. Each heater was mounted on a precision rotating stage for optimizing the angle between the input beam and the crystal. The output beam from the glass rod amplifier chain was down-collimated to ~25 mm diameter in order to introduce it into the CLBO crystals. In all of our experiments described in this paper the temporal character of the input 1064-nm beam was monitored using a photo-diode, the incident beam energy was determined using calibrated power meter and the spatial profile was measured using charge-coupled device (CCD) camera. The output energy and spatial profile of green beam generated in each crystal were also measured using the calibrated power meter and CCD camera, respectively.
Figure 1 shows the total 532-nm second-harmonic output pulse energy from the two crystals as a function of the input 1064-nm fundamental laser pulse energy. The second-harmonic output energies from each crystal are also indicated in this figure. There was no compensation for optical losses due to reflection, absorption, and scattering of the crystals. As seen from the figure, a maximum total second-harmonic output pulse energy of 25 J was obtained with 34 J of input 1064-nm fundamental laser pulse energy. From Fig. 1, it is seen that 532-nm second-harmonic output pulse energy generated in first crystal is about five times as much as that generated in second.
Figure 2 shows the 532-nm second-harmonic energy conversion efficiency plotted as a function of the input 1064-nm laser intensity. As seen, a maximum conversion efficiency of 74 % was achieved for the dual outputs with an input laser intensity of 368 MW/cm2. This intensity, which was calculated from the measured values of pulse duration, pulse energy and beam diameter, is much lower than the several GW/cm2 levels used in previous work using the 10 cm aperture KDP crystal . The results clearly demonstrate the fact that CLBO crystal is suitable for efficient SHG of high power neodymium-doped lasers at low input intensities. The high efficiency enables effective use of energy as well as hardware. The low input intensity of less than 500 MW/cm2 avoids intensity-dependent damage of the nonlinear crystals and other optical components.
Figure 3 shows the near-field spatial profiles of the second-harmonic beam from each crystal. The intensity distribution along the vertical and horizontal cross-sections of these beams is also shown. The near-field spatial profiles were imaged by a CCD camera through a set of image-relay optics. As seen, the beam profiles had near homogeneous flat-top spatial intensity distribution which are suitable for pumping the Ti:sapphire amplifiers without optical damage of the Ti:sapphire crystal.
We have obtained 25 J of total SHG energy from 34 J of incident 1064-nm laser pulse energy using the two-stage CLBO crystal architecture. The energy conversion efficiency as high as 74 % was achieved with an input laser intensity of only 368 MW/cm2. This scheme can be easily scaled up in energy by increasing the sizes of CLBO crystals to accommodate larger input fundamental laser beam cross-section. These experiments clearly demonstrate that the CLBO crystal, with its excellent nonlinear properties as well as availability in large sizes, is highly useful for the SHG of high-energy neodymium-doped lasers. This scheme is currently being applied for pumping an 80-mm diameter Ti:sapphire amplifier with an object to produce more than several hundred terawatt of 800-nm radiation.
The authors would like to acknowledge K. Yagi, T. Nagai, M. Aoyama and M. Fujino for their technical assistance. The authors sincerely thank Y. Kato for his encouragement and are also grateful to N. Srinivasan of Instruments Research and Development Establishment, Dehradun, India for several helpful comments on the manuscript.
References and links
1. W. Koechner, Solid-State Laser Engineering 4th ed. (Springer-Verlag, Berlin, 1996), Chap. 10.
2. V. G. Dmitriev, G. G. Gurzadyan, and D. N. Nikogosyan, Handbook of Nonlinear Optical Crystals 2nd ed. (Springer-Verlag, Berlin, 1991), Chap. 4.
4. J. P. Chambaret, C. Le Blanc, G. Chériaux, P. Curley, G. Darpentigny, P. Rousseau, G. Hamoniaux, A. Antonetti, and F. Salin, “Generation of 25-TW, 32-fs pulses at 10 Hz,” Opt. Lett. 21, 1921–1923 (1996). [CrossRef] [PubMed]
5. K. Yamakawa, M. Aoyama, S. Matsuoka, T. Kase, Y. Akahane, and H. Takuma, “100-TW sub-20-fs Ti:sapphire laser system operating at a 10-Hz repetition rate,” Opt. Lett. 23, 1468–1470 (1998). [CrossRef]
6. M. A. Rhodes, C. D. Boley, A. G. Tarditi, and B. S. Bauer, “Plasma electrode pockels cell for ICF lasers,” in Solid State Lasers for Application to Inertial Confinement Fusion, M. André and H. T. Powell eds., Proc. SPIE2633, 94–104 (1995).
7. G. J. Linford, B. C. Johnson, J. S. Hildum, W. E. Martin, K. Snyder, R. D. Boyd, W. L. Smith, C. L. Vercimak, D. Eimerl, and J. T. Hunt, “Large aperture harmonic conversion experiments at Lawrence Livermore National Laboratory,” Appl. Opt. 21, 3633–3643 (1982). [CrossRef] [PubMed]
8. Y. Mori, I. Kuroda, S. Nakajima, T. Sasaki, and S. Nakai, “New nonlinear optical crystal: Cesium lithium borate,” Appl. Phys. Lett. 67, 1818–1820 (1995). [CrossRef]
9. Y. K. Yap, M. Inagaki, S. Nakajima, Y. Mori, and T. Sasaki, “High-power fourth- and fifth-harmonic generation of a Nd:YAG laser by means of a CsLiB6O10,” Opt. Lett. 21, 1348–1350 (1996). [CrossRef] [PubMed]
10. Y. Mori and T. Sasaki, “CsLiB6O10 crystal: Growth and Properties,” in Nonlinear Frequency Generation and Conversion, M. C. Gupta, W. J. Kozlovsky, and D. C. MacPherson. eds., Proc. SPIE2700, 20–27 (1996). [CrossRef]
11. Y. K. Yap, S. Haramura, A. Taguchi, Y. Mori, and T. Sasaki, “CsLiB6O10 crystal for frequency doubling the Nd:YAG laser,” Opt. Commun. 145, 101–104 (1998). [CrossRef]
12. Y. Mori, S. Nakajima, A. Miyamoto, M. Inagaki, and T. Sasaki, “Generation of ultraviolet light by using new nonlinear optical crystal CsLiB6O10,” in Solid State Lasers for Application to Inertial Confinement Fusion, M. Andrè and H. T. Powell eds., Proc. SPIE2633, 299–307 (1995). [CrossRef]
14. J. A. Armstrong, N. Bloembergen, J. Ducuing, and P. S. Pershan, “Interactions between light waves in a nonlinear dielectric,” Phys. Rev. 127, 1918–1939 (1962). [CrossRef]
15. Y. K. Yap, T. Inoue, H. Sakai, Y. Kagebayashi, Y. Mori, T. Sasaki, K. Deki, and M. Horiguchi, “Long-term operation of CsLiB6O10 at elevated crystal temperature,” Opt. Lett. 23, 34–36 (1998). [CrossRef]