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JP0050759 JAERI-Conf 2000-006 JP0050759 JAERI-Conf 2000-006 21. Development of High Efficiency Second Harmonic Frequency Converter Hiromitsu Kiriyama, Shinichi Matsuoka, Yoichiro Maruyama and Takashi Arisawa Advanced Photon Research Center, Kansai Research Establishment, Japan Atomic Energy Research Institute 8-1 Umemidai, Kizu-cho, Souraku-gun, Kyoto, 619-0215, Japan An efficient four-pass quadrature frequency conversion scheme was developed. A high conversion efficiency in excess of 80 % has been achieved for frequency doubling of 1064-nm in KTP with a low input fundamental laser intensity of 76 MW/cm2. A second-harmonic output of 486 mJ has been obtained with 607 mJ of the input 1064-nm fundamental laser at 10 Hz. Keywords: quadrature frequency conversion, second-harmonic generatiom, Nd:YAG, KTP 1. INTRODUCTION The technique of using nonlinear optical crystals to convert the frequency is an important and popular method to extend the utility of existing lasers. In particular, generation of pulsed green output is important for application for pumping Ti: sapphire amplifier. The most effective way to accomplish this has been through second-harmonic generation (SHG) of 1064-nm Nd:YAG lasers in nonlinear optical crystals. For SHG with the 1064-nm Nd:YAG lasers, a commonly employed laser source, conversion efficiencies exceeding 50 % are commonplace [1-6]. Efficient SHG from the fundamental laser wavelength is also essentially important for the frequency up-conversion because the performance of higher-order harmonic generation is very much dependent on this. Additionally, in many applications and experiments, a high second-harmonic conversion efficiency is desirable for the purpose of enhancing the signal-to-noise ratio or reducing the size of the required laser source. In order to achieve high conversion efficiency, a quadrature frequency conversion scheme has been proposed [7]. Figure 1 shows the quadrature doubling scheme used for SHG. In this scheme, the planes formed by the input laser beam propagation direction and optic axes of two type II crystals are arranged to be orthogonal. The input laser beam left unconverted from the first crystal due to nonuniformities of the input beam intensity can be converted efficiently in the second crystal. Thus, the sensitivity to input laser beam profile, angle and thermal misadjustment is lower than that of single crystal scheme. The second-harmonic output produced in the first crystal is not at the correct polarization for interaction in the second. Thus, the second-harmonic output from the first crystal passes through the second without back-conversion. In this paper, we describe a four-pass quadrature frequency conversion scheme by using polarization rotation for pumping Ti.sapphire amplifier and report a frequency conversion efficiency of above 80 % for SHG with a low input fundamental laser beam intensity of 76 MW/cm2. The incorporation of quadrature and multi-pass features in this scheme provides a frequency conversion performance superior to that of comparable lasers in its - 105 - JAERI-Conf 2000-006 class. This scheme can be easily scaled up by increasing the size of the nonlinear optical crystal to accommodate larger input fundamental laser beam cross-section. '2u> input laser E2o, •IOJ first crystal optical axes in orthogonal planes Fig. 1 quadrature doubling scheme used for SHG. 2. EXPERIMENTAL SETUP The experimental arrangement of four-pass quadrature frequency conversion scheme is shown in Fig. 2. The laser source used in this experiment was a Q-switched Nd: YAG laser (Continuum, Powerlite 910), operated at a repetition rate of 10 Hz. The input 1064-nm fundamental laser beam diameter was 8.5 mm and the pulse duration was 15 ns (full width at half maximum (FWHM)). This scheme consists of a thin-film polarizer, a high- reflection mirror, two dichroic mirrors, a half-wave plate, two type II KTP crystals and a quarter-wave plate. KTiOPO4 (KTP) was chosen as the nonlinear optical crystal because of its high effective nonlinear coefficient, large acceptance angle, large temperature bandwidth, and reasonably high damage threshold. The KTP crystal (Crystal Associates, gray-tracking-resistant KTP) size was lOmmX lOmmx 10mm. The crystal was oriented for type II phase matching for SHG of input 1064-nm laser radiation at room temperature. The input faces of the crystals were antireflection coated at both 532-nm and 1064-nm. The crystals were mounted on a rotation stage for optimizing the angle between the input beam and the crystals at room ambient without any crystal temperature control. A dual-output scheme was used to lower the second harmonic power loading in the crystals. The second harmonic output beam on each direction which are elliptically polarized will be separated into two linear polarized beams by thin-film polarizers. The polarization of the two linear polarized beams are rotated in half-wave plates for correct orientation to the Ti:sapphire amplifier for efficient absorption. optical isojator relay optics Q-switched Nd:YAG laser thin-film polarizer second harmonic output A /4 plate second harmonic output dichroic mirror dichroic mirror Mi type II KTP crystal M 2 Fig. 2 Experimental arrangement of four- pass quadrature scheme. - 106 - JAERI-Conf 2000-006 The input laser beam for this scheme, which was /7-polarized, was relayed and passed through an optical isolator and a thin-film polarizer. The optical isolator (EOT, 1211064) was placed between the Q-switched Nd:YAG laser and the frequency conversion part in order to provide sufficient optical isolation between the two sections. The input beam was injected into the two KTP crystals in quadrature frequency conversion scheme by dichroic mirror M,, which was high-reflection coated at 1064-nm and anti-reflection coated at 532-nm, and then converted, after rotating the polarization by 45° in a half-wave plate for correct orientation to the crystals for efficient conversion. Furthermore, the polarization of the input beam was rotated by 90° after a round-trip pass through the quarter-wave plate before retracing its path. The input beam made two more passes through the two KTP crystals as it was s -polarized at the thin-film polarizer. Thus, the input beam passed through the two KTP crystals in quadrature frequency conversion scheme a total of four times and the second-harmonic output generated was extracted through the dichroic mirrors M, and M2. 3. RESULTS Figure 3 shows the 532-nm second-harmonic conversion efficiency as a function of the input 1064-nm fundamental laser intensity for conventional scheme and four-pass quadrature scheme. In the four-pass quadrature scheme, we measured the second-harmonic output energy generated in two opposite direction as 532- nm second-harmonic output energy. For comparison, we also measured the SHG charactenstic in a single pass with a single crystal scheme as a conventional scheme. 100 u '3 80 • § • 60 <>* o- o o o .0. 40 > o o • < 20 o conventional scheme 1 1 • four-pass quadrature scheme 1 u 0 • ••iii i 0 10 20 30 40 50 60 70 80 input 1064-nm fundamental laser intensity [MW/cm Fig.3 532-nm second-harmonic conversion efficiency as a function of the input 1064-nm fundamental laser intensity for conventional scheme and four-pass quadrature scheme. The second-harmonic output and the fundamental laser power were measured by a calibrated power meter - 107 - JAERI-Conf 2000-006 (OPHIR, ATN). The intensity was calculated from the measured pulse duration, the measured energy, and measured beam diameter. There were no compensation for optical losses such as reflection and absorption of the crystals, and reflection and transmission of the dichroic mirrors. As can be seen from this figure, the conversion efficiency of four-pass quadrature frequency conversion scheme is clearly higher than that of the conventional scheme. A maximum second-harmonic conversion efficiency of above 80 % was achieved with a low input laser intensity of 76 MW/cm2. The high efficiency enables efficient use of energy and hardware. The low input laser intensity enables the use of a smaller laser source, and ensures no photochromic damage (gray-tracking) [8,9] in KTP. Though the threshold of gray-tracking depends on the repetition rate of the laser and the growth technique of the crystal, laser damage thresholds ranging from 100 MW/cm2 to about 10 GW/cm2 have been reported. 4. CONCLUSION In conclusion, we have demonstrated efficient SITG in four-pass quadrature frequency conversion scheme. A high second-harmonic conversion efficiency of 80 % has been achieved with low input laser intensity of 76 MW/cm2. The successful operation of the scheme demonstrates that it is applicable and scalable to the design of a high power laser system with high efficiency. REFERENCES [1] W. Koechner, Solid-State Laser Engineering, Springer Berlin, 1996. [2] V. G. Dmitriev, G. G. Gurzadyan, D. N. Nikogosyan, Handbook of Nonlinear Optical Crystals, Springer Berlin, 1991. [3] T. A. Driscoll, H. J. Hoffman, R. E. Stone, Efficient second-harmonic generation in KTP crystals, J. Opt. Soc. Am. B 3, 683(1986). [4] R. A. Stolzenborger, Nonlinear optical properties of flux growth KTiOPO4, Appl. Opt. 27, 3883 (1988). [5] R. J. Bolt and M. van der Mooren, Single shot bulk damage threshold and conversion efficiency measurements on flux grown KTiOPO4 (KTP), Opt. Comm. 100, 399 (1993). [6] Y. K. Yap, S. Haramura, A. Taguchi, Y. Mori, T. Sasaki, CsLiB6O10 crystal for frequency doubling the Nd:YAG laser, Opt. Comm. 145, 101 (1998). [7] D. Eimerl, Quadrature frequency conversion, IEEE J. Quantum Electron. 23, 1361 (1987). [8] B. Boulanger, M. M. Fejer, R. Blachman, P. F. Bordui, Study of KTiOPO4 gray-tracking at 1064, 532, and 355 ran, Appl. Phys. Lett. 65, 2401 (1994). [9] J. P. Feve, B. Boulanger, G. Marnier, II. Albrecht, Repetition rate dependence of gray-tracking in KTiOPO4 during second-harmonic generation at 532 run, Appl.
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