Applied Mechanics and Materials Vols. 26-28 (21) pp 12-123 Online: 21-6-3 (21) Trans Tech Publications, Switzerland doi:1.428/www.scientific.net/amm.26-28.12 The Narrow Pulse-Width Laser-Diode End-Pumped Nd:Yvo4/Lbo Green Laser Wenbing Yu 1,2,a, Chenpen He 3,b 1 School of Sciences Wuhan Institute of Technology, Wuhan, china 2 Hubei Province Key Laboratory of Intelligent Robot, Wuhan, China 3 Wuhan Sunic Photoelectricity Equipment Manufacture Co.,LTD, Wuhan, China a yuwhict@163.com, b hechp666@tom.com Keywords: Narrow Pulse-Width; Double Acousto-Optic Q-Switch; Intracavity Frequency Doubling; All-Solid-State Green Laser Abstract. A high stable 9.2W and 12.6 ns pulse-width acousto-optic Q- switch solid- state green laser was reported. Through theoretical analysis and experimental research, the resonator was designed and optimized in order to compress pulse-width. In experiment, the double acousto-optic Q- switch was employed with single Nd:YVO 4 rod and LBO crystal was applied for frequency doubling. The resonator is the flat-concave type. Under the pumping current of 3A,a maximum green power of 9.2W was generated at 4kHz repetition rate and 8.164 ns pulse-width. Introduction Laser-diode pumped solid-state lasers are now becoming the mainstream in the developing field of solid state lasers. Laser-diode pumped all-solid-state high-power green laser has the virtue of being small size, high efficiency, long life and good stability. It has been widely used in military, scientific research, laser measurement,laser micro-machining, and they can also be applied to femtosecond Kerr lens mode-locked lasers based on Cr3+:LiSrAlF6, Cr3+:LiSrGaF6 and Cr3+:LiSrCaAlF6 crystals light[1]. Foreign and domestic have made a series of studies on side-pumped high-power lasers. In 1996 B.J. LeGarrec adopted 3 continuous diode laser side-pumped and output 27kHz, 16W high power green light[2],in 1999 Moon HeeJong adopted diode side-pumped three-mirror folded cavity to output 13kHz, an average power of 3W green light[3], but with a very low optical conversion efficiency and poor beam quality for its side pumping style. The end-pumping technique, which is a more effective way to match the size of pump beam and cavity mode, can improve the beam quality and increase the optical conversion efficiency. In 24 Yaojianquan adopted LBO SHG crystal to generate green light with pulse width of 13ns,power of 14W[4].High-power green laser is not uncommon[5-9],but High-power, high-repetition-frequency narrow pulse End-pumped green lasers are few reported of. This paper reports a single LD end-pumped Nd:YVO4 crystal,lbo intracavity frequency doubling narrow pulse high-efficiency acousto-optic Q-switch solid-state green laser.its repetition frequency is 4kHz, pulse width is 8.164ns and the average output power is 9.2W. All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans Tech Publications, www.ttp.net. (ID: 13.23.136.75, Pennsylvania State University, University Park, USA-18/9/15,22:58:25)
Applied Mechanics and Materials Vols. 26-28 121 Experimental Setup The experimental setup of intracavity second-harmonic generation (SHG), which had a L-configuration resonator, was shown schematically in Figure 1. The pump source was a commercially available high power fiber-coupled diode-laser-arrays produced by Germany DILAS Company. The core diameter and numerical aperture (N.A.) of the fiber were.8 mm and.22, respectively. The pump beams from the fiber at the wavelength of 88 nm were focused into the laser crystal by optical imaging system. The a-cut Nd:YVO 4 crystal with Nd 3+ concentration of.3 at.% and dimensions of 4 4 12 mm 3 was antireflection(ar) coated at 88 nm on one of its outside surface and high-reflectance(hr) coated at 164 nm on one of its inside surface. To remove the heat generated at high pump power levels from the crystal, it was wrapped with indium foil and held in a water-cooled copper block. A temperature sensor was mounted in the copper block near the laser crystal to monitor its surface temperature. The surface temperature of Nd:YVO 4 crystal was kept at about 3 C during the experiments. The output coupler M2 was a flat mirror, HR coated at 164 nm and AR coated at 532 nm on the inside surface, and HT coated at 532 nm on the outside surface. An acousto-optical Q-switch with high diffraction loss at 164 nm was placed between M2 and Nd:YVO 4 crystal, and its repetition rate could be tuned continuously from 1 khz to 6 khz. The LBO crystal produced by CASTECH INC. with dimensions of 3 3 1 mm 3 for type-i critical phase-matching at 164 nm was used as the frequency doubler. To minimize the internal losses caused by Fresnel reflection, it was AR coated at 164 nm and 532 nm on both end faces. It was cooled in the same way as the Nd:YVO 4 crystal. The end mirror M3 was a concave mirror with a radius of curvature of 1 mm and with a dual-wavelength HR coating at 164 nm and 532 nm on its curved surface. Fig.1 Nd:YVO4/LBOLaser experimental setup Fig.2 8.164ns Q-Pulse Theoretical Analysis and Experimental Results The thermal lens effect of laser crystal is the central issue of resonator design, which affects the stability of the laser, the cavity mode size, mode coupling efficiency, the quality of output beam and so on. At different pumping power, the thermal lens focal length of laser crystal is different. But the laser resonator structure is fixed, which will lead to the instability of resonator).weakening the thermal lens effect of laser crystal is one of the tasks of laser designs. Fiber-coupled LD pumped VIS intensity distribution can be approximated by Gaussian function. When pumped by medium-power LD the Nd:YVO 4 laser crystal generates equivalent Convex Mirrors. The expression for the focal length is:
122 Advanced Mechanical Engineering 1 f ξp = n 4πK in [ dn 2 / dt + ( 1)] cωp α T (1) The heat load ratio in Equation (1) is ξ =. 4,thermal conductivity is =.523W /(K cm), K c Pin is the absorption of pump power, pump spot radius is ω P = 4µm,the refractive index of ambient temperature is n, Temperature coefficient of refractive index of crystal is 5 6 dn / dt = 5.92 1, thermal expansion coefficient of crystal is T = 4.43 1 K α When pumped by the power of 5w, the thermal lens focal length is f = 12mm Reasonably choosing the location of LBO crystal and Nd: YVO 4 rod in the cavity makes pumping beam and the resonant cavity mode matching well, the absorption of the gain medium for pumping light is sufficient, the output green laser achieve TEM mode easily. Compared with vertical cavity, L cavity has advantages of flexible adjustment of model parameters, large incavity effective space, generating smaller beam waist by the incavity double frequency, improving the pumping light utilization of the laser, easy achievement of the output of the basic mode, high efficiency of the light - optical conversion. In the experiment, the total cavity length L = 365 mm, the distance from the back mirror to the ND: YVO4 rod center is 22 mm, from output mirror to the Nd:YVO4 rod center is 145 mm, from LBO to output mirror is 65 mm. When the pump current is 3 A, the maximum average output power at 532 nm was measured to be 9.2 W, while the pulse width was 8.164 ns (Full Width Half Maximum, FWHM) and the repetition frequency was 4 khz, respectively. As the pump current was increased to more than 4 A, the output power was saturated and not increased along with the pump current any more. This was due primarily to the too serious thermal effect making the resonator away from the center of stable region. The laser pulse signal was detected at different pump powers and modulation frequency by using a fast photodiode detector and it was observed and measured with a 3 MHz oscilloscope. Figure 2 shows a single pulse profile with the repetition frequency of 4 khz at the incident pump current of 3 A. Figure 3 shows the Q-switched pulse sequence with equal spaces in the screen of the oscilloscope. From this figure, it could be seen that the pulse sequence was very stable when the laser operated at a high output power level. Fig.3 8.164ns Q-Pulse Sequence Fig.4 repetition frequency vs average power
Applied Mechanics and Materials Vols. 26-28 123 Change the repetition frequency and pumping current, as is shown in Figure 4 we can obtain the output of green power. Seen from Figure 4, maintaining the same pumping current, increasing the repetition rate led to decline in green power Conclusion In a single LD end-pumped narrow pulse solid-state green laser, we use L-unstable resonator and Chinese-made laser frequency doubling crystal.when the pumping current is 3A,it achieves an output of high-repetition-rate, narrow pulse width green light (532nm),whose repetition frequency is 4kHz, pulse width is 8.164ns, and the output power is 9.2W. After 12 hours of continuous operation, the fallen of power is no more than.5w and the variation of pulse width is less than.5ns. Laser has been wuhan sunic photoelectricity equipment manufacture Co., Ltd. laser scribing machine on trial run. Acknowledgments We thank wuhan sunic photoelectricity equipment manufacture co., LTD for their valuable feedback and their assistance with experiment. References [1]C. L. Du, S. C. Ruan, Y. Q. Yu, and F. Zeng, 6-W diode-end-pumped Nd:GdVO4/LBO quasi-continuouswave red laser at 671 nm, Opt. Express,25, 13, 213-218. [2]Le Garrec B J,Raze G J,Thro P Y. High average power diode array pumped frequency doubled YAG laser[j]. Opt.Lett.,1996,21(24):199-1992. [3]Moon Hee-Jong,Yi Jonghoon,Rhee Yongjoo,et al. Efficient intracavity frequency doubling from diffusive reflector type diode side pumped Nd: YAG laser using a KTP crystal[a]. Laser and Electro-Optics, 1999. CLEO/Pacific Rim'99. The Pacific Rim Conference on, 1999, 2:461-462. [4]Xu Degang, Yao Jianquan, Guo Li et al.. 14 W laser diode-pump intracavity frequency doubled Nd:YAG green light laser [J]. Acta Optica Sinica, 24, 24(7):925-928. [5]Yao Zhenyu, Jiang Jianfeng, Tu Bo et al.. 162 W laser diode pumped Nd:YAG intracavity doubled laser[j]. Chinese J. Laser, 25, 32(11):1459-1462. [6]YUAN Ligang, JIANG Dongsheng,WANG Jianjun et al.. Green solid-state laser with 23 W output power [J]. INFRARED AND LASER ENGINEERING, 28,37(6): 98-983. [7]Liu Yuan,Li Zhengjia,Wu Qibin. Laser Scribing Application on Film a-si and poly-si Solar Cell[J]. Laser & Optronics Progress,25,42(5):43~47. [8]Dai Houmei, Bai Jintao. LD-pumped Nd:YAG/KTP intra-cavity double frequency 16W CW green laser[j]. Laser Technology, 28,32(3):312~313. [9]Zhang Huiyun, Zhang Yuping, Zhong Kai et al..efficient High Power Diode-Side-Pumped Intracavity-Doubled Continuous Wave Green Laser[J]. Chinese J. Lasers, 28,35(1):3~5.
Advanced Mechanical Engineering 1.428/www.scientific.net/AMM.26-28 The Narrow Pulse-Width Laser-Diode End-Pumped Nd:Yvo4/Lbo Green Laser 1.428/www.scientific.net/AMM.26-28.12 DOI References [1] C. L. Du, S. C. Ruan, Y. Q. Yu, and F. Zeng, 6-W diode-end-pumped Nd:GdVO4/LBO quasicontinuouswave red laser at 671 nm, Opt. Express,25, 13, 213-218. doi:1.1364/opex.13.213 [1] C. L. Du, S. C. Ruan, Y. Q. Yu, and F. Zeng, 6-W diode-end-pumped Nd:GdVO4/LBO uasicontinuouswave red laser at 671 nm, Opt. Express,25, 13, 213-218. doi:1.1364/opex.13.213