A diode stack end-pumped Nd:YVO4 slab laser at 1342 nm with near-diffraction-limited beam quality by using a hybrid resonator was presented. At a pump power of 139.5 W, laser power of 35.4 W was obtained with a conversion efficiency of 25.4% of the laser diode to laser output. The beam quality M-2 factors were measured to be 1.2 in the unstable direction and 1.3 in the stable direction at the output power of 29 W. (C) 2009 Optical Society of Americ

He JL

Liu Y

Xin JG

Yan Y

Zhang HJ

Zhang HL

余西龙

Institute Of Mechanics,Chinese Academy of Sciences

July 15, 2009 / Vol. 34, No. 14 / OPTICS LETTERS 2105Near-diffraction-limited,35.4 W laser-diode end-pumped Nd:YVO4slab laser operating at 1342 nmYing Yan,1 Hengli Zhang,1,* Yang Liu,1 Xilong Yu,2 Huaijin Zhang,3Jingliang He,3 and Jianguo Xin11School of Optoelectronics, Beijing Institute of Technology, Beijing 100081, China2Institute of Mechanics, Chinese Academy of Sciences, Beijing 100190, China3State Key Laboratory of Crystal Materials, Shandong University, Jinan 250000, China*Corresponding author: zhl040325@sina.comReceived April 30, 2009; revised June 4, 2009; accepted June 5, 2009;posted June 10, 2009 (Doc. ID 110618); published July 7, 2009A diode stack end-pumped Nd:YVO4 slab laser at 1342 nm with near-diffraction-limited beam quality byusing a hybrid resonator was presented. At a pump power of 139.5 W, laser power of 35.4 W was obtainedwith a conversion efficiency of 25.4% of the laser diode to laser output. The beam quality M2 factors weremeasured to be 1.2 in the unstable direction and 1.3 in the stable direction at the output power of 29 W.© 2009 Optical Society of AmericaOCIS codes: 140.3480, 140.3530.Laser emission around 1.3 m has great applicationsin fiber communication, medical treatment, and laserradar. It also can be used to generate red and blue la-sers by nonlinear frequency conversion and as thepumping source for a special laser, such as Co:MgF2.Nd3+-doped gain media, such as Nd:YVO4 [1–4],Nd:GdVO4 [5,6], Nd:YAP [7], Nd:LuVO4 [8], andNd:YAG [9], were considered to be common lasergain medium to generate 1.3 m radiation in laser-diode (LD)-pumped rod lasers, and much researchhad been done to achieve high output power. Al-though output power of more than 100 W was gener-ated [7,9] by LD side-pumped solid-state lasers, gen-erally speaking, its efficiency and beam quality werenot as good as those of end-pumped lasers. Nd:YVO4has strong absorption at 808 nm in a rather broadbandwidth, a large emission cross section, and lim-ited thermally induced beam aberration. In 2003, DiLieto et al. [1] presented a 1342 nm laser with outputpower of 7.3 W by a single crystal and 12 W poweroutput by twin Nd:YVO4 crystal configuration. Oneyear later, Minassian et al. [2] demonstrated a13.7 W grazing-incidence Nd:YVO4 slab laser with adiode-edge-pumped structure. In the same year, Yaoet al. [3] obtained 11 W output at 1342 nm with aFig. 1. Experimental setup of LD end-pumped Nd:YVO4vertical directions.0146-9592/09/142105-3/$15.00 ©slope efficiency of 31.4% with a double-end-pumpedNd:YVO4 rod laser. In 2008, Lu et al. [4] got 16.4 Woutput at 1342 nm by using two double-end-pumpedNd:YVO4 rod crystals, and the beam quality wasmeasured to be 1.7 and 1.9 in horizontal and verticaldirections at an output power of 12 W. Nd:GdVO4was also a favorite gain medium in 1342 nm lasersowing to its good thermal conduction. Recently, Zhouet al. [6] achieved a 26.3 W cw by an LD end-pumpedNd:GdVO4 rod laser with beam qualityM2=30 at theoutput power of 24 W. To the best of our knowledge,so far this is the highest power generated by LD end-pumped 1.3 m solid-state lasers.LD end-pumped slab lasers with hybrid resonatorswere proved to be a promising approach to generatelaser with both high power and high beam quality[10,11]. However, by now most of the research workwas focused on the emission line of 4F3/2→ 4I11/2 forNd3+-doped crystals. Recently, we adopted a positive-branch hybrid resonator to obtain a 1342 nm laser atpumping power of 100 W; the highest output powerof 20.2 W was achieved with a slope efficiency of26.3%, and M2 factors were 1.2 in the unstable direc-tion and 2.3 in the stable direction at an outputpower of 16 W [12]. To further improve the outputlaser with hybrid resonator in the (a) horizontal and (b)slab2009 Optical Society of America2106 OPTICS LETTERS / Vol. 34, No. 14 / July 15, 2009power and beam quality, a negative branch with alonger cavity length was designed in this Letter. A35.4 W output laser was created with a slope effi-ciency of 33.3%; beam quality M2 factors were 1.2 inthe unstable direction and 1.3 in the stable directionat an output power of 29 W.Figure 1 is the schematic diagram of an LD end-pumped Nd:YVO4 slab laser. The central wavelengthof the LD was 808.6 nm, and the emission from eachdiode laser bar was individually collimated by micro-lens. A 12 mm0.4 mm homogeneous pumping linewas generated inside the Nd:YVO4 slab. TheNd:YVO4 laser crystal with the size of 12 mm10 mm1 mm was 0.3 at. % doped and mountedbetween two water-cooled heats sinks with two largefaces 12 mm10 mm. Indium foil was used for ef-fective and uniform thermal contact and cooling.Both the LD stacks and the laser crystal were tem-perature controlled by circulating water. Line-shapedpumping line and efficient heat removal through twolarge faces induced a quasi-one-dimensional tem-perature gradient along the y axis.As shown in Fig. 1, mirrors M1 and M2 built up anoff-axis negative-branch confocal unstable resonatorin the x–z plane and a thermal-induced stable reso-nator in the y–z plane. The resonator mirrors (M1,M2) were two concave spherical mirrors, with radii of450 mm and 400 mm respectively, both of which werecoated for high reflection (HR) at 1342 nm and hightransmission (HT) at 808 nm and 1064 nm. M2 wascut and polished at one edge where the laser beamwas coupled out. The magnification of unstable direc-tion was M=R1/R2=1.125, and the outcoupling wasT=1− 1/M11%. Considering the insertion of theNd:YVO4 crystal in the resonator, the cavity lengthwas adjusted to about 430 mm. In the unstable direc-tion, an automatically collimated output beam wascoupled out from the edge of M2 with a beam width of1.3 mm. While in the stable direction, there was arather small Fresnel number N0.44 (N=a2 /L, a=0.5 mm was the half-thickness of the slab and L=425 mm was the cavity length), which induced atypically single TEM00 mode operating. Here, theFig. 2. Output power as a function of LD pump power.thermal lens of gain medium was ignored.The output power as a function of LD pumpingpower is shown in Fig. 2. When the pumping powerwas 139.5 W, the maximum output power of 35.4 Wwas achieved with slope efficiency of 33.3% andoptical-to-optical efficiency of 25.4%. Taking the effi-ciency of the couple system 88%  into account, theslope efficiency and the optical-to-optical efficiencywould be 37.8% and 28.8%, respectively.To measure the beam quality, a lens with focallength of 350 mm and a thin knife were used to getthe beam diameters at different positions along thepropagation. Using the M2 factor, the propagation ofthe high-order laser beam can be described as dz2=d021+ 4M2z /d022, [13], where d0 is the diam-eter of beam waist, d(z) is the beam diameter at thepoint z,  is the wavelength, and z is the distance tothe beam waist. The squared beam diameters in un-stable and stable directions at an output power of29 W were tested and fitted as shown in Fig. 3; thebeam quality M2 factors were 1.2 in the unstable di-rection and 1.3 in the stable direction, respectively.As we know, a side lobe does exist in the far field as aresult of diffraction at output coupling mirror [13,14],and its intensity has been measured to be lower thanFig. 3. Beam-quality measurements at output power of29 W: (a) unstable direction, (b) stable direction.July 15, 2009 / Vol. 34, No. 14 / OPTICS LETTERS 210710% of peak value. Thus it is not included in beamdiameters of Gaussian profile 1/e2=13.5% , and itsinfluence on the testing results of beam size and M2factor can be neglected.Compared with positive-branch hybrid resonator,both the efficiency and beam quality were increasedin this experiment. It was mainly because the cavitylength in positive-branch hybrid resonator was muchshorter L= R1+R2 /2=25 mm, and the Fresnelnumber was as large as 7.5. The beam radiuses ofTEM00 mode at the end of slab in stable direction arecalculated in Fig. 4. From the figure, it could be seenFig. 4. TEM00-mode beam radius in the stable direction asa function of pumping power.Fig. 5. Time stability of an LD end-pumped Nd:YVO4 slablaser.that as the pumping power changed from50 W to 150 W, the TEM00 mode of negative-branchresonator had a larger beam radius and better over-lap with the pumping volume than the positive-branch resonator.A significant astigmatism was also shown in Fig. 3.It was because that the almost-square laser beam atthe output mirror was divergent in the stable direc-tion and automatically collimated in the unstable di-rection. So an off-axis cylindrical telescoped systemwould be designed to adjust the beam waist in theunstable direction the same size as that in the stabledirection.We measured the stability of the LD end-pumpedslab laser at an output power of 29 W for 30 min, andthe results are shown in Fig. 5. No obvious fluctua-tions were observed. The stability was about 0.6%.In conclusion, 35.4 W cw 1342 nm laser was gener-ated with the pumping power of 139.5 W; the slopeefficiency and optical-to-optical efficiency were 33.3%and 25.4%, respectively. At the output power of 29 W,the beam quality M2 factors were measured to be 1.2in the unstable direction and 1.3 in the stable direc-tion, and the stability was about 0.6%.References1. A. Di Lieto, P. Minguzzi, A. Pirastu, and V. Magni,IEEE J. Quantum Electron. 7, 903 (2003).2. A. Minassian and M. J. Damzen, Opt. Commun. 230,191 (2004).3. A. Y. Yao, W. Hou, Y. P. Kong, L. Guo, L. A. Wu, R. N.Li, D. F. Cui, and Z. Y. Xu, J. Opt. Soc. Am. B 22, 2129(2005).4. C. Lu, M. Gong, Q. Liu, L. Huang, and F. He, LaserPhys. Lett. 5, 21 (2008).5. C. Du, S. Ruan, H. Zhang, Y. Yu, F. Zeng, J. Wang, andM. Jiang, Appl. Phys. B 80, 45 (2005).6. R. Zhou, S. Ruan, C. Du, and J. Yao, Opt. Commun.281, 3510 (2008).7. H. Zhu, C. Huang, G. Zhang, Y. Wei, L. Huang, J.Chen, W. Chen, and Z. Chen, Opt. Commun. 270, 196(2007).8. F. Liu, J. He, B. Zhang, J. Xu, X. Dong, K. Yang, H. Xia,and H. Zhang, Opt. Express 16, 11759 (2008).9. Y. Inoue and S. Fujikawa, IEEE J. Quantum Electron.36, 751 (2000).10. K. M. Du, N. L. Wu, J. D. Xu, J. Giesekus, P. Loosen, R.Poprawe, Opt. Lett. 5, 370 (1998).11. K. M. Du, D. J. Li, H. L. Zhang, P. Shi, X. Y. Wei, andR. Diart, Opt. Lett. 2, 87 (2003).12. Y. Yan, H. Zhang, Y. Liu, J. Li, P. Sha, J. He, H. Zhang,and J. Xin, Opt. Commun. 282, 3124 (2009).13. A. E. Siegman, Lasers (University Science, 1986), p.864.14. P. Shi, D. Li, H. Zhang, Y. Wang, and K. Du, Opt.Commun. 229, 349 (2004).

Near-diffraction-limited, 35.4 W laser-diode end-pumped Nd:YVO4 slab laser operating at 1342 nm

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Near-diffraction-limited, 35.4 W laser-diode end-pumped Nd:YVO4 slab laser operating at 1342 nm

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Institute Of Mechanics,Chinese Academy of Sciences