IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 21, NO. 1, JANUARY/FEBRUARY 2015 1601006 Power Scaling in a Diode-End-Pumped Multisegmented Nd:YVO 4 Laser With Double-Pass Power Ampliﬁcation Yu-Jen Huang, Wei-Zhe Zhuang, Kuan-Wei Su, and Yung-Fu Chen Abstract—We demonstrate a high-power master oscillator power ampliﬁer with the double-pass conﬁguration based on the specially designed multisegmented Nd:YVO 4 crystals. A powerful mathematical technique on the basis of the Fourier eigenfunction expansion method is developed for precisely calculating the tem- perature distribution inside the gain medium. A seed Nd:YVO 4 oscillator under dual-end pumping is subsequently constructed for efﬁciently emitting the output power of up to 50 W. Moreover, under a total incident pump power of 244 W at 808 nm, as high as 108 W of the output power at 1064 nm is further generated in our developed master oscillator power ampliﬁer system. Theoret- ical and experimental results clearly reveal that the gain medium with multiple doping concentrations is practically valuable for con- structing a high-power end-pumped laser without bringing in sig- niﬁcantly thermal effects. Index Terms—Diode-pumped laser, high-power laser, master oscillator power ampliﬁer, multisegmented laser crystal. I. INTRODUCTION O VER the past few decades, high-power solid-state lasers were rapidly developed because they are useful for many scientiﬁc studies and industrial applications [1]–[3]. For the ex- tension of the power scale-up in the end-pumped oscillator, the noticeable thermal gradient and accompanied mechanical stress inside the gain medium are the most critical issues to be solved. This is due to the fact that the homogeneous doping proﬁle in the active element leads to the exponential decay of the pump light along the longitudinal direction. With an undoped mate- rial to effectively serve as a heat sink, the composite crystal has recently proven its feasibility in reducing the spatial gra- dient of the temperature and the thermally induced mechanical stress [4]–[7]. More recently, the Nd:YAG crystal with increas- ing doping concentrations was proposed to show that employing the so-called multi-segmented crystal could not only avoid the risk of the thermal fracture inside the laser material, but also maintain the high optical conversion efﬁciency [8], [9]. Manuscript received March 31, 2014; revised June 10, 2014; accepted June 25, 2014. This work was supported by the National Science Council under Grant NSC-100–2628-M-009-001-MY3. Y.-J. Huang, W.-Z. Zhuang, and K.-W. Su are with the Department of Electrophysics, National Chiao Tung University, Hsinchu 30010, Taiwan (e-mail: yujenhuang@nctu.edu.tw; edward10517@yahoo.com.tw; sukuanwei @mail.nctu.edu.tw). Y.-F. Chen is with the Department of Electrophysics, National Chiao Tung University, Hsinchu 30010, Taiwan, and also with the Department of Electronics Engineering, National Chiao Tung University, Hsinchu 30010, Taiwan (e-mail: yfchen@cc.nctu.edu.tw). Color versions of one or more of the ﬁgures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identiﬁer 10.1109/JSTQE.2014.2336541 The concept of the master oscillator power ampliﬁer (MOPA) offers another means for boosting the output power of a laser, and it has been widely realized in ﬁber- and bulk-based architec- tures [10]–[14]. The required high-power performance can be relatively easily achieved in the MOPA by partially decoupling the problems usually encountered in the high-power laser oscil- lator, including the possible instability caused by the multi-mode interactions, thermal-lensing effect, and so on. The Nd:YVO 4 crystal, due to the large product of the stimulated emission cross section and the upper-state lifetime, can produce much higher optical gain as compared with other Nd-doped laser ma- terials. To date, most of the Nd:YVO 4 ampliﬁers are based on the single-pass conﬁguration [12]–[14]. However, some recent works have shown that the double-pass architecture seems to be a more efﬁcient design for scaling the output power of the pulsed oscillator [15], [16]. In this work, an efﬁcient high-power MOPA based on the multi-segmented Nd:YVO 4 crystal is suc- cessfully realized for emitting output power greater than one hundred watts in the continuous-wave operation. We ﬁrst utilize the Fourier eigenfunction expansion method to develop a pow- erful mathematical technique for analytically solving the heat conduction equation of the anisotropic crystal with a rectangu- lar geometry. Theoretical analysis manifestly reveals that the smoother temperature distribution could be achieved inside the multi-segmented crystal than the conventional composite one. Based on the calculated results, we construct a dual-end-pumped multi-segmented Nd:YVO 4 oscillator for efﬁciently producing the output power of 50 W. We subsequently design two types of MOPA and make a systematical comparison between both conﬁgurations. It is experimentally found that the power gain obtained from the double-pass MOPA is generally larger than that obtained from the single-pass one. Consequently, the output power could be further scaled to reach 108 W at 1064 nm under a total incident pump power of 244 W at 808 nm, corresponding to the optical conversion efﬁciency of up to 44.3%. II. THEORETICAL ANALYSIS ON TEMPERATURE DISTRIBUTION Fig. 1(a) schematically depicts the thermal model of the multi-segmented crystal with a rectangular geometry used for our theoretical analysis, which is end-pumped from two sides. The lengths for each side of the crystal are a, b, and c, re- spectively. The current multi-segmented crystal is made up of ﬁve sections with three doped materials between two undoped end-caps, where the doped parts are characterized by pump ab- sorptions α 1 , α 2 , and α 3 , and heat source densities q 1 (x, y, z), 1077-260X © 2014 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications standards/publications/rights/index.html for more information.