IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 27, NO. 4, FEBRUARY 15, 2015 407 Efficient High-Peak-Power Wavelength-Switchable Femtosecond Yb:LGGG Laser Fei Lou, Zhi-Tai Jia, Jing-Liang He, Ru-Wei Zhao, Jia Hou, Zhao-Wei Wang, Shan-De Liu, Bai-Tao Zhang, and Chun-Ming Dong Abstract—A systematic study of the spectral properties and ultrafast lasing characteristics of Yb:(Lu x Gd 1-x ) 3 Ga 5 O 12 ( x = 0.062) (Yb:LGGG) disordered crystal was reported for the first time. The broad fluorescence spectral lines indicate a great potential of Yb:LGGG for tunable and ultrafast laser application. In the lasing experiment, a passively mode-locked Yb:LGGG laser was realized, which generated a maximum average power of 1.3 W with pulse duration of 324 fs at the central wavelength of 1029.6 nm, corresponding to a slope efficiency as high as 41.6%. The pulse energy and peak power were calculated to be 22.4 nJ and 69.2 kW. By optimizing the cavity design, the shortest pulse duration of 188 fs was achieved at the central wavelength of 1026.8 nm. In this case, the maximum average output power of 0.77 W was generated with a slope efficiency of 27.1%. The experimental results show the excellent potential of Yb:LGGG crystal for high-efficiency and high-peak- power femtosecond laser. Index Terms— High-peak-power, wavelength-switchable, passively mode-locked, Yb:LGGG. I. I NTRODUCTION D UE to the stable structure, remarkable thermal resis- tance and optical properties, garnets have attracted great attention in laser application. Typical garnet host materials (YAG, GGG, YGG, etc.) for doping of rare earth active ions, have been widely investigated with impressive results given out [1]–[5]. In the gadolinium gallium garnet, which can be expressed as Gd 3 [Gd 0.02 Ga 1.98 ]Ga 3 O 12 [6], the Gd 3+ ions occupying octahedral sites and dodecahedral sites can be partially replaced by Lu 3+ ions of smaller ionic radius, forming an additional class of lutetium gadolin- ium gallium garnets having a general chemical formula of (Lu x Gd 1-x ) 3 Ga 5 O 12 (LGGG). So far, the lasing character- istics of Nd 3+ -doped LGGG crystal have been extensively studied [7]–[9]. Relative to Nd 3+ ion, Yb 3+ ion has great advantages of low quantum defect, broad absorption and emission bandwidths, allows low-concentration quenching even at high doping Manuscript received September 14, 2014; accepted November 21, 2014. Date of publication November 26, 2014; date of current version January 28, 2015. This work was supported in part by the National Natural Science Foun- dation of China under Grant 51321091, Grant 61275142, Grant 51202128, and Grant 91022003, and in part by the Program of Introducing Talents of Disciplines to Universities (111 program) of China under Grant b06015. (Corresponding author: Jing-Liang He.) The authors are with the State Key Laboratory of Crystal Materials, Shandong University, Jinan 250100, China (e-mail: louphy@126.com; z.jia@sdu.edu.cn; jlhe@sdu.edu.cn; Wangzw_369@126.com; houjia334@ sina.com; lingyinzi@yeah.net; pepsl_liu@163.com; bai3697@126.com; 36315034@qq.com). Color versions of one or more of the figures in this letter are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/LPT.2014.2375178 concentrations. It is therefore interesting to explore the laser performance of the Yb 3+ ion doped in such kind of garnet. Recently, with lutetium replacing yttrium in the YGG crystal, LGG has been developed [10], [11]. The continuous-wave and Q-switched lasers are achieved and with impressive results given out [12], [13]. Besides, the scintillating properties of Yb:LGG is also studied [14]. As for the ultrafast laser performance of Yb:LGG crystal, there is no related report. Compared to the Yb:LGG crystal, more degree of disorder will be induced inside the Yb:LGGG crystal due to the random distribution of Lu 3+ and Gd 3+ ions at the same lattice sites, which will give rise to inhomogeneous broadening in the absorption or emission spectra. Therefore, Yb:LGGG could be a potential laser crystal for the high efficiency ultrafast laser operation, which possess both the advantages of Yb 3+ ion and disordered crystal structure. Up to now, Yb: LGGG crystal has never been investigated as far as we know. In this letter, the spectroscopic properties of Yb:(Lu x Gd 1-x ) 3 Ga 5 O 12 (x = 0.062) (Yb:LGGG) disordered crystal were systematically studied. Moreover, with a semiconductor saturable absorber mirror (SESAM) for passive mode locking and two Gires-Tournois interferometer (GTI) mirrors for dispersion compensation, the highly efficient wavelength-switchable femtosecond operation of Yb:LGGG crystal was achieved. The mode-locked laser wavelength could be switched between 1026-nm band and 1030-nm band with different cavity design. II. SPECTROSCOPIC PROPERTIES In this letter, Yb:LGGG single crystal with a Yb 3+ ion dopant concentration of 7.74 × 10 20 cm -3 was grown by the Czochralski method. Considering the strong electron- phonon coupling in Yb 3+ -doped materials, which disturbs the Yb 3+ energy transition and generates some additional emission peaks [15], [16], it is difficult to interpret the infor- mation from the emission spectra at room temperature (RT). Measuring the low-temperature fluorescence spectrum can avoid these problems and provide the accurate energy level scheme. Based on charge transfer luminescence of Yb 3+ ion [17], the emission spectrum at 77 K was measured with an excitation wavelength of 275 nm as shown in Fig. 1. The Yb 3+ energy level diagram of Yb:LGGG has been identified. The RT absorption spectra of Yb:LGGG measured over a wavelength range of 850 to 1100 nm are shown in Fig. 2. The strongest absorption peak located around 935 nm and it is similar with Yb:YAG (940 nm) [18], while it is different from those of Yb:YGG (970 nm) [1] and Yb:GGG (971 nm) [14]. The maximum absorption cross-section is 0.75 × 10 -20 cm 2 1041-1135 © 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.