Spectroscopic Properties and Laser Performance of ${\hbox {Er}}^{3+

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IEEE JOURNAL OF QUANTUM ELECTRONICS, VOL. 43, NO. 10, OCTOBER 2007

Spectroscopic Properties and Laser Performance of Er3+ and Yb3+ Co-Doped GdAl3(BO3)4 Crystal Yujin Chen, Yanfu Lin, Xinghong Gong, Zundu Luo, and Yidong Huang

Abstract—An Er:Yb:GdAl3 (BO3 )4 crystal was grown and room-temperature polarized absorption, emission, and gain spectra were investigated. Fluorescence decay curves of Er3+ at 1530 nm and Yb3+ at 1040 nm in the crystal were measured. Efficient laser operation of Er:Yb:GdAl3 (BO3 )4 crystal at 1.5–1.6 m was realized. Quasi-continuous-wave output powers of 1.8 W with slope efficiency of 19% and 0.78 W with slope efficiency of 14% were achieved in diode-pumped c-cut and a-cut crystals, respectively. The output spectrum and polarization of Er:Yb:GdAl3 (BO3 )4 laser were also investigated. Index Terms—Diode-pumped solid-state laser, Erbium–Ytterbium laser, Er:Yb:GdAl3 (BO3 )4 crystal, spectroscopic property.

(GAB) crystal is in the same family as the GdAl BO YAB crystal, which belongs to the trigonal system with the space group R32. It possesses good chemical and physical properties, and has been proved as excellent laser and nonlinear optical crystals. Growth, spectra, and laser operation have been investigated in Nd and Yb single-doped GAB crystals [5]–[7]. In this paper, detailed spectroscopic properties and efficient laser operation at 1.5–1.6 m of Er:Yb:GAB crystal are reported. The output spectrum and polarization of laser are also investigated. Er:Yb:GdAl BO II. SPECTROSCOPIC PROPERTIES

I. INTRODUCTION

L

ASER radiation at 1.5–1.6 m has excellent transparency in both the atmosphere and fused-silica fiber, and is located in the eye-safe wavelength range and sensitive region of roomtemperature infrared light detectors (such as Ge and InGaAs photodiodes). Therefore, 1.5–1.6- m lasers have many practical applications, such as optical fibers communication, medicine, laser-range-finding, and lidar [1]–[4]. Erbium laser at 1.5–1.6 m corresponding to the transition operates in quasi-three-level scheme. To reduce the lasing threshold and improve the absorption efficiency of Erbium laser, a large amount of Yb ions is co-doped into the materials as sensitizers and then transfers energy to Er ions [3]. Many investigation have shown that the hosts with high effective phonon energy, good mechanical and thermal properties are more favorable for realizing a high 1.5–1.6- m laser performance and avoiding fracture at high pump power [2]–[4]. Borate crystals have been considered as one of the most ideal hosts because they have high effective phonon energy of about 1400 cm [2]–[4], and better thermal property than the phosphate glass, which is the main host of 1.5–1.6- m laser [1]. Manuscript received May 17, 2007; revised July 21, 2007. This work was supported in part by the National Natural Science Foundation of China under Grants 50590405 and 50572105, in part by the Major Programs of Science and Technology Foundation of Fujian Province under Grant 2005HZ1024, and in part by the Natural Science Foundation of Fujian Province under Grant A0610031. Y. Chen is with the National Engineering Research Center for Optoelectronic Crystalline Materials, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian, 350002, China and Graduate School of the Chinese Academy of Sciences, Beijing 10039, China, e-mail addresses: [email protected], fax number: +86-591-83714946. The authors are with the National Engineering Research Center for Optoelectronic Crystalline Materials, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, China (e-mail: [email protected]; [email protected]; [email protected]; [email protected]. cn). 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/JQE.2007.904308

A. Absorption Properties An Er:Yb:GAB crystal with size of 30 20 20 mm was grown by the top seeded solution method and the growth procedure is similar to that of Yb -doped GAB crystals [6]. Er and Yb concentrations in the grown crystal were measured to be 1.1 and 20.7 at.%, respectively, corresponding to ion densities of 0.61 10 and 11.44 10 cm , respectively. Taking 1.1 at.% Er and 20 at.% Yb concentrations in the initial melt into account, the segregation coefficients of Er and Yb ions in the GAB crystal were calculated to be close to unity, which shows that Er and Yb ions are distributed uniformly in the GAB crystal. The ion densities of the dopants were close to those used in YCOB crystal (0.62 10 and 13.2 10 cm for Er and Yb ions, respectively) [2], and may not be the optimum values for GAB crystal. Furthermore, the X-ray powder diffraction (XRD) pattern of the Er:Yb:GAB crystal shows that all of the diffraction peaks are in agreement with those of pure GAB crystal. Therefore, the influence of the large Yb concentration on the crystallographic parameters of host GAB crystal is slight. The polarized absorption spectra were recorded at room temperature using a spectrophotometer (Lambda900, Perkin-Elmer) and are shown in Figs. 1–3. Except the absorption band around 976 nm, which is consisted of the transitions of Yb and of Er , of the other bands are related to the transitions originated from ground multiplet to different excited multiplets the of Er ions, which are depicted in the figures. The absorption spectra show strong polarized dependence due to the anisotropy of the crystal. Similar to that of Yb:GAB crystal [6], the peak -polarized absorption cross section at 976 nm is approximately an order of magnitude larger than that of the polarization, as shown in Fig. 2. The peak absorption cross section and full-width at half-maximum (FWHM) of cm and 20 nm, Er:Yb:GAB crystal at 976 nm (3.4 10

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CHEN et al.: SPECTROSCOPIC PROPERTIES AND LASER PERFORMANCE OF Er

Fig. 1. Room-temperature polarized absorption spectra of Er:Yb:GAB crystal in a range from 300 to 850 nm.

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B. Infrared Emission Properties


respectively, for polarization) are comparable to those of cm and 27 nm, respectively, Er:Yb:YAB crystal (2.5 10 for polarization) [8], and larger than those of Er:Yb:YCOB crystal (about 0.9 10 cm and 3 nm, respectively) [9], [10]. It implies that compared with Er:Yb:YCOB crystal, which has been demonstrated to be the most efficient crystal for 1.5–1.6 m laser till now [2], Er:Yb:GAB crystal may be a more suitable gain medium for diode-pumped 1.5–1.6 m laser [11], [12]. When an unpolarized pump source is adopted, a c-cut Er:Yb:GAB crystal with Yb concentration of 20.7 at.% and thickness of 0.4 mm can absorb about 80% incident pump power at 976 nm. It is well known that the magnetic-dipole has a significant and of Er contribution to the transition between ions [13]. So, the complete polarized absorption spectra related to the transition, i.e., , , and polarizations [14], were recorded and are shown in Fig. 3. Due to the polarization effect of magnetic-dipole transition between different Stark levels of and multiplets, there is distinction in the central region of the absorption bands for and polarizations. By means of the Judd–Ofelt theory [15], [16], three intensity parameters of Er ions in the crystal can be obtained and its detailed calculated process can be found in many papers [17], [18].

Polarized fluorescence spectra of the crystal in a range from 900 to 1700 nm were recorded at room temperature by a spectrophotometer (FL920, Edinburgh, U.K.) when the exciting wavelength was 976 nm. The sensitive region of detected system is located around 1 m. For all polarizations, an intensive fluorescence band around 1530 nm, originated from transition of Er ions, and a weak band the around 1040 nm were observed. The fluorescence around 1040 transition nm is mainly originated from the ions, because the multiphonon relaxation from the of Yb to multiplets of Er ions is rapid due to the high effective phonon energy of GAB crystal. Using the calculated intensity parameters, the values of elec, magnetic-dipole trantric-dipole transition probabilities , and total spontaneous emission probsition probabilities abilities with different polarizations for the transition in Er:Yb:GAB crystal were estimated [17], [18], and are listed in Table I. The stimulated-emission cross section can be calculated by the Fuchtbauer–Ladenburg ( - ) formula [18], [19] (1) where indicates the polarization of the fluorescence spectra, is the fluorescence intensity at wavelength , is refractive index of crystal and the value of Er:YAB crystal was adopted in the calculation [20]. The wavelength dependences of the stimulated-emission cross sections around 1530 nm with different polarizations for Er:Yb:GAB crystal are shown in Fig. 4. Similar to the absorption cross section, the difference of the stimulatedemission cross sections between and polarizations is also mainly in the central region of emission bands. Furthermore, for transition, the stimulated-emission cross the sections can be calculated alternately from the absorption cross sections by the reciprocity method (RM) [21]. Because the Stark energy-level structure of Er ions in GAB crystal is unavailable, the energy-level diagram of Er:YAB crystal is adopted approximately in the calculation [22], which is acceptable due to

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POLARIZED SPECTROSCOPIC PARAMETERS OF THE

I

!

TABLE I

Fig. 4. Room-temperature polarized emission spectra of Er:Yb:GAB crystal in a range from 1450 to 1650 nm calculated by the Fuchtbauer-Ladenburg (F -L) formula. The -polarized emission cross section calculated by the reciprocity method (RM) is also shown for comparison.

the similar crystal structures of both crystals. For the brevity, only the -polarized emission cross section calculated by the RM is shown in Fig. 4. The good agreement between emission cross sections obtained from above two methods showed that the fluorescence spectra around 1530 nm of Er:Yb:GAB crystal recorded in this work were not distorted by radiation trapping effect [23], [24] and the influence of this effect on spectroscopic properties of Er ion can be neglected in this crystal. The deviation of cross sections around 1600 nm is mainly caused by the low signal-noise ratio of absorption spectrum in this region, which is exponentially expanded in the calculation [25]. Following the absorption and emission cross sections calculated by the - formula, the gain cross section can be obtained. Here, is the population inversion of Er ions. The wavelength dependences of polarized gain cross sections around 1530 nm of the crystal for 0.3, 0.4, 0.5 and 0.6) are shown in Fig. 5. several values ( For all polarizations, the wavelengths of the laser oscillating under the lowest population inversion are all located around 1600 nm and when increases, a laser oscillating at shorter wavelength can be realized. The radiative lifetime of a multiplet is the reciprocal of the total spontaneous emission probabilities from this multiplet. For a uniaxial crystal, the radiative lifetime is . According to the data in multiplet of Er Table I, the radiative lifetime of the ions in the crystal was calculated to be 3.72 ms. , Edinburgh, U.K.) as Using a microsecond flash lamp ( the exciting source, the fluorescence decay signals at 1530 and

I

TRANSITION OF Er

IONS IN Er:Yb:GAB CRYSTAL

Fig. 5. Room-temperature polarized gain spectra of Er:Yb:GAB crystal in a range from 1500 to 1650 nm.

Fig. 6. Room-temperature fluorescence decay curves of Er at 1530 nm and Yb at 1040 nm in Er:Yb:GAB crystal. The exciting wavelength is 976 nm.

1040 nm were recorded by a spectrophotometer (FL920, Edinburgh) when the exciting wavelength was 976 nm and the results are shown in Fig. 6 in a semilog scale. By linear fitting, the flumultiplet of Er and mulorescence lifetimes of tiplet of Yb ions in the crystal were estimated to be about 300 and 20 s, respectively. The presence of Yb ions may affect the measurement of the fluorescence lifetime of multiplet of Er ions. However, due to the fast energy transfer from Yb to Er ions and the weak energy transfer from Er to Yb ions, the influence may be slight [26]. By comparing the multiple, the fluorescence and radiative lifetimes of the fluorescence quantum efficiency of this multiplet was calculated to be 8%, which is similar to that of Er:YAB crystal [17]. The multiplet in low fluorescence quantum efficiency of the the crystal is caused by the high effective phonon energy of borate crystal and becomes a disadvantage for laser operation, because the nonradiative relaxation process is one of the main rea-


SPECTROSCOPIC PARAMETERS OF Er

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TABLE II Yb CO-DOPED GAB, YAB, AND YCOB CRYSTALS

sons for the pumped-induced thermal load in the crystal. Therefore, for reducing the influence of the thermal load on laser performance of Er:Yb:GAB crystal [27], some techniques must be adopted in the laser experiment. The efficiency of energy transfer from Yb to Er ions, , in co-doped system is quantified by [2] (2)

where and are the fluorescence lifetimes of Yb ions in the co-doped and single-doped materials, respectively. According to the fluorescence lifetimes of the high Yb -doped GAB crystals reported previously (993, 298, and 45 s for 10, 12.5, and 50.9 at.%, respectively) [6], [7], an approximation that the fluorescence lifetime of 20.7 at.% Yb:GAB crystal is in the level of hundred microseconds should be acceptable. Therefore, the energy transfer efficiency in the Er:Yb:GAB crystal was close to 90%, which is similar to that of Er:Yb:YCOB crystal [9]. For comparison, spectroscopic parameters of Er and Yb co-doped GAB, YAB, and YCOB crystals are listed in Table II [8], [9], [17], [28]. Compared with those of Er:Yb:YAB and Er:Yb:YCOB crystals, the larger absorption cross section for Er:Yb:GAB crystal implies that a thinner medium at the same dopant concentration can be used, which can reduce the internal loss originated from the crystal and realize a better coupling between tightly focused pump light with large divergence and laser beam in the crystal. The broader FWHM of absorption band of Er:Yb:GAB crystal than that of Er:Yb:YCOB crystal shows that the wavelength control of diode laser used as pump source is less stringent. Combining with the high energy transfer efficiency, it is expected that Er and Yb co-doped GAB crystal can become a good medium for realizing 1.5–1.6- m laser. Furthermore, it is well known that the threshold power of 1.5–1.6- m laser is inverse proportional to the product of emission cross section and fluorescence lifetime multiplet of Er ions [29]. It can be seen from of the the Table II that this product for Er:Yb:GAB crystal is between those of Er:Yb:YAB and Er:Yb:YCOB crystals. Then, it can be expected that the threshold power of Er:Yb:GAB laser is moderate among them.

III. LASER EXPERIMENTAL SETUP A -cut, 0.7-mm-thick and an -cut, 1.34-mm-thick Er:Yb:GAB crystals were end-pumped by a 970-nm fiber-coupled diode laser (800- m diameter core) from Coherent Inc. The flat input mirror had 90% transmission at 970 nm and 99.8% reflectivity at 1.5–1.6 m. The uncoated crystal was held in an aluminum mount and placed as close as possible to the input mirror. No special care was taken to ensure good thermal contact or cooling of the crystals; for reducing the influence of the pump-induced thermal load on laser performance and avoiding the fracture of crystal at high pump power [27], square pulse mode of the diode laser was used. The width of pump pulse was 2 ms and duty cycle was 2%. The pump laser was collimated and focused into the crystals. Two radii of the focused pump 110 and 145 m were used. The - and -polarized spot absorption cross sections of the crystal at 970 nm are about and 0.13 10 cm (see Fig. 2), resulting in 2.16 10 about 80% and 70% absorption of the unpolarized incident pump power in a single pass of the c-cut and a-cut crystals, respectively. Two output couplers (OC) with the same radius of curvature 1.0% and 1.5%) (RoC) 50 mm and different transmissions ( at 1.5–1.6 m were used to complete the cavity. The reflectivities at 970 nm of both OCs were higher than 98%. The cavity length was kept at about 50 mm. The distance between the flat input mirror and the input face of the crystal was about 1.0 mm and formed a thin-air–space etalon. The output laser spectra were recorded with a monochromator (Triax550, Jobin-Yvon). IV. RESULTS AND DISSCUSSIONS The laser experimental results of c-cut and a-cut Er:Yb:GAB crystals are summarized in Table III. The highest output power obtained in Er:Yb:GAB crystal is lower than that in Er:Yb:YAB crystal (2.0 W) [4] but still far higher than those reported in other Er and Yb co-doped materials operating in 1.5–1.6 m [1]–[3]. Because the duty cycle of the quasi-continuous-wave (quasi-CW) diode laser was 2%, the values of power given in the figure were obtained by multiplying the measured average values by fifty. The thermal fracture of the crystals was not observed at any point under our experimental conditions. When the duty cycle was increased, the output laser power at the same pump power was reduced due to the influence of the larger pump-induced thermal load.

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TABLE III LASER EXPERIMENTAL RESULTS OF DIODE-PUMPED Er:Yb:GAB CRYSTALS

Fig. 7. 1.5–1.6 m laser output power versus absorbed pump power at 970 nm for c-cut Er:Yb:GAB crystal.

Fig. 8. Spectra of c-cut Er:Yb:GAB laser when the transmission of the output coupler is 1.0% and absorbed pump power is 13.6 W. The spot radius of pump beam is 145 m.

A. Laser Performance of c-Cut Crystal Fig. 7 shows the measured laser output power versus absorbed pump power for different OC transmissions and pump spot radii . As shown in Fig. 7 and Table III, the results of 1.0% and 1.5% are similar and only those of are discussed for the sake of brevity. m and , the maximum output For power of 1.1 W was achieved when the absorbed pump power was 10.8 W. The absorbed pump threshold power was 2.45 W and slope efficiency was 15% when absorbed pump power was higher than 7.0 W. The lower slope efficiency near the threshold is caused by the quasi-three-level nature of Er laser operating at 1.5–1.6 m [29]. The amplitude instability of the output power was measured to be less than 3.0% in a 10-min period. increased to 145 m, the maximum absorbed pump When power increased to 13.6 W because of the better coupling of pump laser beam and collimate-focused system. In this case, the maximum output power, threshold power, and slope efficiency increased to 1.80 W, 3.38 W, and 19%, respectively, for . With the optimization of Er and Yb concentrations in GAB crystal and better matching between pump and cavity mode in the future, improvement of laser performance can be expected. From Table III, it can be seen that the absorbed pump threshold power obtained in Er:Yb:GAB crystal is between those of Er:Yb:YAB (3.3–4.7 W) [4] and Er:Yb:YCOB (0.8–1.0 W) [2] crystals. This result is in agreement with the prediction obtained from the spectroscopic parameters listed in Table II. However, Er:Yb:GAB crystal can be considered as a more suitable medium to realize high-power laser operation because of its better thermal property than Er:Yb:YCOB crystal. For c-cut Er:Yb:GAB crystal, which belongs to uniaxial crystal,

the isotropy of the thermal conductivity and thermal expansion coefficient in input and output faces of the crystal leads to a higher thermal fracture limit than that of Er:Yb:YCOB crystal, which belongs to biaxial crystal as the Er:Yb:GdCa O BO (Er:Yb:GCOB) crystal [30]. m and , there was Except the case of always only one longitudinal mode group around 1600 nm observed in the output spectra of Er:Yb:GAB laser for all the absorbed pump power in this work. As an example, the output and spectrum at absorbed pump power of 13.6 W when are 145 m and 1.0%, respectively, is shown in Fig. 8. It is consistent with the -polarized gain curve for a small value shown in Fig. 5. There were six modes in the longitudinal mode group, as shown in the inset of Fig. 8. The wavelength spacing between these laser modes was approximately 0.7 nm and corresponding to the free spectral range of the air etalon with spacing m and of about 1.0 mm [31]. For the case of , when absorbed pump power was lower than 7.0 W, there was also only one longitudinal mode group around 1600 nm in the output spectrum, as shown in the Fig. 9(a). However, when absorbed pump power increased, it can be seen from Fig. 9 that the longitudinal mode groups around 1580 and 1550 nm appeared and the relative intensities for different longitudinal mode groups changed with absorbed pump power. It shows that with the increment of absorbed pump power, the main output laser power gradually flows from the longitudinal mode group around 1600 nm to that around 1550 nm. The phenomenon is similar to that observed in Er:Yb:YAB crystal [4] and may be originated from the change of the gain and cavity losses [4], [32]. The fact that the oscillating wavelength of Er:Yb:GAB


Fig. 9. Spectra of c-cut Er:Yb:GAB laser when the transmission of the output coupler is 1.5% and the spot radius of pump beam is 145 m. The absorbed pump powers are: (a) 7.0 W, (b) 9.0 W, (c) 11.0 W, and (d) 13.6 W.

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power and lower slope efficiency in the a-cut crystal than in the c-cut crystal is caused by the larger internal loss for the thicker crystal. Furthermore, there was always only one longitudinal mode group around 1600 nm in the output spectra of all the a-cut Er:Yb:GAB lasers in this work. The shape of output spectra was similar to that in the c-cut crystal except a slight blue-shift of oscillating wavelength (in a range from 1599 to 1602 nm). According to the gain curves shown in Fig. 5, the -polarized gain cross section was larger than the -polarized cm and 0.10 10 cm at , re(0.17 10 spectively). Then, it should be expected that -polarized laser radiation might be more favorable in the a-cut Er:Yb:GAB crystal. However, -polarized laser radiation was always generated in the a-cut Er:Yb:GAB crystal in this work, which may be originated from anisotropy of thermal conductivity and asymmetric cooling in the a-cut crystal. The linearly polarized laser generated in a-cut Er:Yb:GAB crystal may be advantageous for many practical applications. But from the viewpoint of thermal property, the anisotropic a-cut crystal may not be an appropriate gain medium used in high-power laser. V. CONCLUSION

Fig. 10. 1.6 m laser output power versus absorbed pump power at 970 nm for a-cut Er:Yb:GAB crystal.

laser varies with the condition of cavity and pump light must be considered for some technique applications, such as passively -switched operation and selecting optimal wavelength for optical communication and medicine. Theoretically, the polarization state of output laser for c-cut Er:Yb:GAB laser would be undefined. However, an elliptical polarization of output laser was always observed for all the above cases. The ratio of output powers between horizontal and vertical polarizations was measured to be about two. It may be associated with the induced anisotropy by thermal stress in the gain medium [33]. In our experiment, the thermal dissipation in the Er:Yb:GAB crystal was not uniform due to the lack of effective cooling of the crystal. B. Laser Performance of a-Cut Crystal Efficient laser operation was also realized in the 1.34-mmthick, a-cut Er:Yb:GAB crystal (see Fig. 10). For m , the threshold power, and slope efficiency were and 4.20 W and 10%, respectively, and the maximum output power of 0.55 W was achieved when the absorbed pump power was increased to 145 m and kept at 1.0%, 9.45 W. When the maximum output power increased to 0.78 W when the absorbed pump power was 11.9 W and slope efficiency increased to 14%. The absorbed pump threshold power was 5.90 W. It can also be seen from Fig. 10 and Table III that the change of OC transmissions from 1.0% to 1.5% has not significant influence on laser output power and slope efficiency of the crystal for the pump power scale used in this work. The higher threshold

and An Er:Yb:GAB crystal doped with 1.1 at.% Er 20.7 at.% Yb concentrations was grown and polarized absorption and fluorescence spectra were investigated in detail at multiroom temperature. The fluorescence lifetimes of multiplet of Yb in the crystal were plet of Er and measured to be about 300 and 20 s, respectively. Spectroscopic investigations showed that the energy transfer efficiency from Yb to Er ions in the crystal was close to 90% and Er and Yb co-doped GAB crystal is a good medium for realizing 1.5–1.6- m laser. High-power laser performance at 1.5–1.6 m of Er:Yb:GAB crystal were also investigated. In a diode-pumped hemispherical cavity, quasi-CW output powers of 1.8 W with slope efficiency of 19% and 0.78 W with slope efficiency of 14% were achieved in c-cut and a-cut crystals, respectively, when the output coupler transmission was 1.0%. The influences of the pump power and output coupler transmission on output laser spectra were studied. The elliptical and -polarized laser radiations were obtained in c-cut and a-cut crystals, respectively. REFERENCES [1] P. Laporta, S. Taccheo, S. Longhi, O. Svelto, and C. Svelto, “Erbiumytterbium microlasers: Optical properties and lasing characteristics,” Opt. Mater., vol. 11, pp. 269–288, 1999. [2] P. Burns, J. M. Dawes, P. Dekker, J. A. Piper, H. Jiang, and J. Wang, “Optimization of Er, Yb:YCOB for CW laser operation,” IEEE J. Quantum. Electron., vol. 40, no. 11, pp. 1575–1582, Nov. 2004. [3] B. Denker, B. Galagan, L. Ivleva, V. Osiko, S. Sverchkov, I. Voronina, J. E. Hellstrom, G. Karlsson, and F. Laurell, “Luminescence and laser properties of Yb-Er:GdCa O(BO ) : A new crystal for eye-safe 1.5-m lasers,” Appl. Phys. B, vol. 79, pp. 577–581, 2004. [4] Y. J. Chen, Y. F. Lin, X. H. Gong, Q. G. Tan, Z. D. Luo, and Y. D. Huang, “2.0 W diode-pumped Er:Yb:YAl (BO ) laser at 1.5–1.6 m,” Appl. Phys. Lett., vol. 89, p. 241111, 2006. [5] A. Brenier, C. Tu, Z. Zhu, and B. Wu, “Red-green-blue generation from a lone dual-wavelength GdAl (BO ) :Nd laser,” Appl. Phys. Lett., vol. 84, pp. 2034–2036, 2004. [6] J. Liao, Y. Lin, Y. Chen, Z. Luo, and Y. Huang, “Growth and optical properties of Yb Gd Al (BO ) single crystals with different Yb concentration,” J. Alloys Comp., vol. 397, pp. 211–215, 2005.

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Yujin Chen received the B. Sc. degree in material from the Lanzhou University, Lanzhou, China, in 1999 and the M.Sc. degree in condensed matter physics from Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Beijing, China, in 2004, where he is currently working toward the Ph. D. degree. He is currently an Assistant Professor at the Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Beijing, China. His fields of research include solid-state lasers and materials.

Yanfu Lin received the B.Sc. degree in material from the Fuzhou University, Fuzhou, China, in 1986. He is currently a Senior Engineer at the Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Beijing, China, where he is responsible for growing and characterizing optoelectronic crystals.

Xinghong Gong received the B.Sc. degree in material from the Wuhan University of Technology, Wuhan, China, in 2001 and the M.Sc. degree in condensed matter physics from Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, in 2007. He is currently an Assistant Professor at the Fujian Institute, where he is responsible for growing and characterizing laser crystals.

Zundu Luo received the B.Sc. degree in physics from the Xiamen University, Xiamen, China, in 1960. He is currently a Research Professor at the Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Beijing, China. His fields of research include solid-state laser materials and solid-state spectraoscopy.

Yidong Huang received the B.Sc. degree from the University of Science and Technology of China, Hefei, China, in 1986, and the Ph.D. degree from the University of Strathclyde, Glasgow, U.K., in 1997, both in physics. He is currently a Research Professor at the Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Beijing, China, where his research interests include solid-state laser materials and devices.