lable at ScienceDirect

Optical Materials 71 (2017) 90e97

Contents lists avai

Optical Materials

journal homepage: www.elsevier .com/locate/optmat

Fabrication, microstructure and laser performance of compositeNd:YAG transparent ceramics

Yuelong Fu a, Lin Ge a, Jiang Li a, *, Yang Liu b, Maxim Ivanov c, Lei Liu b, Hong Zhao b,Yubai Pan a, d, **, Jingkun Guo a

a Key Laboratory of Transparent and Opto-functional Advanced Inorganic Materials, Shanghai Institute of Ceramics, Chinese Academy of Sciences, 1295Dingxi Road, Shanghai 200050, Chinab Science and Technology on Solid-State Lasers Laboratory, North China Research Institute of Electro-Optics, Jiu Xianqiao Road No.4, Beijing 100015, Chinac Institute of Electrophysics, Ural Branch of Russian Academy of Sciences, 106 Amundsena Street, Ekaterinburg 620016, Russiad Shanghai Normal University, 100 Guilin Road, Shanghai 200234, China

a r t i c l e i n f o

Article history:Received 7 March 2016Received in revised form27 April 2016Accepted 7 May 2016Available online 15 May 2016

Keywords:Nd:YAGComposite ceramicsMicrostructureLaser performance

* Corresponding author.** Corresponding author. Shanghai Normal Universi200234, China.

E-mail addresses: lijiang@mail.sic.ac.cn (J. Li), ybp

http://dx.doi.org/10.1016/j.optmat.2016.05.0170925-3467/© 2016 Elsevier B.V. All rights reserved.

a b s t r a c t

“Sandwich” structure YAG/Nd:YAG/YAG ceramics with different core (Nd3þ doped area) lengths of 1 mm,5 mm and doping concentrations of 1 at.%, 2 at.% were prepared by dry pressing and vacuum sintering ofoxide powder mixture. Smaller average grain size was found in the specimen with the larger core andhigher doping concentration. With the increase of core length, the optimum transmission of outputcoupler increases from 10% to 19% and better laser performance can be obtained. However, longer corelength causes the noticeable thermal lens effect, which influences the beam quality significantly. It is alsoreflected in the thermally induced depolarized beam pattern, which becomes more obvious with thelonger core length and higher Nd3þ doping concentration.

© 2016 Elsevier B.V. All rights reserved.

1. Introduction

In the last decade, laser diode (LD) pumped solid-state lasers(DPSSL) have a rapid development due to the improvement ofoptical quality of gain media [1e3]. Among these media, neodym-ium doped yttrium aluminum garnet (Nd:YAG) is the most widelyused laser gain material in solid-state lasers [4e6] for the excellentphysical and chemical properties. However, there still remainsgreat challenges to solve the technical and economic issues of theNd:YAG single crystal growth. Therefore the researchers havemadegreat efforts to find a newmaterial to be used in DPSSL. Since Ikesueet al. firstly fabricated the highly transparent Nd:YAG ceramics [7],which are believed to be a promising candidate for solid-state la-sers because of their obvious advantages over single crystals, suchas high doping concentration, relatively low cost, short fabricationperiod and potential of sophisticated design [8e11]. Henceforward,

ty, 100 Guilin Road, Shanghai

an@mail.sic.ac.cn (Y. Pan).

Nd:YAG transparent ceramics have made a quick progress, espe-cially in the high power laser systems [12,13].

In the high power laser systems, thermal management is a keyissue, which limits the power scaling of the solid-state lasers[14,15]. Inhomogeneous temperature distribution in the gain mediaresults in mechanical and thermal stress and variation of refractiveindex. It will further evolve into the thermal lens effect and thermalinduced depolarization, which decreases the output power andmakes the laser beam quality worse [16,17]. The thermal effect canbe considerably reduced if the gain media has a dopant profile. Inthe case of single crystals the dopant profile can be designed withthermal diffusion bonding techniques, which is complicated andexpensive, while the fabrication of composite ceramics is simpleand rapid [18,19]. Some researchers have been done on the com-posite ceramics [20e22]. Yagi et al. demonstrated the fracturestrength of the composite Nd:YAG ceramics was comparable withone of the single crystal, and the destruction point was not locatedin interface region [23]. Tang et al. prepared Nd:YAG compositeceramics and achieved the laser output [24]. Liu et al. reported thepreparation of composite ceramics by solid-state reactive sintering,its microstructure and optical properties [25].

Y. Fu et al. / Optical Materials 71 (2017) 90e97 91

In view of the advantages of composite structure ceramics, wereported the fabrication of “sandwich” structure YAG/Nd:YAG/YAGceramics with different Nd3þ ion doping profile lengths and con-centrations in this paper. The microstructure, concentration dis-tribution, in-line transmittance of the composite ceramics wereinvestigated in detail. The laser output and thermal effects werealso compared for different composite ceramics.

2. Experimental procedure

Commercially available high-purity powders of a-Al2O3 (99.98%,Alfa Aesar, USA), Y2O3 (99.999%, Alfa Aesar, USA) and Nd2O3(99.99%, Alfa Aesar, USA) were used as starting materials. Tetrae-thoxysilane (TEOS, 99.999%, Alfa Aesar, USA) and magnesium oxide(MgO, 99.99%, Alfa Aesar, USA) were used as sintering aids. Thesepowders were blended together with the stoichiometric ratio of(NdxY1�x)3Al5O12 (x ¼ 0.01, 0.02) and mixed in ethanol for 12 h.Then the slurry was dried and sieved through a 200-mesh screen.After calcining at 800 �C, the powders were uniaxially pressed intodisks at about 15 MPa and further pressed by cold isostatic pressing(CIP) at 250 MPa. The as-obtained green bodies were sintered un-der vacuum at 1810 �C for 50 h. The ideal schematic of designedceramic slab along the thickness direction after cutting and pol-ishing is shown in Fig. 1. Two types of YAG/Nd:YAG/YAG ceramicswith different core (Nd3þ doped area) lengths were employed. Thecross section of all specimens is 3 � 3 mm2. The specimens wereannealed in air at 1450 �C for 10 h to get rid of internal stress and

Fig. 1. Schematic of designed ceramic slabs along the thickness direction afterprocessing.

Fig. 2. FESEM images of the polished and thermally etched surface of the YAG/Nd:YAG/YAG ceramics.

Fig. 3. Concentration distributions of Nd3þ ion along the thickness direction of theYAG/2 at.%Nd:YAG/YAG ceramics with different doping lengths.

Fig. 4. The in-line transmittances of the YAG/Nd:YAG/YAG ceramics.

Fig. 5. A scheme of the las

oxygen vacancies.The microstructures of the polished and thermally etched sur-

face of the specimens were observed by the field emission scanningelectron microscopy (FESEM, S-4800, Hitachi, Japan). Mirror-polished specimens on both surfaces were used to measure trans-mittance and absorption spectra by a UV-VIS-NIR spectrometer(Cary-5000, Varian, USA). The concentration distributions of Nd3þ

ions along the thickness direction of the composite ceramics weredetected by the inductively coupled plasma atomic emissionspectra-mass spectrometry (ICP-MS, Thermo Scientific XSERIES 2,USA).

3. Results and discussion

Fig. 2 shows the FESEM images of the polished and thermallyetched surface of the YAG/Nd:YAG/YAG ceramics. It is seen thatthere are still a few pores and no second phase in the ceramics. Thegrain size changes significantly with the increase of core length. ForFig. 2aed, the grain sizes are 47.1 mm, 43.2 mm, 31.8 mmand 21.2 mm,respectively.

In the case of 5 mm doping core length specimen (Fig. 2c, d),higher concentration of Nd3þ ions is clearly seen to inhibit YAGgrain growth. The influence of Nd3þ concentration on YAG grainsize was observed by Kochawattana [26]. Following his reasoningwe have to admit the decrease of growth rate in Nd:YAG ceramicsmay be explained by solute drag effects, which was firstly exploredby Cahn [27]. The segregation of the dopant at the grain boundary isvery effective for reducing the intrinsic grain boundary mobility.Segregation consists in an enhanced concentration of the dopant inthe grain near its boundary without the formation of a secondphase. Obviously, Nd3þ ion has a lower diffusion coefficient thaneither Y3þ ion or Al3þ ion due to its increase atomic mass and ionicradius; therefore, as the grains grow Nd3þ ions may gather at grainboundaries effectively slowing coarsening in the ceramics. Poreplays a similar role with high concentration. The pores aroundcenter area of the specimen with long core length will go throughlonger distance during sintering, which results in the grainboundary pin effect. So the grain size of the specimen with longcore length is much smaller.

Apparently discrepant data of 1 mm doping length specimen(Fig. 2a), as well as the considerable difference of grain size in thedifferent core length specimens (Fig. 2a, b) need an extrareasoning. As we know, Nd3þ ions diffuse during the sinteringprocess. This phenomenon can be detected by ICP-MS, and thedistributions of Nd3þ ions along the thickness direction of thespecimens were determined by line scanning from one surface toanother (two different distances), as shown in Fig. 3. No matterhow the core length and doping concentration change, the obviousdiffusion of Nd3þ ions always exists in all specimens. The diffusiondistance in this paper is defined as the distance of Nd3þ ionsdiffusing from the interface between core layer and undoped partinto the undoped YAG. It is shown that with the increase of corelength the diffusion distance of Nd3þ ions becomes longer. Thediffusion distance changes from 0.7 mm to 1mm, corresponding to

er experimental setup.

Fig. 7. Output power versus input pump power with different cavity lengths (a) 2 at.%/1 mm core length; (b) 2 at.%/5 mm core length.

Y. Fu et al. / Optical Materials 71 (2017) 90e97 93

the core length of 1 mm and 5mm, respectively. Quite obviously, inall specimens Nd3þ ion concentration in the core area decreasedduring sintering because of the diffusion. The decrease was muchmore sizeable in the case of 1/5/1 mm slab than in 1/1/1 specimen.At the sintering temperature of about 1800 �C the lower Nd3þ ionconcentration, the larger YAG grain size [23]. Agreeably, the YAGgrain size in core area is the larger in 1/1/1 mm slab and smaller in1/5/1 mm one.

Fig. 4 shows the in-line transmittances of the YAG/Nd:YAG/YAGcomposite ceramics. It is seen that most specimens have good op-tical quality, but the transmittances of the specimen with long corelength are lower than that with the short one.

Fig. 5 shows the laser setup of composite ceramics. A radiation of808 nm laser diode used as the pump source was focused on theend face of the ceramic specimen with the diameter of 0.6 mmthrough the fiber coupling and the collimator (lenses 1 and 2). Twosizes of specimens, which are 3 � 3 � 3 mm3 and 3 � 3 � 7 mm3,

Fig. 6. Laser outputs of ceramics with different core (doping) lengths and Nd3þ ionconcentrations at different output coupler transmissions of T ¼ 5%, 10%, 15%, 19%, 30%,55%, 60% (a) 1 at.%/1 mm core length; (b) 1 at.%/5 mm core length; (c) 2 at.%/1 mm corelength; (d) 2 at.%/5 mm core length.

Fig. 8. Experimentally measured CW output power with respect to the cavity lengthfor the two types of 2 at.% Nd:YAG composite ceramics at the pump power (a)Pin ¼ 6 W and (b) Pin ¼ 10.8 W.

Fig. 9. The experiment setup for measur

Y. Fu et al. / Optical Materials 71 (2017) 90e9794

were all used for laser output. The surfaces of 3 � 3 mm2 werecoated. One surface of the composite ceramics was AR-coated at808 nm and HR-coated at 1064 nm. The other surface was AR-coated at 1064 nm. The pump laser energy traveled into spec-imen from the surface of 3 � 3 mm2 with HR-coating, corre-sponding to undoped YAG, and then passed through the Nd:YAGcore layer. The ceramic specimen was cooled by water with acopper heat sink. The output couplers with different transmissionsat 1064 nm were used to obtain the optimum laser performance.

Fig. 6 shows the laser output powers with different outputcoupler transmissions of T ¼ 5%, 10%, 15%, 19%, 30%, 55%, 60%. Theoptimum transmission changes significantly with the different corelengths and Nd3þ ion concentrations. When the core length is1 mm, the optimum transmission is 10% with the slope efficiency of12.9% and 24.4% for 1 at.%, 2 at.%. When the core length is 5 mm, theoptimum transmission is 15%e19% with the slope efficiency of31.5% and 36.2% for 1 at.%, and 2 at.%, respectively.

The pump-induced thermal lens effect in 2 at.% YAG/Nd:YAG/YAG ceramics was investigated by a parallel planar cavity method.When the laser resonator begins to be unstable, namely the outputpower becoming zero, the thermal focal length is the laser cavitylength. Fig. 7 shows the output power versus input pump powerwith different cavity lengths. All the output power increases line-arly with the pump power increasing at first, then saturates, anddecreases to zero in the end. Although longer core length ceramicscan demonstrate higher output power and slope efficiency, it re-sults in more serious thermal lens effect which leads to decrease ofthe beam quality.

Fig. 8 shows the experimentally measured CW output powerwith respect to the cavity length for two types of 2 at.% Nd:YAGcomposite ceramics. At lower pump power (Fig. 8a) the thermaleffect in specimen is not strong enough, thus the change of outputpower is not so obvious. However, at higher pump power (Fig. 8b)the decrease of output power becomes remarkable except for thatof the composite ceramics with 1 mm core length because of itsweaker absorption to pump power. The serious thermal lens effectwill break the stability of laser cavity, limiting the further improveof laser output power and beam quality.

The thermally induced birefringence changing with the pumppower in the ceramics can be observed by depolarized beam pat-terns. The pump-probe method was adopted to measure thedepolarized beam patterns. Fig. 9 shows the experimental setup to

ing the depolarized beam patterns.

Fig. 10. The depolarized beam patterns of YAG/1 at.%Nd:YAG/YAG composite ceramics (a) 1 mm core length; (b) 5 mm core length.

Y. Fu et al. / Optical Materials 71 (2017) 90e97 95

measure the depolarized beam patterns. A He-Ne laser was used asthe probe source. The laser beamwas collimated to 1 mm aperture.The probe beam was reflected by a dichroic splitter after passingthe polarizer. Both polarizer and analyzer were set to be orthog-onal. The depolarization was measured under the non-lasingcondition.

Fig. 10 and Fig. 11 show the depolarized beam patterns fordifferent composite ceramics. The depolarized beam patterns werefound to differ significantly. With the increase of pump power, corelength and doping concentration, the depolarized beam patternchanges from a four-leaves-like pattern to a ring-like patterndemonstrating the aggravation of thermal effects. Because of thestronger thermally induced birefringence in the specimens with

the higher doping concentration and longer core length, the cor-responding depolarized beam pattern changes more sizably.

4. Conclusions

YAG/Nd:YAG/YAG composite ceramicswith different core lengthsand doping concentrations were made by dry pressing and vacuumsintering of nanopowdersmixture. The higher doping concentrationand longer core length of the ceramics were found to result in thehigher laser output power, however, both contribute tomore sizablethermal effects. The contradiction between doping concentration,core length and thermal effects of the composite ceramics should besettled by new design and improved fabrication method.

Fig. 11. The depolarized beam patterns of YAG/2 at.% Nd:YAG/YAG composite ceramics (a) 1 mm core length; (b) 5 mm core length.

Y. Fu et al. / Optical Materials 71 (2017) 90e9796

Acknowledgments

This work was supported by National Natural Science Founda-tion of China (Grant Nos. 61575212 and 50990301), the Project ofInternational Cooperation and Exchange NSFC-RFBR (Grant No.1151101157 and No. 16-52-53059-GФЕН_а), Program of Presidiumof RAS (No. 15-17-2-20) and the Russia-China Inter-governmentalS&T cooperation program.

References

[1] A. Ikesue, Y.L. Aung, Ceramic laser materials, Nat. Photonics 2 (2008) 721e727.[2] G.L. Messing, A.J. Stevenson, Toward pore-free ceramics, Science 322 (2008)

383e384.[3] R. Boulesteix, A. Maitre, L. Chretien, Y. Rabinovitch, C. Salle, Microstructural

evolution during vacuum sintering of yttrium aluminum garnet transparentceramics: toward the origin of residual porosity affecting the transparency,J. Am. Ceram. Soc. 96 (2013) 1724e1731.

[4] W. Koechner, Absorbed pumb power, thermal profile and stresses in a cwpumped Nd-YAG crystal, Appl. Opt. 9 (1970) 1429e1434.

[5] V. Lupei, N. Pavel, T. Taira, Laser emission in highly doped Nd:YAG crystalsunder F-4(5/2) and F-4(3/2) pumping, Opt. Lett. 26 (2001) 1678e1680.

[6] M.G. Ivanov, Yu.L. Kopylov, V.B. Kravchenko, K.V. Lopukhin, V.V. Shemet, YAG

and Y2O3 laser ceramics from nonagglomerated nanopowders, Inorg. Mater.50 (2014) 951e959.

[7] A. Ikesue, T. Kinoshita, K. Kamata, K. Yoshida, Fabrication and optical-properties of high-performance polycrystalline Nd-YAG ceramics for solid-state lasers, J. Am. Ceram. Soc. 78 (1995) 1033e1040.

[8] S.H. Lee, S. Kochawattana, G.L. Messing, J.Q. Dumm, G. Quarles, V. Castillo,Solid-state reactive sintering of transparent polycrystalline Nd:YAG ceramics,J. Am. Ceram. Soc. 89 (2006) 1945e1950.

[9] J. Li, Y.S. Wu, Y.B. Pan, W.B. Liu, L.P. Huang, J.K. Guo, Fabrication, microstruc-ture and properties of highly transparent Nd:YAG laser ceramics, Opt. Mater.31 (2008) 6e17.

[10] H. Yang, X.P. Qin, J. Zhang, J. Ma, D.Y. Tang, S.W. Wang, Q.T. Zhang, The effectof MgO and SiO2 codoping on the properties of Nd:YAG transparent ceramic,Opt. Mater. 34 (2012) 940e943.

[11] W. Zhang, T.C. Lu, N.A. Wei, B.Y. Ma, F. Li, Z.W. Lu, J.Q. Qi, Effect of annealing onthe optical properties of Nd:YAG transparent ceramics, Opt. Mater. 34 (2012)685e690.

[12] Y.L. Fu, J. Li, Y. Liu, L. Liu, H. Zhao, Y.B. Pan, Effect of air annealing on the opticalproperties and laser performance of Nd:YAG transparent ceramics, Opt.Mater. Express 4 (2014) 2108e2115.

[13] B.L. Liu, J. Li, M. Ivanov, W.B. Liu, L. Ge, T.F. Xie, H.M. Kou, S.J. Zhuo, Y.B. Pan,J.K. Guo, Diffusion-controlled solid-state reactive sintering of Nd:YAG trans-parent ceramics, Ceram. Int. 41 (2015) 11293e11300.

[14] L.M. Osterink, Thermal effects and transverse mode control in a Nd:YAG laser,Appl. Phys. Lett. 12 (1968) 128.

[15] W. Koechner, Thermal lensing in a Nd-YAG laser rod, Appl. Opt. 9 (1970)

Y. Fu et al. / Optical Materials 71 (2017) 90e97 97

2548e2553.[16] I. Shoji, Y. Sato, S. Kurimura, V. Lupei, T. Taira, A. Ikesue, K. Yoshida, Thermal-

birefringence-induced depolarization in Nd:YAG ceramics, Opt. Lett. 27 (2002)234e236.

[17] H.M. Yang, G.Y. Feng, S.H. Zhou, Thermal effects in high-power Nd:YAG disk-type solid state laser, Opt. Laser Technol. 43 (2011) 1006e1015.

[18] J. Sulc, H. Jelinkova, V. Kubecek, K. Nejezchleb, K. Blazek, Comparison ofdifferent composite Nd:YAG rods thermal properties under diode pumping,Proc. SPIE 4630 (2002) 128e134.

[19] D. Kracht, M. Frede, R. Wilhelm, C. Fallnich, Comparison of crystalline andceramic composite Nd:YAG for high power diode end-pumping, Opt. Express13 (2005) 6212e6216.

[20] G.J. Exarhos, K. Yoshida, H. Ishii, T. Kumada, T. Kamimura, A. Ikesue,T. Okamoto, A.H. Guenther, N. Kaiser, K.L. Lewis, M.J. Soileau, C.J. Stolz, All-ceramic composite with layer-by-layer structure by advanced ceramic tech-nology, in: Proc. SPIE, 5647, 2005, pp. 247e254.

[21] C.Y. Ma, F. Tang, H.F. Lin, W.D. Chen, G. Zhang, Y.G. Cao, W.C. Wang, X.Y. Yuan,Z. Dai, Fabrication and planar waveguide laser behavior of YAG/Nd:YAG/YAGcomposite ceramics by tape casting, J. Alloy Compd. 640 (2015) 317e320.

[22] L. Ge, J. Li, Z.W. Zhou, H.Y. Qu, M.J. Dong, Y. Zhu, T.F. Xie, W. Li, M. Chen,H.M. Kou, Y. Shi, Y.B. Pan, X.Q. Feng, J.K. Guo, Fabrication of composite YAG/Nd:YAG/YAG transparent ceramics for planar waveguide laser, Opt. Mater.Express 4 (2014) 1042e1049.

[23] H. Yagi, K. Takaichi, K. Ueda, Y. Yamasaki, T. Yanagitani, A.A. Kaminskii, Thephysical properties of composite YAG ceramics, Laser Phys. 15 (2005)1338e1344.

[24] F. Tang, Y.G. Cao, J.Q. Huang, W. Guo, H.G. Liu, Q.F. Huang, W.C. Wang,Multilayer YAG/RE: YAG/YAG laser ceramic prepared by tape casting andvacuum sintering method, J. Eur. Ceram. Soc. 32 (2012) 3995e4002.

[25] W.B. Liu, Y.P. Zeng, J. Li, Y.Q. Shen, Y. Bo, N. Zong, P. Wang, Y. Xu, J. Xu, D. Cui,Q. Peng, Z. Xu, D. Zhang, Y.B. Pan, Sintering and laser behavior of compositeYAG/Nd:YAG/YAG transparent ceramics, J. Alloy Compd. 527 (2012) 66e70.

[26] S. Kochawattana, Phase Formation and Sintering of YAG Ceramics, PhD Thesisin Materials Science and Engineering, The Pennsylvania State University,2007.

[27] J.W. Cahn, The impurity drag effect in grain boundary motion, Acta. Metall. 10(1962) 789e798.