IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 21, NO. 1, JANUARY/FEBRUARY 2015 1602705

Multilayer YAG/Yb:YAG Composite Ceramic LaserKangwen Yang, Xuewei Ba, Jiang Li, Yubai Pan, and Heping Zeng

Abstract—Laser performance of a multilayer YAG/Yb:YAGcomposite ceramic fabricated by nonaqueous tape casting wasdemonstrated in a diode-pumped three-mirror cavity. We obtaineda maximum continuous-wave output power of 3.6 W at 1051 nmwith an absorbed pump power of 20.1 W, corresponding to aslope efficiency of 28.6%. The composite ceramic laser wavelength-tuning characteristics were experimentally compared with fusedsilica, SF10, SF14, and SF57 dispersive prisms inserted in the lasercavity. An optimum smooth wavelength-tuning curve from 1008 to1053 nm was achieved by using an SF57 Brewster prism.

Index Terms—Continuous-wave (CW), Ytterbium lasers,ceramics, composite materials.

I. INTRODUCTION

O PTICAL ceramics have been well-developed with re-markable advantages allowing for high doping concentra-

tion, large-size sample fabrication, homogeneous dopant distri-bution, optical grade post-processing of low cost, low-thresholdbroadly tenability, as well as sophisticated but convenient engi-neering of multilayer structures, which surpass mono-crystalsand other laser materials and are regarded as promising substi-tutes for the next generation of laser gain media [1]–[7]. Amongthose advantages, multilayer ceramic structures are quite attrac-tive as well-designed ceramic multilayers can facilitate materialoptimization and multifunction exploration. Advanced multi-layer ceramic technology enables mode and pattern control oflaser beam as well as addition of Q-switching and miniaturiza-tion of the laser oscillator [8]. In particular, effective thermalmanagement of ceramic laser can be achieved by using multi-layer ceramics with different doping concentrations, which was

Manuscript received March 28, 2014; revised June 23, 2014, August 9, 2014,and September 8, 2014; accepted October 17, 2014. This work was supported inpart by the National Natural Science Fund under Grant 61127014), National KeyScientific Instrument Project under Grant 2012YQ150092, the Major Programof National Natural Science Foundation of China under Grant 50990301, theProject for Young Scientists Fund of National Natural Science Foundation ofChina under Grant 51002172, and the Key Program of Shanghai Association ofScience and Technology under Grant 11JC1412400.

K. Yang is with Shanghai Key Laboratory of Modern Optical System, En-gineering Research Center of Optical Instrument and System (Ministry of Ed-ucation), School of Optical-Electrical and Computer Engineering, Universityof Shanghai for Science and Technology, Shanghai 200093, China (e-mail:kangwenyang@yeah.net).

X. Ba, J. Li, and Y. Pan are with the Key Laboratory of TransparentOpto-functional Inorganic Materials, Shanghai Institute of Ceramics, ChineseAcademy of Science, Shanghai 200050, China (e-mail: bxw03@sina.com;lijiang@mail.sic.ac.cn; ybpan@mail.sic.ac.cn).

H. Zeng is with the State Key Laboratory of Precision Spectroscopy, EastChina Normal University, Shanghai 200062, China and also with the ShanghaiKey Laboratory of Modern Optical System, Engineering Research Center ofOptical Instrument and System (Ministry of Education), School of Optical-Electrical and Computer Engineering, University of Shanghai for Science andTechnology, Shanghai 200093, China (e-mail: hpzeng@phy.ecnu.edu.cn).

Color versions of one or more of the figures in this paper are available onlineat http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/JSTQE.2014.2365576

suitable for the application of a high-power laser because thepure YAG layer could act as the heat-sink system due to thehigher thermal conductivity than the doping layers [9], [10]. Byadjusting the doping concentration of rare earth ions into thehost materials, longitudinal temperature and mechanical stressgradient in end-pumped solid-state lasers can be minimized[11], [12]. In 2008, A. Ikesue and Y. L. Aung demonstratedthe heat distribution of a conventional Nd:YAG single crystalwith uniform doping and advanced Nd:YAG ceramic rods witha smooth gradient configuration, the experimental results in-dicate that the abrupt heat generation was well suppressed bythe ceramic composite element due to the gradient distributionof doping ions [13]. In 2012, Takunori Taira’s team reportedgeneration of passively Q-switched pulses with energy up to3.2 mJ and duration down to 500 ps from a composite,all-ceramics laser cavity, which was suitable for single- andmultiple-point laser igniter [14]–[16].

The Yb system has many advantages compared with Nd sys-tem such as high quantum efficiency, long lifetime of upper state,and low thermal conversion efficiency towards pump laser [17]–[19]. The ytterbium-doped laser systems have been focused asa high-power high-efficiency or an ultra-short pulse laser be-cause of their high quantum efficiency and very wide emissionspectrum [20]–[23]. The high doping concentration and energyextraction ability of Yb-doped laser materials have contributedto the miniaturization and commercialization of Yb-doped highpower solid-state lasers. Recently, a low repetition rate, pas-sively Q-switched micro-laser with peak power of 2.8 MW andpulse energy of 3.6 mJ was developed by Yb:YAG/Cr:YAG lasermaterial [24]. However, the severe thermal load of quasi-three-level Yb3+ ions imposes restrictions on higher output power,thus design and fabricating laser materials with more superiorheat dissipation performance become a hot topic of practicalhigh-power solid-state laser field [25], [26].

Tape casting is one of the most important method in indus-trial manufacture for producing multilayer ceramic packagingsubstrates, solid oxide fuel cells, ceramic semiconductors withnegative temperature coefficient, and thick-film piezoelectricresonators [27]–[31]. The acquired cast film has high solid load-ing and density due to the good dispersibility caused by the lowviscosity of the slurry, which would benefit the densification ofceramic microstructure [32]. Meanwhile, the combination of theorganic solvents enables better uniformity for dissolution anddispersion of the binders and surfactants [33]. Therefore, betterstability for the prepared slurry should be achieved. Moreover,the drying rate of the non-aqueous tape casting slurry is veryfast, which will reduce the preparation cycle greatly and makemassive manufacture of ceramic laser materials possible.

In the previous reports of composite ceramic laser fabricatedby tape casting method, the emphasis was on the thermal man-agement and beam control to achieve efficient laser oscillation

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1602705 IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 21, NO. 1, JANUARY/FEBRUARY 2015

with remarkable beam quality [34]. For example, In 2012, all-ceramic multilayer composite Yb:YAG laser gain medium withdoping concentration distribution of 0–5–10–15–20–15–10–5–0 at.% Yb ions was successfully fabricated by tape casting pro-cess and CW laser performance with this sample was demon-strated, indicating low-cost but efficient fabrication method formultilayer composite ceramic [35]. However, there is very littledescription about frequency tunability of the composite ceramiclaser, which is desirable in many applications such as high-resolution spectroscopy and holographic memory. The widesttunability of 85 nm from 1024.1 to 1108.6 nm for a Yb:YAGcrystal laser was reported in 2000 by Saikawa et al. [36]. In2009, Nakamura’s group reported the widest tuning range of118.31 nm from 992.52 to 1110.83 nm for a 9.8 at.% Yb:YAGceramic laser, they found that ceramic Yb:YAG had higher emis-sion intensity and possibility of producing shorter-wavelengthoscillation than the crystal Yb:YAG [37]. To obtain wider fre-quency tunability, laser gain medium with larger emission sec-tion, higher fluorescence intensity and longer fluorescence life-time would be desirable, which could be achieved by heavydoping concentration [38]. Meanwhile, novel laser architecturewith small active thickness such as the thin disk or microchipconfigurations will reduce not only thermal problems but alsoreabsorption loss, which would be better for practical wider tun-ing applications [39]. Especially, owing to the advanced ceramicfabrication method, it is possible to prepare multilayer compos-ite ceramic with high doping concentrations in small dopinglayer thickness, the active gain layers could be surrounded bythe un-doped layers or designed with gradient doping concen-tration, which will improve the heat-dissipation, reduce the re-absorption loss and thus support wider tuning range. In thispaper, we present experimental results of CW laser oscillationand frequency tuning characteristic from diode-pumped ceramicYAG/Yb:YAG composite laser fabricated by non-aqueous tapecasting and vacuum sintering method. The tunability of the com-posite ceramic laser was investigated by using prisms of fusedsilica, SF10, SF14 and SF57. The widest tunable range of theYAG/Yb:YAG ceramic laser was 45 nm from 1008 to 1053 nm.

II. EXPERIMENTAL SETUP

We used commercially available high-purity powders offine-grained α-Al2O3 (99.99%, Alfa Aesra, USA) and coarse-grained Y2O3 (99.99%, Alfa Aesra, USA) as the raw material.The anhydrous alcohol was used as solvent while the tetraethylorthosilicate (TEOS, 99.99%, Alfa Aesra, USA) and magne-sium oxide (MgO, 99.99%, Alfa Aesra, USA) were used as thesintering aids. The powder mixture was ball milled as the pri-mary slurry by the planetary ball mill for at least 8 h. Then thebinder (PVB, polyvinyl butyral) and plasticizer (PEG-400 andBBP, butyl benzyl phthalate) were weighted and added to ballgrinder with primary slurry for 24 h. The dosage of the binderand the plasticizer were 3.3 and 3.4 wt%, respectively. The newslurry was sieved and degassed under a vacuum condition. Thetape was prepared in casting film machine with the gap height ofthe blade of 300–500 μm and the casting speed was controlled

Fig. 1. Photograph of the tape prepared by non-aqueous tape casting method.

Fig. 2. Comparison of EPMA morphologies of YAG ceramics molded by(a) dry pressing, (b) aqueous tape casting, and (c) non-aqueous tape casting.

to 100 mm/min. The preparation process was implemented atroom temperature.

Fig. 1 is the photograph of the tape prepared by non-aqueoustape casting method, which shows the cape has sufficient tough-ness for bending and twisting several times without mechanicaldamage. The tape contained 12 layers in 1 mm fracture whichcould be seen by using a field emission scanning electron micro-scope. The green bodies of composite ceramic were fabricatedby laminating 30 layers of Yb3+ -doped cast film onto 30 layersof undoped cast film, which were compacted by cold isostaticpressing to get homogeneous bodies. The obtained coating sur-face of green bodies is very compact and smooth. Subsequently,the green bodies were sintered in a high vacuum environmentto exclude the residual air and increase the densification ofceramic. The sintering temperature was set to 1760 °C and theholding time was 10 h with the pressure was below 1 × 10−3 Pa.After that, the samples were annealed in a box resistor-stove for10 h at 1450 °C to eliminate lattice defects such as oxygenvacancy and improve the optical quality of samples.

In addition to the non-aqueous tape casting method, compos-ite YAG transparent ceramics with multilayer structure couldalso be prepared by dry pressing and aqueous tape casting.Fig. 2 shows the Electron Probe Micro-Analysis (EPMA) mor-phologies of YAG ceramics molded by different methods. Thegrain size of tape casting sample is smaller than that of drypressing method, which is preferable to improve the mechani-cal properties of ceramic. It should be noticed that the samplefabricated by aqueous tape casting method has pores both at thegrain boundaries and inside the grains, the latter are difficult tobe eliminated by vacuum sintering. As for the dry pressing andnon-aqueous tape casting sample, most of the pores are at thegrain boundaries, and the quantity are much smaller than that ofaqueous tape casting. The existence of the pores could cause thelower optical quality of the aqueous sample, which is mainlyrelated to the high surface tension of water.

Fig. 3 shows the photograph of the as-sintered compositemultilayer transparent ceramics prepared by non-aqueous tapecasing (Left) and dry pressing (Right). The interface of drypressing sample was bended and the greater the pressure is,

YANG et al.: MULTILAYER YAG/Yb:YAG COMPOSITE CERAMIC LASER 1602705

Fig. 3. Photograph of the as-sintered composite transparent ceramic preparedby non-aqueous tape casting (Left) and dry pressing (Right).

Fig. 4. Energy diagram of Yb3+ ions in YAG crystals.

more severe bend occurs. This could cause obvious diffusionlayer existing at the interface of different doping layers. Theinterface of non-aqueous sample was very straight and flat, thetransition layer was hardly observed. After equal reduction andoptical polishing in the top and bottom surface of the as-obtainedsample, the YAG/Yb:YAG ceramic was cut 4-mm long with5 × 5 mm2 aperture for laser test without any anti-reflection(AR) coatings on both surfaces.

As shown in Fig. 4, the Yb3+ ions only have a ground state of2F7/2 and an excited state of 2F5/2 , the gap between two energylevels is about 10 000 cm−1. Four energy levels of the groundstate and three energy levels of excited state were generateddue to the Stark effect caused by the crystalline field at roomtemperature. The terminal level is hundreds of cm−1 higher thanthe ground state energy level, therefore a Yb3+ :YAG laser wascalled a quasi-three level laser, which needs high intensive andhigh brightness pump source to saturate absorption [40].

In our experiment, the Yb3+ concentration of the ceramics is5 at.%. To efficiently remove the thermal loads during the exper-iment, we wrapped the ceramic with indium foil and mountedit tightly in a water-cooled copper heat sink. The temperatureof the gain medium was controlled to 14 °C. The experimen-tal setup is shown in Fig. 5. The ceramic was end-pumped by ahigh-brightness fiber-coupled laser diode at 976 nm with a max-imum output power of 30 W. The diameter of the fiber core was50 μm and the numerical aperture was 0.22. We used a 1:1 imag-ing system to focus the pump laser beam to a spot size of 50 μmon the YAG/Yb:YAG ceramic. A stable three-mirror laser cav-ity was employed to characterize the laser performance, whichconsisted of a flat mirror M1 (AR coated at 940–976 nm, HRcoated at 1020–1120 nm), a concave mirror M2 (R = 300 mm,

Fig. 5. Experimental setup of a diode-pumped YAG/Yb:YAG ceramic laser.

Fig. 6. (a) The typical output spectrum of ceramic laser. (b) Average outputpower versus absorbed pump power with OC of T = 2%, 5%, and 10%.

HR coated at 1020–1120 nm), and an output coupler (OC) mir-ror with different transmission at 1020–1120 nm.

III. RESULTS AND DISCUSSIONS

Firstly, we selected an OC of T = 2%, CW Yb:YAG ce-ramic laser emission was attained at 1051 nm in TEM00 mode.The CW laser output 47 mW at the threshold absorbed pumppower of 1.7 W. In order to achieve the maximum laser effi-ciency, we compared the laser output by using different outputcouplers with transmissions T = 2%, T = 5% and 10%. Theoutput spectrum exhibited no observable changes at three dif-ferent output couplers, the center wavelength was 1051 nm,as shown in Fig. 6(a). The optimized maximum output powerwas obtained with the T = 10% output coupler. Under an ab-sorbed pump power of 20.1 W (correspond to an incident pumppower of 28.7 W), the maximum output power was 3.6 W witha slope efficiency of 28.6% and an optical-to-optical conversionefficiency of 17.9%. The output power versus absorbed pumppower was shown in Fig. 6(b) for CW laser cavity using threedifferent output couplers. Compared of the optimum laser per-formance with OC at T = 10%, the output power with T = 2%and T = 5% OC at the same maximum incident pump powerwere 1.8, 3.0 W, corresponding to a slope efficiency of 19.1%,23.9%, respectively. The maximum output power was obtainedat the highest OC transmission (T = 10%), indicating enoughpump absorption and sufficient population inversion in the lasercavity.

The slope efficiency in our experiment is similar to the re-sults reported in F. Tang’s work and the 3.6-W output power issmaller compared to our previous work with Yb:YAG ceramic.The dissatisfactory laser performance might be caused by theuncoated ceramic surface and incomplete annealing process,

1602705 IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 21, NO. 1, JANUARY/FEBRUARY 2015

Fig. 7. Tuning curve obtained for a YAG/Yb:YAG ceramic laser with fusedsilica (a) SF10, (b) SF14, (c) and SF57. (d) Brewster prism in the laser cavity.

the latter will decrease the optical absorptivity of Yb3+ irons.By improving the ceramic fabrication process and optimizingcavity arrangement, higher slope efficiency and output powerwould be expected.

We next tuned the laser wavelength by inserting a silica dis-persive prism into the laser cavity as the adjustable element. Theprism was placed between the concave mirror and the OC atBrewster’s angle to obtain the optimum transmission efficiency.Fig. 7(a) gives the typical tuning curves with a fused-silica dis-persive prism and T = 10% transmission output coupler. Weobserved laser radiation around 1030 and 1048 nm, correspond-ing to the two major emission bands of Yb3+ ion from the low-est levels of 2F5/2 to the highest and secondary levels of 2F7/2manifold, while the absorption peak of 976 nm from pump lasercorresponding to the zero-line transition from the lowest levelsof 2F5/2 to the lowest level of 2F7/2 manifold.

We also used SF10 dispersive prism to characterize the tun-ing range of YAG/Yb:YAG ceramic laser, as shown in Fig. 7(b).The laser oscillation also occurs around 1030 and 1050 nm,but the tuning range around these two wavelengths was a littlewider than that using fused silica prism. When the fused-silicaprism was adjusted to other wavelengths for tuning, laser oscil-lation from 1036 to 1046 nm with small emission cross sectionswas suppressed by the gain competition with wavelengths oflarge emission cross sections due to the insufficient separationof nearby wavelengths. In order to obtain broadband spectra ofsimultaneous laser emissions, the chromatic dispersion inducedby the prism should be much larger to sufficiently separate thenearby wavelengths inside the laser cavity. Meanwhile, the cav-ity losses should be well controlled to overcome the gain differ-ence at different wavelengths and cavity misalignment causedby the angular dispersion of prism. This could be achievedby selecting prisms with large chromatic dispersion and care-fully adjusting the output mirror to match the optimum beamposition of the silica prism. The refractive indexes for SF14

and SF57 are (n = 1.80 around 1051 nm, respectively) muchlarger than that of fused silica (n = 1.45 around 1051 nm) andSF10 (n = 1.70 around 1051 nm). The chromatic dispersion offused silica, SF10, SF14, and SF57 was calculated to be –0.012,–0.025, –0.029, and –0.034 μm−1, respectively. As a result, largeangular separation of SF14 and SF57 Brewster prism ensuredbroadband wavelength tuning from 1010 to 1050 nm. As shownin Fig. 7(d), a continuous and smooth tuning range of 45 nmwas achieved by inserting an intra-cavity SF57 dispersive prism,while the tuning range using SF14 dispersive prism was 28 nmfrom 1024 to 1052 nm [see Fig. 7(c)]. We investigated the fre-quency tuning range of mono-layered Yb:YAG ceramic withthe same cavity configuration. The Yb concentration was 5 at.%and the ceramic size was 5 × 4 × 4.5 mm3. The achieved tuningrange was 38 nm from 1015 to 1053 nm, which was smaller thanthat of two-layered composite ceramic. However, the differenceof the thickness and pump absorption of the active region wouldinfluence the results of frequency tuning range. Therefore, fur-ther experiments with around 100 nm tuning range or moredistinct analyses would be required to prove the advantages offrequency tuning of multilayer composite ceramic. The tuningrange was probably limited by ytterbium doping concentrationand strong re-absorption of Yb3+ ions. By employing ceramicswith higher doping concentration and thinner size, a broadbandtuning range with more laser oscillation wavelengths was ex-pected.

IV. CONCLUSION

In conclusion, diode-pumped CW laser emissions have beendemonstrated with a transparent YAG/Yb:YAG ceramic. Themaximum output power of 3.6 W was obtained under an ab-sorbed pump power of 20.1 W in a stable three-mirror cavity,corresponding to a slope efficiency of 28.6%. The wavelength-tuning characteristics were experimentally compared by usingdifferent intra-cavity Brewster prisms. A smooth tunable curvefrom 1008 to 1053 nm with a tuning range of 45 nm was achievedby using SF57 prism as the wavelength-tuning element. Theflexible engineering structures of multilayer composite laser ce-ramics allow for high ytterbium-doping concentrations, whichare anticipated to support higher output power and wider tuningrange by using a thinner ceramic in the laser cavity. Multilayercomposite ceramics are promising to build broadband tunablehigh-power chip laser sources.

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Kangwen Yang was born in Gansu, China, in 1987. He received the B.S. degreein optical information science and technology from the Huazhong University ofScience and Technology, Hubei, China.

His research interests include high-power femtosecond pulse generation.

Xuewei Ba was born in Heilongjiang, China, in 1974. He received the B.S.degree in material sciences from Jinan University, Guangdong, China, in 1998and the Ph.D. degree from the Shanghai Institute of Ceramics, Chinese Academyof Sciences, Beijing, China, in 2013.

His research interests include laser ceramics and luminescent materials.

Jiang Li was born in Zhejiang, China, in 1977. He received the B.S. degreein material sciences from Jiangsu University, Jiangsu, China, in 2000 and thePh.D. degree from the Shanghai Institute of Ceramics, Chinese Academy ofSciences, Beijing, China, in 2007.

His research interests include laser ceramics and optical transparent ceramics.

Yubai Pan was born in Shanghai, China, in 1961. He received the B.S. degreein chemical engineering from Tianjin University, Tianjin, China, in 1984 andthe Ph.D. degree from the Toyama Prefectural University, Toyama, Japan, in1999.

His research interest include transparent and optofunctional ceramics.

Heping Zeng was born in Hunan, China, in 1966. He received the B.S. degreein physics from Peking University, Beijing, China, in 1990 and the Ph.D. degreefrom the Shanghai Institute of Optics and Fine Mechanics, Chinese Academyof Science, Shanghai, China, in 1995.

His research interests include ultrafast photonics and single-photon optics.