Polycrystalline Ho:YAG Transparent Ceramics for Eye-SafeSolid State Laser Applications

Hao Yang,‡,§ Jian Zhang,¶,k,† Xianpeng Qin,k,‡‡ Dewei Luo,‡‡ Jan Ma,§,¶

Dingyuan Tang,‡‡ Hao Chen,§§ Deyuan Shen,§§ and Qitu Zhang‡,†

‡College of Materials Science and Engineering, Nanjing University of Technology, Nanjing, 210009, China

§School of Materials Science and Engineering, Nanyang Technological University, Singapore 639798

¶Temasek Laboratories of Nanyang Technological University, Singapore 639798

kShanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, China

‡‡School of Electrical and Electronic Engineering, Nanyang Technological University, Singapore 639798

§§Department of Optical Science and Engineering, Fudan University, Shanghai 200433, China

High optical quality holmium doped yttrium aluminum garnet

(Ho:YAG) transparent ceramics that contained 0.5–2.0 at.%

holmium additives were fabricated by a reactive sinteringmethod under vacuum. Fully dense Ho:YAG ceramics with the

average grain size of ~10 lm were obtained after vacuum

sintered at 1780°C for 8 h. The in-line transmittances in thevisible and infrared region are higher than 82% and 84%. The

absorption coefficients of the 0.8, 1.0, 1.5, and 2.0 at.% Ho:

YAG ceramics at 1907 nm was calculated to be 0.72, 0.89,

1.32, and 1.78 cm�1, respectively. The absorption cross-section

of Ho:YAG ceramic was 0.645 3 10�20 cm2. The 1.5 and

2.0 at.% Ho:YAG ceramic slabs were in-band pumped by a

Tm:fiber laser at 1907 nm. A maximum output power of

20.6 W at 2097 nm was obtained with a corresponding slopeefficiency of 60.9%.

I. Introduction

THE quality of polycrystalline transparent ceramic lasermaterials has been greatly improved in recent years and

can be competitive with the corresponding single crystalmaterials, especially in the case of yttrium aluminum garnet(YAG).1,2 YAG-based transparent ceramics have beenproved to be one of the most promising laser materials forsolid state laser applications. Compared with single crystals,YAG-based ceramic exhibits similar optical spectroscopicproperties. Moreover, ceramic YAG has several advantages,such as ease of fabrication into large sizes and complicatedstructures, short fabrication time and the possibility for themass production.3,4

YAG-based ceramics have been studied widely to obtainlaser output near 1 lm wavelengths. Ikesue et al. firstachieved lasing using Nd:YAG ceramics gain media in 1995and demonstrated the high functionality and flexibility of

Nd:YAG ceramics.5,6 In 2006, the LLNL succeeded inachieving a world record power of 67 kW from its solid stateheat capacity laser system using commercially Nd:YAGceramics supplied by Konoshima Corp. Ltd. Compared with1 lm laser, the 2 lm laser emission is considered eye-safe asthe cornea absorbs wavelengths greater than ~1.4 lm, whileshorter wavelengths are transmitted to the more sensitive ret-ina. The potential for eye damage from scattered laser lightis therefore much lower for lasers operating above 1.4 lmwavelengths, an important consideration for use in applica-tions including coherent Doppler wind detection LIDAR,laser ranging, time-resolved spectroscopy, and pumpinglasers for mid-infrared OPO.7–9 It is well known that hol-mium ions doped YAG are suitable for producing ~2.1 lmlaser emission arising from the 5I7 ?

5I8 transition of Ho3+.10

Shen et al. reported a Ho:YAG single crystal laser endpumped by a tunable Tm-fiber laser.11 The performance ofthe Ho:YAG single crystal has been widely studied and thelaser operation has been successfully realized.12 Consideringthe advantages of ceramics, Ho:YAG transparent ceramicscould be another attractive candidate of the laser gain mediafor 2 lm eye-safe laser applications.

In this study, high optical quality of 0.5–2.0 at.% Ho:YAG transparent ceramics were successfully fabricated by areactive sintering method under vacuum. The microstruc-tures, absorption spectrum, fluorescence spectrum, and thelaser performances of Ho:YAG ceramics were systematicallyinvestigated.

II. Experimental Procedures

(1) Ceramic FabricationThe reactive sintering method was employed for the fabrica-tion of Ho:YAG ceramics and it was very similar to themethod reported in the previous literature.5,13 Commerciala-Al2O3 powder (99.99% purity; Shanghai Wusong ChemicalCo. Ltd, Bao Shan Town, Shanghai, China) and co-precipi-tated Y2O3 and Ho2O3 powders were selected as the startingmaterials. The powder synthesis process for Y2O3 and Ho2O3

was similar to the method described by Zhang et al.14 Thepowders were weighed precisely according to the chemicallystoichiometric composition (Y1�xHox)3Al5O12 where x =0.008, 0.01, 0.015, 0.02 and mixed with 99.99% ethanol (ana-lytical pure; Merck, Darmstadt, Germany). The mixed slurry

T. Stefanik—contributing editor

Manuscript No. 30052. Received July 22, 2011; approved October 12, 2011.This study was supported by NSFC (50902139, 60928010), Nanjing University of

Technology PhD thesis innovation fund (BSCX200902) and Priority Academic ProgramDevelopment of Jiangsu Higher Education Institutions (PAPD).

†Authors to whom correspondence should be addressed. e-mails: njzqt@126.comand jianzhang@ntu.edu.sg

52

J. Am. Ceram. Soc., 95 [1] 52–55 (2012)

DOI: 10.1111/j.1551-2916.2011.04953.x

© 2011 The American Ceramic Society

Journal

was then ball milled using a planetary milling machine(PM400; Retsch, Haan, Germany) for 15 h. Tetraethyl ortho-silicate (TEOS; Sigma-Aldrich, 99.999%, Singapore, Singa-pore) was introduced at 0.5 wt% relative to the total powdermass to introduce SiO2 as a sintering aid. After milling, thepowder mixtures were dried at 120°C for 24 h in an ovenand then sieved through a 200-mesh screen. After removingorganic components by calcining at 800°C for 3 h, the pow-ders were dry pressed in a stainless steel die at 15 MPa. Thegreen body pellets were further cold isostatically pressed(CIPed) at 200 MPa. After CIP, the relative density of thegreen body was ~53%. The green bodies were then sinteredat 1780°C for 8 h in a high temperature vacuum sinteringfurnace utilizing a tungsten heating elements under a vacuumof 10�3 Pa. The sintered pellets were then annealed in air at1400°C for 15 h to completely remove internal stress andeliminate the oxygen vacancies. The as-prepared ceramicpellets were mirror polished on both surfaces with differentlevel of diamond slurries.

The microstructures of the transparent ceramics wereobserved using scanning electron microscopy (Model JSM-6360LV; JEOM, Tokyo, Japan). The optical transmittance ofHo:YAG transparent ceramics were measured using aUV-VIS-NIR spectrophotometer (Cary-5000; VARIAN,Blackburn, Vic., Australia). The photoluminescence (PL)spectra was measured at room temperature using a spectro-fluorometer (Fluorolog-3; Jobin Yvon, Edison), equippedwith a PbS detector. The slit was fixed at 2 nm and all spec-tra were measured at room temperature.

(2) Laser ExperimentFigure 1 shows the schematic diagram of the laser experi-ment setup. The dimension of the Ho:YAG slab was2 mm 9 3 mm 9 14 mm. Both end surfaces of the sample(2 mm 9 3 mm) were antireflection coated at 1800–2100 nm.The Ho:YAG ceramics were in-band pumped by a Tm:fiberlaser whose wavelength is locked at 1907 nm by volumeBragg gratings. The pump beam size was focused to 240 lm.The laser cavity consisted of two mirrors M1 and M2, wherethe rear mirror M1 was high reflection coated at 2050–2205 nm and high transmission coated at 1850–1960 nm.The output coupler M2 of 6% transmission at 2100 nm washigh reflection coated at 1850–1960 nm. The total length ofthe laser cavity was 20 mm.

III. Results and Discussion

Grain sizes, grain boundary phases, and pores are usuallyconsidered as the main factors which will affect the opticalquality of ceramics.15,16 Figure 2 shows the microstructuresof Ho:YAG ceramics after thermal etching at 1500°C for1 h. The Ho:YAG ceramics are composed of grains of theaverage size ~10 lm. It can be seen that the grain size chan-ged very little with the increase of holmium concentration.

No grain-boundary phases and pores can be observed in themicrostructures of Ho:YAG ceramics.

The photo of the transparent Ho:YAG ceramics and thetransmittances are shown in Fig. 3. The thickness of all sam-ples is polished to be 3 mm. As shown in Fig. 3, differentHo3+ concentration doped YAG ceramics exhibited the simi-lar transmittance. The transmittances of Ho:YAG ceramicsat 2500 nm is above 84%. No obvious transmittance drop isobserved at UV and VIS wavelength region. The transmit-tances of all samples at 400 nm are higher than 82%, whichmeans that none scattering center existed in the microstruc-tures. This is consistent with the SEM results.

From the transmittance spectra and the thickness of theceramics, the absorption coefficients of Ho:YAG ceramicscan be obtained. As shown in Fig. 4(a), all strongest absorp-tion peaks are centered at 1907 nm, which is exactly thesame as that measured in Ho:YAG single crystals. Thisabsorption peak was selected as the pump wavelength in thelaser testing experiments. As the Ho3+ doping concentrationincreased from 0.8 to 2.0 at.%, the absorption coefficient at1907 nm increased linearly from 0.72 to 1.78 cm�1, which isshown in Fig. 4(a) inset. From the absorption coefficient, theabsorption cross-section of Ho:YAG ceramic is derived to be0.645 9 10�20 cm2, which is also comparable to the datareported in the previous literature.12,17 The emission spec-trum of 1.0 at.% Ho:YAG ceramic from 1700 to 2600 nm

Fig. 1. Schematic of the Ho:YAG ceramic laser experiment.

(a) (b)

(d)(c)

Fig. 2. SEM microstructure of mirror-polished surface of Ho:YAGfollowing thermal etching at 1500°C. Holmium concentrations are(a) 0.8 at.%, (b) 1.0 at.%, (c) 1.5 at.%, and (d) 2.0 at.%

Fig. 3. Optical transmittance and photograph (inset) of Ho:YAGceramics.

January 2012 Rapid Communications of the American Ceramic Society 53

at room temperature is shown in Fig. 4(b). The main emis-sion band is centered at 2093 nm, which corresponds to the5I7 ?

5I8 transition of Ho3+.Laser performance of the 1.5 and 2.0 at.% Ho:YAG cera-

mic slabs were first evaluated with an output coupler of 6%transmission. The laser output power as a function ofincident pump power is shown in Fig. 5. It is obvious thatthe 1.5 at.% Ho:YAG ceramic exhibited better performancethan that of 2.0 at.% Ho:YAG. By using the 1.5 at.% Ho:YAG ceramic as laser media, a maximum output power of5.74 W at 2097 nm was obtained under an incident pump

power of 13.35 W with a threshold of 1.42 W. For the2.0 at.% Ho:YAG ceramic, a maximum output power of4.77 W was obtained under an incident pump power of13.5 W with a threshold of 2.56 W. The slope efficiencies for1.5 and 2.0 at.% Ho:YAG samples were 48.1% and 42.7%,respectively. This situation may be attributed to theenhanced upconversion loss in the 2.0 at.% Ho:YAG. It isknown that high holmium concentration in YAG single crys-tal results in a significant increase in upconversion loss, anddegradation in laser performance.18 By further optimizingthe pumping scheme, 20.6 W laser output at 2097 nm under35 W of incident pump power was achieved using the 1.5 at.% Ho:YAG ceramic, as reported in our recent article.19 Thiscorresponds to a slope efficiency of 60.9% and an opticalconversion efficiency of 58.8%. Zhang et al. reported thelaser oscillation of Ho:YAG ceramic in 2010.20 In that study,a maximum output power of 1.95 W was yielded with aslope efficiency of 44.19%. The increased slope efficiencydemonstrated here is likely a result of improved optical qual-ity of the Ho:YAG ceramics. Duan et al. reported the Ho:YAG single crystal laser pumped by a diode-pumped Tm:YLF laser. An output power of 10.5 W at 2090 nm wasobtained when pumped at an incident pump power of18.1 W, corresponding to a slope efficiency of 65.7% and anoptical conversion efficiency of 58.0%.21 The slope efficiencyof the Ho:YAG ceramic produced in this study is very closeto that of Ho:YAG single crystal. Moreover, it is worth not-ing that the output power is essentially linear with respect tothe incident pump power at even the highest power level,suggesting there is still room for further power scaling. Thesefindings indicate that Ho:YAG ceramic is a very promisinglaser material for future 2.0 lm laser applications.

IV. Conclusions

High quality Ho:YAG transparent ceramics were fabricatedusing a reactive sintering method under vacuum at 1780°Cfor 8 h. Defect free microstructures were observed for all thesamples produced independent of doping level. The averagegrain size of the ceramics was about 10 lm. The optical spec-troscopic properties and the laser testing results have shownthe as-prepared Ho:YAG ceramics to be a promising lasergain media for 2.0 lm solid state laser applications. Byfurther optimizing the ceramic processing, polycrystallineHo:YAG ceramics could replace single crystal Ho:YAG inthe future.

References

1A. Ilesue, Y. L. Aung, T. Taira, T. Kamimura, K. Yoshida, and G. L.Messing, “Progress in Ceramic Lasers,” Annu. Rev. Mater. Res., 36, 397–429(2006).

2A. Ikesue, T. Kinoshita, K. Kamata, and K. Yoshida, “Fabrication andOptics Properties of High-Performance Polycrystalline Nd:YAG Ceramics forSolid State Lasers,” J. Am. Ceram. Soc., 78 [4] 1033–40 (1995).

3H. Yagi, T. Yanagitani, T. Numazawa, and K. Ueda, “The Physical Prop-erties of Transparent YAG Elastic Modulus at High Temperature and Ther-mal Conductivity at low Temperature,” Ceram. Int., 33, 711–4 (2007).

4K. A. Appiagyei, G. L. Messing, and J. Q. Dumm, “Aqueous Slip Castingof Transparent Yttrium Aluminum Garnet (YAG) Ceramics,” Ceram. Int., 34,1309–13 (2008).

5A. Ikesue, I. Furusato, and K. Kamata, “Fabrication of Polycrystalline,Transparent YAG Ceramics by a Solid-State Reaction Method,” J. Am.Ceram. Soc., 78 [1] 225–8 (1995).

6A. Ikesue and Y. L. Aung, “Ceramic Laser Materials,” Nat. Photonics,2 [12] 721–7 (2008).

7F. Wang, D. Y. Shen, D. Y. Fan, and Q. S. Lu, “High Power Widely Tun-able Tm:Fiber Laser with Spectral Linewidth of 10 pm,” Laser Phys. Lett., 7[6] 450–3 (2010).

8S. W. Henderson, C. P. Hale, J. R. Magee, M. J. Kavaya, and A. V.Huffaker, “Eye-Safe Coherent Laser Radar System at 2.1 lm Using Tm, Ho:YAG Lasers,” Opt. Lett., 16 [10] 773–5 (1991).

9E. Lippert, S. Nicolas, G. Arisholm, K. Stenersen, and G. Rustad, “Midin-frared Laser Source with High Power and Beam Quality,” Appl. Opt., 45 [16]3839–45 (2006).

10A. Godard, “Infrared (2–12 lm) Solid-State Laser Sources: A Review,”C. R. Physique, 8, 1100–28 (2007).

Fig. 4. (a) Absorption spectrum of Ho:YAG ceramics andabsorption coefficients at 1907 nm as a function of Ho3+

concentration (inset); (b) the fluorescence spectrum of 1.0 at.% Ho:YAG ceramic.

Fig. 5. Output power versus incident pump power for 1.5 and2.0 at.% Ho:YAG ceramics. Inset, output spectrum of the Ho:YAGceramic laser at 2097 nm.

54 Rapid Communications of the American Ceramic Society Vol. 95, No. 1

11D. Y. Shen, A. Abdolvand, L. J. Cooper, and W. A. Clarkson, “EfficientHo:YAG Laser Pumped by a Cladding-Pumped Tunable Tm:Silica-FiberLaser,” Appl. Phys. B, 79, 559–61 (2004).

12M. Malinowski, Z. Frukacz, M. Szuflinska, A. Wnuk, and M. Kaczkan,“Optical Transitions of Ho3+ in YAG,” J. Alloys Compd., 300–301, 389–94(2000).

13D. W. Luo, C. W. Xu, J. Zhang, X. P. Qin, H. Yang, W. D. Tan, Z. H.Cong, and D. Y. Tang, “Diode Pumped and Mode-Locked Yb:GdYAG Cera-mic Lasers,” Laser Phys. Lett., 8 [10] 719–22 (2011).

14J. Zhang, S. W. Wang, T. J. Rong, and L. D. Chen, “Upconversion Lumi-nescence in Er3+ Doped and Yb3+/Er3+ Codoped Yttria NanocrystallinePowders,” J. Am. Ceram. Soc., 87 [6] 1072–5 (2004).

15A. Ikesue, K. Yoshida, T. Yamamoto, and I. Yamaga, “Optical ScatteringCenters in Polycrystalline Nd:YAG Laser,” J. Am. Ceram. Soc., 80 [6] 1517–22 (1997).

16X. D. Li, J. G. Li, Z. M. Xiu, D. Huo, and X. D. Sun, “Transparent Nd:YAG Ceramics Fabricated Using Nanosized c-Alumina and Yttria Powders,”J. Am. Ceram. Soc., 92 [1] 241–4 (2009).

17R. C. Stoneman and L. Esterowitz, “Intracavity-Pumped 2.09-lmHo:YAG Laser,” Opt. Lett., 17 [10] 736–8 (1992).

18N. P. Barnes, B. M. Walsh, and E. D. Filer, “Ho Upconversion: Applica-tions to Ho Lasers,” J. Opt. Soc. Am. B, 20 [6] 1212–9 (2003).

19H. Chen, D. Y. Shen, J. Zhang, H. Yang, D. Y. Tang, T. Zhao, and X.F. Yang, “In-Band Pumped Highly Efficient Ho:YAG Ceramic Laser with21 W Output Power at 2097 nm,” Opt. Lett., 36 [9] 1575–7 (2011).

20W. X. Zhang, J. Zhou, W. B. Liu, J. Li, L. Wang, B. X. Jiang, Y. B.Pan, X. J. Cheng, and J. Q. Xu, “Fabrication, Properties and Laser Perfor-mance of Ho:YAG Transparent Ceramic,” J. Alloys Compd., 506, 745–8(2010).

21X. M. Duan, B. Q. Yao, C. W. Song, J. Gao, and Y. Z. Wang, “RoomTemperature Efficient Continuous Wave and Q-Switched Ho:YAG LaserDouble-Pass Pumped by a Diode-Pumped Tm:YLF Laser,” Laser Phys. Lett.,5 [11] 800–3 (2008). h

January 2012 Rapid Communications of the American Ceramic Society 55