Diode-Pumped Tm:YAG Ceramic Laser

Wen-Xin Zhang,w,z,y Yu-Bai Pan,z Jun Zhou,z,y Wen-Bin Liu,z Jiang Li,z Ben-Xue Jiang,z Xiao-Jin Cheng,z

and Jian-Qiu Xuz

zKey Laboratory of Transparent and Opto-Functional Advanced Inorganic Materials, Shanghai Institute of Ceramics,Chinese Academy of Sciences, Shanghai 200050, China

yGraduate School of the Chinese Academy of Sciences, Beijing 100039, ChinazShanghai Institute of Optics and Fine Mechanics, Chinese Academy of Science, Shanghai 201800, China

Highly transparent Tm:YAG ceramic was fabricated by a solid-state reaction and vacuum sintering. The optical properties, themicrostructure, and the laser performance of the Tm:YAG ce-ramic were investigated. A Tm:YAG ceramic with an averagegrain size of B10 lm was obtained by sintering at 17501C for10 h. The in-line transmittance was 84.0% at 2015 nm. Theabsorption coefficients at 681 and 785 nm were 8.23 and 3.27cm�1, respectively. The grain boundaries were clean and no sec-

ondary phase was observed. The 6 at.% Tm:YAG ceramic slab(1.2 mm� 5 mm� 6 mm) was end-pumped by a laser diode at792 nm, and the maximum output power of 4.5 W was obtainedwith a slope efficiency of 20.5% at 2015 nm.

I. Introduction

THE interest in all solid-state lasers operating in the eye-safe spectral region near 2 mm is acknowledged for appli-

cations in medicine, range-finding, coherent laser radar, andatmospheric sensing.1 Most solid-state 1.6–2.0 mm laser op-erations are based on the transition of 3F4-

3H6 in thuliumions. The upper laser level (3F4) is populated through thecross-relaxation process 3H41

3H6-3F41

3F4. Two Tm31 ionswill be excited to the 3F4 level through this relaxation processfor each photon absorbed in the 3H4 level. The Tm31 laserhas a high-quantum efficiency (B2). This advantage cancompensate for its disadvantage in a small emitting crosssection.2–4

Yttrium aluminum garnet (YAG, Y3Al5O12) is a well-estab-lished laser host material due to its attractive thermal and op-tical properties. The performance of the thulium ions in YAGsingle crystal (Tm:YAG) has been widely studied, and the laseroperation has been successfully realized.5–9 Most widely usedTm-doped YAG single crystals are grown by the traditionalCzochralski method. Although this method provides relativelylarge single crystals for different applications, it still hasseveral disadvantages. For example, an expensive Ir crucible isrequired for the growth of YAG single crystal, and contamina-tion from it is hard to avoid. The growing technique for YAGsingle crystal requires great skills and a very long productionperiod.10

Compared with the single crystal, the Tm:YAG ceramic lasermaterial has many advantages, for example ease of fabrication,

less cost, ability of mass production, feasibility of large size, ahigh thulium concentration, multiple layers, and multifunctionalceramic structure.

In this paper, highly transparent Tm:YAG ceramic by a sim-ple solid-state reaction method using commercial a-Al2O3,Y2O3, and Tm2O3 powders was successfully fabricated. The ab-sorption spectrum, the fluorescence spectrum, the microstruc-ture, and the laser property of the Tm:YAG ceramic wereinvestigated.

II. Experimental Procedures

(1) Ceramic Fabrication

High-purity powders of a-Al2O3 (99.99%, Shanghai WusongChemical Co. Ltd., Shanghai, China), Y2O3 (99.99%, ShanghaiYuelong NewMaterials Co. Ltd., Shanghai, China), and Tm2O3

(99.99%, Conghua Jianfeng Rare-Earth Co. Ltd., Guangzhou,China) were used as starting materials. The starting powderswere weighed according to the chemical composition of 6 at.%Tm:YAG, and then mixed by ball milling in anhydrous alcoholfor 12 h, with 0.5 wt% tetraethyl orthosilicate as the sinteringaid. The dried powder mixture was ground and sieved througha 200-mesh screen. After removing the organic component bycalcining at 4001–8001C for 2 h, the powder mixture was dry-pressed with a low pressure into F25 mm disks in a steel mold,and then cold-isostatic-pressed at 250 MPa. Then the specimenswere vacuum sintered at 17501C for 10 h. After sintering, thespecimens were annealed at 15001C for 2 h in air to relieve in-ternal stresses and fill oxygen vacancies formed during the vac-uum-sintering process. Finally, highly transparent Tm:YAGceramics were obtained.

The sample was mirror-polished on both surfaces and used tomeasure the optical transmittance and absorption spectra(Model U-2800 Spectrophotometer, Hitachi, Tokyo, Japan).For measuring the fluorescence spectrum (Model Fluorolog-3,Jobin Yvon, Paris, France), the specimen was excited with a 785nm Ti-sapphire laser diode. The microstructure of the mirror-polished surface of the sample was observed by an electronprobe micro-analyzer (EPMA,Model JXA-8100, JEOL, Tokyo,Japan). The microstructure of the grain boundary was charac-terized by field emission transmission electron microscopy(FETEM, Model EM 2100, JEOL).

(2) Laser Experiment

Figure 1 shows a schematic diagram of the experimental setup.11

The dimension of the sample is 1.2 mm� 5 mm� 6 mm. Bothsurfaces (1.2 mm� 5 mm) of the Tm:YAG ceramic were mirror-polished, parallel, and antireflection-coated at 792 nm and 2.01mm. The sample was end-pumped by a laser diode with an emis-sion wavelength of around 792 nm. The LD output was shapedand focused by a series of convex lenses. The pump diode light

R.-J. Xie—contributing editor

This work was supported by the 863 project (no. AA03Z523) and the Major BasicResearch Programs of Shanghai (no. 07DJ14001).

wAuthor to whom correspondence should be addressed. e-mail: zwx1985@mail.sic.ac.cn

Manuscript No. 26189. Received April 30, 2009; approved May 26, 2009.

Journal

J. Am. Ceram. Soc., 92 [10] 2434–2437 (2009)

DOI: 10.1111/j.1551-2916.2009.03220.x

r 2009 The American Ceramic Society

2434

was focused into the Tm:YAG ceramic slab as a beam(0.5 mm� 1.5 mm). The laser cavity consisted of two mirrors(M1 and M2), where M1 was antireflection-coated at 792 nmand high-reflection-coated at 2015 nm, and M2 was the outputcoupler with 5% transmission at 2015 nm.

III. Results and Discussion

The mirror-polished 6 at.% Tm:YAG sample is shown inFig. 2(a). When the 20 mW He–Ne laser beam passed throughthe Tm:YAG sample (from left to right), the light beam wasalmost unseen with the naked eye, which indicated very fewscattering particles in the Tm:YAG ceramic. The in-line trans-mittances in the visible region and the infrared region are bothover 82%, as shown in Fig. 2(b). No obvious decline of trans-mittance in the visible region is revealed due to a limited numberof scattering centers in the sample. The in-line transmittancesare 84.0% at 2015 nm and 57.2% at 792 nm, respectively.

The absorption spectrum of the Tm:YAG ceramic from200 to 2000 nm at room temperature is shown in Fig. 3.It can be seen that Tm:YAG has seven strong absorptionbands, which are associated with the energy level transitionsin thulium ions from the ground state 3H6 to the excited states3P2,

1D2,1G4,

3F3–3F2,

3H4,3H5, and 3F4, respectively. The

central wavelengths are 262, 357, 460, 681, 785, 1172, and1622 nm, respectively. Two strong absorption peaks locatedat 681 nm (3H6-

3F3–3F2) and 785 nm (3H6-

3H4) are con-sistent with the emission wavelengths of the near-infrareddiode and the infrared diode. Therefore, Tm:YAG could bean excellent diode-pumped laser medium.12,13 The absorption

coefficients are 8.23 cm�1 at 681 nm and 3.27 cm�1 at 785 nm,respectively.

Figure 4(a) is the room temperature fluorescence spectrumexcited at 785 nm by a Ti–sapphire laser diode. The emissionregion centered at 2015 nm is from 1600 to 2200 nm, whichcomes from the transition 3F4-

3H6, as shown in the energylevel diagram14 of the Tm:YAG ceramic (Fig. 4(b)).

The Tm:YAG ceramic sample was thermal etched after mir-ror polishing in order to observe the microstructure of the grainsand the grain boundaries by EPMA. Figure 5 shows that theaverage grain size is about 10 mm and there are no pores andimpurities in or between the grains.

A high-resolution TEM micrograph of the grain boundary isshown in Fig. 6. It can be seen that the grain boundary is cleanand no secondary phase is detected. The interplanar spacing ofthe left lower grain is about 0.62 nm, which corresponds to thedistance between two (200) planes of cubic YAG. The interpla-nar spacing of the right upper grain is about 0.49 nm, whichcorresponds to the distance between two (121) planes of cubicYAG. The insets in Fig. 6 are the corresponding selected areaelectron diffraction patterns of both the Tm:YAG grains. Op-tical scattering caused by clean boundaries in cubic YAGceramic is insignificant, which has been reported in previouspapers.15,16

To the best of our knowledge, a 2 mm laser from Tm:YAGceramic is reported for the first time. The Tm:YAG ceramic slabwas end-pumped by a laser diode at 792 nm. Figure 7(a) showsthe laser output power versus the absorbed pump power for the6 at.% Tm:YAG ceramic. With a maximum absorbed pumppower of 31.2 W, a laser output power of 4.5 W has been ob-tained. The slope efficiency and optic–optic transformation

LD laser diode; M1 rear mirror; M2 output coupler;

L1: Plano-convex cylindrical lens 1;

L2: plano-convex cylindrical lens 2;

L3: plano-convex spherical lens

M2 M1L1 L2LD L3

Tm:YAGCeramic

Fig. 1. The scheme of the Tm:YAG ceramic laser experiment.11

Fig. 2. The appearance (a) and the transmittance spectrum (b) of the mirror-polished 6 at.% Tm:YAG ceramic.

Fig. 3. The absorption spectrum of the Tm:YAG ceramic from 200 to2000 nm at room temperature.

October 2009 Communications of the American Ceramic Society 2435

efficiency of the Tm:YAG ceramic are 20.5% and 14.1%, re-spectively, which are just a little lower than those of theTm:YAG crystal (the slope efficiency and optic–optic transfor-mation efficiency of Tm:YAG crystal are 26.8% and 14.2%,respectively).17 The laser threshold is 9.3 W. Figure 7(b) displays

the laser spectrum of the 6 at.% Tm:YAG ceramic. The laserspectrum of the Tm:YAG ceramic is centered at 2015 nm. Theresults of the laser experiment show that the quality of suchceramic is good enough to be used as a highly efficient lasermaterial.

Fig. 4. The fluorescence spectrum (a) and the energy level diagram14 (b) of the Tm:YAG ceramic at room temperature.

Fig. 5. The EPMA micrograph of the mirror-polished and thermal-etched surface of the Tm:YAG ceramic.

Fig. 6. The high-resolution TEM micrograph of the grain boundary.

Fig. 7. The laser output power versus the absorbed pump power (a) and the laser spectrum (b) for the 6 at.% Tm:YAG ceramic.

2436 Communications of the American Ceramic Society Vol. 92, No. 10

IV. Conclusions

Highly transparent Tm:YAG ceramic with an average grain sizeof B10 mm was obtained by a solid-state reaction and vacuumsintering. The in-line transmittances are 84.0% at 2015 nm and57.2% at 792 nm, respectively. The 6 at.% Tm:YAG ceramicslab was end-pumped by a laser diode at 792 nm. The maximumoutput power of 4.5W was obtained with a slope efficiency of20.5% at 2015 nm. The results of the laser experiment show thatthe quality of such ceramics is good enough to be used as ahighly efficient laser material.

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