Optical properties and laser characteristics of highly Nd3+-dopedY3Al5O12 ceramicsIchiro Shoji, Sunao Kurimura, Yoichi Sato, Takunori Taira, Akio Ikesue et al. Citation: Appl. Phys. Lett. 77, 939 (2000); doi: 10.1063/1.1289039 View online: http://dx.doi.org/10.1063/1.1289039 View Table of Contents: http://apl.aip.org/resource/1/APPLAB/v77/i7 Published by the American Institute of Physics. Related ArticlesFourier transform infrared spectroscopy approach for measurements of photoluminescence andelectroluminescence in mid-infrared Rev. Sci. Instrum. 83, 053106 (2012) Effect of Si-induced defects on 1µm absorption losses in laser-grade YAG ceramics J. Appl. Phys. 111, 093104 (2012) Electrically switchable random to photonic band-edge laser emission in chiral nematic liquid crystals Appl. Phys. Lett. 100, 071110 (2012) Spectroscopic features and laser performance at 1.06 μm of Nd3+-doped Gd1−xLuxCa4O(BO3)3 single crystal J. Appl. Phys. 111, 013102 (2012) Modified long-range surface plasmon polariton modes for laser nanoresonators J. Appl. Phys. 110, 063106 (2011) Additional information on Appl. Phys. Lett.Journal Homepage: http://apl.aip.org/ Journal Information: http://apl.aip.org/about/about_the_journal Top downloads: http://apl.aip.org/features/most_downloaded Information for Authors: http://apl.aip.org/authors

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APPLIED PHYSICS LETTERS VOLUME 77, NUMBER 7 14 AUGUST 2000

Optical properties and laser characteristics of highly Nd 3¿-dopedY3Al5O12 ceramics

Ichiro Shoji, Sunao Kurimura, Yoichi Sato, and Takunori Tairaa)

Laser Research Center for Molecular Science, Institute for Molecular Science, 38 Nishigonaka, Myodaiji,Okazaki 444-8585, Japan

Akio IkesueJapan Fine Ceramics Center, 2-4-1 Mutsuno, Atsuta-ku, Nagoya 456-8587, Japan

Kunio YoshidaInstitute of Laser Engineering, Osaka Institute of Technology, 5-16-1 Omiya, Asahi-ku, Osaka 535-8585,Japan

~Received 7 March 2000; accepted for publication 20 June 2000!

Diode-pumped laser oscillation in highly Nd31-doped polycrystalline Y3Al5O12 ~YAG! ceramicshas been demonstrated. The Nd:YAG ceramics are highly transparent; the loss of a 2.3 at. %neodymium-doped ceramic is as low as that of a 0.9 at. % Nd:YAG single crystal. The high dopingof Nd31 ions realizes large pump absorption; a 6.6 at. %-doped ceramic has an absorption coefficientof 60.4 cm21 at 808 nm. The same concentration quenching parameter is obtained between theNd:YAG ceramics and Nd:YAG single crystals. A laser using an 847-mm-thick 3.4 at. % Nd:YAGceramic as a gain medium operates at 2.3 times higher output power than the same laser with a719-mm-thick 0.9 at. % Nd:YAG single-crystal gain medium. ©2000 American Institute ofPhysics.@S0003-6951~00!03233-2#

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Diode-pumped microchip solid-state lasers are attraclight sources because of advantages such as compacthigh efficiency, high power, and a small number of longidinal and low-order transverse modes.1 A wide variety ofmaterials has been investigated to develop more efficienthigher power microchip lasers. Nd:YVO4 is a popular mate-rial for a highly efficient microchip laser owing to its largabsorption cross section.2,3 However, Nd:YVO4 is not suit-able for high-power operation because its thermal conducity is only half that of Nd:Y3Al5O12 ~Nd:YAG! and it haspoor mechanical properties. On the other hand, althoNd:YAG has good thermo-mechanical properties, highlyficient microchip operation is difficult; YAG crystals withNd31 doped beyond a certain amount~typically 1.5 at. %! arehardly obtainable by the standard Czochralski method wout deterioration such as segregation, so that the pumpsorption is limited. Higher concentration~up to 4.5 at. %!was reported for crystals grown with a flux method,4 but onlysmall samples are available for quite a long growth time

We have recently developed polycrystalline Nd:YAceramics that have transparency comparable to Nd:Ysingle crystals,5,6 and we have succeeded in highly dopiNd31 into the YAG ceramics to overcome its small absotion cross section.7 We applied a simple sintering methowith which larger samples can be made in much shorter tthan the conventional crystal growth techniques. Tthermo-mechanical properties of the Nd:YAG ceramicshardly degraded; the thermal conductivity was measurebe 9.0 W/mK at 20 °C at the Nd31 concentration of 6.6at. %,8 while that of a YAG single crystal was 10.7 W/m K

a!Author to whom correspondence should be addressed; electronictaira@ims.ac.jp

9390003-6951/2000/77(7)/939/3/$17.00

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In this letter we report measurements of the absorption sptra and the fluorescence lifetime of the ceramic Nd:YAG ashow that it is promising as a highly efficient, high powmicrochip laser material. Moreover, we characterized itsser performance.

The Nd:YAG ceramic samples were fabricated with tfollowing process, the details of which described in Ref.High-purity (.99.99 wt. %) powders of Al2O3, Y2O3, andNd2O3, which were weighed in accordance with thNd:YAG composition, were mixed, milled, spray driegranulated, and pressed at 140 MPa into disks. These dof powder compacts were then sintered at 1750 °C for 2under a vacuum of 1.331023 Pa. The input and output facets of the samples were polished for optical measuremeThe Nd31 concentration of the samples was measured wan electron probe microanalyzer~EPMA; Shimadzu Model8705!, which ranged from 1.0 to 8.2 at. %. Single crystsamples grown by Scientific Materials, the Nd31 concentra-tion of which ranged from 0.7 and 1.4 at. %, were also ppared for comparison.

Figure 1 shows the absorption spectra of the 2.0, 3.4,6.6 at. % Nd:YAG ceramics and the 1.0 at. %-doped sincrystal. The measurements were carried out using a spephotometer~Hitachi Model U3500! at a 0.2 nm resolutionThe spectrum of the 1.0 at. %-doped ceramic, which isshown in Fig. 1, was nearly identical to that of the singcrystal. The absorption increases in proportion to the Nd31

concentration. At the peak absorption wavelength of 808 nthe 3.4 at. % Nd:YAG ceramic has an absorption coefficiof 30.4 cm21, which is as large as that of a typical 1 at. %doped Nd:YVO4. The absorption spectrum of the 8.2 at. %doped ceramic is not shown in Fig. 1 because its absorbawas too large to accurately obtain the absorption coefficiearound the peak absorption wavelength of 808 nm, butil:

© 2000 American Institute of Physics

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940 Appl. Phys. Lett., Vol. 77, No. 7, 14 August 2000 Shoji et al.

absorption coefficients at the other measured wavelenwere eight times larger than that of the 1.0 at. %-dopsingle crystal.

Fluorescence lifetimest’s were determined by the measurement of the time dependence of the fluorescence insity which decayed exponentially. We used as a pump soan output from an optical parametric oscillator~LambdaPhysik SCANMATE OPPO! pumped by a frequency-tripleQ-switched Nd:YAG laser~Coherent Infinity 40-100!. Thepump wavelength and the pump pulse width were 808and 10 ns, respectively. The measurement was performethe 1.0, 2.0, 3.4, 6.6, and 8.2 at. %-doped ceramics and0.7, 0.9, 1.0, 1.2, 1.3, and 1.4 at. %-doped single crystThe dependence of the lifetime on the Nd31 concentration isshown in Fig. 2. The solid curve is the fit to the data usthe following equation:

t5t0

11~CNd /C0!2 , ~1!

whereCNd is the Nd31 concentration,t0 is the lifetime atCNd50, andC0 is the quenching parameter. Nearly the sat0’s, 248 ms for the ceramics and 250ms for the singlecrystals, and the sameC0 , 2.8 at. %, were obtained for thceramics and the single crystals. Those values are in gagreement with a previously reported value,9 and it has beenshown that the quenching of Nd:YAG obeys Eq.~1! up to ahigh concentration region of Nd31.

FIG. 1. Absorption spectra of 2.0, 3.4, and 6.6 at. % Nd:YAG ceram~solid curves! and 1.0 at. % Nd:YAG single crystal~dashed curve!.

FIG. 2. Fluorescence lifetime as a function of Nd31 concentration. The opensquares show the experimental data for the ceramics and the closed triafor the single crystals. The solid curve is the fitting with Eq.~1!.

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Figure 3 shows the schematic of the laser oscillatmeasurement. We used a diode laser oscillating at 809 nma pump source. The pump beam was collimated with a fiand a rod lens and focused onto the sample. The samthicknesses were less than 1 mm. Although the beam radeach focal point in the fast and slow-axis direction wereand 90mm, respectively, a nearly circular pump beam sha;90mm radius, was realized in the medium because ofaberration. The input facet of the sample was high transmsion and high-reflection coated at 808 and 1064 nm, resptively, and the output facet was antireflection coated at 10nm. An output coupler with a curvature of 100 mm was usand the cavity length was set to be 50 mm. We haveserved laser oscillation in ceramic samples with up to theat. % doping. Figure 4 shows the input–output power retionships of the 2.3 and 3.4 at. %-doped ceramics and theat. %-doped single crystal when an output coupler with tramittance of 4.4% was used. For the 3.4 at. %-doped cera2.3 times higher output was achieved than for the sincrystal, which indicates the advantages of Nd:YAG ceramas highly efficient miniature or microchip lasers.

We estimated the cavity losses by obtaining the sloefficiencieshs’s, which were determined from the relatiobetween the absorbed pump and the laser output powechanging the transmittance of the output coupler. The routrip cavity lossLi is given by

Li5~T2 /T1!T12~h2 /h1!T2

h2 /h12T2 /T1, ~2!

whereh1 andh2 are the slope efficiencies when the outp

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FIG. 3. Schematic of the diode-pumped laser oscillation.

FIG. 4. Dependence of the output power on the input pump power for3.4 and 2.3 at. % Nd:YAG ceramics and the 0.9 at. % Nd:YAG single crtal when an output coupler~OC! with transmittance of 4.4% was usedThicknesses of the 3.4, 2.3, and 0.9 at. %-doped samples were 847, 868719 mm, respectively.

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941Appl. Phys. Lett., Vol. 77, No. 7, 14 August 2000 Shoji et al.

couplers with transmittance ofT154.4% and T251.1%were used, respectively. Absorbed power was estimatedmonitoring the transmitted pump power at each input powthe input-to-absorbed power ratio was not constant at difent input powers~different diode currents! because of spectral changes of the pump diode. This is why the output powfrom the 3.4 at. %-doped ceramic sample decreased athe input pump power of 550 mW, as shown in Fig. 4, nbecause of thermal effects. The relation between thesorbed and the output power for the 3.4 at. %-doped cerais shown in Fig. 5, and Table I summarizes the slope eciencies and the cavity losses of each sample. It was fothat the loss of the 2.3 at. %-doped ceramic is as low asof the single crystal. On the other hand, the 3.4 at. %-doceramic has much higher loss coefficient. It is presumabecause the scattering loss at the grain boundaries becsignificant, since the ceramics with higher neodymium c

FIG. 5. Dependence of the output power on the absorbed pump powethe 3.4 at. % Nd:YAG ceramic. The open squares show the experimedata obtained for the output coupler with transmittance of 4.4%, whileopen circles show that of 1.1 at. %.

TABLE I. Slope efficiencies and cavity losses.

Nd31 concentration~at. %! h1 ~%! h2 ~%! Li ~%! a i(cm21)a

Single Crystal0.9 20.7 15.7 0.5 0.04

Ceramics2.3 24.7 19.0 0.5 0.033.4 27.4 13.7 2.2 0.13

aLoss coefficient.

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centration have smaller grain sizes, i.e., the increased numof the grain boundaries.7 The slope efficiency of the ceramicwas found to be higher than that of the single crystal. Thabecause the mode-matching efficiencies of the ceramicshigher than that of the single crystal; owing to their largabsorption coefficients, the absorption depths in the ceramare shorter, resulting in larger mode overlap betweenpump and the laser beams in the sample. We have notsucceeded in laser oscillation of the ceramic samples wneodymium doped higher than 3.4 at. %, probably becaustheir much higher scattering losses. However, using thinsamples may enable laser oscillation since their absorpcoefficients are large enough, although of course we havlower the loss by optimization of the sample fabrication pcesses.

In summary, we have shown that the Nd:YAG ceramhave large pump absorption coefficients and low scattelosses. Using a 3.4 at. % Nd:YAG ceramic we have demstrated 2.3 times higher laser power than with a conventioNd:YAG single crystal. It is concluded that the highNd31-doped YAG ceramics are promising as highly efficieand high power microchip laser materials.

The authors are grateful to M. Nakamura of The Univesity of Tokyo for his technical support of the EPMA mesurement. The authors thank Okamoto Optics Work, Inc.polishing the samples. This work was partially supporteda Grant-in-Aid for Scientific Research Grant No. 1055501from The Ministry of Education, Science, Sports, and Cture of Japan.

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