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Optics & Laser Technology 39 (2007) 1295–1300

www.elsevier.com/locate/optlastec

The optical properties and laser characteristics of Cr3+ and Nd3+

co-doped Y3Al5O12 ceramics

H. Yagia,b,�, T. Yanagitania, H. Yoshidac, M. Nakatsukac, K. Uedab

aTakuma Works, Konoshima Chemical Co. Ltd., 80 Koda, Takuma-cho, Mitoyo-gun, Kagawa 769-1103, JapanbInstitute for Laser Science, University of Electro-Communications, 1-5-1 Chofugaoka, Chofu, Tokyo 182-8585, Japan

cInstitute of Laser Engineering, Osaka University, 2-6 Yamada-oka, Suita, Osaka 565-0871, Japan

Received 27 January 2006; received in revised form 27 June 2006; accepted 30 June 2006

Available online 1 September 2006

Abstract

We have fabricated Cr3+ and Nd3+ co-doped YAG (Cr;Nd:YAG) ceramics, and investigated their optical properties and laser

characteristics. The Cr;Nd:YAG has two broad absorption bands at around 440 nm (4A2-4T1) and 600 nm (4A2-

4T2) respectively,

caused by Cr3+ ions. In the case of pumping at 440 nm, the maximum effective lifetime of the Cr;Nd:YAG was 737 ms with a 0.1 at%

Cr3+ and 1.0 at% Nd3+ co-doped YAG sample. Cr3+ ions take a role of an effective sensitizer to convert the UV light of flashlamp. For

single-shot laser operation, a 10.4 J output energy at 1064 nm was obtained with 0.1 at% Cr3+ and 1.0 at% Nd3+ co-doped YAG

ceramic rod with a laser efficiency of 4.9%. The laser efficiency was found to be more than twice that of a 1.0 at % Nd3+:YAG ceramic

rod.

r 2006 Elsevier Ltd. All rights reserved.

Keywords: Ceramic laser; Cr3+ and Nd3+ co-doped: YAG; Lamp-pumped laser

1. Introduction

Neodymium-doped yttrium aluminum garnet(Nd:YAG) is the most widely used of the solid-state lasermaterials. Generally, the optical conversion efficiency oflamp-pumped Nd:YAG laser is only about 2–3%, whereasa laser diode (LD) pumping scheme can give higherefficiency because a pump light from the LD can beefficiently absorbed by the laser gain medium with asmaller quantum defect. However, lamps are much morecost-effective than LDs even today, so high-power lamp-pumped Nd:YAG lasers are widely used in industrialapplications. Therefore, it is worthwhile to consider waysto improve the efficiency of lamp-pumped Nd:YAG lasers.

To improve the efficiency of lamp-pumped Nd-dopedlasers, a cross-pumped Cr3+ and Nd3+ co-doped YAG

e front matter r 2006 Elsevier Ltd. All rights reserved.

tlastec.2006.06.016

ing author. Takuma Works, Konoshima Chemical Co.

Takuma-cho, Mitoyo-gun, Kagawa 769-1103, Japan.

3 3155; fax: +81 875 83 8188.

ess: yagi@konoshima.co.jp (H. Yagi).

(Cr;Nd:YAG) laser system was studied in 1964 [1]. Thesensitizer Cr3+ ions, have broad absorption bands in thevisible region, and their spectra overlap with the emissionspectra of a Xe flashlamp. By the 4T2-

4A2 transition ofCr3+ ions, energy is transferred from Cr3+ to Nd3+ ions.As a result of this transfer, pumping efficiency is drasticallyincreased. Ideally, this cross-pumped laser system is usefulto industrial lamp-pumped lasers, and it is also suitable forsolar-pumped lasers, because the absorption of Cr3+ ionsis matched with the emission of solar light.Kvapil et al. [2] reported laser properties of the

Cr;Nd:YAG single crystal, and they showed that the laserproperty was improved by doping a trace of Cr3+ ions. Forthe Cr;Nd:YAG single crystal, the luminescence propertiesof the Cr3+ ions and the energy transfer between the Cr3+

and Nd3+ ions has studied quantitatively [3]. In 1980s, aCr3+ and Nd3+ co-doped gadolinium scandium galliumgarnet (Cr;Nd:GSGG) laser crystal was also developed andstudied [4,5]. From laser experiments by using a flashlamp,high laser efficiency was obtained when compared with thatof a Nd:YAG laser. The GSGG crystal is suitable for the

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Nd:YAG

0123456

Cr,Nd:YAG

0123456

Cr:YAG

00.5

11.5

22.5

3

350 450 550 650 750

Wavelength (nm)

Abs

orpt

ion

coef

fici

ent (

cm-1

)

Fig. 1. Absorption spectra of 1.0 at% Nd3+:YAG, 0.1 at% Cr3+ and

1.0 at% Nd3+ co-doped:YAG and 0.1 at% Cr3+:YAG ceramics at room

temperature.

H. Yagi et al. / Optics & Laser Technology 39 (2007) 1295–13001296

co-doped host material when compared with the YAGcrystal because energy transfer rate of the Cr;Nd:GSGG ishigher than that of the YAG [6].

Although theoretical and experimental studies of Cr3+

and Nd3+ co-doped lasers have been carried out for a longtime, they are not conclusive. For a YAG single crystal, itis difficult to grow Cr3+ and Nd3+ co-doped YAG crystalswith high optical quality. On the other hand, it is easy togrow the Cr;Nd:GSGG single crystals and to fabricatelarge crystals with high optical quality. However, thermalconductivity of a Nd:GSGG crystal is �0.06W/cmK, andit is lower than that of the YAG crystal (�0.13W/cmK).Therefore, the GSGG crystal is not suitable for high-powerlaser materials.

Ceramic YAG laser materials have recently become anattractive alternative to the single crystal. In comparison withYAG single crystals, the YAG ceramics have severaladvantages: easy fabrication, scalability to large size, avail-ability of high Nd3+ ion concentration, the relative ease ofachieving composite structure, and so on. We have been ableto fabricate highly transparent solid-state laser ceramics bythe vacuum sintering technique and nanocrystalline technol-ogy [7]. LD end-pumped Nd:YAG ceramic lasers weredemonstrated in 2000 and 2001 [8,9], and their laserefficiencies were comparable with the best available Nd:YAGsingle-crystal lasers. For a side-pumped high-power Nd:YAGceramic rod laser, the laser output power of 110W wasobtained with a slope efficiency of about 41% in 2004 [10].

Ceramic technology makes it easier to incorporateseveral dopant ions into the YAG material compared tosingle crystal grown from the melt. A 0.4 at% Cr3+ and0.8 at% Nd3+ co-doped transparent YAG ceramic wasfabricated by a solid-state reaction method, and the opticalproperties of this ceramic were investigated by Ikesue et al.[11]. However, this ceramic was a small thin disk, so thelaser performance had not been studied.

Recently, we had fabricated Cr;Nd:YAG ceramics withsufficient size. In this work, optical properties and laserperformance of highly transparent Cr;Nd:YAG ceramicsare reported. First, we described the fabrication method ofour Cr;Nd:YAG ceramics. Next, the optical properties ofthese ceramics were investigated. Finally, a flashlamp-pumped Cr;Nd:YAG ceramic laser operated at 1064 nmwavelength was demonstrated.

2. Fabrication of YAG ceramics

The ceramics samples were fabricated by the slip castingand vacuum sintering method [7]. The fabrication processof the YAG ceramics involves the following steps. First,aqueous solutions of aluminum, yttrium, and each metal(neodymium, chromium) chloride were mixed together.The mixed aqueous solution was added drop-wise andmixed with aqueous solution of ammonium hydrogencarbonate. Then steps of filtration and washing with waterwere repeated several times and the obtained powdermaterial was dried for 2 days at 120 1C in an oven. The

obtained precursor was calcinated at approximately1200 1C to produce raw YAG oxide powder with anaverage particle diameter of about 200 nm. Next, theobtained YAG powder and amount of SiO2 powder weremilled with a solvent, a binder and a dispersion medium,and mixed for 24 h. The SiO2 was used as the sintering aid.The milled slurry was put into a gypsum mold and dried toobtain a desired form. Finally, after removing the organiccomponents by calcinations, the materials were sintered invacuum at 1700 1C for 20 h. After annealing, highlytransparent YAG ceramics were obtained. TheCr;Nd:YAG, Nd3+:YAG and Cr3+:YAG ceramics wereprepared for these works.

3. Optical properties of Cr;Nd:YAG ceramics

Absorption spectra of the 1.0 at%. Nd3+:YAG, 0.1 at%Cr3+ and 1.0 at% Nd3+ co-doped YAG and 0.1 at%Cr3+:YAG ceramics are shown in Fig. 1. The absorptionspectra were measured with a spectrometer (U4100, HitachiLtd, Japan) at room temperature. Both Cr;Nd:YAG andCr:YAG have two broad absorption bands at around440 nm (4A2-

4T1) and 600 nm (4A2-4T2). From Fig. 1,

absorption of the Cr;Nd:YAG ceramic is superposition ofabsorption of the Nd:YAG and the Cr:YAG. Fig. 2 showsthe absorption spectra of the 1.0 at% Nd3+, 0.2 and 0.7 at%Cr3+ co-doped YAG ceramics. Two absorption intensitiesat 4A2-

4T1 and 4A2-4T2 increase by increasing the

concentration of co-doped Cr3+ ions.The emission spectra of Cr3+:YAG (4T2 - 4A2) are

shown in Fig. 3. It was measured at room temperature withan optical spectrum analyzer (AQ-6315A, ANDO, Japan),and the ceramic sample was excited by an argon ion laser at476.5 nm wavelength. The R lines were observed at around

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0

2

4

6

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10

350 450 550 650 750

Wavelength (nm)

Abs

orpt

ion

coef

fici

ent (

cm-1

)

Nd 1Cr0 .2

Nd 1Cr0 .7

Fig. 2. Absorption spectra of the 1.0 at% Nd, 0.2 and 0.7 at% Cr co-

doped YAG ceramics at room temperature.

Wavelength (nm)

600 650 700 750 800

Abs

orpt

ion

(a. u

.)E

mis

sion

inte

nsity

(a.

u.) Cr3+:YAG

4T24A2

R linesλ = 688 nm

Nd3+:YAG 4I9/24F9/2

4I9/2

4I9/24F7/2 + 4S3/2

4F5/2 + 2H9/2(2)

Fig. 3. Emission spectra of 0.1 at% Cr3+:YAG excited by an argon ion

laser at 476.5 nm at room temperature.

Table 1

The effective lifetime of Cr;Nd:YAG at 1064 nm (ms)

Sample Pumped at 440 nm Pumped at 590 nm

1at% Nd, 0.1 at% Cr 732 228

1 at% Nd, 0.7 at% Cr 459 306

1 at% Nd, 3.0 at% Cr 389 365

H. Yagi et al. / Optics & Laser Technology 39 (2007) 1295–1300 1297

688 nm. The emission transition 4T2-4A2 of the Cr

3+ ionsoverlaps with the absorption lines 4I9/2-

4F9/2,4F7/2,

4S3/2,4F5/2, and

2H9/2(2) of the Nd3+ ions. As a result, energy ofthe Cr3+ ions is transferred to Nd3+.

It is very important to investigate the fluorescence at Cr/Nd:YAG ions system, so the effective lifetimes of Nd3+

ions at 1064 nm of Cr;Nd:YAG samples (0.1, 0.7, 3 at% Crand 1 at% Nd co-doped) were measured at room tempera-ture. The samples were excited by 440 and 590 nmwavelength and the result is shown in Table 1. In the caseof pumping at 440 nm, a maximum effective lifetime is737 ms with the 0.1 at% Cr3+ co-doped ceramic. Theeffective lifetime of Cr;Nd:YAG decreases by increasingthe concentration of co-doped Cr3+ ions. The degradationof lifetime is thought to be caused by concentration

quenching. On the other hand, the effective lifetime ofCr;Nd:YAG increases by increasing the concentration ofco-doped Cr3+ ions in the case of pumping at 590 nm. Amaximum effective lifetime was 365 ms with 3.0 at% Cr3+

co-doped ceramic, and a minimum effective lifetime was228 ms with 0.1 at% Cr3+ co-doped ceramic.Generally, the Nd3+ fluorescence lifetime of 1.0 at%

Nd:YAG single crystal or ceramic is about 230 ms. There-fore, the effective lifetime of Cr;Nd:YAG is longer thanthat of Nd:YAG. Especially, the absorption band at UVregion (4A2-

4T1) affected the effective lifetime ofCr;Nd:YAG. Of course, the fluorescence lifetime of theCr;Nd:YAG and Nd:YAG ceramics is the same, whensamples were excited by 808 nm wavelength. In order tounderstand increasing of effective lifetime of Cr;Nd:YAG,the fluorescence lifetime of Cr3+:YAG ceramic was alsomeasured at room temperature. When a 0.1 at% Cr3+-doped YAG ceramic was excited by 440 nm wavelength,the fluorescence lifetime was about 1.8ms. Wall et al. [12]reported temperature dependence of fluorescence lifetimeof a 0.5% Cr:YAG single crystal , and it was 1.6ms atroom temperature. This is in agreement with our experi-mental result. Therefore, the effective lifetime of Nd3+ ionsof Cr;Nd:YAG is elongated because the lifetime of Cr3+

ions is very long.

4. Laser experimental setup

High-intensity ultraviolet (UV) radiation from a flash-lamp is unfavorable for laser gain media because thesolarization phenomenon may occur during the laseroperation. The solarization phenomenon of our Nd:YAGceramic was reported previously [13], and we confirmedthat there were no solarization problems when the UVlights below 350 nm wavelength was cut by a UV cut filter.Moreover, it has been gradually solved by improvement ofquality. However, it is not easy to solve the solarizationproblem of the Cr3+ co-doped Nd:YAG ceramics, becausethe valence of Cr ions is comparatively unstable. More-over, temperature of the laser gain media is raised by theUV lights. As a result, laser performance decreases becausethermal lens effect occurs. Therefore, the UV cut filter isimportant to realize the highly efficient oscillation ofCr;Nd:YAG ceramic laser. Two types of UV cut filters(cut-off wavelength: 420, 370 nm) were prepared, and laseroperation was carried out for first time.The schematic diagram of the laser experimental setup is

shown in Fig. 4. A YAG ceramic rod and two Xe

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YAG ceramic rodXe Flashlamp

UV-cut filter(420 or 370 nm cutoff)

Ba or Aucoating

750 mm

plane mirrorr = 1m

R = 99.5%

YAG ceramic

Fig. 4. Schematic diagram of the laser experimental setup.

0

2

4

6

8

10

0 100 200 300 400

Bank energy (J)

Out

put e

nerg

y (J

)

Nd:YAG: 370nmcut

Nd:YAG: 420nmcut

Cr;Nd:YAG: 370nmcut

Cr;Nd:YAG:420nm cut

Fig. 5. Laser output energy versus the bank energy with each ceramic rod

and UV cut filters. The 0.8 at% Nd:YAG and the 0.01 at% Cr;0.8 at%

Nd:YAG: Au-coated chamber was used.

H. Yagi et al. / Optics & Laser Technology 39 (2007) 1295–13001298

flashlamps were placed into a pumping enclosure of whichthe surface was coated with Au. A 0.8 at% Nd3+ and0.01 at% Cr3+ co-doped YAG ceramic rod with size off10� 150mm2 was used for this experiment. A 0.8 at%Nd3+:YAG ceramic rod with the same size was used forcomparison. Both end faces of each rod were polished witha surface flatness of less than l/10, where l is 633 nm, andcoated with an anti-reflection (AR) coating at 1064 nm. Xeflashlamps had an arc length of 125mm and their tubeconsisted of fused silica. Two UV cut glass filters wereinserted between the flashlamps and the YAG ceramic rod.Each UV cut filters had cut-off wavelength of approxi-mately 420 and 370 nm, respectively. Thickness of thesefilters was 2mm. The cavity length was 750mm. The lasercavity consisted of a concave mirror of 99.5% reflectivity and1m radius of curvature and a plane mirror of 57% reflectivity.

Fig. 5 shows the laser output energy versus the bankenergy with each ceramic rod and UV cut filters. The laseroutput energy of Nd:YAG was nearly the same with bothUV cut filters, and it was approximately 7.6 J. When thecut-off wavelength of the UV filters was 420 nm, the laseroutput of Cr;Nd:YAG was similar to that of Nd:YAG. Inthis case, maximum laser output energy and efficiency wereapproximately 7.65 J and 2.1%, respectively with 360 J of

bank energy. By using the 370 nm UV cut filters, the laseroutput energy of Cr;Nd:YAG rod had a large increase. Themaximum output energy of 8.9 J was obtained by bankenergy of 360 J. The efficiency was about 2.5%.In order to check coloration of ceramics rods, transmis-

sion spectra of each ceramics rods before and after thelasing experiments, were measured. As a result, it wasknown that the spectra of each rod did not changeappreciably. So a coloration caused by solarizationphenomenon did not occur with both ceramic rods, whenthe UV cut filter was used. In Fig. 2, Cr;Nd:YAG has anabsorption band at around 440 nm (4A2-

4T1), and anedge of absorption is approximately 360 nm. Therefore,this absorption band is very important, and the 370 nm UVcut filter was used for the below experiment in order toimprove the laser characteristic of the Cr;Nd:YAG.

5. Highly efficient Cr;Nd:YAG laser

The schematic diagram of the laser experimental setup issimilar same as in Fig. 4. The surface of the pumpingenclosure was coated with barium (Ba) oxide powder inorder to reflect UV light. A 1.0 at% Nd3+ and 0.1 at%Cr3+ co-doped YAG ceramic rod with size off10� 80mm2 was used for this experiment. A 1.0 at%Nd3+:YAG ceramic rod with the same size was used forcomparison. The UV cut filters had a cut-off wavelength ofabout 370 nm and a thickness of 2mm. The laser cavityconsisted of a plane mirror with reflectivity ranging from37% to 74%.First, laser experiments were carried out with single-shot

operation. The ceramic YAG rod was pumped by twoflashlamps, and each has effective pumping length of70mm. The electro-efficiency of discharge capacitor tolamp was 98%. Fig. 6 shows the laser output energy versusthe bank energy with the Cr;Nd:YAG ceramic rod andseveral plane mirrors; each reflectivity was 74%, 57%,50%, 42%, and 37% at 1064 nm.With the mirror of 74%, 57%, 42%, and 37%

reflectivity, output energy of 9.6, 10.1, 10.1 and 10.3 Jwere obtained respectively, when a bank energy reached390 J. With the mirror of 50% reflectivity, the maximumoutput energy of 10.4 J was obtained with a bank energy of390 J.A similar laser experiment with the Nd:YAG ceramic

rod was carried out for comparison. Fig. 7 shows the laser

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0

2

4

6

8

10

12

0 50 100 150 200 250 300 350 400

Bank energy (J)

Out

put e

nerg

y (J

)

:R74% :R57%

:R50% :R42%

:R37%

Fig. 6. Laser output energy versus the bank energy with the 0.1 at%

Cr;1.0 at% Nd:YAG ceramic rod: Ba-coated chamber was used, and cut

of wavelength of UV filter was 370 nm. Reflectivity of the plane mirror was

74%, 57%, 50%, 42%, and 37%, respectively.

0

1

2

3

4

5

0 50 100 150 200 250 300 350 400

Bank energy (J)

Out

put e

nerg

y (J

)

:R74% :R57%

:R50% :R42%

:R37% :R24.8%

Fig. 7. The laser output energy versus the bank energy with the 1.0 at%

Nd:YAG ceramic rod: Ba-coated chamber was used, and cut of

wavelength of UV filter was 370 nm. Reflectivity of the plane mirror was

74%, 57%, 50%, 42%, 37% and 24.8%, respectively.

0

0.01

0.02

0.03

0.04

0.05

0.06

0 50 100 150 200 250 300 350 400

Effi

cien

cy (

×10

0%)

Bank energy (J)

Cr, Nd:YAG

Nd:YAG

Fig. 8. The laser efficiency versus the bank energy with the 0.1 at%-

Cr;1.0 at% Nd:YAG rod (R ¼ 50%) and the 1.0 at% Nd:YAG rod

(R ¼ 42%).

H. Yagi et al. / Optics & Laser Technology 39 (2007) 1295–1300 1299

output energy versus the bank energy in using the severalmirrors. In the case of Nd:YAG ceramic, similar outputenergy was obtained when the bank energy was lower than140 J. With an increase in the bank energy, the laserperformances depended on the reflectivity of the planemirror. However, the output energy of the Nd:YAG laserwas much lower than that of the Cr;Nd:YAG laser. Whenthe bank energy reached 390 J, maximum output energy of4.9 J was obtained with the plane mirror of 42%reflectivity. In the same condition, output energy of over10 J was achieved with the Cr;Nd:YAG ceramic rod.

Fig. 8 shows the laser efficiency versus the bank energywith each ceramic rod. The efficiencies were calculated bythe ratio of the effective pumping length (70mm) to an arclength of a flashlamp (125mm), at the each best condition;50% reflective mirror for the Cr;Nd:YAG laser and 42%reflective mirror for the Nd:YAG laser. Maximumefficiency of the Nd:YAG laser was 2.3% for a bankenergy of 320 J. This efficiency is a general value for lamp-pumped lasers. On the other hand, in the case of theCr;Nd:YAG, the highest efficient was obtained, andmaximum efficiency of 5.0% was obtained for a bankenergy of 250 J. And an efficiency of 4.9% was obtained atmaximum bank energy of 390 J. The efficiency of theCr;Nd:YAG laser was over twice higher than that of theNd:YAG laser.Next, the laser experiments were carried out with each

rods, and several repetition rates at best conditions: a 50%reflective mirror for the Cr;Nd:YAG laser and a 42%reflective mirror for the Nd:YAG laser. The pumpingenergy and pulse duration were 175 J and 200 ms, respec-tively. Fig. 9 shows an average output power as a functionof pulse-repetition rates, with each ceramics rods. Fig. 9also shows the ratio of the average output power of bothrods. At each repetition rates, the average output power ofthe Cr;Nd:YAG rod was higher than that of a Nd:YAGrod. The ratio of the average output power of both rodswas 1.42 at 4Hz.However, the ratio gradually decreased with increasing

repetition rates. At maximum repetition of 28.5Hz, theaverage output power of the Cr;Nd;YAG rods, andthe Nd:YAG rod were 113 and 97.2W, respectively, andthe ratio was about 1.2. By increasing of repetition rates,the heat accumulated in the Cr;Nd:YAG rod, and it washigher than that of a Nd:YAG rod, because theCr;Nd:YAG had large absorption bands. Therefore, thethermal effect of the Cr;Nd:YAG rod is higher than that ofa Nd:YAG rod at high-repetition operation.

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0

20

40

60

80

100

120

0 5 10 15 20 25 30

Repetition rates (Hz)

Ave

rage

out

put p

ower

(W

)

1

1.2

1.4

1.6

1.8

2

2.2

Rat

io

Nd:YAG

CrNd:YAG

ratio

Output power

Fig. 9. An average output power as a function of pulse-repetition rates

with the 0.1 at% Cr;1.0 at% Nd:YAG rod and the 1.0 at% Nd:YAG rod.

H. Yagi et al. / Optics & Laser Technology 39 (2007) 1295–13001300

Although the Cr;Nd:YAG may be not suitable for high-repetition laser operation, the laser performance of theCr;Nd:YAG rod is higher than that of the Nd:YAG atlower repetition rates according to the result of experi-ments. It is very promising for lamp-pumped or solar-pumped lasers because of the energy transfer between Cr3+

and Nd3+ ions. The efficiency of energy transfer increasesby increasing the concentration of Cr3+ ions, but on acontrary, the thermal problem will become serious. There-fore, the optimization of concentration of Cr ions andrepetition rates are required to achieve more high-efficiencylasers. In future, a more detailed discussion and demon-stration of laser operation will be reported.

6. Conclusions

We have fabricated Cr3+ and Nd3+ co-doped YAGceramics, and investigated those optical properties of theseceramics. The Cr;Nd:YAG has two broad absorptionbands at around 440 nm (4A2-

4T1) and 600 nm(4A2-

4T2). In the case of pumping at 440 nm, a maximumeffective lifetime was measured to be 737 ms with 0.1 at%Cr3+ co-doped ceramics, and the effective lifetimedecreased by increasing the concentration of co-dopedCr3+ ions. On the other hand, the effective lifetimeincreased by increasing the concentration of co-dopedCr3+ ions in the case of pumping at 590 nm. In this case, a

maximum effective lifetime was 365 ms with 3.0 at% Cr3+

co-doped ceramic, and a minimum effective lifetime was228 ms with 0.1 at% Cr3+ co-doped ceramic.A flashlamp-pumped Cr;Nd:YAG ceramic laser oper-

ated at 1064 nm wavelength, was demonstrated with single-shot operation. A 10.4 J output energy at 1064 nm wasobtained, with a laser efficiency of 4.9%. The laserefficiency of the Cr;Nd:YAG ceramic rod was more thantwice that of the 1.0 at% Nd:YAG ceramic rod. The laserexperiments were also carried out with each rod at severalrepetition rates. At each repetition rates, the averageoutput power of Cr;Nd:YAG rod was higher than that ofNd:YAG rod. However, the ratio gradually decreasedaccording to increase repetition rates. The thermal effect ofCr;Nd:YAG rod is higher than that of Nd:YAG rod in thecase of high-repetition operation.

Acknowledgments

The authors wish to thank A.A. Kaminskii and J. Lu forfruitful discussions. We also would like to thank theceramic division of Konoshima Chemical Co. Ltd., forfabricating the ceramic samples.

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