Optical Materials 33 (2011) 667–669

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Optical Materials

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Broadband-gain Nd3+-doped Ba(Zr, Mg, Ta)O3 ceramic lasersfor ultrashort pulse generation

H. Kurokawa a, A. Shirakawa a,⇑, M. Tokurakawa a, K. Ueda a, S. Kuretake b, N. Tanaka b,Y. Kintaka b, K. Kageyama b, H. Takagi b, A.A. Kaminskii c

a Institute for Laser Science, University of Electro-Communications, 1-5-1 Chofugaoka, Chofu, Tokyo 182-8585, Japanb Murata Manufacturing Co., Ltd., 1-10-1 Higashi-kotari, Nagaokakyo, Kyoto 617-0855, Japanc Institute of Crystallography, Russian Academy of Science, Leninskii pr. 59, Moscow 119333, Russia

a r t i c l e i n f o a b s t r a c t

Article history:Received 15 April 2010Accepted 15 October 2010Available online 16 November 2010

Keywords:Ceramic laserDisordered crystalPerovskiteNeodymiumUltrashort pulse laser

0925-3467/$ - see front matter � 2010 Elsevier B.V. Adoi:10.1016/j.optmat.2010.10.024

⇑ Corresponding author. Tel.: +81 42 443 5714; faxE-mail address: akira@ils.uec.ac.jp (A. Shirakawa).

We report Nd3+-doped Ba(Zr, Mg, Ta)O3 ceramic lasers. The fluorescence of the ceramics shows character-istic broadband spectra due to the disordered crystalline system and can be tailored by the compositionadjustment. The continuous-wave laser operations as well as the first mode-locked laser operation arepresented.

� 2010 Elsevier B.V. All rights reserved.

1. Introduction

Demands are increasing for high-average power, high-efficiency,compact, and low-cost femtosecond laser for industrial applications.Laser-diode (LD) pumped mode-locked lasers based on Nd3+-doped[1] and Yb3+-doped [2] gain materials have been extensively inves-tigated to alternate the conventional Ti:sapphire laser. The principallimiting factor of operating pulse duration is the gain bandwidth ofthe material. Murata Manufacturing recently developed a newhighly transparent ceramic Ba(Zr, Mg, Ta)O3 (‘‘Lumicera�’’ type-Z;called here BZMT) for high refractive optical lenses [3,4]. It has a‘‘disordered’’ perovskite structure, and doping with rare-earth ionsexhibits very broadband fluorescence spectra. In addition, mechan-ical and thermal properties of BZMT are superior to glasses as a hostmaterial (Table 1). Therefore rare-earth doped BZMTs are attractivefor ultrashort pulse lasers and tunable lasers. We reported LD-pumped Nd3+:BZMT ceramic laser in CW operation with a177 mW output power at 1075 nm [5]. In comparison with Yb3+,Nd3+ has advantages such as no need of high-intensity pumpingand large emission cross section, and Nd3+:BZMT ceramics will bepromising material for ultrashort pulse lasers.

In this paper, we report Nd3+:BZMT ceramics with their broad-band gain profiles tailored for ultrashort pulse laser and high laser

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efficiency. 1.4 ps pulses at 1062 nm have been successfully gener-ated with passive mode locking and LD pumping.

2. Experiments and results

Using high-purity BaCO3, ZrO2, MgCO3, Ta2O5, and Nd2O3 pow-ders, the transparent Nd3+:BZMT ceramics were prepared by opti-mized ceramic fabrication process. The Nd3+ ions can occupy anyplace of B-site cation’s sites, and the disordered nature originatesthe characteristic broad fluorescence spectra. The effectiveemission cross section spectra of three types of Nd3+:BZMT(CNd � 1 mol%) ceramics in the 4F3/2 ?

4I11/2 transition are shownin Fig. 1. The profiles are changed by composition adjustment ofZr, Mg, and Ta. The BZMT-1 is the ceramic reported in Ref. [5],which has a 26 nm bandwidth. The newly fabricated BZMT-2 and3 show broader bandwidths (30 nm) with a different peak position.The effective emission cross section does not change so much. Allceramics have similar peak values of around 2.4 � 10�20 cm2. Thefluorescence lifetimes of the new ceramics were measured to be290 ls, almost similar to the lifetime of BZMT-1 (270 ls) [5].

The laser properties of those ceramics are compared in Fig. 2.Here we used a simple two-mirror resonator pumped by a fiber-coupled LD, as was in Ref. [5]. The thickness of BZMT-1 was4 mm, whereas those of BZMT-2 and 3 were 2 mm. All ceramicswere uncoated. The output powers as functions of the absorbedpump power are shown in Fig. 2. Because of no coating, the signif-icant reflection loss (�12% for one surface) due to the high indices

0

0.5

1

1.5

2

2.5

1000 1050 1100 1150 1200

BZMT-1BZMT-2BZMT-3

Effe

ctiv

e em

issi

on c

ross

sec

tion

(10-2

0 cm2)

Wavelength (nm)

Fig. 1. Effective emission cross-section spectra of the three types of 1 mol%Nd3+:BZMT ceramics.

Table 1Mechanical and thermal properties.

BZMT Silicate(Q-246)

Phosphate(HAP-4)

YAG

Nd3+ emission bandwidth (nm) 30 28 27 0.8Nd3+ emission cross-section

(10�20 cm2)2.4 2.9 3.6 26

Thermal conductivity (W/mK) 3.1 1.3 1.023 14Thermal expansion coefficient

(�10�6/K)8.9 9.0 8.5 7.8

Young’s modulus (GPa) 228 84.0 68.8 310.9Thermal shock parameter (W/m) 302 96 140 790

M2

M3

OC

CM1

SESAM

809nm5 W LD

1 95µm

3.3mm, 1mol.% Nd3+:BZMT

M1

CM2

M2

M3

OC

CM1

SESAM

809nm5 W LD

1 95µm

3.3mm, 1mol.% Nd3+:BZMT

M1

CM2

Fig. 4. Experimental setup of the mode-locked Nd3+:BZMT ceramic laser.

-70

-60

-50

-40

-30

-20

-10

Inte

nsity

(dB

m)

-70

-60

-50

-40

-30

-20

-10

0

Inte

nsity

(dB

m)

-70

-60

-50

-40

-30

-20

1050 1055 1060 1065 1070 1075 1080

Inte

nsity

(dB

m)

Wavelength (nm)

BZMT-1

BZMT-2

BZMT-3

1062nm 1075nm

Fig. 3. Spectra of the Nd3+:BZMT ceramic lasers at the maximum output powers.

668 H. Kurokawa et al. / Optical Materials 33 (2011) 667–669

(>2.0) limits the efficiency, but the slope efficiencies of the newceramics increase by about twice in comparison with BZMT-1.Since BZMT-3 has the lowest laser threshold and highest slope effi-ciency (6.1%), it was used in the mode-locked laser. The outputspectra at the maximum powers are shown in Fig. 3. BZMT-1showed single-wavelength operation at 1075 nm, whereas BZMT-2 and 3 showed multi-wavelength and very broadband operations,indicating more inhomogeneous spectral broadening.

Fig. 4 shows the schematic of the mode-locked Nd3+:BZMT cera-mic laser. A z-shaped astigmatically compensated cavity was em-ployed. A 3.3 mm thick 1 mol% Nd3+:BZMT ceramic (BZMT-3) wasarranged at the Brewster angle and pumped by a broad-stripe LD(emission area of 1 � 95 lm, maximum power of 5 W around809 nm). The pump beam was focused into the ceramic to 1/e2

diameters of 25 � 134 lm by the beam shaping optics. The maxi-mum incident pump power was 3.97 W. The folding mirrors (M1,M2) have a 100 mm radius of curvature (ROC) and AR coated below1000 nm and HR coated above 1020 nm. The laser beam was

0

20

40

60

80

100

0 0.5 1 1.5 2 2.5 3 3.5 4

BZMT-1BZMT-2BZMT-3

Out

put p

ower

(m

W)

Absorbed pump power (W)

Slope eff.6.1%

Slope eff.5.6%

Slope eff.2.9%

Fig. 2. Laser properties of the Nd3+:BZMT ceramics.

0

2

4

6

8

10

12

0 1 2 3 4 5

Out

put p

ower

(m

W)

Incident pump power (W)

mode locked

Fig. 5. Output power evolution of the mode-locked Nd3+:BZMT ceramic laser.

0

0.2

0.4

0.6

0.8

1

1.2

1058 1060 1062 1064 1066

Nor

mal

ized

inte

nsity

Wavelength (nm)

(a)

0

0.5

1

1.5

2

2.5

-10 -5 0 5 10

SH

Int

ensi

ty (

arb.

uni

ts)

Delay time (ps)

(b)

Fig. 6. (a) Spectrum and (b) autocorrelation trace of the output pulses. The experimental data (dots) and sech2-fitting curve (solid curve) are shown.

H. Kurokawa et al. / Optical Materials 33 (2011) 667–669 669

focused onto a semiconductor saturable absorber mirror (SESAM,BATOP GmbH, 2% saturable absorption) by a concave mirror (M3,ROC = 50 mm). The chirped mirror pair (CM1, CM2) has a group de-lay dispersion of �2000 fs2 per one round trip. The output coupler(OC) has a 1% transmittance.

Fig. 5 shows the power evolution. At the incident pump powerof 2.6 W, mode locking self-started. The maximum average powerwas 11 mW. The spectrum and autocorrelation trace of the outputare shown in Fig. 6. The center wavelength was 1062 nm and thespectral bandwidth was 1.5 nm. The sech2-fit pulse width was1.4 ps. Much shorter pulse generation is being pursued by optimiz-ing the setup and improving the ceramics to utilizing the full rangeof the gain spectrum.

3. Conclusion

We have demonstrated for the first time mode-lockedNd3+:BZMT ceramic laser. The mechanical and thermal propertiessuperior to glasses as well as the broadband-gain profile tailoringproperty in the disordered Nd3+:BZMT system are advantageousfor ultrashort pulse lasers.

Acknowledgements

This research was partly supported by Grant-in-Aid for Scien-tific Research and the Photon Frontier Network Program of Minis-try of Education, Culture, Sports, Science and Technology of Japan.

References

[1] For example J. Aus der Au, D. Kopf, F. Morier-Genoud, M. Moser, U. Keller, 60-fspulses from a diode-pumped Nd:glass laser, Opt. Lett. 22 (1997) 307.

[2] For example M. Tokurakawa, A. Shirakawa, K. Ueda, H. Yagi, M. Noriyuki, T.Yanagitani, A.A. Kaminskii, Diode-pumped ultrashort-pulse generation based onYb3+:Sc2O3 and Yb3+:Y2O3 ceramic multi-gain-media oscillator, Opt. Express 17(2009) 3353.

[3] N. Tanaka, Y. Kintaka, S. Kuretake, N. Wada, Y. Sakabe, Transparent ceramics foroptical application, in: Proceedings of the 2nd Laser Ceramic Symposium,November 10–11, Tokyo, 2006, paper 10pAS6.

[4] Y. Kintaka, S. Kuretake, N. Tanaka, K. Kageyama, H. Takagi, Crystal structuresand optical properties of transparent ceramics based on complex perovskiteBa(M4+, B2þ

1 , B5þ2 )O3 (M4+ = Ti, Sn, Zr, Hf; B2þ

1 = Mg, Zn; B5þ2 = Ta, Nb), J. Am.

Ceram. Soc. 93 (2010) 1114.[5] A.A. Kaminskii, H. Kurokawa, A. Shirakawa, K. Ueda, N. Tanaka, P. Becker, L.

Bohaty, M. Akchurin, M. Tokurakawa, S. Kuretake, Y. Kintaka, K. Kageyama, H.Takagi, Ba(Mg, Zr, Ta)O3:Nd3+ fine-grained ceramics: a novel laser-gain materialwith disordered structure for high-power laser systems, Laser Phys. Lett. 6(2009) 304.