2005 Conference on Lasers and Electro-Optics Europe

Heat generation in Nd: GdVO4 crystal with and without lasing at 1.3JmMasahito Okida, Masahide Itoh, Toyohiko YatagaiInstitute ofApplied Physics, University of Tsukuba1-1-1, Tenn6dai, Tsukuba, Ibaraki, 305-8573, Japan

Hamish Ogilvy, James A. PiperCentrefor Lasers and Applications, Macquarie University

NSW2109, Australia

Takashige OmatsuDepartment ofInformation and Image science, Chiba University

1-33 Yayoi-cho, Inage-ku, Chiba, 263-0022, JapanTel. +81-43-290-3477, Fax. +81-43-290-3490

omatsu@.faculty.chiba-u.jp

Neodymium-doped gadolinium orthovanadate (GdVO4) is one of promising laser crystals for diode-pumped solid-state lasers because of its relatively larger stimulated emission cross-section as well as good thermal properties. However,in the case of 1.3 pm laser oscillation the fractional thermal loading mainly due to larger quantum defect with respectto 808 nm pumping limits frequently a power scaling even by using low-doped crystals. Key issues to overcome thislimitation are quantitative characterization and management ofthermal loading in the crystal with 1.3 pm laser action. Inthis paper, we present interferometric measurements of thermal lens effects in GdVO4 crystal with and without 1.3 rmlaser action.

A crystal used is 0.3 at.% Nd3+ doping a-cut GdVO4 crystal with dimensions of 4 x 4 x 7 mm3. The 4 mm x 4 mmfaces of the crystal were AR-coated for diode pumping. The crystal was longitudinally pumped by a fiber-coupled 15 WCW 808 nm diode laser array. The diode output was focused to a spot of 350 x 350 pm2 on the crystal face. The cavitywas composed of a plane-parallel resonator with an end mirror and a partial reflective output coupler. The cavity lengthwas -110 mm. The slope efficiency of the laser was 19 % in a system optimized for output coupler. The output powerwas 1.2 W at the pump power of 8.4 W. A shutter placed inside the cavity was closed to prevent the laser action. Tomeasure the thermal lens power holographic lateral-shearing interferometry was employed[l].

Fig. I shows the measured thermal lens power as a function of the absorbed pump power. The thermal lens powerwithout laser action was proportional to the absorbed pump power with a slope of 1.3 m'l/W. Once the laser turning on,the slope increased to 2.6 m- /W. This value was 2 times larger than the slope without laser action. Above threshold,excited state absorption (a process in which the excited ions lying in the upper laser level transit to the higher excited levelthrough 1.3 pm lasing photon absorption) as well as quantum defect contribute to the thermal loading. The excited stateabsorption is evidenced by visible emission from the pumped crystal due to visible fluorescence from higher lying levelssuch as 4G7/2. We also observed the fluorescence by a spectrometer and an intensified CCD camera. The fluorescencespectrum is shown in Fig. 2. Above threshold, two peaks appeared around 544 nm and 618 nm, while there was nosignificant peak near threshold. These correspond to the 'G7/2 - 'I 1/2 and 4G712 _. 419/2 transitions, respectively.These show that the excited state absorption through 1.3 pm lasing photons is a significant contributor to the thermalloading with 1.3 pm laser action.

20 lasing * * 2500 3.5Wnonlasing 2 8.0wOW

- 15 2.60W'1 1- 2000

EA 1500

*~~~~~~10 0~ ~ ~~~~~~~10

5 *5 5 500

0 00 1 2 3 4 5 6 7 8 9 540 560 580 600 620 640

Absorbed power [W] Wavelength [nm]

Fig. 1: Thermal lens power as a function of the absorbed pump Fig. 2: Fluorescence spectnum in visible region with different ab-power sorbed power

References

[1] Justin L Blows, Takashige Omatsu, Judith Dawes, Helen Pask, and Mitsuhiro Tateda. Heat generation in Nd:YVO4with and without laser action. IEEE Photon. Tech. Lett., 10(12):1727-1729, Dec. 1998.

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