May 15, 2006 / Vol. 31, No. 10 / OPTICS LETTERS
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Diode-pumped continuous-wave Nd: LuVO4 laser operating at 916 nm Chunyu Zhang, Ling Zhang, Zhiyi Wei, Chi Zhang, Yongbing Long, and Zhiguo Zhang Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100080, China
HuaiJin Zhang and Jiyang Wang National Laboratory of Crystal Materialis and Institute of Crystal Materials, Shandong University Jinan 250100, China Received January 3, 2006; revised February 15, 2006; accepted February 22, 2006; posted February 27, 2006 (Doc. ID 67011) We realized an efficient diode-pumped Nd: LuVO4 continuous-wave (CW) laser operating at 916 nm. Laser experiments with 0.5 at. % Nd-doped Nd: LuVO4 crystals of various lengths and cutting directions were also investigated. The maximum output power of 930 mW was obtained with a slope efficiency of 27.2% and an optical conversion efficiency of 20.8% at the absorbed pump power of 4.5 W. The laser experiment shows that Nd: LuVO4 crystal can be used for an efficient diode-pumped laser system. © 2006 Optical Society of America OCIS codes: 140.3530, 140.3580, 140.3380.
Diode-pumped solid-state lasers have been widely applied in the fields of industry, scientific research, medical treatment, and so on because of their compactness, high efficiency, and high stability. The laser crystal is one of the most important parts of a laser. Neodymium-doped vanadate crystals are characterized by their high absorption and emission cross sections. A well-known representative is Nd: YVO4, which has been widely used both in research and in commercial devices.1 Another vanadate crystal, Nd: GdVO4,2–5 which was confirmed to be superior to Nd: YVO4 in thermal properties, was also investigated extensively. Recently another member from the vanadate family, Nd: LuVO4, has attracted much attention because it has larger absorption and emission cross sections than those of Nd: YVO4 and Nd: GdVO4.6–8 Moreover, Nd: LuVO4 crystal has a high damage threshold, and it is easy to process.7 Nd: LuVO4 laser operation at 1.06 and 1.34 m was reported recently,9,10 demonstrating its great potential for practical applications. The 1.06 and 1.34 m lasers operate in different four-level systems. However, to the best of our knowledge, the Nd: LuVO4 laser operating in a quasi-three-level system has not been reported until now because it is difficult to obtain, owing to the reabsorption loss of the quasithree-level system. In this Letter we report that we have realized a compact diode-pumped 916 nm Nd: LuVO4 laser operating in a quasi-three-level system for the first time to our knowledge. Laser experiments of 0.5 at. % Nd-doped Nd: LuVO4 crystals with different lengths and cutting directions were also investigated. The maximum output power of 930 mW was obtained with a slope efficiency of 27.2% and an optical conversion efficiency of 20.8% at the absorbed pump power of 4.5 W. The laser experiment shows that the Nd: LuVO4 crystal can be used for an efficient diode-pumped lasers system. 0146-9592/06/101435-3/$15.00
Figure 1 shows the experimental setup. A linear resonator was employed to make the system very simple and compact with a cavity length of 18 mm. The pump source used in our experiment was a highbrightness fiber-coupled diode laser (LD). The pump power from the fiber with a core diameter of 200 m and a numerical aperture of 0.22 was coupled into the gain medium by the coupling system. The c-cut Nd: LuVO4 crystal had a 0.5 at. % Nd3+ concentration and dimensions of 3 mm⫻ 3 mm⫻ 3 mm. The laser crystal was mounted in a water-cooled heat sink 共T = 6 ° C兲. For simplicity and reduction of cavity loss, the pump facet of the Nd: LuVO4 crystal was coated for high reflection at 916 nm, antireflection at 808 nm, and high transmission at 1.06 and 1.34 m. The other end of the laser crystal was antireflection coated at 1.06 m, 1.34 m, and 916 nm. The output coupler had 3.6% transmission at 916 nm. The central wavelength of the pump source used in our experiment was 806 nm. The absorption coefficient of 0.5 at. % Nd-doped Nd: LuVO4 crystal corresponding to 806 nm was very low, only 3.5 cm−1 according to the absorption spectrum. From Fig. 2, we can see that the absorption coefficient of Nd: LuVO4 crystal corresponding to 810 nm was much higher than that of 806 nm. So we could obtain higher output power at 916 nm if we could use a pump source with a central wavelength of 810 nm. Figure 3 shows the CW output power of Nd: LuVO4 crystals with various lengths and cutting directions as a function of the incident pump power with an output mirror of 50 mm radius of curvature. From Fig. 3, we can see
Fig. 1. Schematic of the diode-pumped Nd: LuVO4 laser. © 2006 Optical Society of America
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Fig. 2. Absorption spectrum of 0.5 at. % Nd-doped Nd: LuVO4 crystal.
pump power of 4.5 W. The optical conversion efficiencies were 6% with respect to the incident pump power of 15.5 W and 20.8% with respect to the absorbed pump power of 4.5 W. When the incident pump power exceeded 15.5 W, the output power of the 916 nm laser began to drop. This phenomenon occurred probably because of a thermal-lensing effect. A higher output power of 916 nm should be achieved if we use the Nd: LuVO4 crystal with optimum Nd concentration and length. This is our next step. The far-field beam spatial profile of the 916 nm laser at the output power of 800 mW was measured. The result is shown in Fig. 5 along with a Gaussian fitting curve, which indicates that the laser was oscillating nearly in the fundamental transverse mode. We observed the stability of the laser by monitoring the output power with a powermeter. The output noise was 0.3% (rms) for 30 min at the output power level of 800 mW. In conclusion, we demonstrated an efficient diodepumped Nd: LuVO4 CW laser operating at 916 nm. Laser experiments with 0.5 at. % Nd-doped Nd: LuVO4 crystals with different lengths and cutting directions were also investigated. The maximum output power of 930 mW was obtained with a slope efficiency of 27.2% and an optical conversion efficiency of 20.8% at the absorbed pump power of 4.5 W.
Fig. 3. Output power of Nd: LuVO4 crystals with various lengths and different cutting directions as a function of the incident pump power.
that the 2 mm long c-cut Nd: LuVO4 crystal is the best crystal in our experiment. When the incident pump power was 14.4 W, the maximum output power of the 2 mm long c-cut Nd: LuVO4 crystal was 777 mW with a slope efficiency of 9.3% and an optical conversion efficiency of 5.4%. With further increases in the incident pump power, the output power of 916 nm in the 2 mm long c-cut Nd: LuVO4 crystal began to drop. This phenomenon occurred probably because of a thermal-lensing effect. We estimated that the thermal-lensing length is about 26 mm at the incident pump power of 14.4 W by using the method mentioned in Ref. 11. To obtain an efficient mode match between the pump light and the oscillating light, we used the output coupler with a 100 mm radius of curvature. The spot radius of the pump light in the Nd: LuVO4 crystal was about 120 m. The spot radius of the oscillating light in the laser crystal was calculated to be about 105 m by using an ABCD matrix and considering the thermal-lensing effect. The threshold pump power of 916 nm laser was 6.4 W owing to the efficient mode match between the pump light and the oscillating light. Figure 4 shows the CW output power of 916 nm in the 2 mm long c-cut Nd: LuVO4 crystal as a function of the incident pump power. The maximum output power of 930 mW was obtained at an incident pump power of 15.5 W and an absorbed pump power of 4.5 W. The slope efficiencies were 10.2% with respect to the incident pump power of 15.5 W and 27.2% with respect to the absorbed
Fig. 4. Output power at 916 nm in the 2 mm long c-cut Nd: LuVO4 crystal as a function of the incident pump power.
Fig. 5. Far-field beam spatial profile of the 916 nm laser.
May 15, 2006 / Vol. 31, No. 10 / OPTICS LETTERS
We gratefully acknowledge support from the Foundation (60490280, 60225005, 10227401), the National Natural Science Foundation of China (50572054, 50590401), and the Grant for State Key Program of China (2004CB619002). C. Zhang’s e-mail address is
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