Passively Q-switched operation at 1.34 μm in which V:YAG was employed as saturable absorber has been demonstrated with a series of mixed Nd:GdxY1- xVO4 crystals. The Q-switched laser pulse performance with these Nd-doped mixed vanadate crystals, especially Nd:Gd0.63Y0.37VO4, was enhanced in respect to Nd:GdVO4 under the identical conditions. The average output power of 171.3 mW with the shortest pulse width of 44.4 ns, largest single pulse energy of 21.7 μJ, and highest peak power of 489 W at the incident pump power of 5.04 W were obtained with Nd:Gd0.63Y0.37VO4 crystal.
©2010 Optical Society of America
Laser gain medium is the key factor that determines the output performance and has attracted much attention in recent years. Neodymium doped single vanadate crystals have been proved to be excellent laser materials for diode-pumped solid-state lasers. Nd:YVO4 [1–8] has become the most common medium due to its broad absorption bandwidth and large absorption coefficient. Nd:GdVO4, an isomorphism of Nd:YVO4, which has a large thermal conductivity coefficient has also attracted much attention [9–14]. However, the limited energy storage ability makes it difficult to generate large pulse energy and high peak power laser pulse.
A new class of Nd-doped mixed vanadate crystals (Nd:GdxY1- xVO4) has been discovered . They are grown as single crystals with tetragonal ZrSiO4 structure because YVO4, LuVO4 and GdVO4 possess the same structures, and their relatively broad fluorescence spectra can enhance pulse energy and peak power in diode-pumped solid-state laser operation. They have been proved to be good laser gain media for Q-switched operation [16,17].
Employing the saturable absorber in the resonator is an effect way to obtain Q-switched laser operation. Up to now, some solid-state saturable absorbers such as Co:LMA and V:YAG have been proved to be good passively Q-switched components. V:YAG can work as a saturable absorber in the range from 1.05 μm to 1.54 μm , and the transition of 3A2 → 3T2(3F) makes it possible for the passive Q-switching at 1.34 μm. The lifetime of the 3T2(3F) level is found to be 22 ± 6 ns, the ground-state absorption cross section σg = (7.2 ± 2.6) × 10− 18 cm2, and the excited-state absorption cross section σe = (7.4 ± 2.8) × 10− 19 cm2 for 1.34 μm . So V:YAG is a suitable saturable absorber for passively Q-switched operation.
In this paper, we report the passively Q-switched 1.34 μm laser performance with a class of Nd:GdxY1- xVO4 in which x = 0.19, 0.3, 0.63, 0.83, and 1. The experiment results indicated that the output characteristics altered with the Gd composition x, and the Q-switched performance at 1.34 μm including pulse energy and peak power were enhanced with mixed Nd:GdxY1- xVO4 compared to that of Nd:GdVO4. At the incident pump power of 5.04 W the largest pulse energy of 21.7 μJ was produced with Nd:Gd0.63Y0.37VO4, with the highest peak power of 489 W and the shortest pulse width of 44.4 ns.
2. Experimental setup
The arrangement of the experimental setup is shown in Fig. 1 in which a simple flat-concave cavity with the length of 7 cm was employed. The pump mirror M1 which was AR (anti-reflection)-coated at 808 nm on the pump face, HT (high-transmission)-coated at 808 nm and HR (high-reflection)-coated at 1.34 μm on the other side was a concave mirror with 200 mm curvature radius. The output coupler M2 was a flat mirror with transmission of 10% at 1.34 μm. The laser gain medium was a class of a-cut 0.5 at.% Nd-doped Nd:GdxY1- xVO4 crystals with dimensions of 3 × 3 × 6 mm3, and their end sides were HT-coated at 808 nm, AR-coated at 1.34 μm and 1.06 μm. They were wrapped with indium foil and held in a copper blocks cooled at 20 °C using semiconductor cooler. The V:YAG with the initial transmissions of 89% and 96% were employed as the saturable absorbers, AR-coated at 1.34 μm on both surfaces and placed close to M2. The pump source was a commercially used fiber-coupled laser-diode (FAP system, Coherent Inc., USA) that worked at 808 nm. The beam generated by it was focused into the laser crystal with a spot size of 400 μm by a focusing optical system. The generated average output power was measured by the EPM 2000 energy/power meter (Molectron Detector Inc., USA). The pulse width and repetition rate of the laser pulses were measured by TED 6208 digital oscilloscope (500MHz bandwidth, Tektronix Inc., USA) with a quick photoelectric detector which works at 1.34 μm.
3. Results and discussion
The variations of cw output power as well as average output power in Q-switched operation versus the Gd composition x at the incident pump power of 5.04 W is presented in Fig. 2 . The cw output power dropped from 1.307 W to 1.207 W, corresponding to the optical conversion efficiencies of 26% and 24% as x increased from 0.19 to 0.63, then augmented to the maximum of 1.524 W corresponding to the optical conversion efficiency of 30% with x = 1. Both the maximum and minimum of optical conversion efficiency with Nd:GdxY1- xVO4 mixed crystals in our experiments were larger than those of Nd: GdxLu1- xVO4 mixed crystals (18.8% and 12.5%) . The average output power in Q-switched operation decreased from 266 mW with x = 1 to 128.2 mW with x = 0.83 for the V:YAG initial transmission of T 0 = 89%, and decreased from 405 mW with x = 1 to 252.1 mW with x = 0.83 for T 0 = 96%. The average output power of 171.3 mW was measured with Nd:Gd0.63Y0.37VO4 crystal. The threshold pump power of cw operation also altered with the Gd composition x as shown in Fig. 3 . It increased from 1.045 W to 1.123 W with x rising from 0.19 to 0.63, and then dropped to 0.89 W with x sequentially increased to 1. The threshold of cw operation with Nd:Gd0.63Y0.37VO4 in our experiments was smaller than that obtained in  (1.55 W).
The Nd:GdxY1- xVO4 mixed crystals were grown by replacing a fraction of Y ions with Gd ions in Nd:YVO4 crystals bars which were used as the seed in the Czochralski method. Due to the random distribution of Y ions and Gd ions at the Y-ion sites neighboring the Nd ions, the inhomogeneous broadening of the fluorescence lines occurred. The fluorescence lifetime of the excited state level (4F3/2) for the Nd-doped vanadate crystals has been measured . It augmented at first and then decreased as x increased from 0 to 1, and the maximum fluorescence lifetime was owned by the Nd-doped mixed vanadate crystal with x = 0.64, so it can be supposed that the Nd:Gd0.63Y0.37VO4 crystal had the largest upper-level lifetime compared to other laser crystals employed in the experiments.
On the other hand, the inhomogeneous broadening in Nd-doped mixed crystals led to the reduction of the stimulated emission cross section. It can be approximately deduced by the well-known expression :Eq. (1) we can see that σ is of inverse proportion to τ and . The estimated stimulated emission cross sections have been shown in Fig. 3, and the values were close to the data in . Both the upper-level lifetime τ and threshold pump power firstly augmented when x increased to 0.63 then dropped as x reached 1, so the stimulated emission cross section σ decreased as x increased from 0.19 to 0.63, then increased as x rose from 0.63 to 1. The relatively small stimulated emission cross section induced low output power for Nd-doped mixed vanadate crystals.
Figure 4 shows the variation of the pulse width versus the Gd composition x. It was obvious that at the pump power of 5.04 W, the pulse width of the Nd:GdxY1- xVO4 crystals were shorter than that of Nd:GdVO4. The pulse width of T 0 = 96% was shorten from 86.2 ns to 66.3 ns as x raised from 0.19 to 0.63, then enlarged to 122 ns when x approached 1. The system with T 0 = 89% produced narrower laser pulses than that with T 0 = 96% all the time, and the shortest pulse width of 44.4 ns was obtained with Nd:Gd0.63Y0.37VO4.
From Fig. 5 which shows the pulse repetition rate versus the Gd composition x we can see the variation of the pulse repetition rate was also dependent on Gd composition x. In the case of T 0 = 96%, the pulse repetition rate decreased from 82.5 kHz for Nd:GdVO4 to 25.3 kHz for Nd:Gd0.63Y0.37VO4. The reduction of pulse repetition rate with Nd-doped mixed vanadate crystals was supposed to be resulted from the small emission cross section compared to that of Nd-doped single vanadate crystals.
The relationship between the pulse energy and the Gd composition x at the incident pump power of 5.04 W is shown in Fig. 6 . For each crystal, the output single pulse energy was larger with T 0 = 89% than that with T 0 = 96%. That’s because the saturable absorber with small initial transmission can hold more inversion populations and store larger energy in the resonator during the interspace of producing two laser pulses. Both the pulse energy of T 0 = 89% and 96% increased as x rose from 0.19 to 0.63, and began to drop when x was beyond 0.63. This trend was more distinct with T 0 = 96%. The largest single pulse energy of 10.3 μJ was obtained in the system of T 0 = 96% with x = 0.63 which was 2.1 times in respect to 4.9 μJ with x = 1. Meanwhile, the pulse energy of Nd:Gd0.63Y0.37VO4 (x = 0.63) with T 0 = 89% was 21.7 μJ, in respect to 12.5 μJ of Nd:GdVO4 (x = 1).
By using the relation Ppeak = E/W, where E is the single pulse energy, W is the pulse width measured in the experiments, the calculated peak power at the incident pump power of 5.04 W is also shown in Fig. 6. As we seen, the curves had the same trends as the pulse energy curves in Fig. 6. The output peak power distinctly increased as x approaching 0.63, and then decreased when x sequentially increased to 1, especially in the situation of T 0 = 96%. Under the same experimental conditions, the largest peak power of 155 W in the system of T 0 = 96% was produced with Nd:Gd0.63Y0.37VO4, and it was nearly enhanced by 4 times compared to 40 W which was obtained with Nd:GdVO4. The maximum and minimum peak power of T 0 = 89% at the pump power of 5.04 W were 489 W and 219 W, respectively. The upper-level lifetime was a factor that determined the Q-switched performance of laser crystals. The variety of fluorescence lifetime depended on the Gd composition x. As described above, the Nd:Gd0.63Y0.37VO4 crystal had the largest upper-level lifetime compared to other laser crystals employed in the experiments. The longer fluorescence lifetime a laser medium had the stronger capacity for storing energy it owned. The stimulated emission cross section was also a factor that enhanced the Q-switched performance. It decreased as x increased from 0.19 to 0.63, then increased as x rose from 0.63 to 1 as shown in Fig. 3. So the Q-switched performance was enhanced with Nd-doped mixed vanadate crystals and the Nd:Gd0.63Y0.37VO4 crystal produced the maximum pulse energy and peak power. Both the relatively long upper-level lifetime and small simulated emission cross section made the Nd:GdxY1- xVO4 crystals to be excellent gain medium for Q-switched laser operation.
In conclusion, diode-pumped passively Q-switched Nd:GdxY1- xVO4/V:YAG laser at 1.34 μm has been demonstrated, The pulse energy and peak power with mixed vanadate crystals were enhanced compared to that with Nd:GdVO4. At the incident pump power of 5.04 W, the largest pulse energy produced with Nd:Gd0.63Y0.37VO4 was 21.7 μJ, with the highest peak power of 489 W and the shortest pulse width of 44.4 ns.
The work was supported by the National Natural Science Foundation of China (NNSFC) (Grants: 60678015 and 50990303, NSFDYS: 50925205).
References and links
1. R. A. Fields, M. Birnbaum, and C. L. Fincher, “Highly efficient Nd:YVO4 diode-laser end-pumped laser,” Appl. Phys. Lett. 51(23), 1885–1886 (1987). [CrossRef]
2. N. MacKinnon and B. D. Sinclair, “A laser diode array pumped, Nd: YVO4/KTP, composite material microchip laser,” Opt. Commun. 105(3-4), 183–187 (1994). [CrossRef]
3. D. G. Matthews, J. R. Boon, R. S. Conroy, and B. D. Sinclair, “A comparative study of diode pumped microchip laser materials: Nd-doped YVO4, YOS, SFAP and SVAP,” J. Mod. Opt. 43, 1079–1087 (1996). [CrossRef]
4. R. Zhou, B. G. Zhang, X. Ding, Z. Q. Cai, W. Q. Wen, P. Wang, and J. Q. Yao, “Continuous-wave operation at 1386 nm in a diode-end-pumped Nd:YVO4 laser,” Opt. Express 13(15), 5818–5824 (2005). [CrossRef] [PubMed]
5. G. Li, S. Zhao, K. Yan, D. Li, and J. Zou, “Pulse shape symmetry and pulse width reduction in diode-pumped doubly Q-switched Nd:YVO4/KTP green laser with AO and GaAs,” Opt. Express 13(4), 1178–1187 (2005). [CrossRef] [PubMed]
6. H. Ogilvy and J. Piper, “Compact, all solid-state, high-repetition-rate 336nm source based on a frequency quadrupled, Q-switched, diode-pumped Nd:YVO4 laser,” Opt. Express 13(23), 9465–9471 (2005). [CrossRef] [PubMed]
8. T. Li, S. Z. Zhao, Z. Zhuo, K. J. Yang, G. Q. Li, and D. C. Li, “Pulse compression in an electro-optic Q-switched diode-pumped YVO4/Nd:YVO4 laser with a Cr4+:YAG saturable absorber,” Appl. Opt. 48(12), 2243–2248 (2009). [CrossRef] [PubMed]
9. A. I. Zagumennyĭ, V. G. Ostroumov, I. A. Shcherbakov, T. Jensen, J. P. Meyen, and G. Huber, “The Nd:GdVO4 crystal: a new material for diode-pumped lasers,” Sov. J. Quantum Electron. 22(12), 1071–1072 (1992). [CrossRef]
10. C. Li, J. Song, D. Shen, N. S. Kim, J. Lu, and K. Ueda, “Diode-pumped passively Q-switched Nd:GdVO4 lasers operating at 1.06 μm wavelength,” Appl. Phys. B 70(4), 471–474 (2000). [CrossRef]
11. J. L. He, C. K. Lee, J. Y. Huang, S. C. Wang, C. L. Pan, and K. F. Huang, “Diode-pumped passively mode-locked multiwatt Nd:GdVO4 laser with a saturable Bragg reflector,” Appl. Opt. 42(27), 5496–5499 (2003). [CrossRef] [PubMed]
13. G. Q. Li, S. Z. Zhao, K. J. Yang, and P. Song, “Control of the pulse width in a diode-pumped passively Q-switched Nd:GdVO4/KTP green laser with a Cr4+:YAG saturable absorber,” Appl. Opt. 44(28), 5990–5995 (2005). [CrossRef] [PubMed]
14. W. W. Ge, H. J. Zhang, J. Y. Wang, X. F. Cheng, M. H. Jiang, C. L. Du, and S. C. Yuan, “Pulsed laser output of LD-end-pumped 1.34μm Nd: GdVO4 laser with Co: LaMgAl11O19 crystal as saturable absorber,” Opt. Express 13(10), 3883–3889 (2005). [CrossRef] [PubMed]
15. Y. G. Yu, J. Y. Wang, H. J. Zhang, H. H. Yu, Z. P. Wang, M. H. Jiang, H. R. Xia, and R. I. Boughton, “Growth and characterization of Nd:YxGd1-xVO4 series laser crystals,” J. Opt. Soc. Am. B 25(6), 995–1001 (2008). [CrossRef]
16. J. H. Liu, Z. P. Wang, X. L. Meng, Z. S. Shao, B. Ozygus, A. Ding, and H. Weber, “Improvement of passive Q-switching performance reached with a new Nd-doped mixed vanadate crystal Nd:Gd0.64Y0.36VO4.,” Opt. Lett. 28(23), 2330–2332 (2003). [CrossRef] [PubMed]
17. J. Liu, Y. Wan, W. Han, H. Yang, H. Zhang, and J. Wang, “Actively Q-switched laser performance of Nd:GdxY1−xVO4: a class of mixed vanadate crystals,” Appl. Phys. B 98(1), 69–76 (2010). [CrossRef]
18. S. A. Zolotovskaya, K. V. Yumashev, N. V. Kuleshov, and A. V. Sandulenko, “Diode-pumped Yb,Er:glass laser passively Q switched with a V3+:YAG crystal,” Appl. Opt. 44(9), 1704–1708 (2005). [CrossRef] [PubMed]
19. A. M. Malyarevich, I. A. Denisov, K. V. Yumashev, V. P. Mikhailov, R. S. Conroy, and B. D. Sinclair, “V:YAG-a new passive Q-switch for diode-pumped solid-state lasers,” Appl. Phys. B 67(5), 555–558 (1998). [CrossRef]
20. H. H. Yu, H. J. Zhang, Z. P. Wang, J. Y. Wang, Y. G. Yu, and M. H. Jiang, “Continuous-wave laser performance of Nd:LuxGd1-xVO4 operating at 1.34 μm,” Laser Phys. Lett. 5(3), 181–184 (2008). [CrossRef]
21. J. H. Liu, X. L. Meng, Z. S. Shao, M. H. Jiang, B. Ozygus, A. Ding, and H. Weber, “Pulse energy enhancement in passive Q-switching operation with a class of Nd:GdxY1-xVO4 crystals,” Appl. Phys. Lett. 83(7), 1289–1291 (2003). [CrossRef]
22. T. Y. Fan and R. L. Byer, “Diode laser-pumped solid-state lasers,” IEEE J. Quantum Electron. 24(6), 895–912 (1988). [CrossRef]