The continuous-wave (cw) and pulsed laser performances of laser-diode (LD)-end-pumped c-cut Nd:LuVO4 at 1.06 μm were achieved for the first time. Maximum cw output of 2.35W was obtained. Compared with the cw laser output of a-cut Nd:LuVO4 crystal, the efficient emission cross-section of the c-cut crystal has been calculated to be 3.24×10-19 cm2. The largest pulse repetition rate, shortest pulse width, largest pulse energy and highest peak power were measured to be 20 kHz, 12 ns, 84.93 μJ and 7.02 kW, respectively, by using Cr4+:YAG crystals as the saturable absorbers in the passive pulsed laser output experiments.
©2007 Optical Society of America
Laser-diode (LD)-pumped solid-state lasers have been applied in many fields, including military, industry, medical treatment and scientific research, due to their excellent properties, including the high efficiency, high reliability, compact structure and high beam quality. As laser materials, the neodymium-doped vanadate crystals, such as Nd:YVO4 [1–3], Nd:GdVO4 [4–7] and Nd:LuVO4 [8–13], have been proved to be the promising materials for LD-pumped lasers due to their good laser properties and high chemical stability. Nd:YVO4  and Nd:GdVO4  were identified that the crystals cut along a and c axes have different properties: the a-cut crystals have the advantages of large emission cross-sections and polarized laser output, and are suitable for the application in the continuous-wave (cw) laser experiments, but the c-cut crystals have the properties of the non-polarized laser output and small emission cross-section, and are suitable for the application in the pulsed laser experiments. Nd:LuVO4 is a new laser crystal and possess the same ZrSiO4 structure with that of Nd:YVO4 and Nd:GdVO4. Our group have studied the growth and laser properties of Nd:LuVO4 [9–12] and shown that Nd:LuVO4 has high thermal conductivity (9.9 Wm-1K-1)  and appropriate fluorescence lifetime (95 μs)  for the 4F3/2 level, and the a-cut crystal has the same excellent laser properties with that of Nd:YVO4 and Nd:GdVO4. But the laser properties at 1.06μm of c-cut Nd:LuVO4 crystals have been largely ignored. In this letter, the cw and pulsed LD-end-pumped laser performances at 1.06 μm of c-cut Nd:LuVO4 have been demonstrated and the efficient emission cross-section has been calculated. All the results show that the c-cut Nd:LuVO4 is more suitable than a-cut one for the application in pulsed laser experiments.
2. Experimental details
Nd:LuVO4 crystals used for the laser were grown by the Czochralski method along a- and c-direction under a nitrogen atmosphere containing 2% oxygen (v/v) in an iridium crucible and the growing process was same with that of Nd:GdVO4 and Nd:YVO4. The Nd-doped concentration in LuVO4 crystal was 0.5 at.%. The samples were cut along a and c axes with the same dimensions of 3×3×8 mm3. The 3×3 mm2 faces were polished and anti-reflection (AR) coated at 808 nm and 1.06 μm. Two Cr4+:YAG samples were used for the saturable absorbers, of which the initial transmissions were 91% and 84%, respectively. Their dimensions were Φ10×1.2 mm2 and Φ10×1.6 mm2, and their end faces were HR coated at 1.06 μm.
The cw and Q-switched laser experiments were carried out in the plano-concave resonator shown in Fig. 1. The pump source employed in the experiment was a fiber-coupled LD with the central wavelength around 808 nm. Through the focusing optics (N. A.= 0.22), the output of the source was put into the laser crystal with a spot radius of 0.256 mm. M1 was a concave mirror with a curvature radius of 200 mm. It was AR coated at 808 nm on the flat face, HR coated at 1.06μm and high-transmission (HT) coated at 808 nm on the concave face. M2 was a flat mirror which was HR coated at 808 nm, and its transmission at 1.06 μm was 10%. In order to remove heat generated from the laser crystals and saturable absorbers in the laser experiments, Nd:LuVO4 crystals were wrapped with indium foil and held in a water-cooled copper block, and the Cr4+:YAG were held in a copper block but without cooling water. The cooling water was controlled to be 15 °C during all the laser experiments. The laser crystal was placed close to M1. The length of the cavity was tuned to be as short as 38 mm and the cw laser operation was studied first. The pulsed laser operation was carried out after the cw experiment by inserting the saturable absorber into the cavity at the position as close as possible to M2 where the size of the laser spot got its minimum. The cw and average pulsed laser output power were measured by the power meter (EPM 2000. Melectron Inc.). The temporal behaviors of the Q-switched laser were recorded by a TED620B digital oscilloscope (500-MHZ bandwidth and 2.5-Gs/s sampling rate, Tektronix Inc.). Through average output power, pulse width and repetition rate, the pulse energy and peak power can be obtained.
3. Results and discussion
Figure 2 shows the cw output performances of a- and c-cut Nd:LuVO4 crystals at 1.06μm using the output coupler with transmission of 10%. The highest output powers of 5.03 W and 2.35 W were obtained under a pump power of 13.75 W, the threshold pump powers were measured to be 0.21 W and 1.1 W, and the optical conversion efficiencies were 36.6% and 17.1%, respectively. N. Mermilliod et. al.  have proved that the stimulated emission cross section σ is inversely proportional to the absorbed pump power at threshold when the laser is operated in the cw mode. At laser threshold, we measured that the ratios of absorbed to incident pump powers of the a- and c-cut crystals were 72.4% and 62.3%, respectively. Using σ =1.46×10-18 cm2 for a-cut Nd:LuVO4 , a value of σ =3.24×10-19 cm2 for the c-cut crystal was obtained which is a bit larger than that of c-cut Nd:GdVO4 (σ=1.2×10-19 cm2)  and smaller than that of c-cut Nd:YVO4 (σ=6.5×10-19 cm2), and agree well with that obtained by C. Maunier et.al from the spectroscopy (about 3.5×10-19 cm2) .
Figure 3 shows the average output power of c-cut crystal at 1.06 μm when the Cr4+:YAG saturable absorbers with initial transmissions of T0 = 91% and 84% were inserted into the cavity. Under the pump power of 13.75 W, the maximum output powers were 0.98 W and 0.81 W, the optical conversion efficiencies were 7.1% and 5.9%, and the slop efficiencies were 10.7% and 13.8%, respectively. The repetition rate, pulse width, pulse energy and peak power versus incident power are shown in Figs. 4, 5, 6 and 7, which show the similar behaviors with those of c-cut Nd:YVO4  and Nd:GdVO4 . The highest repetition rate of 9.6 kHz, shortest pulse width of 12 ns, largest pulse energy of 84.93 μJ and highest peak power of 7.02 kW were obtained by using the T0 = 84% saturable absorber, at the incident pump power of 13.75, 13.75, 11.96 and 13.75 W, respectively. Compared with the pulsed energy of 65 μJ  obtained with the c-cut Nd:GdVO4, we have achieved the larger pulse energy. When T0=91%, the repetition rate and pulse width became larger, meanwhile, the pulse energy and peak power were lower than those obtained in the experiment using T0 = 84% saturable absorber, and the highest repetition rate of 20 kHz, largest pulse energy of 49 μJ, highest peak power of 2.23 kW and shortest pulse width of 22 ns were obtained at the incident pump power of 13.75 W. From Fig. 6, it can be noted that the variation of energy versus the incident pump power is small, which agree with the analysis of cw pumped Q-switched lasers by Degnan  and X. Zhang . Figures 8 and 9 show the pulse profile with a pulse width of 12 ns and the pulse train with 9.6 kHz, respectively, when T0=84% under the incident pump power of 13.75 W.
Using the a-cut Nd:LuVO4 and saturable absorber with T0=84%, the shortest pulse width of 42 ns, largest pulse energy of 7.35 μJ and peak power of 175 W were obtained in this configuration. Compared with the previous results [13, 17] and this work using the a-cut Nd:LuVO4 and saturable absorbers, we have gotten the shorter pulse width, larger pulse energy and higher peak power by the c-cut one, which show that the c-cut crystal is more suitable than that cut along a-axis for the application in the pulsed laser experiments.
Degnan  has derived that the total useful energy stored in the crystal can be given by
where A = πω2 0/2 is the effective beam cross-sectional area of the fundamental laser in the laser crystal, ω0≈0.2 mm is the radii of the fundamental laser beam, l is the length of the laser crystal, hν is the laser photon energy, ni is the initial population inversion density at the start of Q-switching, γ is the inversion reduction factor which can be assumed to be 0.50  according to the analysis by Degnan , R is the reflectivity of the output mirror and L is the round-trip loss inside the cavity which can be assumed to be 2%  in the pulsed laser experiments. From (1) and (2), we calculated that ni=5.95×1014 mm-3 and 9.04×1014 mm-3, Eu=111 μJ and 169 μJ, and the maximum laser efficiency ηe=44% and 50%, when T0=91% and 84%, respectively. It means that only about a half of energy was used in the pulsed experiments. There are the six aspects which may cause the discrepancies: first, the mode spot sizes were assumed to be uniform in the crystal. Second, thermally induced losses were neglected during the calculation. Third, the reabsorption of Nd:LuVO4 was ignored. Fourth, the inversion reduction factor used in the calculation was considered to be the fractional populations of the upper laser Stark sublevels of Nd:YVO4. Fifth, the excited state absorption of Cr4+:YAG was not considered. Finally, the influences of the spatial distributions of the pump and laser beam on the Q-switched laser output were ignored .
Using the model of passively Q-switched lasers [18, 20], the optimized initial transmission of the saturable absorber of 92% and transmission of the output coupler T=11% were obtained with ni=5.95×1014 mm-3, which maximized the pulse energy of 57.1 μJ. When ni=9.04×1014 mm-3, the optimized T0=87% and T=16%, which maximized the pulse energy of 92.2 μJ. During the process of calculation, the parameters of ground-state and excited-state absorption cross-sections for Cr4+:YAG were considered to be 4.3×10-18 and 8.2×10-19cm2. From the theoretical calculations, we believe that it is possible to get the better passively Q-switched laser if the proper saturbale absorber and output coupler are used.
The cw and pulsed laser performances of LD-end-pumped c-cut Nd:LuVO4 at 1.06 μm were reported for the first time to our knowledge. We believed that the c-cut Nd:LuVO4 is more suitable than a-cut Nd:LuVO4 for the applications in pulse laser domain, and the results will be better if optimized laser cavity and saturable absorber are used. Y. Chen  have demonstrated the passively Q-switched laser with the pulse energy of 18 μJ, peak power of 21 kW and pulse width of 0.85 ns by using c-cut Nd:YVO4 microchip. It’s possible that efficient passively Q-switched laser could be achieved either by using the c-cut Nd:LuVO4 microchip.
This work is supported by the Natural Science Foundation of Shandong Province under Grand No Y2004F01, the National Natural Science Foundation of China under Grand No 50590401, 60508010, and the National Basic Research Program of China under Grand 2004CB619002.
References and links
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