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The absorption spectra of the 0.5at.% and 1at.% Co: LaMgAl11O19 (LaMg1-xCoxAl11O19, x=0.005 and 0.01, abbreviated as Co:LMA) crystals were measured at room temperature, and the results show that the Co: LMA crystals have two absorption bands, and the absorption band located at 1030–1660 nm can be used for a passive saturable absorber Q switch of 1.3–1.6µm laser. The passive pulsed laser output of LD-end-pumped Nd:GdVO4 1.34µm laser was demonstrated for the first time by using the 0.5 at.% Co:LMA crystal as a saturable absorber Q switch. The maximum average output power of 500 mW was obtained under the pumping power of 25 W. The shortest pulse width, the largest pulse energy and the highest peak power were obtained to be 160 ns, 25.5µJ and 150 W, respectively.
©2005 Optical Society of America
1. Introduction
Passive Q-switching of solid-state lasers provides a simple technique to generate laser pulses with short pulse duration and high peak power [1]. This technique is of practical importance in many areas, such as industry, medicine, military applications and basic scientific research, where compact, reliable and cost-effective nanosecond pulsed lasers are required. Passive Q-switching laser outputs of flashlamp pumped Nd:YAlO3 1.34 µm and laser diode pumped Nd:KGd(WO4)2 (Nd:KGW) (for passive Q-switch laser of Nd:KGW crystal at 1.34 µm, only 3 mW average output power was obtained under pumping power of 700 mW laser diode) have been reported when Co-doped LiGa5O8, MgAl2O4, LaMgAl11O19, and ZnS crystals are used as saturable absorber passive Q switching[2–5]. Laser experimental results show Nd-doped vanadate crystals, such as Nd:YVO4 and Nd:GdVO4, are efficient laser materials for 1.3 µm laser output[6–9], for vanadate crystals have higher thermal conductivity and emission section at 1.3µm than Nd:KGW. In this letter, the passive Q switching of a fiber-coupled laser-diode end-pumped Nd:GdVO4 laser at 1.34µm was demonstrated using a Co: LMA as a saturable absorber.
2 Experimental details
Nd:GdVO4 crystal used for laser output and Co:LMA crystal used for saturable absorber were grown by the Czochralski method, and Co-doped concentrations in LMA crystal were 0.5 and 1 at.%. The absorption spectra of the as-grown Co: LMA crystals were measured with Hitachi U-3500 spectrophotometer in the wavelength of 190nm–3200nm at room temperature. These samples were polished wafers and the thickness is 2.420 mm (X2), 2.540 mm (X3), 2.180 mm (X2) and 1.820 mm (X3) for 0.5 at.% and 1 at.% Co: LMA crystals. The incident light was perpendicular to the wafers in the absorption spectra measurements.
The experiments for continuous wave (CW) and Q-switched laser operations were carried out in a three-mirror folded resonator, as shown in Fig. 1 (when the CW laser was demonstrated, the Co:LMA crystal was not put into the laser cavity). The length between M2 and M3 is about 70 mm, and the total cavity length accounted for suitable Q-switching is about 305 mm. The pump source employed in the experiment was a fiber-coupled laser-diode with center wavelength around 808nm. Its output beam was focused into the Nd:GdVO4 laser crystal with a spot radius of about 0.2mm and numerical aperture of 0.22 by focusing optics. M1 was a concave mirror with radius of curvature of 150 mm, anti-reflection (AR) coated at 808 nm on the flat face, high-reflection (HR) coated at 1.34µm and high-transmission (HT) coated at 808 nm on the curved face. M2 is a concave mirror with radius of curvature of 100 mm, it was also coated a HR film at 1.34µm on the curved face. M3 is flat mirror with different output transmissions of 7.25% and 11.3% at 1.34µm, and in order to suppress oscillation of the 4F3/2→4I11/2 transition, the output mirrors had sufficient transmission (>90%) at 1.06µm. The laser crystal used in the experiments was a 0.3 at.% Nd -doped GdVO4 crystal with dimension of 3×3×8 mm3 (b×c×a) and the two faces (cross-section 3mm×3mm) of the laser crystal were coated for antireflection (AR) at pumping wavelength of 0.808 µm and lasing wavelength of 1.34µm to reduce intracavity reflection loss. In order to remove the heat from the Nd:GdVO4 crystal generated in lasing, it was wrapped with indium foil, held in a copper block, and cooled by using thermoelectric coolers. The face temperature of the laser crystal was controlled to be about 20°C during the laser experiments. The saturable absorber used for passive Q-switched operation was one 0.5 at.% Co: LMA crystals, with dimension of 3×3×0.2mm3 (X2×X3×X1), and Co:LMA crystal was not coated for HR at 1.34µm, and it was placed close to output coupler mirror M3, as shown in the Fig. 1.
3 Results and discussion
Figure 2 shows the non-polarized room-temperature absorption spectra of Co: LMA crystals. It has been shown that in LaMgAl11O19 crystal, the Co2+ ions substitute for the Mg2+ ions which are in tetrahedral sites and the absorption spectra of Co:LMA are associated with these tetrahedrally coordinated Co2+ ions[10]. A schematic energy-level diagram for Co2+ ions (electronic configuration 3d7) in a tetrahedral crystal field is shown in the inset of the Fig. 2. The ground state energy level 4F of a free ion Co2+ is split into three levels: 4T1, 4T2, and 4A2, by the tetrahedral crystal field. The strong absorption band centered at 591nm is assigned to the spin- and electric-dipole-allowed 4A2(4F)→4T1(4P) transition. This transition leads to the Co:LMA crystal appearing blue color. The broad near-infrared absorption band located at 1030–1660 nm is assigned to the 4A2(4F)→4T1(4F) transition and indicates potential application of the Co:LMA crystal as a saturable absorber Q switch for the 1.3–1.6µm lasers. The complicated structure of the absorption bands is explained by splitting of the 4T1 (4P) and 4T1 (4F) states in slightly distorted tetrahedral crystal field of oxygen environment around the Co2+ ions. It can be seen that the non-polarized absorption spectra almost have the same shape when the non-polarized light through the crystal wafers along X2 and X3 crystallophysical directions in the visible absorption band except that the absorption coefficient increases with the cobalt ions concentration, while there are a little difference in the near-infrared absorption band. The absorption coefficients at 1.34µm were 5.79 cm-1 (1 at.% Co: LMA X2), 6.27 cm -1 (1 at.% Co: LMA X3), 3.32 cm -1 (0.5 at.% Co: LMA X2) and 3.51cm -1 (0.5 at.% Co: LMA X3). It can be seen that the absorption coefficients almost linear increases with the concentration of Co2+ ions, and the absorption cross-section of Co:LMA crystal was calculated to be 2.6×10-19cm2.
Figure 3 shows the output power of Nd:GdVO4 crystal at 1.34 µm when output coupler transmissions are 7.2% and 11.3%, the highest output power of 6.6W was obtained under pumping power of 46.6W when T=11.3%, and output power was 2.7W under pumping power of 25 W when T=7.2%. Figure 4 shows the average output power of Nd:GdVO4 crystal at 1.34 µm when a Co:LMA crystal was used as the saturable absorber, the average output power is 500 mW under the pumping power of 25 W when T=7.2%, the optical-optical conversion efficiency is 2%, and Q-switching conversion efficiency is 18.5%. The pulse parameters with the different transmission output couplers versus incident power are shown in Figs. 5, 6 and 7. From the Figs.5–7, it can be seen that the pulse width decreased with the increasing of the incident pump power, while the repetition frequency, the pulse energy and the peak power increased with the increasing of the incident pump power when T=7.2%, the shortest pulse width of 160 ns, the largest pulse energy of 22.7µJ and the highest peak power of 113.6 W were obtained at the incident pump power of 30 W, 25 W and 25 W, respectively. When T=11.3%, the pulse width, the pulse energy and the peak power decreased with the increasing of incident pump power. The shortest pulse width of 160 ns, the largest pulse energy of 25.5µJ and the highest peak power of 150.8 W were obtained at the incident pump power of 30 W, 17.8 W and 17.8 W, respectively.
We can believe that the laser results will be better if the Co:LMA crystal is coated AR film for 1.34 µm and optimum laser cavity is designed in future experiment. Because Nd:GdVO4 crystal has much larger emission cross section and shorter lifetimes, these are unfavorable for the energy storage in laser crystals under high power pumping (this is the reason why increasing pumping power, the average output power is not increased); if vanadate mixed crystal, such as Nd:Gd1-xYxVO4, can be used for laser crystal, the pulsed laser output power will be increased (Because the cross-section of vanadate mixed crystal can be reduced for the inhomogeneous broadening of the fluorescence lines [11]).
4 Conclusions
The LD-end-pumped passively Q-switched 1.34µm Nd:GdVO4 laser was demonstrated for the first time to our knowledge by using the 0.5 at.% Co: LMA crystal as a saturable absorber Q switch. Better laser results can be obtained by improving cavity design and coating AR film at 1.34µm for Co:LMA crystal.
Acknowledgments
This work is supported by the National Natural Science Foundation of China (No. 90201017).
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