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The static and dynamic 1.34 μm laser output performance of the Nd:YxGd1-xVO4 mixed crystal, pumped by a flash-lamp, is investigated with different transmissions of output couplers (T =15%, 30%, and 40%) in a plano-concave laser cavity. With the output coupler transmission of T=30%, the static output energy of 62 mJ is obtained when the pump energy is 24.2 J, and the corresponding electric-optical conversion efficiency is 0.26%. By use of a new saturable absorber, Co2+:LaMgAl11O19 (Co:LMA), the passive Q-switching at 1.34 μm is realized when the second threshold condition of passive Q-switching is satisfied. With the cavity length of 245 mm and pump energy of 24.2 J, the single-pulse output energy, pulse width, and peak power are obtained to be 3.5 mJ, 45 ns, and 77.8 kW, respectively.
©2006 Optical Society of America
1. Introduction
The Nd:YVO4 and Nd:GdVO4 laser crystals have been widely used in many applications such as industry, medicine, basic scientific research, military fields, and so on, because of their good mechanical, thermal, physical, chemical and laser properties. By synthesizing Nd:YVO4 and Nd:GdVO4 polycrystalline materials with the liquid-phase method, the different Gd/Y Nd:YxGd1-xVO4 mixed crystals with low Nd ion concentrations, where the value of x is between 0 and 1, were successfully grown by the Czochralski method [1,2]. The Nd:YxGd1-xVO4 mixed crystals have good properties of larger stimulated emission cross-section and higher thermal conductivity like the Nd:YVO4 and Nd:GdVO4 crystals. Furthermore, because the radius of the Gadolinium ion is larger than that of Yttrium, the field of the mixed crystal lattice has some changes compared with those of Nd:YVO4 and Nd:GdVO4 crystals, and these changes may improve laser performance of the mixed crystal. The studies of static and dynamic 1.06 μm laser output performance of laser-diode pumped Nd:YxGd1-xVO4 laser have been reported [3–5].
There is a widespread need for the lasers that operate near 1.3 μm, which is the transmission window of silica optical fibers, and the frequency-doubling of these lasers provides an effective way to generate red lasers. Saturable absorbers like Co-doped crystals of LiGa5O8, MgAl2O4, LaMgAl11O19 and ZnS can be used as a passive Q-switch in the lasers with output wavelength ranging from 1.3 μm to 1.6 μm [6–9]. A saturable absorber, Co:LMA, for Q-switching of the flash-lamp pumped 1.34 μm Nd:YAlO3 laser [6] and laser-diode pumped 1.34 μm Nd:GdVO4 laser [10] has been reported.
In this paper, we have demonstrated, for the first time to our knowledge, the static output performance of a flash-lamp pumped Nd:YxGd1-xVO4 laser operating at 1.34 μm in a planoconcave laser cavity. Experimental results show that the Nd:YxGd1-xVO4 crystal has good static output performance at 1.34 μm. The passive Q-switching at 1.34 μm with Co:LMA as saturable absorber is realized by focusing in a plano-concave cavity. Under the pump energy of 24.2 J, the single-pulse output is obtained, and the corresponding pulse energy, pulse width, and peak power are 3.5 mJ, 45 ns and 77.8 kW, respectively.
2. Experimental setup
The experimental setup of Q-switched laser cavity structure is schematically given in Fig. 1. The laser cavity is composed of a concave mirror M1 (R=250 mm) with high-transmission (HT) coating at 1.06 μm and high-reflection (HR) coating at 1.34 μm, and a plane output coupler M2 with HT at 1.06 μm and partial-reflection at 1.34 μm. The laser cavity is 245 mm long. In our experiment, two Nd:YxGd1-xVO4 laser crystals with different Gd/Y values are used. One is the Nd:YxGd1-xVO4 I (x=0.36, Nd:Y0.36Gd0.64VO4); the other is the Nd:YxGd1-xVO4 II (x=0.58, Nd:Y0.58Gd0.42VO4). Neither of them has an HT coating. The Nd:YxGd1-xVO4 crystal (4×4×11 mm3, 0.5 at.% Nd-doped), placed close to M1, is pumped by a Φ5×50 mm xenon flash-lamp in a 50 mm-long single silver-coated elliptical cylinder reflector. The sample of the Co:LMA with the initial transmission T0 of 81% and the size of 4×4×0.5 mm3 is placed close to M2, and no HT film is coated on the surface. The measurement of the output energy is carried out by an EPM2000 energy meter produced by Molectron Corp., USA. The output pulse shapes are measured with a germanium detector (Judson Technologies, response range: 800nm–1800nm) and a TDS3032B oscilloscope (Tectronix Inc., USA).
3. Results and discussion
3.1 Static laser performance
Fig. 2. Static output energy of Nd:YxGd1-xVO4 I versus pump energy with different output couplers transmissions.
The Co:LMA crystal is removed from the cavity in Fig. 1. The variations of output energy versus pump energy for different transmissions (T=15%, 30% and 40%) of output couplers are plotted in Fig. 2. It is well known that the pump threshold power for a four-level system is given by [11]:
where Pth is the pump threshold power, L is the intracavity round-trip dissipative optical loss, R is the reflectivity of output mirror, A is the cross-section of Nd:YxGd1-xVO4 mixed crystal, hυp is the single pump photon energy, σ is the stimulated emission cross-section of the gain medium, τ is the fluorescence lifetime of the gain medium, and ηge is the efficiency factor. According to Eq. (1), we find that the lower threshold pump power can be expected when R is relatively larger. Figure 2 shows that the laser output energy increases almost linearly with the pump energy for these output coupler transmissions. For T=15%, the pump threshold energy is 3.9 J, but the curve is saturated when the pump energy reaches 21.8 J. According to the formula of optimum output coupler transmission, , where g0 is the round-trip small signal power gain and l is the length of gain medium, the higher transmission of the output coupler is required when g0 or the pump energy increases. However, the increase of output coupler transmission, corresponding to an increasing loss of the resonator, will result in higher pump threshold energy and lower electric-optical conversion efficiency. For T=40%, although the slope efficiency of the curve is higher, the pump threshold energy is highest at 7.3 J, and the electric-optical conversion efficiency is relatively lower. The output curve of T=30% has the lower pump threshold energy of 6 J and higher slope efficiency, and the saturation does not occur. Under the pump energy of 24.2 J, the laser output energy is 62 mJ, and the corresponding electric-optical conversion efficiency is 0.26%.
3.2 Dynamic laser performance
For optimization of passive Q-switching, the gain medium with large stimulated emission cross-section is not helpful, because the Q-switch works well only if the saturation of the absorber occurs before that of the gain medium (the second threshold condition) [12]. According to the analysis of the coupled rate equations, the criterion for good passive Q-switching is given by [12]:
where T0 is the initial transmission of saturable absorber, A/AS is the ratio of the effective area in the gain medium to that in the saturable absorber, R is the reflectivity of output couple, L is the nonsaturable intracavity round-trip dissipative optical loss, σgs is the ground-state absorption cross-section of saturable absorber, σ is the stimulated emission cross-section of gain medium, γ is the inversion reduction factor (γ=1 and γ=2 correspond to four-level and three-level systems), and β is the ratio of the excited-state absorption cross-section to that of the ground-state absorption in the saturable absorber.
In order to satisfy the second threshold condition, the value of A/AS must be increased because the stimulated emission cross-section of the Nd:YxGd1-xVO4 mixed crystal is relatively larger. Therefore, the intracavity focusing in a plano-concave laser cavity is necessary. The plano-concave laser cavity is composed of a concave mirror with the 250 mm radius of curvature and a plane output mirror. In order to get larger A/AS, Nd:YxGd1-xVO4 and Co:LMA crystals are placed close to the concave mirror and the output mirror, respectively, and the cavity length is increased to 245 mm, because the beam waist of oscillating lights is located on the output mirror in the plano-concave laser cavity. In our experiment, the Q-switched pulses at 1.34 μm are obtained with the output coupler transmission of T=30% and the initial transmission T0=81% of Co:LMA. All of the Q-switched pulses are single-pulse when the flash-lamp pump energy is lower than 25 J; if we continue to increase the pump energy, the laser begins to produce multi-pulses.
Figure 3 shows the variation of Q-switched pulse output energy versus the pump energy. The experimental results show that the output energy increases with the enhancement of pump energy and that the pump threshold energy values of the two mixed crystals are 11.9 J and 13.6 J, respectively. The fluorescence lifetime of the Nd:YxGd1-xVO4 I mixed crystal is longer than that of the Nd:YxGd1-xVO4 II [3], which makes it helpful for passive Q-switching, so the single-pulse output energy of the former is relatively larger. Under the pump energy of 24.2 J, the single-pulse output energy values of the two mixed crystals are 3.5 mJ and 3.1 mJ, respectively. At the same time, the slope efficiency of the Nd:YxGd1-xVO4 II mixed crystal is notably higher than that of Nd:YxGd1-xVO4 I, so a higher output energy of the Q-switched single-pulse can be expected if the transmission of the output coupler increases.
Figures 4 and 5 show the variations of Q-switched pulse width and peak power versus the pump energy, respectively. We can see that the pulse width decreases and the peak power increases gradually with the enhancement of pump energy. Under the same pump energy, the pulse width of Nd:YxGd1-xVO4 I is shorter than that of Nd:YxGd1-xVO4 II, and the peak power of the former is larger than that of the latter. The main reason for this situation is attributed to the longer fluorescence lifetime of the former, which results in the enhancement of its energy storage capacity. Under the pump energy of 24.2 J, the values of the shortest single-pulse width of the two mixed crystals are 45 ns and 60 ns, and the corresponding peak power values are 77.8 kW and 51.7 kW, respectively. Figures 6 and 7 respectively show the single-pulse profiles of the two mixed crystals with pump energy of 24.2 J.
4. Conclusions
In conclusion, we have reported for the first time, the static and dynamic output performances of a flash-lamp pumped Nd:YxGd1-xVO4 laser operating at 1.34 μm. With the optimum output coupler transmission of T=30% and pump energy of 24.2 J, the static output energy of 62 mJ is obtained, and the corresponding electric-optical conversion efficiency is 0.26%. The passive Q-switching of two Nd:YxGd1-xVO4 mixed crystals with different Gd/Y values at 1.34 μm has been realized, with Co:LMA as the saturable absorber, by focusing in a plano-concave cavity. The experimental results indicate that the Nd:YxGd1-xVO4 I mixed crystal with a larger Gd/Y value has better Q-switching effects than the Nd:YxGd1-xVO4 II mixed crystal. Under the lower pump energy of 24.2 J, the single-pulse output is obtained, and the corresponding pulse energy, pulse width, and peak power are 3.5 mJ, 45 ns, and 77.8 kW, respectively. If the Nd:YxGd1-xVO4 mixed crystal and the Co:LMA crystal are coated with HT film at 1.34 μm, the single-pulse output with higher pulse energy can be expected. Therefore, it shows wide applied foregrounds of the medium- and high-energy 1.34 μm laser.
Acknowledgments
This work is supported by the National Natural Science Foundation of China (No. 90201017).
References and links
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