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Continuous-wave (CW) and passively Q-switched operations of LD-end-pumped Nd:Gd3AlxGa5-xO12 (Nd:GAGG) laser at 1062 nm were reported. The highest CW output power of 5.7 W was obtained, corresponding to an optical conversion efficiency and slope efficiency of 51.0% and 54.5%, respectively. The CW output efficiency of Nd:GAGG laser is comparable and even better than that of Nd:GGG. The passively Q-switched output was realized for the first time to our knowledge. In addition, a maximum output power of 1.12 W, a maximum pulse repetition rate of 39 kHz and a minimum pulse width of 6 ns were obtained by using Cr4+:YAG as the saturable absorber.
©2010 Optical Society of America
1. Introduction
Diode pumped solid-state lasers (DPSSLs) based on Nd-doped crystals, such as Nd:YAG, Nd:GGG, Nd:CLNGG, etc, have been extensively developed due to their promising practical applications in medical treatment, materials processing, military and so on [1–3]. In recent years, considerable efforts have been devoted to searching for new single crystals of high quality and optimized performance for DPSSLs.
Compared with other laser crystals, Nd:GGG crystal has the merits of larger size (no core growth), higher Nd-ions concentration (greater than 4 at.%), wider phase homogeneity with high pulling rate (up to 5 mm/h) and so on [4,5]. It has been considered as one of the best hosts for solid-state heat capacity lasers. However, the applications of Nd:GGG crystal are limited by the high cost of Ga2O3. Besides, the serious decomposition and evaporation of Ga2O3 show deleterious impacts on the crystal quality. In order to overcome these disadvantages and retain the excellent properties of Nd:GGG crystals, we have grown Nd:GAGG crystals by substituting some gallium ions with aluminum ions.
Previous studies indicated that GAGG crystal has comparable thermal properties to that of GGG [6], and the Nd-doped GAGG crystal possesses broadened spectra due to the random substitution of Al in the Ga sites. For instance, the width of the absorption peak located at 808 nm is 7 nm [7], which is larger than that of Nd:GGG(5.9 nm measured under the same conditions in our lab) and Nd:YAG (0.9-2.1 nm) [8,9]. Based on the analysis of the inhomogeneous broadening [10], we proposed that Nd:GAGG crystals should have smaller emission cross-section and larger energy storage capacity than that of Nd:GGG. This means that Nd:GAGG crystals may have excellent Q-switching properties. However, up to now, no work on Q-switching performance of Nd:GAGG crystal has been reported. In addition, the CW laser of Nd:GAGG has not been optimized.
In this paper, the CW and passively Q-switched operation of a diode-end-pumped Nd:GAGG laser at 1062 nm was realized. The highest CW laser output power of 5.7 W was achieved with an optimized cavity. With Cr4+:YAG as the saturable absorber, the passively Q-switched Nd:GAGG laser was achieved for the first time to our knowledge.
2. Experiments
The Nd:GAGG single crystal was grown in a radio frequency (RF) furnace by the Czochralski method. The Nd concentration was measured to be 0.74 at % by X-ray fluorescence (XRF) method. The sample used in the laser experiments was cut along the <111> direction with dimensions of 3 × 3 × 5.2 mm3, and both faces were anti-reflection (AR) coated at 808 nm and 1.06 µm. During the experiments, the sample was wrapped with indium foil and held in a water-cooling aluminum block to maintain a temperature of 18 °C.
The CW and passively Q-switched operations were carried out in a compact concave-plano resonator shown in Fig. 1 . M1 was a concave mirror with a curvature radius of 200 mm. Its flat face was AR coated at 808 nm (R<0.5%), and the concave face was high-reflectance (HR) coated at 1.06 µm (R>99.8%) and high-transmission (HT) coated at 808 nm. The flat mirror M2 was used as the output coupler, and different transmissions of 5%, 10%, 15% and 27% at 1062 nm were employed during the experiments. Two Cr4+:YAG crystals (AR coated on both sides at 1.06 µm, R<0.5%) with the initial transmission of 94% and 85%, respectively, were used as the saturable absorbers. They were placed as close as possible to M2 where the minimum beam size located. The pump source was a fiber coupled diode laser emitting at 808 nm. Its radiation was coupled into the laser crystal by a focusing optical system with a focal length of 25 mm. The beam-waist radius in the Nd:GAGG crystal was estimated to be 200 µm. The laser pulse signal was recorded by a Tektronix TDS2022B digital oscilloscope (200 MHz bandwidth, 2 Gs/s sampling rate) and a photo detector (New focus, model 1623 with 1 ns rise time).
3. Results and discussions
The CW operation of Nd:GAGG laser was performed firstly by optimizing the cavity length to be 20 mm. The laser spectrum was measured by a fiber optical spectrometer. The emission peak was located at 1062 nm. The dependence of output power on the absorbed pump power for different output couplers was shown in Fig. 2 .As can be seen, the threshold pump powers (Pth) were 0.25, 0.47, 0.7 and 1.44 W with the output coupler of 5%, 10%, 15% and 27%, respectively. With the augment of the output mirror transmission, the threshold increased correspondingly and the slop efficiency decreased apparently. According to the analysis by N. Mermilliod et al. [11], this phenomenon can be explained by the inhomogeneous broadening such that only some of the excited ions give rise to stimulated emission and this effect is more evident as the augment of output mirror transmission. Similar phenomena have been reported in Nd:glass, Nd:CLNGG and other disordered laser media [12,13].The Pth for the CW laser can be expressed by [11]
where hνp denotes the pump photon energy; ηp = 1 stands for the excited quantum efficiency; ωp and ωc are the pump and laser beam waist, and in our experiment ωp is 200 μm and ωc is calculated to be 160 μm by ABCD theory. T, L and τ represent the transmission of output coupler, the value of the optical losses and the excited state fluorescence lifetime, respectively. By using the measured thresholds, L was calculated to be 1.16%, indicating that Nd:GAGG crystal used in the experiment is of good quality. Using τ = 240 μs for Nd:GGG [11], the emission cross-section σ for Nd:GAGG crystal is estimated to be 1.2 × 10−19 cm2 by Eq. (1) just about half of that of Nd:GGG (about 2.46 × 10−19 cm2) [11], which can be ascribed to the inhomogeneous broadening of fluorescence line in the mixed Nd:GAGG crystal. Under an absorbed pump power of 11.2 W, the highest output power of 5.7 W was obtained with the output coupler of 5%, which gives the maximum slope efficiency of 54.5% and the optical conversion efficiency of 51.0%. These results are much higher than that of our previous studies [7], in which we reported an output power of 2.44 W, a maximum slope efficiency of 28.8% and an optical conversion efficiency of 28.5%, respectively. To our knowledge, what we report here is the most efficient Nd:GAGG laser with the highest CW output power. In addition, compared with Nd:GGG laser [14], Nd:GAGG laser has comparable and even better CW laser output efficiency. Using the knife edge method, the beam quality parameter M2 of the CW laser beam at the maximum output power was measured to be 2.34. As a result, we expect that Nd:GAGG crystal could be a new potential candidate material for DPSSLs.
Fig. 2 The CW output power versus the absorbed pump power with output coupler of 5%, 10%, 15% and 27%, respectively.
The passively Q-switched operation was realized by using Cr4+:YAG as the saturable absorbers. Figure 3 showed the experimental results of the average output power versus the absorbed pump power with the output coupler of 5% and 10% and the saturable absorber initial transmission of 94% and 85%. The oscillating thresholds were 1.15, 1.44, 1.21 and 2.88 W, respectively. Under the absorbed pump power of 5.34 W, the maximum average output power was measured to be 1.12, 0.52, 1.02 and 0.5 W, respectively. The maximum average output power of 1.12 W was obtained with the saturable absorber of T0 = 94% and the output coupler of Toc = 5%, and accordingly the maximum slope efficiency and optical conversion efficiency were determined to be 26.2% and 21.0%, respectively.
Fig. 3 The Q-switched average output power versus the absorbed pump power with output coupler of 5% and 10% and the saturable absorber initial transmission of 85% and 94% exploited.
The variations of the pulse width and the repetition rate versus the absorbed pump power under different lasing situations were shown in Fig. 4 and Fig. 5 , respectively. From Fig. 4, it can be found that the pulse width decreased with the increase of the absorbed pump power and the decrease of the saturable absorber initial transmission, which is coincident with the basic theory of Q-switching [15] and similar experimental results had been shown in Ref [14]. When the output coupler was 5%, the pulse width decreased from 17.6 to 14.4 ns and from 8.5 to 6.3 ns with the saturable absorber initial transmission of 94% and 85%, respectively. While it decreased from 16.4 to 13.5 ns and from 7.1 to 6 ns when the 10% output coupler was used. The minimum pulse width of 6 ns was obtained with Toc = 10% and T0 = 85%, which is shorter than that of Nd:GGG laser under the same situation [14]. The shorter pulse width of Nd:GAGG crystal could be correlated to the larger inhomogeneous broadening of the emission spectra [10]. The single pulse profile with a width of 6 ns was shown in Fig. 6 (a) . As illustrated by its profile, the laser pulse was not symmetrically shaped and the falling edge was slower than the rising edge, which suggests that the output coupling used in the experiment (Toc = 10%) was not sufficiently large in view of optimizing the laser pulse width [16]. Nonetheless, the average output power would be much decreased if the output coupler with a high transmission was used in the Q-switched operation. Based on the analysis by J. J. Zayhowski et al. [16], it is possible that the performance of Nd:GAGG Q-switched laser can be further optimized by balancing the pulse width and the output power.
Fig. 4 The pulse width versus the absorbed pump power with output coupler of 5% and 10% and the saturable absorber initial transmission of 85% and 94% exploited.
Fig. 5 The pulse repetition rate versus the absorbed pump power with output coupler of 5% and 10% and the saturable absorber initial transmission of 85% and 94% exploited.
Fig. 6 (a) The pulse profile with the pulse width of 6 ns taken at the absorbed pump power of 5.34 W with output coupler of 10% and the saturable absorber initial transmission of 85% exploited; (b) the corresponding pulse train with the repetition rate of 39 kHz.
As shown in Fig. 5, we can see that the repetition rate increased with the augment of the absorbed pump power and saturable absorber initial transmission under the same pump power, which is in accordance with the analysis of Q-switched laser shown in Ref [14]. With the output coupler of 5% and the saturable absorber initial transmission of 94%, the maximum repetition rate reached to 39.0 kHz under the absorbed pump power of 5.34 W. The pulse train with the repetition rate of 39 kHz was presented in Fig. 6 (b), which indicated good pulse homogeneity and stability in the long-time operation. The largest single pulse energy and peak power for the passively Q-switched laser are determined to be 66.7 µJ and 11.1 kW, respectively. It is believed that the better passively Q-switched laser output could be obtained if proper saturable absorbers and output coupler were used. The passively Q-switched microchip laser output of Nd:YAG crystal has been demonstrated in Ref [17], with the pulse width of 218 ps, pulse energy of 250 µJ, and peak power up to 565 kW, without any switching electronics. It’s possible that efficient passively Q-switched laser could be obtained by using Nd:GAGG microchip.
4. Conclusion
In summary, we have demonstrated the CW and passively Q-switched operations of a diode-end-pumped Nd:GAGG laser at 1062 nm in a concave-plano cavity. The CW output efficiency of Nd:GAGG laser is comparable and even better than that of Nd:GGG laser. The results will be much improved if more suitable saturable absorbers were used. By using Cr4+:YAG crystals as saturable absorbers, the passively Q-switched regime at 1062 nm was reported for the first time to our knowledge. The maximum average output power, the minimum pulse width and the maximum repetition rate were obtained to be 1.12 W, 6 ns and 39 kHz, respectively. The maximum peak power of 11.1 kW and single pulse energy of 66.7 µJ were achieved with the saturable absorber initial transmission of T0 = 85% and the output coupler of Toc = 10%. Our experimental results indicate that Nd:GAGG crystal could be a new potential laser gain medium suitable for LD pumping and passive Q-switching.
Acknowledgement
This work was supported by the National Natural Science Foundation of China (NNSFC) (grants 50721002, 50590403, and 50802054) and the 973 Program of China (grant 2010CB630702).
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