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A composite YAG/Nd:YAG/YAG ceramic was fabricated and the laser performance was investigated in this paper. It was proved to have a better thermal diffusion ability when compared to a traditional non-composite Nd:YAG ceramic. In a continuous-wave operation, the maximum output power was 8.2 W for a YAG/Nd:YAG/YAG ceramic laser with Nd3+ ion concentrations of 1 at%, and that for a traditional Nd:YAG ceramic laser was 6.5 W. For the laser beam quality factor measurement, it was found to be 1.6 and 1.7 for horizontal and vertical directions, respectively, for a YAG/Nd:YAG/YAG ceramic laser at an absorbed incident power of 13.5 W, and for a Nd:YAG ceramic laser it was 2.6 and 2.8, respectively. In the passively Q-switched operation, at an absorbed pump power of 19 W, the average output power and pulse energy for Cr4+:YAG crystal with an initial transmission of T0 = 80% and T0 = 85% was 3.25 W, 59.4 μJ and 4.14 W, 54.8 μJ, respectively. The pulse width was about 11 ns and 15.5 ns for Cr4+:YAG crystal with T0 = 80% and T0 = 85%, respectively.
© 2016 Optical Society of America
1. Introduction
Q-switched lasers with high repetition rate and high pulse energy are widely used in many areas, such as laser lidar, remote sensing and laser ignition [1–4]. Passive Q-switching technique has the advantages of obviously lower cost, compactness, simplicity in set-up and operation since it does not require external control. Due to the merit of improved thermo-mechanical properties, large absorption cross section, low saturable intensity and high damage threshold, Cr4+:YAG crystal is widely adopted as saturable absorber for passively Q-switched laser generation [5,6]. Compared with single crystal material, the ceramic has many favorable characteristics, such as higher doping concentration [7], more function design freedom [8], lower cost, and especially superior resistance to fracture [9].
Thermal management is an important problem for laser gain media. Theoretical and experimental results have demonstrated that the thermal lens effect of laser media, as one of the most serious results caused by the thermal effects, cannot only influence the stability of cavity but also change the spot sizes of the TEM00 mode, which will distinctly limit the power scaling of TEM00 laser [10] and have significant impact on the output performance of both active Q-switching [11] and passive Q-switching [12,13]. For example, good mode matching resulted from weak thermal effects is beneficial to stabilize the repetition rate of passively Q-switched lasers. The active ions such as Nd3+ doped into YAG can degrade its thermal properties significantly. For example, the thermal conductivity of 1.0 at% Nd:YAG is about 1.4 times lower than that of pure YAG [14]. Composite medium was proved to be an effective method to reduce the thermal lens effect due to the superior thermal conductivity of undoped section [15,16].
In this paper, we demonstrated a continuous-wave (CW) and passively Q-switched 1.06 μm laser using a novel composite YAG/Nd:YAG/YAG ceramic with a sandwich structure. The temperature distribution in YAG/Nd:YAG/YAG ceramic was simulated and the thermal focal length was measured. The laser performance was studied when YAG/Nd:YAG/YAG ceramics with different Nd3+ ion doped concentrations were employed. For comparison, under the same conditions, the thermal effect and output performance of non-composite Nd:YAG ceramic were investigated.
2. Experimental
2.1 YAG/Nd:YAG/YAG ceramic fabrication
High purity commercial α-Al2O3 (99.98%), Y2O3 (99.999%) and Nd2O3 (99.99%) powders were used as staring materials. Y2O3, α-Al2O3 and Nd2O3 powders were weighted with chemical compositions of Y3Al5O12, Nd0.03Y2.97Al5O12 (1.0 at% Nd:YAG), Nd0.045Y2.955Al5O12 (1.5 at% Nd:YAG) and Nd0.06Y2.94Al5O12 (2.0 at% Nd:YAG). Appropriate amount of MgO (99.999%) and TEOS (99.999%) were added as sintering aids. These powders were mixed and milled for 10 h in the mixed solvent of ethanol and xylene, with fish oil as dispersant. Then the binder (PVB) and the plasticizer (PEG-400 and BBP) were added into the slurry and milled for another 12 h. The obtained slurries were cast at a rate of 100 cm/min with a gap height (the distance between the blade and the substrate) of 450 μm. The thickness of the dried tapes was about 150 μm. By using a mold with round shape, the dried tapes were cut into dozens of pieces. 20 layers of α-Al2O3 + Y2O3 component tape slice were laminated at both sides of 10 layers α-Al2O3 + Y2O3 + Nd2O3 component tape slice respectively, with the temperature of 70 °C under 57 MPa for 30 min. The containing organics were removed by calcining at 600 °C for 10 h, and then the green bodies were cold isostatically pressed at 250 MPa to obtain homogeneous bodies. Composite YAG/Nd:YAG/YAG transparent ceramics with different doping concentrations were fabricated by vacuum sintering at 1760 °C for 10 hours. As-sintered ceramics were annealed at 1450 °C for 10 h in air to eliminate the stress and the oxygen vacancies. Finally, the composite YAG/Nd:YAG/YAG ceramics were cut, polished and coated for laser testing.
2.2 Experimental setup
The experimental configuration of passively Q-switched YAG/Nd:YAG/YAG ceramic laser under 808 nm laser-diode end-pumping is shown in Fig. 1.
The 808 nm pumping source was a fiber coupled LD. The pump beam was shaped using a set of collimating and focusing lenses, L1 and L2, and the diameter of beam waist was about 500 μm. The composite YAG/Nd:YAG/YAG ceramic was 3.5 mm, 3.0 mm and 3.5 mm in width, height and length, respectively. The doped part had a height of 1 mm and had different Nd3+ ion concentrations of 1 at%, 1.5 at% and 2 at%. The non-composite Nd:YAG ceramic had a same dimensions with the composite one and the doped concentration was 1 at%. The ceramics were wrapped with indium foil and placed into water-cooled copper heat sink with microchannel structure. M1 was a flat mirror with antireflection at 808 nm and high reflectivity at the 1064 nm. The output coupler M2 had different transmissions (T) of 10%, 15% and 20% at 1064 nm. Cr4+:YAG crystals as the saturable absorbers with initial transmissions (T0) of 80% and 85% at 1064 nm were used to test the properties of the passively Q-switched lasers. The total cavity length was 50 mm.
2.3 Thermal effect analysis and simulation
The measured thermal conductivity of pure YAG and 1.0 at% Nd:YAG transparent ceramics was 13.1 W/m·K [17] and 9.7 W/m·K [18], respectively, at room temperature. In order to investigate the performance improvement of composite YAG/Nd:YAG/YAG ceramic, we simulated its temperature distribution according to the experimental conditions by LASCAD software, and the result was compared with that of traditional non-composite Nd:YAG ceramic. The Nd3+ ion doping concentration was 1 at% and the absorbed pump power was set to 20 W. Figure 2 shows the simulated results of three dimensional temperature fields. The peak value temperature appeared in the pumped surface and the highest temperature of the conventional ceramic is 339.2 K, in comparison, that of the composite ceramic is 325 K. The temperature curve of pumping surface for these two ceramics are displayed in Fig. 3. The origin of coordinates was located at the center of the ceramics. Due to the sandwich structure of YAG/Nd:YAG/YAG ceramic in Y direction, the temperature variation curve of Fig. 3(b) looks different from Fig. 3(d) of non-composite Nd:YAG creamic. These simulated results indicated that the composite ceramic suffered much weaker thermal lens effect, so it could scale up the laser power and improve the beam quality.
Fig. 2 Three dimensional temperature distribution for different ceramics under the same conditions: (a) traditional non-composite Nd:YAG ceramic; (b) composite YAG/Nd:YAG/YAG ceramic.
Fig. 3 Temperature curve for different ceramics on pumping surface: (a) X direction for composite YAG/Nd:YAG/YAG ceramic; (b) Y direction for composite YAG/Nd:YAG/YAG ceramic; (c) X direction for traditional non-composite Nd:YAG ceramic; (d). Y direction for traditional non-composite Nd:YAG ceramic.
3. Results and discussions
3.1 CW operation
Without Cr4+:YAG, the CW operations were made firstly with three kinds of output couplers which had different transmission of 10%, 15% and 20%. The measured results are shown in Fig. 4 when the Nd3+ ion doping concentration CNd3+ was 1 at%.
Fig. 4 CW output power as a function of absorbed pump power for CNd3+ of 1 at%: (a) composite YAG/Nd:YAG/YAG ceramic; (b) traditional non-composite Nd:YAG ceramic.
From Fig. 4, we can see, with increasing of the incident pump power, the CW output power increased firstly. The maximum CW output power of 8.2 W for YAG/Nd:YAG/YAG ceramic laser was obtained at an absorbed pump power of 22.4 W when the output coupler with transmission of 15% was used. With further increasing absorbed pump power, due to the gain saturation and thermal effect, the output power decreased. For the Nd:YAG ceramic laser shown in Fig. 4(b), the maximum output power was 6.5 W when the absorbed pump power was 20.2 W and the optimum transmission of output coupler was 15%. Compared to traditional Nd:YAG ceramic, the increase of output power for YAG/Nd:YAG/YAG ceramic laser came from the superior heat dissipation of composite structure.
The output beam profiles for the two ceramic lasers at different absorbed pump power are displayed in Fig. 5. We can find that, at a low absorbed incident power, both YAG/Nd:YAG/YAG laser and Nd:YAG laser had good beam quality. When the absorbed incident power increased, beam distributions became worse. The beam quality of YAG/Nd:YAG/YAG laser was better than that of Nd:YAG laser, especially when the absorbed incident power was high. It should be noticed that the vertical direction was larger than the horizontal direction for YAG/Nd:YAG/YAG ceramic laser beam distribution at the five different absorbed pump power. Pure YAG had a higher thermal conductivity than that of Nd:YAG. It was in the vertical direction of YAG/Nd:YAG/YAG composite ceramic. Therefore in the YAG/Nd:YAG/YAG ceramic laser, the vertical direction may had a smaller thermal induced loss than that in the horizontal direction. This resulted in a larger size of laser beam in vertical direction. At an absorbed incident power of 13.5 W, with the help of a focusing lens, the laser beam quality factor M2 was measured by a 90/10 traveling knife-edge method in the far field. For YAG/Nd:YAG/YAG laser, it was found to be 1.6 and 1.7 for horizontal and vertical directions, respectively. And for Nd:YAG laser, it was 2.6 and 2.8, respectively.
When the YAG/Nd:YAG/YAG ceramic with Nd3+ doping concentration of 1.5 at% was used, the tested results are shown in Fig. 6. It was obvious that the optical conversion efficiency was lower than the former one with Nd3+ doped concentration of 1.0 at%. The maximum output power of 2.83 W was obtained when the absorbed pump power was 13.5 W and the output coupler with transmission of 15% was used. Compared to YAG/Nd:YAG/YAG ceramic laser with Nd3+ doped concentration CNd3+ of 1.0 at%, the decreased performance when YAG/Nd:YAG/YAG ceramic with CNd3+ of 1.5 at% was used resulted from the following reasons. On one hand, ceramic with higher doping concentration suffered more serious thermal effect therefore the thermally induced loss was higher. On the other hand, the increased scattering loss due to the higher doping concentration contributed to decreased efficiency. Finally, when YAG/Nd:YAG/YAG ceramic with CNd3+ of 2.0 at% was employed, only several hundreds milliwatts CW output power was obtained. In this research, the YAG/Nd:YAG/YAG ceramic was full of red fluorescence, and we thought this phenomenon confirmed the strong scattering loss at high doping concentration.
Fig. 6 CW output power of composite YAG/Nd:YAG/YAG ceramic laser as a function of absorbed pump power when CNd3+ was 1.5 at%.
3.2 Passively Q-switched operation
In the passively Q-switched research, YAG/Nd:YAG/YAG ceramic with CNd3+ of 1.0 at% was adopted due to the optimum performance in the above CW operations. The output coupler with transmission of 15% was selected, and Cr4+:YAG crystals with initial transmission T0 of 80% and 85% at 1.06 μm as the saturable absorbers were used to test the properties of passively Q-switched YAG/Nd:YAG/YAG ceramic laser. The average output power and repetition rate as a function of absorbed pump power are shown in Fig. 7.
Fig. 7 Passively Q-switched YAG/Nd:YAG/YAG ceramic laser performance as a function of absorbed pump power: (a) average output power; (b) repetition rate.
As shown in Fig. 7(a), the average output power increased linearly with absorbed pump power. A higher average output power and a lower pump threshold were obtained when Cr4+:YAG crystal with higher T0 of 85% was used. This is due to lower intracavity loss of Cr4+:YAG with T0 = 85%. At an absorbed pump power of 19 W, the average output power of 3.25 W and 4.14 W was achieved for T0 = 80% and T0 = 85%, respectively. Figure 7(b) indicates the repetition rate as a function of absorbed pump power. As shown in this figure, the repetition rate increased with increasing of the absorbed pump power. The higher repetition rate would be obtained when a saturable absorber with higher T0 was used. At an absorbed pump power of 19 W, the repetition rate of 54.7 kHz and 75.6 kHz was achieved for T0 = 80% and T0 = 85%, respectively.
The pulse width as a function of absorbed pump power is depicted in Fig. 8. It can be see that the pulse width was insensitive to the absorbed pump power. When the Cr4+:YAG crystal was fully saturated, the modulation depth of Cr4+:YAG was constant. Therefore the pulse width will not change [12]. The pulse width was about 11 ns and 15.5 ns for Cr4+:YAG crystals with T0 = 80% and T0 = 85%, respectively.
Fig. 8 The pulse width of passively Q-switched YAG/Nd:YAG/YAG ceramic laser as a function of absorbed pump power.
The pulse energy and pulse peak power as a function of absorbed pump power is shown in Fig. 9(a) and Fig. 9(b), respectively. When Cr4+:YAG crystal with T0 = 80% was used, the pulse energy and pulse peak power were higher than those using T0 = 85%. At an absorbed pump power of 19 W, the pulse energy and pulse peak power for Cr4+:YAG crystal with T0 = 80% and T0 = 85% was 59.4 μJ, 54.8 μJ and 5.2 kW, 3.4 kW, respectively.
Fig. 9 Passively Q-switched YAG/Nd:YAG/YAG ceramic laser performance as a function of absorbed pump power: (a) pulse energy; (b) pulse peak power.
4. Conclusions
In conclusion, a CW and passively Q-switched 1.06 μm laser using a novel composite YAG/Nd:YAG/YAG ceramic was demonstrated in this paper. YAG/Nd:YAG/YAG ceramic was proved to have a better thermal diffusion ability through the simulation of temperature distribution and the measurement of thermal focal length in YAG/Nd:YAG/YAG ceramic and traditional non-composite Nd:YAG ceramic. In CW operation, the laser performance was tested when YAG/Nd:YAG/YAG ceramic with different Nd3+ ion doping concentration was employed. For comparison, under the same conditions, the output performance of traditional Nd:YAG ceramic was investigated. The maximum CW output power was 8.2 W for YAG/Nd:YAG/YAG ceramic laser, and that for Nd:YAG ceramic laser was 6.5 W. For the laser beam quality factor measurement, it was found to be 1.6 and 1.7 for horizontal and vertical directions, respectively for YAG/Nd:YAG/YAG laser at an absorbed incident power of 13.5 W, and for Nd:YAG laser it was 2.6 and 2.8, respectively. In the passively Q-switched operation, at an absorbed pump power of 19 W, the average output power, repetition rate, pulse energy and pulse peak power for Cr4+:YAG crystal with T0 = 80% and T0 = 85% was 3.25 W, 54.7 kHz, 59.4 μJ, 5.2 kW and 4.14 W, 75.6 kHz, 54.8 μJ, 3.4 kW, respectively. The pulse width was about 11 ns and 15.5 ns for Cr4+:YAG crystal with T0 = 80% and T0 = 85%, respectively.
Acknowledgments
This work was supported by the National Natural Science Foundation of China (Grant No. 61505041, 61505042 and 61575212), the Natural Science Foundation of Heilongjiang Province of China (Grant No. F2015011), the General Financial Grant from the China Postdoctoral Science Foundation (Grant No. 2014M560262 and 2013M531040), the Special Financial Grant from the China Postdoctoral Science Foundation (Grant No. 2015T80350 and 2014T70336), the Special Financial Grant from the Heilongjiang Province Postdoctoral Foundation (Grant No. LBH-TZ0602 and LBH-TZ0507), the Financial Grant from the Heilongjiang Province Postdoctoral Foundation (Grant No. LBH-Z14074 and LBH-Z13081), the Fundamental Research Funds for the Central Universities (Grant No. HIT. NSRIF. 2015044), and the National Key Scientific Instrument and Equipment Development Projects of China (Grant No. 2012YQ04016401).
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