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High power operation of high repetition Q-switched ceramic Nd:YAG laser is demonstrated using a master-oscillator power amplifier with transversely-pumped bounce geometry. The laser system produced >40W high spatial quality output (M 2 < 1.3) at a pulse repetition frequency of >100kHz. Pulse width of 35–80ns was achieved in pulse repetition frequency range of 10–150kHz.
©2006 Optical Society of America
1. Introduction
High repetition rate (>100kHz) Q-switched lasers have received intense attention in several applications including laser machining, remote sensing, laser display etc. High repetition rate Q-switching requires large gain as well as a short storage time of population inversion in the laser amplifier.
Side-pumped bounce amplifier configuration [1, 2] produces intense inversion density in a shallow absorption depth below the pump face resulting in an extremely high single-pass gain. Lasers based on bounce amplifier configuration, with high power and high repetition rate Q-switched output, have been successfully demonstrated in diode-pumped Nd:YVO4 [3], Nd:GdVO4 [4], and mixed vanadate Nd:GdxY1-xVO4 [5] materials.
Polycrystalline ceramic neodymium (Nd) doped Y3Al12O5 (YAG), have been investigated intensely because of their attractive features including excellent thermal properties, high Nd doping, no limitation of shape and size, and low cost [6–9]. The high Nd doping capability results in strong diode absorption and relatively shallow absorption depth, thereby yielding high inversion density as well as shorter storage time of population inversion. Highly doped ceramic Nd:YAG material is a promising candidate for high repetition rate Q-switched lasers based on the side-pumped bounce amplifier.
In most research and commercial systems based on Nd:YAG, the repetition frequency is generally limited in the range 10-50 kHz [10]. Recently, we have demonstrated >100kHz high repetition rate Q-switched operation of a side-pumped highly Nd-doped (2%) ceramic YAG bounce laser for the first time. The maximum average output power of 10W was achieved [11].
In this paper, we demonstrate the first power scaled operation of a Q-switched highly Nd-doped (2%) ceramic YAG bounce geometry master oscillator power amplifier (MOPA) laser system. Over 40-watt diffraction-limited output, in 30–80ns pulses were obtained in the 10–150 Hz pulse repetition frequency range.
2. Experiments
2.1 Master oscillator
A schematic diagram of the master oscillator is shown in Fig. 1(a). The amplifier used was a ceramic Nd:YAG slab (Baikowski co.) with 2% Nd doping with dimensions of 2 × 5 × 20mm3. Absorption coefficient of the slab was 20cm-1 around 809nm, and the expected absorption depth was ~1mm. Fluorescence lifetime was 174μs [12]. The end faces were AR-coated for 1064 nm and the pump face was AR-coated for 808 nm. The end faces were wedged at 3° relative to the normal of the pump face. The slab was mounted onto a copper block for heat removal from the intensely pumped laser material. The temperature of the copper block was maintained at ~12°C by a water re-circulating chiller.
The ceramic Nd:YAG slab amplifier in the bounce geometry was transversely pumped by a fast-axis lensed continuous-wave (CW) three-bar diode array stack. The diode output was line-focused by a 12.7mm cylindrical lens on the pump face of the slab. A high reflectivity (HR) plane mirror, M1, for 1064 nm and a partially reflecting plane output coupler, M2, (reflectivity 60%, 70% or 80% for 1064 nm) were used for the laser cavity. Two vertical cylindrical lenses, VCL1 and VCL2, (f=50mm) in the cavity were used to provide good spatial overlap between the laser mode and the small gain region in the vertical dimension. In this arrangement, the laser mode experienced internal bounce angle of 11° with respect to the pump face. The total cavity length was ~20cm (L1=L2~10cm).
Figure 2 shows the experimental CW output power versus pump diode power. Using the 70% reflectivity output coupler, 42 W of output power with a 32% slope efficiency was obtained at 142 W of pump power. Threshold for lasing was achieved for ~15W pump power. The laser output had a multi-mode profile along the horizontal dimension (x-axis) but single mode in the vertical dimension (y-axis). The beam propagation factor, M 2, along the x-axis was measured to be ~9.
To improve the beam quality along the x-axis, the cavity length was extended and made asymmetric with respect to the position of the ceramic Nd:YAG amplifier as shown in Fig. 1(b). Total cavity length was ~60cm with L1=20cm and L2=40cm. As reported previously [11], with this configuration, the gain medium forms a limiting aperture for TEM00 mode by utilizing the thermal lensing of the laser amplifier allowing the laser to operate in TEM00 mode.
Fig. 1. Experimental Nd-doped ceramic YAG laser with bounce amplifier geometry, showing (a) CW multi-mode laser cavity, and (b) Asymmetric cavity for high beam quality Q-switched output.
As shown in Fig. 2, an output power of 27W was obtained for 142 W of pump power. The kink seen in the power curve around 80 W of pump power was induced by the thermal lens in the amplifier making the cavity unstable. Above this power level, the cavity became stable again, and the laser started to operate with TEM00 mode. The output exhibited a TEM00 profile, with measured M 2 values of < 1.3 in both dimensions. These results are consistent with our previous experiments [11].
Fig. 2. CW output power as a function of pump diode power for various output coupler reflectivities. Red solid squares show experimental output power from the long asymmetric cavity.
To operate the laser in Q-switched mode an acousto-optic modulator (AOM) was placed between the output coupler and cylindrical lens CL1 as shown in Fig. 1(b). Experimental results for average output power, pulse duration at various pulse repetition frequencies (PRF) and typical Q-switched pulse profile are summarized in Fig. 3. Stable Q-switching operation was observed in the range of 10–150kHz. Above PRF of 160kHz, missing pulses were observed due to insufficient energy storage in the amplifier. Experimental average output power and pulse width were 16–25W and 35–80ns in the range of 10–150kHz, respectively. Average output power reached saturation level (~25W) around the PRF of 60kHz. Beam quality of the output was slightly improved with M 2~1.2 due to the aperturing effect of the AOM for the laser mode. Figure 3(c) shows temporal evolution of the Q-switched output from the laser at the PRF of 100 kHz.
Fig. 3. (a). Experimental average output power and (b) pulse width versus pulse repetition frequency. (c) A snap shot of temporal evolution of the Q-switched pulse at the PRF of 100kHz.
2.2 Master-oscillator power amplifier
High quality, high power Q-switched output can explore several applications including efficient nonlinear frequency conversion as well as precise machining. For the further power scaling of the high quality Q-switched output, a MOPA system based on two ceramic Nd:YAG slab amplifiers was investigated. A schematic diagram of the system is shown in Fig. 4. The slab used for the power amplifier was identical to that used for the master-oscillator. The pump power in the master-oscillator was fixed at ~140W. The power amplifier was also pumped by second fast-axis lensed continuous-wave (CW) three-bar diode array stack. To reduce thermal lens in the power amplifier, the diode output was loosely line-focused by a 25mm cylindrical lens onto the amplifier pump face.
The output from the Q-switched master oscillator was focused to be an ellipsoidal spot inside the power amplifier using a horizontal cylindrical lens, HCL, (f = 300mm) and a vertical cylindrical lens, VCL3, (f = 60mm), so that the focused beam and the pumped region in the power amplifier could have optimum spatial overlap. The amplified output was re-collimated and directed towards diagnostic equipment. The bounce angle inside the power amplifier was slightly shallower (~9°), thereby increasing the gain for improved extraction.
Experimental average output powers at various pump levels are shown in Fig. 5. Average amplified output power reached 41W at pump power of ~125W. The Q-switched input power from the master oscillator was 23W at 100kHz. The corresponding saturated gain was ~1.8. Above the pump power of 125W saturation of the output power was observed. This is likely to be caused by increased thermal loading due to insufficient energy extraction in the amplifier thus limiting further power scaling of the MOPA above this pump level. Experimental average output power as a function of PRF is shown in Fig. 6. In the PRF range of 10–150kHz, the average output power of 30–41W was achieved. We also investigated the beam quality of the output from the MOPA. The spatial form of the output was TEM00-mode as shown in Fig. 7 and its M 2 was measured to be <1.3. Though the pulse contrast of the output was not measured accurately, it was expected to be almost same as that of the master-laser output, because the saturation gain of the amplifier was ~2.
When the AOM was removed from the master oscillator (CW operation), the increased output power from the master oscillator slightly improved the energy extraction of the power amplifier. A CW output power of 45W was obtained with the of ~1.6. The brightness of the output obtained by the MOPA was 7 times higher than that of the output from the master-oscillator under multi-mode operation. The high quality, high power MOPA system based on the ceramic Nd:YAG slabs must be a promising candidate of light sources for efficient nonlinear frequency conversion as well as precise machining.
3. Conclusion
We have demonstrated power-scaling of high repetition Q-switched Nd doped ceramic YAG bounce laser by the use of a master-oscillator power amplifier system. The master oscillator produced diffraction-limited Q-switched output with a pulse width of 35–80ns in pulse repetition frequency range of 10–150kHz. Maximum output power of 25W was obtained at 142 W of pump power. We also investigated further power scaling of the laser system by use of a power amplifier. The system produced 41W high quality Q-switched output with beam propagation factor (M 2) of <1.3. This is the highest value, to the best of our knowledge, obtained by Q-switched ceramic YAG lasers with a high Nd doping of 2%. Further power scaling should be possible by use of a multi-pass amplification geometry, which allows improved energy extraction as well as reduction of the heat generation in the amplifier [13].
Acknowledgments
The authors acknowledge support from the Joint Research Project of the Japan Society for the Promotion of Science, and from The Engineering and Physical Sciences Research Council (UK) under grant number GR/T08555/01. T. Omatsu’s e-mail address is omatsu@faculty.chiba-u.jp.
References and links
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