Efficient high-pulse-energy green-beam generation by intracavity frequency doubling of a quasi-continuous-wave laser-diode-pumped Nd:YAG laser Susumu Konno, Yoko Inoue, Tetsuo Kojima, Shuichi Fujikawa, and Koji Yasui
A 50-mJ green beam was generated at a 1-kHz repetition rate by intracavity frequency doubling of a quasi-cw laser-diode-pumped Nd:YAG laser. The green laser was used for 12-mJ fourth-harmonic beam generation with a CsLiB6O10 crystal. © 2001 Optical Society of America OCIS codes: 140.3480, 140.3540, 140.3610, 140.7300.
1. Introduction
With the recent development of high-power laser diodes and high-quality frequency-conversion crystals, greater than 100-W green average output power has been reported in intracavity frequency-conversion systems.1– 4 The pulse energies of these lasers, however, are limited to less than 20 mJ and, high average powers are achieved only at high repetition rates of more than 10 kHz, because the short upper-state lifetime 230 s of Nd:YAG results in inefficient Q-switching operation at low repetition rates.5 Nd: YLF could be an alternative to Nd:YAG for generation of high-pulse-energy green beams. However, Nd:YLF has a lower scaling potential than Nd:YAG. Extracavity frequency-conversion systems have proved to have a pulse-energy scaling potential of more than 100 mJ; however, enhancement of the electrical efficiency could be difficult.6,7 Pulse-energy enhancement of the green beam, while high electrical efficiency is maintained, is highly desirable in many fields related to information technology, such as large-area material treatment, UV beam generation, and femtosecond Ti:sapphire 共Ti:S兲 laser pumping. For example, in Ti:S lasers, because the gain is a function of the pump-beam fluence, a low-repetition-rate high-pulse-energy pump
The authors are with the Advanced Technology R&D Center, Mitsubishi Electric Corporation, 8-1-1 Tsukaguchi, Amagasaki, Hyogo 661-8661, Japan. The e-mail address for S. Konno is
[email protected]. Received 29 January 2001; revised manuscript received 26 April 2001. 0003-6935兾01兾244341-03$15.00兾0 © 2001 Optical Society of America
source with sufficient focusability is required for high-power Ti:S laser systems.8 Here we report an intracavity frequency-doubled quasi-cw laser-diode-pumped Nd:YAG laser that produces a 53-mJ, 532-nm beam with 6% electrical-tooptical conversion efficiency at a repetition rate of 1 kHz. The green pulse energy has to our knowledge the highest value obtained with an intracavitydoubled Nd:YAG laser. The electrical efficiency is comparable with those of conventional highrepetition-rate low-pulse-energy green-beam sources. We have also generated a 12-mJ fourth-harmonic beam with a CsLiB6O10 共CLBO兲 crystal. The UV pulse energy has what we believe is the highest value obtained with ⬎1-kHz repetition rate. A diffusive reflector pumping configuration has been used for efficient pumping. The fluorescence decay energy loss of a low-repetition-rate Q-switched Nd:YAG laser has been compensated for by quasi-cw diode pumping. 2. Experiment
Figure 1 shows schematic drawings of the experimental setup. A quartz 90-deg polarization rotator is placed between two uniformly pumped Nd:YAG rods for polarization-dependent bifocusing compensation.9 The pump heads consist of four modules. Each module contains four 1-cm-long linear quasi-cw diode arrays 共808-nm wavelength, 60-W rated peak output power, 20% duty cycle兲. The laser diodes are operated at a 1-kHz repetition rate with a 170-s pulse width. We rotated each module 22.5 deg from the others about the optical axis to produce uniform pump-light distribution within the rod’s cross section. Figure 1共a兲 is a schematic edge view of the pump module. A Nd:YAG rod 共4 mm in diameter, 105 mm 20 August 2001 兾 Vol. 40, No. 24 兾 APPLIED OPTICS
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Fig. 2. Second-harmonic generation and IR output power.
Fig. 1. Schematic view of 共a兲 the pump module and 共b兲 the resonator configuration.
in length, with 0.6% Nd doping兲 is surrounded by a flow tube and a diffusive reflector. Pump light is coupled into the diffusive reflector through thin optical plates 共the end surfaces are both antireflection coated for pump light兲 by internal total reflection with more than 97% transfer efficiency. Details of the pumping configuration have been reported elsewhere.8 Figure 1共b兲 is a schematic drawing of the z-shaped 2.8-m-long concave– concave cavity. The distance between the end mirrors 共radius of curvature, 1 m兲 and the rod ends is 1.3 m. Based on the calculations reported in Refs. 9 and 11, the resonator is designed to avoid damage to the optical components and to provide an appropriate beam diameter at the frequency-conversion crystal. The resonator is folded by a harmonic separator mirror 共T ⬎ 98% at 532 nm, R ⬎ 99.5% at 1064 nm, where T is the transmittance and R is the reflectance兲 and a total reflector. The pumping head and acousto-optic 共AO兲 Q switches are placed in one arm of the resonator. The Q switches are operated at a 1-kHz repetition rate. A type II phase-matched dual-wavelength antireflection-coated LiB3O5 crystal is placed in another arm of the resonator. The beam diameter at the second-harmonic-generating 共SHG兲 crystal is 2 mm. The 532-nm green beam is extracted from the harmonic separator mirror in one direction. In Fig. 2 we compare the laser performance for 1064- and 532-nm operation. We obtained 1064-nm operation by replacing one of the total reflection end mirrors with a partial output coupler 共T ⫽ 22%兲 and removing the LiB3O5 crystal from the resonator. The green beam was extracted in one direction from the harmonic separator mirror. At 296-W average 4342
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total diode output power and 880-W electrical input power, 110 W of 1064-nm output power was obtained in quasi-cw operation, and 68 W in Q-switching operation. Although we confirmed the Q-switch holdoff, the Q-switched output power was saturated at more than 1400-W laser-diode 共LD兲 peak pump power. An investigation of the reason for the output power saturation is now under way. We then replaced the transmitting output coupler with a total reflector and placed the LiB3O5 crystal in the resonator again. A maximum 532-nm green output power of 53 W was generated, with 296 W of total diode output power and 880 W of electrical input power, corresponding to a maximum green-pulse energy of 53 mJ and an electrical efficiency of 6%. The ratio of green output to Q-switched IR output was approximately 80%. The beam quality of the green output, measured with a CCD camera, was M 2 ⫽ 15 at the maximum output power. The pulse width of the green beam was 70 ns at the maximum green output power. After at least 200 h of operation, we observed no degradation of green output power. We used the green laser described above for fourthharmonic generation. The experimental setup is shown in Fig. 3. The green beam extracted from the green laser was focused into a 15-mm-long type I CLBO crystal by a lens. The focusing diameter at the crystal was 1.3 mm. The CLBO crystal was used in an oven whose temperature was maintained at 140 °C. The generated fourth-harmonic beam was
Fig. 3. Experimental setup for fourth-harmonic generation.
The authors thank M. Tanaka for supporting and encouraging their research. References
Fig. 4. UV output power as a function of green incident power.
separated from the green beam by two separator mirrors 共T ⬎ 98% at 532 nm, R ⬎ 95% at 266 nm, where T is transmittance and R is reflectance兲; HR means highly reflective and HT means highly transmissive. In Fig. 4 we show the UV output power as a function of the incident green power. The maximum UV output power of 12 W was obtained with a green incident power of 40 W, corresponding to a conversion efficiency of 30%. Although we attempted to enhance the fourth-harmonic output power with tighter focusing, the conversion efficiency from green to UV saturated near 30%. We believe that the saturation could be attributed to a thermally induced change in the phase-matching condition.12 3. Summary
In conclusion, we have enhanced the electrical efficiency of a high-pulse-energy green laser by intracavity frequency doubling of a quasi-cw laser-diodepumped Nd:YAG laser. The highest green pulse energy of 53 mJ was generated at a repetition rate of 1 kHz with 6% electrical efficiency. The green pulse energy has to our knowledge the highest value obtained with intracavity-double Nd:YAG lasers, and the UV pulse energy also has the highest value obtained at a ⬎1-kHz repetition rate. A green laser could be a reliable and efficient beam source for material processing and femtosecond Ti:S laser pumping.
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