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High power and high beam quality CW green beam generated by diode-side-pumped intracavity frequency doubled Nd:YAG laser
Optics Communications 282 (2009) 4288–4291
Contents lists available at ScienceDirect
Optics Communications journal homepage: www.elsevier.com/locate/optcom
High power and high beam quality CW green beam generated by diode-side-pumped intracavity frequency doubled Nd:YAG laser Xian-Kun Cheng a,c,*, Qian-Jin Cui a,c, Yong Zhou a,c, Zhi-Min Wang b, Jia-Lin Xu b, Yong Bo b, Qin-Jun Peng b, Da-Fu Cui a,b, Zu-Yan Xu a,b a b c
Laboratory of Optical Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China Research Center for Laser Physics and Technique, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, China Graduate School of the Chinese Academy of Sciences, Beijing 100190, China
a r t i c l e
i n f o
Article history: Received 4 March 2009 Received in revised form 21 July 2009 Accepted 23 July 2009
PACS: 42.55.Rz 42.55.Xi 42.65.Ky
a b s t r a c t We report a stable high power and high beam quality diode-side-pumped CW green laser from intracavity frequency doubled Nd:YAG laser with LBO crystal. By using a advanced resonator, a large fundamental mode size in the laser crystal and a tight focus in the nonlinear crystal could be obtained simultaneously, which are favorable for high power and high beam quality CW green laser generation. The green laser delivered a maximum 532 nm output power of 40 W. The corresponding optical-to-optical conversion efficiency and electrical-to-optical conversion efficiency were 8.6% and 5.0%, respectively. Under 532 nm output power of 34 W, the beam quality factor was measured to be 1.6. Crown Copyright Ó 2009 Published by Elsevier B.V. All rights reserved.
Keywords: Diode-side-pumped Continuous-wave Frequency doubled
1. Introduction Stable high power and high beam quality green lasers have been widely studied for various applications such as material processing [1], pumping tunable lasers and optical parametric oscillators [2], laser display [3], annealing and crystallization of glass and so on. At present, a main method to produce high power continuouswave (CW) green output is to take intracavity frequency doubled all-solid-state lasers. Compared with large-frame argon-ion lasers, which were the first to offer powers in excess of 30 W in a near diffraction-limited beam, intracavity frequency doubled all-solidstate lasers have higher efficiency and no need of high-flow-rate water cooling. Most of intracavity frequency doubled all-solidstate lasers are diode-end-pumped configuration. In this kind of system, high efficiency and high beam quality could be easily gotten. Diode-side-pumped intracavity frequency doubled all-solidstate lasers are another approach to generate high power green * Corresponding author. Address: Laboratory of Optical Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100080, China. Tel.: +86 10 82543473. E-mail address: [email protected] (X.-K. Cheng).
output, and it has potential to provide higher CW green output power. With this method, Mukhopadhyay et al. [4] have achieved 30.5 W green output power with a special laser head. However, their beam quality factor M2 was almost 20. By compensating thermal lensing of the nonlinear crystal and that of the Nd:YAG rods with an advanced resonator design, Kojima et al. [5] have reported 27 W green output power at 532 nm with M2 = 8. In our previous work, based on a resonator with two Nd:YAG rods in L-shaped concave–convex cavity, 23.2 W green output power with M2 = 4.1 was obtained [6]. However, all of those lasers adopted KTP as their nonlinear crystal, which has the effect of the gray-tracking and would lead to the output power declined as operation time increased. In this letter, we report a high power and high beam quality CW diode-side-pumped Nd:YAG intracavity frequency doubled all-solid-state laser based on LBO crystal. Compared with the Nd:YLF and Nd:YVO4, Nd:YAG cannot emit polarized light, but the YAG host is harder, of better optical quality, and has a higher thermal conductivity, which lead to the potential of providing higher fundamental frequency power. The main reason of choosing LBO as the frequency doubled crystal is that it not only has high damage threshold, wide acceptance angle and no effect of the gray-tracking but also can achieve type I temperature-tuned noncritical phase
0030-4018/$ - see front matter Crown Copyright Ó 2009 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2009.07.046
X.-K. Cheng et al. / Optics Communications 282 (2009) 4288–4291
matching (NCPM). There is no walk-off effect, which is very important for high power CW intracavity frequency doubled all-solidstate laser. As compared with Q-switch mode, CW mode has lower power density. In order to achieve high convert efficiency, we should focus a tight spot in the LBO crystal. In type II phase matching, the beam cannot be focused to too small since the walk-off effect, so it is hard to achieve high convert efficiency. Finally, we designed a resonator to provide a large fundamental mode volume in the rod for high fundamental power with high beam quality, and a tight focus inside the LBO crystal, which results in high fundamental power density as well as high nonlinear conversion efficiency. Moreover, the resonator has very low misalignment sensitivity for stable and reliable operation. As a result, the maximum of 40 W output power at 532 nm was obtained. Under the 532 nm output power of 34 W, the beam quality factor M2 was measured to be 1.6 and the fluctuation of the output power was 1.03% in 300 s.
2. Experiment setup The schematic diagram of the resonator is shown in Fig. 1. In order to obtain higher fundamental frequency power, we used two laser heads with a quartz 90° polarization rotator between them for thermally-induced birefringence compensation [7]. Each laser head consisted of a Nd:YAG rod (diameter, 3 mm; length, 76 mm; Nd doping, 0.7 at%) and three diode arrays. Each array contained four maximum 20 W CW diode bars. The rods were surrounded by glass tubes with cooling water, and the temperature of the water was maintained at 20 °C (±0.1 °C) [6]. The LBO crystal (3 3 20 mm3), with both faces antireflection coated at 1064 nm and 532 nm, was mounted in an oven for temperaturetuned NCPM, and the phase match temperature corresponding to the cutting angle of this LBO (H = 90°, U = 0°) was 150 °C. The convex end mirror M1 (R = 224 mm), which was coated at 1064 nm (R > 99.9%), was placed here to provide large fundamental mode volume in the rods. With this method, larger power of fundamental mode in the cavity can be obtained. Moreover, it makes the loss of the high order mode more serious than that of the fundamental mode, which contributes to achieving a high fundamental beam quality. The calculated result from ABCD matrix indicated that with this convex end mirror, the fundamental mode diameter at the end of the laser crystal was about 1.7 mm, which is the value needed for optimum filling of the rod. Plane mirror M2 as the polarizer (45°, R > 99.5%) was also coated at 1064 nm. Concave mirror M3 (R = 200 mm) as the harmonic-separator mirror was HR-coated at 1064 nm (R > 99.8%) and HT-coated at 532 nm (T > 98%). To create a focal point in the middle of the LBO crystal, we used a concave mirror M4 (R = 100) and a plane mirror M5, which were both HRcoated at 1064 nm (R > 99.8%) and HR-coated at 532 nm
4289
(R > 99.5%). Compared with a single concave end mirror, which was adopted in most of intracavity frequency doubled lasers traditionally, our cavity structure could not only provide the same small spot size in the middle of the nonlinear crystal, but also had a much lower misalignment sensitivity [8]. The waist radius in the middle of the LBO was about 50 lm. The small waist radius in the LBO leading to high power density, so high conversion efficiency can be achieved. The distance from M5 to the LBO should be adjusted to compensate for the dispersion of air, so the green beams generated in the two directions would combine in phase match. At last, we kept the tilting angle as small as possible (6°). Then the astigmatism effect could be neglected.
3. Result and discussion The laser was first operated without heating the LBO crystal, and mirror M5 was replaced by a mirror with 75% reflectivity for the 1064 nm (optimum output coupling). At a pump power of 465 W, the laser delivered a 72.3 W output power at 1064 nm. Then the LBO crystal was heated to the phase matching temperature and the output coupler was replaced by the dual high-reflectivity mirror M5. The laser provided a maximum output power of 40 W at 532 nm, corresponding to an IR-to-green conversion efficiency of 55.3%. The optical-to-optical conversion efficiency and electrical-to-optical conversion efficiency were 8.6% and 5.0%, respectively. The ‘‘green problem”, commonly known as noise, caused by longitudinal-mode coupling [9–12], is the major issue in the development of high power intracavity frequency doubled all-solid-state lasers. It causes the rise of power instabilities in the microsecond to millisecond range. At the present time, two methods are usually adopted to solve the problem, one of which is to make the laser operate in a single-longitudinal-mode, achieved by inserting a Fabry–Perot etalon in a unidirectional ring oscillator for singlemode selection. The other one is to design a long linear resonator to allow a large number of longitudinal-modes to oscillate in the cavity simultaneously, thus the effect of sum-frequency mixing between modes could be averaged out. If the single-longitudinalmode is not a requirement, we think the later method is more practical, as it requires fewer components. The total length of the resonator is 1010 mm and the corresponding longitudinal-mode spacing is 148.5 MHz. The optical spectral width of 1064 nm was 22 GHz, measured by a laser spectrum analyser (WS-7 HighFinesse Inc.), so there were 148 longitudinal-modes oscillating in this cavity simultaneously. The output power of 532 nm was monitored by a 1 ns risetime photodiode and a 1 GHz bandwidth oscilloscope. The RMS noise is 0.015%, as shown in Fig. 2a, as well as b, which is the close-up view. In order to demonstrate the effect of reducing noise by creasing the number of longitudinal-modes, we compare
Fig. 1. Schematic diagram of the green laser system.
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X.-K. Cheng et al. / Optics Communications 282 (2009) 4288–4291
1.01
1.01
a
c
Green power (a.u.)
1.00
1.00
0.99 0
4
8
12
16
20
1.001
0.99 0
4
8
12
16
20
1.001
b
d
RMS=0.015%
1.000
0.999 0.0
RMS=0.025%
1.000
0.1
0.2
0.3
0.4
0.999 0.0
0.1
0.2
0.3
0.4
Time (ms) Fig. 2. Oscilloscope trace of the SH power: (a) shows the noise observed from this laser and (b) is close-up view; (c) shows the noise observed from previous one and (d) is the close-up view.
Green Output Power (W)
40
30
20
10
0
0
50
100
150
200
250
300
Time (s) Fig. 4. Beam 2D profile of 532 nm for 34 W output. The inset is the 3D profile. Fig. 3. Stability measurement of 532 nm for 34 W output.
the noise with that in the previous laser [6]. Being operated under the same pump power, it has the same optical spectral width, differing only in the length of the cavity of 800 mm. Under this circumstance, the corresponding longitudinal-mode spacing is 187.5 MHz, thus 117 longitudinal-modes oscillated in the cavity. The RMS noise was 0.025%, as shown in Fig. 2c and d is the close-up view. The result indicates that as the number of longitudinal-modes increasing the noise reduces significantly. Besides the noise problem, we should also pay some attention to the fluctuation of the output power. In fact, both of them are indicating the stability of the output power, differentiated only in time range. Specifically saying, the former one refers to the instability of the output power in the microsecond to millisecond range, while the latter one, above second range. There are in all three aspects that call for special attention, and the cooling water’s temperature comes first. It is known to all that the 532 nm output
power will be dramatically affected by the changes of the Nd:YAG rods’ thermal focal length. Through experiments, we found that when the fluctuation of the cooling water’s temperature was ±0.5 °C, the 532 nm output power fluctuated seriously, while the fluctuation of the temperature was ±0.1 °C, the 532 nm output power remained constant. Secondly, the convert efficiency of the LBO crystal is very sensitive to its temperature, so it also should be strictly controlled (±0.1 °C). At last, the laser should be operated in the bottom of the stable zone, where the beam diameter on the ends of the YAG rods has a low sensitivity to the laser rods’ thermal focal length. With the methods mentioned above, when the 532 nm output power was 34 W, the fluctuation was 1.03% over 300 s. The stable line is shown in Fig. 3. In order to display the changing process of the output power in great detail, we read the output power value by the power meter with a step of 5 s for 300 s. The beam profile was shown in Fig. 4, and the corresponding beam quality factor M2 was 1.6 (M2–200 Spiricon Inc.).
X.-K. Cheng et al. / Optics Communications 282 (2009) 4288–4291
4. Conclusion In summary, we have designed an intracavity frequency doubled Nd:YAG/LBO laser. As a result, the maximum of 40 W output power at 532 nm was obtained. When the 532 nm output power was 34 W, the beam quality factor M2 was measured to be 1.6 and the fluctuation of the output power was 1.03% over 300 s. Furthermore, it had very low noise and very low misalignment sensitivity. In fact, higher output power at 532 nm could be achieved while higher pump power diodes were adopted. Acknowledgments This work is supported by the National Basic Research Programme of China (Grant Nos. 2004CB619006 and 2004CB619001),
4291
and the National Natural Science Foundation of China (Grant Nos. 60578030 and 50590404). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12]
J. Golden, Laser Focus World 28 (1992) 75. H.Q. Li, H.B. Zhang, Z.Y. Xu, et al., Opt. Commun. 232 (2004) 411. Z.Y. Xu, Y. Bi, Proc. SPIE 5632 (2005) 115. P.K. Mukhopadhyay, S.K. Sharma, T.P.S. Nathan, et al., Opt. Commun. 259 (2006) 805. T.S. Kojima, S. Fujikawa, K. Yasui, IEEE J. Quantum Electron. 35 (1999) 377. X.K. Cheng, Y. Zhou, Q.J. Cui, et al., Chin. Phys. Lett. 26 (2009) 034201. S. Konno, S. Fujikawa, K. Yasui, Appl. Phys. Lett. 70 (1997) 2650. V. Magni, G. Cerullo, et al., Opt. Lett. 18 (1993) 2111. M. Oka, S. Kubota, Opt. Lett. 13 (1988) 805. G.E. James, E.M. Harrel II, R. Roy, Opt. Lett. 15 (1990) 1141. D.W. Anthon, D.L. Sipes, IEEE J. Quantum Electron. 28 (1992) 1148. T. Baer, J. Opt. Soc. Am. B 3 (1986) 1175.
Contents lists available at ScienceDirect
Optics Communications journal homepage: www.elsevier.com/locate/optcom
High power and high beam quality CW green beam generated by diode-side-pumped intracavity frequency doubled Nd:YAG laser Xian-Kun Cheng a,c,*, Qian-Jin Cui a,c, Yong Zhou a,c, Zhi-Min Wang b, Jia-Lin Xu b, Yong Bo b, Qin-Jun Peng b, Da-Fu Cui a,b, Zu-Yan Xu a,b a b c
Laboratory of Optical Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China Research Center for Laser Physics and Technique, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, China Graduate School of the Chinese Academy of Sciences, Beijing 100190, China
a r t i c l e
i n f o
Article history: Received 4 March 2009 Received in revised form 21 July 2009 Accepted 23 July 2009
PACS: 42.55.Rz 42.55.Xi 42.65.Ky
a b s t r a c t We report a stable high power and high beam quality diode-side-pumped CW green laser from intracavity frequency doubled Nd:YAG laser with LBO crystal. By using a advanced resonator, a large fundamental mode size in the laser crystal and a tight focus in the nonlinear crystal could be obtained simultaneously, which are favorable for high power and high beam quality CW green laser generation. The green laser delivered a maximum 532 nm output power of 40 W. The corresponding optical-to-optical conversion efficiency and electrical-to-optical conversion efficiency were 8.6% and 5.0%, respectively. Under 532 nm output power of 34 W, the beam quality factor was measured to be 1.6. Crown Copyright Ó 2009 Published by Elsevier B.V. All rights reserved.
Keywords: Diode-side-pumped Continuous-wave Frequency doubled
1. Introduction Stable high power and high beam quality green lasers have been widely studied for various applications such as material processing [1], pumping tunable lasers and optical parametric oscillators [2], laser display [3], annealing and crystallization of glass and so on. At present, a main method to produce high power continuouswave (CW) green output is to take intracavity frequency doubled all-solid-state lasers. Compared with large-frame argon-ion lasers, which were the first to offer powers in excess of 30 W in a near diffraction-limited beam, intracavity frequency doubled all-solidstate lasers have higher efficiency and no need of high-flow-rate water cooling. Most of intracavity frequency doubled all-solidstate lasers are diode-end-pumped configuration. In this kind of system, high efficiency and high beam quality could be easily gotten. Diode-side-pumped intracavity frequency doubled all-solidstate lasers are another approach to generate high power green * Corresponding author. Address: Laboratory of Optical Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100080, China. Tel.: +86 10 82543473. E-mail address: [email protected] (X.-K. Cheng).
output, and it has potential to provide higher CW green output power. With this method, Mukhopadhyay et al. [4] have achieved 30.5 W green output power with a special laser head. However, their beam quality factor M2 was almost 20. By compensating thermal lensing of the nonlinear crystal and that of the Nd:YAG rods with an advanced resonator design, Kojima et al. [5] have reported 27 W green output power at 532 nm with M2 = 8. In our previous work, based on a resonator with two Nd:YAG rods in L-shaped concave–convex cavity, 23.2 W green output power with M2 = 4.1 was obtained [6]. However, all of those lasers adopted KTP as their nonlinear crystal, which has the effect of the gray-tracking and would lead to the output power declined as operation time increased. In this letter, we report a high power and high beam quality CW diode-side-pumped Nd:YAG intracavity frequency doubled all-solid-state laser based on LBO crystal. Compared with the Nd:YLF and Nd:YVO4, Nd:YAG cannot emit polarized light, but the YAG host is harder, of better optical quality, and has a higher thermal conductivity, which lead to the potential of providing higher fundamental frequency power. The main reason of choosing LBO as the frequency doubled crystal is that it not only has high damage threshold, wide acceptance angle and no effect of the gray-tracking but also can achieve type I temperature-tuned noncritical phase
0030-4018/$ - see front matter Crown Copyright Ó 2009 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2009.07.046
X.-K. Cheng et al. / Optics Communications 282 (2009) 4288–4291
matching (NCPM). There is no walk-off effect, which is very important for high power CW intracavity frequency doubled all-solidstate laser. As compared with Q-switch mode, CW mode has lower power density. In order to achieve high convert efficiency, we should focus a tight spot in the LBO crystal. In type II phase matching, the beam cannot be focused to too small since the walk-off effect, so it is hard to achieve high convert efficiency. Finally, we designed a resonator to provide a large fundamental mode volume in the rod for high fundamental power with high beam quality, and a tight focus inside the LBO crystal, which results in high fundamental power density as well as high nonlinear conversion efficiency. Moreover, the resonator has very low misalignment sensitivity for stable and reliable operation. As a result, the maximum of 40 W output power at 532 nm was obtained. Under the 532 nm output power of 34 W, the beam quality factor M2 was measured to be 1.6 and the fluctuation of the output power was 1.03% in 300 s.
2. Experiment setup The schematic diagram of the resonator is shown in Fig. 1. In order to obtain higher fundamental frequency power, we used two laser heads with a quartz 90° polarization rotator between them for thermally-induced birefringence compensation [7]. Each laser head consisted of a Nd:YAG rod (diameter, 3 mm; length, 76 mm; Nd doping, 0.7 at%) and three diode arrays. Each array contained four maximum 20 W CW diode bars. The rods were surrounded by glass tubes with cooling water, and the temperature of the water was maintained at 20 °C (±0.1 °C) [6]. The LBO crystal (3 3 20 mm3), with both faces antireflection coated at 1064 nm and 532 nm, was mounted in an oven for temperaturetuned NCPM, and the phase match temperature corresponding to the cutting angle of this LBO (H = 90°, U = 0°) was 150 °C. The convex end mirror M1 (R = 224 mm), which was coated at 1064 nm (R > 99.9%), was placed here to provide large fundamental mode volume in the rods. With this method, larger power of fundamental mode in the cavity can be obtained. Moreover, it makes the loss of the high order mode more serious than that of the fundamental mode, which contributes to achieving a high fundamental beam quality. The calculated result from ABCD matrix indicated that with this convex end mirror, the fundamental mode diameter at the end of the laser crystal was about 1.7 mm, which is the value needed for optimum filling of the rod. Plane mirror M2 as the polarizer (45°, R > 99.5%) was also coated at 1064 nm. Concave mirror M3 (R = 200 mm) as the harmonic-separator mirror was HR-coated at 1064 nm (R > 99.8%) and HT-coated at 532 nm (T > 98%). To create a focal point in the middle of the LBO crystal, we used a concave mirror M4 (R = 100) and a plane mirror M5, which were both HRcoated at 1064 nm (R > 99.8%) and HR-coated at 532 nm
4289
(R > 99.5%). Compared with a single concave end mirror, which was adopted in most of intracavity frequency doubled lasers traditionally, our cavity structure could not only provide the same small spot size in the middle of the nonlinear crystal, but also had a much lower misalignment sensitivity [8]. The waist radius in the middle of the LBO was about 50 lm. The small waist radius in the LBO leading to high power density, so high conversion efficiency can be achieved. The distance from M5 to the LBO should be adjusted to compensate for the dispersion of air, so the green beams generated in the two directions would combine in phase match. At last, we kept the tilting angle as small as possible (6°). Then the astigmatism effect could be neglected.
3. Result and discussion The laser was first operated without heating the LBO crystal, and mirror M5 was replaced by a mirror with 75% reflectivity for the 1064 nm (optimum output coupling). At a pump power of 465 W, the laser delivered a 72.3 W output power at 1064 nm. Then the LBO crystal was heated to the phase matching temperature and the output coupler was replaced by the dual high-reflectivity mirror M5. The laser provided a maximum output power of 40 W at 532 nm, corresponding to an IR-to-green conversion efficiency of 55.3%. The optical-to-optical conversion efficiency and electrical-to-optical conversion efficiency were 8.6% and 5.0%, respectively. The ‘‘green problem”, commonly known as noise, caused by longitudinal-mode coupling [9–12], is the major issue in the development of high power intracavity frequency doubled all-solid-state lasers. It causes the rise of power instabilities in the microsecond to millisecond range. At the present time, two methods are usually adopted to solve the problem, one of which is to make the laser operate in a single-longitudinal-mode, achieved by inserting a Fabry–Perot etalon in a unidirectional ring oscillator for singlemode selection. The other one is to design a long linear resonator to allow a large number of longitudinal-modes to oscillate in the cavity simultaneously, thus the effect of sum-frequency mixing between modes could be averaged out. If the single-longitudinalmode is not a requirement, we think the later method is more practical, as it requires fewer components. The total length of the resonator is 1010 mm and the corresponding longitudinal-mode spacing is 148.5 MHz. The optical spectral width of 1064 nm was 22 GHz, measured by a laser spectrum analyser (WS-7 HighFinesse Inc.), so there were 148 longitudinal-modes oscillating in this cavity simultaneously. The output power of 532 nm was monitored by a 1 ns risetime photodiode and a 1 GHz bandwidth oscilloscope. The RMS noise is 0.015%, as shown in Fig. 2a, as well as b, which is the close-up view. In order to demonstrate the effect of reducing noise by creasing the number of longitudinal-modes, we compare
Fig. 1. Schematic diagram of the green laser system.
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X.-K. Cheng et al. / Optics Communications 282 (2009) 4288–4291
1.01
1.01
a
c
Green power (a.u.)
1.00
1.00
0.99 0
4
8
12
16
20
1.001
0.99 0
4
8
12
16
20
1.001
b
d
RMS=0.015%
1.000
0.999 0.0
RMS=0.025%
1.000
0.1
0.2
0.3
0.4
0.999 0.0
0.1
0.2
0.3
0.4
Time (ms) Fig. 2. Oscilloscope trace of the SH power: (a) shows the noise observed from this laser and (b) is close-up view; (c) shows the noise observed from previous one and (d) is the close-up view.
Green Output Power (W)
40
30
20
10
0
0
50
100
150
200
250
300
Time (s) Fig. 4. Beam 2D profile of 532 nm for 34 W output. The inset is the 3D profile. Fig. 3. Stability measurement of 532 nm for 34 W output.
the noise with that in the previous laser [6]. Being operated under the same pump power, it has the same optical spectral width, differing only in the length of the cavity of 800 mm. Under this circumstance, the corresponding longitudinal-mode spacing is 187.5 MHz, thus 117 longitudinal-modes oscillated in the cavity. The RMS noise was 0.025%, as shown in Fig. 2c and d is the close-up view. The result indicates that as the number of longitudinal-modes increasing the noise reduces significantly. Besides the noise problem, we should also pay some attention to the fluctuation of the output power. In fact, both of them are indicating the stability of the output power, differentiated only in time range. Specifically saying, the former one refers to the instability of the output power in the microsecond to millisecond range, while the latter one, above second range. There are in all three aspects that call for special attention, and the cooling water’s temperature comes first. It is known to all that the 532 nm output
power will be dramatically affected by the changes of the Nd:YAG rods’ thermal focal length. Through experiments, we found that when the fluctuation of the cooling water’s temperature was ±0.5 °C, the 532 nm output power fluctuated seriously, while the fluctuation of the temperature was ±0.1 °C, the 532 nm output power remained constant. Secondly, the convert efficiency of the LBO crystal is very sensitive to its temperature, so it also should be strictly controlled (±0.1 °C). At last, the laser should be operated in the bottom of the stable zone, where the beam diameter on the ends of the YAG rods has a low sensitivity to the laser rods’ thermal focal length. With the methods mentioned above, when the 532 nm output power was 34 W, the fluctuation was 1.03% over 300 s. The stable line is shown in Fig. 3. In order to display the changing process of the output power in great detail, we read the output power value by the power meter with a step of 5 s for 300 s. The beam profile was shown in Fig. 4, and the corresponding beam quality factor M2 was 1.6 (M2–200 Spiricon Inc.).
X.-K. Cheng et al. / Optics Communications 282 (2009) 4288–4291
4. Conclusion In summary, we have designed an intracavity frequency doubled Nd:YAG/LBO laser. As a result, the maximum of 40 W output power at 532 nm was obtained. When the 532 nm output power was 34 W, the beam quality factor M2 was measured to be 1.6 and the fluctuation of the output power was 1.03% over 300 s. Furthermore, it had very low noise and very low misalignment sensitivity. In fact, higher output power at 532 nm could be achieved while higher pump power diodes were adopted. Acknowledgments This work is supported by the National Basic Research Programme of China (Grant Nos. 2004CB619006 and 2004CB619001),
4291
and the National Natural Science Foundation of China (Grant Nos. 60578030 and 50590404). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12]
J. Golden, Laser Focus World 28 (1992) 75. H.Q. Li, H.B. Zhang, Z.Y. Xu, et al., Opt. Commun. 232 (2004) 411. Z.Y. Xu, Y. Bi, Proc. SPIE 5632 (2005) 115. P.K. Mukhopadhyay, S.K. Sharma, T.P.S. Nathan, et al., Opt. Commun. 259 (2006) 805. T.S. Kojima, S. Fujikawa, K. Yasui, IEEE J. Quantum Electron. 35 (1999) 377. X.K. Cheng, Y. Zhou, Q.J. Cui, et al., Chin. Phys. Lett. 26 (2009) 034201. S. Konno, S. Fujikawa, K. Yasui, Appl. Phys. Lett. 70 (1997) 2650. V. Magni, G. Cerullo, et al., Opt. Lett. 18 (1993) 2111. M. Oka, S. Kubota, Opt. Lett. 13 (1988) 805. G.E. James, E.M. Harrel II, R. Roy, Opt. Lett. 15 (1990) 1141. D.W. Anthon, D.L. Sipes, IEEE J. Quantum Electron. 28 (1992) 1148. T. Baer, J. Opt. Soc. Am. B 3 (1986) 1175.