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High-power diode-end-pumped CW Nd:GdVO4 laser
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Optics & Laser Technology 37 (2004) 51 – 54 www.elsevier.com/locate/optlastec
High-power diode-end-pumped CW Nd:GdVO4 laser J. Konga;∗ , D.Y. Tanga , S.P. Nga , L.M. Zhaoa , L.J. Qinb , X.L. Mengb a School
of Electrical and Electronic Engineering, Nanyang Technological University, Singapore 639798, Singapore b National Laboratory of Crystal Materials, Shangdong University, Jinan 250100, China Received 13 October 2003; received in revised form 5 February 2004; accepted 6 February 2004
Abstract We report on a high-power diode-end-pumped CW Nd:GdVO4 laser. Under the pump power of 39:5 W, a maximum output power of 19:8 W and a slope eAciency of 58.5% were obtained. The beam quality M 2 at maximum output power was measured to be around 2.62. The thermal focal length in Nd:GdVO4 crystal under the pump power from 22 to 40:6 W was measured. The pump-induced damage threshold was estimated to be 28:6 kW=cm2 . ? 2004 Elsevier Ltd. All rights reserved. Keywords: Nd:GdVO4 laser; Diode-end-pumped; Thermal lens eGect
Recently, the Nd 3+ -doped GdVO4 crystal has attracted considerable attention as a novel diode-pumped solid-state laser material. The Nd:GdVO4 crystal was Irst developed by Zaguniennyi et al. in 1992 [1]. It has the same space group as Nd:YVO4 [2]. The zircon structure neodymium-doped vanadate crystal Nd:YVO4 has proved to be an excellent laser material [3]. In comparison with the Nd:YVO4 crystal, Nd:GdVO4 has a even higher absorption coeAcient and larger emission cross-section [4]. The most attracting property of the Nd:GdVO4 crystal is its excellent thermal conductivity which is about two times larger than that of the Nd:YVO4 [2]. Therefore, the Nd:GdVO4 is more suitable to be used in high-power diode-pumped solid-state laser systems. Up to now, signiIcant progress has been achieved on high-power diode-pumped Nd:GdVO4 lasers [5–7]. In particular, Liu et al. reported a 14:3 W CW output in a diode-end-pumped Nd:GdVO4 laser in 1999 [8]. To our knowledge, this is the highest CW output power of the end-pumped Nd:GdVO4 lasers ever reported. However, limited by the Inite pump power, the potential of the Nd:GdVO4 as a high-power laser crystal was not fully exploited in their experiment. In this paper, we report on a high-power diode-endpumped CW Nd:GdVO4 laser. Under the pump power of 39:5 W, a maximum output power of 19:8 W and a
∗
Corresponding author. Tel.: +65-67904036; fax: +65-67904161. E-mail address: j [email protected] (J. Kong).
0030-3992/$ - see front matter ? 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.optlastec.2004.02.008
general slope eAciency of 58.5% were obtained. The slope eAciency increased to 63% if only the linear output part was considered. The beam quality M 2 at maximum output power was measured to be 2:62 ± 0:14. The thermal focal length in Nd:GdVO4 crystal under the pump power from 22 to 40:6 W was measured and the pump-induced damage threshold was estimated. The thermal properties of laser crystals are very important for laser applications. In a high-power Nd:GdVO4 crystal laser, pump-induced thermal lens eGect usually introduces signiIcant diGraction loss to the cavity, which greatly aGects the laser performance. The cavity stability and beam radius are also determined by the thermal focal length of the laser media. For the end-pumped Nd:GdVO4 laser, the thermal focal length under pump power from 0 to 26 W has been well addressed [9]. However, the experimental investigation on the thermal focal length of Nd:GdVO4 crystals under higher pump power levels has not been reported. Therefore, to design and optimize the cavity conIguration of the Nd:GdVO4 laser, the thermal focal lengths of Nd:GdVO4 under various pump power levels were Irstly measured. The laser setup used in our experiment is schematically shown in Fig. 1. A commercial 60 W laser-diode-bar (Apollo F60-808-6) was used as the pump source. The pump light was focused into the Nd:GdVO4 sample by two coupling lenses. The focused pump beam in the laser medium has a minimum diameter of about 400 m. The Nd:GdVO4 crystal sample in the laser has a Nd-doping concentration of 0.5 at%. It has a dimension of 3×3 mm in cross-section and
52
J. Kong et al. / Optics & Laser Technology 37 (2004) 51 – 54
of the laser medium; R1 and R2 are the radii of curvature of input mirror (M1) and output mirror (M2), respectively; d1 and d2 are the distance from the laser crystal to M1 and M2, respectively. The stable condition of the resonator in this case will be
Laser crystal
Coupling lenses
0 ¡ g1∗ g2∗ ¡ 1: LD
Input Mirror Water-chilling holder
Output Mirror
Fig. 1. Schematic of the laser setup. LD: laser diode.
In our laser, d1 equals zero and d2 equals L. R1 and R2 are both inInite for Qat mirrors. Then the stable condition for our laser can be achieved by 0¡1 −
d2
M1
M2
Crystal
R1
fT
R2
L=d1+d2 Fig. 2. Schematic of a two-mirror cavity with the internal thermal lens.
4 mm in length. The sample is water-cooled to 13:8◦ C. One side of the sample is HR coated at 1064 nm and AR coated at 808 nm, which served as the input mirror (M1). The other side is AR coated both at 1064 and 808 nm. A Qat mirror (M2) with 90% reQectivity at 1064 nm was used as the output coupler. The thermal focal lengths of the Nd:GdVO4 sample at various pump power levels were Irstly measured. The method used is simple but was proved to be eGective in determining the thermal focal length of the Nd:YVO4 lasers [10]. According to the stability theory of laser resonators, for a given cavity conIguration in which thermal lens eGect exists, the stable condition will be determined by the thermal focal length. For a two-mirror resonator with an internal thin thermal lens, the equivalent g-parameters can be written as [11] d1 d2 g1∗ = g1 − 1− ; fT R1 d2 d1 1− ; g2∗ = g2 − fT R2 g1 = 1 − L=R1 ;
L ¡ 1: fT
(3)
From Eq. (3) it can be concluded that the resonator will keep stable only if the cavity length is shorter than the pumpinduced thermal focal length. For a Ixed cavity length, if we increase the pump power, the laser output power would Irst increase and then fall oG when the thermal focal length becomes less than the cavity length. Therefore, the thermal focal length at diGerent pump power levels can be simply determined by observing the relationship between the output power and the pump power. By use of this method, the refractive power as a function of the pump power was measured and shown in Fig. 3. From this Igure, one can Ind that when the pump power increased from 22 to 40:6 W, the thermal focal length decreased from 22 to 6 cm. It is worth noting that the thermal focal length at a pump power of 40:6 W obviously departures with those at other pump powers, which suggests the crystal structure might have been broken under this pump power level. To verify this, we measured the thermal focal length again with the same crystal. It was found that the laser slope eAciency was dramatically reduced and the results were totally diGerent with the data previously measured. This proved that the laser crystal had been broken and the result indicates that in our laser the maximum pump power that the crystal can endure could be around 40:6 W. 20
Refractive Power (1/m)
d1
(2)
16
12
8
g2 = 1 − L=R2 ; L = d1 + d2 :
(1)
The terms used in Eq. (1) is schematically shown in Fig. 2, where L is the cavity length; fT is the thermal focal length
4 25
30
35
Pump Power (W) Fig. 3. The refractive power versus the pump power.
40
J. Kong et al. / Optics & Laser Technology 37 (2004) 51 – 54
20
Output Power (W)
Slope efficiency=58.5% 15
10
5
0 0
5
10
15
20
25
30
35
40
45
Pump Power (W) Fig. 4. Output power versus pump power relationship of the laser.
Based on the measured thermal focal length a Qat–Qat cavity conIguration was designed for the laser. Normally Qat–Qat cavity is seldom used in end-pumped solid-state lasers as it works at the boundary of the stable condition. To select it we have made the following considerations. Firstly, under high-pump power levels the pump area in the laser crystal should be kept as large as possible to avoid possible pump-induced crystal damage. For a better mode matching, a large laser mode size is thus needed. Secondly, because of the existence of strong thermal lens eGect, the stable state of Qat–Qat cavity will be driven deep into the stable region. Consequently, a stable output can be achieved in spite of that a Qat–Qat cavity was used. Finally, the Qat–Qat cavity can oGer a better beam quality as compared with the concave– plano cavity. It is because a plano–concave cavity can result in a comparable beam radius in the laser material if the thermal focal length is considered, which greatly decreases the mode-matching eAciency between pump light and the laser. As can be seen in Fig. 3, with a 40 W pump power the thermal focal length can be reduced to 6 cm, which means that to achieve a stable output power under high-pump power, a short cavity less than 6 cm is needed in the laser. With an appropriate alignment of the laser cavity, stable 1064 nm laser emission has been achieved. Fig. 4 shows the measured output power versus pump power in the laser. The laser had a threshold of about 3:6 W and a general slope eAciency of 58.5% when a Qat output coupler with 90% reQectivity was used. It is to note that with the increase of the pump power, the slope eAciency also increased. The slope eAciency was 62.2% in the range of pump power higher than 10 W and further increased to 63% when the pump power was higher than 15 W. The increased slope eAciency was attributed to an improved mode-matching eAciency between laser mode and pump light, which was caused by the reduced thermal focal length of the laser crystal. With a pump power of 39:5 W, a maximum output power of 19:8 W was obtained. By use of a laser beam proIler, the far-Ield beam radius was measured. It is shown that the laser beam had a
53
beam diameter of 5 mm at the distance of 0:6 m away from the output coupler. The beam radius at the output coupler was about 106 m. The beam quality M 2 thus was calculated to be 2:62 ± 0:14. It is worth mentioning that in our laser, the distribution of the pump light in the laser medium is a ring shape instead of a Gaussian shape or a uniform distribution. The light intensity in the center of pump beam spot is slightly lower than that in the edge of the pump beam spot. This kind of pump light distribution may be due to a bad coupling when the pump laser light is coupled into the Iber. This ring-shape pump light distribution depressed the generation of low-order laser modes and caused a bad beam quality. Consequently, the M 2 of the beam was higher than that reported in Ref. [8] even though a Qat–Qat cavity was used in our laser. We also investigated the relationship between the output power and the cavity length. Theoretically, the optimum cavity length can be selected by keeping an optimum ratio of pump spot size and mode diameter. If the thermal focal length is taken in account, according to the ABCD law, the optimum cavity length can be determined. Our simulation results shows that when the cavity length was about 5:0 cm, the mode diameter is about 400 m in the laser crystal, which is close to the pump spot size. However, experimentally, the maximum output power was achieved when the laser cavity length was set at about 2:5 cm. In this case, the mode diameter in the crystal is only about 270 m, which is far smaller than the pump spot size. Both reducing and increasing from this length would cause a decrease of the laser slope eAciency. Why the best performance of the laser was achieved at this cavity length needs to be further studied. From Fig. 4 it seems that the laser output power could be further increased if the pump power increased. However, in our experiments the Nd:GdVO4 sample was broken when we raised the pump power to 41 W. This coincides with the phenomenon observed in the thermal focal length measurement, which suggests that the pump-induced damage threshold of the crystal could be about 28:6 kW=cm2 . In conclusion, a diode-end-pumped high-power Nd: GdVO4 crystal laser has been demonstrated. The laser has a threshold of about 3:6 W and a general slope eAciency of 58.5%. In the range of linear output part, namely when the pump power is higher than 15 W, the slope eAciency increases to 63%. With 39:5 W pump power at 808 nm a maximum output power of 19:8 W at 1064 nm has been obtained. To our knowledge, both the output power and the slope eAciency for linear output part were the highest ever reported for end-pumped Nd:GdVO4 lasers. The beam quality M 2 at maximum output power was measured to be around 2.62. The thermal focal length in Nd:GdVO4 crystal under various pump power levels was also measured. Our experiments show that the thermal focal length changed from 22 to 6 cm when the pump power increased from 22 to 40:6 W. The Nd:GdVO4 crystal in our laser was broken when the pump power was increased to 41 W. The pump-induced damage threshold thus was estimated to be 28:6 kW=cm2 . Our
54
J. Kong et al. / Optics & Laser Technology 37 (2004) 51 – 54
experimental results further reveal the promising potential of Nd:GdVO4 crystals as a high-power laser material. References [1] Zagumennyi AI, Ostroumov VG, Shcherbakov IA, Jenson T, Meyn JP, Huber G. Nd:GdVO4 crystal: a new material for diode-pumped lasers. Sov J Quant Electron 1992;22:1071–2. [2] Zhang H, Meng XL, Zhu L, Yang ZH. Growth and thermal properties of Nd:GdVO4 single crystal. Mater Res Bull 1999;34:1589–93. [3] Fields RA, Birnbaum M, Fincher CL. Highly eAcient Nd:YVO4 diode-laser end-pumped laser. Appl Phys Lett 1987;51:1885–6. [4] Liu J, Shao Z, Zhang H, Meng XL, Zhu L, Jiang M. High-power end-pumped CW Nd:GdVO4 laser formed with a Qat–Qat cavity. Opt Laser Technol 1999;31:459–62. [5] Shcherbakov IA, Zagumennyi AI. Characterization of Nd:GdVO4 crystals for high-eAciency diode-pumped lasers. Proc SPIE Int Soc Opt Eng 1995;2498:241–9.
[6] Czeranowsky C, Schmidt M, Heumann E, Huber G, Kutovoi S, Zavartsev Y. Continuous wave diode-pumped intracavity doubled Nd:GdVO4 laser with 840 mW output power at 456 nm. Opt Comm 2002;205:361–5. [7] Wyss CP, Luthy W, Weber HP, Vlasov VI, Zavartsev YD, Studenikin PA, Zagumennyi AI, Shcherbakov IA. Performance of a diode-pumped 5 W Nd3+ :GdVO4 microchip laser at 1:06 m. Appl Phys B 1999;68:659–61. [8] Liu J, Shao Z, Zhang H, Meng XL, Zhu L, Jiang M. Diode-laser-array end-pumped 14:3 W CW Nd:GdVO4 solid-state laser at 1:06 m. Appl Phys B 1999;69:241–3. [9] Zhang HJ, Liu J, Wang JY, Wang CQ, Zhu L, Shao ZS, Meng XL, Hu XB, Jiang MH, Chow YT. Characterization of the laser crystal Nd:GdVO4 . J Opt Soc Am B 2002;19:18–27. [10] Song F, Zhang C, Ding X, Xu J, Zhang G, Leigh M, Peyghambarian N. Determination of thermal focal length and pumping radius in gain medium in laser-diode-pumped Nd:YVO4 Lasers. Appl Phys Lett 2002;81:2145–7. [11] Hodgson N, Weber H. Optical resonators. London: Springer; 1997 [Chapter 12].
Optics & Laser Technology 37 (2004) 51 – 54 www.elsevier.com/locate/optlastec
High-power diode-end-pumped CW Nd:GdVO4 laser J. Konga;∗ , D.Y. Tanga , S.P. Nga , L.M. Zhaoa , L.J. Qinb , X.L. Mengb a School
of Electrical and Electronic Engineering, Nanyang Technological University, Singapore 639798, Singapore b National Laboratory of Crystal Materials, Shangdong University, Jinan 250100, China Received 13 October 2003; received in revised form 5 February 2004; accepted 6 February 2004
Abstract We report on a high-power diode-end-pumped CW Nd:GdVO4 laser. Under the pump power of 39:5 W, a maximum output power of 19:8 W and a slope eAciency of 58.5% were obtained. The beam quality M 2 at maximum output power was measured to be around 2.62. The thermal focal length in Nd:GdVO4 crystal under the pump power from 22 to 40:6 W was measured. The pump-induced damage threshold was estimated to be 28:6 kW=cm2 . ? 2004 Elsevier Ltd. All rights reserved. Keywords: Nd:GdVO4 laser; Diode-end-pumped; Thermal lens eGect
Recently, the Nd 3+ -doped GdVO4 crystal has attracted considerable attention as a novel diode-pumped solid-state laser material. The Nd:GdVO4 crystal was Irst developed by Zaguniennyi et al. in 1992 [1]. It has the same space group as Nd:YVO4 [2]. The zircon structure neodymium-doped vanadate crystal Nd:YVO4 has proved to be an excellent laser material [3]. In comparison with the Nd:YVO4 crystal, Nd:GdVO4 has a even higher absorption coeAcient and larger emission cross-section [4]. The most attracting property of the Nd:GdVO4 crystal is its excellent thermal conductivity which is about two times larger than that of the Nd:YVO4 [2]. Therefore, the Nd:GdVO4 is more suitable to be used in high-power diode-pumped solid-state laser systems. Up to now, signiIcant progress has been achieved on high-power diode-pumped Nd:GdVO4 lasers [5–7]. In particular, Liu et al. reported a 14:3 W CW output in a diode-end-pumped Nd:GdVO4 laser in 1999 [8]. To our knowledge, this is the highest CW output power of the end-pumped Nd:GdVO4 lasers ever reported. However, limited by the Inite pump power, the potential of the Nd:GdVO4 as a high-power laser crystal was not fully exploited in their experiment. In this paper, we report on a high-power diode-endpumped CW Nd:GdVO4 laser. Under the pump power of 39:5 W, a maximum output power of 19:8 W and a
∗
Corresponding author. Tel.: +65-67904036; fax: +65-67904161. E-mail address: j [email protected] (J. Kong).
0030-3992/$ - see front matter ? 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.optlastec.2004.02.008
general slope eAciency of 58.5% were obtained. The slope eAciency increased to 63% if only the linear output part was considered. The beam quality M 2 at maximum output power was measured to be 2:62 ± 0:14. The thermal focal length in Nd:GdVO4 crystal under the pump power from 22 to 40:6 W was measured and the pump-induced damage threshold was estimated. The thermal properties of laser crystals are very important for laser applications. In a high-power Nd:GdVO4 crystal laser, pump-induced thermal lens eGect usually introduces signiIcant diGraction loss to the cavity, which greatly aGects the laser performance. The cavity stability and beam radius are also determined by the thermal focal length of the laser media. For the end-pumped Nd:GdVO4 laser, the thermal focal length under pump power from 0 to 26 W has been well addressed [9]. However, the experimental investigation on the thermal focal length of Nd:GdVO4 crystals under higher pump power levels has not been reported. Therefore, to design and optimize the cavity conIguration of the Nd:GdVO4 laser, the thermal focal lengths of Nd:GdVO4 under various pump power levels were Irstly measured. The laser setup used in our experiment is schematically shown in Fig. 1. A commercial 60 W laser-diode-bar (Apollo F60-808-6) was used as the pump source. The pump light was focused into the Nd:GdVO4 sample by two coupling lenses. The focused pump beam in the laser medium has a minimum diameter of about 400 m. The Nd:GdVO4 crystal sample in the laser has a Nd-doping concentration of 0.5 at%. It has a dimension of 3×3 mm in cross-section and
52
J. Kong et al. / Optics & Laser Technology 37 (2004) 51 – 54
of the laser medium; R1 and R2 are the radii of curvature of input mirror (M1) and output mirror (M2), respectively; d1 and d2 are the distance from the laser crystal to M1 and M2, respectively. The stable condition of the resonator in this case will be
Laser crystal
Coupling lenses
0 ¡ g1∗ g2∗ ¡ 1: LD
Input Mirror Water-chilling holder
Output Mirror
Fig. 1. Schematic of the laser setup. LD: laser diode.
In our laser, d1 equals zero and d2 equals L. R1 and R2 are both inInite for Qat mirrors. Then the stable condition for our laser can be achieved by 0¡1 −
d2
M1
M2
Crystal
R1
fT
R2
L=d1+d2 Fig. 2. Schematic of a two-mirror cavity with the internal thermal lens.
4 mm in length. The sample is water-cooled to 13:8◦ C. One side of the sample is HR coated at 1064 nm and AR coated at 808 nm, which served as the input mirror (M1). The other side is AR coated both at 1064 and 808 nm. A Qat mirror (M2) with 90% reQectivity at 1064 nm was used as the output coupler. The thermal focal lengths of the Nd:GdVO4 sample at various pump power levels were Irstly measured. The method used is simple but was proved to be eGective in determining the thermal focal length of the Nd:YVO4 lasers [10]. According to the stability theory of laser resonators, for a given cavity conIguration in which thermal lens eGect exists, the stable condition will be determined by the thermal focal length. For a two-mirror resonator with an internal thin thermal lens, the equivalent g-parameters can be written as [11] d1 d2 g1∗ = g1 − 1− ; fT R1 d2 d1 1− ; g2∗ = g2 − fT R2 g1 = 1 − L=R1 ;
L ¡ 1: fT
(3)
From Eq. (3) it can be concluded that the resonator will keep stable only if the cavity length is shorter than the pumpinduced thermal focal length. For a Ixed cavity length, if we increase the pump power, the laser output power would Irst increase and then fall oG when the thermal focal length becomes less than the cavity length. Therefore, the thermal focal length at diGerent pump power levels can be simply determined by observing the relationship between the output power and the pump power. By use of this method, the refractive power as a function of the pump power was measured and shown in Fig. 3. From this Igure, one can Ind that when the pump power increased from 22 to 40:6 W, the thermal focal length decreased from 22 to 6 cm. It is worth noting that the thermal focal length at a pump power of 40:6 W obviously departures with those at other pump powers, which suggests the crystal structure might have been broken under this pump power level. To verify this, we measured the thermal focal length again with the same crystal. It was found that the laser slope eAciency was dramatically reduced and the results were totally diGerent with the data previously measured. This proved that the laser crystal had been broken and the result indicates that in our laser the maximum pump power that the crystal can endure could be around 40:6 W. 20
Refractive Power (1/m)
d1
(2)
16
12
8
g2 = 1 − L=R2 ; L = d1 + d2 :
(1)
The terms used in Eq. (1) is schematically shown in Fig. 2, where L is the cavity length; fT is the thermal focal length
4 25
30
35
Pump Power (W) Fig. 3. The refractive power versus the pump power.
40
J. Kong et al. / Optics & Laser Technology 37 (2004) 51 – 54
20
Output Power (W)
Slope efficiency=58.5% 15
10
5
0 0
5
10
15
20
25
30
35
40
45
Pump Power (W) Fig. 4. Output power versus pump power relationship of the laser.
Based on the measured thermal focal length a Qat–Qat cavity conIguration was designed for the laser. Normally Qat–Qat cavity is seldom used in end-pumped solid-state lasers as it works at the boundary of the stable condition. To select it we have made the following considerations. Firstly, under high-pump power levels the pump area in the laser crystal should be kept as large as possible to avoid possible pump-induced crystal damage. For a better mode matching, a large laser mode size is thus needed. Secondly, because of the existence of strong thermal lens eGect, the stable state of Qat–Qat cavity will be driven deep into the stable region. Consequently, a stable output can be achieved in spite of that a Qat–Qat cavity was used. Finally, the Qat–Qat cavity can oGer a better beam quality as compared with the concave– plano cavity. It is because a plano–concave cavity can result in a comparable beam radius in the laser material if the thermal focal length is considered, which greatly decreases the mode-matching eAciency between pump light and the laser. As can be seen in Fig. 3, with a 40 W pump power the thermal focal length can be reduced to 6 cm, which means that to achieve a stable output power under high-pump power, a short cavity less than 6 cm is needed in the laser. With an appropriate alignment of the laser cavity, stable 1064 nm laser emission has been achieved. Fig. 4 shows the measured output power versus pump power in the laser. The laser had a threshold of about 3:6 W and a general slope eAciency of 58.5% when a Qat output coupler with 90% reQectivity was used. It is to note that with the increase of the pump power, the slope eAciency also increased. The slope eAciency was 62.2% in the range of pump power higher than 10 W and further increased to 63% when the pump power was higher than 15 W. The increased slope eAciency was attributed to an improved mode-matching eAciency between laser mode and pump light, which was caused by the reduced thermal focal length of the laser crystal. With a pump power of 39:5 W, a maximum output power of 19:8 W was obtained. By use of a laser beam proIler, the far-Ield beam radius was measured. It is shown that the laser beam had a
53
beam diameter of 5 mm at the distance of 0:6 m away from the output coupler. The beam radius at the output coupler was about 106 m. The beam quality M 2 thus was calculated to be 2:62 ± 0:14. It is worth mentioning that in our laser, the distribution of the pump light in the laser medium is a ring shape instead of a Gaussian shape or a uniform distribution. The light intensity in the center of pump beam spot is slightly lower than that in the edge of the pump beam spot. This kind of pump light distribution may be due to a bad coupling when the pump laser light is coupled into the Iber. This ring-shape pump light distribution depressed the generation of low-order laser modes and caused a bad beam quality. Consequently, the M 2 of the beam was higher than that reported in Ref. [8] even though a Qat–Qat cavity was used in our laser. We also investigated the relationship between the output power and the cavity length. Theoretically, the optimum cavity length can be selected by keeping an optimum ratio of pump spot size and mode diameter. If the thermal focal length is taken in account, according to the ABCD law, the optimum cavity length can be determined. Our simulation results shows that when the cavity length was about 5:0 cm, the mode diameter is about 400 m in the laser crystal, which is close to the pump spot size. However, experimentally, the maximum output power was achieved when the laser cavity length was set at about 2:5 cm. In this case, the mode diameter in the crystal is only about 270 m, which is far smaller than the pump spot size. Both reducing and increasing from this length would cause a decrease of the laser slope eAciency. Why the best performance of the laser was achieved at this cavity length needs to be further studied. From Fig. 4 it seems that the laser output power could be further increased if the pump power increased. However, in our experiments the Nd:GdVO4 sample was broken when we raised the pump power to 41 W. This coincides with the phenomenon observed in the thermal focal length measurement, which suggests that the pump-induced damage threshold of the crystal could be about 28:6 kW=cm2 . In conclusion, a diode-end-pumped high-power Nd: GdVO4 crystal laser has been demonstrated. The laser has a threshold of about 3:6 W and a general slope eAciency of 58.5%. In the range of linear output part, namely when the pump power is higher than 15 W, the slope eAciency increases to 63%. With 39:5 W pump power at 808 nm a maximum output power of 19:8 W at 1064 nm has been obtained. To our knowledge, both the output power and the slope eAciency for linear output part were the highest ever reported for end-pumped Nd:GdVO4 lasers. The beam quality M 2 at maximum output power was measured to be around 2.62. The thermal focal length in Nd:GdVO4 crystal under various pump power levels was also measured. Our experiments show that the thermal focal length changed from 22 to 6 cm when the pump power increased from 22 to 40:6 W. The Nd:GdVO4 crystal in our laser was broken when the pump power was increased to 41 W. The pump-induced damage threshold thus was estimated to be 28:6 kW=cm2 . Our
54
J. Kong et al. / Optics & Laser Technology 37 (2004) 51 – 54
experimental results further reveal the promising potential of Nd:GdVO4 crystals as a high-power laser material. References [1] Zagumennyi AI, Ostroumov VG, Shcherbakov IA, Jenson T, Meyn JP, Huber G. Nd:GdVO4 crystal: a new material for diode-pumped lasers. Sov J Quant Electron 1992;22:1071–2. [2] Zhang H, Meng XL, Zhu L, Yang ZH. Growth and thermal properties of Nd:GdVO4 single crystal. Mater Res Bull 1999;34:1589–93. [3] Fields RA, Birnbaum M, Fincher CL. Highly eAcient Nd:YVO4 diode-laser end-pumped laser. Appl Phys Lett 1987;51:1885–6. [4] Liu J, Shao Z, Zhang H, Meng XL, Zhu L, Jiang M. High-power end-pumped CW Nd:GdVO4 laser formed with a Qat–Qat cavity. Opt Laser Technol 1999;31:459–62. [5] Shcherbakov IA, Zagumennyi AI. Characterization of Nd:GdVO4 crystals for high-eAciency diode-pumped lasers. Proc SPIE Int Soc Opt Eng 1995;2498:241–9.
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