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High-power continuous-wave doubly resonant all-intracavity sum-frequency mixing deep blue laser at 447 nm
Optik
Optics
Optik 121 (2010) 1625–1629 www.elsevier.de/ijleo
High-power continuous-wave doubly resonant all-intracavity sum-frequency mixing deep blue laser at 447 nm Yong-ji Yu, Guang-yong Jin, Xin-yu Chen, Ming Ling, Chao Wang, Zhu Liang Changchun University of Science and Technology, School of Science, Institute of Laser Technology, Changchun 130022, China Received 21 November 2008; accepted 3 March 2009
Abstract In this paper, a high-power continuous-wave deep blue laser at 447 nm with intracavity tripling was achieved. The deep blue laser at 447 nm is obtained by using a doubly cavity, and type-II critical phase matching KTP crystal for intracavity sum-frequency mixing. Through designing of the cavity, the optimum matching of modes and gains for the two wavelengths was obtained. With incident pump power of 30 W for the Nd:YVO4 crystal and 16 W for the other Nd:YVO4 crystal, the deep blue laser output of 3.5 W at 447 nm with TEM00 mode was obtained, the beam quality M2 value was equal to 1.8 in both horizontal and vertical directions at the maximum output power, and the power stability is better than 3% at the maximum output power during half an hour. The experimental results show that the intracavity sum-frequency mixing by doubly resonant is an effective method for high-power blue laser. Crown Copyright r 2009 Published by Elsevier GmbH. All rights reserved. Keywords: Deep blue laser; Sum-frequency; Doubly cavity; LD pumped
1. Introduction High-power blue laser source is desired for many applications, such as optical data storage, submarine communications, laser color display, laser spectroscopy and medical diagnostics. For these applications, the high-power blue laser source must have a high electrical to optical efficiency, compact package, long and reliable life time. A diode-pumped solid-state blue laser is a promising method to such a blue laser source. Recently, continue-wave blue laser generated by frequency doubling of the diode-end pumped neodymium-doped lasers operating at the 4F3/2-4I9/2 transition have been extensively explored [1–6]. But this way is limited by the considerable re-absorption loss Corresponding author.
E-mail address: [email protected] (Y.-j. Yu).
caused by thermal population of the lower laser level for the oscillation of quasi-three-level laser. Another efficient way to obtain blue laser is based on summing frequency of the neodymium-doped laser operating at the 4F3/2-4I13/2 transition. Unlike the three-level system of the 4F3/2-4I9/2 transition, stimulated emission at the 4 F3/2-4I13/2 transition is a four-level system that can provide a low-threshold and stable laser output due to the lack of sensitive temperature dependence of the transition rate [7]. High-power blue laser has been achieved in this way. In this paper, we firstly report a 447 nm deep blue laser by a new resonator, which doubly cavity on intracavity sum-frequency mixing of two Nd:YVO4 crystals were pumped by two laser diode arrays, which coupled by optical fiber, respectively. In the two cavities, wavelengths of 1342 nm from 4F3/2-4I13/2 transition in Nd:YVO4 and 671 nm from intracavity frequency
0030-4026/$ - see front matter Crown Copyright r 2009 Published by Elsevier GmbH. All rights reserved. doi:10.1016/j.ijleo.2009.03.009
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doubling with type-I critical phase matching LBO crystal of 4F3/2-4I13/2 transition in another Nd:YVO4 were chosen to be mixed into 447 nm deep blue laser. In the overlapping of the cavities, sum-frequency mixing was generated with a type-II critical phase matching KTP crystal. The deep blue laser output of 3.5 W at 447 nm with TEM00 mode was obtained at the incident pump power of 30 W for the Nd:YVO4 crystal and 20 W for the other Nd:YVO4 crystal. The beam quality M2 value was equal to 1.8 in both horizontal and vertical directions at the maximum output power. The power stability is better than 3% at the maximum output power during half an hour.
2. Experiment setup The schematic diagram of the deep blue laser is shown in Fig. 1. The double resonator consisted of one shared and two separated arms. The separated arms include the laser crystals and independent alignment of the lasers, which are joined with a diachronics beam splitter (M1). This design could shift the stability range to higher pump power, with the advantage of dividing the pump power between the Nd:YVO4 rods. Two pump sources used in the experiments were commercially available fiber-coupled laser diode arrays, which delivered the maximum output power of 30 and 16 W, respectively at the center wavelength of 808 nm from the fiber bundle ends. The fibers were drawn into round bundles of 200 mm core radius with the numerical aperture of 0.22. The pump light is focused by four achromatic lenses into the laser crystals, where the diameter at the focus is 15 mm. Two Nd:YVO4 crystals with dimension of 3 3 5(mm) were 0.5% Nd3+ doping, a-axis cut-off. One side of the laser crystals were coated high-transmission (HT) at 808 nm
16W LD
couple lens Nd:YVO4 Crystal 2
Nd:YVO4 Crystal 1
M2
couple lens
M4 LBO M1 double-frequency KTP sum-frequency
30W LD
M3
Fig. 1. Schematic diagram of the deep blue laser sytem.
(T499.8%) and high-reflection (HR) at 1342 nm (R499.8%) and the other side were HT at 1342 nm (T499.9%). In addition, considering that the stimulated emission cross-section at 1342 nm (7.6 1019 cm2) is estimated to be 30% of that at 1064 nm (25 1019 cm2) and quantum efficiency at 1342 nm is also lower than that at 1064 nm, the HR coating of the laser crystals were also of low reflectance near 1064 nm to avoid laser oscillation at this wavelength [8]. In order to decrease the influence of the thermal effects, the laser crystals were wrapped with indium foil and mounted in a semiconductor cooled copper blocks. The beam splitter M1 (R ¼ N) was made 1 mm thickness of K9 glass by double-sided coating. Right side of M1 was coated HT for 1342 nm (T499.9%) at 451 and the other side was coated HT for 1342 nm (T499.9%) at 451 and HR for 671 nm (R499.9%) at 451. Concave mirror M2 (R ¼ 200 mm) was coated HR at both 1342 nm and 671 nm (R499.9%). Concave of output mirror M3 (R ¼ 50 mm) was coated HR at 1342 nm and 671 nm (R499.9%) and HT at 447 nm (T495%) and other side was HT at 447 nm (T495%). One side of M4 (R ¼ 200 mm) was coated HR at 1342, 671, 447 nm (R499.9%). In order to restrain the oscillation of the 1064 nm strongest line, all mirrors were coated AR at 1064 nm (T470%). To provide an efficient second-harmonic generation (SHG), an LBO crystal was used with dimensions of 2 2 15 (mm) and type-I critical phase matching for 1342 nm (e) and 1342 nm (e), producing 671 nm (o). Calculated by SNLO [9], the LBO crystal cut at y ¼ 86.11, j ¼ 01. Both end faces of the LBO crystal were coated with AR films for both 1342 and 671 nm. The LBO was chosen due to a high anti-damage threshold (18 GW/cm2) and a much smaller walk-off angle (3.45 mrad). To provide an efficient third harmonic generation (THG), a KTP crystal was used with dimensions of 2 2 10 (mm) and type-II critical phase matching for 1342 nm (o) and 671 (e), producing 447 nm (o). Calculated by SNLO [9], the KTP crystal cut at y ¼ 79.51, j ¼ 01. Both end faces of the KTP crystal were coated with AR films for both 1342, 671 and 447 nm. The KTP provided the THG 447 nm deep blue laser generation due to a larger effective nonlinear coefficient (4.06 pm/V) and a smaller change of the phase matching direction in the temperature dependence [10]. Both nonlinear crystals were provided by Qingdao CRYSTECH crystal Inc. Each of the nonlinear crystals was wrapped with a thin indium foil and mounted in a copper holder whose temperature was maintained at 300 K by a thermodelectric cooler with an accuracy of 70.1 K. Based on the doubly cavity design, the KTP crystal was set the beam waist of the folded arms M3 and M4 to take advantage of the strongest power density of the beam in the doubly cavity. The
Y.-j. Yu et al. / Optik 121 (2010) 1625–1629 1 L1=60mm L1=70mm L1=80mm
0.9 0.8 0.7 Radius/mm
second-harmonic wave at 671 nm was generated by frequency doubling of the fundamental wave at 1342 nm in the SHG LBO crystal. The fundamental wave at 1342 nm and the second-harmonic wave at 671 nm were mixed in the THG KTP to generate the third harmonic at 447 nm.
1627
0.6 0.5 0.4 0.3
3. Resonator stability analysis
0.2 0.1
The equivalent resonator of the one presented in Fig. 1 is shown in Fig. 2. The laser crystals in the resonator can be considered as a thin thermal lens with an effective focal length f, which decreases in nonlinearly with an increase in the pump power. The stability of the resonator can be investigated by an ABCD ray transfer matrix. For the maximum optimization of the resonator parameters, repeated calculations were completed with Matlab software
0
0
20
40
60
80
100
120
140
180
200
Fig. 4. Dependence of the radius of the beam spot on the Nd:YVO4 crystal 1 on the thermal focal length for different L1.
1 L2=100mm L2=85mm
0.9 0.8
L2=70mm L1=60mm
Radius/mm
0.7
Nd:YVO4 Crystal 2
160
Thermal focal length/mm
0.6 0.5 0.4 0.3
M2
0.2 0.1
Nd:YVO4 Crystal 1 L2
M4
M3
Fig. 2. Equivalent resonator corresponding to a thermal lens.
1 0.9 0.8
Radius/mm
0.7 0.6 0.5 0.4 0.3 0.2 0.1 20
40
60
80
100
20
40
60
80
100
120
140
160
180
200
Fig. 5. Dependence of the radius of the beam spot on the Nd:YVO4 crystal 1 and the Nd:YVO4 crystal 2 on the thermal focal length for different L2.
L3
0
0
Thermal focal length/mm
L1
0
0
120
140
160
180
200
Thermal focal length/mm
Fig. 3. Dependence of the radius of the beam spot on the Nd:YVO4 crystal 2 on the thermal focal length at L2 ¼ 70 mm for different lengths of the resonator: L3 ¼ 20 mm (green dotted lines), L3 ¼ 30 mm (black solid lines), L3 ¼ 40 mm (red dashed lines).
according to the experimental conditions. Fig. 3 shows the dependence of the radius of the beam spot on the Nd:YVO4 crystal 2 on the thermal focal length for three different groups of the cavity parameters. The transition from a stable to an unstable domain is characterized by an infinite increase in the beam radius, which means very high diffraction losses and thus vanishing laser action. Fig. 3 shows that, for resonator parameters L2 ¼ 70 mm and L3 ¼ 30 mm, the stable domain is the widest one. Fig. 4 shows the dependence of the radius of the beam spot on the Nd:YVO4 crystal 1 on the thermal focal length for different L1. With increase in L1, the stable domain gradually narrows. This will limit the input power and thus reduce the maximum output power. So according to practical situation on the experiment, we choice shorter of L1 as possible. For L1 ¼ 60 mm, L3 ¼ 30 mm, and different L2, the dependence of the beam spot on the Nd:YVO4 crystal 1 and the Nd:YVO4 crystal 2 versus the thermal focal length is shown in Fig. 5. Fig. 5 shows that, with increase in pump power, the laser gradually approaches the boundary of the stability domain, when
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L2 ¼ 85 mm, the Nd:YVO4 crystal 1 of laser and the Nd:YVO4 crystal 2 of laser simultaneous enters into the boundary region, thus the sum-frequency light of 447 nm will obtained quite well matched. In a word, the cavity length was 60 mm for 671 nm oscillation and the other cavity length was 115 mm for 1342 nm oscillation.
4. Experiment results and discussion After 808, 1064, 1342 and 671 nm lights were filtered, a power meter with precision of 10 nW was used to measure 447 nm laser output. The output power as a function of incident pump power is shown in Fig. 6. The black curve is the output power of 447 nm deep blue laser versus pump power injected into Nd:YVO4 crystal 1 when pump power for Nd:YVO4 crystal 2 is 16 W, red curve is the output power of 447 nm deep blue laser versus pump power injected into Nd:YVO4 crystal 2 when pump power for Nd:YVO4 crystal 1 is 30 W. The deep blue laser output started at pumping power around 4 W of the Nd:YVO4 crystal 1, the starting point of power out is about 0.15 W. It increases rapidly as the
Fig. 8. The blue light spot.
Fig. 9. Far-field intensity distribution of the blue beam.
Output power /(W)
4.0 3.5
30W pump power for 671 nm Nd: YVO4 laser
3.0
16W pump power for 1342 nm Nd: YVO4 laser
2.5 2.0 1.5 1.0 0.5 0.0 0
5
10
15
20
25
30
Incident pump power /w
Output power at 447 nm (W)
Fig. 6. 447 nm deep blue laser output power versus incident pump power. 7.0 6.5 6.0 5.5 5.0 4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 Time (minute)
Fig. 7. Stability of the 447 nm deep blue laser output power around the 3.5 W operation point.
pumping power of the Nd:YVO4 crystal 1 increases and is not saturated at 30 W, the corresponding maximum output power is 3.5 W, the slope efficiency is about 13%. The deep blue laser output started at pumping power around 4 W of the Nd:YVO4 crystal 2, the starting point of power out is about 0.2 W. It increases rapidly as the pumping power of the Nd:YVO4 crystal 2 increases and is not saturated at 16 W, the corresponding maximum output power is 3.5 W, the slope efficiency is about 28%. Other characteristics of the deep blue laser were also experimentally studied. The time trace of the maximum output power around the 3.5 W operating point is as shown in Fig. 7. At the maximum output power level of 3.5 W, the fluctuation of the blue beam output power was better than 3% in the given 30 min. The blue light spot and the beam profile and 3D stereogram of the far-field were shown in Figs. 8 and 9, respectively. Beam quality is important for many applications. The beam quality factor M2 becomes poorer as the pump power increases. At maximum output of 3.5 W, the M2 value is 1.8 in both horizontal and vertical directions measured by Spiricon beam analyzer. At a lower output, the beam quality is much better. For example, at an output of 1 W, the value of M2 is reduced to about 1.2 in both directions. The poorer quality at higher output is mostly due to that the laser tends to oscillate in high-order transverse modes under high pump power. But, it is enough for laserprojecting application.
Y.-j. Yu et al. / Optik 121 (2010) 1625–1629
5. Conclusion [3]
In summary, a high-power continuous-wave deep blue laser at 447 nm achieved by intracavity sum-frequency mixing of two laser diode arrays pumped two Nd:YVO4 crystals with a doubly cavity has been demonstrated. A maximum output power of 3.5 W at 447 nm was obtained, and the power stability is better than 3% at the maximum output power during half an hour. The beam quality factor M2 is 1.8 in both horizontal and vertical directions. The experimental results show that the intracavity sum-frequency mixing by doubly resonant is an effective method for high-power blue laser.
[6]
Acknowledgments
[7]
This work was supported by the Science and Technology Innovation Foundation of Changchun City under Grant 2008cc06.
[8]
References [1] T.Y. Fan, R. Byer, Continuous-wave operation of a room-temperature diode-laser-pumped 946 nm Nd:YAG laser, Opt. Lett. 12 (1987) 809–811. [2] C. Czeranowsky, E. Heumann, G. Huber, All-solid-state continuous-wave frequency-doubled Nd:YAG BiBO laser
[4]
[5]
[9]
[10]
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with 2.8 W output power at 473 nm, Opt. Lett. 28 (2003) 432–434. P.X. Lia, D.H. Lia, Z.G. Zhang, S.W. Zhang, Diodepumped high-power cw blue laser at 473 nm with a compact three-element cavity, Opt. Commun. 215 (2003) 159–162. Y. Chen, H. Peng, W. Hou, et al., 3.8 W of cw blue light generated by intracavity frequency doubling of a 946 nm Nd:YAG laser with LBO, Appl. Phys. B 83 (2006) 241–243. F. Jia, Q. Xue, Q. Zheng, Y. Bu, L. Qian, 5.3 W deep-blue light generation by intracavity frequency doubling of Nd:GdVO4, Appl. Phys. B 83 (2006) 245–247. Y. Lu, X. Zhang, Z. Yao, F. Zhang, 6.2 W deep blue light generation by intracavity frequency-doubled Nd:GdVO4 using BiBO, Chin. Opt. Lett. 5 (2007) 407–408. Y.F. Chen, Y.S. Chen, T.H. Ou, K.W. Su, Compact efficient diode-pumped Nd:YVO4 Q-switched blue laser with intracavity frequency tripling, Appl. Phys. B 81 (2005) 517–520. Y.F. Chen, cw dual-wavelength operation of a diode-endpumped Nd:YVO4 laser, Appl. Phys. B 70 (2000) 475. G.O. Boyd, D.A. Kleinman, SNLO, free software for modeling nonlinear frequency conversion processes in nonlinear crystals /http://www.sandia.gov/imrl/X1118/ xxta-l.htmS. H.Y. Zhu, G. Zhang, C.H. Huang, et al., Multi-watt power blue light generation by intracavity sum-frequencymixing in KTiOPO4 crystal, Opt. Express 16 (2008) 2989–2994.
Optics
Optik 121 (2010) 1625–1629 www.elsevier.de/ijleo
High-power continuous-wave doubly resonant all-intracavity sum-frequency mixing deep blue laser at 447 nm Yong-ji Yu, Guang-yong Jin, Xin-yu Chen, Ming Ling, Chao Wang, Zhu Liang Changchun University of Science and Technology, School of Science, Institute of Laser Technology, Changchun 130022, China Received 21 November 2008; accepted 3 March 2009
Abstract In this paper, a high-power continuous-wave deep blue laser at 447 nm with intracavity tripling was achieved. The deep blue laser at 447 nm is obtained by using a doubly cavity, and type-II critical phase matching KTP crystal for intracavity sum-frequency mixing. Through designing of the cavity, the optimum matching of modes and gains for the two wavelengths was obtained. With incident pump power of 30 W for the Nd:YVO4 crystal and 16 W for the other Nd:YVO4 crystal, the deep blue laser output of 3.5 W at 447 nm with TEM00 mode was obtained, the beam quality M2 value was equal to 1.8 in both horizontal and vertical directions at the maximum output power, and the power stability is better than 3% at the maximum output power during half an hour. The experimental results show that the intracavity sum-frequency mixing by doubly resonant is an effective method for high-power blue laser. Crown Copyright r 2009 Published by Elsevier GmbH. All rights reserved. Keywords: Deep blue laser; Sum-frequency; Doubly cavity; LD pumped
1. Introduction High-power blue laser source is desired for many applications, such as optical data storage, submarine communications, laser color display, laser spectroscopy and medical diagnostics. For these applications, the high-power blue laser source must have a high electrical to optical efficiency, compact package, long and reliable life time. A diode-pumped solid-state blue laser is a promising method to such a blue laser source. Recently, continue-wave blue laser generated by frequency doubling of the diode-end pumped neodymium-doped lasers operating at the 4F3/2-4I9/2 transition have been extensively explored [1–6]. But this way is limited by the considerable re-absorption loss Corresponding author.
E-mail address: [email protected] (Y.-j. Yu).
caused by thermal population of the lower laser level for the oscillation of quasi-three-level laser. Another efficient way to obtain blue laser is based on summing frequency of the neodymium-doped laser operating at the 4F3/2-4I13/2 transition. Unlike the three-level system of the 4F3/2-4I9/2 transition, stimulated emission at the 4 F3/2-4I13/2 transition is a four-level system that can provide a low-threshold and stable laser output due to the lack of sensitive temperature dependence of the transition rate [7]. High-power blue laser has been achieved in this way. In this paper, we firstly report a 447 nm deep blue laser by a new resonator, which doubly cavity on intracavity sum-frequency mixing of two Nd:YVO4 crystals were pumped by two laser diode arrays, which coupled by optical fiber, respectively. In the two cavities, wavelengths of 1342 nm from 4F3/2-4I13/2 transition in Nd:YVO4 and 671 nm from intracavity frequency
0030-4026/$ - see front matter Crown Copyright r 2009 Published by Elsevier GmbH. All rights reserved. doi:10.1016/j.ijleo.2009.03.009
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Y.-j. Yu et al. / Optik 121 (2010) 1625–1629
doubling with type-I critical phase matching LBO crystal of 4F3/2-4I13/2 transition in another Nd:YVO4 were chosen to be mixed into 447 nm deep blue laser. In the overlapping of the cavities, sum-frequency mixing was generated with a type-II critical phase matching KTP crystal. The deep blue laser output of 3.5 W at 447 nm with TEM00 mode was obtained at the incident pump power of 30 W for the Nd:YVO4 crystal and 20 W for the other Nd:YVO4 crystal. The beam quality M2 value was equal to 1.8 in both horizontal and vertical directions at the maximum output power. The power stability is better than 3% at the maximum output power during half an hour.
2. Experiment setup The schematic diagram of the deep blue laser is shown in Fig. 1. The double resonator consisted of one shared and two separated arms. The separated arms include the laser crystals and independent alignment of the lasers, which are joined with a diachronics beam splitter (M1). This design could shift the stability range to higher pump power, with the advantage of dividing the pump power between the Nd:YVO4 rods. Two pump sources used in the experiments were commercially available fiber-coupled laser diode arrays, which delivered the maximum output power of 30 and 16 W, respectively at the center wavelength of 808 nm from the fiber bundle ends. The fibers were drawn into round bundles of 200 mm core radius with the numerical aperture of 0.22. The pump light is focused by four achromatic lenses into the laser crystals, where the diameter at the focus is 15 mm. Two Nd:YVO4 crystals with dimension of 3 3 5(mm) were 0.5% Nd3+ doping, a-axis cut-off. One side of the laser crystals were coated high-transmission (HT) at 808 nm
16W LD
couple lens Nd:YVO4 Crystal 2
Nd:YVO4 Crystal 1
M2
couple lens
M4 LBO M1 double-frequency KTP sum-frequency
30W LD
M3
Fig. 1. Schematic diagram of the deep blue laser sytem.
(T499.8%) and high-reflection (HR) at 1342 nm (R499.8%) and the other side were HT at 1342 nm (T499.9%). In addition, considering that the stimulated emission cross-section at 1342 nm (7.6 1019 cm2) is estimated to be 30% of that at 1064 nm (25 1019 cm2) and quantum efficiency at 1342 nm is also lower than that at 1064 nm, the HR coating of the laser crystals were also of low reflectance near 1064 nm to avoid laser oscillation at this wavelength [8]. In order to decrease the influence of the thermal effects, the laser crystals were wrapped with indium foil and mounted in a semiconductor cooled copper blocks. The beam splitter M1 (R ¼ N) was made 1 mm thickness of K9 glass by double-sided coating. Right side of M1 was coated HT for 1342 nm (T499.9%) at 451 and the other side was coated HT for 1342 nm (T499.9%) at 451 and HR for 671 nm (R499.9%) at 451. Concave mirror M2 (R ¼ 200 mm) was coated HR at both 1342 nm and 671 nm (R499.9%). Concave of output mirror M3 (R ¼ 50 mm) was coated HR at 1342 nm and 671 nm (R499.9%) and HT at 447 nm (T495%) and other side was HT at 447 nm (T495%). One side of M4 (R ¼ 200 mm) was coated HR at 1342, 671, 447 nm (R499.9%). In order to restrain the oscillation of the 1064 nm strongest line, all mirrors were coated AR at 1064 nm (T470%). To provide an efficient second-harmonic generation (SHG), an LBO crystal was used with dimensions of 2 2 15 (mm) and type-I critical phase matching for 1342 nm (e) and 1342 nm (e), producing 671 nm (o). Calculated by SNLO [9], the LBO crystal cut at y ¼ 86.11, j ¼ 01. Both end faces of the LBO crystal were coated with AR films for both 1342 and 671 nm. The LBO was chosen due to a high anti-damage threshold (18 GW/cm2) and a much smaller walk-off angle (3.45 mrad). To provide an efficient third harmonic generation (THG), a KTP crystal was used with dimensions of 2 2 10 (mm) and type-II critical phase matching for 1342 nm (o) and 671 (e), producing 447 nm (o). Calculated by SNLO [9], the KTP crystal cut at y ¼ 79.51, j ¼ 01. Both end faces of the KTP crystal were coated with AR films for both 1342, 671 and 447 nm. The KTP provided the THG 447 nm deep blue laser generation due to a larger effective nonlinear coefficient (4.06 pm/V) and a smaller change of the phase matching direction in the temperature dependence [10]. Both nonlinear crystals were provided by Qingdao CRYSTECH crystal Inc. Each of the nonlinear crystals was wrapped with a thin indium foil and mounted in a copper holder whose temperature was maintained at 300 K by a thermodelectric cooler with an accuracy of 70.1 K. Based on the doubly cavity design, the KTP crystal was set the beam waist of the folded arms M3 and M4 to take advantage of the strongest power density of the beam in the doubly cavity. The
Y.-j. Yu et al. / Optik 121 (2010) 1625–1629 1 L1=60mm L1=70mm L1=80mm
0.9 0.8 0.7 Radius/mm
second-harmonic wave at 671 nm was generated by frequency doubling of the fundamental wave at 1342 nm in the SHG LBO crystal. The fundamental wave at 1342 nm and the second-harmonic wave at 671 nm were mixed in the THG KTP to generate the third harmonic at 447 nm.
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0.6 0.5 0.4 0.3
3. Resonator stability analysis
0.2 0.1
The equivalent resonator of the one presented in Fig. 1 is shown in Fig. 2. The laser crystals in the resonator can be considered as a thin thermal lens with an effective focal length f, which decreases in nonlinearly with an increase in the pump power. The stability of the resonator can be investigated by an ABCD ray transfer matrix. For the maximum optimization of the resonator parameters, repeated calculations were completed with Matlab software
0
0
20
40
60
80
100
120
140
180
200
Fig. 4. Dependence of the radius of the beam spot on the Nd:YVO4 crystal 1 on the thermal focal length for different L1.
1 L2=100mm L2=85mm
0.9 0.8
L2=70mm L1=60mm
Radius/mm
0.7
Nd:YVO4 Crystal 2
160
Thermal focal length/mm
0.6 0.5 0.4 0.3
M2
0.2 0.1
Nd:YVO4 Crystal 1 L2
M4
M3
Fig. 2. Equivalent resonator corresponding to a thermal lens.
1 0.9 0.8
Radius/mm
0.7 0.6 0.5 0.4 0.3 0.2 0.1 20
40
60
80
100
20
40
60
80
100
120
140
160
180
200
Fig. 5. Dependence of the radius of the beam spot on the Nd:YVO4 crystal 1 and the Nd:YVO4 crystal 2 on the thermal focal length for different L2.
L3
0
0
Thermal focal length/mm
L1
0
0
120
140
160
180
200
Thermal focal length/mm
Fig. 3. Dependence of the radius of the beam spot on the Nd:YVO4 crystal 2 on the thermal focal length at L2 ¼ 70 mm for different lengths of the resonator: L3 ¼ 20 mm (green dotted lines), L3 ¼ 30 mm (black solid lines), L3 ¼ 40 mm (red dashed lines).
according to the experimental conditions. Fig. 3 shows the dependence of the radius of the beam spot on the Nd:YVO4 crystal 2 on the thermal focal length for three different groups of the cavity parameters. The transition from a stable to an unstable domain is characterized by an infinite increase in the beam radius, which means very high diffraction losses and thus vanishing laser action. Fig. 3 shows that, for resonator parameters L2 ¼ 70 mm and L3 ¼ 30 mm, the stable domain is the widest one. Fig. 4 shows the dependence of the radius of the beam spot on the Nd:YVO4 crystal 1 on the thermal focal length for different L1. With increase in L1, the stable domain gradually narrows. This will limit the input power and thus reduce the maximum output power. So according to practical situation on the experiment, we choice shorter of L1 as possible. For L1 ¼ 60 mm, L3 ¼ 30 mm, and different L2, the dependence of the beam spot on the Nd:YVO4 crystal 1 and the Nd:YVO4 crystal 2 versus the thermal focal length is shown in Fig. 5. Fig. 5 shows that, with increase in pump power, the laser gradually approaches the boundary of the stability domain, when
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L2 ¼ 85 mm, the Nd:YVO4 crystal 1 of laser and the Nd:YVO4 crystal 2 of laser simultaneous enters into the boundary region, thus the sum-frequency light of 447 nm will obtained quite well matched. In a word, the cavity length was 60 mm for 671 nm oscillation and the other cavity length was 115 mm for 1342 nm oscillation.
4. Experiment results and discussion After 808, 1064, 1342 and 671 nm lights were filtered, a power meter with precision of 10 nW was used to measure 447 nm laser output. The output power as a function of incident pump power is shown in Fig. 6. The black curve is the output power of 447 nm deep blue laser versus pump power injected into Nd:YVO4 crystal 1 when pump power for Nd:YVO4 crystal 2 is 16 W, red curve is the output power of 447 nm deep blue laser versus pump power injected into Nd:YVO4 crystal 2 when pump power for Nd:YVO4 crystal 1 is 30 W. The deep blue laser output started at pumping power around 4 W of the Nd:YVO4 crystal 1, the starting point of power out is about 0.15 W. It increases rapidly as the
Fig. 8. The blue light spot.
Fig. 9. Far-field intensity distribution of the blue beam.
Output power /(W)
4.0 3.5
30W pump power for 671 nm Nd: YVO4 laser
3.0
16W pump power for 1342 nm Nd: YVO4 laser
2.5 2.0 1.5 1.0 0.5 0.0 0
5
10
15
20
25
30
Incident pump power /w
Output power at 447 nm (W)
Fig. 6. 447 nm deep blue laser output power versus incident pump power. 7.0 6.5 6.0 5.5 5.0 4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 Time (minute)
Fig. 7. Stability of the 447 nm deep blue laser output power around the 3.5 W operation point.
pumping power of the Nd:YVO4 crystal 1 increases and is not saturated at 30 W, the corresponding maximum output power is 3.5 W, the slope efficiency is about 13%. The deep blue laser output started at pumping power around 4 W of the Nd:YVO4 crystal 2, the starting point of power out is about 0.2 W. It increases rapidly as the pumping power of the Nd:YVO4 crystal 2 increases and is not saturated at 16 W, the corresponding maximum output power is 3.5 W, the slope efficiency is about 28%. Other characteristics of the deep blue laser were also experimentally studied. The time trace of the maximum output power around the 3.5 W operating point is as shown in Fig. 7. At the maximum output power level of 3.5 W, the fluctuation of the blue beam output power was better than 3% in the given 30 min. The blue light spot and the beam profile and 3D stereogram of the far-field were shown in Figs. 8 and 9, respectively. Beam quality is important for many applications. The beam quality factor M2 becomes poorer as the pump power increases. At maximum output of 3.5 W, the M2 value is 1.8 in both horizontal and vertical directions measured by Spiricon beam analyzer. At a lower output, the beam quality is much better. For example, at an output of 1 W, the value of M2 is reduced to about 1.2 in both directions. The poorer quality at higher output is mostly due to that the laser tends to oscillate in high-order transverse modes under high pump power. But, it is enough for laserprojecting application.
Y.-j. Yu et al. / Optik 121 (2010) 1625–1629
5. Conclusion [3]
In summary, a high-power continuous-wave deep blue laser at 447 nm achieved by intracavity sum-frequency mixing of two laser diode arrays pumped two Nd:YVO4 crystals with a doubly cavity has been demonstrated. A maximum output power of 3.5 W at 447 nm was obtained, and the power stability is better than 3% at the maximum output power during half an hour. The beam quality factor M2 is 1.8 in both horizontal and vertical directions. The experimental results show that the intracavity sum-frequency mixing by doubly resonant is an effective method for high-power blue laser.
[6]
Acknowledgments
[7]
This work was supported by the Science and Technology Innovation Foundation of Changchun City under Grant 2008cc06.
[8]
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