12.0-W continuous-wave diode-end-pumped Nd:GdVO4 laser with

12.0-W continuous-wave diode-end-pumped Nd:GdVO4 laser with high brightness operating at 912-nm Jing Gao,* Xin Yu, Fei Chen, Xudong Li, Renpeng Yan, Kun Zhang, Junhua Yu, and Yuezhu Wang National Key Laboratory of Tunable Laser Technology, Harbin Institute of Technology, 150080, Harbin, China * Corresponding author: [email protected]

Abstract: We present a diode-pumped high-power continuous-wave (cw) laser at 912 nm based on the quasi-three-level 4F3/2→4I9/2 transition in Nd:GdVO4 crystal. By using a 5-mm-long conventional bulk crystal with 0.2at.% Nd3+-doped concentration, the maximum output power of 12.0 W is obtained, with a slope efficiency of 29.3%. To the best of our knowledge, this is the highest output at 912 nm generated by diode-pumped Nd:GdVO4 lasers. Furthermore, 3.2 W 456 nm deep-blue light is acquired by frequency doubling, resulting in an optical-to-optical efficiency of 8.0%. The shortterm power instability of the blue laser is less than 3%. 2009 Optical Society of America OCIS codes: (140.3580) Lasers, solid state; (140.3530) Lasers, neodymium; (140.3480) Laser, diode-pumped; (140.3515) Lasers, frequency doubled; (140.7300) Visible lasers

References and links 1.

T. Y. Fan and R. L. Byer, “Modeling and CW operation of a quasi-three-level 946 nm Nd:YAG laser,” IEEE J. Quantum Electron. 23, 605–612 (1987). 2. R. Zhou, T. L. Zhang, E. B. Li, X. Ding, Z. Q. Cai, B. G. Zhang, W. Q. Wen, P. Wang, and J. Q. Yao, “8.3 W diode-end-pumped continuous-wave Nd:YAG laser operating at 946-nm,” Opt. Express 13, 10115–10119 (2005), http://www.opticsinfobase.org/abstract.cfm?URI=oe-13-25-10115. 3. R. Zhou, E. B. Li, H. F. Li, P. Wang, and J. Q. Yao, “Continuous-wave, 15.2 W diode-end-pumped Nd:YAG laser operating at 946 nm,” Opt. Lett. 31, 1869–1871 (2006). 4. J. Gao, X. Yu, F. Chen, X. D. Li, Z. Zhang, J. H. Yu, and Y. Z. Wang, “Characteristics of CW and A-O Qswitched Nd:GdVO4 laser operation at 912 nm,” in Advanced Solid-State Photonics, OSA Technical Digest Series (CD) (Optical Society of America, 2008), paper WB7, http://www.opticsinfobase.org/abstract.cfm?URI=ASSP-2008-WB7. 5. J. Gao, M. Larionov, J. Speiser, A. Giesen, A. Douillet, J, Keupp, E. M. Rasel, and W. Ertmer, “Nd:YVO4 thin disk laser with 5.8 watts output power at 914 nm,” in Conference on Lasers and Electro-Optics, Vol. 73 of OSA Trends in Optics and Photonics Series (Optical Society of America, 2002), paper CTuI1. 6. J. Speiser, A. Giesen, and J. Gao, “25-W diode-pumped continuous-wave quasi-three-level Nd:YAG thin disk laser,” in Advanced Solid-State Photonics, Technical Digest (Optical Society of America, 2005), paper TuB34, http://www.opticsinfobase.org/abstract.cfm?URI=ASSP-2005-TuB34. 7. J. W. Dawson, R. Beach, A. Drobshoff, Z. Liao, D. M. Pennington, S. A. P, L. Taylor, W. Hackenberg, and D. Bonaccini, “Scalable 11 W 938 nm Nd3+ doped fiber laser,” in Advanced Solid-State Photonics, (Optical Society of America, 2004), paper MD8, http://www.opticsinfobase.org/viewmedia.cfm?URI=ASSP-2004MD8. 8. C. Czeranowsky, E. Heumann, and G. Huber, “All-solid-state continuous-wave frequency-doubled Nd:YAG-BiBO laser with 2.8-W output power at 473 nm,” Opt. Lett. 28, 432–434 (2003). 9. Q. H. Xue, Q. Zheng, Y. K. Bu, F. Q. Jia, and L. S. Qian, “High-power efficient diode-pumped Nd:YVO4/LiB3O5 457 nm blue laser with 4.6 W of output power,” Opt. Lett. 31, 1070–1072 (2006). 10. Y. F. Lü, X. H. Zhang, Z. H. Yao, and F. D. Zhang, “6.2-W deep blue light generation by intracavity frequency-doubled Nd:GdVO4 using BiBO,” Chin. Opt. Lett. 5, 407–408 (2007). 11. O. Svelto. Principle of Lasers, 4th ed. (Springer, New York, 1998).

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Received 5 Jan 2009; revised 18 Feb 2009; accepted 20 Feb 2009; published 23 Feb 2009

2 March 2009 / Vol. 17, No. 5 / OPTICS EXPRESS 3574

1. Introduction Since Fan and Byer introduced the first laser diode (LD) pumped 946 nm Nd:YAG laser at room temperature in 1987, lasers operating around 900 nm have attracted more and more attention [1]. For one thing, lasers operating around 900 nm with high power and high brightness are charming pump sources for the Ytterbium (Yb)-doped crystals and Yb-doped fibers. For another, they are capable and competent of some unique applications such as water vapor detecting and differential absorption lidar (DIAL) for ozone measurements. Moreover, by frequency doubling, lasers operating around 900 nm can generate blue lasers, which have numerous applications in optical data storage, full-color display, high-resolution printing, Raman spectroscopy, biological medicine, and underwater communication. The 4F3/2→ 4I9/2 transition in neodymium (Nd3+) has been proved to be a convenient and efficient approach to realize laser operating around 900 nm. Recently, there were extensive researches on the lasing characteristics of 4F3/2→ 4I9/2 transition in neodymium with different structures. For the most universal rod-type structure, Zhou reported 8.3 W cw 946 nm laser output with a beam quality factor (M2) of 9.3 by employing a conventional Nd:YAG rod in 2005, resulting in a slope efficiency (ηs) of 33.5% and an optical-to-optical efficiency (ηo-o) of 30.0% [2]. With a diffusion bonded rod, the highest 15.2 W laser power at 946 nm was achieved in 2006, with ηs~45.0%, ηo-o~37.8% and M2~13.1 [3]. In 2008, our group presented an 8.6 W cw laser at 912 nm in a conventional Nd:GdVO4 bulk crystal, the corresponding parameters were ηs~33.3% and ηo-o~21.3% [4]. For the thin-disk structure, 5.8 W cw output power of 914 nm in Nd:YVO4 was firstly demonstrated by Gao in 2002 with ηo-o~12.9% and M2~4.0 [5]. In 2005, Speiser and Giesen realized 14.0 W laser at 946 nm and 6.0 W output at 938 nm in Nd:YAG thin disk when the incident pump power was 138 W, giving ηo-o~10.1%, and M2~30 for 946 nm line and ηo-o~4.3%, and M2~30 for 938 nm line, respectively [6]. Moreover, Nd-doped fiber was also used in a master oscillator power amplifier (MOPA) operating around 900 nm, which results in an 11.0 W output power at 938 nm [7]. The corresponding frequency doubled blue lasers were also in moderate progress. In 2003, Czeranowsky reported a 2.8 W blue laser output at 473 nm in a diffusion bonded Nd:YAG rod with a BiB3O6 (BiBO) crystal and a Z-type cavity [8]. In 2006, Xue presented a 4.6 W deepblue laser at 457 nm in a V-type resonator by utilizing a Nd:YVO4 bulk crystal and a LiB3O5 (LBO) crystal as the frequency doubler, with ηo-o~15.3% and M2~2.5 [9]. In 2007, Lü demonstrated a 6.2 W 456 nm Nd:GdVO4/BiBO laser with ηo-o~17.2% and M2~2.5 by using a double-end-pumped geometry, the output power of which represents the highest level till now [10]. As all the literatures mentioned above, the output power of lasers operating around 900 nm is still low and the brightness is not high enough, which handicaps their further power scaling and also the development of the following frequency doubled blue lasers. The main reason for the dilemma can be ascribed to the nature of quasi-three-level transitions. On the one hand, the stimulated-emission cross section is determined only ~1/10 of the main 1.06 µm transition. On the other hand, the thermal population of lower laser level leads to significant reabsorption loss. So the threshold of such laser is always higher than the commonly four-level laser, and the high-power, efficient operation is difficult to realize. In this paper, we demonstrate an efficient, high power 912 nm laser with 12.0 W cw output power and high brightness, achieving a slope efficiency of 29.3%. To our knowledge, our results represent the highest output level of diode-pumped quasi-three-level Nd:GdVO4 lasers. In addition, efficient frequency doubling was performed and maximum 3.2 W 456 nm deep-blue light was obtained with ηo-o~8.0%. 2. Experimental setups A simple plano-concave cavity was employed to generate the fundamental 912 nm laser, as shown in Fig. 1(a). The fiber coupled LD array purchased from LIMO Inc. served as the pump source, which delivers a maximum output power of 110 W at 808 nm from the end of a fiber

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with a core diameter of 400 µm and a numerical aperture (N.A.) of 0.22. The output radiation was re-imaged into the laser crystal by coupling lenses with a waist spot diameter of ~400 µm. A conventional a-cut Nd:GdVO4 single crystal with Nd3+-doped concentration of 0.2 at.% and 3×3×5 mm3 in dimension was employed as the gain medium. The laser crystal was wrapped by a 0.05 mm thick indium foil, mounted in a well-designed, efficient micro-channel copper heat-sink and kept at 13±0.2 ºC by water cooling. Both facets of the crystal were well polished and coated for high transmission (HT) at 912 nm (T>99.8%) and the pump wavelength (T>99%). The input mirror was coated for high reflection (HR) at 912 nm (R>99.8%) and HT at 808 nm (T>95%). Three plano-concave mirrors (coupler 1, coupler 2 and coupler 3) with the same radius of curvature of 200 mm and coated partial transmission at 912 nm (T=3.6%, T=6.0%, and T=9.0%, correspondingly) were used as the output couplers. In addition, both the input and the output mirrors were HT coated at 1063 nm (T>95%) and 1340 nm (T>90%) to suppress the more efficient four-level transitions in Nd:GdVO4. The geometric cavity length was ~25 mm. The laser output power was measured by a power meter PM30 (Coherent Inc.). M2

M1 Nd:GdVO4

912 nm laser output

Fiber Coupling Lenses

LD

(a) M1

M3

l1

Nd:GdVO4

α

Fiber LD

Coupling Lenses l2

BiBO

456 nm laser output

(b) M4

l3

456 nm laser output M5

Fig. 1. Schematic diagrams of the experimental setup. (a) is for fundamental 912 nm infrared laser and (b) is for the frequency doubled 456 nm blue laser.

To realize efficient frequency doubling, a Z-folded cavity was designed, as shown in Fig. 1(b). All the elements were the same as the corresponding ones in the setup of fundamental 912 nm laser mentioned above. The mirror M3, M4 and M5 were coated HR at 912 nm (R>99.8%), HT at 456 nm (T>96%) and HT at 1063 nm (T>95%). A BiB3O6 (BiBO) crystal with dimension of 2×2×15 mm3 was employed as the frequency doubler and placed very close to M5 in order to achieve high frequency conversion efficiency. The crystal was cut for type I critical-phase-matching condition (θ=159.5°, φ=90°; with deff~3.4 pm/V) and was placed in a copper holder whose temperature was precisely controlled by a thermal electric cooler with 0.1ºC accuracy. Both facets of the nonlinear crystal were well polished and AR coated at 456 nm and 912 nm. To keep the cavity less sensitive to the thermal lens and favorable to frequency doubling, the arm l1, l2 and l3 were selected to be 113 mm, 400 mm and 58 mm, respectively. The radiuses of curvature were chosen as 200 mm, 100 mm and 300 mm for M3, M4 and M5, correspondingly. The folded angle (α) was set to be ~7° to reduce the astigmatism. Thus under these conditions, the radius of the laser mode in the middle of Nd:GdVO4 crystal (ω1) and BiBO (ω02) are shown in Fig. 2, which were calculated by ABCD matrix formalism with the approximation of a thin lens in the middle of laser crystal. From Fig. 2, we can see that the difference of laser mode radius between the tangential and

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sagittal plane is within 10%, so astigmatism induced by the folded angles is not serious. What’s more, both of the spot radiuses in the laser crystal and in the BiBO are nearly constant within a broad region of the thermal-lens focal length (fT) from 50 mm to 400 mm. Therefore, such a cavity is very insensitive to the thermal lens and can stably operate even at high pump level. The radius in the middle of BiBO is about 44 µm, which would yield good frequencydoubling efficiency.

laser beam radius (µm)

100

80

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40

ω1 sagittal ω02 sagittal ω1 tangential ω02 tangential

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0

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fT (mm) Fig. 2. Sagittal radius and tangential radius of laser mode in the middle of Nd:GdVO4 and BiBO versus focal length of the thermal lens (fT).

3. Results and discussions 3.1 Output power characteristics

Output power at 912 nm (W)

14

T=9% R=200mm T=6% R=200mm T=3.6% R=200mm

12 10 8 6 4 2 0 10

15

20

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Incident pump power (W) Fig. 3. Output power at 912 nm laser versus the incident pump power

Figure 3 presents the input and output characteristics when employing three different output couplers. It is noted that the thresholds for all cases were relatively high. This was mainly attributed to the significant reabsorption loss, and partially to the spectral mismatch between LD’s radiation and the absorption peak of the laser crystal. When coupler 2 was used, an output power of 12.0 W at 912 nm was achieved under the incident pump power of 53.4 W, giving ηo-o~22.5% and ηs~29.3%. To the best of our knowledge, this is the highest cw output #105902 - $15.00 USD

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ever generated by diode-pumped quasi-three-level Nd:GdVO4 lasers. Slope efficiencies ηs of 21.0% and 28.0% were obtained for coupler 1 and coupler 3, respectively. In consideration of the relatively low absorption efficiency (~60.0%) of Nd:GdVO4 crystal for the pump radiation, actually the slope efficiencies with respect to the absorbed pump power ( η sa ) were 35.0%, 48.8%, and 46.7% for coupler 1, coupler 2 and coupler 3, respectively. Moreover, the output power kept good linearity and no saturation appearance was observed even in such high pump level. However, to prevent the laser crystal from thermal damage, we didn’t increase the pump power any more. 3.2 Spatial distribution of laser spot and the M2 test The spatial distribution of the 912 nm laser spot at the output power of 10 W (coupler 2) was measured by LBA300 (Spiricon Inc.) and shown in Fig. 4, from which we can see the transverse mode with good symmetry is in nearly Gaussian distribution for both directions. To check the beam quality of this laser, we measured the beam radius for the 912 nm laser at the 10 W power level by the 90/10 traveling knife-edge method. Figure 5 shows the measured beam radius at different distance from the lens with a focal length f=150 mm. By fitting Gaussian beam standard expression to these data, we estimated the beam quality to be M x2 ~ 3.45, M y2 ~ 2.99 , indicating a good beam propagation parameter for the quasi-threelevel 9XX nm lasers.

(a)

(b)

Fig. 4. The spatial beam profile (a) two-dimensional and (b) three-dimensional for 912 nm laser at 10 W output power level. 1.6

y plane x plane

Beam radius (mm)

1.4 1.2 2

Mx=3.45

1.0 0.8

2

My=2.99

0.6 0.4 0.2 0.0

0

50

100

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Relative distance from the lens (mm) Fig. 5. Beam radius for the 912 laser at 10 W output power level

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3.3 Brightness of 912 nm laser Brightness (B) is also a very important factor for lasers. The brightness for a cw laser is determined by the following equation [11]:

B=

P M M y2λ2 2 x

where P is the output power and λ is the central wavelength. Figure 6 displays the variations of measured the calculated brightness for different output levels when using coupler 2. The M2 value (calculated by M x2 M x2 ) was 1.9 for 2 W output and slowly increased to 3.2 for 10 W power level because of the more severe thermal lens. Then based on the M2 value, B can be calculated according to the equation above. Figure 6 also shows a monotonous growing tendency despite of deterioration of the beam quality. Note that the brightness of the 10-W 912 nm laser was determined to be 118 MW/cm2·sr, this is, to our knowledge, the highest value obtained with cw diode-pumped >10-W oscillators operating around 900 nm using Nd3+-doped crystals. 4

130

B 2 M

120

2

3

100

2

M value

B (MW/cm.Sr)

110

90 2

80 70 60

2

4

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1

Laser power at 912 nm (W) Fig. 6. Dependence of the M2 value and brightness on the output power of 912 nm laser.

3.4 456 nm blue laser generation The threshold of blue laser was measured about 10.5 W in this cavity. At an incident pump power of 40.2 W, the maximum blue laser of 1.5 W and 1.7 W emitted from M4 and M5 were achieved, respectively. Thus, a total output power from both ends of 3.2 W at 456 nm was obtained, yielding ηo-o ~8.0%. The measured blue laser output power as a function of incident pump power is exhibited in Fig. 7. The power fluctuation of the blue laser was less than 3% within 20 min at the maximum output level. The M2 was measured about M x2 ~ 2.5, M y2 ~ 2.2 for both the blue laser beam emitted from M4 and from M5 at the maximum output level. We believe that efficient power scaling in 456 nm blue laser can be achieved by using LDs with high brightness, composite Nd:GdVO4 crystals for efficient heat extraction, a double-endpumped geometry for good mode matching between the pump mode and the oscillating mode, and a BiBO nonlinear crystal with optimum length.

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456 nm blue laser output (W)

3.5 3.0

laser power emitted from M5 laser power emitted from M4 total laser power

2.5 2.0 1.5 1.0 0.5 0.0 10

15

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25

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45

Incident pump power (W) Fig. 7. 456 nm blue laser output power as a function of the incident pump power.

4. Conclusion In conclusion, we have developed a high power, high brightness diode-pumped solid-state 912 nm laser. By using a traditional Nd:GdVO4 bulk crystal, we achieved up to 12.0 W cw laser output at 912 nm with ηs~29.3%. To the best of our knowledge, this represents the highest cw output power at 912 nm in diode-pumped Nd:GdVO4 lasers. The brightness of 912 nm laser at 10-W output level was estimated to be 118 MW/cm2·sr, which we believe is the highest value obtained in cw diode-pumped >10-W oscillators operating around 900 nm using Nd3+-doped crystals. By frequency doubling in a Z-type cavity with a 15-mm-long BiBO, 3.2 W deep blue light at 456 nm was obtained with ηo-o~8.0%. The short-term power instability of the blue laser was less than 3%. Acknowledgments This work was supported by the Scientific and Technological Project of Heilongjiang Province under contract No GC06A116 and Program of Excellent Team in Harbin Institute of Technology. J. Gao wishes to gratefully acknowledge Dr. N. Pavel (National Institute for Laser, Plasma and Radiation Physics, Laboratory of Solid-State Quantum Electronics, Bucharest, Romania) for the stimulating and fruitful discussions.

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