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 ⁴ F _3/2→ ⁴ I _9/2 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 ⁴ F _3/2→ ⁴ I _9/2 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 (M ²) of 9.3 by employing a conventional Nd:YAG rod in 2005, resulting in a slope efficiency (η _s) of 33.5% and an optical-to-optical efficiency (η _o-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 η _s~45.0%, η _o-o~37.8% and M ²~13.1 [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 η _s~33.3% and η _o-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 η _o-o~12.9% and M ²~4.0 [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 η _o-o~10.1%, and M ²~30 for 946 nm line and η_o-o~4.3%, and M ²~30 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 deep-blue 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 η_o-o~15.3% and M ²~2.5 [9]. In 2007, Lü demonstrated a 6.2 W 456 nm Nd:GdVO₄/BiBO laser with η _o-o~17.2% and M ²~2.5 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 η_o-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 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 °C 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.).

 

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.

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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 d_eff~3.4 pm/V) and was placed in a copper holder whose temperature was precisely controlled by a thermal electric cooler with 0.1°C 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 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 (f _T) 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 frequency-doubling efficiency.

 

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 (f _T).

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3. Results and discussions

3.1 Output power characteristics

 

Fig. 3. Output power at 912 nm laser versus the incident pump power

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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 η _o-o~22.5% and η _s~29.3%. To the best of our knowledge, this is the highest cw output ever generated by diode-pumped quasi-three-level Nd:GdVO₄ lasers. Slope efficiencies η _s 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_s^a) 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_x ² ~ 3.45, M_y ² ~ 2.99, indicating a good beam propagation parameter for the quasi-three-level 9XX nm lasers.

 

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

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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]:

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 M ² value (calculated by $\sqrt{M_x^2{M}_x^2}$) 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 M ² 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.

 

Fig. 6. Dependence of the M ² value and brightness on the output power of 912 nm laser.

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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 η _o-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 M ² was measured about M_x ² ~ 2.5, M_y ² ~ 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-end-pumped geometry for good mode matching between the pump mode and the oscillating mode, and a BiBO nonlinear crystal with optimum length.

 

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

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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 η _s~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 η _o-o~8.0%. The short-term power instability of the blue laser was less than 3%.