1. Introduction

During the past few years, all-solid-state blue laser sources have attracted a lot of interests because of their potential applications in various areas including high-density optical data storage, biological and medical diagnostics, Raman spectroscopy, colour displays, bio-fluorescence, trace-gas detection, high-resolution printing, and underwater communications. One of the most efficient approaches for generating blue light is through the second harmonic generation (SHG) of Nd:YAG lasers operating at 946 nm. High-power 946-nm Nd:YAG lasers are also attractive pump sources for Yb-doped materials such as Yb:YAG and Yb:Phosphate glass, and can be used for measuring the absorption of water vapors.

High-power Nd:YAG lasers operating at 946-nm, however, are generally much more difficult to achieve than those operating at 1064 nm. One of the reasons is due to the fact that the lower laser level is the upper 857 cm^-1 crystal-field component of the ⁴ F _9/2 ground-state manifold, leading to significant reabsorption losses. Another reason is because that the stimulated-emission cross-section (σ=4×10^-20cm²) is about an order of magnitude lower than that for the 1064 nm transition [1, 2].

Room-temperature diode-pumped CW Nd:YAG laser at 946 nm and LD pumped intracavity frequency-doubled blue laser were firstly reported by Risk and Length [2]. Since then, various resonator configurations (such as compact linear, V-shaped and Z-shaped resonators) and nonlinear crystals (such as KN, LBO, BBO, BIBO, PPLN and PPKTP) were developed [3–8]. In 2001, Abraham [9] reported a diode-end-pumped CW laser at 946-nm with an output power of 7.4 W which, to our knowledge, was the highest output power, except for our work reported here. In 2003, Czeranowsky et al. [10] reported a 2.8 W blue laser output at 473 nm with a BIBO crystal and a Z-shaped resonator.

In this paper, we report a diode-end-pumped CW Nd:YAG laser operating at 946-nm with an output power of 8.3 W and a slope efficiency of 33.5%. To the best of our knowledge, this is the highest output power ever reported. By using intracavity frequency doubling with a 10 mm long LBO crystal and a compact three-element resonator, we also generated a blue laser 473 nm with an output power of 1.2 W.

2. Experiments and results

Figure 1 shows a schematic diagram of the high-power CW 946-nm laser. The pump source was a high-brightness and high-power fibre-coupled 808-nm diode laser with a fibre core diameter of 400 μm and a numerical aperture of 0.22. The gain medium was a plane-parallel polished conventional Nd:YAG rod (Φ4×4 mm), with a Nd³⁺ doping level of 1.1 at.%. An optical coupler consisting of multiple lenses was used to couple the pump radiation into the gain medium with a spot size of about 300 μm in diameter. The transmission of the optical coupler at 808 nm was approximately 90%. The pumping facet of the Nd:YAG crystal had an optical coating with high reflectivity at 946 nm and 473 nm, high transmission at 808 nm, and partial transmission (>15%) at 1064 nm. Another facet was coated with an anti-reflection coating at 946 nm and 473 nm. A plano-concave mirror with a radius of 70 mm was chosen as the output coupler with a transmission of 5% at 946 nm and a transmission > 80% at 1064nm.

 

Fig. 1. Schematic diagram of the high power diode-end-pumped 946-nm laser

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In principle, the pumping beam should be focused to a small spot in the gain medium to overcome the low stimulated-emission cross-section at 946 nm transition. Strong beam focusing, however, will lead to severe thermal effects. Therefore, compromise has to be made between the laser performance and the thermal effects. In our design, a beam spot diameter of 300 μm was selected. For a laser that is end pumped by a fibre-coupled diode and edge cooled, the focal length of the thermal lens is given by [11]

where ξ is the fractional thermal loading, P _abs is the absorbed pump power, K _c is the thermal conductivity, n₀ is the refractive index of the laser crystal, dn/dT is the thermal-optic coefficient, ν is Poissos’s ratio, α_T is the thermal expansion coefficient, C_r is the photoelastic coefficient, and ω_p is the average pump size in the active medium. The calculated and experimental thermal focal lengths as a function of incident pump powers are shown in Fig. 2. The parameters used in the calculation are as follows: ξ = 0.2, K _c = 0.13 WK^-1cm^-1, n₀ = 1.82, dn/dT = 7.3×10^-6K^-1, ν = 0.25, α_T = 7.5×10^-6K^-1, C_r = 0.017 and ω_P = 150 μm. According to Eq. (1), at an incident pump power of 27.7 W, the focal length was calculated as 23.9 mm.

 

Fig. 2. Calculated and measured thermal focal lengths under different incident pump powers.

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In order to mitigate the thermal effects of a conventional Nd: YAG rod at high pump power conditions, we adopted the following approaches to make a balance between the output power and the beam quality. Firstly, a short cavity length of 8 mm was selected to reduce the influence of the thermal effects on cavity stability. Secondly, the laser rod was tightly wrapped in a water-cooled copper mount, and an indium foil was used to increase the thermal contact between the Nd:YAG rod and the copper heat sink. In the experiments, the temperature of the copper mount was kept at 3±0.2 °C to decrease the temperature at the rod center and to improve the laser performance.

Figure 3 shows the measured output powers and beam qualities under different incident pump powers. From the measured results, we can find that the lasing threshold is 1.3 W. At an incident pump power of 27.7 W, a maximum output power of 8.3 W at 946 nm was achieved with an optical-to-optical efficiency of 30 % and a slope efficiency of 33.5%. To the best of our knowledge, this is the highest CW output power generated by LD end-pumped Nd:YAG lasers at 946 nm.

 

Fig. 3. Measured output powers and beam qualities at 946 nm as a function of incident pump powers.

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From some experiments, we found that the crystal could absorb only 59.5% of the pump radiation, resulting in an effective absorption coefficient of α=2.26 cm^-1. When this absorption coefficient is taken into account, the slope efficiency would be as high as 56.2% with respect to the absorbed pump power. In order to characterize the beam quality, we also measured the M² factor under different incident pump power. The results showed that at an output power of 8.3 W, the M² factor was 9.34. The relatively poor beam quality was attributed to the short cavity length (8 mm) and the strong thermal effects.

By inserting a 3×3×10 mm LBO crystal, which was cut for type-I critical phase matching (θ=90°, φ=19.3°), as an SHG generator, and replacing the 946-nm output coupler with a blue laser output coupler, we achieved the 473 nm blue laser output. In this case, both facets of the LBO crystal were coated with anti-reflection coatings at both 473 nm and 946 nm. Since the LBO crystal had a small temperature bandwidth (7.29 K∙cm ), the operating temperature of the crystal was precisely controlled at 24 °C by using an active temperature controller with a stability of ±0.1 °C. The output coupler for 473 nm blue laser output was a plano-concave (50 mm in radius) mirror with following transmission values: <0.2% at 946 nm; >90% at 1064 nm and >96% at 473 nm. As mentioned above, the strong thermal effects in a conventional Nd:YAG crystal under high pump power needed to be carefully considered in the cavity design. To decrease the influence of the thermal effects on the cavity stability, the resonator length was chosen as 20 mm. Therefore, a compact three-element resonator was formed. Under these conditions, the threshold of incident pump power was found to be 0.7 W. At incident pump power of 16.9 W, a maximum laser power of 1.2 W at 473 nm was obtained, showing an optical-to-optical conversion efficiency of 7.1%. The measured output powers of the blue laser as a function of incident pump power are depicted in Fig. 4. The short-term power instability of the blue laser was measured as less than 1 %.

 

Fig. 4. Measured output powers of the blue laser (473 nm) under different incident pump powers.

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3. Conclusions

In conclusion, we have demonstrated a high power diode-end-pumped CW Nd:YAG laser with an output power of 8.3 W at 946 nm, with a slope efficiency of 33.5% with respect to incident pump power of 27.7 W. To the best our knowledge, this is the highest CW output power at 946 nm generated by LD end-pumped Nd:YAG lasers. By using intracavity frequency doubling with a 10 mm long LBO crystal operating under type-I critical phase matching condition, a high-stability and high-power blue laser has obtained. The output power at 473 nm was 1.2 W and optical-to-optical conversion efficiency was 7.1% with respect to an incident pump power of 16.9 W. The short-term power instability of the blue laser was less than 1 %.