We report a high-power diode-end-pumped Q-switched Nd:GdVO4 red laser through intracavity frequency-doubling with a type-I critical phase-matched LBO crystal. The maximum average output power at 671 nm was obtained to be 6 W at the repetition frequency of 47 kHz, with the corresponding optical conversion efficiency of 12.8% and the pulse width of about 97 ns. At the average output power around 5 W, the power stability was better than 5.8% for one hour.
©2005 Optical Society of America
High-power red lasers have wide application fields such as medical treatment and laser color display, and can also be applied to optically excited femtosecond Kerr-lens mode-locked lasers based on Cr3+:LiSrAlF6 (Cr:LiSAF), Cr3+:LiSrGaF6 (Cr:LiSGAF), and Cr3+:LiSrCaAlF6 (Cr:LiSCAF) crystals. Although the power from red laser diodes has been increased, laser diodes are still inadequate for certain applications due to their wide linewidth and poor beam quality. An effective method to generate high-power and high-beam-quality red light is the intracavity frequency doubling of infrared radiation with nonlinear optical crystals. Therefore, diode-pumped all-solid-state red lasers are still important for many application fields.
Recently, quasi-continuous-wave (QCW) coherent red light up to 11.5 W was generated by intracavity frequency doubling of a side-pumped Nd:YAG laser in a 40-mm LBO crystal . However, the optical conversion efficiency was only 2.4%, and the beam quality of M 2 factor was up to 15±3. These are the inherent disadvantages of side-pumping technique . An efficient and good-quality red light can be generated by end-pumping technique. But the thermal-fracture problem of laser crystals limits the maximum laser output power in end-pumped solid-state lasers . For laser crystals, low dopant concentration and high thermal conductivity can reduce the thermal loading and increase the fracture-limited pump power. Therefore, we can use longer laser crystals with lower dopant concentrations and high thermal conductivity to achieve higher laser output power in end-pumped solid-state lasers.
Nd:GdVO4, as a relatively new laser crystal, has attracted great attention due to its excellent physical, optical, and mechanical properties [4–12]. Besides the large absorption coefficient and stimulated emission cross section, Nd:GdVO4 crystals also have a large thermal conductivity along the direction of <110> (about 11.7 W/m∙K), which is even higher than that of Nd:YAG crystals [4,5]. The thermal conductivities of the Nd:GdVO4 crystal along <001> and <100> directions were also measured to be 11.4 W/m∙K and 10.1 W/m∙K, respectively . The emission cross section (1.8×10-19 cm2) of Nd:GdVO4 at 1.34 µm is larger than that (0.7×10-19 cm2) of Nd:YAG, but slightly smaller than that (2.8×10-19 cm2) of Nd:YVO4 [6–8]. However, the smaller thermal conductivity of Nd:YVO4 compared with Nd:GdVO4 makes Nd:YVO4 unsuitable for high-power lasers . So Nd:GdVO4 can be used to generate efficient and high-power 1.34 µm radiation. Recently, with a 0.3 at.% Nd:GdVO4 crystal single-end-pumped by laser-diode-array, the maximum continuous-wave (CW) 1.34 µm output power of 13.3 W was obtained at the incident pump power of 49.2 W .
In this paper, we report high-power QCW 671 nm red light generation by intracavity frequency-doubling with a type-I critical phase-matched LBO crystal in a diode-single-end-pumped Nd:GdVO4 Q-switched laser. The maximum red light output power of 6 W was achieved at the repetition frequency of 47 kHz, with the corresponding optical conversion efficiency of 12.8% and the pulse width of about 97 ns. The excellent laser performance demonstrates that intracavity frequency-doubling of an end-pumped Nd:GdVO4 laser is a promising method for generating red light with high power and high efficiency.
2. Experimental setup
The intracavity second-harmonic generation (SHG) experiments were carried out in a three-mirror folded resonator, as shown schematically in Fig. 1. The pump source employed in the experiments was a commercially available high-power fiber-coupled diode-laser-array. The core diameter and numerical aperture (N.A.) of the fiber were 0.4 mm and 0.22, respectively. The pump beam from the fiber at the wavelength of 808 nm was focused into the laser crystal by an optical imaging system with the imaging ratio of 1:1. The pump mirror M1 was a concave mirror with a radius of curvature of 150 mm, antireflection (AR) coated at 808 nm on the flat face, high-reflectance (HR) coated at 1.34 µm and high-transmittance (HT) coated at 808 nm on the curved face. The a-cut Nd:GdVO4 crystal with Nd3+ concentration of 0.3 at.% and dimensions of 3×3×8 mm3 was AR coated at 808 nm and 1.34 µm on both of its faces, and placed closely to M1. To remove the heat generated at high-pump-power levels from the crystal, it was wrapped with indium foil and held in a water-cooled copper block. A temperature sensor was mounted in the copper block near the laser crystal to monitor its surface temperature. The surface temperature of Nd:GdVO4 crystal was kept to be about 18 °C during the experiments. An acousto-optical (A-O) Q-switch with high diffraction loss at 1342 nm was placed close to the Nd:GdVO4 crystal in the M1M3 arm. Its repetition rate could be tuned continuously from 1 kHz to 100 kHz. The output coupler M3 was also a concave mirror with radius of curvature of 100 mm, HR coated at 1.34 µm and AR coated at 671 nm on the curved surface, and HT coated at 671 nm on the outside surface. A 15-mm-long, (θ, φ)=(86.1°, 0°)-cut LBO crystal for type-I critical phase-matching at 1342 nm was used as the frequency doubler. To minimize the internal losses caused by Fresnel reflection, it was also AR coated at 1342 nm and 671 nm on both end faces. It was also cooled in the same way as in the case of the Nd:GdVO4 crystal. M2, a flat mirror with a dual-wavelength HR coating at 1.34 µm and 671 nm on its inside surface, was mounted on a translation stage. Based on our numerical calculation and previous experiments [12–14], it was found that the fundamental mode size in the laser crystal is very sensitive to the length of M2M3 arm. Thus the mode size in the laser crystal could be changed conveniently by translating M2. The folding angle between the M1M3 arm and the M2M3 arm was kept as small as possible to be about 7° to minimize astigmatism. A filter was placed before the output coupler M3 to absorb the fundamental wave that leaked out of the cavity. To suppress the oscillation of the 4F3/2→4I11/2 transition (1.06 µm) of Nd3+, all three mirrors had sufficient transmission (>90%) at 1.06 µm.
For efficient second-harmonic generation, it is necessary to provide a high power density of fundamental waves in the nonlinear optical crystal. To take advantage of the intense fundamental wave power density, the LBO crystal was placed close to the end mirror M2 where a beam waist existed. In our experiment, the length of the M2M3 arm was experimentally optimized to be about 70 mm by translating the end mirror M2, while the total cavity length was about 305 mm.
3. Results and discussion
We experimentally measured the average output power of the red light as a function of the incident pump power. The second-harmonic output beam was p polarized, and the fundamental infrared field oscillating inside the resonator was s polarized, as it is well known from type-I conversion process. At the maximum incident pump power, the average output power was optimized at the repetition rate of 47 kHz. The red laser pulse signal was detected by using a fast photodiode detector (Newport 818-BB-20), and was observed and measured with a 300 MHz oscilloscope (Tektronix TDS 3032B). Figure 2 shows the average output power at 671 nm as a function of incident pump power. The threshold pump power was measured to be 2.7 W. At the incident pump powers of 15, 30, and 40 W, the average output powers at 671 nm were measured to be 1, 3, and 4.6 W, respectively. The maximum average output power of red light was obtained to be 6 W at the incident pump power of 46.7 W, with the corresponding optical conversion efficiency of 12.8%. The temporal pulse profile of the output red light is shown in Fig. 3. The pulse width was measured to be about 97 ns (Full Width Half Maximum, FWHM).
The output power stability of the red laser was also measured at the average output power of around 5 W. The fluctuation (rms) of the average output power for one hour was measured to be about 5.8%. It was mainly due to the pump wavelength variation of the laser diode because no attempt was made to actively control its temperature. The output power stability is expected to improve significantly with the active temperature control of both the laser diode and the LBO crystal.
The far-field intensity distribution of the red beam was also measured by the CCD laser beam profiler (Newport LBP) at the maximum average output power of about 6 W, which is shown in Fig. 4. From Fig. 4, we can see that a nearly Gaussian beam intensity profile was obtained at the maximum output power. The beam quality parameter M 2 of the red beam at various output powers was also measured, which is shown Fig. 5. The M2 factor increased from 1.33 to 2.47 when the output power was increased from 1 W to 6 W.
When the incident pump power was 46 W, we also measured the average output power and pulse width of the 671 nm light at different repetition frequencies, as shown in Fig. 6. The maximum average output power of 5.5 W was obtained at the repetition frequency of 47 kHz. When the repetition frequency was lower or higher than 47 kHz, the average output power would drop. The shortest pulse width of about 124 ns was obtained at the repetition frequency of 35 kHz. And the pulse width would increase when the repetition frequency was lower or higher than 35 kHz.
In our experiments, the most important step for the significant output power improvement was the choice of the Nd:GdVO4 crystal with low dopant concentration of 0.3 at.% and high thermal conductivity as the active laser material. So the thermal effect of the laser crystal was not strong even under high pump power. It was experimentally proved in our previous work . At the high pump power up to 46.7 W, the thermal lensing of Nd:GdVO4 crystal was not strong enough to drive the resonator approaching its stability boundary . Moreover, its power-dependent diffractive loss was also reduced significantly .
In conclusion, a high-power diode-end-pumped Q-switched intracavity frequency-doubled Nd:GdVO4/LBO red laser has been demonstrated. The maximum average output power up to 6 W at 671 nm was achieved at the repetition frequency of 47 kHz, with the corresponding optical conversion efficiency of 12.8%. The average power fluctuation of less than 5.8% was obtained at the maximum output power. It is expected that higher output power over 10 W at 671 nm can be achieved with the double-end-pumping configuration.
This work was supported by the Science and Technology Project of Guangdong Province of China (2004B16001210), the Natural Science Foundation of Guangdong Province of China (No. 04300858), and the Science and Technology Project of Shenzhen.
References and links
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