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In this paper, we report a high power, high efficiency blue-violet laser obtained by intracavity frequency-doubling of an all-solid-state Q-switched tunable Ti:sapphire laser, which was pumped by a 532nm intracavity frequency-doubled Nd:YAG laser. A β-BaB2O4 (BBO) crystal was used for frequency-doubling of the Ti:sapphire laser and a V-shape folded three-mirror cavity was optimized to obtain high power high efficiency second harmonic generation (SHG). At an incident pump power of 22W, the tunable output from 355nm to 475nm was achieved, involving the maximum average output of 3.5W at 400nm with an optical conversion efficiency of 16% from the 532nm pump laser to the blue-violet output. The beam quality factor M2 was measured to be Mx 2=2.15, My 2=2.38 for characterizing the tunable blue laser.
©2008 Optical Society of America
1. Introduction
Compact package blue-violet sources with high power and high conversion efficiency have attracted much attention for their wide applications in spectroscopy, medical research, color display and undersea communications. Frequency-conversion of the all-solid-state Nd3+ lasers is a popular approach for obtaining blue laser [1–8]. A 2.8W 473nm blue laser was produced by an intracavity frequency doubled 946nm quasi-three-lever Nd:YAG/BiB3O6 (BiBO) laser, which was operated on the 4F3/2→4I9/2 transition [1], and a 7.6W 439.7nm blue laser was obtained by frequency-tripling of a 1319nm Nd:YAG/LiB3O5 (LBO) laser [7]. In addition, a 914nm Nd:YVO4/LBO laser was utilized to generate 4.6W 457nm blue laser [8]. However, the efficiency of the Nd3+ laser is limited because of the small stimulated-emission cross section and the considerable reabsorption loss due to the thermal population at the lower level caused by the quasi-three-level operation.
In comparison with the Nd3+ blue lasers, when high power and tuning are required simultaneously in blue-violet spectral region, frequency conversion of a tunable laser in the near-infrared spectral region is regarded as a flexible solution [9–15]. Average powers of 10.1W at 450nm was obtained in a complicated OPG system pumped by a mode-locked Yb:YAG thin disk laser [15], but it was complex for routine operation. The Ti:sapphire laser, due to wide and proper tuning range, high gaining, high conversion efficiency, high peak power and the inherent stability of a solid state laser, is a better choice for nonlinear frequency conversion to blue-violet spectral region as a fundamental laser. A 48W arc lamp-pumped intracavity frequency-doubled Ti:sapphire laser produced 7W continuous wave (CW) tunable blue light (430nm<λ<460nm) was reported [12].
In this paper, a high efficiency all-solid-state Q-switched tunable intracavity frequency doubled Ti:sapphire laser was demonstrated, which was pumped by an all-solid-state Q-switched intracavity frequency doubled Nd:YAG laser. BBO was selected as the frequency-doubling crystal. At an incident pump power of 22W, the tunable Ti:sapphire laser from 700nm to 950nm was achieved, including the maximum average output power of 5.6W at 800nm with a line-width of 2nm (FWHM) and a pulse-width of 17.2ns (FWHM), which led to an optical conversion efficiency of 25.5% from the 532nm pump laser to the Ti:sapphire laser. Through frequency-doubling with nonlinear crystal BBO, tunable blue-violet laser from 355nm to 475nm was achieved including the maximum average output power of 3.5W at 400nm with high beam quality (Mx 2=2.15, My 2=2.38) and an optical conversion efficiency of 16% from the 532nm pump laser to the blue-violet output. To the best of our knowledge, this is the widest tunable range, the highest output power and optical conversion efficiency in the field of the all-solid-state blue-violet laser system.
Fig. 1. The setup of the all-solid-state Q-switched tunable frequency doubled Ti:sapphire laser system
2. Experimental setup and analysis
The experimental setup is schematically depicted in Fig. 1. The pump source was a 22W 532nm Q-switched intracavity frequency doubled Nd:YAG laser with repetition rate of 6.3 kHz. F was a focus lens with the focal length of 150mm, which was used to enhance the density of pump power and made the coupling of the pump beam and the oscillating beam better. To intensify the output power and the frequency-doubling efficiency of the Ti:sapphire laser, a classical V-shape folded three-mirror cavity was carefully designed. The turning flat mirror (M4) was high-reflection coated in the range of 700nm–950nm (R>99.8%). A folded plano-concave mirror (M5) with the curvature-radius of 100mm was adopted to reduce the length of the cavity and the focusing spot size, the concave surface was high-reflection coated in the range of 700nm–950nm (R>99.8%) and high-transmission coated in the range of 350nm–500nm (T>95%). The end mirror (M6) with the curvature-radius of 100mm was high-reflection coated in the range of 700nm–950nm (R>99.8%) and 350nm–500nm (R>99.8%). Two dense flint glass prisms were served as the dispersion element in the cavity. The Ti:sapphire crystal with dimensions of 7mm×7mm×16mm and FOM value of 150, was cut at Brewster angle at both ends with respect to the direction of c axis for better coupling of the pump source and lower loss of the Ti:sapphire laser. The dimensions of the BBO crystal were 3mm×3mm×7mm and the cutting angle was selected at 29.2°for Type I, angle-tuned phase matching. To extract the deposited heat, the Ti:sapphire crystal and the BBO crystal were wrapped with indium foil and mounted in the water-cooled copper holder through the resonator base plate, and the water temperatures were kept at 7°C and 15°C respectively. In addition, the ends of BBO crystal were high-transmission coated at 700–950nm (T>95%) and 350-500nm (T>93%) for the beams transmission and protecting from air slaking (provided by Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences).
It can be seen from Fig. 2, the folded three-mirror cavity was constituted of two arms. One was the collimating arm (l1), which had a larger beam radius (ω1) in the Ti:sapphire crystal, and the other was the focusing arm (l2), which had a smaller waist spot radius (ω2) in the BBO crystal. When the pump power was 22W, the waist spot radius of the 532nm pump laser in the center of the Ti:sapphire crystal was 200µm and the thermal-lens focus length of the Ti:sapphire crystal was 195mm measured by using the method we proposed in Ref. [16]. To generate high efficiency Ti:sapphire laser, the mode-volume of the Ti:sapphire laser must be well matched with that of 532nm pump laser in the center of the Ti:sapphire crystal. Besides, taking the thermal-lens effect of the Ti:sapphire crystal into account, to obtain higher power output and better beam quality, the Ti:sapphire laser must operate at the stable-power point of the U-shape curve indicating the thermal-lens focus length of the Ti:sapphire crystal. Through theoretical calculating, the length of l1 was 136mm, the length of l2 was 165mm and the distance from the center of the Ti:sapphire crystal to M5 was adjusted to 68mm. As a result, the TEM00 mode spot radius (ω1=214µm) of the Ti:sapphire laser was matched to the pump beam waist spot radius of 200µm well and the Ti:sapphire laser operated at the stable-power point of the U-shape curve when the thermal-lens focus length of the Ti:sapphire crystal was 195mm, as shown in Fig. 3. Furthermore, the V-shape cavity also provided a proper waist spot radius (ω2) of 52µm in the BBO crystal.
Fig. 2. The equivalent straight cavity of the V-shape cavity (the imaginal lines stand for the beam of the Ti:sapphire laser)
Fig. 3. The U-shape curve of the fundamental mode spot radius in the Ti:sapphire crystal as a function of the thermal-lens focus length
3. Results
In the experiment, first, the output power and the conversion efficiency of the Ti:sapphire laser were measured in the folded cavity without the BBO crystal. The plano-concave high reflection mirror (M6) was selected with the transmissivity of 18% in the range of 700nm–950nm. The output power and the spectrum of the tunable Ti:sapphire laser were recordd by a power meter (Molectron: EPM1000) and an optical spectrum analyzer (Agilent Technologie: 6842B) respectively. It can be seen from Fig. 4, the threshold pump power for tunable Ti:sapphire laser was around 6W due to the insertion loss of the prisms and the reflection loss of M4. When the pump power was 22W, through turning the mirror M4, tunable Ti:sapphire laser from 700nm to 950nm was achieved. The maximum output power of 5.6W at 800nm with a line-width of 2nm (FWHM) and a pulse-width of 17.2ns (recorded by an oscilloscope of Tektronix: TDS620B) was achieved, leading to an optical conversion efficiency of 25.5% from the 532nm pump laser to the Ti:sapphire laser, as shown in Fig. 5.
Fig. 5. The output power and optical-to-optical conversion efficiency of the Ti: sapphire laser at 800nm
Then the BBO crystal was placed at the waist of the Gauss beam and M6 was changed into a plano-concave high-reflection mirror (r=100mm) from 700nm to 950nm (R>99.8%) and from 350nm to 500nm (R>99.8%). It can be seen from Fig. 6, when the pump power was 22W, by spinning the BBO crystal in the range of ±5° around the cutting angle of 29.2° in the horizontal direction while tuning the mirror M4, tunable blue-violet laser from 355nm to 475nm was achieved. The maximum output power of 3.5W at 400nm with a frequency-doubling efficiency of 63% was obtained, as shown in Fig. 7. The beam quality factor of the tunable blue laser was Mx 2=2.15, My 2=2.38 measured by a laser beam diagnostics (Spiricon: M2-200).
4. Conclusion
In conclusion, a high power high efficiency blue-violet laser by intracavity frequency-doubling of an all-solid-state Q-switched tunable Ti:sapphire laser has been demonstrated. At an incident pump power of 22W, the tunable Ti:sapphire laser from 700nm to 950nm was achieved, including the maximum average output power of 5.6W at 800nm with a line-width of 2nm (FWHM) and a pulse-width of 17.2ns (FWHM), which led to an optical conversion efficiency of 25.5% from the 532nm pump laser to the Ti:sapphire laser. Through frequency-doubling with nonlinear crystal BBO, tunable blue-violet laser from 355nm to 475nm was achieved involving the maximum average output power of 3.5W at 400nm with an optical conversion efficiency of 16% from the 532nm pump laser to the blue-violet output. The beam quality factor of the tunable blue laser was measured to be Mx 2=2.15, My 2=2.38.
Acknowledgments
This project was supported in part by the National Natural Science Foundation of China (Grant Nos. 10474071, 60637010, 60671036 and 60278001) and Tianjin Applied Fundamental Research Project (07JCZDJC05900).
References and links
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