Diode-pumped laser frequency doubled by CPM LBO
Introduction
It was well known that Nd3+ doped in YVO4 can also emit in addition to . Furthermore, Nd3+ can have a high doped concentration and emit polarized light in YVO4 [1], so Nd:YVO4 is highly suitable for diode-pumping to generate laser, which can be frequency doubled to obtain red laser.
There are already some papers on diode-pumped red lasers [2], [3], [4], but all of them used type-II CPM KTP or NCPM LBO to generate the second harmonic wave. Among them, with a fiber-coupled diode array, Zhang et al. used type-II KTP to achieve output of about [2]; also Zhang et al. used type-I NCPM LBO (T≈5°C) and a V-shaped folded cavity to obtain output, with optical-to-optical conversion efficiency up to 8.3% [3]; Agnesi et al. used type-II NCPM LBO (T≈38°C) and a V-shaped folded cavity to obtain output [4].
Analysis has shown that KTP has a low second harmonic generation (SHG) efficiency and bad beam quality due to its large walk-off angle, while NCPM LBO requires strict temperature control and is very hard to be a commercial product [5], especially for type-II NCPM LBO which has the highest efficiency, however, it strongly requires dry surroundings due to its work temperature being lower than room temperature. As to CPM LBO, we have not found any reports on it for frequency doubling in diode-pumped Nd:YVO4 red lasers.
In this paper, type-I and type-II CPM LBO was used for intracavity frequency doubling. By reasonable design, high SHG efficiency and good beam quality were obtained.
Section snippets
Characteristics of CPM LBO for frequency doubling
We have calculated some important parameters of different CPM crystals. Critical phase-matching angle (θ,φ), effective nonlinear coefficient (deff), walk-off angle (ρ) and acceptance angle of CPM LBO (I), LBO (II) and KTP (II) are listed in Table 1.
Data listed in Table 1 showed that although LBO has less deff than KTP, its little walk-off angle and large acceptance angle can make LBO's effective work length (L) very long for frequency-doubling. Based on the formula [6]where ω
Experimental setup
Based on the above analysis, the optional parameters were designed by computer programming. The equipment is shown in Fig. 1.
A laser diode with maximum output , a central emitting wavelength of at 23°C and a divergent angle of was used as the pumping source. After going through the coupling optics, light emitted from the LD was reshaped to high-quality pumping light (with an ellipticity of 0.91, beam waist's radius is about ) and was injected into Nd:YVO4 (
Results and discussions
In the same setup, type-I and type-II CPM LBO were used for frequency doubling, respectively. After 808 and light were filtered, the laser output power was measured. The red laser's power as a function of pumping light is shown in Fig. 2.
Fig. 2 shows the thresholds of pump power were both about under the two conditions. But above the threshold, CPM LBO (I) is markedly better than LBO (II). When the pump light of was injected, 97 and outputs were obtained
Conclusions
For the first time, type-I and type-II CPM LBO were used for intracavity frequency doubling of a Nd:YVO4 laser. By reasonable design, high SHG efficiency and good beam quality were obtained. With incident pump laser, using type-I and type-II CPM LBO, 97 and mode red laser outputs were obtained, the optical-to-optical conversion efficiencies are up to 12.1% and 6.5%, respectively. Furthermore, the power would be increased by a big margin if a folded cavity was used. We can
Acknowledgements
This work was supported by the National High-tech 863 plan of People's Republic of China (No.863-307-22-51, 863-16-05).
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