Diode-end-pumped solid-state ultraviolet laser based on intracavity third-harmonic generation of in YCa4O(BO3)3 crystal
Introduction
In recent years, short wavelength coherent radiation in ultraviolet (UV) region has attracted wide attention for a variety of applications such as machining, spectroscopy, optical data storage, laser printing, undersea communication and medical treatment. Nonlinear optical (NLO) conversion of solid-state lasers operating in the near-infrared range is currently a very effective method for UV light generation. Compared to traditional ultraviolet gas lasers, diode-pumped solid-state ultraviolet lasers have many advantages, including compactness, high efficiency, long lifetime, high stability and all solid-state construction. Nowadays LiB3O4 (LBO) and β-BaB2O4 (β-BBO) are the most widely used nonlinear optical crystals for UV laser-beam generation. However, these crystals suffer from some sort of limitations, such as the difficulty of growth, hygroscopy, small acceptance angle and large walk-off. Therefore it is very necessary to search for new NLO crystals which can mitigate these limitations.
During the last few years, newly developed NLO crystals of ReCa4O(BO3)3 (ReCOB) (Re=Y,Gd) have attracted much attention due to their good optical properties [1], [2], [3], [4], [5], [6]. They possess many advantages such as non-hygroscopy, large nonlinear coefficient which is comparable to that of LBO, high damage threshold, wide transmission band, large phase-matching range and good mechanical properties allowing easy polishing. What is more, since they melt congruently, they can be grown rapidly and easily to large-size single crystals with high optical quality by using conventional Czochralski technique. While other famous NLO crystals, such as KTP, KDP, LBO and β-BBO, do not possess this advantage. So ReCOB has been recognized as a promising NLO crystal in the frequency-conversion domain.
Until now, most of the research interest involving ReCOB has been focused on the aspects of second-harmonic generation (SHG) and self-frequency doubling (SFD). In 1997, Iwai found that YCOB crystal can reach type-I phase-matching for third-harmonic generation (THG) of by means of sum-frequency mixing (SFM) , while GdCOB cannot [1]. The transmission limit of YCOB can be as short as , while for GdCOB there are several sharp absorption peaks in the region of 200–. So YCOB is more suitable for UV light generation than GdCOB. Recently, our group have fitted the spatial distribution curve of deff (effective NLO coefficient) for the THG of in YCOB according to its second-order NLO susceptibilities. By the calculating and the extracavity THG experiments, we found that the largest deff for THG of is near the direction of (θ,ϕ)=(106°,77.2°), [7].
In this paper, we report a compact diode-pumped solid-state ultraviolet laser, in which a (106°,77.2°)-cut YCOB crystal has been used for type-I sum-frequency mixing of and to generate light. The light was generated by SHG of with type-II phase-matching KTP crystals. Two kinds of NLO process, SHG and SFM, occurred in one resonant cavity, which was very complicated. We investigated the output power with three KTP crystals with different length as frequency-doubler for continuous-wave (CW) and Q-switched operation, respectively.
Section snippets
Experimental setup
The conversion efficiency of THG depends on efficient SHG significantly. In our previous intracavity SHG work [5], [8], [9], a three-mirror folded resonator can generate efficient second-harmonic. So the intracavity THG experiment was carried out in the similar resonator configuration, as shown in Fig. 1. A fiber-coupled laser diode with center wavelength around was employed as the pump source in the intracavity THG experiment. The laser crystal, a , a-cut Nd:YVO4, was AR
Experimental results
The CW operation of intracavity THG was performed by removing A-O Q-switch firstly. We optimized the length of M2M3 arm to by translating M2. Because the output power of with KTP crystal was too low to measure, Fig. 2 only shows the results obtained with 10 and KTP crystals. The corresponding THG conversion efficiency as a function of incident pump power is shown in Fig. 3. From these figures, we can see that the ultraviolet output power and THG conversion efficiency
Discussion
Based on the Sellmeier equations of YCOB crystal given in Ref. [10], the angular acceptance of the YCOB crystal used in our experiments, Δθl and Δφl were calculated to be 34.5 and for Type-I THG , respectively. And the walk-off angle was also calculated to be , which is smaller than that of LBO [11].
Since two NLO processes, SHG and SFM occurred in one arm of the cavity, the operation in the cavity was somewhat complicated. Too many factors such as the balance
Conclusions
In summary, we investigated the intracavity THG of in a (106°,77.2°)-cut YCOB crystal for type-I SFM, with three different-length KTP crystals as frequency-doubler, for CW and Q-switched operation, respectively. The maximum ultraviolet output power of was obtained with the KTP crystal for CW operation, while the maximum ultraviolet output power of was obtained with the KTP crystal for Q-switched operation. Therefore, YCOB crystal is suitable for not only SFD and
Acknowledgements
This work was supported by the Key Program of National Natural Science Foundation of China under Grant No. 69890235, the Research Award Foundation for Prominent Youth Scientists of Shandong Province of China, and the Key Program of Science and Technology Research of the Ministry of Education of China.
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