1. Introduction

Nonlinear optical (NLO) crystals like KH₂PO₄ (KDP), KTP (KTiOPO₄), LBO (LiB₃O₅), and BBO (β-BaB₂O₄) have been extensively applied in the field of NLO frequency conversion. But all of these crystals have some drawbacks such as their maximum available size, hygroscopy, and low effective nonlinear coefficient. None of these crystals can be considered as ideal. Therefore, it is necessary to develop new and better NLO materials that can mitigate these limitations.

Borate crystals such as BBO and LBO possess the attractive versatility in nonlinear optics due to the various structural possibilities for boron atom [1]. They have been attracting much attention due to their outstanding linear and nonlinear optical properties. New borate crystals for NLO applications are still of great interest. As a new developed NLO crystal, BiB₃O₆ (BIBO) is a highly nonsymmetric crystal of point group 2 and space group C2. The effective NLO coefficient d _eff of BIBO is found to be comparable to that of KTP and be larger than those of KDP, LBO or BBO [2,3]. In addition, BIBO is non-hygroscopy, and has high optical damage threshold which is comparable to high quality LBO [3]. So in the past few years, a number of research works on BIBO were performed. These include second-harmonic generation (SHG) [4–8], optical parametric oscillators (OPO) [9], noncollinear SHG [10], and UV-induced two-photon absorption (TPA) [11].

High-power red lasers can be widely applied in many 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. The SHG of infrared radiation with nonlinear optical crystals is an effective method to generate high-power and high-beam-quality red light.

Here we report the phase-matching (PM) curve and d _eff distribution for the SHG of 1342 nm in BiB₃O₆ crystal. High-power intracavity SHG of 1.34 μm in BiB₃O₆ crystals, at a type-I phase-matching direction of (θ, ϕ)=(9.07°, 0°), was performed with a LD-end-pumped Nd:YVO₄ laser. The maximum continuous-wave (CW) and quasi-CW output powers at 671 nm were obtained to be 1.22 W and 4.38 W, with the corresponding optical conversion efficiency of 4.9% and 9.5%, respectively.

2. Phase-matching curve and |d_eff| distribution

With the Sellmeier equation of BIBO presented by Hellwig et al. [3], we calculated the type-I and type-II SHG phase-matching curves at the fundamental wavelength of 1342 nm, as shown in Fig. 1.

 

Fig. 1. Type-I and type-II SHG phase-matching curves at the fundamental wavelength of 1342 nm for the BIBO crystal

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According to the d_ijk values presented by Hellwig et al. [2], the absolute values distribution of d _eff corresponding to the PM directions in Fig.1 was also calculated, which is shown in Fig.2. From the figure, we can see the maximum values of |d _eff| for type-I and type-II PM were calculated to be 3.1 pm/V and 2.83 pm/V, corresponding to the PM directions of (θ, ϕ)=(167.4°, 152.1°) and (45.95°, 7°), respectively. The |d _eff| value for type-I PM direction of (9.07°, 0°) is 2.89 pm/V, which is a little lower than that of (167.4°, 152.1°). So for the simplicity of crystal processing, we chose the type-I PM direction of (9.07°, 0°) as the cutting direction of BIBO crystals used in our experiments.

 

Fig. 2. The |d _eff| distribution corresponding to (a) type-I and (b) type-II PM directions for the SHG of 1342 nm

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3. SHG experimental setup

The intracavity SHG experiments were carried out in a three-mirror folded resonator, as shown schematically in Fig. 3. The pump source employed in the experiments was a 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 bundle at the wavelength of 808 nm was focused into the laser crystal by an optical imaging system with the imaging ratio of 1:1. So the diameter of the pump beam spot in the laser crystal was also 0.4 mm. 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 laser crystal used in the SHG experiments was an a-cut Nd:YVO₄ crystal with Nd³⁺ concentration of 0.3 at.% and dimensions of 3×3×8 mm³. It 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:YVO4 crystal was kept to be about 20 °C during the experiments. An acousto-optical (A-O) Q-switch with high diffraction loss at 1342 nm was placed close to the Nd:YVO₄ crystal in the M₁M₃ arm. Both of its faces were also AR coated at 1342 nm. 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 HT coated at 671 nm on the curved surface, and AR coated at 671 nm on the outside flat surface. The SHG crystal was an 8-mm-long, (θ, ϕ)=(9.07°, 0°)-cut BIBO crystal for type-I critical PM at 1342 nm. 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 laser 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 [4,12,13], 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 the beam 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 ⁴ F _3/2→⁴ I _11/2 transition (1.06 μm) of Nd³⁺, all three mirrors had sufficient transmission (>90%) at 1.06 μm.

 

Fig. 3. Schematic diagram of the experimental SHG laser setup

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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 SHG 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

First, the CW performance of intracavity SHG was investigated by removing the A-O Q-switch out of the resonator. The threshold pump power was measured to be about 625 mW. The CW 671 nm output power as a function of the incident pump power is shown in Fig.4. The maximum CW red output power of 1.22 W was obtained at the incident pump power of 25 W, with the corresponding optical conversion efficiency of 4.9 %.

 

Fig. 4. The average output power of 671 nm light versus incident pump power

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The SHG conversion efficiency can be improved for Q-switched operation due to the higher peak power. So we performed the quasi-CW operation of intracavity SHG by inserting the A-O Q-switch. We investigated the red output powers for different repetition rates of 15, 30, 47, and 70 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). The average output power at 671 nm as a function of incident pump power for various repetition frequencies is also shown in Fig.4. From Fig.4, we can see that, the maximum average output power of quasi-CW red light was obtained to be 4.38 W at the repetition rate of 70 kHz and the incident pump power of 46 W, with the corresponding optical conversion efficiency of 9.5%. The temporal pulse profile and stable pulse train of the output red light are shown in Fig.5. The pulse width was measured to be about 290 ns (Full Width Half Maximum, FWHM). In the case of the repetition rates of 15, 30, and 47 kHz, the average output powers saturated at the lower pump powers. And the maximum output powers were only obtained to be 1.35, 2.17, and 2.98 W at the repetition rates of 15, 30, and 47 kHz, respectively.

 

Fig. 5. The temporal (a) pulse profile and (b) stable pulse train of red light at the incident pump power of 46 W and the repetition rate of 70 kHz.

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In the experiments, the far-field intensity distribution of red laser was very elliptic. It was due to the large difference between the two angular acceptances along the orthogonal directions in the plane perpendicular to the orientation of BIBO crystal. The two angular acceptances, ∆θ and ∆ϕ were calculated to be 0.91 and 74.77 mrad for type-I SHG of 1342 nm in the 8-mm-long (θ, ϕ)=(9.07°, 0°)-cut BIBO crystal, respectively.

4. Conclusion

The PM curves and d _eff distribution for both type-I and type-II SHG of 1342 nm in BiB₃O₆ crystal were calculated, respectively. Taking into consideration of both the |d _eff| value and the simplicity of crystal processing, the type-I PM direction of (9.07°, 0°) was recommended as the optimal cutting direction of BIBO crystals for SHG of 1342 nm. With a LD-end-pumped Nd:YVO₄/BIBO intracavity SHG laser, the maximum CW and quasi-CW output powers at 671 nm were obtained to be 1.22 W and 4.38 W, with the corresponding optical conversion efficiency of 4.9% and 9.5%, respectively.