An important resource for realizing long-distance quantum communication is the 1560 nm nonclassical state light field because it produces a low loss window for optical fiber transmission. The most effective method to generate a 1560 nm nonclassical state light field is to pump an optical parametric amplifier using a 780 nm laser. Additionally, 780 nm lasers correspond to the D2 line of 87Rb used in scientific fields, such as atomic cooling, frequency standard, and quantum storage. Therefore, methods for obtaining a highly efficient and stable 780-nm laser have become a widespread concern. With the rapid development of quasi-phase-matching technology, the high-efficiency frequency conversion of multiple lasers with different wavelengths is realized using the external cavity resonance frequency doubling technology based on periodic polarization nonlinear crystal. The external cavity-resonant frequency doubler with a high efficiency based on customized high-quality frequency doubling cavity mirrors and low loss periodically poled potassium titanyl phosphate (PPKTP) crystals are used. However, the PPKTP crystal relies heavily on imports, which restricts the development of native relevant technologies, thereby making it difficult to realize large-scale industrial applications. A periodically poled lithium niobate (PPLN) crystal has a highly effective nonlinear coefficient, especially because manufacturing PPLN crystal is relatively easy. Many native companies can produce high-quality PPLN crystals. However, PPLN crystals need to operate above 90 ℃ to avoid photorefractive damage effects, which inconvenience practical application. MgO-doped PPLN (MgO∶PPLN) crystals have the same nonlinear coefficient as PPLN, but can greatly improve the photorefractive damage threshold. To develop a practical external cavity-resonant frequency doubler using the MgO∶PPLN crystal, studying the external cavity-resonant frequency doubling process systematically using domestic MgO∶PPLN crystal and cavity mirror is necessary.
First, we theoretically analyze the frequency doubling efficiency of external cavity using the MgO∶PPLN crystal with different lengths, which is affected by the mode-matching efficiency of the frequency doubling cavity caused by the change of the periodically polarized tolerance, crystal absorption loss, and crystal thermal lens effect. For a definite fundamental wavelength and polarization period of the nonlinear crystal, the phase matching of a nonlinear crystal and the highly efficient frequency doubling conversion can be realized by accurately adjusting the crystal temperature. However, the polarization period of the nonlinear crystal deviates due to the polarization processing technology, leading to phase mismatch and reducing the efficiency of the frequency doubling conversion. Additionally, due to the increase of the cavity power, the thermal lens effect intensifies and the waist of the cavity mode changes, whereas the size of the waist of the fundamental light does not change. This results in the reduction of the mode-matching efficiency and the efficiency of the frequency doubling. Then, an external cavity-resonant frequency doubler composed of the domestic MgO∶PPLN crystal and cavity mirror is built experimentally (Fig. 1). The frequency doubling cavity is composed of the MgO∶PPLN crystal and two flat-concave mirrors. Next, the MgO∶PPLN crystal with a polarization period of 19.6 μm is temperature-controlled using an oven and a temperature controller. The length of the frequency doubling cavity is locked using the PDH(Pound-Drever-Hall) technique. Then we experimentally measure the frequency doubling conversion efficiency and output characteristics of the resonant frequency doubling using MgO∶PPLN crystals with different lengths. By optimizing the length of MgO∶PPLN crystal, we obtain the optimal resonant frequency doubling output. Finally, based on the optimal cavity parameters obtained from the experiment, we design a miniaturized external cavity-resonant frequency doubler with a size of 9 cm×6 cm×7 cm.
When the fundamental frequency optical injection power is 100 mW, the single-pass frequency doubling efficiency of MgO∶PPLN crystals with different lengths are measured (Fig. 3). Short crystal lengths produce large polarization period tolerance. Notably, the effect of the polarization period tolerance on the efficiency is limited. The efficiency increases with the increase of crystal length. However, large crystal lengths produce a sharp decline in the efficiency of the single-pass frequency doubling due to the polarization period tolerance. When the fundamental frequency optical injection power is 1 W, the efficiency of the external cavity-resonant frequency doubling of MgO∶PPLN crystals with different lengths are measured (Fig. 4). When the crystal length is short, the fundamental frequency light loss in the inner cavity is small, and the efficiency increases with the increase in crystal length. However, when the crystal length is large, the polarization period tolerance decreases, and the absorption loss of the crystal increases, thereby reducing the efficiency. In the theoretical calculation, the polarization period error is set to 3 nm. The theoretical calculation is consistent with the experimental value. Furthermore, the optimal crystal length with the best resonant frequency doubling efficiency of 85% is 18 mm. For a crystal length of 10 mm, the corresponding frequency doubling cavity length is 59 mm, and the temperature of the MgO∶PPLN crystal is controlled to be 53.45 ℃. The variation curve of the frequency doubling efficiency with the injected fundamental frequency optical power is measured (Fig. 5). With the increase in fundamental optical power, the frequency doubling efficiency of the external cavity increases and tends to saturation. When the injected fundamental optical power is 1.7 W, we obtain the highest efficiency of 84.1%. The stability of the output power of the miniaturized resonant frequency doubler for 3 h is measured (Fig. 6); when the output power is 1.64 W, the power fluctuation is ±0.17%.
In this paper, we design a 1560 nm laser external cavity resonant frequency doubler using MgO∶PPLN crystals. Based on the theoretical analysis of the influence of the mode-matching efficiency change of frequency doubling cavity caused by the change of polarization period tolerance, the absorption loss, and thermal lens effect of MgO∶PPLN crystals with different lengths on the external cavity resonant frequency doubling efficiency , we experimentally build an external cavity resonant frequency doubler composed of domestic MgO∶PPLN crystal and cavity mirrors. The effects of MgO∶PPLN crystals with different lengths on the frequency doubling efficiency are experimentally studied. Furthermore, we design a miniaturized external cavity resonant frequency doubler. The results show that when the temperature of the MgO∶PPLN crystal is controlled to be 53.45 ℃ and the injected 1560 nm fundamental frequency light power is 1.7 W, the optimal resonant frequency doubling efficiency is 84.1% using a 10 mm MgO∶PPLN crystal and the output power stability of the frequency-doubled laser is better than ±0.17% in 3 h. The miniaturized external cavity resonant frequency doubler with stable output at 780 nm can be used to prepare a 1560 nm nonclassical light field and to research the quantum storage based on rubidium atoms, which promotes the industrialization of quantum technology.
