In the free-space quantum communication, the performance of communication systems has a great degradation due to the atmospheric effects, such as atmospheric absorption, scattering and turbulence. However, quantum signals with different wavelengths is differently affected by atmospheric characteristics. In this paper, in order to investigate the effects of different wavelengths quantum signals on free-space quantum key distribution, an entanglement-based continuous variable quantum key distribution transmission model is established. Considering the influence of various atmospheric effects on quantum signals, the secret key generation rates are calculated though the homodyne detectors. The simulation results show that the long-wavelength signal can enhance the secret key generation rates at the same transmission distance. Therefore, the long-wavelength signal is more suitable for the free-space quantum transmission.
In this paper, we investigate the influence of temperature changes on the grating periods of periodically poled lithium niobate (PPLN) with segmented grating structure. Furthermore, the effect of temperature on the conversion performance of segmented gratings based wavelength converter is also studied. Giving the basic constraints of the wavelength conversion characteristics, the grating periods of the segmented gratings at different temperatures is collected when the constraint conditions are met. Then, three mathematical equations are achieved by analyzing the collected data to calculate the optimal grating periods for different temperatures. Broad bandwidth with flat response can be obtained by using these optimal grating periods. Finally, comparison between the designed segmented gratings- and uniform grating-based wavelength converters is carried out. The results show that, for the same temperature, the wavelength converter with segmented gratings has broader bandwidth and more flat conversion efficiency than that with uniform grating.
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