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.
A hybrid genetic algorithm is proposed to optimal design the wavelength converter which using segmented grating
structure and cascaded second-harmonic generation and difference-frequency generation process. Investigation of the
influences of the structure parameters on conversion bandwidth and conversion response are carried out. High conversion
efficiency, flat response and broad conversion bandwidth can be obtained simultaneously, by adding the segment number
of QPM grating and optimizing the poling period of each segment. The utilizing of the hybrid genetic algorithm can not
only make one obtain precise optimal results, but also shorten the simulation time significantly, so it is helpful to the
practical design of the wavelength converters.
Broadband wavelength converters based on difference frequency generation (DFG), single-pass and double-pass
cascaded second harmonic generation and difference frequency generation (SHG+DFG), single-pass and double-pass
cascaded sum and difference frequency generation (SFG+DFG) are fulfilled by utilizing segmented grating structure in
lithium niobate waveguide. Under the small-signal approximation, the effects of the waveguide length and the response
flatness on DFG-based wavelength conversion are investigated by use of matrix operator, and then a feasible scheme to
enhance the bandwidth and stability of signal and pump wave is proposed. For single-pass/double-pass SHG+DFG
wavelength conversion scheme and single-pass/double-pass SFG+DFG wavelength conversion scheme, the effect of
segmented grating structure on conversion bandwidth is studied theoretically and numerically, the results show that the
signal conversion bandwidth can be broadened by optimizing the aperiodic grating. Moreover, the influence of "balance
condition" on double-pass SFG+DFG-based wavelength conversion is analyzed, one can achieve enhanced conversion
efficiency and conversion bandwidth by adjusting the power and wavelengths of pump sources according to the "balance
condition". In the end, a reasonable suggestion is presented to achieve broadband wavelength conversion under different
requirements.
The tunable bandwidth and parametric bandwidth are investigated theoretically and numerically for periodically poled
LiNbO3 based optical parametric amplification. For two different optical parametric amplification geometries, which we
called as geometry I and geometry II respectively according to different positions of the wave vector ks and km with respect to wave vector kp, both the grating period of PPLN and the noncollinear angle θ have optimal values when signal beam can be amplified in a maximum tunable range. With the increase of temperature, the optimal grating period nearlinearly decreased while the optimal noncollinear angle θ almost unchanged. An equation is derived to achieve the
optimal grating period for different temperatures. Under the same conditions, the tunable bandwidth we achieved by
employing geometry I is closed to that of geometry II. Considering the stability of tunable bandwidth, geometry II should
be chose for the actual optical parametric amplification geometry because the tunable bandwidth is insensitive to the
small deviation of θ when employing it. The parametric bandwidth varied with grating period and noncollinear angle θ.
Compared with the change of θ, parametric bandwidth is sensitive to the variation of grating period. When achieving
maximum and flat parametric bandwidth, the optimal grating period and optimal noncollinear angle θ are equal to those
at which we obtained the maximum tunable bandwidth.
A novel scheme of global adaptive dispersion compensation combined with extended control plane in transparent optical
networks is proposed. The performance of transport plane of ASON is monitored by extended control plane in this paper.
When the residual dispersion at optical receiver can't satisfy the transmission performance demand, a module of
compensation budget computing will be activated to compute adjustment quantity of all tunable dispersion
compensators, and then every tunable dispersion compensator adjust their parameters respectively according to the
adjustment quantity to realize global adaptive dispersion compensation. Simulation results indicate that it is more
efficient than separate compensation of using adaptive dispersion compensator.
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