One of the key techniques is the detection of near infrared ( 1310nm and 1550nm ) . because the quantum key is transmitted by one photon , and 1310nm and 1550nm infrared are the lost loss wavelength in fiber . we can detect the visible light now , but it is difficult to detect the near infrared because of its low energy 量子通信的关键技术之一就是红外( 1310nm 、 1550nm )单光子探测，这是因为光量子密钥传输是采用单个光子来实现的， 1 . 3微米和1 . 5微米是现在所使用的光纤通信中损耗最小的波长，现有成熟的单光子探测器工作波长都是在可见光波段，而红外光子因为能量小，信号非常微弱，极易被噪声淹没，因而非常难探测到。
It is composed of thirteen functin ? components including two 3 - db y - junction power splitter , two directional couplers , two waveguide attenuators and one polarizer of waveguide itself . the prominent feature of the device is that it has a beat detection function with respect to the single y - branch mioc device , and it plays an important role in improving the inertial - grade and the high precision steady - aim system because of the beat detection function . farthermore it has direct influence on the improvement of the high flexibility , shockproof and overload - resistance for the steady - aim system 器件的工作波长为1 . 3 m ,集分束器、合束器、起偏器/检偏器、相位调制器、波导吸收器等十三个功能单元于一体,除了单y多功能集成光路器件具备的功能外,它还具有拍频检测功能,因而对提高惯导级和高精度稳瞄系统起着重要的作用,并直接影响着稳瞄系统的高机动性、抗冲击力和过载能力的提高。
1550nm is chosen as the work wavelength , cassegrain transmitting - receiving antenna as the optical antenna of the system through the elucidation of how to choose the work wavelength and optical antenna of comm - unication system . the structure and theoretical design parameter of apt sub - system have been presented through the discussion of the optical signal power estimation of apt sub - system , beam divergence angle , pointing error , and error signal extraction . compared with existed optical wireless communication system , the design scheme proves applicable 通过对通信系统工作波长选择、光学天线选择等阐述，选择了1550nm作为系统工作波长、卡塞格伦（ cassegrain ）收发合一天线作为系统光学天线；通过对apt子系统功率测算、束散角和瞄准误差、误差提取信号等的讨论，给出了apt子系统结构和理论设计指标，并将其与现有的光无线通信系统进行比较，说明该设计方案是可行的。
More information of objects can be obtained from developing the wide - band radar and ultra wide - band radar . with the frequency increasing and the wavelength decreasing , the original objects become electrically large objects . so it is important to study the wide - band electromagnetic scattering characteristics of electrically large objects 发展宽带雷达和超宽带雷达可以获得目标更多、更丰富的信息，而随着雷达频率的提高，工作波长的缩小，被研究的雷达目标成为电大尺寸目标，所以研究电大目标宽带电磁散射特性具有十分重要的意义。
