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中国科学院云南天文台于2016年建立了异地收发1 064 nm激光测距平台,图1为其示意图,53 cm双筒望远镜发射激光,1.2 m望远镜接收激光回波光子,两台望远镜距离约30 m[12]。试验平台主要参数如表1所示,发射系统望远镜有效口径为510 mm,激光器单脉冲能量约为400 mJ,波长为1 064 nm,重复频率为100 Hz;接收系统望远镜有效口径为1 060 mm,单光子超导探测器对1 064 nm波长激光的探测效率优于60%,恢复时间少于50 ns。
表 1 试验平台主要参数
Table 1. Main parameters of testing platform
Item Value Transmitting telescope diameter/mm 510 Receiving telescope diameter/mm 1 060 Laser wavelength/nm 1 064 Laser pulse energy/mJ 400 Frequency/Hz 100 Detecting efficiency of SNSPD(@1064 nm) >60% Dead time/ns <50 激光测距时,激光器输出的激光通过53 cm双筒望远镜1级扩束系统后再经E镜、D镜、C镜、B镜以及A镜等五面反射镜依次反射至望远镜副镜,再经望远镜主副镜完成两级扩束后发射至空间目标;被空间目标反射回地面测站的少部分光子信号进入1.2 m望远镜的接收主副镜,然后被其反射至分光镜、反射镜以及缩束系统后从光纤耦合进入超导单光子探测器。
图2~4分别显示了使用上述试验平台在夜间观测到的Beaconc激光测距卫星、compassi6b北斗卫星以及空间碎片22803的激光测距回波信号情况,横坐标为时间,单位为s,纵坐标为空间目标实测距离与其轨道预报距离之差,单位为ns。观测试验中,探测器工作于无门控的自由模式。
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结合单光子超导探测器的优点和考虑白天激光测距的强背景噪声等特点[13-14],2018年12月22日、2018年12月23日以及2019年1月24日,使用上述测距试验平台进行了基于超导单光子探测器的晨昏卫星激光测距试验,并获得了部分卫星的白天激光测距回波信号,观测结果如表2所示。
表 2 获得回波信号的部分卫星
Table 2. Part of satellites receiving echo signal
Date Satellites Sunset epoch/s Start epoch/s Δt/s 2018-12-22 glonass122 66 297 66 698 401 2018-12-23 glonass134 66 328 65 867 –461 2018-12-23 Beaconc 66 328 67 034 706 2018-12-23 hy2a 66 328 67 444 1 116 2019-01-24 hy2a 67 646 67 498 –148 2019-01-24 glonass103 67 646 67 811 165 由表2可知,2018年12月23日18:17:47开始测到导航卫星glonass134 (约20 000 km)的一段观测数据,这天日落时间为18:25:28[15],因此是在日落前461 s开始观测到数据;2019年1月24日在日落前148秒开始测到测距卫星hy2a (约1 000 km)的一段观测数据。图5和图6分别是glonass134和hy2a卫星的白天跟踪测距界面图。横坐标为时间,单位为s,纵坐标为空间目标实测距离与其轨道预报距离之差,单位为ns。
图 5 glonass134(~20 000 km)激光测距界面截图
Figure 5. glonass134(~20 000 km) laser ranging interface screenshot
图7为2018年12月22日观测到glonass122后于18:38:39用相机拍摄的天空背景亮度图,当天日落时间为18:24:57,因此是日落后13.7 min分拍的。
由于发射望远镜和接收望远镜相隔约30 m,且白天卫星不可见,很难实现高精度同步指向,因此,每次试验时均花了大量的时间进行目标搜索,但在不做任何信号衰减时超导单光子探测器在试验时间内均能正常工作,观测结果表明单光子超导探测器可应用于白天空间目标激光测距。
Superconductivity detector applied to daytime satellite laser ranging experiment and research
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摘要: 超导纳米线单光子探测器是一种新型的单光子探测器,灵敏度高、暗计数低且可工作于恒流模式,对1064 nm波长激光具有较高的探测效率。将该探测器应用于夜间卫星和空间碎片激光测距试验,获得了较好的观测结果。为了使超导探测器的优点在空间目标激光测距中得到充分的应用,通过卫星白天激光测距观测试验以及对白天天空背景光实际响应输出测量等方法,研究分析了超导探测器应用于白天激光测距的可行性。在日落前测到了约20 000 km距离的导航卫星glonass134以及低轨卫星hy2a;实际白天天空背景光测量时,单光子超导探测器最高计数输出可达到约2 MHz。结果表明,使用超导单光子探测器作为回波探测器可实现高性能和高效率的白天激光测距系统。Abstract: As a novel single photon detector, the superconducting nanowire single photon detector can work in constant current mode with high sensitivities, low dark counts, and high detecting efficiencies for 1064 nm wavelength laser. Applying the detector to satellites with retroreflectors and space debris without any retroreflector laser ranging experiments in nighttime, a set of good observing data was obtained. To make advantages of this detector in space target laser ranging, the feasibility of employing it to daylight laser ranging system was studied and analyzed through the daytime laser ranging experiment and the actual response output measurement for daytime sky background light. The navigation satellite glonass134 (~20 000 km) and the low-orbit satellite hy2a were measured before sunset; the maximum counts output of the single-photon superconducting detector could reach up to 2 MHz during the actual daytime sky background light measurement. The results show that using the superconducting single photon detector as echo detector can realize a high performance and high efficiency daytime laser ranging system.
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表 1 试验平台主要参数
Table 1. Main parameters of testing platform
Item Value Transmitting telescope diameter/mm 510 Receiving telescope diameter/mm 1 060 Laser wavelength/nm 1 064 Laser pulse energy/mJ 400 Frequency/Hz 100 Detecting efficiency of SNSPD(@1064 nm) >60% Dead time/ns <50 表 2 获得回波信号的部分卫星
Table 2. Part of satellites receiving echo signal
Date Satellites Sunset epoch/s Start epoch/s Δt/s 2018-12-22 glonass122 66 297 66 698 401 2018-12-23 glonass134 66 328 65 867 –461 2018-12-23 Beaconc 66 328 67 034 706 2018-12-23 hy2a 66 328 67 444 1 116 2019-01-24 hy2a 67 646 67 498 –148 2019-01-24 glonass103 67 646 67 811 165 -
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