A new technique for space debris laser ranging

Scheme of laser ranging experiments. Laser pulses transmit through the telescope and irradiate on the cooperative or non-cooperative targets at a distance of 237 m or 19 km. Echoes reflected by the targets are coupled into the multimode fiber through receiving optical path, and trigger electrical signals by the SSPD, thereby obtaining the ranges of the targets.

Laser ranging is one of the most accurate technologies for the measurement of objects in space and has been applied successfully to areas such as satellite determination, low Earth Orbit space debris surveillance, and 3D laser imaging. However, the widely used single-photon avalanche detector (SPAD) has drawbacks such as high dark count rate and low repetition rate, thereby limiting the detection range of laser ranging systems.

A joint research team comprising Prof. Ming Li’s group from Beijing Institute of Tracking and Telecommunications Technology, Prof. Labao Zhang’s group from Nanjing University, and Prof. Yaoheng Xiong’s group from Yunnan Observatory, Chinese Academy of Sciences, has proposed a new technique for space debris laser ranging by using superconducting nanowire single-photon detectors (SSPD). They have demonstrated the feasibility of using SSPD, which has a broadband spectrum, low dark count rate, and free detection mechanism, for space debris laser ranging through experiments of long-range laser ranging for cooperative and non-cooperative targets. Their findings have been reported recently in Chinese Optics Letters Volume 14, No. 7, 2016 (L. Xue et al., Long-range laser ranging using superconducting nanowire single-photon detectors).

The traditional SPAD usually operates in Geiger mode, and a maximum of only one photon can be detected during one laser pulse. Thus, SPAD may turn blind by the early-returning photon, which limits its photon detection efficiency. The newly developed SSPD operates in free mode and is retriggerable after its dead time. Therefore, multiple photons can be detected during a single laser pulse, thereby improving the photon detection efficiency significantly. Thus, SSPD offers a new mechanism of photon detection that has significant potential for measuring objects in space.

The use of a lensed multimode fiber increases the equivalent detection area of a device using SSPD approximately thirty times compared with its original size. Thus, non-cooperative targets at a distance of 19 km could be ranged successfully with a precision of sub-centimeter or even several millimeters. To the best of the researchers’ knowledge, this is the first demonstration of diffuse reflection laser ranging at a distance of tens of kilometers. A free detection signal-to-noise (SNR) model was used to evaluate the feasibility of space debris laser ranging. Next, ground-based equivalent experiments were conducted to demonstrate the feasibility of space debris laser ranging using SSPD. The reported results demonstrated a significant improvement of the detection range of diffuse reflection laser ranging, thereby establishing the feasibility of space debris laser ranging using SSPD. These findings are expected to advance space situational awareness and space surveillance significantly.

“The work significantly improves the detection range of laser ranging for non-cooperative targets and indicates the feasibility of space debris laser ranging using SSPD. Results show that the SSPD is very attractive for space debris surveillance and space environment protection” said Li Xue, a member of the research team and the first author of the related article.

Future work will focus on improving the target tracking system and quantum efficiency of SSPD, and laser ranging experiments will be conducted with actual space debris. Additionally, laser ranging using SSPD in infrared lasers with eye-safe emission wavelengths is worth exploring.

超导纳米线单光子探测器有望实现空间碎片激光测距

图片说明：实验效果图。激光脉冲经望远镜发射，照射237 m/19 km处的合作/非合作目标，目标反射的回波经接收光路耦合进多模光纤，由SSPD激发出电信号，从而实现目标测距。

激光测距是目前空间目标单站定轨精度最高的方式，已被成功应用于卫星定轨、空间碎片监测、三维激光成像等领域。然而，普遍使用的单光子雪崩探测器(SPAD)存在量子效率低、暗计数率高、工作重复频率低等缺点，限制了光子探测效率，也制约了激光测距系统的作用范围。

北京跟踪与通信技术研究所李明研究员课题组、南京大学超导电子学研究所张蜡宝副教授课题组和中国科学院云南天文台熊耀恒研究员课题组联合提出将超导纳米线单光子探测器(SSPD)用作激光测距光子探测接收端，利用SSPD宽响应波段、低暗计数、自由工作等特点，开展了远距离合作/非合作目标超导激光测距实验，并论证了超导激光测距系统探测空间碎片的可行性。相关研究成果发表在Chinese Optics Letters 2016年第7期上 (L. Xue et al., Long-range laser ranging using superconducting nanowire single-photon detectors)。

传统SPAD通常工作在盖革模式，在单个脉冲周期内仅能够探测一个光子，探测器容易被首先到达光敏面的光子触发而停止该周期工作，不利于回波光子探测。而新兴的SSPD可工作在自由模式，被光子触发后经极短时间便可恢复待触发状态，单个脉冲周期内允许多次光子探测，极大提高了光子探测效率。超导探测方法作为一种新型激光测距探测接收机制，在空间目标激光测距领域具有重要发展前景。

该研究利用光纤-微透镜耦合技术将SSPD等效探测面提高近30倍，成功实现了19 km处非合作目标超导激光测距，测距精度达亚厘米甚至毫米量级，据研究人员所知，这是国际上首次报道数十公里漫反射激光测距的实验。此外，通过信噪比建模和回波衰减地面等效实验，创新性地论证了超导探测器实现空间碎片激光测距的可行性。

该研究团队成员之一、相关论文第一作者薛莉认为，该研究在实验上极大地提高了非合作目标超导激光测距作用距离，提出的超导自由探测信噪比模型以及开展的地面等效实验有效论证了超导探测器具备探测远距离空间碎片的可行性，有利于推动SSPD在空间碎片监测、空间环境维护领域的应用。

后续工作主要是改进望远镜的动态目标跟踪问题，提高超导探测器量子效率，开展空间碎片超导激光测距实验。并尝试利用红外波段激光器探索超导探测器应用于人眼安全激光测距的能力。