Black hole imaging challenges the 3rd generation space VLBI, the Very Long Baseline Interferometry, to operate on a 500 GHz band. The coherent integration timescale needed here is of 450 s though the available space oscillators cannot offer more than 10 s. Self-calibration methods might solve this issue in an interferometer formed by 3 antenna/satellite system, but the need in the 3rd satellite increases mission costs. A frequency transfer is of special interest to alleviate both performance and cost issues. A concept of 2-way optical frequency transfer is examined to investigate its suitability to enable space-to-space interferometry, in particular, to image the 'shadows' of black holes from space. The concept, promising on paper, has been demonstrated by tests. The laboratory test set-up is presented and the verification of the temporal stability using standard analysis tool as TimePod is given. The resulting Allan Deviation is dominated by the 1/$\tau$ phase noise trend since the frequency transfer timescale of interest is shorter than 0.2 s. This trend continues into longer integration times, as proven by the longest tests spanning over a few hours. The Allan Deviation between derived 103.2 GHz oscillators is $1.1\times10^{-14}/\tau$ within 10 ms < $\tau$ < 1,000 s that degrades twice towards the longest delay 0.2 s. The worst case satisfies the requirement with a margin of an order of magnitude. The obtained coherence in range of 0.997-0.9998 is beneficial for space VLBI at 557 GHz. The result is of special interest to future science missions for black hole imaging from space.

中文翻译：

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Event Horizo​​n Imager 本地振荡器概念的实验室演示

黑洞成像挑战了第三代空间 VLBI，即甚长基线干涉测量法，使其在 500 GHz 频段上运行。尽管可用的空间振荡器不能提供超过 10 秒，但此处所需的相干积分时间标度为 450 秒。自校准方法可能会在由 3 个天线/卫星系统组成的干涉仪中解决这个问题，但在第 3 颗卫星中的需要增加了任务成本。频率转移对于缓解性能和成本问题具有特殊意义。研究了 2 路光学频率传输的概念，以研究其是否适用于实现空间到空间干涉测量，特别是从空间对黑洞的“阴影”进行成像。这个概念在纸上很有前景，已经通过测试得到证实。介绍了实验室测试设置，并使用标准分析工具 TimePod 验证了时间稳定性。由于感兴趣的频率传输时间尺度短于 0.2 秒，因此产生的艾伦偏差由 1/$\tau$ 相位噪声趋势主导。这种趋势持续到更长的集成时间，跨越几个小时的最长测试证明了这一点。派生的 103.2 GHz 振荡器之间的艾伦偏差在 10 ms < $\tau$ < 1,000 s 内为 $1.1\times10^{-14}/\tau$，向最长延迟 0.2 s 降低两倍。最坏的情况以一个数量级的余量满足要求。在 0.997-0.9998 范围内获得的相干性有利于 557 GHz 的空间 VLBI。结果对未来从太空进行黑洞成像的科学任务特别感兴趣。