Laboratory Demonstration of the Local Oscillator Concept for the Event Horizon Imager

TL;DR

This paper demonstrates a laboratory setup for 2-way optical frequency transfer, showing it can provide stable, high-coherence signals suitable for space VLBI black hole imaging at 500 GHz, reducing mission complexity and cost.

Contribution

It presents the first laboratory demonstration of a 2-way optical frequency transfer system capable of supporting space VLBI at 500 GHz for black hole imaging.

Findings

Achieved Allan Deviation of 1.1×10⁻¹⁴/τ within 10 ms to 1,000 s

Maintained high coherence (0.997-0.9998) suitable for 557 GHz space VLBI

Demonstrated stability over several hours, meeting mission requirements

Abstract

Black hole imaging challenges the 3rd generation space VLBI, the Very Long Baseline Interferometry, to operate on a 500 GHz band. The coherent integration time needed here is 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 systems, but the need for 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 has been passed. 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 11 times. The obtained coherence in the 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.