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Black hole imaging challenges the 3rd generation space VLBI, the Very LongBaseline Interferometry, to operate on a 500 GHz band. The coherent integrationtime needed here is 450 s though the available space oscillators cannot offermore than 10 s. Self-calibration methods might solve this issue in aninterferometer formed by 3 antenna/satellite systems, but the need for the 3rdsatellite increases mission costs. A frequency transfer is of special interestto alleviate both performance and cost issues. A concept of 2-way opticalfrequency transfer is examined to investigate its suitability to enablespace-to-space interferometry, in particular, to image the 'shadows' of blackholes from space. The concept, promising on paper, has been demonstrated bytests. The laboratory test set-up is presented and the verification of thetemporal stability using standard analysis tool as TimePod has been passed. Theresulting Allan Deviation is dominated by the 1/$\tau$ phase noise trend sincethe frequency transfer timescale of interest is shorter than 0.2 s. This trendcontinues into longer integration times, as proven by the longest testsspanning over a few hours. The Allan Deviation between derived 103.2 GHzoscillators is $1.1\times10^{-14}/\tau$ within 10 ms < $\tau$ < 1,000 s thatdegrades twice towards the longest delay 0.2 s. The worst case satisfies therequirement with a margin of 11 times. The obtained coherence in the range of0.997-0.9998 is beneficial for space VLBI at 557 GHz. The result is of specialinterest to future science missions for black hole imaging from space.

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