Independent measurement of XUV coherence and phase noise

Determine whether measuring the coherence of extreme-ultraviolet light relative to an independent reference, such as a singly ionized helium ion, reveals substantially greater phase noise than measurements using correlated extreme-ultraviolet sources.

Background

The proposed Ramsey-comb spectroscopy method depends on phase-coherent extreme-ultraviolet radiation over interpulse delays of hundreds of nanoseconds to several microseconds. The paper contrasts a short coherence time inferred from an independent phase-jitter measurement with a much longer coherence time reported using two extreme-ultraviolet frequency combs derived from the same near-infrared source. Because the shared source can correlate the phase noise of the two combs, the latter measurement may underestimate the phase noise relevant to spectroscopy against an independent atomic reference. Establishing the coherence relative to a singly ionized helium ion would therefore resolve whether substantially larger phase noise is present and would inform the required filtering-cavity design.